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Exogenous hydrogen sulfide protects against high glucose-induced apoptosis and oxidative stress by inhibiting the STAT3/HIF-1α pathway in H9c2 cardiomyocytes

Hydrogen sulfide (H2S), an endogenous gasotransmitter, possesses multiple physiological and pharmacological properties including anti-apoptotic, anti-oxidative stress and cardiac protective activities in diabetic cardiomyopathy. An increasing body of evidence has suggested that signal transducer and activator of transcription 3 (STAT3) has beneficial effects in the heart. However, the effect of diabetes on the phosphorylation or activation of cardiac STAT3 appears to be controversial. The present study was designed to investigate the precise function of the STAT3/hypoxia-inducible factor-1α (HIF-1α) signaling pathway in high glucose (HG)-induced H9c2 cardiomyocyte injury and the function of the STAT3/HIF-1α pathway in the cardioprotective action of H2S. The results revealed that GYY4137 pretreatment substantially ameliorated the HG-induced decrease in cell viability and the increase in lactate dehydrogenase (LDH) release in H9c2 cells. Additionally, HG treatment resulted in the upregulation of the phosphorylated (p)-STAT3/STAT3 ratio and HIF-1α protein expression in H9c2 cells, indicating that the activation of the STAT3/HIF-1α pathway was induced by HG. STAT3/HIF-1α pathway inhibition induced by transfection with STAT3 small interfering (si)-RNA attenuated the HG-induced downregulation of cell viability and the upregulation of LDH release. Furthermore, STAT3 siRNA transfection and GYY4137 pretreatment combined attenuated HG-induced apoptosis as illustrated by the decrease in the number of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells, caspase-3 activity, apoptosis ratio and BCL2 associated X, apoptosis regulator/BCL2 apoptosis regulator ratio in H9c2 cells. In addition, STAT3 siRNA transfection and GYY4137 blocked HG-induced oxidative stress as evidenced by the decrease in reactive oxygen species generation, malondialdehyde content and NADPH oxidase 2 expression, and the increase in superoxide dismutase activity and glutathione level. Notably, GYY4137 pretreatment was revealed to reduce the p-STAT3/STAT3 ratio and HIF-1α protein expression, resulting in the inhibition of the STAT3/HIF-1α signaling pathway in HG-treated H9c2 cells. Altogether, the present results demonstrated that H2S mitigates HG-induced H9c2 cell damage, and reduces apoptosis and oxidative stress by suppressing the STAT3/HIF-1α signaling pathway.


Introduction
Diabetes mellitus is a common metabolic disorder disease that is characterized by impaired glucose tolerance and is closely associated with excess cardiovascular morbidity and mortality (1). Diabetic cardiomyopathy (DCM), a diabetes-specific complication, is characterized by systolic and autonomic dysfunction independent of hypertension, hyperlipidemia or coronary artery disease (2,3). A number of studies have revealed that a number of mechanisms are involved in the pathogenesis of DCM, including myocardial insulin resistance, oxidative stress, mitochondrion dysfunction, inflammation and cardiomyocyte apoptosis (3,4). Among all these events, persistent hyperglycemia in diabetes provokes the excessive production of reactive oxygen species (ROS), resulting in oxidative stress which contributes to the development and pathogenesis of DCM (5,6). Furthermore, oxidative stress injury may further activate cardiac pro-apoptotic signaling pathways in DCM (7,8). However, the pathogenesis of DCM remains poorly understood and there are presently no effective approaches to prevent DCM clinically. Thus, it is necessary to elucidate the molecular mechanisms underlying DCM and identify a novel therapeutic agent with antioxidant and anti-apoptotic activities that is promising for the effective treatment of DCM.

Exogenous hydrogen sulfide protects against high glucose-induced apoptosis and oxidative stress by inhibiting the STAT3/HIF-1α pathway in H9c2 cardiomyocytes
Hydrogen sulfide (H 2 S) is an endogenous gasotransmitter, along with nitric oxide (NO) and carbon monoxide (CO), which regulate a variety of physiological and pathological processes in body (9). Evidence derived from cell cultures, animal models and clinical studies have identified that H 2 S possesses a variety of potent biological and physiological effects including anti-inflammation, anti-apoptosis, anti-oxidant stress and cardioprotection (10,11). For example, exogenous H 2 S contributes to the recovery of ischemic post-conditioning-induced cardioprotection by decreasing the levels of ROS by downregulating the nuclear factor (NF)-κB and Janus kinase (JAK)-2/transducer and activator of transcription 3 (STAT3) pathways in the aging cardiomyocytes (12). In addition, H 2 S attenuates doxorubicin-induced cardiotoxicity by inhibiting apoptosis and ROS production in H9c2 cardiomyocytes (13). To date, numerous studies have confirmed the protective effect of H 2 S on DCM (14)(15)(16). Furthermore, enhanced levels of H 2 S have been demonstrated to elicit infarct-limiting effects against DCM by reducing cardiac fibrosis and apoptosis (17,18). However, the potential protective mechanisms of H 2 S in DCM remain unclear.
