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1. INTRODUCTION

Diseases of calcium, phosphate, and skeletal metabolism are among the most common group of disorders that the practicing physician will encounter (1). They can involve abnormalities in the serum concentrations of the two minerals, especially calcium; abnormalities of bone; and abnormalities of the major regulating organ systems, especially the parathyroid gland, kidney and gastrointestinal (GI) tract (Table 1). The serum calcium concentration can be abnormally high, as in malignancy and primary hyperparathyroidism, or abnormally low as it is in renal failure and hypoparathyroidism. The skeleton can have low bone density, as occurs in osteoporosis and osteomalacia, or high bone density as Paget's disease of bone and osteopetrosis. The GI tract can exhibit low calcium absorption, as in malabsorptive states, or high calcium absorption, as in vitamin D intoxication and the milk-alkali syndrome. The kidneys can fail to excrete calcium, as occurs in some hypercalcemic disorders; overexcrete calcium, as in some cases of nephrolithiasis; underexcrete phosphorus, as in renal failure; and overexcrete phosphorus, as in some renal tubular disorders. Corresponding events occur for magnesium, but they will not be discussed in this chapter. The goal of this chapter is to discuss the normal regulation of bone mineral metabolism in order to provide the clinician a basis for diagnosis and management of patients with the common disorders that involve this homeostatic system.

As detailed in other chapters, disorders of mineral and skeletal metabolism can be due to a primary disease of one of the involved organ systems, as in primary hyperparathyroidism due to a tumor of the parathyroid gland; secondary hyperparathyroidism, due to a compensatory response of the parathyroid gland to a low serum calcium; perturbations in serum calcium due to malignancy and bone metastases; and the complex mineral and skeletal complications of GI and renal failure. A basis for understanding the pathogenesis of the primary and secondary diseases of bone and its minerals that are discussed in this text is an appreciation of the interplay among hormones, minerals, and organ systems that the regulates normal bone and bone mineral metabolism (Figure 1).

The skeleton is the reservoir of calcium for many physiological functions, and it serves a similar but not so unique role for phosphorus and magnesium (Table 2) (2,3). Skeletal calcium is controlled through the regulatory pathways of the gastrointestinal (GI) tract and the kidney, and this regulation is mediated in bone by the osteoblast, the bone?forming cell, and the osteoclast, the bone?resorbing cell. Calcium reaches the skeleton by being absorbed from the diet in the GI tract. Unabsorbed calcium passes into the feces, which also contains the small amount of calcium secreted into the GI tract. Minor losses occur through perspiration and cell sloughing. In pregnancy, substantial losses can occur across the placenta to the developing fetus and through breast milk. Absorbed dietary calcium then enters the extracellular fluid (ECF) space and becomes incorporated into the skeleton through the process of mineralization of the organic matrix of bone, osteoid. ECF calcium is also filtered by the kidney at a rate of about 6 grams per day, where up to 98 percent of it is reabsorbed (Figure 1).

Figure 1 - Schematic Representation of Calcium and Skeletal Metabolism Abbreviations: A, absorption; S, secretion; ECF, extracellular fluid; GF, glomerular filtration; TR, tubular reabsorption. The dark vertical line between bone and ECF represents bone surface and bone-lining cells. Shaded area represents labile skeletal calcium. The various calcium compartments are not to scale. See text for discussion. (see Acknowledgements)

The regulation of bone and bone mineral metabolism results from the interactions among three hormones - parathyroid hormone (PTH), calcitonin (CT), and Vitamin D (VD) - at these three target organs - bone, kidney, and GI tract - to regulate bone minerals - calcium and phosphorus. Other hormones and mineral also play a role (Table 1). The deviations from this normal regulatory scheme that occur in disease states can be appreciated, addressed, and, in most cases, effectively managed by the clinician when considered in the light of skeletal homeostasis (1-3).

Cellular and intracellular calcium and phosphorous metabolism

Physicians are most interested in the clinical status of calcium and skeletal metabolism in the patient, as is evidence by the circulating concentrations of these minerals in biological fluids, especially blood and urine, and by the structural integrity of the skeleton (1). The actions of the calcemic hormones to regulate mineral concentrations in biological fluids are well understood the target organ level. However, less well understood are the cellular and intracellular mechanisms that underly the clinically important phenomena.

Both calcium and phosphorous, as well as magnesium, are transported to blood from bone, renal, and GI cells, and vice versa (4-6). These transport mechanisms can be through cells (transcellular) and around cells (paracellular). The cellular transport is mediated by the membrane structures illustrated in Figure 2 and by binding transport proteins (7,8). The paracellular transport is generally passive and mediated by mineral gradients. These mechanisms also involve corresponding co-transportation and exchange-transportation with other ions, notably sodium, potassium, chloride, hydrogen, and bicarbonate, some of which are powered by ATP hydrolysis. Similar mechanisms allow for the intracellular distribution of calcium, where it partitions primarily between the mitochondria and cytosol. The details of the regulation of these cellular and intracellular mineral transport are not as well understood as are the whole organ mechanisms that they effectuate. However, some evidence along with inferences lead to the tentative clinical conclusion that changes in ambient concentrations of mineral in extracellular fluids are mirrored by corresponding intracellular changes and redistribution (Figure 2).

Figure 2. Schematic representation of cellular transport of bone minerals. The model can be applied to transport of calcium, magnesium, and phosphorus for cells of the renal tubules, gastrointestinal tract enterocytes, and bone cells. The mineral transport can be with (downhill) or against (uphill) a gradient. Lumen refers to GI and renal tracts; for bone, it can refer to bone marrow, blood, and/or matrix space. The site of the indicated membrane transport structures is schematic. Microsomes designate other intracellular organelles such as secretory vesicles and endoplasmic reticulum. See text for details.

