MALIGNANT TRANSFORMATION BY H-RAS RESULTS IN ABERRANT REGULATION OF RIBONUCLEOTIDE REDUCTASE GENE-EXPRESSION BY TRANSFORMING GROWTH FACTOR-BETA(1)

Ribonucleotide reductase is a key rate-limiting and regulatory step in DNA synthesis and plays a crucial role in the coordination of DNA synthesis, DNA repair, and cell proliferation. The present study demonstrates a link between alterations in TGF-beta(1) regulation during malignant conversion and the expression of ribonucleotide reductase. H-ras-transformed mouse 10T1/2 cell lines exhibiting malignant potential were examined for possible TGF-beta(1)-mediated alterations in ribonucleotide reductase expression. Selective induction of ribonucleotide reductase gene expression occurred, since only H-ras-transformed highly metastatic cells exhibited marked elevations in ribonucleotide reductase expression, whereas nontransformed normal 10T1/2 cells were unaffected by TGF-beta(1) treatment. These changes occurred without any detectable modifications in DNA synthesis rates, suggesting that these changes were regulated by a novel mechanism independent of the S-phase of the cell cycle. Furthermore, this TGF-beta(1)-mediated regulation of ribonucleotide reductase expression was shown to occur through an autocrine mechanism. TGF-beta(1)-modulated regulation of ribonucleotide reductase expression requires de novo protein synthesis and involves, at least in part, transcriptional and post-transcriptional events. Furthermore, evidence is presented to suggest a possible role for protein kinase C-mediated events, protein phosphatases, and G-protein-coupled events in the TGF-beta(1)-mediated regulation of ribonucleotide reductase expression in H-ras-transformed malignant cells. TGF-beta(1) regulation of ribonucleotide reductase in highly malignant cells appears to be complex and multifaceted and constitutes an integral part of an altered growth regulatory program. (C) 1995 Wiley-Liss, Inc.