STAT3 is a cytoplasmic transcription and signaling molecule that modulates transcription and mitochondrial function, serving necessary functions in a diverse range of biological processes (19,20). Numerous studies have revealed that endogenous STAT3 is beneficial for the heart, serving a function in prevention against multiple heart disease types, including age-associated and postpartum heart failure, cardiotoxic doxorubicin or ischaemia/reperfusion injury (20)(21)(22). However, the effects of diabetes on cardiac STAT3 phosphorylation, expression and activation appear to be rather controversial. A number of publications have reported a substantial decrease in cardiac phosphorylated (p)-STAT3 levels and/or activation in various models of diabetes (23,24). In contrast to these studies, a couple of reports demonstrated that the p-STAT3 level or the activation of STAT3 were substantially increased in diabetic hearts and certain potential cardioprotective agents have been demonstrated to attenuate STAT3 dysregulation in diabetes (25,26). Hence, the function of STAT3 in DCM is worth further investigation. In addition, hypoxia-inducible factor-1α (HIF-1α) is the regulatory subunit of a master regulator of hypoxia-HIF-1, serving a notable function in an important transcription factor whose expression is increased in hypoxia (27). A previous study also indicated that, in addition to hypoxia, glucose also affects the expression and activation of HIF-1α in human pharyngeal carcinoma, fibrosarcoma cells and rat cardiomyocytes (28). Notably, a study from Guilian Niu et al (29) reported that STAT3 is a necessary molecular target for inhibiting the expression of HIF-1 induced by hypoxia and overactive growth pathways prevalent in cancer. However, the function of the STAT3/HIF-1α signaling pathway in the cardioprotection of H 2 S has not been reported.
The present study examines whether exogenous H 2 S inhibits apoptosis and oxidative stress in DCM, and aimed to investigate whether the STAT3/HIF-1α signaling pathway participates in this process.
Cells culture and treatment. Embryonic rat heart-derived H9c2 cells obtained from the Sun Yat-Sen University Experimental Animal Center (Guangzhou, China) were cultured in DMEM supplemented with glucose (5.5 mM), 10% FBS and 1% penicillin-streptomycin in a humidified atmosphere containing 5% CO 2 and 95% air at 37˚C. To investigate the protective effect of exogenous H 2 S on high glucose (HG)-induced H9c2 cell injury, cells were pretreated with GYY4137 (50, 100 or 200 µM), an exogenous H 2 S donor (30), using a range of concentrations based on a previous study (16) for 30 min at 37˚C prior to treatment with HG (33 mM) for 48 h at 37˚C. The control H9c2 cells were cultured in normal glucose (5.5 mM) for 48 h at 37˚C. To confirm the function of the STAT3/HIF-1α signaling pathway, cells were transfected with STAT3 small interfering (si)-RNA or scrambled siRNA followed by treatment with HG (33 mM) for 48 h at 37˚C. STAT3 siRNA transfection. Once grown to 70% confluence in antibiotic-free medium, H9c2 cells were transfected with siRNA against STAT3 (Guangzhou RiboBio Co., Ltd., Guangzhou, China; 50 nM, 5'-CUGUCUUUAGGCUGAUCAU-3') or scrambled siRNA (Guangzhou RiboBio, Co., Ltd.; 50 nM) using Lipofectamine ® RNAiMAX (Invitrogen; Thermo Fisher Scientific, Inc.) according to the manufacturer's protocol. The scrambled siRNA was used as a control for off-target changes. Following 6 h, the transfection mixtures were replaced with DMEM supplemented with 5.5 mM glucose, 10% FBS and 1% penicillin-streptomycin medium, and the cells were treated as described above. The transfection efficiency was confirmed at the mRNA level by reverse transcription-quantitative polymerase chain reaction (RT-qPCR).
