Transforming Growth Factor-β and Receptor Tyrosine Kinase–Activating Growth Factors Negatively Regulate Collagen Genes in Smooth Muscle of Hypertensive Rats

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Abstract
Abstract —Previous studies have suggested that differences in vascular smooth muscle cell (VSMC) proliferative responses between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats can be attributed to transforming growth factor-β (TGF-β) actions. Because vascular collagen content is reported to be lower in SHR than in WKY rats, in this study we investigated in cell culture whether the differences in collagen content might also be attributed to differential actions of TGF-β on VSMCs from the two strains. Exposure of VSMCs from WKY to the TGF-β isoforms -β 1 , -β 2 , or -β 3 induced rapid, transient elevations in mRNAs encoding collagens α 1 (I), α 2 (I), and α 1 (III); maximum increases were apparent by 2 hours and ranged from twofold [collagen α 1 (III)] to ninefold [collagen α 1 (I)]. Thereafter they returned to near basal levels. When VSMCs from SHR were exposed to these TGF-β isoforms, only reductions in collagen mRNA levels were observed, persisting for 24 hours. Basic fibroblast growth factor and epidermal growth factor, factors known to stimulate production of the TGF-β 1 isoform in VSMCs, also induced a pattern of gene responses similar to those induced by the TGF-β isoforms in VSMCs from SHR and WKY rats. The simultaneous presence of TGF-β did not affect the time course or magnitude of the changes in collagens α 1 (I), α 2 (I), or α 1 (III) mRNA levels in SHR or WKY VSMCs. Examination of the induction of c -myc mRNA and immunoreactive oncoprotein content indicated that c- myc is a likely contributor to the downregulation of the collagen gene activity in both SHR and WKY VSMCs despite the differential regulation of its mRNA by TGF-β 1 in the two VSMC lines. Together these data suggest that in VSMCs from SHR, a number of gene responses to TGF-β, in addition to cell proliferation, appear to be abnormal compared with WKY rats, and the lower than normal collagen levels observed in the vasculature of SHR may be in part due to abnormalities in TGF-β responsiveness.