Mitochondria are the main sites for aerobic respiration and are also primary targets for ischemic injury. Mitochondrial dysfunction plays a crucial role in MIRI (24,25). During ischemia, decreased oxygen levels impair mitochondrial ATP production and induce an increase in intracellular Ca²⁺. During reperfusion, the concentration of intracellular Ca²⁺ increases further, resulting in a calcium overload in the cytoplasm and mitochondria. Simultaneously, hypoxia damages the mitochondrial electron transport chain (ETC), resulting in increased reactive oxygen species (ROS) production. The increases in ROS and Ca²⁺ levels lead to the opening of non-selective, highly conductive permeability transition pores (PTPs) in the mitochondrial inner membrane, as well as changes in mitochondrial membrane permeability (26). PTP opening further increases mitochondrial Ca²⁺ and ROS levels and stimulates the oxidation of proteins and lipids in mitochondria (27). Calcium overload and oxidative stress may cause mitochondrial dysfunction, which in turn induces cardiomyocyte apoptosis or necrosis. On the one hand, the decrease in the intracellular oxygen concentration can activate HIF-1α by inhibiting PHD proteins during ischemia. On the other hand, HIF-1α can regulate the expression levels of mitochondria-specific genes to adapt to hypoxic stress and improve mitochondrial function (28). Nanayakkara et al (29) suggested that HIF-1α could transcriptionally regulate frataxin expression levels in response to hypoxia and acted as a cardioprotective factor against ischemic injury. Increased levels of frataxin can mitigate mitochondrial iron overload and subsequent ROS production, thereby preserving mitochondrial membrane integrity and the viability of cardiomyocytes. Thus, HIF-1α preserves the integrity of the mitochondrial membrane, promotes cell survival and protects against MIRI. Moreover, HIF-1α can also improve mitochondrial respiratory function by activating different cardioprotective signaling pathways, such as the PI3K/AKT and Janus kinase 2/STAT3 pathways, to protect the heart following IRI (30). Additionally, some studies have suggested that mitochondria can regulate HIF-1α stability and that ROS produced by the mitochondrial ETC can stabilize HIF-1α under hypoxic conditions (16). Mitochondria-derived ROS have multiple, opposing effects. They can impair mitochondrial function and promote cardiomyocyte death or stabilize HIF-1α to improve mitochondrial function (31). However, this finding is controversial and may be associated with oxygen levels, as oxygen concentration affects both mitochondrial function and HIF-1α stability and activity. The determination of the oxygen levels required to optimize mutual beneficial effects of mitochondria and HIF-1α may provide a novel direction for the treatment of MIRI.

ROS are generally considered to be toxic by-products of aerobic metabolism and are the primary cause of macromolecular destruction (32,33). However, it has been demonstrated that ROS also contribute substantially to numerous physiological and pathological conditions (34). In MIRI, ROS generation begins during ischemia, and large amounts of ROS are produced during reperfusion. Excessive ROS accumulation in cells is one of the primary causes of MIRI. This deleterious effect is mediated by the oxidative modification of proteins, lipids and histone-free mitochondrial DNA (35). Therefore, the elimination of excessive ROS in cells and the alleviation of oxidative stress in cardiomyocytes can promote myocardial cell survival and decrease the severity of MIRI (36,37).

The mitochondrial ETC is an important source of ROS and may contribute to MIRI (38). In ischemia, an abnormal increase in ROS causes the accumulation of succinate, resulting in decreased ETC complex activity. Following reperfusion, succinate is rapidly oxidized to generate large amounts of ROS. A notable increase in ROS and IR-induced Ca²⁺ influx leads to mitochondrial PTP opening, which decreases ETC activity and further increases the formation of ROS. In addition, the NADPH oxidase (Nox) family produces large amounts of ROS. The Nox2 and Nox4 isoforms are major components of the Nox family that produce reactive oxidants in the heart, leading to MIRI. Decreased ROS production by mitochondria and Nox can attenuate the severity of MIRI, whereas low levels of ROS can also modulate HIF-1α expression levels (32,39).

