Abstract 13040: Insulin-Like Growth Factor-1 Upregulates Larp6 via Translational and Post-Translational Mechanisms in Vascular Smooth Muscle Cells, Leading to Elevated Collagen Production

Abstract

Introduction: Collagen content in atherosclerotic plaque is a hallmark of plaque stability. We have shown that insulin-like growth factor-1 (IGF1) increased collagen content in atherosclerotic plaques of Apoe -/- mice. We have identified LARP6 (La Ribonucleoprotein Domain Family, Member 6), a type I collagen mRNA-binding protein, as a mediator of IGF1-dependent type I collagen upregulation and increased mature fibril formation by smooth muscle cells (SMC), however specific mechanism has not been elucidated yet. Methods and Results: Micro-RNAs, including miR-1976 have been shown to bind to LARP6 mRNA. We found that SMC treatment with IGF1 significantly downregulated miR-1976 by 41±5% after 3h (P<0.05), consistent with the IGF1-induced LARP6 upregulation. In fact, miR-1976 mimic (20 nM) decreased LARP6 by 67% (P<0.05) and reduced type I procollagen protein levels by 85% (P<0.05). MiR-1976 mimic inhibits IGF1-induced LARP6 upregulation showing miR-1976 involvement in IGF1 effect on LARP6. It has been reported that LARP6-mediated promotion of procollagen synthesis may be regulated by phosphorylation of LARP6, therefore we investigated whether IGF1 induces LARP6 phosphorylation in cultured human aortic SMCs. For this experiment, SMCs were serum-starved and treated for 0-48h with 10 ng/mL IGF1. LARP6 protein was detected by immunoblots as a single 67kDa band in untreated SMC. However, after 6h of IGF1 treatment there was additional phosphorylated LARP6 band detected at 70 kDa. The levels of 70kDa LARP6 time-dependently increased up to 18h of treatment, with a concomitant increase of type I procollagen and collagen expression levels. Conclusions: In summary, our data suggests that (i) IGF1 downregulates miR-1976, thereby increasing LARP6 expression levels and (ii) IGF1 induces phosphorylation of LARP6, thereby promoting LARP6 activity. Our study revealed novel mechanisms underlying IGF1-induced type I collagen upregulation in vascular SMC and potentially in atherosclerotic plaques.