Figure 2 provides a simplified version of the cellular regulation of bone mineral metabolism and transport. Mineral homeostasis requires the transport of calcium, magnesium, and phosphate across their target cells in bone, intestine, and kidney. This transport can be across cells (transcellular) and around cells (pericellular). The pericellular transport is usually diffusional, down a gradient ("downhill"), and not hormonally regulated. Diffusion can also occur through cell channels, which can be gated. Transport across cells is more complex and usually against a gradient ("uphill"). This active transport is energized by either ATP hydrolysis or electrochemical gradients and involves membrane structures that are generally termed porters, exchangers, or pumps. Three types of porters have been described, uniporters of a single substance; symporters for more than one substance in the same direction; and anti-porters for more than one substance in opposite directions (7,8).

Once through the lumenal cell membrane, the minerals can cross the cell into the extracellular fluid compartment, blood for enterocytes and urine for renal epithelium cells (5,6). For bone cells, the corresponding compartments are marrow and blood (1,2). For calcium, the transcellular transport is ferried by the interaction among a family of proteins that include calmodulin, calbindin, integral membrane protein, and alkaline phosphatase; the latter three are vitamin D dependent (6). Cytoskeletal interactions are likely important for transcellular transport as well. Exit from the cell is regulated by membrane structures similar to those that mediate entry. There do not appear to be any corresponding binding proteins for phosphorous, so diffusional gradients and cytoskeletal interactions seem to regulate cellular transport.

The molecular details of the hormonal regulation of cellular bone mineral transport have not been fully elucidated. It is reasonable to hypothecate that PTH, CT, and Vitamin D regulate these molecular mechanisms through their biological effects on the participating membrane structures and transport proteins. For the enterocyte, vitamin D is central in enhancing the movement of calcium into the cell through its stimulation of calbindin synthesis (6). For kidney tubules, PTH is the key regulator in a corresponding manner for the transport of phosphate and calcium (5). For bone, PTH and CT are the major regulators of cellular calcium and phosphate transport, while vitamin D provides appropriate concentrations of these minerals through its renal and GI actions (1-3).

It is important to note that these minerals translocations not only mediate the organ mineral metabolism represented in Figure 2, but also the cellular effects summarized in Table 3.

2. CALCIUM METABOLISM

Serum and extracellular calcium concentrations in mammals are closely regulated within a narrow physiologic range that is optimal for the many normal cellular functions affected by calcium in many tissues (1,2). It is the ionized component of serum calcium that is closely regulated, as it subserves the physiological functions of this divalent cation (Table 3). Ambient calcium is so close to its saturation point in the respect to phosphates that deviations in concentrations of either can cause precipitation. Intracellular calcium, which serves as second messenger in many signal transduction pathways, is also tightly controlled, but at concentrations several orders of magnitude lower than extracellular calcium. Extraskeletal calcium accounts for only 1% of the total body calcium and is primarily sequestered in bone (Table 4-6). The average diet can be considered to contain about 1 gm of calcium, but there are great variations. About 500 mg undergoes net absorption from the diet, and the unabsorbed and secreted components appear in the stool (Table 6-9). Approximately 10,000 mg/day is filtered at the glomerulus and most is reabsorbed by the renal tubules, with only a few hundred milligrams appearing in urine each day (Tables 10 and 11). The skeleton turns over about 250 mg/day of calcium, but there is wide variation. This turnover is attributed to a labile calcium pool near bone surfaces, but it is different to give anatomical assignment to either labile or non-labile calcium compartments. The turnover is mediated by bone-forming osteoblasts and bone resorbing osteoclasts. In disease states, the turnover can be increased or decreased with corresponding changes in blood and urinary calcium. The calcium regulating hormones that control this homeostatic system are PTH and vitamin D, which act at bone, kidney, and GI tract to increase serum calcium and calcitonin, which an correspondingly act to decrease serum calcium (Figure 1).

Approximately 50% of the total calcium in serum is ionized, with the rest bound primarily to albumin or complexed with counter?ions, including phosphates (Table 6) (1,2). The ionized calcium concentration averages 1.25 + 0.07 mmol/L and the total serum calcium concentrations range from 8.5 to 10.5 mg/dL. Since ionized calcium has the primary regulatory role, it is in turn the regulated component that maintains homeostasis. This regulation takes place through the complex interactions at their target organs of the primary calcium regulating hormones, parathyroid hormone (PTH), calcitonin (CT), and vitamin D and its metabolites (Tables 4-11).

3. PHOSPHORUS METABOLISM

Phosphorus in more widely distributed than calcium and also serves a variety of biological functions (Table 2) (3,4). While most of phosphorus is skeletal as hydroxyapatite, 15 % is distributed among extraskeletal sites like phosphoproteins, phospholipids, and nucleic acids (Table 13). In blood, phosphorus exists as the phosphates, H2PO4G and HPO4=, but its concentration is measured as phosphorus, with a normal range of 2.5 - 4.5 mg/100 ml. The regulation is not as tight as it is for calcium, with substantial perturbations due to diet and alimentation.

Dietary phosphorus comes from most foods, averaging about 1 gm per day (Table 14), with the most important sources being dairy products, grains, meats, and food additives (2,4). Absorption takes place at a site distal to duodenum and utilizes both calcium dependent and calcium independent mechanisms that can be active or passive. The most significant quantitatively is post-prandial passive absorption. Approximately 60-80% is absorbed primarily by a diffusional process without a significant saturable component; however, there is regulation by the calcitropic hormones, especially Vitamin D, whose active metabolites increases absorption, while PTH and CT have only minor direct effects (Tables 13 and 14). Calcium- and aluminum-containing phosphate binders can inhibit absorption and are used to do so in the treatment of renal disease. Fecal phosphate comprises non-absorbed and secreted components (Table 14).

Renal phosphate reabsorption controls the concentration of phosphate in serum, and it is usually quantified as the tubular resorption of phosphorus and expressed as the renal phosphate threshold (TmP/GFR), which closely mirrors the normal range of serum phosphorus (5). Although the TmP/GFR can be measured, it is usually estimated by a nomogram from measurements of serum and urinary phosphorus and creatinine. The proximal convoluted tubule reabsorbs about 75 percent of filtered phosphate, and most of the remainder is reabsorbed in the proximal straight tubule; the distal tubule segments may have a limited capacity for reabsorption, about 5 percent of filtered load (1,5).

