Abstract 19616: Ischemic Stress Reduces Cardiac Progenitor Cell Self-renewal Through Gsk3β Activation and Reduction in Cmyc Stability

Abstract

Introduction: The adult heart harbors a population of resident stem cells capable of self-renewal and differentiation into cardiac, smooth muscle and endothelial lineages. Cardiac progenitor cells (CPCs) have been shown to promote cardiac regeneration and improve heart function. However, despite their potential use in cardiac repair, evidence suggests that resident CPCs regenerative capacity is limited in conditions of severe hypoxia such as ischemic cardiomyopathy. Elucidation of the mechanisms involved in CPC protection against hypoxic stress is essential to maximize their cardioprotective and therapeutic potential. Methods and Results: We investigated the growth pattern of murine CPCs under normoxia (21% oxygen) and hypoxia (0.5% oxygen) conditions along a period of 96 hours. We found that CPC proliferation in hypoxia was significantly reduced by 40% after 72-96 hours (p<0.05, n=4-7), without evidence of increase in cell death. CPC proliferation was accompanied with a time-dependent increase in senescence-associated β-galactosidase activity by 30% in hypoxia relative to normoxia control (p<0.02, n=7). Western blot analysis of CPCs grown under normoxia and hypoxia revealed a time-dependent decrease in the expression of the transcription factor cMyc and its downstream target Bmi-1 by 40% and 30% (p<0.05, n=4-5) relative to normoxia control, respectively. Interestingly, reduction in cMyc protein expression was not associated with a decrease in cMyc transcripts, suggesting that cMyc protein degradation is activated under hypoxia. Treatment of CPCs with cycloheximide to block protein synthesis showed that the rate of cMyc degradation under hypoxia is increased by 30% (p<0.05, n=5) relative to normoxia. GSK3β plays a central role in the control of cMyc stability. Growth of CPCs in the presence of GSK3β inhibitor rescued proliferation in hypoxia by approximately 40% (n=4). Conclusions: Our results suggest that hypoxic stress reduces CPC self-renewal potential by inducing CPC senescence through a mechanism that involves decrease of cMyc stability through GSK3β activation. Modulation of GSK3β activity may be used as a therapeutic approach for protection of endogenous CPCs self-renewal potential under ischemic conditions.