CCK-8 assay. The viability of H9c2 cells was measured using the CCK-8 kit according to the manufacturer's protocol. Briefly, once the cells reached 70-80% confluence, H9c2 cells (at a density of 1x10 4 cells/well) were seeded into a 96-well plate overnight and were administered different treatments as aforementioned. Subsequently, 10 µl CCK-8 solution was added to each well and co-incubated for 3 h at 37˚C. The absorbance at 570 nm was determined with a Multiskan FC microplate absorbance reader (Thermo Fisher Scientific, Inc.).
LDH release assay. Cytotoxicity was quantitatively evaluated using an LDH cytotoxicity Colorimetric Assay kit by examining the release of LDH into the culture supernatant. In brief, following treatment as aforementioned, 100 µl culture supernatant from each group was transferred into a different 96-well plate, and 100 µl reaction mixture included in the kit was added and co-incubated for 30 min at room temperature. The optical density (OD) value at 490 nm was measured with a microplate enzyme-linked immunosorbent assay reader. The LDH release was calculated using the following equation: LDH release (%)=[(OD value treated well -OD value blank control )/(OD value control -OD value blank control )] x100%.
Hoechst 33258 staining analysis of cell apoptosis. The apoptosis apoptotic morphology of H9c2 cells was also analyzed using Hoechst 33258 staining (Beyotime Institute of Biotechnology) according to the manufacturer's protocol. Briefly, H9c2 cells were seeded at a density of 2x10 5 /well in 6-plates and treated with NaHS or transfected with STAT3 siRNA followed by HG treatment. Following 48 h incubation, cells were fixed with 4% formaldehyde (EMD Millipore, Billerica, MA, USA) for 10 min at room temperature, washed with phosphate buffered saline (PBS) and stained using Hoechst 33258 staining solution for 10 min in the dark at 37˚C. Following washing with PBS, the cells were observed under fluorescence microscopy (magnification, x200; BX51TRF; Olympus Corporation, Tokyo, Japan). The morphology of the nuclei in apoptotic cells was defined as either the tight and hyperchromatic or fragmental block structure.
Detection of cell apoptosis by flow cytometry. The apoptotic cells were detected using the Annexin V-FITC/PI cell apoptosis detection kit (BD Biosciences) followed by flow cytometry. To measure the apoptotic rate, H9c2 cells in the different groups were collected and washed twice with PBS. Then, cells were re-suspended in 1x binding buffer at a concentration of 1x10 6 cells/ml. Subsequently, Annexin-V-FITC (10 µl) and PI (10 µl) were added to 500 µl cell suspension and co-incubated for 15 min in the dark at room temperature. The rate of apoptosis was analyzed using FACSCantoII Flow cytometry (Becton, Dickenson and Company) and quantified using FACSDiva 6.0 software (Becton Dickenson, and Company) within 1 h. Each experiment was performed at least three times.
Measurement of caspase-3 activity. Caspase-3 activity was detected using a caspase-3 Colorimetric Assay kit in accordance with the manufacturer's protocol. H9c2 cells were seeded at a density of 1x10 6 cells/well into 6-well culture dishes, and harvested using cell lysis buffer (provided in the kit) following 48 h treatment at 37˚C. Following centrifugation at 12,000 x g for 10 min at 4˚C, the supernatants were collected and quantified using a BCA assay kit (Beyotime Institute of Biotechnology) according to the manufacturer's protocol.
The reaction mixture, including the cell lysate (50 µl) and caspase-3 substrate (Ac-DEVD-pNA, 5 µl) in assay buffer was co-incubated at 37˚C for 2 h. The OD at 405 nm was measured using a microplate spectrophotometer. The caspase-3 activity in each treatment group was presented as the fold-change compared with the control group.

Measurement of intracellular ROS by flow cytometry.
Intracellular ROS production was quantified using a DCFH-DA fluorescence probe followed by flow cytometry. Under normal conditions, DCFH-DA is non-fluorescent; however, upon oxidation by ROS, it converts to 2,7-dichlorofluorescein (DCF), a fluorescent marker, which emits green fluorescence (32). Cells were seeded in a 6-well plate at density of 1x10 6 cells/well. Following treatment for 48 h, cells were washed with PBS twice to remove the original medium, and then incubated with DCFH-DA (10 µM) for 20 min at 37˚C. Next, the cells were washed three times with PBS again to remove the additional dyes and the DCF fluorescence intensity was measured using FACSCantoII flow cytometry (Becton, Dickenson and Company) with 488 nm excitation and 538 nm emission filters. Results were quantified using FACSDiva 6.0 software (Becton Dickenson, San Jose, CA). The experiment was performed three times.
