Abstract Body: Stem/progenitor cells are being investigated as a potential cell source to reverse cardiac remodeling and restore cardiac function, but the molecular mechanisms required for improved repair are not well understood. Circulating BMP9 is negatively associated with heart disease risk factors. BMP9/ALK1 signaling is a critical pathway involved in several mechanisms of vascular repair such as angiogenesis and inflammation. We recently identified ALK1 expression in human highly proliferative cells (hHiPCs) with progenitor capabilities isolated from patients undergoing coronary artery bypass graft (CABG) surgery as measured by flow cytometry. We have determined that circulating BMP9 in patients is negatively associated with number of hHiPC (p<0.001) and positively associated with endothelial cell (EC) number (p=0.008) suggesting that BMP9 may play a role in hHiPC differentiation into EC. Further, we found upregulation of novel BMP9 regulated proteins, IGFBP3, SOST, and ISLR, in hHiPC upon BMP9 treatment using LC-MS/MS analysis of the secretome (p<0.01, Fold-change (FC)>2.0) and RT-qPCR (p<0.001, FC>2.0). Conditioned media collected from BMP9 treated hHiPCs or ECs improved tube formation in the tube formation assay. IGFBP3, SOST, and ISLR upregulation by BMP9 is inhibited upon knockdown and pharmacological inhibition of ALK1 as measured by proteomics (p<0.01) and RT-qPCR analysis (p<0.05). ISLR is a novel marker of mesenchymal stem cells found to have an important role in maintaining “stemness” and colony formation. hHiPCs treated with novel BMP9 target, ISLR, were analyzed using LC-MS/MS and STRING. We found ISLR treatment enriched for differential regulation of cell adhesion, cell migration, and angiogenesis proteins such as CXCL12, EMILIN1, and ANXA5 (p<0.05). Together, these data lay the foundation for investigating BMP9/ALK1 autocrine signaling in potential regulation of progenitor cell maintenance and angiogenesis in heart disease.