Urinary phosphate is the major route of homeostatic regulation (5). About 90% is filtered, with reabsorption being the major regulatory step, primarily at the proximal tubule, where 80% is reabsorbed, mostly in the convoluted segment. PTH and CT increase phosphorus excretion, and Vitamin D decreases its excretion (Table 15). The most important factors regulating proximal phosphate reabsorption are PTH and the dietary phosphate intake. By resetting the renal phosphate threshold, PTH allows the kidney to prevent increases in serum phosphate with calcium. This protective mechanism is compromised in renal failure. Other factors that regulate phosphate reabsorption include growth hormone and insulin which increase proximal phosphate reabsorption. Glucocorticoids and calcitonin have a phosphaturic effect; and acidosis, both acute and chronic, causes phosphaturia. At all three sites of phosphate reabsorption, the proximal convoluted tubule, proximal straight tubule, and distal tubule, PTH decreases phosphate reabsorption. Calcitonin inhibits phosphate reabsorption in the proximal convoluted and proximal straight tubule; an action of calcitonin on the distal tubule is uncertain.

4. SKELETAL METABOLISM

The metabolic function of bone is to provide a homeostasic mineral reservoir, primarily for calcium, but also for other minerals, especially magnesium and phosphorus (1-3). These bone minerals can be mobilized to maintain systemic mineral homeostasis. This metabolic function of bone prevails over its structural function in that calcium and other minerals are removed from and replaced in bone to serve systemic homeostatic needs irrespective of loss of skeletal structural integrity. Bone is also a depository for certain cytokines and growth factors that can be released upon its resorption and can exert their effects locally and systemically.

Bone consists of a mineral phase and an organic phase (Table 5) (2). The major component of the mineral phase is hydroxyapatite crystal and the major component of the organic phase is type 1 collagen which, with other bone proteins, comprises the osteoid matrix of bone. The organic components of bone are products of the osteoblast. Bone mineral is present in two forms in the skeleton. Hydroxyapatite crystals, represented by the formula Ca10(PO4)6(OH)2, are the major forms and occur in mature bone. Amorphous calcium phosphate comprises the remainder; it occurs in areas of active bone formation and matures through several intermediate stages to hydroxyapatite. The end result is a highly organized amalgam of protein, primarily collagen, and mineral, primarily hydroxyapatite, that has sufficient structural integrity to serve the mechanical functions of the skeleton. Upon completion of this process, the osteoblast becomes encased in bone and become an osteocyte. Mineralization can occur if there is a functionally adequate local concentration of these ions, if nucleators are present to promote crystallization, and if local inhibitors of mineralization are removed. While Vitamin D is key to providing sufficient ambient concentrations of calcium and other minerals to promote mineralization of osteoid, this hormone does not seem to exert a direct regulatory effect on mineralization.

Cortical bone comprises approximately 80% of the skeleton and trabecular bone 20% (1,3). However, the surface area of cortical bone is only one fifth that of trabecular bone, so trabecular bone is metabolically more active than cortical bone, with an annual turnover (remodeling) of approximately 20% to 30% for the former and 3% to 10% for the latter. A given skeletal site in the adult is remodeled approximately every 3 years. Bone mass is acquired up to the fourth to fifth decade, with a rapid phase during adolescent growth. Most of peak bone mass is genetically determined. Women have approximately 30% less peak bone mass than men and experience an accelerated loss after the menopause. Both genders experience age-related loss of bone mass.

Bone Cells

Skeletal metabolism is regulated by bone cells and their progenitors (Figure 3). Among the population of bone cells are osteoblasts, osteocytes, osteoclasts, and lining cells (Table 16) (1-3). Monocytes, macrophages, and mast cells may also mediate certain aspects of skeletal metabolism. Marrow cells contribute to the population of bone cells. The osteoblast forms bone. Osteoblasts express receptors to many bone?active agents such as PTH, PTHrP, vitamin D metabolites, gonadal and adrenal steroids, and certain cytokines and growth factors. The major product of osteoblasts is type 1 collagen, which along with other proteins, forms the organic osteoid matrix that is mineralized to hydroxyapatite.

Figure 3 - Schematic Representation of Osteoclast and Osteoblast Lineages Schematic representation of the osteoclast (top) and osteoblast (bottom) lineages. The two lineages are distinct, but there is regulatory interaction among the cells (vertical arrows). Osteoclasts originate from a hematopoietic stem cell that can also differentiate into a macrophage, granulocyte, erythrocyte, megakaryocyte, mast cell, B-cell, or T-cell. Osteoblasts originate from a mesenchymal stem cell that can also differentiate into a chondrocyte, myocyte, fibroblast, or adipocyte. The terminology for these lineages is still evolving and is herein [over] simplified. Many intermediate steps and regulatory factors are involved in lineage development. (see Acknowledgements)

Osteocytes are osteoblasts that become encased in bone during its formation and mineralization and reside in the resulting lacuna (2,3). While their synthetic activity decreases, the cells develop processes that communicate as canaliculi with other osteocytes, osteoblasts, and the vasculature. Osteocytes thus present acres of cellular syncytium that permits translocation of bone mineral during times of metabolic activity and can provide minute-to-minute exchanges of minerals from bone matrix. Osteocytes are also the likely transducers through their canaliculi of mechanical forces on bone and mediate the complex remodeling response to mechanical stimuli of the skeleton that causes appropriate changes in formation and resorption in response to skeletal loading.

The osteoclast resorbs bone. It is a terminally?differentiated, large, multinucleated giant cell that arises from hematopoietic marrow precursors under the influences of hormones, growth factors, and cytokines (3). The osteoclast resorbs bone by attachment with a ruffled border through adhesion molecules and by secretion of hydrogen and chloride ions that dissolve mineral and lytic proteases, notably lysosomal proteases active at low pH and metalloproteinases and cysteine proteinases that dissolve matrix. In contrast to the receptor?rich osteoblast, the mature osteoclast has few receptors, but it robustly expresses the receptor for CT. After completing its function, the terminally?differentiated osteoclast undergoes apoptosis.