MDA content assay. The MDA content was measured using a Lipid Peroxidation MDA Assay kit (colorimetric method). Briefly, treated H9c2 cells were collected and re-suspended in 300 µl MDA lysis buffer on ice for 30 min. Following homogenization using a Dounce homogenizer (10-50 passes) on ice, the mixture was then centrifuged (13,000 x g for 10 min at 4˚C) to remove insoluble materials. A total of 200 µl sample was mixed with 600 µl thiobarbituric acid and incubated at 95˚C for 60 min, prior to cooling to room temperature. The intensity of the absorbance was measured at 532 nm. The MDA content was expressed as mmol/mg protein.

Measurement of SOD activity and GSH level.
To evaluate antioxidant enzymes including SOD activity and GSH levels, they were measured using commercial assay kits according to the manufacturer's protocols. H9c2 cells were seeded into a 6-well plate at a density of 2x10 5 /well and treated for 48 h. Following lysis in the extraction buffer on ice for 30 min, the mixture was centrifuged at 12,000 x g for 10 min at 4˚C and the supernatant was isolated. The protein concentration of the samples was determined using a BCA protein assay kit (Beyotime Institute of Biotechnology) according to the manufacturer's protocol. The SOD activity was measured at 550 nm based on the superoxide radicals generated by xanthine oxidase and hypoxanthine. The activity was expressed as U/mg protein. Intracellular GSH levels were monitored at 405 nm, via the produced enzyme-catalyzed reaction product (reduced glutathione). The GSH levels were expressed as µmol/g protein.
Western blot analysis. H9c2 cells were lysed in radioimmunoprecipitation assay lysis buffer (Beyotime Institute of  Biotechnology) supplemented with 1% (v/v) phenylmethylsulfonyl fluoride at 4˚C for 30 min. Following centrifugation at 12,000 x g for 10 min at 4˚C, the supernatant was collected and protein concentration was quantified using a BCA assay kit. Equal amounts of protein (50 µg) was subjected to 12% SDS-PAGE gels and then transferred to a polyvinylidene difluoride membrane. The membranes were blocked with 5% free-fat milk in tris-buffered saline with 1% (v/v) Tween-20 (TBS-T) for 2 h at room temperature, and subsequently incubated with primary antibodies at 4˚C overnight specific to p-STAT3 (cat no. 9145), STAT3 (cat no. 12640), HIF-1α (cat no. 20960-1-AP), NOX2 (cat no. 19013-1-AP), Bax (cat no. 14796), Bcl-2 (cat no. 4223) and GAPDH (cat no. 10494-1-AP). Each antibody was diluted to 1:2,000. Following overnight incubation, the membranes were washed three times with TBS-T for 10 min and incubated with HRP-conjugated secondary antibody (cat no. KC-RB-035; 1:5,000) for 2 h at room temperature. Following washing three times with TBS-T, the membranes were developed by using enhanced chemiluminescence (Beyotime Institute of Biotechnology) and imaged using X-ray film. The intensities of the protein bands were quantified by Bio-Rad Quantity One v4.62 software (Bio-Rad Laboratories, Inc.).
Statistical analysis. All data were expressed as the mean ± standard deviation from at least three independent experiments. Statistical comparisons were analyzed by one-way analysis of variance followed by Tukey's test with Graph Pad Prism 5 for Windows software (GraphPad Software, Inc., La Jolla, CA, USA). P<0.05 was considered to indicate a statistically significant difference.

GYY4137, an exogenous H 2 S donor, attenuates HG-induced cytotoxicity in H9c2 cardiomyocytes.
