AbstractNegative growth regulators such as the transforming growth factor beta (TGF‐β) family appear to be important inhibitors in most tissue types. However, inhibition of DNA synthesis and cell proliferation is frequently lost during malignant transformation, and in some cases, tumor cell proliferation is actually stimulated by TGF‐β. The present study demonstrates a novel link between alterations in TGF‐β regulation during malignant conversion, and the expression of ornithine decarboxylase, a key rate‐limiting activity in the biosynthesis of polyamines, and an enzyme that plays an important role in cell growth and differentiation. A panel of radiation and H‐rastransformed mouse 10T1/2 cell lines exhibiting increasing malignant potential was investigated for possible TGF‐β1mediated changes in ornithine decarboxylase gene expression. Selective induction of gene expression was observed since only H‐rastransformed cell lines with malignant potential exhibited marked elevations in ornithine decarboxylase message levels. Ornithine decarboxylase gene expression in nontransformed 10T1/2 cells and cell lines capable of only benign tumor formation was unaffected by TGF‐β1treatment. H‐rastransformed cells were transfected with a plasmid placing the TGF‐β1coding region under the control of a zinc sensitive metallothionein promoter. When these cells were cultured in the presence of zinc an elevation of TGF‐β1mRNA was observed within 30 min. This increase in TGF‐β1message closely coincided with an elevation in ornithine decarboxylase message, and preceded an induction ofjun‐B, an early response gene in cells sensitive to TGF‐β1stimulation. Evidence for regulation of ornithine decarboxylase gene expression by TGF‐β1at both transcription and posttranscription was found. Actinomycin D pretreatment of malignant cells prior to TGF‐β1exposure prevented the increase in ornithine decarboxylase message. Marked differences in the rates of ornithine decarboxylase message decay were observed when cells treated with TGF‐β1were compared to untreated controls, with the half‐life of ornithine decarboxylase mRNA increasing from 2.5 h in untreated cells to 17.5 h in cells exposed to TGF‐β1. In addition, evidence was obtained for a cycloheximide sensitive regulator of ornithine decarboxylase gene expression, since the presence of this protein synthesis inhibitor increased the levels of ornithine decarboxylase message, and this effect was synergistically augmented by exposure of cells to cycloheximide and induction of TGF‐β1gene expression together. These results show for the first time that TGF‐β1can regulate ornithine decarboxylase expression in malignant H‐rastransformed cells, and suggest a mechanism of growth factor stimulation of malignant cells, in which early alterations in the control of ornithine decarboxy
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