Bone-lining cells are flat, elongated cells that cover inactive bone surfaces. Their function is unknown, but they may be osteoblast precursors or function to clean up resorption and formation debris. Mast cells can be seen at sites of bone resorption and may also participate in this process. Cells of the immune system play a key role in bone metabolism, especially resorption, by their interactions with bone cells that are described later.

Bone Growth, Modeling and Remodeling

Growth, modeling, and remodeling are important processes that allow the skeleton to play its many important roles (1). Bone grows and models under the influence of metabolic, mechanical, and gravitational forces during growth through adolescence, changing its size and shape in the process. Bone growth continues until approximately the third decade. Bone mass continues to increase until the fourth decade (Figure 4).

Figure 4. Peak Bone Mass Schematic representation in relative units of normal skeletal development, demonstrating changes in bone resorption and formation. The crossover of formation/resorption occurs during the fourth decade. In osteoporosis, there is an accelerated loss of bone because of increased resorption and decreased formation. (see Acknowledgements)

Bone in adults renews itself by remodeling, a cycle in which old bone is first resorbed and new bone is then formed to replace it (2,3). Both cortical bone and trabecular bone remodel, but the latter is more metabolically active. Bone remodeling can be divided into several stages that include resorption by osteoclasts and formation by osteoblasts. Remodeling serves to repair skeletal microdamage and to improve skeletal strength in response to mechanical forces. Osteoclasts and osteoblasts communicate with each other during remodeling in a process that is referred to as coupling and mediated by local regulatory signals that are discussed subsequently. Coupling assures a balance of bone formation and bone resorption in the adult skeleton. The process of bone formation is thus balanced by the process of bone resorption.

Cortical bone is resorbed by "cutting cones" of osteoclasts that tunnel through it (2). Trabecular bone remodels on its surface. Most remodeling occurs in trabecular bone and on the endosteal surfaces of cortical bone, with little periosteal remodeling. However in diseases like hyperparathyroidism, subperiosteal resorption is activated. With aging, periosteal remodeling and expansion seems to compensate (mechanically) for bone loss at other sites.

Bone resorption is mediated by the osteoclast, a large, multinucleated cell that is molecularly equipped to dissolve both the mineral and organic phases of bone (1,3). The processes of osteoblast?mediated bone formation and osteoclast?mediated bone resorption can be assessed by measurement of bone markers. Approximately 20% of adult bone surface is undergoing remodeling at any time. The homeostatic end?point of skeletal metabolism is to provide the appropriate amount of ambient calcium for the many biological functions that this ion serves. These metabolic activities of bone cells can release into blood and urine certain bone cells and matrix products that can serve as clinically useful markers of skeletal metabolism (Figure 5).

Figure 5 - Schematic Representation of the Cellular and Skeletal Sources of Serum and/or Urinary Markers of Bone Formation and Bone Resorption Abbreviations: BGP, bone gamma carboxyglutamic acitd (GLA) protein (osteocalcin); PICP, C-terminal propeptide of type I procollagen; PINP, N-terminal propeptide of type I procollagen; BAP, bone-specific alkaline phosphatase; AP, alkaline phosphate; TRAP, tartrate-resistance acid phosphatase; NTX, N-terminal cross-linked telopeptide of type I collagen; CTX, C-terminal cross-linked telopeptide of type I collagen; OH, hydroxyproline glycoside; OL, hydroxylysine glycoside; PYD, pyridinoline (total, free); DPD, deoxypyridinoline (total, free). (see Acknowledgments)

RANKL, RANK, and OPG

The recent elucidation of this novel pathway of molecular regulation has provided both a physiologic link among bone cell functions as well as a pathogenetic link among cancer cells, the immune system, and bone cells in the regulation of the osteoclastic bone resorption that is the final cellular mediator of most cases of hypercalcemia (Figure 1) (9,10). The molecular participants in this pathway are the membrane-associated protein named RANKL (receptor activator of nuclear factor kappa B ligand,) a member of the tumor necrosis factor family of cytokines; its cognate receptor, RANK, and OPG (osteoprotegerin), a soluble "decoy" receptor for RANKL.

RANKL is expressed on the surface of osteoblastic stromal cells (9). By binding to RANK, its receptor, on osteoclast precursors, RANKL enhances their recruitment into the osteoclastogenesis pathway in the physiology of bone metabolism. RANKL also activates mature osteoclasts to resorb bone. RANKL is considered as the long-sought "coupling factor" through which osteoblasts regulate osteoclasts and bone formation is coupled to bone resorption. In the pathophysiology of hypercalcemia, many of the tumor cell types that are associated with cancer-stimulated bone resorption express a soluble form of RANKL, sRANKL. Furthermore, during the inflammation that can be associated with malignancy, activated T-lymphocytes also express increased amounts of RANKL, which can stimulate osteoclasts. The activated lymphocytes also expresses interferon gamma (INF), which opposes the effect of RANKL on osteoclast mediated bone resorption. The osteoclastic effects of RANKL can also be attenuated by its soluble decoy receptor, OPG, also produced by osteoblasts and tumor cells. Hypercalcemia results when these opposing regulatory interactions of RANKL, RANK, OPG, and INF allow osteoclastic activation to predominate (Figure 5).

These molecular participants in the interaction between bone cells, tumor cells, and the immune system are also regulated by several hormones, growth factors, and cytokines that mediate increased bone resorption, both physiologic and pathophysiologic. They include PTH, PTHrP, TNF, PGE2, vitamin D metabolites, IL-1, and TGF (10).

