Abstract
Background: Adult cardiac stem cells (CSCs) capable of self-renewal and differentiation into cardiac, smooth muscle and endothelial lineages have been shown to promote tissue repair after ischemic injury and improve heart function. Despite their potential role in cardiac regeneration, evidence suggests that resident CSCs are impaired in conditions such as myocardial infarction, limiting their regenerative capacity. Methods and Results: The growth rate of CSCs is significantly reduced by 40% (p<0.05) under hypoxic stress (0.5% O 2 ). Using an extracellular-matrix (ECM) and adhesion-focused PCR array, we found that incubation of CSCs for 18 hours under hypoxia leads to down-regulation of several genes, including osteopontin, thrombospondin-1 and integrins. Due to their critical role in communicating extracellular signals regulating stem cell mobilization, proliferation, survival, migration and differentiation we decided to investigate the role of integrins in CSCs self-renewal and differentiation. Our gene expression studies showed that integrin-β1 (Itgb1) is the most abundant integrin expressed in CSCs. Therefore, we knocked down (KD) Itgb1 in CSCs and evaluated these cells regarding adhesion, growth and differentiation properties. Our results showed that reduction in Itgb1 levels in CSCs significantly decreased their interaction with extracellular matrix components fibronectin, laminin and vitronectin by 32%, 20% and 24% (p<0.05), respectively. KD of Itgb1 in CSCs reduced growth by 40% (p<0.05), similar to what we observed by with wild-type cells in hypoxia. Assessment of endothelial differentiation showed that KD cells have a higher angiogenic potential vs. NS-control, as demonstrated by upregulation of vWF (3-fold), VEGF (2.5-fold), Pecam1 (2.3-fold) and Nos3 (1.9-fold) (p<0.05). Interestingly, Itgb1 KD per se induced upregulation of these vascular markers in CSCs. Conclusions: Our results suggest that Itgb1 plays an important role in CSCs self-renewal, differentiation and cell adhesion. It is possible that loss of self-renewal is a consequence of changes in CSCs fate as knockdown of Itgb1 leads to upregulation of endothelial lineage markers in these cells. This would limit the expansion of CSCs and their regenerative capacity.
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