The Integrated Stress Response Regulates Cell Health of Cardiac Progenitors

The discovery of mammalian cardiac progenitor cells has suggested that the heart consists of not only terminally differentiated beating cardiomyocytes, but also a population of self-renewing stem cells with the potential to generate new cardiomyocytes (Anderson, D., Self, T., Mellor, I. R., Goh, G., Hill, S. J., and Denning, C. 2007. Transgenic enrichment of cardiomyocytes from human embryonic stem cells. Mol. Ther. 15, 2027-2036; Bearzi, C., Rota, M., Hosoda, T., Tillmanns, J., Nascimbene, A., De Angelis, A., Yasuzawa-Amano, S., Trofimova, I., Siggins, R. W., Lecapitaine, N., Cascapera, S., Beltrami, A. P., D'Alessandro, D. A., Zias, E., Quaini, F., Urbanek, K., Michler, R. E., Bolli, R., Kajstura, J., Leri, A., et al. 2007. Human cardiac stem cells. Proc. Natl. Acad. Sci. U.S.A. 104, 14068-14073; Wu, S. M., Chien, K. R., and Mummery, C. 2008. Origins and fates of cardiovascular progenitor cells. Cell 132, 537-543). A consequence of longevity is continual exposure to environmental and xenobiotic stresses, and recent literature suggests that hematopoietic stem cell pools tightly control cell health through upregulation of the integrated stress response and consequent cellular mechanisms such as apoptosis. However, whether or not this biological response is conserved in progenitor cells for later lineages of tissue-specific stem cells is not well understood. Using human-induced pluripotent stem cells (iPSC) of both cardiac progenitor and mature cardiomyocyte lineages, we found that the integrated stress response was upregulated in the iPSC cardiac progenitors leading to an increased sensitivity for apoptosis relative to the mature cardiomyocytes. Of interest, C/EBP homologous protein (CHOP) signaling plays a mechanistic role in the cell death phenotype observed in iPSC progenitors, by which depletion of CHOP prevents cell death following cellular stress by thapsigargin exposure. Our studies suggest that the integrated stress response plays a unique role in maintaining iPSC cardiac progenitor cellular integrity by removing unhealthy cells via apoptosis following environmental and xenobiotic stresses, thus preventing differentiation and self-renewal of damaged cells.