Abstract
The potentiation of the naturally limited regenerative capacity of the heart is dependent on an understanding of the mechanisms that are activated in response to pathological conditions such as hypoxia. Under these conditions, the expression of genes suggested to support cardiomyocyte survival and heart adaptation is triggered. Particularly important are changes in the expression of myosin heavy chain (MHC) isoforms. We propose here that alterations in the expression profiles of MHC genes are induced in response to hypoxia and are primarily mediated by hypoxia inducible factor (HIF). In in vitro models of mouse embryonic stem cell-derived cardiomyocytes, we showed that hypoxia (1% O2) or the pharmacological stabilization of HIFs significantly increased MHCbeta (Myh7) gene expression. The key role of HIF-1alpha is supported by the absence of these effects in HIF-1alpha-deficient cells, even in the presence of HIF-2alpha. Interestingly, ChIP analysis did not confirm the direct interaction of HIF-1alpha with putative HIF response elements predicted in the MHCalpha and beta encoding DNA region. Further analyses showed the significant effect of the mTOR signaling inhibitor rapamycin in inducing Myh7 expression and a hypoxia-triggered reduction in the levels of antisense RNA transcripts associated with the Myh7 gene locus. Overall, the recognized and important role of HIF in the regulation of heart regenerative processes could be highly significant for the development of novel therapeutic interventions in heart failure.
Highlights
The sensitivity of the mammalian myocardium to pathological alterations is related to its highly limited regenerative potential [1, 2]
In in vitro models of mouse embryonic stem cell-derived cardiomyocytes, we showed that hypoxia (1% O2) or the pharmacological stabilization of hypoxia inducible factor (HIF) significantly increased MHCbeta (Myh7) gene expression
Aware of the fact that in the HG8 model the MHCα expression profile might be biased by cardiomyocyte pre-selection based on Myh6 promoter activity, we tested the expression of both cardiac myosin heavy chain (MHC) isoforms upon hypoxic exposure in the parental R1 cell line and again observed the upregulation of Myh7 mRNA expression (Figure 1D) and total MHCα and β protein levels (Figure 1E)
Summary
The sensitivity of the mammalian myocardium to pathological alterations is related to its highly limited regenerative potential [1, 2]. In a variety of pathophysiological conditions including ischemia, hypertrophy, and atrophy, a common response of the heart is the induction of fetal gene reprogramming associated with complex changes in gene expression profiles, often suggested to support cell adaptation and survival [3]. These include modulation of the expression of genes coding for myosin heavy chains (MHCs). The critical pathological event responsible for such regulation is not yet well understood; it has been suggested that it is most likely related to alterations in the myocardium microenvironment [7]
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