Bone marrow-derived MSCs have become therapeutically important agents because of their multilineage potentials, immuno-modulatory properties and ability to localize specifically to injured sites, to reduce scar tissue formation and to increase neovascularization
[4, 40]. Although many MSC transplantation studies have shown beneficial effects in treating ischemic injury, it is currently limited by the poor engraftment of implanted MSCs due to the harsh microenvironment in the ischemic region
[4, 5]. Thus, additional strategy is required to enhance the therapeutic efficiency of cell therapy by improving cell homing, survival, engraftment and repair capacity of transplanted cells. One promising strategy may be the combination of cell and gene therapy.
Our results demonstrate that intramyocardial injection of Ad-HIF-1α combined with MSC transplantation was potent to promote angiogenesis, and led to improved cardiac performance after myocardial infarction in the rats. Our study also showed that overexpression of HIF-1α after Ad-HIF-1α transfection allows for supranormal levels of HIF-1α mRNA in peri-infarcted myocardium. Consistent with previous studies
[29, 33], we found that in the SHAM group, without hypoxia, the mRNA level of HIF-1α was extremely low (Figure
2A1-A3). While in the Control group, which presented the endogenous HIF-1α expression under hypoxia conditions, the mRNA level of HIF-1α was significantly increased. The mRNA levels of HIF-1α in the HIF-1α + MSCs and HIF-1α groups were expressed at much higher levels than that of the MSCs and Control groups due to the exogeneous HIF-1α expression. However, the HIF-1α mRNA was much lower in the HIF-1α-MSCs group compared to the HIF-1α + MSCs and HIF-1α groups. The HIF-1α expression level in the HIF-1α-MSC group depends on the numbers of engrafted MSCs, which have very low survival rates as evidenced by our data. In contrast, the injected virus could transfect any cell type in the infarcted area, which may explain the observed differences of HIF-1α expressions among different groups.
We found that the SDF-1α mRNA expressions were also significantly increased in the HIF-1α + MSCs and HIF-1α groups than in that of other groups (Figure
2B1-B3). Previous research found that the SDF-1α/CXCR4 axis mediated the migration and homing of bone marrow-derived cells and endothelial progenitor cells in vivo[30, 41–43]. Consistent with these findings, we found that more MSCs survived and engrafted in the infarcted hearts in the HIF-1α + MSCs and HIF-1α-MSCs groups than in the MSCs group. Enhanced angiogenesis, better blood flow and the beneficial effect of several cytokines may contribute to the improvement of the ability of MSCs to survive in hypoxic environments, the migration to the ischemic fibrotic tissue from the border zone, and the angiogenesis in the ischemic area. Since more MSCs survived in the infarcted hearts in the HIF-1α + MSCs and HIF-1α-MSCs groups, we speculated that the combined therapy may further enhance the angiogenesis at the peri-infarcted and infarct regions compared to the HIF-1α group, partly due to the MSC-dependent paracrine mechanism.
The mRNA levels of VEGF, regulated directly by HIF-1α, were expressed at much higher level in the HIF-1α + MSCs and HIF-1α groups compared to other groups (Figure
2C1-C3). It partly explained why a marked increase in capillary density in the peri-infarcted and infarcted regions was observed in these two groups. Sufficient blood flow as a result of increased blood vessel formation might be instrumental in preventing the loss of cardiomyocytes in these zones over time, preservation of contractility in the border zone adjacent to the infarct, and suppression of post-infarction cardiac failure during left ventricular remodeling
Our findings suggest that myocardial deterioration after infarction in the HIF + MSCs group may be limited not only as a result of stimulation of angiogenesis through a VEGF-related pathway, but also through additional HIF-1α-mediated local adaptations to low oxygen tension, and significantly improved microenvironment and increased survival, engraftment and repair ability of MSCs. Furthermore, our research demonstrated that the HIF + MSCs group showed better capacity for cardiac repair in terms of the expression of certain important cytokines, such as VEGF,、SDF-1α, pro-angiogenesis, anti-apoptosis and restoration of heart function than the HIF-MSCs group. Our report suggests the possibility of using the combination of cell and gene therapy to improve the cardiac repair, without reported side effects, such as fragile and immature vessels and angioma formation
Additionally, our study demonstrated that HIF-1α + MSC treatment was superior to HIF-1α transfection alone in terms of pro-angiogenesis, anti-apoptosis and the capacity for cardiac function repair. Consistent with previous references
[46, 47], our study showed that MSCs contribute to the angiogenesis and anti-apoptosis, which may be partly due to the MSC-dependent paracrine mechanism and their potential for trans-differentiation
Although we demonstrated that the cell survival rate was higher in the HIF-1α + MSCs and HIF-1α-MSCs groups when compared to the MSCs group, the engraftment of cells decreased significantly in all three groups. One hypothesis that may explain the drop-off in engraftment and survival rate is that the exogenous HIF-1α may have been degraded or metabolized. In our future study, Western blotting will be used to track the amount of HIF-1α available to cells over time in vivo in order to clarify the role of HIF-1α in cardiac repair. Furthermore, several studies recently demonstrated encouraging results via the use of biomaterials, such as a dendrimer-type bio-reducible polymer or facial amphipathic bile acid-conjugated polyethyleneimine, to aid the localization of therapeutic genes to the target cells, thus improving the transfection efficiency and enhancing the cardiac repair
[52, 53]. Application of such biomaterials in gene therapy holds promise as a potential novel therapy for the treatment of myocardial ischemia and infarction.
So far, the low engraftment of transplanted cells is still the major obstacle to the wide application of stem cell transplantation to treat MI. A series of studies have demonstrated several potential strategies to optimize stem cell engraftment, such as using tissue-engineered collagen-based scaffolds to provide a suitable microenvironment to support cell attachment and proliferation
[54, 55], or using dynamic three-dimensional culture techniques to enhance MSCs’ properties and increase therapeutic potential
[56, 57]. Recently, our team found that magnetic targeting enhanced the retention of magnetized stem cell in a rat model of myocardial infarction, suggesting that magnetic targeting offers new perspectives for enhancing the cell retention and subsequent functional benefit in heart diseases
. And in our future study, we plan to combine physics and biological methods to optimize the engraftment rate.