Attenuating Mkrn1 E3 Ligase-Mediated Ampk Alpha Suppression Increases Tolerance Against Metabolic Stresses In Mice

The 5' adenosine monophosphate-activated protein kinase (AMPK) is an essential energy sensor in the cell, which, at low energy levels, instigates the cellular energy-generating systems along with suppression of the anabolic signaling pathways. The activation of AMPK through phosphorylation is a well-known process; however, activation alone is not sufficient, and knowledge about the other regulatory networks of post-translational modifications connecting the activities of AMPK to systemic metabolic syndromes is important, which is still lacking. The recent studies on Makorin Ring Finger Protein 1 (MKRN1) mediating the ubiquitination and proteasome-dependent degradation of AMPK alpha implicate that the post-translational modification of AMPK, regulating its protein homeo-stasis, could impose significant systemic metabolic effects (Lee et al. Nat Commun 9:3404). In this study, MKRN1 was identified as a novel E3 ligase for both AMPK alpha 1 and alpha 2. Mouse embryonic fibroblasts, genetically deleted for Mkrmn1, and Ampk alpha 1 and alpha 2, became stabilized with the suppression of lipogenesis pathways and an increase in nutrient consumption and mitochondria regeneration. Of note, the Mkm1-knockout mice fed normal chow displayed no obvious phenotypic defects or abnormality, whereas the Mkm1-null mice exhibited strong tolerance to metabolic stresses induced by high-fat diet (HFD). Thus, these mice, when compared with the HFD-induced wild type, were resistant to obesity, diabetes, and non-alcoholic fatty liver disease. Interestingly, in whole-body Mkm1 knockout mouse, only the liver and white and brown adipose tissues displayed anincrease in the active phosphorylated AMPK levels, but no other organs, such as the hypothalamus, skeletal muscles, or pancreas, displayed such increases. Specific ablation of MKRN1 in the mouse liver using adenovirus prevented HFD-induced lipid accumulation in the liver and blood, implicating MKRN1 as a possible therapeutic target for metabolic syndromes, such as obesity, type II diabetes, and fat liver diseases. This study would provide a crucial perspective on the importance of post-translational regulation of AMPK in metabolic pathways and will help researchers develop novel therapeutic strategies that target not only AMPK but also its regulators.