AbstractTransforming growth factor‐β1(TGF‐β1) stimulated DNA synthesis (3‐fold) in BALBc/3T3 fibroblasts following 24 hours of growth factor exposure. Since ribonucleotide reductase is important for the coordination of DNA synthesis and cell proliferation, we investigated the hypothesis that cells like BALB/c 3T3, which are TGF‐β1responsive, would exhibit modifications in expression of the gene for ribonucleotide reductase following growth factor treatment. We observed 2.6, 4.1, and 4.8‐fold increases in ribonucleotide reductase activity following TGF‐β1exposure for 6, 12, and 24 hours, respectively. Increased ribonucleotide reductase R2 gene expression (3, 3.7, and 4.5‐fold) and R1 gene expression (2, 2.5, and 2.6‐fold) were observed following 6, 12, and 24 hours of TGF‐β1treatment, respectively. Western blots indicated 2.2, 3.1, and 4.1‐fold increases in protein R2 levels at 6, 12, and 24 hours exposure to TGF‐β1, whereas 2.6 and 3.3‐fold elevations in R1 protein levels were observed at 12 and 24 hours postTGF‐β1exposure. These TGF‐β1mediated modifications in ribonucleotide reductase gene expression occurred, in part, prior to any detectable changes in the rate of DNA synthesis, demonstrating alterations in the normal regulation of ribonucleotide reductase. Furthermore, these alterations could be markedly reduced by prolonged pretreatment with 12‐0‐tetradecanoylphorbol‐13‐acetate (R2 gene expression increased by only 1.3, 1.5 and 2.3‐fold after 6, 12, and 24 hours of TGF‐β1treatment, respectively), suggesting a role for a protein kinase C pathway in the TGF‐β1regulated changes in ribonucleotide reductase gene expression. These results indicate for the first time that TGF‐β1can regulate the expression of the two genes for ribonucleotide reductase in BALB/c 3T3 fibroblasts, and suggest that regulation of these genes plays an important role in critical events involved in growth factor modulation of normal and
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