HIF-1α contributes to MIRI through numerous mechanisms. It can activate the HIF pathway, thereby activating target genes involved in the regulation of the redox state of cells as well as decreasing ROS production and apoptosis in the cardiomyocytes of patients with IRI (6). Additionally, HIF-1α stabilizes mitochondrial function and promotes the production of mitochondrial antioxidants in cells. For instance, HIF-1α can enhance the antioxidant capacity of cells through the antioxidant transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and by upregulating the synthesis of the antioxidant tripeptide glutathione and superoxide dismutase 2 (32,40,41). Furthermore, HIF-1α mediates a shift from oxidative metbolism to glycolysis (thus decreasing the production of mitochondrial oxidants), decreases ETC activity and attenuates mitochondrial ROS production, thereby avoiding cell death (42). HIF-1α mediates ROS production by mitochondria and the Nox family and regulates the redox state of cells, which decrease the severity of MIRI (36).

A number of studies support the beneficial effects of HIF-1α on the maintenance of cellular redox balance (43–45). However, this view is also somewhat controversial. Tang et al (46) demonstrated that during MIRI, the polyol pathway increases the cytosolic NADH/NAD⁺ ratio, resulting in HIF-1α activation and transferrin receptor upregulation, which exacerbates oxidative damage and increases lipid peroxidation. However, the effect of HIF-1α on oxidative stress was not evaluated separately in this previous study. Further investigation is needed to determine the dynamic regulatory effects of HIF-1α on different types of redox indicators at different time points in MIRI.

Since the first reported target gene of HIF-1α (erythropoietin), hundreds of downstream targets have been identified, demonstrating the complexity and importance of the HIF-1α signaling pathway (15). In MIRI, HIF-1α can regulate and participate in a number of signaling pathways that protect the heart (47). MIRI initially leads to hypoxia, resulting in AKT phosphorylation and activation of HIF-1α and numerous protective genes (48). Dong et al (49) demonstrated that sevoflurane pretreatment can increase the expression levels of VEGF by activating the AKT/HIF-1α/VEGF signaling pathway. VEGF is closely associated with angiogenesis. In MIRI, increased angiogenesis can effectively improve hypoxia in lesions, thereby protecting the heart (50). iNOS acts downstream of HIF-1α and has a cardioprotective effect in MIRI (51,52). In addition, heme oxygenase-1 (HO-1), adiponectin, insulin-like growth factor-2, GLUT and other loci are involved in the protective effect of HIF-1α against MIRI (18).

In MIRI, HIF-1α can also act by directly or indirectly regulating numerous signaling pathways (42,49). In terms of mitochondria, HIF-1α can directly or indirectly influence mitochondrial function, decrease mitochondrial damage and attenuate the severity of MIRI (53). HIF-1α can induce myocardial mitochondrial autophagy via the HIF-1α/Bcl2 and adenovirus E1B 19-kDa-interacting protein 3 (BNIP3) signaling pathway, thereby promoting myocardial cell survival following MIRI. However, this is limited to HIF-1α-mediated mitochondrial autophagy in the early stage of IR, which may lead to protective responses, whereas prolonged autophagy may promote cardiomyocyte death. In terms of oxidative stress, HIF-1α also plays a significant role. For instance, HIF-1α can upregulate Nrf2, which then activates antioxidant enzymes to protect cells by enhancing intrinsic ROS clearance (23,53). In terms of inflammation, the phosphorylation of IκBα during hypoxia results in the degradation of IκBα and activation of NF-κB (48). However, HIF-1α activation can inhibit the NF-κB pathway and induce HO-1, thereby attenuating the production of pro-inflammatory cytokines, inhibiting tissue inflammation and decreasing the severity of MIRI. In addition, HIF-1α can regulate numerous signaling pathways, such as GSK3β/mitochondrial PTP, β-catenin, ERK1/2, Bcl-2, PI3K/AKT and mTOR, which are involved in the regulation of broad-spectrum cell functions, and thus can decrease the severity of MIRI (54–57).

