Cardiomyocyte death post-MI starts immediately after coronary occlusion and occurs downstream from the occlusion. The therapeutic goal to decrease infarct size to less than 20% of the left ventricle has shown to improve long-term prognosis . Early diagnosis and treatment are critical but presently inadequate. Administering a recombinant protein alone or in conjunction with grafting cells has reduced early injury as well as adverse remodeling and subsequent cardiac dysfunction –. Clinically, only reperfusion therapy with angioplasty or thrombolysis has been proven to reduce early injury. Although late remodeling cannot be significantly altered, it is managed with pharmacologic agents which sustain ventricular function but this merely delays the progression to heart failure. Current therapeutics, such as early myocardial revascularization after myocardial infarction, are geared toward minimizing cardiac myocyte necrosis and may modulate apoptosis and autophagy . Research that explores the number and type of cells for grafting alone or in combination with gene and protein-based therapeutic strategies are aimed at augmenting the cardiac regenerative potential and promoting tissue salvage. Due to poor reproducibility and efficacy, as well as untoward side effects, none of these experimental paradigms have been successfully translated into clinical therapies. Due to these factors and the narrow window of opportunity between the initial cardiac insult and the cellular demise, developing newer therapies for “cellular resuscitation” are of paramount importance –.
Diabetes has long been associated with greater incidence of myocardial infarction, inadequate healing, and susceptibility to further events. Hyperglycemia, the principal manifestation of all types of diabetes, results in perturbations in cellular metabolism, oxidative stress, exacerbated inflammation, and AGE-modified proteins that culminate in tissue damage, cardiac dysfunction, and impaired wound healing responses –. Sweeney et al. (2012) observed decreased glucose uptake and increased autophagy after acute I/R injury in STZ hearts, however, long term studies reveal detrimental effects on remodeling that ultimately compromise performance .
The EphA2-R has historically been known to regulate apoptosis in developing vasculature and nervous system and cell death during tumorigenesis ,–, the EphA2-R also modulates vascular permeability, inflammation, and ischemic injury ,,,–. Knockdown of EphA2-R in HUVECs suppresses thrombin-induced phosphorylation of NF-κB, resulting in decreased ICAM-1 expression . We observed increased sICAM-1 in serum of WT mice post-MI but this is blunted in hyperglycemic mice. Interestingly, sICAM-1 is decreased in hyperglycemic EphA2-R-M mice compared to normoglycemic mice and is not detected post-MI but is increased in hyperglycemic EphA2-R-M post-MI. EphrinA1 and EphA2 are both upregulated in response to inflammation –. Although we did not detect differences in NF-kB expression, we did observe decreased ephrinA1 expression in EphA2-R-M mice post-MI, and propose that the deficiency of EphA2-R prevents timely induction of inflammatory cascades necessary for mediating repair. Further, cardiac ephrinA1 expression exhibited an non-significant trend to increase slightly in hyperglycemic WT mice post-MI but not to the extent that they are expressed in uninjured normoglycemic mice, and is thus likely ineffective in eliciting the protective effects known to occur from exogenous administration of ephrinA1-Fc –. TNF-α, IL-1α, MIG, and TREM were only detectable in serum of hyperglycemic EphA2-R-M mice, and significant elevation of MCP-1, TIMP-1, and C5a, which typically correlate with the severity of injury, suggests exacerbated wound healing in EphA2-R-M mice –. Additionally, KC and M-CSF are reduced in hyperglycemic EphA2-R-M mice post-MI, implying poor monocyte adhesion and differentiation ,–. This is supported by the reduced leukocyte infiltration observed in this group, and although cardiac function is not worsened, the increased mortality in this group indicates that inadequate healing mechanisms acutely result in lethality. Indeed, plasma sICAM-1 level is associated with vascular inflammation and has been shown to correlate with increased risk of cardiovascular events ,, providing further support for the continuity of our findings.
We have previously shown that intramyocardial administration of ephrinA1-Fc reduces early injury in non-reperfused myocardium via reduced inflammatory cell infiltration and changes in gene expression of several EphA-Rs. The mechanisms by which this protection occurs can be multifaceted depending on the EphA receptors activated/inhibited and the expression profile of the various cells involved. Goichberg et al. (2012) showed ephrinA1-Fc enhanced motility of EphA2-expressing hCSCs, promoting infarct repair . In adult mouse immortalized HL-1 cardiomyocytes, increased EphA2 expression and reduced phosphorylation following lithocholic acid treatment prevented doxazosin-induced apoptosis via inhibition of SHP-2 . Clearly, the EphA2 receptor plays a vital role in myocardial tissue viability and more studies to understand the signaling pathways involved are warranted.
Expression of the EphA6 receptor gene decreases in response to injury and stays decreased with ephrinA1-Fc . Previous work has shown it to be expressed in genital tubercle vascular endothelia and expression is regulated by HOXA13, a homeobox gene . HOXA13 is also a transcriptional regulator of appropriate vessel morphology and function of the developing placenta . In a recent study, we showed that the EphA6-R localizes to the vasculature and increased in response to injury in EphA2-R-M mouse hearts , suggesting that EphA6-R compensates for the absence of EphA2-R. In the current study, only EphA6 was different between uninjured normoglycemic and hyperglycemic WT and EphA2-R-M mouse hearts. While not altered significantly by hyperglycemia at 4 days post-MI in either strain, it was elevated post-MI in all groups. EphA6 is expressed in adult human monocytes and coronary artery endothelial cells . Therefore, given that several ephrins and Eph receptors are upregulated during injury in various cell types, they may play a role in chemotaxis and extravasation and provide insights into the mechanisms of inflammatory diseases and damage resolution –.