Salient findings
Our study is the first to document that HDAC inhibition preserves cardiac performance and suppresses cardiac remodeling in diabetic cardiomyopathy. Specifically, (1) Serial echocardiographic evaluation indicates that HDAC inhibition resulted in the preservation of ventricular function in STZ-induced diabetic heart; (2) HDAC inhibition plays a profound effect in suppressing interstitial fibrosis and attenuating myocyte hypertrophy in the diabetic myocardium; (3) HDAC inhibition mitigated the frequencies of apoptosis in the diabetic myocardium by decreasing active caspase 3 and TUNEL positive signals; (4) HDAC inhibition resulted in the increase of SOD1, GLUT1, and GLUT4 protein levels in diabetic hearts, and acetylation of GLUT1 was elevated following HDAC inhibition; (5) Diabetic hearts exhibited significant decreases in CD31 and α-SMA positive microvessels, which was prevented following HDAC inhibition.
Recent evidence has indicated a genetic association between diabetes and HDACs. HDAC inhibitors promote β-cell development and function that positively affect diabetic microvascular complications [
31]. Ventricular hypertrophy in diabetic hearts was mitigated by inhibition of HDACs [
32]. Transient hyperglycemia is considered to promote gene-activating epigenetic changes critical in the progression of vascular complications [
33]. Therefore, the function of epigenetics in the development of diabetes is largely recognized [
34]. However, its implications in diabetes-associated cardiomyopathy remain to be determined. Our study showed that HDAC inhibition attenuates progressive dysfunction, suggesting that HDACs play an important role in controlling the progression of heart failure.
Clinical studies demonstrate that diabetes mellitus increased the susceptibility of the myocardium to ischemic injury [
35,
36]. In the present study, when we employed a STZ-induced diabetic model [
37], we have demonstrated that the diabetic myocardium (DM) presents with progressive LV systolic failure following STZ treatment for 21 weeks, which is consistent with observations in the development of cardiac dysfunction in STZ-induced diabetic rats [
38]. This was supported by an observation that HDAC inhibition prevented the heart from effects of diet-induced obesity and insulin resistance in mouse models [
39]. Further estimation of diastolic function in this model will determine whether diabetic cardiomyopathy developed. In addition, the low dose of STZ could be used as a diabetogenic method to induce diabetes to avoid the high rate of mortality caused by high dose of drug. Treatment of sodium butyrate also decreased HDAC activity and HDAC4 levels in DM myocardium, which is in agreement with our recent observations that specific inhibition of HDAC4, by reducing HDAC4 protein, promotes stem cell-derived myocardial repair [
40]. Furthermore, we could not find significant changes in other HDAC isoforms following HDAC inhibition in diabetic myocardium. Myocardial enhancing factor 2 (MEF2) is reported to be associated with class II HDACs to mediate cardiac growth and remodeling [
41]. We found that HDAC inhibition only slightly increased MEF2 protein levels in STZ-induced myocardium (not shown), implying that MEF2 may not be a major target following HDAC inhibition in STZ-induced diabetic myocardium. An important pathological feature of diabetic cardiomyopathy is cardiac hypertrophy [
42]. In the initial stage, this hypertrophic cardiomyopathy may be an adaptive response responsible for enhancing cardiac performance [
43]. However, sustained hypertrophic growth of the myocardium may be associated with the occurrence of myocardial remodeling [
20]. HDAC inhibitors blocked cardiac hypertrophy induced by angiotensin II infusion and aortic banding (18.19). We observed that HDAC inhibition remarkably prevented these hypertrophic features in DM. The anti-hypertrophic effect of HDAC inhibition in the DM heart is well mirrored by our recent studies in which cardiac hypertrophy was mitigated in the infarcted hearts following global HDAC inhibition or infarcted hearts engrafted with cardiac stem cells treated with HDAC inhibitors [
27].
Myocardial fibrosis is another important hallmark of diabetic cardiomyopathy, and it is also featured by the accumulation of interstitial collagens in the hearts [
44]. The present results demonstrate that interstitial collagen in DM hearts was reduced after treatment with HDAC inhibition, indicating the function of HDAC in preventing interstitial fibrosis. Another finding is an increase of myocyte apoptosis in DM hearts. Although the significance of myocyte apoptosis in diabetic myocardium remains speculative, progressive loss of myocytes could exacerbate cardiac dysfunction and structure deterioration.
