ROS and NF B are closely related to
ROS and NF-κB are closely related to cardiac cell apoptosis, hypertrophy, and fibrosis in hyperglycemia , . As one of the hallmarks of DCM, apoptosis induced by HG has been considered to be closely interrelated with enhanced oxidative stress, endoplasmic reticulum (ER) stress, inflammation and mitochondrial injury , . Recently, the emerging role of mitochondria in regulating myocardial survival and death has gained much attention . Previous studies have reported that mitochondria play a key role in the development of DCM . Our results showed that HG/diabetes induced an obvious loss of MMP, cytochrome c release, caspase-9/3 activation and subsequent apoptosis (Fig. 3, Fig. 7). Klotho treatment significantly reversed all these abnormalities, likely through its antioxidative and anti-inflammatory properties. Cardiac hypertrophy and fibrosis are also two key factors that participate in the progression of DCM. Consistent with previous reports , we observed a marked increase in the expression of several pro-fibrotic genes in HG-stimulated cardiac MF498 and diabetic hearts (Figs. 4E and 6B). However, Klotho treatment alleviated these pathological changes, indicating the anti-fibrotic effect of Klotho in DCM. Inflammatory responses and oxidative stress participate in the pathogenesis of cardiac hypertrophy in DCM. The pivotal molecular and phenotypic markers for cardiac hypertrophy are elevated expression of pro-hypertrophic proteins such as ANP, BNP and β-MyHC and increased cell surface area. Interestingly, a recent report revealed that Klotho could counteract indoxyl sulfate-induced myocardial hypertrophy . In the current study, we found that Klotho was able to attenuate the cardiac hypertrophic phenotype caused by HG/diabetes (Figs. 4A–B and 6D–E and S5A).
Recently, a potential link between Klotho and insulin homeostasis has been described but still remains controversial , . In our study, we found Klotho treatment attenuated cardiac pathological changes and dysfunction by inactivating ROS and NF-kB-mediated inflammation without affecting blood glucose in the diabetic mice (Fig. S6A). As we all know, insulin is essential in the regulation of blood glucose levels. These findings suggested that Klotho may exert cardioprotective role via its antioxidative and anti-inflammatory properties instead of regulating insulin and blood glucose metabolism , , , .
Oxidative stress and inflammatory responses interact and engage in cross-talk throughout the development of DCM . In the current study, we found that targeting Nrf2 and NF-κB may be a potential therapeutic approach to treat DCM. However, our study has several limitations: (1) although some information regarding signaling pathways activated by Klotho has been widely reported, no specific receptor for soluble Klotho has yet been identified. However, soluble Klotho has been shown to be capable of modulating the PI3K/Akt and Wnt/β-catenin pathways , both of which are involved in oxidative stress and inflammation. In current study, Nrf2 and NF-κB pathways are merely associated with the pathological and functional changes. Further studies are needed to elucidate the underlying mechanism by which Klotho regulates the Nrf2 pathway and NF-κB-mediated inflammatory responses; (2) as we all know, hyperlipidemia (palmitate) can also induce diabetic cardiac damage , , . It is also important to investigate how Klotho protects the heart from hyperlipidemia (palmitate)-induced cardiac injury. In the future, we will conduct further in-depth studies to fully clarify the mechanisms underlying the effect of Klotho on hyperlipidemia, especially palmitate-induced cardiac injury.
Conclusions In summary, to our knowledge, our study demonstrated for the first time that Klotho protects against inflammatory responses, oxidative stress, mitochondrial damage, fibrosis, hypertrophy and apoptosis in DCM (Fig. 7C), eventually improving cardiac function (Table 1). The cardioprotective role of Klotho is closely related to its ability to inhibit HG-stimulated oxidative stress and inflammatory responses by augmenting the Nrf2 pathway and suppressing the NF-κB pathway (Fig. 7C). Our observations strongly suggest that Klotho may be a potential therapeutic agent to treat DCM. Moreover, our data provide a deeper understanding of the regulatory roles of NF-κB and Nrf2 in diabetic heart injury, showing that targeting them may be a potential therapeutic approach to treat diabetic complications.