Expression of human transforming growth factor α by chinese hamster ovarian tumors in nude mice causes hypercalcemia and increased osteoclastic bone resorption

Transforming growth factor alpha (TGF-alpha) is a polypeptide regulator of cell growth produced by many malignant tumors. It stimulates osteoclastic resorption in bone organ culture and osteoclast-like cell formation in marrow culture. To determine whether tumor production of TGF-alpha can cause hypercalcemia in vivo, we used Chinese hamster ovarian (CHO) cells transfected with the human TGF-alpha gene (TCHO), which stably express and secrete TGF-alpha. We used nontransfected CHO cells as controls (CCHO). TCHO and CCHO were inoculated intramuscularly into one hindlimb of nude mice and grew as local solid tumors. After 4 weeks of TCHO tumor growth, plasma ionized calcium (Ca2+) increased to reach 1.48 +/- 0.03 mM (mean +/- SEM), whereas mice bearing similarly sized CCHO tumors and non-tumor-bearing mice (NTB) remained normocalcemic (normal range for Ca2+, 1.15-1.30 mM). Plasma TGF-alpha was undetectable by an ELIFA assay in all NTB mice, was markedly increased in all TCHO mice (5.75 +/- 0.78 ng/ml), and was slightly increased in CCHO mice (0.50 +/- 0.22 ng/ml). Quantitative bone histomorphometry showed a prominent increase in osteoclastic bone resorption in TCHO mice. These data suggest that TGF-alpha is a mediator of hypercalcemia and increased osteoclastic bone resorption in tumors that produce it in sufficient quantity.