Neoplastic transformation of normal rat embryo fibroblasts by a mutated p53 and an activated ras oncogene induces parathyroid hormone-related peptide gene expression and causes hypercalcemia in nude mice.

Parathyroid hormone-related peptide (PTHRP) is a 141-amino acid protein identified in various carcinomas associated with humoral hypercalcemia of malignancy (HHM). Although the causal role of PTHRP in HHM syndrome has been established, the molecular and cellular mechanism by which PTHRP gene is overexpressed in certain malignancies remains unknown. We have demonstrated in the present study that PTHRP secretion was markedly induced concomitantly with the formation of transformed foci after normal rat embryo fibroblasts (REFs) were co-transfected with an activated ras (ras) and a mutated form of p53 (p53-mt) genes. In either ras- or p53-mt-transfected (nontransformed) cells, only modest or barely detectable secretion of PTHRP was observed, respectively. Northern blot analysis revealed that PTHRP mRNA was markedly induced in fully transformed cells 11 days after transfection with both ras and p53-mt genes. Inhibition of RNA synthesis with actinomycin D resulted in almost complete disappearance of PTHRP mRNA at 2-3 h, suggesting a transcriptional mechanism. Transient transfection experiments revealed that PTHRP promoter activity was induced in ras + p53-mt transfectants. REFs transformed by ras and p53-mt genes and thereby induced to secrete PTHRP in vitro produced aggressively growing tumors associated with HHM syndrome when injected into nude mice. These results suggest that activation of PTHRP gene is closely related to malignant transformation of normal mammalian cells and that ras and p53 may be important regulators of PTHRP gene transcription. The transfection-focus formation system of REFs should provide an excellent model to study the molecular and cellular mechanism underlying concomitant overexpression of PTHRP gene with carcinogenesis.