Initially, the present study examined the toxicity of HG on H9c2 cardiac cells. The CCK-8 results revealed that the cell viability was significantly decreased following 33 and 44 mM glucose treatment for 48 h when compared with the normal glucose group (5.5 mM; P<0.01; Fig. 1A), with a significant decrease in cell viability (P<0.01) observed in the 33 mM HG group. As viability was reduced to 50-60%, the present study selected HG (33 mM) treatment for 48 h as the model group for subsequent experiments as the conditions were suitable to mimic hyperglycemia in vivo. Then, in order to investigate the effects of exogenous H 2 S supplementation on HG-induced H9c2 cell injury, cells were pretreated with GYY4137 (50, 100 or 200 µM), an exogenous H 2 S donor, for 30 min prior to HG (33 mM) treatment for 48 h. The CCK-8 assay revealed that GYY4137 significantly increased cell viability when compared with HG treatment (P<0.05; Fig. 1B). In addition, the results of the LDH release assay revealed that GYY4137 pretreatment significantly reversed the HG-induced increase in LDH release in H9c2 cells (P<0.05; further experimentation Fig. 1C). At a concentration of 100 mM, GYY4137 achieved its most significant effect, and as such was selected for further studies. GYY4137 treatment alone produced no such effect. These results indicated that exogenous H 2 S prevents H9c2 cells against HG-induced H9c2 cell injury.
HG results in the activation of the STAT3/HIF-1α signaling pathway in H9c2 cardiomyocytes. The pivotal function of the STAT3 signaling pathway in the control of cardiac contractile function and cardiomyocyte survival is well established (25,33). In addition, various transcription factors including STAT3 are critical for regulating HIF-1α levels (34). Therefore, the present study investigated the effects of HG on the STAT3/HIF-1α signaling pathway in H9c2 cells. Western blot analysis  Fig. 2C). These results suggested that the activation of the STAT3/HIF-1α signaling pathway was induced by HG in H9c2 cells.

Inhibition of the STAT3/HIF-1α signaling pathway prevents HG injury in H9c2 cardiomyocytes.
To further confirm the function of the STAT3/HIF-1α signaling pathway in HG injury, H9c2 cells were transfected with STAT3 siRNA to knockdown the STAT3/HIF-1α signaling pathway. The RT-qPCR results revealed that STAT3 siRNA transfection successfully significantly reduced the levels of STAT3 mRNA in the presence (P<0.01) or absence (P<0.01) of HG treatment in H9c2 cells when compared with the scramble siRNA transfection group (Fig. 3A). In addition, STAT3 siRNA resulted in a significant decrease in the levels of HIF-1α mRNA when compared with the scramble siRNA transfection group (P<0.01; Fig. 3B), indicating that STAT3 siRNA results in the inhibition of the STAT3/HIF-1α signaling pathway. On this basis, the present study discovered that the cell viability in STAT3 siRNA transfection and HG co-treatment groups was significantly increased (P<0.05; Fig. 3C) while LDH release was decreased (P<0.01; Fig. 3D) compared with STAT3 scramble transfection and HG co-treatment. These results indicated that the STAT3/HIF-1α signaling pathway mediates HG-induced cytotoxicity in H9c2 cells.
NaHS and inhibition of the STAT3/HIF-1α signaling pathway attenuates HG-induced H9c2 cardiomyocyte apoptosis. Next, the present study further investigated the effects of NaHS and STAT3 knockdown on apoptosis in HG-treated H9c2 cells. Hoechst 33258 staining demonstrated that HG treatment resulted in typical apoptotic morphology in the cells with pyknosis, dense and dark staining, chromatin edge gathering and bright blue strong fluorescence, while NaHS and STAT3 siRNA improved these phenomena (Fig. 4A). Annexin V/PI double staining followed by a flow cytometry assay revealed that when compared with the blank group, the cell apoptotic rate in the HG-treated group was significantly increased (P<0.01; Fig. 4B and C). However, this effect of HG was significantly reversed by NaHS pretreatment or STAT3 siRNA transfection (P<0.01). In addition, NaHS and STAT3 siRNA mitigated HG-induced increases in caspase-3 activity (P<0.01; Fig. 4D) and Bax, apoptosis regulator expression levels (P<0.01; Fig. 4E), and the decreases in Bcl-2 apoptosis regulator expression levels (P<0.01; Fig. 4F). These results indicate that NaHS attenuates HG-induced cytotoxicity and apoptosis and that the STAT3/HIF-1α signaling pathway contributes to HG-induced apoptosis in H9c2 cells.
