Cellular Biology Regulated Expression and Role of c-Myb in the Cardiovascular-Directed Differentiation of Mouse Embryonic Stem Cells

Rationale: c-myb null (knockout) embryonic stem cells (ESC) can differentiate into cardiomyocytes but not contractile smooth muscle cells (SMC) in embryoid bodies (EB). Objective: To define the role of c-Myb in SMC differentiation from ESC. Methods and Results: In wild-type (WT) EB, high c-Myb levels on days 0–2 of differentiation undergo ubiquitin-mediated proteosomal degradation on days 2.5–3, resurging on days 4–6, without changing c-myb mRNA levels. Activin-A and bone morphogenetic protein 4–induced cardiovascular progenitors were isolated by FACS for expression of vascular endothelial growth factor receptor (VEGFR)2 and platelet-derived growth factor receptor (PDGFR) . By day 3.75, hematopoesis-capable VEGFR2 cells were fewer, whereas cardiomyocyte-directed VEGFR2 /PDGFR cells did not differ in abundance in knockout versus WT EB. Importantly, highest and lowest levels of c-Myb were observed in VEGFR2 and VEGFR2 /PDGFR cells, respectively. Proteosome inhibitor MG132 and lentiviruses enabling inducible expression or knockdown of c-myb were used to regulate c-Myb in WT and knockout EB. These experiments showed that c-Myb promotes expression of VEGFR2 over PDGFR , with chromatin immunopreciptation and promoter-reporter assays defining specific c-Myb–responsive binding sites in the VEGFR2 promoter. Next, FACS-sorted VEGFR2 cells expressed highest and lowest levels of SMCand fibroblast-specific markers, respectively, at days 7–14 after retinoic acid (RA) as compared with VEGFR2 /PDGFR cells. By contrast, VEGFR2 /PDGFR cells cultured without RA beat spontaneously, like cardiomyocytes between days 7 and 14, and expressed cardiac troponin. Notably, RA was required to more fully differentiate SMC from VEGFR2 cells and completely blocked differentiation of cardiomyocytes from VEGFR2 /PDGFR cells. Conclusions: c-Myb is tightly regulated by proteosomal degradation during cardiovascular-directed differentiation of ESC, expanding early-stage VEGFR2 progenitors capable of RA–responsive SMC formation. (Circ Res. 2012;110:253-264.)