Hypercalcemia and tumor-prostaglandins: the VX2 carcinoma model in the rabbit

The VX2 carcinoma produces profound hypercalcemia (17-22 mg/100 ml) in the rabbit about 3-4 wk after transplantation. A bone resorption-stimulation factor (assayed in vitro with mouse calvaria in culture) has been extracted with diethyl ether from the tumor tissue and from the medium of a clonal strain of VX2 cells grown in culture. Serologic methods reveal that the tumors contain 294 plus or minus 51 ng/g fresh weight (mean plus or minus SE, 25 tumors) of prostaglandin E2 (PGE2), a potent bone resorption-stimulating agent. VX2 cells in culture produce 0.5-3.0 mug PGE2 per mg cell protein per 24 hr. The production of bone resorption-stimulating activity and PGE2 by VX2 cells in culture were both inhibited by indomethacin (100 ng/ml). Tumors from normocalcemic, indomethacin-treated rabbits (10-40 mg/rabbit/24 hr) contained little or no bone resorption-stimulating activity nor PGE2. Tumor-bearing rabbits receiving indomethacin continuously did not develop hypercalcemia, however, following cessation of indomethacin administration, hypercalcemia developed rapidly and was again reversed by reinstitution of indomethacin feeding. In untreated, hypercalcemic, tumor-bearing rabbits, initiation of indomethacin treatment was followed by a rapid return of the plasma calcium to the normal range. Systemic venous plasma from hypercalcemic tumor-bearing plasma contained higher concentrations of PGE2 than plasma from normocalcemic control rabbits. Venous drainage of the tumor contained even higher plasma PGE2 concentrations than systemic venous plasma in hypercalcemic animals; plasma PGE2 concentrations locally and in systemic plasma were unmeasurable (less than 70 pg/ml) in normocalcemic, indomethacin-treated, tumor-bearing rabbits. We conclude that PGE2 is a bone resorption-stimulating factor produced by VX2 tumor cells, and that secretion of PGE2 by the tumor in vivo may well be responsible for the hypercalcemia observed in tumor-bearing rabbits.