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Background and purpose TGR5 plays an important role in many physiological processes. However, the functions of TGR5 in the regulation of the glucose metabolism and insulin sensitivity in the skeletal muscles have not been fully elucidated. We synthesized MN6 as a potent and selective TGR5 agonist. Here, the effect of MN6 on insulin resistance in skeletal muscles was evaluated in diet-induced obese (DIO) mice and C2C12 myotubes, and the underlying mechanisms were explored. Methods The activation of MN6 on human and mouse TGR5 was evaluated by a cAMP assay in HEK293 cell lines stable expressing hTGR5/CRE or mTGR5/CRE cells. GLP-1 secretion was measured in NCI-H716 cells and CD1 mice. The acute and chronic effects of MN6 on regulating metabolic abnormalities were observed in ob/ob and DIO mice. 2-deoxyglucose uptake was examined in isolated skeletal muscles. Akt phosphorylation, glucose uptake and glycogen synthesis were examined to assess the effects of MN6 on palmitate-induced insulin resistance in C2C12 myotubes. Results MN6 potently activated human and mouse TGR5 with EC50 values of 15.9 and 17.9 nmol/L, respectively, and stimulated GLP-1 secretion in NCI-H716 cells and CD1 mice. A single oral dose of MN6 significantly decreased the blood glucose levels in ob/ob mice. Treatment with MN6 for 15 days reduced the fasting blood glucose and HbA1c levels in ob/ob mice. MN6 improved glucose and insulin tolerance and enhanced the insulin-stimulated glucose uptake of skeletal muscles in DIO mice. The palmitate-induced impairment of insulin-stimulated Akt phosphorylation, glucose uptake and glycogen synthesis in C2C12 myotubes could be prevented by MN6. The effect of MN6 on palmitate-impaired insulin-stimulated Akt phosphorylation was abolished by siRNA-mediated knockdown of TGR5 or by the inhibition of adenylate cyclase or protein kinase A, suggesting that this effect is dependent on the activation of TGR5 and the cAMP/PKA pathway. Conclusions Our study identified that a TGR5 agonist could ameliorate insulin resistance by the cAMP/PKA pathway in skeletal muscles; this uncovered a new effect of the TGR5 agonist on regulating the glucose metabolism and insulin sensitivity in skeletal muscles and further strengthened its potential value for the treatment of type 2 diabetes.

Objective Salt-induced kinase 1 (SIK1) acts as a key modulator in many physiological processes. However, the effects of SIK1 on gluconeogenesis and the underlying mechanisms have not been fully elucidated. In this study, we found that a natural compound phanginin A could activate SIK1 and further inhibit gluconeogenesis. The mechanisms by which phanginin A activates SIK1 and inhibits gluconeogenesis were explored in primary mouse hepatocytes, and the effects of phanginin A on glucose homeostasis were investigated in ob/ob mice. Methods The effects of phanginin A on gluconeogenesis and SIK1 phosphorylation were examined in primary mouse hepatocytes. Pan-SIK inhibitor and siRNA-mediated knockdown were used to elucidate the involvement of SIK1 activation in phanginin A-reduced gluconeogenesis. LKB1 knockdown was used to explore how phanginin A activated SIK1. SIK1 overexpression was used to evaluate its effect on gluconeogenesis, PDE4 activity, and the cAMP pathway. The acute and chronic effects of phanginin A on metabolic abnormalities were observed in ob/ob mice. Results Phanginin A significantly increased SIK1 phosphorylation through LKB1 and further suppressed gluconeogenesis by increasing PDE4 activity and inhibiting the cAMP/PKA/CREB pathway in primary mouse hepatocytes, and this effect was blocked by pan-SIK inhibitor HG-9-91-01 or siRNA-mediated knockdown of SIK1. Overexpression of SIK1 in hepatocytes increased PDE4 activity, reduced cAMP accumulation, and thereby inhibited gluconeogenesis. Acute treatment with phanginin A reduced gluconeogenesis in vivo, accompanied by increased SIK1 phosphorylation and PDE4 activity in the liver. Long-term treatment of phanginin A profoundly reduced blood glucose levels and improved glucose tolerance and dyslipidemia in ob/ob mice. Conclusion We discovered an unrecognized effect of phanginin A in suppressing hepatic gluconeogenesis and revealed a novel mechanism that activation of SIK1 by phanginin A could inhibit gluconeogenesis by increasing PDE4 activity and suppressing the cAMP/PKA/CREB pathway in the liver. We also highlighted the potential value of phanginin A as a lead compound for treating type 2 diabetes., Highlights • Phanginin A inhibits gluconeogenesis in primary mouse hepatocytes. • Phanginin A increases hepatic SIK1 phosphorylation both in vitro and in vivo. • Activation of SIK1 increases PDE4 activity and suppresses the cAMP signaling pathway. • Activation of SIK1 inhibits gluconeogenesis by regulating the PDE4/cAMP/PKA/CREB pathway. • Phanginin A improves metabolic disorders in ob/ob mice.