TRANSFORMING GROWTH FACTOR-BETA(1) SELECTIVELY REGULATES ORNITHINE DECARBOXYLASE GENE-EXPRESSION IN MALIGNANT H-RAS TRANSFORMED FIBROSARCOMA CELL-LINES

Negative growth regulators such as the transforming growth factor beta (TGF-beta) family appear to be important inhibitors in most tissue ty pes. However, inhibition of DNA synthesis and cell proliferation is fr equently lost during malignant transformation, and in some cases, tumo r cell proliferation is actually stimulated by TGF-beta. The present s tudy demonstrates a novel link between alterations in TGF-beta regulat ion during malignant conversion, and the expression of ornithine decar boxylase, a key rate-limiting activity in the biosynthesis of polyamin es, and an enzyme that plays an important role in cell growth and diff erentiation. A panel of radiation and H-ras transformed mouse 10T 1/2 cell lines exhibiting increasing malignant potential was investigated for possible TGF-beta1 mediated changes in ornithine decarboxylase gen e expression. Selective induction of gene expression was observed sinc e only H-ras transformed cell lines with malignant potential exhibited marked elevations in ornithine decarboxylase message levels. Ornithin e decarboxylase gene expression in nontransformed 10T 1/2 cells and ce ll lines capable of only benign tumor formation was unaffected by TGF- beta1 treatment. H-ras transformed cells were transfected with a plasm id placing the TGF-beta1 coding region under the control of a zinc sen sitive metallothionein promoter. When these cells were cultured in the presence of zinc an elevation of TGF-beta1 mRNA was observed within 3 0 min. This increase in TGF-beta1 message closely coincided with an el evation in ornithine decarboxylase message, and preceded an induction of jun-B, an early response gene in cells sensitive to TGF-beta1 stimu lation. Evidence for regulation of ornithine decarboxylase gene expres sion by TGF-beta1 at both transcription and posttranscription was foun d. Actinomycin D pretreatment of malignant cells prior to TGF-beta1 ex posure prevented the increase in ornithine decarboxylase message. Mark ed differences in the rates of ornithine decarboxylase message decay w ere observed when cells treated with TGF-beta1 were compared to untrea ted controls, with the half-life of ornithine decarboxylase mRNA incre asing from 2.5 h in untreated cells to 17.5 h in cells exposed to TGF- beta1. In addition, evidence was obtained for a cycloheximide sensitiv e regulator of ornithine decarboxylase gene expression, since the pres ence of this protein synthesis inhibitor increased the levels of ornit hine decarboxylase message, and this effect was synergistically augmen ted by exposure of cells to cycloheximide and induction of TGF-beta1 g ene expression together. These results show for the first time that TG F-beta1 can regulate ornithine decarboxylase expression in malignant H -ras transformed cells, and suggest a mechanism of growth factor stimu lation of malignant cells, in which early alterations in the control o f ornithine decarboxylase, gene expression are important. (C) 1993 Wil ey-Liss, Inc.