TGF-β superfamily members do not promote smooth muscle-specific alternative splicing, a late marker of vascular smooth muscle cell differentiation

Smooth muscle (SM) specific alternate splicing of a number of genes is a late marker of the differentiated vascular smooth muscle cell (VSMC) phenotype and is one of the first differentiation characteristics to be lost during de-differentiation and in disease. An understanding of how this aspect of VSMC phenotype is regulated may provide insights into the earliest events of the atherosclerotic process. TGF-beta 1 is a potent regulator of VSMC differentiation and can induce expression of SM-specific contractile proteins in both pluripotent stem cells and de-differentiated VSMCs. The purpose of this study was to test the hypothesis that members of the TGF beta-superfamily can also effect SM-specific alternative splicing. Firstly, we established that SM-specific splicing of alpha-tropomyosin, vinculin and SM-myosin heavy chain (MI-IC) increases during rat fetal/neonatal development and is decreased in VSMCs following balloon-induced carotid injury in the rat. Treatment of cultured rat VSMCs with TGF beta-superfamily members resulted in a significant reduction in the ratio of SM to non-muscle (NM) alpha-tropomyosin, but did not effect SM-specific alternative splicing of vinculin or SM-MHC. Treatment of pluripotent C3H10T1/2 cells with TGF-beta 1, which increased SM differentiation marker expression, did not increase SM-specific alpha-tropomyosin splicing. Taken together, these results demonstrate differential regulation of SM-specific alternative splicing and indicate that although TGF-beta 1 promotes VSMC differentiation marker expression, TGF-beta 1 cannot act as the sole trigger of VSMC differentiation.