GYY4137 and inhibition of the STAT3/HIF-1α signaling pathway attenuate HG-induced oxidative stress in H9c2 cardiomyocytes. Increasing oxidative stress is associated with the development of DCM (5). In the present study, the results of DCFH-DA staining revealed that HG treatment for 48 h significantly increased ROS generation (P<0.01; Fig. 5A) and MDA content (P<0.01; Fig. 5B) compared with the control, while these effects were significantly blocked by GYY4137 treatment and STAT3 siRNA transfection (P<0.01). NOX2 is an enzyme that generates ROS as its primary function, serving an essential function in the development of cardiovascular disease (35). These results further revealed that GYY4137 also reversed the HG-induced increase in the expression of NOX2 (Fig. 5C). Similarly, the upregulation of NOX2 expression was also significantly attenuated by STAT3 siRNA transfection (P<0.05; Fig. 5D). In addition, HG treatment significantly decreased SOD activity (P<0.01; Fig. 5E) and GSH levels (P<0.01; Fig. 5F) compared with control cells in H9c2 cells, while these effects were blocked by GYY4137 and STAT3 siRNA. These results implied that GYY4137 and STAT3/HIF-1α pathway inhibition attenuated HG-induced oxidative stress in H9c2 cells.
GYY4137 mitigates HG-induced STAT3/HIF-1α signaling pathway activation in H9c2 cardiomyocytes. To further demonstrate whether the STAT3/HIF-1α signaling pathway is involved in the protective mechanism of H 2 S-induced protection against HG-induced H9c2 cell injury, the effects of GYY4137 on this pathway in the presence or absence of HG were measured. The results from western blot analyses (Fig. 6A) revealed that GYY4137 pretreatment significantly decreased the expression levels of p-STAT3 (P<0.01; Fig. 6B) and HIF-1α (P<0.01; Fig. 6C) when compared with the HG treatment group of H9c2 cells. GYY4137 treatment alone had no effect on this pathway. These results indicate that H 2 S may alleviate the HG-induced activation of the STAT3/HIF-1α signaling pathway, resulting in cardioprotection against HG-induced H9c2 cell injury.

Discussion
DCM is a critical complication of diabetes (1). A comprehensive understanding of the mechanisms underlying the pathogenesis of DCM and the identification of effective intervention drugs are urgently required. In present study, the function of the STAT3/HIF-1α signaling pathway was demonstrated in HG-induced H9c2 cardiac injury, and the cardioprotection of H 2 S on HG injury was investigated, in addition to the underlying mechanism associated with STAT3/HIF-1α pathway inhibition. The present results provide insight into a novel mechanism of H 2 S therapeutic use in the treatment of DCM.
H 2 S, an endogenously-generated gas, elicits cardioprotection in various injury models (36,37). Previously, a substantial amount of attention has been focused on investigating whether exogenous H 2 S protects cardiac cells from diabetes-induced injury. Research has confirmed that H 2 S serves a cytoprotective function in the pathophysiological processes of DCM (14,16,38). Once produced, H 2 S is rapidly reduced and causes a switch amongst cell death pathways during hyperglycaemia (17). A number of studies have reported that exogenous H 2 S prevents HG-induced cytotoxicity in cardiac cells (39,40). Similarly, the present study revealed that GYY4137, a recognized exogenous H 2 S donor, reversed the HG-induced downregulation of cell viability and the upregulation of LDH release in H9c2 cells. These results are indicative of the protective effect of exogenous H 2 S against HG-induced cardiac injury.
STAT3, an important member of the STAT family of proteins, is activated through its phosphorylation in response to cytokines and growth factors, functioning as a transcription activator to modulate numerous cellular processes including cell growth and apoptosis (19). Notably, previous studies have also associated STAT3 with normal and myocardial damage in complications of diabetes (26,41,42). Previous studies have reported that STAT3 may also be an important mediator of the cardiac survival pathway, and that the STAT3 signaling pathway may participate in various cardiac physiological or pathological processes, including DCM (24,43). Previous studies have revealed that the activation of STAT3 was increased in HG-cultured cardiomyocytes and the hearts of streptozotocin-treated rats (24,25). In the present study, it was revealed that HG treatment increased the expression of p-STAT3, which is indicative of the activation of STAT3 signaling. Notably, a study by Papadakis et al (44) provided specific genetic evidence supporting the notion that STAT3 phosphorylation may upregulate the transcription of the HIF-1α gene, which is a key molecule in the regulation of hypoxia and tumor glycometabolism (45). In the present study, the results revealed that HG also markedly upregulated HIF-1α expression. In addition, the present study demonstrated that the inhibition of STAT3 induced by STAT3 siRNA resulted in the downregulation of the STAT3/HIF-1α pathway in HG-treated H9c2 cells, thereby mitigating HG-induced H9c2 cell injury and apoptosis, which was consistent with numerous other studies in which baseline phosphorylation and/or the activation of STAT3 levels also increased in certain in vitro studies, including in H9c2 cells subjected to a high glucose conditions (46) and the inhibition of the STAT3 signaling pathway attenuating cardiac injury in DCM (25,26). However, in contrast to the aforementioned research and the present results, a number of publications revealed a substantial decrease in cardiac STAT3 phosphorylation or activation in various experimental models of diabetes (23,24,47). The reasons for these controversies remain unclear and may contain substantial differences in the method of induction, severity, type and duration of diabetes in addition to differences in the method of STAT3 phosphorylation and expression detection. Altogether, these results indicated that the STAT3/HIF-1α signaling pathway contributes to the development of DCM.
