Transforming Growth Factor-β 1 Stimulates l -Arginine Transport and Metabolism in Vascular Smooth Muscle Cells

Abstract

Background —Transforming growth factor-β 1 (TGF-β 1 ) contributes to arterial remodeling by stimulating vascular smooth muscle cell (VSMC) growth and collagen synthesis at sites of vascular injury. Because l -arginine is metabolized to growth-stimulatory polyamines and to the essential collagen precursor l -proline, we examined whether TGF-β 1 regulates the transcellular transport and metabolism of l -arginine by VSMCs. Methods and Results —TGF-β 1 increased l -arginine uptake, and this was associated with a selective increase in cationic amino acid transporter-1 (CAT-1) mRNA. In addition, TGF-β 1 stimulated l -arginine metabolism by inducing arginase I mRNA and arginase activity. TGF-β 1 also stimulated l -ornithine catabolism by elevating ornithine decarboxylase (ODC) and ornithine aminotransferase (OAT) activity. TGF-β 1 markedly increased the capacity of VSMCs to generate the polyamine putrescine and l -proline from extracellular l -arginine. The TGF-β 1 –mediated increase in putrescine and l -proline production was reversed by methyl- l -arginine, a competitive inhibitor of cationic amino acid transport, or by hydroxy- l -arginine, an arginase inhibitor. Furthermore, the formation of putrescine was inhibited by the ODC inhibitor α-difluoromethylornithine, and l -proline generation was blocked by the OAT inhibitor l -canaline. l -Canaline also inhibited TGF-β 1 –stimulated type I collagen synthesis. Conclusions —These results demonstrate that TGF-β 1 stimulates polyamine and l -proline synthesis by inducing the genes that regulate the transport and metabolism of l -arginine. In addition, they show that TGF-β 1 –stimulated collagen production is dependent on l -proline formation. The ability of TGF-β 1 to upregulate l -arginine transport and direct its metabolism to polyamines and l -proline may contribute to arterial remodeling at sites of vascular damage.