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.