Circulating miRNAs have become ideal and non-invasive molecular markers to evaluate the pathophysiological status of various diseases due to their stable changes in diseases, stable existence in serum, plasma, urine, saliva and other body fluids, and easy to obtain and preserve clinically (19). According to some previous studies, the levels of miR-126, miR-15a and miR-223 in peripheral blood were decreased several years before the onset of T2D; compared with patients with diabetes mellitus (DM) and metabolic syndrome (MS), the serum levels of miR-9, miR-29, miR-30d, miR-34a, miR-124a, miR-146a and miR-375 were higher in newly diagnosed T2D patients (20); compared with the control group, the serum levels of miR-375 and miR-9 were higher in patients with pre diabetes mellitus. These two miRNAs were directly related to the presence of pre diabetes and T2D, and miR-375 was independently related to the occurrence of T2D. miR-375 alone or combined with miR-9 can be used as early detection markers of pre diabetes and T2D (21). In addition, studies have shown that miR-378, miR-126-3p and miR-223 in blood are indicators of disease staging and prognosis in elderly patients with T2D (22). In conclusion, serum miRNAs are noninvasive and ideal indicators for the diagnosis and prognosis of T2D (23). However, no miRNA has been recognized as a specific diagnostic marker for T2D. In this study, we found that the serum miR-720 level of 50 newly diagnosed T2D patients was significantly higher than that of healthy control group (P < 0.05), and miR-720 was positively correlated with blood glucose level and negatively correlated with insulin level (P < 0.05). This suggests that miR-720 is highly expressed in T2D patients and is related to blood glucose and insulin levels. This suggests that miR-720 may reflect the disorder of glucose metabolism in T2D patients to a certain extent, and may become a potential biomarker for the diagnosis of T2D.
Insulin release from pancreatic β cells is a necessary condition for maintaining normal glucose homeostasis in humans and many other animals. miRNAs are known to be involved in the regulation of insulin signaling pathway, but the mechanism is still unclear. Therefore, it may have important clinical value to explore the mechanism of miRNAs regulating insulin secretion by pancreatic beta cells and to understand the significance of miRNAs in T2D. Evidence suggests that miRNAs, as key regulators of gene expression, play an important role in the production, transport and secretion of insulin. The change of miRNAs expression can lead to insulin secretion dysfunction and IR, destroy insulin signaling pathway and various physiological processes, and then lead to the occurrence and development of DM (24). As a result, the potential role of miRNAs in the treatment of T2D has been widely concerned (25, 26).
The production of insulin is the key function of pancreatic beta cells, and the release of insulin is necessary to maintain glucose homeostasis. Among the known miRNAs that play an important role in insulin secretion and glucose homeostasis, miR-375 is the most concerned (27). Some studies suggest that miR-375 is an important regulator of pancreatic beta cells function (28, 29); overexpression of miR-375 can reduce the number and viability of pancreatic beta cells (29, 30). It can regulate the insulin secretion by directly targeting genes related to exocytosis (28, 31). In addition, miR-103 and miR-107 can regulate insulin and glucose homeostasis in vivo. They play an important role in insulin sensitivity and may be potential targets for the treatment of T2D (20). However, the mechanism of miR-720 in regulating insulin secretion by pancreatic beta cells is still unclear. Many studies have explored the function and target of miR-720 in disease progression. Known targets of miR-720 include TWIST1 in breast cancer (11), Rab35 in cervical cancer (12), StarD13 in colorectal cancer (13) and CCND1 in pancreatic cancer (14).
Other studies have shown that miR-720 can negatively regulate p63 and promote epithelial development (32); miR-720 participates in the control of human dental pulp cell stem cell phenotype by directly inhibiting NANOG’s level (33); targeting miR-720 can help restore the immunity of patients with chronic hepatitis B (34). Our study found that overexpression of miR-720 inhibited glucose stimulated insulin secretion in MIN6 cells, while down-regulation of miR-720 promoted insulin secretion. Subsequently, we found that Rab35 was the target of miR-720 in MIN6 cells by luciferase reporter gene.
Rab proteins are also known to be important participants in exocytosis and secretion of insulin by pancreatic β cells. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and plasma membrane that are regulated by upstream regulators (16, 35). In our previous study, we found that Rab35 is closely related to the exocytotic and secretory function of pancreatic β cells (16). PI3K/Akt/mTOR signaling pathway regulates the life activities of many kinds of cells, including cell growth, proliferation and differentiation. Over activation of PI3K/Akt/mTOR is involved in diabetic retinopathy, diabetic nephropathy and IR (36). PI3K/Akt/mTOR is a signal pathway closely related to insulin signal transduction (6, 24). The expression of PI3K and phosphorylation of Akt in kidney, liver, skeletal muscle and adipose tissue of DM rats were significantly decreased; in the development of DM, persistent hyperglycemia can promote the activation of PI3K / Akt signaling pathway, and ultimately accelerate the development of DM (37). This study found that overexpression of Rab35 inhibited PI3K/Akt/mTOR signaling pathway and promoted insulin secretion; when Rab35 was inhibited, PI3K/ Akt/mTOR pathway was activated and insulin secretion was down regulated. Therefore, we speculate that Rab35 can regulate PI3K/Akt/mTOR signaling pathway and thus affect insulin secretion.