The Regulation of Beta-catenin Function in Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) maintain the ability to switch between a quiescent and proliferative phenotype based on external stimuli; this is an important feature for vascular repair. However, it leaves VSMCs predisposed to be affected by aberrant stimuli which results in improper cell proliferation, contributing to cardiovascular disease. Therefore, understanding the molecular mechanisms involved in activating VSMCs is of great clinical and therapeutic importance.

-catenin is a multifunctional protein that plays an essential role in cell adhesion at the membrane and is required for Wnt dependent target gene expression in the nucleus. -catenin has been shown to play a significant role in VSMCs during both development and wound repair, and -catenins role is wholly dependent on specific protein-protein interactions in the cell, thus the aim of this project was to identify novel interacting partners and determine their effect on -catenin function in VSMCs.

Our lab has previously identified the myocyte enhancer factor 2 (MEF2) family of transcription factors as potential -catenin interacting partners. The MEF2s are involved in vascular development during embryogenesis, but are also upregulated in the neointima following carotid artery injury in mice. In the first study, a direct interaction between MEF2 and -catenin was documented. This interaction was enhanced by p38 phosphorylation of MEF2, which resulted in increased nuclear retention of -catenin. Active p38 in primary VSMCs was able to stimulate Wnt signalling in a MEF2-dependent manner, and the MEF2--catenin interaction also synergistically increased cell proliferation.

The purpose of the second study was to identify novel -catenin interacting partners in VSMCs using mass spectrometry. The investigation revealed 131 interactions, and gene ontology enrichment analysis emphasized translation as a significant biological process. Examination of one potential interactor, the fragile X mental retardation protein (FMRP), revealed a direct interaction with -catenin. siRNA silencing of -catenin increased global protein synthesis, and biochemical analysis determined that -catenin associates at the pre-initiation complex with FMRP. Wnt-dependent nuclear shuttling of -catenin de-repressed protein translation, and we concluded that cytoplasmic -catenin can inhibit translation. Thus, we established a completely novel role for -catenin function in the regulation of translation by interaction with FMRP.