Parathyroid Hormone-Related Protein and Interleukinla 1α Synergistically Stimulate Bone Resorption<i>in Vitro</i>and Increase the Serum Calcium Concentration in Mice<i>in Vivo</i>*

To elucidate the mechanism of humoral hypercalcemia elicited by human esophageal carcinoma cells (EC-GI), which constitutively produced interleukin-1 alpha (IL-1 alpha) and PTH-like factor, the effects of IL-1 alpha and PTH-related protein (PTH-rP) on bone resorption in vitro and on serum calcium concentrations in vivo were investigated. Nude mice transplanted with EC-GI cells invariably developed hypercalcemia, although their urinary cAMP excretion remained within the normal range. IL-1 alpha or PTH-rP-(1-34) stimulated 45Ca release from prelabeled fetal mouse forearm bones in a concentration-dependent manner, and when combined, IL-1 alpha and PTH-rP-(1-34) synergistically stimulated bone resorption in vitro. Injection of PTH-rP-(1-34) into mice three times a day for 2 days increased the serum calcium concentration in a dose-dependent manner. Continuous infusion of IL-1 alpha occasionally increased the serum calcium concentration. Simultaneous administration of IL-1 alpha at rates of 1-2.7 micrograms/day and PTH-rP-(1-34) at doses of 15-30 micrograms/day synergistically increased the serum calcium concentration in vivo. These findings suggest that PTH-rP and IL-1 alpha produced by the tumor cells were synergistically responsible for the humoral hypercalcemia observed in both the original patient and the tumor-bearing nude mice, and that at least two bone-resorbing factors [PTH-rP and another nonadenylate cyclase-stimulating bone-resorbing factor(s)] are active in patients with malignancy-associated hypercalcemia, in whom nephrogenous cAMP excretion is neither increased nor decreased.