Figure 6. Growth factor-regulated PTHrP production in tumor states. Tumor cells at a distance from bone may be stimulated by autocrine growth factors (GF) to increase production of PTHrP which can then travel to bone via the circulation and enhance bone resorption. Tumor cells metastatic to bone (inset) may secrete PTHrP which can resorb bone and release growth factors which in turn can act in a paracrine manner to further enhance PTHrP production. PTHrP may itself promote tumor growth and progression.