Oct4 cooperates with c-Myc to improve mesenchymal-to-endothelial transition and myocardial repair of cardiac-resident mesenchymal stem cells.

Background: Cardiac-resident mesenchymal stem cells (cMSCs) can exhibit fibrotic, proinflammatory, and proangiogenic phenotype in response to myocardial ischemia (Isch). How their phenotypic fate decisions are determined remains poorly understood. Here, we demonstrate that the cooperation of Oct4 and c-Myc in cMSCs creates a preferable mesenchymal-to-endothelial transition (MEndoT) to promote angiogenesis and consequent myocardial repair.

Methods: We collected MSCs from cardiac and peripheral blood of rat with left ventricular Isch (LV Isch) 30 days after myocardial infarction (MI) or sham operation. After a comparison of characterization between cMSCs and peripheral blood MSCs (pbMSCs), we conducted transcriptome analysis and RNA sequencing of cMSCs. Using loss/gain-of-function approaches to understand the cooperation of c-Myc and Oct4 on MEndoT of cMSCs under hypoxic condition, we explored the mechanisms through transcriptome and functional experiment, and chromatin immunoprecipitation. Next, we transplanted male cMSCs with overexpression or inhibition of c-Myc/Oct4 into the infarcted myocardium of female rats and evaluated infarct size, cell retention, inflammation, remodeling, and function after 30 days.

Results: LV Isch switched cMSCs toward both inflammatory and proangiogenic phenotypes, with increased secretion of inflammatory cytokines as well as decreased expression of proangiogenic factors. The effect of LV Isch on pbMSCs was less remarkable. Gene expression heatmap showed imbalance in expression of Oct4 and c-Myc regulating genes associated with remodeling of cMSCs. We provided evidence that cMSCs-specific c-Myc- versus Oct4-overexpression showed divergent genomic signatures, and their corresponding target genes play an important role in regulating cMSCs phenotypic changes. In particular, Oct4 accelerated angiogenesis induced by c-Myc overexpression in cMSCs and inhibited their phenotypic transition into inflammatory cells and fibroblast. Mechanistically, exogenous Oct4 caused c-Myc to translocate from the nucleus to the cytoplasm and activated some of its target signalings including VEGF signaling. Although transplantation of cMSCs alone did not improve LV remodeling and function, cMSCs co-transfected with c-Myc and Oct4 promoted a more positive effect in their survival and reparative properties, increased animal survival, reduced infarct size, decreased scar thickness, inhibited LV remodeling, and improved heart function 30 days after MI. Significantly, Oct4 promoted MEndoT ("Rescue me" signal) of cMSCs after both c-Myc stimulation in vitro and transplantation into the infarcted heart.

Conclusions: Myocardial Isch drives resident cMSCs toward multiple phenotypes. Oct4 interacts with c-Myc to promote MEndoT capacity of cMSCs and improve their survival and reparative effects through upregulation of angiogenesis-related signaling pathways. These findings may identify novel targets for stem cell therapy.