Figure 6. Schematic representation of the cellular and molecular mechanisms of the effects of OPG, RANK, and RANKL on skeletal metabolism. A variety of skeletal and non-skeletal cells can express several cell products [in brackets] that regulate the balance between osteoblastic bone formation (left) and osteoclastic bone resorption (right). They include PTHrP (parathyroid hormone related protein); 1, 25 VitD (1, 25- dihydroxyvitamin D); prostaglandins, especially of the PGE2 series; cytokines, especially interleukin 1 (IL-1); growth factors, especially TGF beta; RANKL (receptor activator of nuclear factor kappa B ligand), a cell membrane-associated member of the tumor necrosis factor family of cytokines; soluble RANKL (sRANKL); and their cognate receptor, RANK; and OPG (osteoprotegerin), a soluble "decoy" receptor for RANKL. The latter group are also expressed by osteoblast precursors as they develop into osteoblasts in the osteoblastic cascade (left). In addition to OPG, the stimulation of osteoclastic bone resorption by RANKL is opposed by activation of the gamma interferon receptor (INFR) by gamma interferon (INF) production by activated lymphocytes and by the peptide hormone, calcitonin. The relative activity of the osteoclast stimulatory effects of RANKL and sRANKL and the inhibitory effects of OPG and INF determine the balance between bone resorption and formation. Arrows indicate a positive (stimulatory) effect except where indicated by the negative sign, (-). Several growth factors in addition to TGF beta reside in bone matrix and can be released upon resorption to exert their biological effects, often osteoclast stimulation. They include BMP (bone morphogenetic proteins, especially BMP-2); FGF (fibroblast growth factor); PDGF (platelet derived growth factor); and IGFs in (insulin like growth factors). Macrophages may fuse into giant cells and resorb bone. (see Acknowledgements)

5. HORMONAL REGULATION OF SKELETAL AND MINERAL METABOLISM PARATHYROID HORMONE

Parathyroid hormone is an 84-amino-acid peptide secreted by two pairs of parathyroid glands located adjacent to the back of the thyroid gland in the neck. There can also be ectopic parathyroid glands along their developmental route between the thyroid gland and mediastinum. The mature PTH is packaged into dense secretory granules for regulated secretion (1,2).

Secretory Regulation of Parathyroid Hormone and the Calcium Sensor

The major regulatory signal for PTH secretion is serum calcium (Table 17) (11). Serum calcium inversely affects PTH secretion, with the steep portion of the sigmoidal response curve corresponding to the normal range of both. An increase in ionized calcium inhibits PTH secretion by increasing intracellular calcium through the release of calcium from intracellular stores and the influx of extracellular calcium through cell membranes and channels. This mechanism differs from most cells, where secretion of their product is stimulated by increased calcium. Intracellular magnesium may serve this secretory function in the parathyroids in that hypermagnesemia can inhibit PTH secretion and hypomagnesemia can stimulate PTH secretion. However, prolonged depletion of magnesium will inhibit PTH biosynthesis and secretion, as it will the function of many cells. Hypomagnesemia also attenuates the biological effect of PTH by interfering with its signal transduction. Serum calcium also inversely regulates transcription of the PTH gene, and increased levels of 1,25?dihydroxyvitamin D (1,25?D) inhibit PTH gene transcription. The parathyroid gland senses the concentration of extracellular ionized calcium through a cell?surface calcium?sensing receptor (CSR) for which calcium is an agonist. The same sensor also regulates the responses to calcium of thyroid C cells, which secrete CT in direct relationship to extracellular calcium; the distal nephron of the kidney, where calcium excretion is regulated; the placenta, where fetal?maternal calcium fluxes occur; and the brain and gastrointestinal (GI) tract, where its function is unknown, and bone cells. Compounds have been identified that act selectively on the CaSR and may be useful treatment agents.

Most studies fail to demonstrate a direct effect of serum phosphate on PTH secretion, but some recent studies show that high phosphate increases PTH biosynthesis and vice versa (4,11). However, serum phosphate has an inverse effect on calcium concentration and ambient phosphate directly increases 1,25?D production. Thus, serum phosphate may directly and indirectly regulate PTH expression.

Metabolism and Clearance of Parathyroid Hormone

Parathyroid hormone has a circulating half-life of less than 5 minutes (2,12). The hormone is metabolized to amino-terminal and carboxyl-terminal fragments primarily in the liver, also in the kidney, and perhaps in the parathyroid gland and blood. The carboxyl-terminal fragments are cleared by glomerular filtration (GF), so they accumulate in renal failure. All of the classic biological effects of PTH are mediated by the amino terminus, PTH1?34, and likely a subpeptide of this sequence, but other fragments may have their own biologic actions.

As a result of the biosynthesis, secretion, and metabolism of PTH, the circulation contains several forms of the molecule (12). The forms that comprise this heterogenous collection of PTH species include primarily native PTH1-84 and mid-region and carboxy terminal PTH fragments. Overall, 10-20% of circulating PTH immunoreactivity comprises the intact hormone, with the remainder being a heterogeneous collection of peptide fragments corresponding to the middle and carboxy regions of the molecule. Recent studies have demonstrated a PTH 7- 84 fragment that accumulates in renal failure and may even be secreted by the normal as well as abnormal parathyroid gland. While only the amino terminus of PTH can bind to the PTH receptor and mediate its classical biological effects that result in hypercalcemia, PTH 7 - 84 may act as an antagonist and/or weak agonist to PTH at its receptor. Nevertheless, it should be kept in mind that each of the circulating forms of PTH, regardless of biological activity, contain within them peptide sequences that can be recognized by a variety of immunoassay systems and thus complicate clinical interpretation.