MicroRNAs (miRNAs or miRs) are small non-coding RNA molecules ~22 nucleotides in length. They primarily bind to the 3′ untranslated region of mRNAs to control stability and translation, thereby decreasing protein levels. More than 30% of genes in the human genome are regulated by miRNAs. A number of studies have reported that miRNAs serve a vital role in cardiovascular disease (58). Sheng et al (59) suggested that the overexpression of miR-7b inhibits IR-induced apoptosis in H9C2 cells by targeting the HIF-1α/phosphorylated-P38 pathway. In vivo experiments have demonstrated that overexpression of miR-335 enhances the transcriptional activity of HIF-1α, increases the expression levels of HO-1 and iNOS and inhibits the opening of mitochondrial PTP, thereby decreasing myocardial infarct size and myocardial apoptosis as well as improving MIRI (41). Liu et al (60) demonstrated that sirtuin 1 (SIRT1) also acts as a transcriptional repressor to suppress the expression levels of miR-138 in adult sensory neurons in response to peripheral nerve injury. Therefore, miR-138 and SIRT1 can form a mutual negative feedback regulatory loop, which provides a novel mechanism for controlling intrinsic axon regeneration. The overexpression of miRNA-138 can decrease apoptosis following myocardial IR by decreasing the expression levels of HIF-1α. This may be explained by the difference between early and prolonged hypoxia (60). During early hypoxia, changes in miRNA levels may contribute to the accumulation of HIF-1α and maintain the steady-state levels of HIF-2 and HIF-3 (61). In addition, HIF-1α can act as a modulator of miRNA function. Wu et al (62) demonstrated that the accumulation of HIF-1α following myocardial infarction leads to a decrease in miRNA-10b-5p, which mediates the apoptosis of cardiomyocytes. Although the number of known miRNAs associated with HIF-1α is increasing, the majority of studies have concentrated on cancer cell lines, and relatively little is known about their role in cardiovascular disease, particularly in MIRI (63,64).

Long non-coding RNAs (lncRNAs) are >200 nucleotides in length and have no protein-coding properties; they mediate numerous biological processes, such as cell proliferation, cell differentiation and apoptosis (65). lncRNAs negatively or positively regulate the expression levels of protein-coding genes by numerous modes of action. For example, one type of lncRNA, referred to as competitive endogenous RNAs (ceRNAs) (66) decrease the availability of functional miRNAs by acting as a complementary sequence for miRNA binding. lncRNAs are closely associated with a number of biological functions and pathological processes (e.g. cardiovascular diseases) (67). There is increasing evidence that lncRNAs serve a significant role in the regulation of the myocardial IR process (68). For example, Ren et al (69) investigated the effect of lncRNA nuclear enriched abundant transcript 1 (lnc-NEAT1) on cell proliferation and apoptosis in MIRI and observed that lnc-NEAT1 is overexpressed in MIRI compared with levels in normal cardiomyocytes. Downregulation of lnc-NEAT1 enhances cell proliferation and inhibits cell apoptosis by targeting miR-193a in IR injury H9C2 cells (69). Li et al (70) assessed the role of lncRNA H19 in the regulation of MIRI and suggested that H19 expression levels are downregulated in IR hearts of mice and cardiomyocytes treated with H₂O₂. They also demonstrated that H19 functions as a ceRNA, decreasing the expression levels of miR-877-3p via the aforementioned base-pairing mechanism. However, the association between HIF-1α and lncRNAs has not yet been fully elucidated. Yang et al (71) assessed the association between HIF-1α and hypoxia-responsive lncRNA-p21 and demonstrated that lncRNA-p21 is essential for enhancing cell glycolysis under hypoxic conditions. VHL-mediated HIF-1α ubiquitination is attenuated and leads to the accumulation of HIF-1α, promoting glycolysis under hypoxic conditions (72). Xue and Luo (73) investigated the mechanism by which lncRNA HIF-1α-antisense RNA 1 (AS1) regulates cytokine signaling inhibitor 2 (SOCS2) via miR-204 in MIRI ventricular remodeling: The silencing of HIF1α-AS1 and upregulation of miR-204 inhibited apoptosis, and lncRNA HIF1A-AS1 served as a ceRNA to adsorb miR-204, thereby inhibiting miR-204 and increasing SOCS2 expression levels. The downregulation of HIF1A-AS1 and upregulation of miR-204 can attenuate ventricular remodeling and improve cardiac function in mice following MIRI by regulating SOCS2. There is extensive evidence that interactions between HIF-1α and lncRNA play pivotal roles in many diseases (73); however, further research on their involvement in MIRI is needed. In-depth studies of the specific mechanisms of action may provide a new direction for the treatment of MIRI.