Abstract 1229: The Reconstitution of Large Coronary Arteries by Cardiac Stem Cells is Mediated by Hypoxia
Cardiac stem cells (CSCs) are self-renewing, clonogenic and multipotent and, thereby, have the ability to differentiate into myocytes, and smooth muscle cells (SMCs) and endothelial cells (ECs) organized in coronary arterioles and capillary structures. Importantly, c-kit-positive CSCs can create large branches of the main coronary vessels when injected intramyocardially above, laterally and below the ligation site of the left coronary artery. The objective of this study was to document whether changes in myocardial environment created by coronary occlusion favor the differentiation of CSCs into vascular SMCs and ECs. Potential candidates include the hypoxia-inducible factor-1α (HIF-1α), which is a transcriptional regulator of the SDF-1 chemokine. HIF-1α and SDF-1 are upregulated with ischemia and may correlate with the oxygen gradient within the tissue. In longitudinal sections of the infarcted heart, hypoxia increased progressively from the base to the mid-portion and apex of the infarcted ventricle. The expression of HIF-1α and SDF-1 was determined by Western blot and was higher in the ischemic heart with respect to control. In the region of the heart above the ligature, the levels of HIF-1α and SDF-1 were comparable to those detected in non-infarcted hearts, but were markedly increased in the ischemic tissue located below the ligature. By immunohistochemistry, SDF-1 and HIF-1α were mostly distributed in the endothelial lining of the vessel wall. SDF-1 was also present in the cardiac interstitium while HIF-1α was abundant in ischemic myocytes. The upregulation of HIF-1α and SDF-1 occurred at 1 hour after infarction and remained elevated up to 12 hours. In vitro studies were then performed. CSCs express CXCR4, the receptor of SDF-1, but do not synthesize and release SDF-1 in the culture medium. In the presence of SDF-1, there was a significant increase in the differentiation of CSCs into SMCs and ECs. Moreover, CSCs co-cultured with HUVEC under hypoxic conditions showed an enhanced formation of SMCs and ECs. However, hypoxic CSCs failed to mimic the in vivo situation. In conclusion, myocardial ischemia leads to an upregulation of HIF-1α, which in turn potentiates the synthesis of SDF-1 and the generation of coronary vessels by lineage commitment of CSCs.