PROJECT SUMMARY Cardiac fibroblasts (CF) contribute to the inflammatory response and subsequent scar formation following myocardial infarction (MI); however, the mechanisms coordinating these processes are poorly understood. Using mouse models and human heart tissue, we have found that Small proline rich proteins (Sprr) are uniquely expressed in CF, and are among the most upregulated genes during disease. Conversely, Sprr gene expression is extinguished in CF during exercise. Our preliminary data reveals that SPRR2B binds USP7 and regulates USP7 / MDM2 - E3 ubiquitin ligase substrate selection, facilitating p53 ubiquitination and degradation and relieving constraints on CF accumulation in disease. In contrast, SPRR1A harbors an interaction motif for tumor necrosis factor (TNF) receptor-associated factor (TRAF)-2, an E3 ubiquitin ligase that controls inflammatory signaling. Based on our preliminary data and supporting literature, we hypothesize that context dependent control of E3 ubiquitin-ligase substrate selection by SPRR1A and SPRR2B modulates the inflammatory response following an ischemic event and drives the accumulation of pathological CF, which results in cardiac fibrosis and the progression of HF. A corollary to this hypothesis is that novel MDM2-E3 ubiquitin ligase antagonists, currently in clinical trials as anti-cancer agents, may be useful for the treatment of cardiac fibrosis. Here, we propose the following aims to define a transcriptional paradigm of CF gene regulation in health and disease, and elucidate a novel and druggable mechanism of E3 ubiquitin ligase substrate selection in CF that coordinates the inflammatory response and the development of cardiac fibrosis. If successful, the proposed experiments will refine our understanding of the molecular mechanisms that drive cardiac fibrosis and the progression of HF. Our study may also uncover a novel mechanism of ubiquitin-mediated protein degradation that is broadly applicable to the fields of cardiology, progenitor cell-based therapeutics, and cancer biology and further efforts to harness novel E3-ubiquitin ligase antagonists for therapeutic benefit.