We reported that HDAC inhibition protects the heart against ischemia/reperfusion through blocking of reactive oxidant species [
25]. Oxidative stress has been suggested by several studies to underlie hyperglycemia-induced myocardial cell deaths [
45‐
47]. High glucose-induced cardiomyocyte apoptosis is associated with the generation of reactive oxygen species [
48], and normalization of SOD1 activity was associated with consequent bolstering of anti-oxidant defenses in diabetes [
49].The present study shows that SOD1 was markedly increased by HDAC inhibition, suggesting that the increased anti-oxidant stress elicited by HDAC inhibition accounts for the suppression of myocardial remodeling in the diabetic heart. In addition, the production of superoxides was increased in the diabetic myocardium, but HDAC inhibition attenuated the superoxide production of diabetic hearts, revealing a link between HDAC inhibition and suppression of superoxide.
It is known that pathological cardiac hypertrophy with reduced contractility is accompanied by impaired coronary angiogenesis [
50]. The reductions in capillary and arteriolar densities were shown in the myocardial infarction model of diabetic conditions [
51,
52]. Our study demonstrates a significant reduction in microvessel density in diabetic myocardium, but vascular growth was stimulated by treatment with HDAC inhibition. This increase in microvessel density following HDAC inhibition in diabetic myocardium may also be attributable to the improvement in cardiac function and attenuation of remodeling.
Two glucose transporter proteins (GLUT1 and GLUT4) were reported to be reduced in the diabetic myocardium [
53]. We have shown that GLP-1-induced myocardial protection is associated with the increase of GLUT4 [
53]. In this study, both GLUT1 and GLUT4 were decreased in the diabetic myocardium, but the decrease of both GLUT1 and GLUT4 was prevented by HDAC inhibition. Interestingly, our results show that acetylation of GLUT1 was increased by HDAC inhibition. However, HDAC inhibition did not result in a significant change in the acetylation of GLUT4 (data not shown). This suggests that acetylation of GLUT1 and GLUT4 may respond differently following HDAC inhibitions or depend on the magnitudes of HDAC inhibition. Although the content of GLUT1 and GLUT4 were increased by HDAC inhibition in the diabetic animals, blood glucose level was not reduced by HDAC inhibition in this studies. It is likely that HDAC inhibition could not reduce the peripheral glucose concentration, but instead modulate insulin resistance in the diabetic status, which will be an interesting subject to investigate in the future. It is interesting to elucidate whether acetylation of GLUT1 could mediate its physiological functions in the future. In addition, we have shown that p38 phosphorylation was associated with cardioprotection induced by glucagon-like peptide (GLP-1) in myocardial ischemia and reperfusion [
54]. The present study again reveals that HDAC inhibition increased p38 phosphorylation in diabetic hearts. The future study may attempt to document whether there exists a direct relationship between p38 phosphorylation and GLUT1 acetylation in diabetic hearts. It is not clear whether the PI3 kinase/Akt-1 signaling pathway is also involved in the protective effects induced by HDAC inhibition. On the other hand, mitochondrial oxygen consumption was impaired in the diabetes related mouse model [
55].Our previous observation also indicates that stimulation of GLP-1R could increase mitochondrial oxygen consumption in myoblasts exposed to hypoxia/reoxygenation [
56]. It is not clear whether HDAC inhibition could also modulate mitochondrial oxygen consumptions in this observation. It has been reported that pathological stress activates the chromatin repressor complex containing HDAC to inhibit the transcription of Mhrt, a long noncoding RNA in the heart [
57]. It would be interesting to define whether long non-coding transcripts would also involve the protective effects of HDAC inhibitors in the prevention of diabetic pathology. In this study, we utilized a high dose of STZ to induce the diabetic model. Because the dose of STZ that was used could be cytotoxic in the animal strain, this could be associated with the high mortality rate and abundant myocyte death during the development of diabetes. This could complicate the type 1 diabetic phenotype. This is a limitation for this observation.