Regulation of collagen gene expression in 3T3-L1 cells. Effects of adipocyte differentiation and tumor necrosis factor .alpha.

Abstract

An early feature in the development of adipocytes from fibroblast-like precursor cells is the biogenesis of an extracellular basement membrane (Napolitano, 1963; Kuri-Harcuh et al., 1984). Interactions between components of the basement membrane (e.g., collagens) and the surfaces of differentiating adipocytes are thought to regulate subsequent phases of the development program. Since fibroblasts principally secrete type I and III collagens whereas type IV collagen is abundant in basement membrane, it appears that a switch in collagen gene expression is a key element in adipocyte differentiation. Little is known about the mechanisms underlying differentiation-dependent changes in collagen expression or the effects of the potent lipolytic cytokine TNF-.alpha. on collagen mRNA accumulation in preadipocytes and adipocytes. In this study, 3T3-L1 preadipocytes were found to express mRNAs encoding type I, III, and IV procollagens. When 3T3-L1 cells were stimulated to differentiate into adipocytes, the relative concentrations of type I and type III procollagen mRNAs declined by 80-90%. Parallel decreases in the rates of transcription of the procollagen I and procollagen III genes appear to account for the diminished levels of these mRNAs. In contrast, the relative rate of transcription of the procollagen IV gene increased 2.6-fold during adipocyte development. As a consequence, the abundance of type IV procollagen transcripts was elevated in adipocytes. Tumor necrosis factor .alpha. (TNF-.alpha.) is a cytokine that stimulates lipolysis, an apparent "dedifferentiation" of adipocytes, and inhibits transcription of certain adipocyte-specific genes. The effects of TNF-.alpha. on collagen mRNA levels were dependent upon the state of differentiation of 3T3-L1 cells. In preadipocytes, 5 nM TNF-.alpha. elicited 60-80% decreases in the transcription rates of the three procollagen genes, thereby promoting a coordinated decline of 75% in the amounts of type I, III, and IV procollagen mRNAs. However, TNF-.alpha. provoked the accumulation of all three procollagen mRNAs in 3T3-L1 adipocytes via posttranscriptional mechanism(s). Thus, the responsiveness of procollagen genes to TNF-.alpha. is drastically altered in degree, direction, and underlying mechanism as preadipocytes differentiate into adipocytes. Transforming growth factor .beta. (TGF-.beta.) also inhibits adipocyte differentiation and alters extracellular matrix protein synthesis. While TGF-.beta. mRNA was expressed in preadipocytes, its abundance declined markedly (3.3-fold) during adipogenesis. In contrast, TNF-.alpha. effected a moderate (2-fold) increase in TGF-.beta. mRNA content in both preadipocytes and adipocytes by a posttranscriptional process.