It is becoming increasingly apparent that increases in ROS and oxidative stress levels are necessary in the pathogenesis of DCM (48,49). In addition, a unifying molecular mechanism of hyperglycemia-induced myocardial cellular damage was proposed, linking elevated glucose levels with oxidative stress (50). However, therapeutic strategies to alleviate oxidative stress in clinical trials have not proved efficacious. Multiple signaling pathways containing the transcription factors nuclear factor-κB, STAT3, HIF-1α, cytokines and other proteins, in addition to enzymes which are involved in modulation of ROS generation, have been associated with proliferation, differentiation, survival, apoptosis, oxidative stress and metabolism (19,51). Until now, the effect of the STAT3/HIF-1α signaling pathway on oxidative stress in DCM was unclear. In the present study, the inhibition of the STAT3/HIF-1α pathway reduced ROS generation, MDA content and NOX2 expression, and increased SOD activity and GSH level, attenuating oxidative stress and promoting the antioxidant defense system. On the other hand, previous studies have suggested that exogenous H 2 S alleviates the development of DCM by inhibiting oxidative stress (52,53). Consistent with these observations, the present study also revealed that GYY4137 pretreatment eliminates HG-induced oxidative stress, which was a similar effect to that of inhibition of the STAT3/HIF-1α pathway.
A number of cardioprotective strategies and agents that activate the STAT3 pathway may successfully rescue injured cardiomyocytes, including cardiotrophin-1, opioids, insulin, leptin, resveratrol and erythropoietin (54)(55)(56). At present, an accumulating body of evidence has indicated that there is a connection between H 2 S and the STAT3 pathway and revealed that suppressing the activation of the STAT3 pathway participates in H 2 S-confered beneficial effects in a variety of disease types (12,57,58). Exogenous H 2 S contributes to cardioprotection by decreasing ROS levels via downregulation of the JAK2-STAT3 pathway in the aging cardiomyocytes (12). However, the effect of H 2 S on HIF-1α has not yet been reported. Consistent with these observations, the present study also proved that GYY4137 pretreatment eliminated the HG-induced activation of STAT3 and the upregulation of HIF-1α expression, which are indicative of the inhibition of the STAT3/HIF-1α pathway induced by H 2 S under HG conditions in cardiomyocytes. These results indicated that the STAT3/HIF-1α pathway inhibition contributes to the cardioprotection provided by H 2 S in DCM.
However, it must be acknowledged that there are limitations in the present study. Firstly, the present did not directly investigate the effects of overexpression or inhibition of the STAT3/HIF-1α pathway on the protection of GYY4137 on HG-induced H9c2 cells injury, which should be investigated in the future; secondly, the details of how GYY4137 attenuated the STAT3/HIF-1α pathway requires further study; finally, further studies are required to examine the association between H 2 S-induced myocardial protection and the STAT3/HIF-1α pathway in in vitro experiments.
In conclusion, the results of the present study demonstrate that exogenous H 2 S exerts cardioprotection against HG-induced cardiac cell apoptosis and oxidative stress via suppressing STAT3/HIF-1α signaling pathway activation. Therefore, a better understanding of the molecular mechanisms underlying H 2 S action in heart disease may be helpful to attenuate the risks of DCM disease in the future. It is noteworthy that H 2 S therapy has only entered a preliminary stage, whether in basic medical research or preclinical research, due to the difficulties in obtaining and maintaining constant concentrations, in addition to the potentially toxic effects of H 2 S in excess, and detailed H 2 S release profiles and byproducts under real biological systems are still unclear for numerous H 2 S donors (10). Hence, developing a suitable donor and using that donor for providing precise and sustained release of H 2 S may possess the potential to be developed as a therapeutic method to prevent DCM injury.