NHX-5, an Endosomal Na+/H+ Exchanger, Is Associated with Metformin Action

Abstract

Diabetes is one of the most impactful diseases worldwide. The most commonly prescribed anti-diabetic drug is metformin. In this study, we identified an endosomal Na ؉ /H ؉ exchanger (NHE) as a new potential target of metformin from an unbiased screen in Caenorhabditis elegans. The same NHE homolog also exists in flies, where it too mediates the effects of metformin. Our results suggest that endosomal NHEs could be a metformin target and provide an insight into a novel mechanism of action of metformin on regulating the endocytic cycle.Diabetes, a common consequence of the growing obesity epidemic, has emerged as a global health threat with over 422 million people worldwide being diagnosed in 2014 (1). Type 2 diabetes mellitus, which is characterized by insulin resistance, is the most common type of diabetes (2).The most prescribed anti-diabetic drug is metformin, 1,1-dimethylbiguanide (3). Metformin reduces glucose production in the liver and increases glucose uptake in muscle and adipose tissue, ameliorating diabetes (4). In addition, metformin exerts beneficial effects on other diseases such as polycystic ovary syndrome, certain cancers, and neurological disorders (2, 5). Several studies identified molecular targets of metformin: it inhibits complex I in mitochondria and decreases NADH oxidation, while reducing proton pumping across the inner mitochondrial membrane and decreasing oxygen consumption rate (6, 7). These effects in turn reduce the proton gradient and increase the AMP/ATP ratio, resulting in activation of AMP-activated protein kinase (AMPK) 2 and triggering a cascade that inhibits gluconeogenic gene expression and energy-consuming processes such as lipogenesis (5, 8). Metformin increases AMP and inhibits adenylate cyclase activity, leading to a reduction in cAMP levels, thereby inhibiting PKA activity and glucagon-dependent glucose production in mice (9). Metformin in physiological doses also suppresses hepatic gluconeogenesis by inhibiting mitochondrial glycerophosphate dehydrogenase (10). Given its versatile effects, it is likely that there are multiple targets of metformin with a wide range of affinities. A better understanding of the targets of metformin will not only allow for a more fine-tuned control of treatment with the drug, but also potentially provide insight into new therapeutic targets for diabetes and other metabolic disorders.Caenorhabditis elegans has been used for studying metabolism, reproduction, aging and other physiological processes, providing useful knowledge of the biology and molecular pathways underpinning human diseases. In this study, we report that metformin reduces C. elegans L1 longevity, a survival span of starvation. Using this phenotype, we performed an unbiased genetic screen and found an endosomal Na ϩ /H ϩ exchanger (NHE) as a potential molecular target of metformin. Moreover, the same NHE homolog in flies also mediates the effects of metformin, suggesting that an NHE could be a conserved target for metformin.

Regulation of organelle function by metformin

“…The level did not fully recover to that of the wild type that was not treated with metformin, but the difference between the mutant and the wild type was large and clear. The data support that the eNHE might be the site of action for metformin's function during starvation in C. elegans .…”

“…Metformin inhibition of mitochondrial complex I is conserved in Caenorhabditis elegans ; metformin delays development in well‐fed conditions and reduces life span during starvation . Kim et al found metformin reduces oxygen consumption in C. elegans .…”

“…Two recent studies suggest that metformin interacts with organelle Na + /H + exchangers (eNHE) and the V‐type‐ATPase (V‐ATPase), promoting the idea that the late endosome/lysosome could be another site of metformin action . Figure summarizes all of the currently known organelles that metformin targets.…”

Activating AMPK to Restore Tight Junction Assembly in Intestinal Epithelium and to Attenuate Experimental Colitis by Metformin

“…Since metformin has been reported to have some other targets ( Kim et al, 2016 ), we further used Compound C, a specific inhibitor of AMPK and another AMPK activator, AICAR, to examine whether metformin functioned via AMPK activation. Mice were divided into 5 groups ( n = 6–8 per group): normal, DSS with vehicle, DSS with metformin (500 mg/kg, p.o.…”

Histone deacetylase–mediated regulation of endolysosomal pH

“…Increased Nhx1 expression in glucose-depleted cells might therefore help promote endocytosis and provide resources for survival during glucose starvation (65). Intriguingly, eNHE mutants in Caenorhabditis elegans (Nhx5) and fly (DmNHE3) were found to be resistant to metformin, a biguanide drug commonly used to treat type-2 diabetes, potentially through dysregulation of autophagy (66). Given that metformin is known to induce a caloric restriction-like state (67), our findings suggest that up-regulation of eNHE and alkalinization of endosomal pH may underlie response to metformin therapy.…”