Metformin is one of the most widely prescribed therapeutics for type 2 diabetes. But exactly how it works is still unclear. Gregory Steinberg and colleagues now show that it does so by activation of the enzyme AMP-activated protein kinase (Ampk) and Ampk's obligate targeting of two key enzymes involved in lipid homeostasis. The obesity epidemic has led to an increased incidence of nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes. AMP-activated protein kinase (Ampk) regulates energy homeostasis and is activated by cellular stress, hormones and the widely prescribed type 2 diabetes drug metformin1,2. Ampk phosphorylates mouse acetyl-CoA carboxylase 1 (Acc1; refs. 3,4) at Ser79 and Acc2 at Ser212, inhibiting the conversion of acetyl-CoA to malonyl-CoA. The latter metabolite is a precursor in fatty acid synthesis5 and an allosteric inhibitor of fatty acid transport into mitochondria for oxidation6. To test the physiological impact of these phosphorylation events, we generated mice with alanine knock-in mutations in both Acc1 (at Ser79) and Acc2 (at Ser212) (Acc double knock-in, AccDKI). Compared to wild-type mice, these mice have elevated lipogenesis and lower fatty acid oxidation, which contribute to the progression of insulin resistance, glucose intolerance and NAFLD, but not obesity. Notably, AccDKI mice made obese by high-fat feeding are refractory to the lipid-lowering and insulin-sensitizing effects of metformin. These findings establish that inhibitory phosphorylation of Acc by Ampk is essential for the control of lipid metabolism and, in the setting of obesity, for metformin-induced improvements in insulin action.

Lysosomes are the main proteolytic compartments of mammalian cells comprising of a battery of hydrolases. Lysosomes dispose and recycle extracellular or intracellular macromolecules by fusing with endosomes or autophagosomes through specific waste clearance processes such as chaperone-mediated autophagy or microautophagy. The proteolytic end product is transported out of lysosomes via transporters or vesicular membrane trafficking. Recent studies have demonstrated lysosomes as a signaling node which sense, adapt and respond to changes in substrate metabolism to maintain cellular function. Lysosomal dysfunction not only influence pathways mediating membrane trafficking that culminate in the lysosome but also govern metabolic and signaling processes regulating protein sorting and targeting. In this review, we describe the current knowledge of lysosome in influencing sorting and nutrient signaling. We further present a mechanistic overview of intra-lysosomal processes, along with extra-lysosomal processes, governing lysosomal fusion and fission, exocytosis, positioning and membrane contact site formation. This review compiles existing knowledge in the field of lysosomal biology by describing various lysosomal events necessary to maintain cellular homeostasis facilitating development of therapies maintaining lysosomal function.

Rationale : Cardiovascular physiology and pathophysiology vary dramatically over the course of the day. For example, myocardial infarction onset occurs with greater incidence during the early morning hours in humans. However, whether myocardial infarction tolerance exhibits a time-of-day dependence is unknown. Objective : To investigate whether time of day of an ischemic insult influences clinically relevant outcomes in mice. Methods and Results : Wild-type mice were subjected to ischemia/reperfusion (I/R) (45 minutes of ischemia followed by 1 day or 1 month of reperfusion) at distinct times of the day, using the closed-chest left anterior descending coronary artery occlusion model. Following 1 day of reperfusion, hearts subjected to ischemia at the sleep-to-wake transition (zeitgeber time [ZT]12) resulted in 3.5-fold increases in infarct size compared to hearts subjected to ischemia at the wake-to-sleep transition (ZT0). Following 1 month of reperfusion, prior ischemic event at ZT12 versus ZT0 resulted in significantly greater infarct volume, fibrosis, and adverse remodeling, as well as greater depression of contractile function. Genetic ablation of the cardiomyocyte circadian clock (termed cardiomyocyte-specific circadian clock mutant [CCM] mice) attenuated/abolished time-of-day variations in I/R outcomes observed in wild-type hearts. Investigation of Akt and glycogen synthase kinase-3β in wild-type and CCM hearts identified these kinases as potential mechanistic ties between the cardiomyocyte circadian clock and I/R tolerance. Conclusions : We expose a profound time-of-day dependence for I/R tolerance, which is mediated by the cardiomyocyte circadian clock. Further understanding of I/R tolerance rhythms will potentially provide novel insight regarding the etiology and treatment of ischemia-induced cardiac dysfunction.

Emerging evidence suggests that impaired regulation of adipocyte lipolysis contributes to the proinflammatory immune cell infiltration of metabolic tissues in obesity, a process that is proposed to contribute to the development and exacerbation of insulin resistance. To test this hypothesis in vivo, we generated mice with adipocyte-specific deletion of adipose triglyceride lipase (ATGL), the rate-limiting enzyme catalyzing triacylglycerol hydrolysis. In contrast to previous models, adiponectin-driven Cre expression was used for targeted ATGL deletion. The resulting adipocyte-specific ATGL knockout (AAKO) mice were then characterized for metabolic and immune phenotypes. Lean and diet-induced obese AAKO mice had reduced adipocyte lipolysis, serum lipids, systemic lipid oxidation, and expression of peroxisome proliferator-activated receptor alpha target genes in adipose tissue (AT) and liver. These changes did not increase overall body weight or fat mass in AAKO mice by 24 weeks of age, in part due to reduced expression of genes involved in lipid uptake, synthesis, and adipogenesis. Systemic glucose and insulin tolerance were improved in AAKO mice, primarily due to enhanced hepatic insulin signaling, which was accompanied by marked reduction in diet-induced hepatic steatosis as well as hepatic immune cell infiltration and activation. In contrast, although adipocyte ATGL deletion reduced AT immune cell infiltration in response to an acute lipolytic stimulus, it was not sufficient to ameliorate, and may even exacerbate, chronic inflammatory changes that occur in AT in response to diet-induced obesity.