Discussion
The present study investigated the therapeutic effects of CAG , the major metabolite of AST , for treating type 2 diabetes, and its underlying mechanism for the first time. AST is a glucoside isolated from traditional Chinese herb Astragalus Membranaceus (fisch) bunge , which is one of the most widely used traditional Chinese medicine for diabetes therapy (Wang et al., 2015). Till today, AST shows broadly pharmacological effects, such as reducing the symptoms of metabolic syndrome (Yue et al., 2017; Zhang et al., 2011), preventing the cardiovascular pathological changes and protecting against cardiovascular injury (Xu et al., 2006) and improving renal function (Lu et al., 2015), or reducing the progression of diabetic peripheral neuropathy (Yu et al., 2006). Despite these findings, the molecular mechanism by which AST fights diabetes remains elusive. In the present study, we tried to study the anti-diabetes mechanism of AST from another perspective, that is, to study the anti-diabetes effect and possible pathways of CAG , which is the aglycone of AST , as well as the major metabolic form ofAST in vivo . The study of CAG not only helps to explain the antidiabetic action of AST in vivo but also helps to discover new active molecules of non-glycosides structure, which is undoubtedly meant for the research in this field. In fact, CAGshowed an improvement of pharmacokinetic profile over AST(Zhou et al., 2012), but whetherCAG had a promising anti-diabetes effect is not reported.
We examined the effect of CAG in the diabetic ZDF rat, which is a well-recognized model with characteristics towards diabetes as a manifestation of accompanied metabolic syndromes (MetS) in humans. Our results showed that CAG efficiently decreased plasma glucose, improved glucose tolerance, heart and kidney fibrosis, and coronary artery sclerosis. To be noted, the anti-diabetic effects of CAG shared a similar manner with Dapagliflozin rather than metformin, especially the manner in glucose uptake. Together with our further finding that CAG could down-regulated SGLT2’s expression, CAG displays its anti-diabetes effects, at least in part, through the SGLT2 protein. These therapeutic effects suggest that CAG may be a considerable potential anti-diabetes candidate for further development.
SGLT2 is a novel plasma glucose regulator that responds for glucose (re)absorption by regulating glucose reabsorption in the kidney. SGLT2 is tissue-specific expressed in the kidney, and its expression level is inducible in response to the change of plasma glucose concentration (Chonlaket et al., 2018). Therefore, SGLT2 inhibitors are a new class of antidiabetic drugs with an insulin-independent mechanism. In particular, SGLT2 inhibitors reduce hospitalization for heart failure or cardiovascular mortality in patients with type 2 diabetes (Reid et al., 2020). We observed a reduction in the thickness of the artery wall in the mice-treated with CAG, together with a decrease of the expression of SGLT2 in cells and tissues. Moreover, CAG increased the urinary glucose, urinary sodium but decreased the urinary efflux volume, which are precisely the characteristics of SGLT2 inhibitors.
As talking above, there are lots of severe complications related to T2DM arising from chronic hyperglycemia. The majority of individuals with diabetes mainly die from cardiovascular disease. Recently, numerous trials have been carried out to ascertain the benefits of intensive glucose lowering on cardiovascular outcomes diabetes, such as using the SGLT2 inhibitors or combination therapy (Birkeland et al., 2017). The mechanism for SGLT2 inhibitor to reduce cardiovascular failure in T2DM patients includes reducing the oxygen consumption, improving the energy metabolism of the heart, and inhibiting the fibrosis of cardiomyocytes (Lam et al., 2016). Extensive evidence shows that there exists fibrosis in T2DM patients, including myocardial fibrosis (Guido et al., 2017) and kidney fibrosis (Tang et al., 2017). Diabetes-associated fibrosis is mainly mediated by activated fibroblasts, but may also involve fibrogenic actions of macrophages, fibroblasts, and vascular cells (Monami et al., 2017). To our delight, CAG showed an excellent effect in reversing myocardial fibrosis and renal fibrosis, although it was not as significant as Dapagliflozin in lowering blood glucose. To be specific, we found CAG significantly decreased the fiber contents in the tissues of the heart and kidney, accompanied by a significant reduction in the expression of TGF-β, which is the primary transcription activator involved in the regulation of fibrosis.
In conclusion, the present study identified the treating effects ofCAG on both metabolic factors and diabetes-related comprehensions. These data showed that CAG treatment significantly reduced plasma glucose and improved hyperinsulinemia in ZDF diabetic rats. CAG prevents cardio and renal dysfunction by inhibiting the accumulation of fibrosis in the heart and kidney of ZDF rats. Be noted, these treating effects of CAG were in line with an inhibition of plasma reabsorption in kidneys with SGLT2 as a key effector (Figure 8). Also, this study further provided new clues regarding the role of the SGLT2 inhibitor Dapagliflozin in treating diabetes and its related complications, such as cardiovascular disease and kidney dysfunction. Overall, these findings may provide a new insight into the novel application of CAG in the prevention of diabetes as well as the related metabolic comprehensions.