In the present study, K2Cr2O7-induced toxicity in cardiovascular disease was used to further investigate the mechanisms for the cardio-protective effects of apelin-13 in H9c2 cardiomyocyte cell cultures and adult male SD rats. Results show that apelin-13 may attenuate K2Cr2O7-induced toxicity in cardiac tissue by inhibiting ROS-mediated DNA damage and regulating ROS initiated multiple signaling pathways, such as MAPKs and PI3K/AKT.
K2Cr2O7-induced toxicity in H9c2 cells is a common cell model of Cr (VI) exposure to evaluate the cardio-protective profiles of bioactive reagents [6]. Apelin-13 can regulate the cardiovascular system [7]. Hence, the potential of apelin-13 was tested for its myocardiaprotective effect against toxicity induced by the toxins K2Cr2O7 in H9c2 cells. We assessed K2Cr2O7-induced cardiotoxicity through CCK-8 and confirmed that K2Cr2O7 significantly decreased cell viability. However, treatment with different concentrations of apelin-13 markedly attenuated the decrease in cell viability. H9c2 displays cell shrinkage and nuclear condensation under K2Cr2O7 stimuli, and these apoptotic features were detected by phase contract imaging. Interestingly, in the present study, apelin-13 treatment completely inhibited these apoptotic features. The results indicate that apelin-13 may inhibit K2Cr2O7-induced apoptosis.
Oxidative stress is as an essential incentive of K2Cr2O7-induced cardiac injury [8]. DNA damage activates Cleaved-PARP and induces and accelerates the apoptosis of myocardial cells [9]. In response to the caspase stimulus, caspase-9, caspase-3, and caspase-7 have distinct roles during intrinsic myocardial cells apoptosis [10]. “Initiator” caspase-9 is required for ROS production and can activate “effector” caspase-3 and caspase-7, while “effector” caspase-3, as the dominant executioner caspase, results in efficient apoptosis execution, and “effector” caspase-7 is responsible for cell detachment by cleaving various actin and cytoskeleton components during intrinsic myocardial cell death [11]. Thus, cleaved-PARP, caspase-9, caspase-3, and caspase-7 form a feedback loop to regulate upstream and downstream functions [12]. In these situations, apoptotic stimulation results in DNA damage [13]. Results show that apelin-13 treatment dramatically reversed K2Cr2O7-induced DNA damage by inhibiting the expression levels of Cleaved-PARP and caspase activation.
DNA damage and myocardial cell apoptosis stimulate ROS overproduction [14]. Free radicals can cause tissue damage and worsen the Cr (VI) exposure of myocardial cells [15]. ROS induced by K2Cr2O7 impairs Cleaved-PARP and caspases, resulting in the collapse of superoxide anion, thereby leading to ATR, which constitutes a significant factor for the activation of p53; subsequently, H2A is activated, thus inducing DNA damage to inhibit cell viability and leading to myocardial cell apoptosis [16]. According to the results of the present study, apelin-13 ameliorated the deleterious effects of ROS by inhibiting the expression levels of ATR, p53, and H2A in H9c2 cells treated with K2Cr2O7.
Further investigation indicated that apelin-13 remarkably suppressed K2Cr2O7-induced cardiotoxicity and blocked or activated the downstream signaling pathways. ROS overproduction activates the MAPK signaling pathway, and both pathways modulate cardiac pathophysiological signal transduction [17]. An increasing number of studies has proposed that p38 MAPK as a key target involve in cell biological response to an external stimulus, such as the activation of the p53 apoptotic pathway [18]. The JNK MAPK pathway contributes to functional activation of ATR in response to oxidative stress, resulting in the lack of p53 [19]. Similar to p38 and JNK MAPK, ERK MAPK plays a pivotal role in various cellular responses to ROS overproduction and DNA damage and regulates the activity of H2A to conduct cell apoptosis [20]. The PI3K/Akt signaling pathway participates in vascular homeostasis and angiogenesis, can block the fibrotic area, and protect cardiomyocyte area; subsequently, eNOS phosphorylation occurs, leading to the regulation of myocardial inflammatory responses [21]. The PI3K/Akt signaling pathway plays an important role in the Cr (VI) exposure of heart, in which the elimination of p-Akt protein promotes cell death. The PI3K/Akt pathway plays a crucial role in the regulation of energy homeostasis, cell growth, and apoptosis, especially in cardioprotection [22]. In the present study, K2Cr2O7 treatment causes the phosphorylation of JNK, p38, and ERK but inhibits the phosphorylation of AKT. Apelin-13 treatment effectively could K2Cr2O7-induced dysfunction of MAPKs and PI3K/Akt signaling pathways. The results suggest that apelin-13 prevents K2Cr2O7-induced toxicity via the inactivation of the MAPK and the activation of the PI3K/Akt pathways.
Further studies are required to evaluate the functional effect of apelin-13 against K2Cr2O7 in vivo and to further test its cardio-protective potential. SD rats were used to explore the protective role of apelin-13 against the cardiotoxicity of K2Cr2O7 exposure and reveal the potential molecular mechanism. In the present study, apelin-13 inhibited inflammation, further ameliorating heart damage, and ultimately protecting the heart from Cr (VI) poisoning.
Apoptotic heart tissue can form fibrotic area and destroy the cardiomyocyte area, which further promote heart tissue inflammatory response. Our studies have confirmed that apelin-13 treatment obviously attenuated K2Cr2O7-induced fibrotic area and cardiomyocyte hypertrophy area.
Cardiac inflammation is important in the pathogenesis of Cr (VI) exposure [23]. eNOS plays a major role in the toxicity of heart tissue [24]. The results demonstrated that apelin-13 treatment counteracted the K2Cr2O7-induced decrease in eNOS expression. Treatment with apelin-13 could substantially inhibit inflammatory cells. Our data show that apelin-13 alleviated active caspase-3 and Ki67 and increased microvessel density induced by K2Cr2O7. These results confirm the protection of apelin-13 against K2Cr2O7 exposure and provide a new perspective to understand the mechanism of apelin-13 to protect against heart injury.
Subsequently, the effect of apelin-13 toward the heart, liver, spleen, lung, kidney, and brain in the absence of other stimuli were examined. Importantly, apelin-13 alone do not cause these tissues injury.
In conclusion, results show that apelin-13 exerts its cardioprotective effect on Cr (VI)-induced death in cardiac tissue through direct scavenging of intracellular ROS induced by K2Cr2O7 and inhibiting ROS-mediated DNA damage and downstream signaling pathways MAPK activation and PI3K/Akt inhibition (Fig. 8). Our findings validated the strategy of the use of apelin-13 is a potential novel pharmaceutical target for Cr (VI)-induced cardiovascular disease.