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We hypothesized that inhibition of the cGMP-specific enzyme phosphodiesterase 5A (PDE5A) promoted cGMP/protein kinase G (PKG) activity to condition stem cells for enhanced survival and proliferation. One-time tadalafil treatment (1 [micro]M for 30 min) of mesenchymal stem cells ([sup.Tada]MSCs) provided sustained protection of cells for 36 h. Higher cGMP activity with concomitantly increased PKGI activity was observed in [sup.Tada]MSCs, which peaked within 12 h after tadalafil treatment. Pretreatment with PKG1 blockers (1 [micro]M KT-5823 or 20 nM K-252a) or transduction with adenoviral PKG1-short-hairpin RNA abolished tadalafil-induced cytoprotection of the cells. A higher proliferation rate was observed in 7adaMSCs compared with nontreated MSCs ([sup.Cont]MSCs). In a rat model of acute myocardial infarction, [sup.Tada]MSCs transplanted 0 and 24 h after tadalafil treatment showed higher survival compared with [sup.cont]MSCs on day 2 and day 4 after engraftment. [sup.Tada]MSCs transplanted 48 h after tadalafil treatment lost their protection on both day 2 and day 4 after engraftment, and their rate of survival was similar to [sup.cont]MSCs. Reduced terminal dUTP nick end-labeling positivity (P < 0.01 vs. [sup.Cont]MSCs) and higher proliferation of [sup.Tada]MSCs (P < 0.01 vs. [sup.Cont]MSCs) was observed in the infarcted heart. Fluorescence immunostaining revealed neomyogenesis in both the infarct and peri-infarct areas. Blood vessel density was significantly increased in group 2 compared with group 1. Transthoracic echocardiographic heart function revealed significant preservation of the indexes of left ventricle contractility and attenuation of remodeling in [sup.Tada]MSC-engrafted animal hearts (group 2) compared with [sup.Cont]MSCs (group 1). PDE5A inhibition using long-acting tadalafil is an innovative approach to promote stem cell survival and proliferation in the infarcted heart. cytoprotection; phosphodiesterase; preconditioning; stem cells; tadalafil doi: 10.1152/ajpheart.00437.2010.

We have previously reported that preconditioning of bone marrow-derived mesenchymal stem cells (MSCs) with diazoxide (DZ) significantly improved cell survival via NF-[kappa]B signaling. Since microRNAs (miRNAs) are critical regulators of a wide variety of biological events, including apoptosis, proliferation, and differentiation, it is likely that DZ-induced survival is mediated by miRNAs. Here we show that miR-146a expressed during preconditioning with DZ is a key regulator of stem cell survival, Treatment of MSCs with DZ (200 [micro]M) markedly increased miR-146a expression and promoted cell survival, as evaluated by lactate dehydrogenase release and transferase-mediated dUTP nick-end labeling staining. Interestingly, blocking NF-[kappa]B by IKK-[gamma] NEMO binding domain inhibitor peptide did not induce miR-146a expression, indicating NF-[kappa]B regulates miR- 146a expression. Moreover, blockade of miR- 146a expression by antisense miR-146a inhibitor abolished DZ-induced cytoprotective effects, suggesting a critical role of miR-146a in MSC survival. Computational analysis found a consensus putative target site of miR-146a relevant to apoptosis in the 3' untranslated region of Fas mRNA. The role of Fas as a target gene was substantiated by abrogation of miR-146a, which markedly increased Fas protein expression. This was verified by luciferase reporter assay, which showed that forced expression of miR-146a downregulated Fas expression via targeting its 3'-UTR of this gene. Taken together, these data demonstrated that cytoprotection afforded by preconditioning of MSCs with DZ was regulated by miR-146a induction, which may be a novel therapeutic target in cardiac ischemic diseases. apoptosis; preconditioning doi: 10.1152/aipheart.00212.2010.

Cardiomyocytes respond to physiological or pathological stress only by hypertrophy and not by an increase in the number of functioning cardiomyocytes. However, recent evidence suggests that adult cardiomyocytes have the ability, albeit limited, to divide to compensate for the cardiomyocyte loss in the event of myocardial injury. Similarly, the presence of stem cells in the myocardium is a good omen. Their activation to participate in the repair process is, however, hindered by some as-yet-undetermined biological impediments. The rationale behind the use of adult stem cell transplantation is to supplement the inadequacies of the intrinsic repair mechanism of the heart and compensate for the cardiomyocyte loss in the event of injury. Various cell types including embryonic, fetal, and adult cardiomyocytes, smooth muscle cells, and stable cell lines have been used to augment the declining cardiomyocyte number and cardiac function. More recently, the focus has been shifted to the use of autologous skeletal myoblasts and bone marrow-derived stem cells. This review is a synopsis of some interesting aspects of the fast-emerging field of bone marrow-derived stem cell therapy for cardiac repair. myocardial infarction; cardiomyocyte; plasticity; transdifferentiation