Activation of the cAMP-PKA pathway Antagonizes Metformin Suppression of Hepatic Glucose Production

Abstract

Metformin is the most commonly prescribed oral anti-diabetic agent worldwide. Surprisingly, about 35% of diabetic patients either lack or have a delayed response to metformin treatment, and many patients become less responsive to metformin over time. It remains unknown how metformin resistance or insensitivity occurs. Recently, we found that therapeutic metformin concentrations suppressed glucose production in primary hepatocytes through AMPK; activation of the cAMP-PKA pathway negatively regulates AMPK activity by phosphorylating AMPK␣ subunit at Ser-485, which in turn reduces AMPK activity. In this study, we find that metformin failed to suppress glucose production in primary hepatocytes with constitutively activated PKA and did not improve hyperglycemia in mice with hyperglucagonemia. Expression of the AMPK␣1(S485A) mutant, which is unable to be phosphorylated by PKA, increased both AMPK␣ activation and the suppression of glucose production in primary hepatocytes treated with metformin. Intriguingly, salicylate/ aspirin prevents the phosphorylation of AMPK␣ at Ser-485, blocks cAMP-PKA negative regulation of AMPK, and improves metformin resistance. We propose that aspirin/salicylate may augment metformin's hepatic action to suppress glucose production.Diabetes is the fastest-growing chronic disease worldwide, and type 2 diabetes (T2D) 2 accounts for more than 90% of diabetes cases. Metformin has been used to treat T2D since the 1950s and works primarily by controlling fasting hyperglycemia (1). Now, over 150 million people worldwide take this medication; guidelines for the treatment of T2D published by the American Diabetes Association and the European Association for the Study of Diabetes in 2012 jointly recommended metformin as the initial drug for T2D treatment (2).Surprisingly, about 35% of diabetic patients either lack or have a delayed response to metformin treatment (3-6), and many patients become less responsive to metformin over time (4). It remains unknown how metformin resistance/ insensitivity occurs. Several genes have been reported to be associated with glycemic control by metformin. In particular, variants of the organic cation transporter and the metformin transporter, have been linked to a reduction in metformin action (7-9). Because of the high occurrence of metformin resistance in diabetic patients, genetic variants in known genes alone cannot explain this phenomenon; therefore, nongenetic factors may also contribute to the response to metformin.Recently, we found that treatment with metformin prior to the addition of cAMP led to greater suppression of glucose production and AMPK activation when compared with simultaneous treatment with metformin and cAMP (10), suggesting that the cAMP-PKA pathway exerts a negative effect on AMPK activation. In both diabetic animal models and human subjects, serum glucagon levels and the ratios of glucagon to insulin are often elevated (11)(12)(13)(14) and are responsible for increased hepatic glucose output and hyperglycemia in T2D (15). Elevated glucagon l...

Controlled Heat Stress Promotes Myofibrillogenesis during Myogenesis

“…Western blot analysis was carried out as described earlier [19]. Briefly, cultured cells were harvested in cell lysis buffer (Cell Signaling Technology) and denatured in SDS loading buffer.…”

AMPK-Mediated Regulation of Lipid Metabolism by Phosphorylation

“…20) Multiple additional phosphorylation sites in the α subunit have also been studied. 21) The α1Ser485 residue is involved in negative regulation of AMPK by cAMP-PKA. 22) In additional, phosphorylation of AMPKα Ser485/491, when stimulated with insulin, has been shown to lead to a decrease in AMPK activity.…”

“…37) Moreover, AMPKα Ser485/491, which was confirmed by using recombinant protein, 35) was also autophosphorylated to limit AMPK activation in response to energy depletion or other regulators. 21) The functional roles of additional AMPK autophosphorylation sites, such as α1Thr477, α1Ser349, β1Thr158, and β1Thr80, 38) requires further investigation.…”

Increased serum circulating asprosin levels in children with obesity

“…Human and experimental animal studies showed that asprosin is significantly associated with glucose and insulin production in the liver during fasting, which means that the presence of asprosin could be a risk factor with regard to insulin resistance . Furthermore, decreasing the level of asprosin leads to the prevention of hyperinsulinemia, which is associated with metabolic syndrome It has been shown that increased asprosin levels in the liver trigger cyclic adenosine 3′, 5′‐monophosphate (cAMP) and protein kinase (PKA) signaling pathways, which has resulted in higher release of glucose from the liver into the circulation . In a recent animal study it was shown that aerobic exercise decreased the level of hepatic asprosin …”