BCKDH kinase promotes hepatic gluconeogenesis independent of BCKDHA.

Elevated circulating branched-chain amino acids (BCAAs) are tightly linked to an increased risk in the development of type 2 diabetes mellitus. The rate limiting enzyme of BCAA catabolism branched-chain α-ketoacid dehydrogenase (BCKDH) is phosphorylated at E1α subunit (BCKDHA) by its kinase (BCKDK) and inactivated. Here, the liver-specific BCKDK or BCKDHA knockout mice displayed normal glucose tolerance and insulin sensitivity. However, knockout of BCKDK in the liver inhibited hepatic glucose production as well as the expression of key gluconeogenic enzymes. No abnormal gluconeogenesis was found in mice lacking hepatic BCKDHA. Consistent with the vivo results, BT2-mediated inhibition or genetic knockdown of BCKDK decreased hepatic glucose production and gluconeogenic gene expressions in primary mouse hepatocytes while BCKDK overexpression exhibited an opposite effect. Whereas, gluconeogenic gene expressions were not altered in BCKDHA-silenced hepatocytes. Mechanistically, BT2 treatment attenuated the interaction of cAMP response element binding protein (CREB) with CREB-binding protein and promoted FOXO1 protein degradation by increasing its ubiquitination. Our findings suggest that BCKDK regulates hepatic gluconeogenesis through CREB and FOXO1 signalings, independent of BCKDHA-mediated BCAA catabolism.

Role of serum- and glucocorticoid-inducible kinase 1 in the regulation of hepatic gluconeogenesis.

Excessive hepatic gluconeogenesis partially accounts for the occurrence of type 2 diabetes mellitus. Serum- and glucocorticoid inducible-kinase 1 (SGK1) is linked to the development of metabolic syndrome, such as obesity, hypertension, and hyperglycemia. However, the regulatory role of SGK1 in glucose metabolism of liver remains uncertain. Our microarray analysis showed that SGK1 expression was strongly induced by 8-Br-cAMP and suppressed by metformin in primary mouse hepatocytes. Hepatic SGK1 expression was markedly increased in obese and diabetic mice. Metformin treatment decreased hepatic SGK1 expression levels in db/db mice. Inhibition or knockdown of SGK1 suppressed gluconeogenesis in primary mouse hepatocytes, with decreased expressions of key gluconeogenic genes. Furthermore, SGK1 silencing in liver decreased hepatic glucose production in C57BL/6 mice. Knockdown of SGK1 had no impact on CREB phosphorylation level but increased AKT and FoxO1 phosphorylation levels with decreased expressions of transcription factors including FoxO1 and hepatocyte nuclear factors. Adenovirus-mediated expression of dominant-negative AMPK antagonized metformin-suppressed SGK1 expression induced by 8-Br-cAMP. These findings demonstrate that hepatic specific silence of SGK1 might be a potential therapeutic strategy for type 2 diabetes.

SIRT2 ablation inhibits glucose-stimulated insulin secretion through decreasing glycolytic flux.

Rationale: Sirtuins are NAD+-dependent protein deacylases known to have protective effects against age-related diseases such as diabetes, cancer, and neurodegenerative disease. SIRT2 is the only primarily cytoplasmic isoform and its overall role in glucose homeostasis remains uncertain. Methods: SIRT2-knockout (KO) rats were constructed to evaluate the role of SIRT2 in glucose homeostasis. The effect of SIRT2 on ß-cell function was detected by investigating the morphology, insulin secretion, and metabolomic state of islets. The deacetylation and stabilization of GKRP in ß-cells by SIRT2 were determined by western blot, adenoviral infection, and immunoprecipitation. Results: SIRT2-KO rats exhibited impaired glucose tolerance and glucose-stimulated insulin secretion (GSIS), without change in insulin sensitivity. SIRT2 deficiency or inhibition by AGK2 decreased GSIS in isolated rat islets, with lowered oxygen consumption rate. Adenovirus-mediated overexpression of SIRT2 enhanced insulin secretion from rat islets. Metabolomics analysis revealed a decrease in metabolites of glycolysis and tricarboxylic acid cycle in SIRT2-KO islets compared with control islets. Our study further demonstrated that glucokinase regulatory protein (GKRP), an endogenous inhibitor of glucokinase (GCK), was expressed in rat islets. SIRT2 overexpression deacetylated GKRP in INS-1 ß-cells. SIRT2 knockout or inhibition elevated GKRP protein stability in islet ß-cells, leading to an increase in the interaction of GKRP and GCK. On the contrary, SIRT2 inhibition promoted the protein degradation of ALDOA, a glycolytic enzyme. Conclusions: SIRT2 ablation inhibits GSIS through blocking GKRP protein degradation and promoting ALDOA protein degradation, resulting in a decrease in glycolytic flux.