YY1 Mediated Competitve Regulation: A Governing Principle Behind Phenotype-Specific Gene Expression in Vascular Smooth Muscle Cells

Leslie Don Roberts, YYI Mediated Competitive Regulation: A governing principle behind phenotype-specific gene expression in vascular smooth muscle cells. Doctor of Philosophy (Biomedical Science), July 2005. The vascular smooth muscle cell (VSMC) lacks the typical phenotypic restriction that limit most cell-types to expressing a single phenotype, as a result, these cells are uniquely suited to wound repair, as well as, exacerbating several vascular disease-states. While much is known regarding the specific transcription factors that drive phenotype-specific gene expression the mechanisms that regulate the transition between phenotype-specific gene programs remain poorly defined. To further explore these mechanisms, we sought to better understand how VSMCs stably express their default contractile-specific gene program despite the inherent instabilities of their environment. This study explored the regulatory implications of a yet undescribed regulatory domain, that resides with a high-frequency in the promoters of the most contractile-specific gene. These domains, which we term dual regulatory domains (DRD), orient the core binding site for the transcriptional repressor Ying Yang-1 in close proximity to, or overlapping with, the core binding site for a variety of transcriptional activators. This study specifically examines the regulatory implications at two DRD where YY1 competes with the transcriptional activators C/EBPβ (C/CAAT-enhancer binding protein beta). Our findings demonstrate: i.) YY1 acts as a dominant, negative, regulator of the smooth muscle myosin heavy chain (SM-MHC) gene promoter; ii.) YY1 binds to, and repressing from, multiple sites within the regulatory context of this promoter; and iii.) The transactivation potential of C/EBPβ completes with transrepressive potential of YY1 for regulatory control over SM-MHC promoter activity and does so in a stoichiometric-dependent fashion. These findings argue that the relative concentrations of YY1 define the effective dose required of specific transcriptional activators to complete with and override the repressive effect of YY1, and by doing so, directly dictate which genes will be expressed.