Dihydromyricetin Improves Skeletal Muscle Insulin Resistance By Inducing Autophagy Via AMPK Signaling Pathway

Diabetes was considered to be the third-leading health threat to human besides cardiovascular diseases and cancer worldwide. Recently eports reveal that the overall prevalence of diabetes was estimated to be 11.6% in the Chinese adult population, which makes China become the first country having the most patients of diabetes in word. Clinical statistics show that more than 90% of diabetics are type 2 diabetes, and insulin resistance, which is an initial factor of the type 2 diabetes occurs and also the key factor of its progression. The most commonly used insulin-sensitizing drugs on type 2 diabetes mellitus are rosiglitazone and other thiazolidinedions(TZDs), but long-term use of TZDs can cause serious side effects such as fat storage and sodium retention, etc, which greatly limits its clinical use. Recent studies have found that a variety of flavonoid phytochemicals can significantly improved insulin resistance. Flavonoids are widespread in plant foods such as fruits and vegetables, and have minimal side effects compared with chemical drugs, with high security, therefore, it has recently been identified as potential candidates in the prevention and treatment of type 2 diabetes.Dihydromyricetin(DHM) is a kind of flavonoid. It mainly exists in many edible and medicinal plants such as hovenia dulcis thunb, Syzygium cumini, especially in Ampelopsis, which almost have up to 20%(dry weight) in the leaves. DHM has been shown to have a lot of biological effects of anti-oxidation, anti-tumor and hypolepidemic capabilities. A preliminary study found that DHM has a good hypoglycemic effect in diabetic animal, however the mechanism of the action is still unclear. Therefore, further study on the improvement of insulin resistance by DHM has a high potential application in preventing and treating Type 2 Diabetes Mellitus, and also provide an experimental basis for the development of natural hypoglycemic drugs.Autophagy is a highly conserved process widely present in eukaryotic cell, which is involved in the turnover of long-lived proteins, cytosolic components, or damaged organelles that is essential for maintaining cellular homeostasis. Oxidative stress, endoplasmic reticulum stress and inflammation are closely related to insulin resistance and leading cell dysfunction and the accumulation ofdamaged components. Autophagy plays an important role in the clear of these damaged organelles. Moreover, autophagy is usually inhibited in many tissues under insulin resistance. Therefore, the regulation of autophagy could be a valuable strategy for the prevention and treatment of Type 2 diabetes.In the current study, palmitate-induced insulin resistance in skeletal muscle cells was established as the in vitro model, high fat diet-induced insulin resistance in rats as the in vivo model, and Sirt3 knockout mice were also used. As well as, plasmid transfection, confocal laser scanning microscopy, transmission electron microscopy, western blot, flow cytometry, HPLC and other techniques were used to study the effect of DHM on skeletal muscle insulin resistance, as well as the molecular mechanism of its improvement of autophagy and the impact on AMPK signaling pathways.The main results and conclusions are as follows:1. DHM significantly improved skeletal muscle insulin resistance and increased insulin sensitivity.2. DHM increased autophagy in a dose- and time-dependent manner in skeletal muscle cells. When autophagy was suppressed, this effect of DHM was significantly abolished. The results suggestd that autophagy plays a very important role in DHM’s improvement of skeletal muscle insulin resistance.3. DHM inhibited the activity of ATP synthase enzyme, F1F0-ATP, increased AMP/ATP ratio, thereby activated the expression of AMPK signaling pathway. In the presence of specific inhibitors of AMPK, PGC-1α or Sirt3, or the use of RNAi to silence these three genes, DHM-induced autophagy was significantly undermined. These results indicated that AMPK signaling pathway plays an important role in mediating DHM-induced autophagy in skeletal muscle.In summary, DHM induced autophagy via activating AMPK signaling pathway,thereby improved insulin resistance in skeletal muscle.