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).
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; they are available for treatment of the increased PTH secretion that occurs in primary and secondary hyperparathyroidism, especially the latter. Genetic and functional disorders of the CSR have been described: activating defects cause hypocalcaemia and inactivating defects cause hypercalcemia.
Table 17. Regulation of PTH Biosynthesis and Secretion
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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.
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. For example, the carboxy terminus may regulate calcium channel flux.
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 amino terminal, 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 at a site that mediate its classical biological effects, which 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. It has become recently appreciated that the so-called intact PTH assays do not recognize the far amino-terminus of the molecule, a sequence need for full biological activity. Newer assays, designated “bio-intact” or “whole” apparently do, but any resulting clinical advantage has not been fully documented (12)
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 has been applicable to osteoporosis treatment by daily injections of PTH. 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. This communication seems mediated through the RANK-OPG pathway (9,10).
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. In fact, a send PTH receptor, specific for this peptide, has been cloned (12, 13). For PTH, a carboxy-terminal peptide seems to mediate cellular calcium flux; for PTHrP, a nuclear localizing sequence (NLS) has been identified (12).
Effects of Parathyroid Hormone on Calcium and Skeletal Metabolism
Bone
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Increases resorption
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Increases formation, especially at low and intermittent concentrations
Kidney
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Decreases calcium excretion (clearance)
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Increases phosphorus excretion
Gastrointestinal Tract
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Increases calcium and phosphorus absorption
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Indirect effect via 1,25-D production
Blood
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Increases calcium
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Decreases phosphorus
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 tissues, but as development proceeds its expression becomes restricted. PTHrP expression reappears in adult tissues when injury or malignancy occurs.
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 As discussed later, 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.
PTHrP is required for normal development as a regulator of the proliferation and mineralization of cartilage cells and as a regulator of local 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.
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.
PTHRP expression was initially noted to be common in squamous cell cancers, but it has been subsequently shown that many other cancer types can overexpress PTHRP. PTHRP production and secretion by breast and prostate cancers is especially common, occurring in more than half of the cases, with even a higher incidence in breast when the patient is hypercalcemic. Breast tumors that produce PTHRP are more likely to metastasize to bone, and breast cancers that metastasize to bone are even more likely to produce PTHRP. PTHRP is commonly expressed in lung cancer, especially in those lung cancers that metastasize to bone. While breast and lung cancer are among the most common PTHrP producing tumors that cause hypercalcemia, this pathway has been described in most cancers. PTHrP production that often accompanies prostate cancer does not usually cause hypercalcemia, perhaps because this tumor processes the polypeptide to a non-hypercalcemic peptide. It is notable that some non-malignant PTHrP-producing tumors can also be associated with hypercalcemia. And it has recently been observed that PTHrP produced by these macrophages can also mediate the hypercalcemia of granulomatous diseases like sarcoidosis. Since PTHrP also regulates vitamin D activation and the RANKL-mediated osteoclast-activating pathway, another interactive regulatory pathway exists for the hypercalcemia of this disease (Figure1).
While PTHrP is the most common Ahumour@ produced by malignant cell to cause osteoclast-mediated hypercalcemia, increased 1,25-dihydroxy vitamin D is causal in lymphomas and some leukemias. And certain cytokines, notably IL-1, and growth factors, notably TGF beta, can also produce hypercalcemia by stimulating osteoclastic bone resorption; but excess prostaglandin production is no longer considered an important hypercalcemic Ahumour@ in malignancy.
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.
Table 18. Regulation of Calcitonin Secretion
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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
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Inhibits resorption
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? Promotes formation
Kidney
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Increases calcium excretion
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Increases phosphorus excretion
Gastrointestinal Tract
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? Inhibitory effect on calcium/phosphorus absorptionBlood
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Decreases calcium
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Decreases phosphorus
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 DAbbreviations: 25-D, 25-hydroxyvitamin D; 1,25-dihydroxyvitamin D; VDR, vitamin D receptorVitamin 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 seem generall inactive but 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 (e.g., in sarcoidosis) and lymphocytes (e.g., in lymphoma) 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.
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).
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 occur 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%.
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 the expression several bone proteins, notable osteocalcin. It promotes the transcription of osteocalcin and has bidirectional effects on type I collagen and alkaline phosphatase gene transcription
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).
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
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Increases resorption at high doses
Kidney
-
Decreases calcium excretion
-
Decreases phosphorus excretion
Gastrointestinal Tract
-
Increases calcium absorption
-
Increases phosphorus absorption
Blood
-
Increases calcium
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Increases phosphorus
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 Metabolism
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