Biologic Effects of Parathyroid Hormone

Parathyroid hormone regulates serum calcium and phosphorus concentrations through its receptor-mediated, combined actions on bone, intestine, and kidney (3,12). The skeletal effects of PTH on bone are complex. High levels of PTH, as seen in primary and secondary hyperparathyroidism, increase osteoclastic bone resorption. Low levels, especially if delivered episodically, seem to increase osteoblastic bone formation, an effect that can be applicable to osteoporosis treatment. The skeletal effects of PTH are mediated through the osteoblast, since they are the major expressor of the PTH receptor. However, osteoblasts communicate with osteoclasts to mediate PTH effects. Any direct gastrointestinal (GI) effect of PTH on intestinal calcium or phosphate absorption is weak. However, PTH through its stimulating effects on the renal production of 1,25?D, discussed later, promotes the absorption of both. In the kidney, PTH increases the reabsorption of calcium, predominantly in the distal convoluted tubule, and inhibits the reabsorption of phosphate in the renal proximal tubule, causing hypercalcemia and hypophosphatemia. PTH also inhibits NA+?H+ antiporter activity and bicarbonate reabsorption, causing a mild hyperchloremic metabolic acidosis.

PTH mediates its effects through the PTH receptor (13). This receptor is an 80,000-MW membrane glycoprotein of the G protein receptor superfamily. The classic PTH receptor recognizes the amino?terminus of PTH and the homologous terminus of the parathyroid hormone?related protein (PTHrP) with indistinguishable affinity; it is therefore designated the PTH/PTHrP receptor. Both PTH and PTHrP generate cyclic adenosine monophosphate (cAMP) as a cellular second messenger by activating protein kinase A (PKA), and the phospholipase C effector system increasing cellular IP3 and calcium and activating protein kinase C (PKC). There may be some tissue specificity as to which pathway dominates. In addition to this shared receptor, there is accumulating evidence for the existence of receptors that are respectively specific for PTH and PTHrP and for some of their subpeptides.

Effects of Parathyroid Hormone on Calcium and Skeletal Metabolism

Bone

• Increases resorption
• Increases formation, especially at low and intermittent concentrations

Kidney

• Decreases calcium excretion (clearance)
• Increases phosphorus excretion

Gastrointestinal Tract

• Increases calcium and phosphorus absorption
• Indirect effect via 1,25?D production

Blood

• Increases calcium
• Decreases phosphorus

Parathyroid Hormone-related Protein (PTHrP)

PTHrP is the major humoral mediator of the hypercalcemia of malignancy (1,3,12). The polypeptide is a product of many normal and malignant tissues (14). PTHrP is secreted by many types of malignant tumors, notably by breast and lung cancer, and produces hypercalcemia by activating the PTH/PTHrP receptor. PTHrP is produced in many fetal and adult tissues. PTHrP is required for normal development as a regulator of the proliferation and mineralization of cartilage cells and as a regulator of placental calcium transport. The amino terminus of PTHrP reacts with the PTH/PTHrP receptor and produces most of he biological effects of native PTH, including hypercalcemia. The PTHrP gene expresses three forms of polypeptide through alternate messenger ribonucleic acid (mRNA) splicing. In addition to mRNA splicing, processing of PTHrP into peptides is an important regulatory mechanism. Distinct biological properties have been attributed to the different PTHrP peptides, and specific receptors and effects have been identified.

Although multiple, the functions of PTHrP in malignant and normal tissues seem to be growth- and proliferation-related (12,14). In most physiologic circumstances, PTHrP carries out local rather than systemic actions. When produced in excess by malignancy, PTHrP has systemic effects, especially hypercalcemia. Because of its protean and developmental effects, PTHrP can be considered an oncofetal protein.

Malignancy and PTHrP

The hypercalcemia of malignancy is usually due to increased bone resorption that is caused by skeletal metastases or the production by the tumor of a "humour" that stimulates osteoclasts (10,12). It is likely that the first mechanism also involves the second, since most tumor cells do not have the capacity to directly resorb bone and more likely stimulate the neighboring osteoclast to do so through their "humours." Many cell types and their products participate in and many tumor products have been implicated in the pathogenesis of the hypercalcemia of malignancy (Figure 5). The most common seems to be PTHrP, especially in solid tumors where abnormal PTHrP expression can be implicated in up to 80% of patients. Originally discovered as a product of malignant cells that produce hypercalcemia, PTHrP has been demonstrated to be a product of many normal and malignant tissues. The growing appreciation of the key role of PTHRP in the pathogenesis of the hypercalcemia of malignancy has revealed that ectopic PTH production by cancer cells is a rare event.

The PTHRP gene expresses three native forms of the polypeptide through alternate mRNA splicing, PTHRP 1-141, a truncated 139 residue form, and a 173 residue form expressed primarily in humans (12). Whereas PTHRP 1-139 is quite similar to PTHRP 1?141, PTHRP 1-173 completely diverges from both at its own carboxy terminus. The amino?terminus of PTHRP reacts with the shared PTH/PTHRP receptor and has the potential to produce most of the biological effects of native PTH, including hypercalcemia. Other cell products, such as cytokines and growth factors, are also likely to play a casual role in the hypercalcemia because of their direct and indirect skeletal actions These can be produced by the tumor cells or immune cells. TGF beta can also participate in pathogenesis by stimulating PTHrP production from tumors or immune cells as it is released from its skeletal reservoir upon resorption.

CALCITONIN

Calcitonin is a 32-amino acid peptide whose main effect is to inhibit osteoclast?mediated bone resorption (15). CT is secreted by parafollicular C cells of the thyroid and other neuroendocrine cells. In a homeostatically-appropriate contrast to PTH, hypercalcemia increases secretion of hypocalcemia-inducing CT while hypocalcemia inhibits secretion (16). CT secretion is controlled by serum calcium through the same CaSR that regulates PTH secretion, but in an inverse manner and at higher concentrations of calcium. The major effect of CT is to indirectly inhibit bone resorption by inactivating the CT-receptor rich osteoclast. CT also inhibits the renal reabsorption of phosphate, thus promoting renal phosphate excretion. CT also induces a mild natriuresis and calciureses, the latter contributing to its hypocalcemic effect.

The CT receptor, like the PTH and calcium?sensing receptor, is a heptahelical G protein?coupled receptor coupled to the PKA, PKC, and Ca++ signal transduction pathways (17). Several isoforms of the receptor have been identified at different organ sites and may play a tissue?specific effects.

