Bone Resorption and Humoral Hypercalcemia of Malignancy: Stimulation of Bone Resorptionin Vitroby Tumor Extracts is Inhibited by Prostaglandin Synthesis Inhibitors*

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Abstract
Extracts of tumors from patients with humoral hypercalcemia of malignancy (HHM) were tested using an in vitro bone resorption assay in order to investigate the pathogenesis of the hypercalcemia. Bone resorption was assessed by comparing the percent release of previously incorporated 45Cafrom paired halves of newborn mouse calvaria. Saline extracts of three out of five tumors from HHM patients caused a significant increase in 45Ca release relative to controls. Extracts of liver and non-HHM tumor did not cause significant resorption. Tumorstimulated bone resorption was blocked by indomethacin and eicosatetraynoic acid, inhibitors of the synthesis of prostaglandins (PGs) and related metabolites of arachidonic acid, whereas resorption stimulated by parathyroid hormone (PTH), PGE2, or 1,25-(OH)2D3 was not. Furthermore, levels of immunoreactive PTH or PGE2 in tumor extracts were not sufficient to account for the degree of resorption observed. These observations indicate that PTH or PGE2 are not responsible for the bone resorption caused by extracts of tumors from these patients with HHM. Furthermore, they suggest that hypercalcemia in these patients may result from bone resorption stimulated by the local production in bone of PGs or related metabolites of arachidonic acid in response to a humoral factor elaborated by the tumor.