Metformin action in human hepatocytes: coactivation of atypical protein kinase C alters 5′-AMP-activated protein kinase effects on lipogenic and gluconeogenic enzyme expression

Abstract

Aims/Hypothesis Levels and activity of atypical protein kinase C (aPKC) are elevated in hepatocytes of type 2 diabetic (T2DM) humans and cause excessive increases in expression of lipogenic and gluconeogenic enzymes; aPKC inhibitors largely correct these aberrations. Metformin improves hepatic gluconeogenesis by activating 5′-AMP-activated protein kinase (AMPK). However, metformin also activates aPKC in certain tissues; in liver, this activation could amplify T2DM aberrations and offset salutary effects of AMPK. Here, we examined whether metformin activates aPKC in human hepatocytes and metabolic its consequences. Methods We compared protein kinase activities and alterations of lipogenic and gluconeogenic enzyme expression during actions of AMPK activators, metformin and AICAR, relative to those of an aPKC-ι inhibitor, in hepatocytes of non-diabetic and T2DM humans. Results Metformin and AICAR activated aPKC at concentrations comparable to those required for AMPK activation. Moreover, both agents increased lipogenic enzyme expression by an aPKC-dependent mechanism. Thus, whereas insulin-dependent and T2DM-dependent increases in lipogenic enzyme expression were reversed by aPKC inhibition, such expression was increased in non-diabetic hepatocytes and remained elevated in T2DM hepatocytes following metformin and AICAR treatment. Also, whereas aPKC inhibition diminished gluconeogenic enzyme expression in the absence and presence of insulin in both non-diabetic and T2DM hepatocytes, metformin and AICAR increased gluconeogenic enzyme expression in non-diabetic hepatocytes, but nevertheless diminished gluconeogenic enzyme expression in insulin-treated T2DM hepatocytes. Conclusions/Interpretations Metformin and AICAR activate aPKC along with AMPK in human hepatocytes. aPKC activation increases lipogenic enzyme expression, alters gluconeogenic enzyme expression and therefore appears to offset salutary effects of AMPK.

Hepatic insulin resistance in ob/ob mice involves increases in ceramide, aPKC activity, and selective impairment of Akt-dependent FoxO1 phosphorylation

“…Its specifi city was reported previously ( 5,24 ). In addition, we presently found that ACPD did not inhibit kinases, Akt2, FGFR1/2/3/4, mTOR, GSK3 ␤ , IRAK1/4, JAK1/2, MEK1, ERK1/2, JNK1/2, PKA, Src, ROCK2, ROS1, or PI3K ␣ / ␣ , as tested by Life Technologies (Madison WI).…”

“…As described ( 8,24 ), hepatocytes of lean, obese, and type 2 diabetic humans were incubated overnight and then incubated for 4 h in the presence of 2 Ci uniformly-labeled 14 C-acetate (Perkin Elmer NEN) and 10 M sodium acetate carrier, and, where indicated, 2 M ACPD; following which, tissue lipids were extracted by the method of Bligh and Dyer and counted for radioactivity.…”

“…Liver and muscle samples were homogenized as described (3)(4)(5)(6)(7)(8)24 ). Nuclei were isolated as described ( 4-7 ).…”

“…Presently, we studied these obese-phase ob/ob mice over 10 weeks, during which they consumed low-fat chow, with or without an agent that selectively targets and inhibits hepatic aPKC ( 5,24 ). We found that these ob/ob mice are different from younger overtly diabetic ob/ob mice in that hepatic Akt activity/activation is clearly excessive, rather than diminished.…”

AMPK-derived peptides reduce blood glucose levels but lead to fat retention in the liver of obese mice

“…Similarly, AICAR also led to fat accumulation in this cell line. Indeed, it has recently been shown that AICAR activates atypical protein kinase C together with AMPK in human hepatocytes, leading to an increase in lipogenic enzyme levels (Sajan et al 2013). Nevertheless, other reports have shown significant reduction in hepatic triglyceride accumulation in both chow-and HF diet-fed animals treated with AICAR (Tomita et al 2005, Henriksen et al 2013.…”