The CT gene through alternative exon splicing and polypeptide processing ultimately encodes two peptide products, CT in thyroid C?cells which is processed from a 141?amino acid precursor, and a 37?amino peptide called gene?related peptide (CGRP) in neural tissues which is processed from a 128?amino acid precursor (1,15). CGRP is weakly recognized by the CT receptor and thereby has a CT?like effect on osteoclasts and osteoblasts. CGRP also acts through its own receptor to produce vasodilation and to act as a neurotransmitter. In addition to its role in calcium and skeletal metabolism, CT is important as a tumor marker in medullary thyroid carcinoma and other neuroendocrine tumors. As an inhibitor of osteoclastic bone resorption, CT is used to treat osteoporosis, Paget's disease, and hypercalcemia. The receptor that mediate the effects of the peptide products of the CT gene can be modulated by accessory proteins to alter binding characteristics (18,19).

Effects of Calcitonin on Mineral Metabolism

Bone

• Inhibits resorption
• ? Promotes formation

Kidney

• Increases calcium excretion
• Increases phosphorus excretion

Gastrointestinal Tract

• ? Inhibitory effect on calcium/phosphorus absorption
  Blood
• Decreases calcium
• Decreases phosphorus

VITAMIN D

Metabolism and Activation

Vitamin D is a secosterol hormone that is present in humans in an endogenous (vitamin D3) and exogenous (vitamin D2) form (20,21). The endogenous form of vitamin D, cholecalciferol (vitamin D3), is synthesized in the skin from the cholesterol metabolite 7?dehydrocholesterol under the influence of ultraviolet radiation. The exogenous form of vitamin D2 (ergocalciferol) is produced by ultraviolet irradiation of the plant sterol ergosterol and is available through the diet. Both forms of vitamin D require further metabolism to be activated, and their respective metabolism is indistinguishable. Vitamin D metabolites are solubilized for transport in blood by specific vitamin D?binding proteins (Figure 7).

Figure 7. The Metabolic Activation of Vitamin D Abbreviations: 25-D, 25-hydroxyvitamin D; 1,25-dihydroxyvitamin D; VDR, vitamin D receptor Vitamin D from the diet or the conversion from precursors in skin through ultraviolet radiation (light) provides the substrate of the indicated steps in metabolic activation. The pathways apply to both the endogenous animal form of vitamin D (vitamin D3, cholecalciferol) and the exogenous plant form of vitamin D (vitamin D2, ergocalciferol), both of which are present in humans at a ratio of approximately 2:1. In the kidney, 25-D is also converted to 24-hydroxylated metabolites which may have unique effects on chondrogenesis and intramembranous ossification. The many effects (Table 2.8) of vitamin D metabolites are mediated through nuclear receptors or effects on target-cell membranes (see Acknowledgments).

In the liver, vitamin D is converted by an hydroxylase to 25?hydroxyvitamin D (25?D), the principal fat storage form of vitamin D (20). Thus, the serum level of 25?D is the best measure of overall vitamin D status. In the proximal tubule of the kidney, 25?D is 1a?hydroxylated to produce 1,25?D, the most active form of the hormone. The more important animal form is referred to as 1,25?dihydroxycholecalciferol. This hydroxylation step is up-regulated by several factors, the most important of which are PTH and low ambient concentrations of calcium, phosphorus, and 1,25?D itself. The 1a?hydroxylase that mediates this conversion in the kidney is also produced in the placenta and in keratinocytes. In certain disease states, macrophages and lymphocytes overexpress 1a?hydroxylase and produce hypercalcemia.

The normal serum concentration of 1,25-D is 20-60 pg/ml. The kidney can also convert 25?hydroxyvitamin D to 24,25?dihydroxyvitamin D. Although this metabolite circulates at 100?fold higher than the concentration of 1,25?D, its biologic role is unclear. Some studies suggest that it is a degradation product with no important biological effects; others suggest that it is important in chondrogenesis and bone formation, especially intramembranous. Vitamin D and its metabolites are inactivated in the liver by conjugation to glucuronides or sulfates and oxidation of their side chains. The requirements for vitamin D are usually satisfied by endogenous synthesis, and milk supplementation with vitamin D makes dietary vitamin D deficiency uncommon in the United States. However, in the winter months, especially in northern climates, the institutionalized elderly may have borderline serum levels of 25?D and may be clinically vitamin D deficient, with 15 ng/dL considered borderline.

Biological Effects of Vitamin D and its Mechanism of Action 

Vitamin D mediates its biological effects through its own member of the nuclear hormone receptor superfamily, the vitamin D receptor (VDR) (20,22). The receptor binds many vitamin D metabolites with affinities that generally mirror their biological effects, and 1,25?D thus has the highest affinity. The VDR regulates gene transcription by homodimerization and by heterodimerization to a retinoic acid x receptor (RXR). The complex binds to target DNA sequences and regulates the transcription of several genes important in mediating vitamin D's effects on calcium and skeletal metabolism and its diverse biological effects. Vitamin D metabolites, as well as other steroid hormones, may also act through a membrane receptor to produce rapid changes in cellular calcium flux (Figure 7) (23).

Intestinal Calcium Absorption

Vitamin D increases intestinal calcium absorption, primarily in the jejunum and ileum, by increasing calcium uptake through the brush border membrane of the enterocyte (Tables 8, 9, and 19). For this action, vitamin D induces the calcium?binding calbindins, which participate in calcium transport across the cell, and through its action on calcium transporting membrane structures (Figure 2), it promotes the efflux of calcium from the basolateral side of the enterocyte into the circulation. The initial effects of vitamin D on intestinal calcium absorption occurs within minutes, so the actions of vitamin D on intestinal calcium transport may be also mediated by a membranous nongenomic receptor. The net result is an increase in the efficiency of intestinal calcium transport. In a vitamin D-deficient state, only 10 to 15% of dietary calcium is absorbed by the gastrointestinal tract, but with adequate vitamin D adults absorb approximately 30% of dietary calcium. During pregnancy, lactation, and growth, increased circulating concentrations of 1,25-D promote the efficiency of intestinal calcium absorption by as much as 50% to 80%. Vitamin D also regulates skeletal metabolism through the RANK pathway (Figure 6). 1,25-D also increases the efficiency of dietary phosphorus absorption by about 15 to 20%.

