Abstract 4139404: Post-Translational Regulation of Larp6 by IGF-1 Modulates Collagen Synthesis in Smooth Muscle Cells

Introduction: Vascular smooth muscle cells (SMCs) play a crucial role in atherosclerosis, contributing to plaque stability by forming the main cellular component of the fibrous cap and synthesizing extracellular matrix. We previously showed that insulin-like growth factor-1 (IGF-1) increases expression of the collagen mRNA binding protein La ribonucleoprotein domain family member 6 (Larp6) and of collagen in atherosclerotic plaques. However, molecular mechanisms remain unclear. Hypothesis: We hypothesized that IGF-1 increases collagen synthesis via a post-translational regulation mechanism of Larp6. Methods: An SMC-specific Larp6 overexpression mouse model (SMC-Larp6) was generated using the Myh11 promoter. Entire aortas and aortic roots were isolated for plaque analysis. IGF-1 was injected in WT mice at a dosage of 1.5 mg/kg. For in vitro assays, human aortic SMCs were transduced with an adenoviral vector to overexpress Larp6 and treated with 50 ng/mL IGF-1 for 18 h. Results: SMC-Larp6 mice had no significant change in plaque collagen content. Additionally, IGF-1 increased Larp6 protein but not mRNA levels suggesting that IGF-1 likely regulated Larp6 via a post-transcriptional mechanism. Western blotting identified two major Larp6 bands at 67 kDa and 70 kDa. We observed a clear band shift from the lower to the upper band after IGF-1 treatment, with a concomitant increase in Procollagen I, suggesting that IGF-1 enhances Larp6's role in promoting collagen through post-translational modification. Mass spectrometry analysis revealed multiple phosphorylation sites on the LaM and LSA domains of Larp6, including S451, which is phosphorylated by the IGF-1/PI3K/AKT axis. We also observed this protein modification pattern in mouse aortic tissue lysates following IGF-1 injection. Conclusions: IGF-1 regulates Larp6 phosphorylation in SMC, thereby likely playing an important role in IGF-1 induced collagen synthesis. This study provides insight into molecular mechanisms underlying collagen production in SMCs and could inform therapeutic strategies for plaque stabilization.