Transforming growth factor-β1 stimulates L-arginine transport and metabolism in vascular smooth muscle cells: Role in polyamine and collagen synthesis

Abstract

Background - Transforming growth factor-β1 (TGF-β1) 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-β1 regulates the transcellular transport and metabolism of L-arginine by VSMCs. Methods and Results - TGF-β1 increased L-arginine uptake, and this was associated with a selective increase in cationic amino acid transporter-1 (CAT-1) mRNA. In addition. TGF-β1 stimulated L-arginine metabolism by inducing arginase I mRNA and arginase activity. TGF-β1 also stimulated L-ornithine catabolism by elevating omithine decarboxylase (ODC) and ornithine aminotransferase (OAT) activity. TGF-β1 markedly increased the capacity of VSMCs to generate the polyamine putrescine and L-proline from extracellular L-arginine. The TGF-β1-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-β1-stimulated type I collagen synthesis. Conclusions - These results demonstrate that TGF-β1 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-β1-stimulated collagen production is dependent on L-proline formation. The ability of TGF-β1 to upregulate L-arginine transport and direct its metabolism to polyamines and L-proline may contribute to arterial remodeling at sites of vascular damage.