Abstract 464: Arginase Promotes Vascular Smooth Muscle Cell Proliferation and Neointima Formation
Arginase is the central enzyme of the hepatic urea cycle catabolizing L-arginine to L-ornithine which is subsequently metabolized to polyamines and L-proline. We recently discovered that vascular smooth muscle cells (SMC) express arginase activity and that arginase stimulates the synthesis of polyamines from L-arginine. Since polyamines are essential for cell growth, we examined the role of arginase in regulating vascular SMC proliferation. Treatment of cultured rat aortic SMC with platelet-derived growth factor (PDGF; 0.1–30 nM for 0–24h) stimulated arginase activity in a concentration and time-dependent manner. The increase in arginase activity by PDGF was associated with the selective expression of arginase I mRNA and protein, and a two-fold increase in intracellular polyamine concentration. In contrast, arginase II expression was unaffected by PDGF. Interestingly, the highly specific arginase inhibitors, (S)-2-boronoethyl)-L-cysteine (BEC; 100 μM) and N-hydroxy-nor-L-arginine (100 μM), inhibited the PDGF-mediated increases in polyamine levels, DNA synthesis, and cell proliferation. The inhibitory effect on SMC growth by the arginase inhibitors was reversed by the exogenous addition of polyamines (10 μM). To investigate the functional effect of arginase in vivo, we performed carotid artery balloon injury in rats. Ribonuclease protection assays, western blots, and immunohistochemistry revealed that arterial injury induced a rapid (1 day) expression of arginase I mRNA and protein in vascular SMC that persisted for nearly two weeks. Perivascular application of BEC (1 mg) via a pluronic gel immediately following balloon injury markedly attenuated neointima formation two weeks after injury (intima/media ratio, 0.213 ± 0.096 versus 0.588 ± 0.049 in control). Furthermore, PCNA-staining indicated that BEC inhibited DNA synthesis two days after arterial injury (3.44 ± 0.66 % versus 16.32 ± 0.97% in control). In conclusion, these results demonstrate that PDGF and arterial injury stimulate arginase I gene expression and that arginase I promotes vascular SMC growth and neointima formation by increasing cellular polyamine levels. Arginase I represents a potentially novel therapeutic target for limiting the vascular response to injury.