Bone

The effects of vitamin D metabolites on bone are complex (1). By providing sufficient ambient calcium and/or through some other unappreciated direct effect, vitamin D promotes the mineralization of osteoid. Vitamin D causes bone resorption by mature osteoclasts, but this effect is indirect, requiring cell recruitment and interaction with osteoblasts. Vitamin D also promotes the fusion of monocytic precursors to osteoclasts. Vitamin D regulates several bone proteins. It promotes the transcription of osteocalcin and has bidirectional effects on type I collagen and alkaline phosphatase gene transcription

Kidney

The VDR is robustly expressed in the kidney, and acting through it, 1,25?D stimulates renal proximal phosphate reabsorption and maintenance of normal calcium reabsorption. However, compared to PTH, these effects are relatively weak (20).

Other Tissues

Vitamin D and its metabolites have protean effects on cell function and signaling (22). Although vitamin D has many in vitro effects on the immune system, no major immune defect is apparent in individuals who are deficient or who lack vitamin D or its receptor. Vitamin D also inhibits proliferation and stimulates maturation of epidermal keratinocytes, which robustly express the VDR. This antiproliferative effect is being used for the treatment of psoriasis, a hyperproliferative skin disorder. Since many persons who lack vitamin D receptors have lifelong alopecia totalis, vitamin D may play a role in the maturation of the hair follicle (21).

Effects of 1,25?D (1,25?dihydroxyvitamin D) on Mineral Metabolism

Bone

• Promotes mineralization of osteoid
• Increases resorption at high doses

Kidney

• Decreases calcium excretion
• Decreases phosphorus excretion

Gastrointestinal Tract

• Increases calcium absorption
• Increases phosphorus absorption

Blood

• Increases calcium
• Increases phosphorus

OTHER HORMONES

In addition to the primary calcemic hormones, other hormones play an important role in calcium and skeletal metabolism (1-3). Gonadal steroids maintain skeletal mass. Glucocorticoids are deleterious to all skeletal functions. Insulin, growth hormone, and thyroid hormones promote skeletal growth and maturation. Excess production of the latter can cause hypercalcemia (Table 20).

Table 20: Hormones that Regulate Bone MetabolismDecrease Bone Resorption Calcitonin EstrogensIncrease Bone Resorption PTH/PTHrP Glucocorticoids Thyroid Hormones High dose vitamin DIncrease Bone Formation Growth Hormone Vitamin D Metabolites Androgens Insulin Low-dose PTH/PTHrPDecrease Bone Formation Glucocorticoids

6. SUMMARY

Through their actions and interactions on bone, kidney and the gastrointestinal (GI) tract, the three calcitropic hormones, parathyroid hormone (PTH), calcitonin (CT), and vitamin D metabolites, especially the 1,25?dihydroxyvitamin D (1,25?D) metabolite, act to maintain serum (and extracellular fluid) calcium within a normal range, a range that optimally subserves many calcium?requiring physiological functions. A perturbation in serum calcium, which plays an important role in regulating the concentrations of the calcitropic hormones, will cause a homeostatically appropriate and often reciprocal change in the secretion of PTH by the parathyroid glands, CT by the thyroidal C cells, and in the production of 1,25?D by the kidneys. These responses are designed to return the serum calcium, and, to a lesser extent, the serum phosphorus and magnesium to normal, with the skeleton acting as a resevoir for these minerals that can be emptied or filled.

Thus, the patient with hypocalcemia (nonparathyroid) will have an increased serum PTH and 1,25?D and a decreased serum CT. This will result in increased GI absorption of calcium, increased bone resorption, and decreased renal calcium excretion all acting to increase the serum calcium toward normal. The patient with hypercalcemia (nonparathyroid) will have a decreased serum PTH and 1,25?D and an increased serum CT. This will result in decreased GI absorption of calcium, decreased bone resorption, and increased renal calcium excretion all acting to decrease the serum calcium toward normal. In diseases that involve one of the three calcitropic hormones, the serum concentrations of the other two will change to either amplify the effect of the primary abnormality or to defend against the calcium perturbation. Although these compensatory mechanisms act to restore serum calcium to normal, the homeostasis will not be complete until the primary abnormality has been corrected. In addition to these three calcitropic hormones, other hormones, cytokines, and growth factors play an important role in calcium metabolism. Among the other important hormones are insulin, growth hormone, and the gonadal and adrenal steroids and thyroid hormone (Table 20). They are discussed in other chapters.

ACKNOWLEDGEMENTS

The author substantially and expressly relied on the following publications for the information presented in this text: Deftos, LJ: Immunoassays for PTH and PTHrP In: The Parathyroids, Second Edition, JP Bilezikian, R Marcus, and A Levine (eds.), Chapter 9, pp.143-165, 2001. Deftos LJ and Gagel R: Calcitonin and Medullary Thyroid Carcinoma In: Cecil Textbook of Medicine, Twentieth First Edition, JB Wyngarden and JC Bennett, Chapter 265, pp.1406-1409, 2000. Deftos, LJ: Clinical Essentials of Calcium and Skeletal Metabolism, Professional Communication Inc, First Edition, pp. 1-208, (Figures 1,3-5 and Table 2) 1998 (Published on-line at Medscape.com). The following Chapters in Felig, P and Frohmer, LA. Endocrinology and Metabolism, 4th Edition, McGraw-Hill, 2001: Chapter 22, Mineral Metabolism, Bruder, Guise, and Mundy. Chapter 23, Metabolic Bone Disease, Singer. Chapter 27. Multiglandular Endocrine Disorders, Deftos, Sherman, and Gagel. Deftos, LJ: Hypercalcemia in malignant and inflammatory diseases. Endocrinology and Metabolism Clinics of North America, 31:1-18, (Figure 2) 2002.