Induction of parathyroid hormone-related peptide by the Ras oncogene: role of Ras farnesylation inhibitors as potential therapeutic agents for hypercalcemia of malignancy.

Parathyroid hormone related peptide (PTHRP) is the major causal agent in the syndrome of malignancy-associated hypercalcemia (MAH). Several studies have shown that PTHRP production is increased in response to growth factors and oncogenes, such as Tpr-Met, that are associated with the tyrosine kinase signaling pathway. Using site-directed mutagenesis of Tpr-Met and chemical inhibitors of phosphotidylinositol-3 kinase and Ras isoprenylation, we demonstrated previously that induction of PTHRP is mediated via the Ras signaling pathway. In the present study, we have directly investigated the role of the Ras oncogene in MAH. As a model system, we used Fisher rat 3T3 fibroblasts stably transfected with a Ras oncogene (Ras-3T3). Ras transfection enhanced PTHRP production 5-10-fold in these cells, and inoculation of this cell line into nude mice led to the development of hypercalcemia within 2 weeks. We used this system to evaluate the effect of a potent inhibitor of Ras processing, B-1086, on cell growth, PTHRP production, plasma calcium, and tumor growth. Treatment of Ras-3T3 cells in vitro with B-1086 at 0.1-10 microg/ml produced a significant reduction in PTHRP mRNA expression and PTHRP secretion and a significant decrease in cell proliferation. Treatment in vivo of BALB/c/nu/nu mice bearing Ras-3T3 tumors with B-1086 resulted in a significant inhibition in tumor growth. In addition, this treatment produced near normalization of serum Ca2+, a significant decrease in plasma PTHRP, and a reduction in tumoral PTHRP mRNA levels. These results show that the Ras pathway is involved in PTHRP production by tumors, identifies Ras as a potential target for treatment of MAH, and demonstrates Ras processing inhibitors as candidate therapeutic agents against this syndrome.