TGF-BETA(1) PREVENTS THE DOWN-REGULATION OF TYPE-I PROCOLLAGEN FIBRONECTIN, AND TGF-BETA(1) GENE-EXPRESSION ASSOCIATED WITH 3T3-L1 PRE-ADIPOCYTE DIFFERENTIATION

Pre-adipocyte 3T3-L1 cells, after an appropriate induction stimulus, p roceed through a defined change in morphology as differentiation progr esses. Transforming growth factor beta(1) (TGF beta(1)) is able to blo ck the morphological and biochemical changes which occur with differen tiation of these cells if given within 36-40 h of induction [Ignotz an d Massague (1985): Proc Natl Acad Sci USA 82:8530-8534]. To begin to e lucidate the role of the extracellular matrix in adipogenesis, as well as the mechanism whereby TGF beta(1) inhibits differentiation, we exa mined the expression of two extracellular matrix genes, type I (alpha( 1)) procollagen and fibronectin, as well as endogenous TGF beta(1). Co nfluent cells were induced to differentiate by treatment with insulin, dexamethasone, and isobutylmethylxanthine in the presence or absence of TGF beta(1). Following 6 days of treatment, the cells in the differ entiated group acquired the rounded shape of mature adipocytes; the cy tosol of these cells also contained numerous lipid-filled vesicles, as demonstrated by oil red O staining. Cells treated with the differenti ation compounds in the presence of TGF beta(1) maintained the fibrobla st-like appearance of control cells and did not stain with oil red O. At the level of gene expression, both procollagen and fibronectin mRNA S were down-regulated during differentiation of 3T3-L1 cells. When cel ls from the control or differentiation groups were treated with TGF be ta(1) there was a 2-5-fold induction of procollagen and fibronectin mR NAS throughout the 6-day time course. No charge in type I procollagen transcription was observed by nuclear run-on analysis, suggesting that the increase in procollagen mRNA with TGF beta(1) treatment was due t o a post-transcriptional process(es). However, both transcriptional an d post-transcriptional components were observed in the regulation of f ibronectin gene expression by TGF beta(1). In addition, TGF beta(1) wa s found to positively regulate its own expression, as treatment of the cells with TGF beta(1) enhanced endogenous TGF beta(1) expression and prevented the small decrease in TGF beta(1) mRNA levels which occurre d early during the differentiation program. Thus, our data demonstrate that down-regulation of type I procollagen, fibronectin, and TGF beta (1) gene expression was prevented during TGF beta inhibition of 3T3-L1 differentiation. Taken together, these data suggest that TGF beta may inhibit differentiation of 3T3-L1 cells by maintaining the fibroblast -like extracellular matrix, thus preventing the changes in cell shape that accompany differentiation. (C) 1994 Wiley-Liss, Inc.