Abstract 211: ATF6 and the Adaptive ER Stress Response Enhances Proliferation and Viability in Mouse Cardiac Stem Cells

Rationale: Cardiac stem cells (CSCs) are beneficial when administered to infarcted mouse or rat hearts. Though the mechanism of these benefits is unknown, CSC vitality likely plays a major role. Thus, investigating the factors governing CSC survival in the ischemic heart may lead to more effective therapeutic strategies. Our previous studies showed that misfolded proteins accumulate in the sarco/endoplasmic reticulum (SR/ER) of the ischemic heart. The transcription factor, ATF6, is a key component of the adaptive ER stress response because it induces genes that reduce the accumulation of misfolded proteins, improving myocyte survival during ischemic stress. While our lab has shown that, in cardiac myocytes, ATF6 is cardioprotective in the ischemic heart, neither the ER stress response nor ATF6 have been examined in CSCs. We hypothesize that ATF6 and the adaptive ER stress response are critical for optimal survival of CSCs. Objective/Methods: To gauge the relevance of the ER stress response in CSCs, we used MTT assays to compare the viabilities of mouse CSCs to neonatal rat ventricular myocytes (NRVM) subjected to treatments that mimic ischemic ER stress in the heart. We also assessed the effect of inhibiting ATF6 on both the ER stress response and CSC viability by using chemical inhibition of ATF6 activation or siRNA-mediated ATF6 knock down. Results: We found that, compared to NRVM, CSCs exhibited lower levels of adaptive ER stress response gene expression and decreased viability in response to ER stress. Thus, relative to NRVM, the adaptive ER stress response is not fully developed in CSCs. We also found that either chemical inhibition of ATF6 activation or ATF6 knock down decreased adaptive ER stress response gene expression. Strikingly, ATF6 inhibition or knockdown decreased CSC viability and cell number by as much as 70%. Conclusions: Thus, compared to cardiac myocytes, CSCs exhibit a reduced adaptive ER stress response and are more sensitive to ER stress, suggesting that enhancement of the ATF6-mediated adaptive ER stress response in CSCs may be a viable therapeutic approach for enhancing stem cell-mediated myocardial repair.