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

Background-Transforming growth factor-beta (1) (TGF-beta (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-beta (1) regulates the transcellular transport and metabolism of L-arginine by VSMCs. Methods and Results-TGF-beta (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-beta (1) stimulated L-arginine metabolism by inducing arginase I mRNA and arginase activity. TGF-beta (1) also stimulated L-ornithine catabolism by elevating ornithine decarboxylase (ODC) and ornithine aminotransferase (OAT) activity. TGF-beta (1) markedly increased the capacity of VSMCs to generate the polyamine putrescine and L-proline from extracellular L-arginine. The TGF-beta (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 cu-difluoromethylornithine, and L-proline generation was blocked by the OAT inhibitor L-canaline. L-Canaline also inhibited TGF-beta (1)-stimulated type I collagen synthesis. Conclusions-These results demonstrate that TGF-beta (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-beta (1)-stimulated collagen production is dependent on L-proline formation. The ability of TGF-beta (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.