Arsenic, Nrf2 and Autophagy Dysfunction in Type II Diabetes

Project Summary Chronic arsenic exposure is a worldwide public health concern correlated with an increasedrisk of developing certain types of cancer, as well as metabolic diseases including type II diabetes.Arsenic-linked type II diabetes has recently been shown among populations in the United States,Mexico, Canada, Taiwan, and Bangladesh. We have previously reported that low, environmentallyrelevant doses of arsenic block autophagy at a later stage. Autophagy is a key cellular degradationpathway and autophagic dysfunction is thought to be an integral part of pathogenic changes thatoccur in adipocytes, ?-islet cells, hepatocytes, skeletal muscle, and kidney mesangial cells duringdiabetes progression. We have also shown that dysregulation of the autophagy pathway results inprolonged activation of the key antioxidant transcription factor nuclear factor erytheroid-derived-2-like2 (Nrf2). Nrf2, which is normally bound and degraded in the cytosol by Kelch-like ECH-associatedprotein 1 (Keap1), is upregulated at the protein level following oxidative modification of Keap1(Keap1-C151 dependent, canonical) or by sequestration of Keap1 into autophagosomes duringautophagy dysfunction (p62-dependent, non-canonical). While controlled Nrf2 activation through thecanonical mechanism is protective, prolonged non-canonical activation of Nrf2 causes cellulardysfunction and tissue damage, indicative of a ?dark side? to Nrf2. Therefore, we hypothesize thatthe prolonged activation of Nrf2, resulting from arsenic-mediated blockage of autophagy flux,is essential for arsenic in promoting type II diabetes. This hypothesis is supported by thefollowing evidence: 1) chronic arsenic exposure decreases glucose uptake and insulin signaling in3T3-L1 adipocytes, 2) Keap11 KD/Lepob/ob mice, a genetic mouse model for diabetes with persistentNrf2 activation, display impaired adipogenesis, as well as decreased insulin sensitivity and glucosetolerance compared to Lepob/ob controls, and 3) ?-cell specific knockdown of Atg7, a key autophagyinitiation protein, results in increased levels of p62 and poly-ubiquitinated proteins, accompanied by ?-cell loss and decreased insulin production. We have generated a large amount of data indicating that arsenic blocks autophagy at theautophagosome-lysosome fusion step. Three core SNARE proteins Stx17, SNAP29, and VAMP8mediate fusion, with SNAP29 mediating the interaction between Stx17 on the outer autophagosomalmembrane and VAMP8 on the lysosomal membrane. We believe that genetic ablation of any of thesethree fusion proteins will hinder autophagosome-lysosome fusion and result in prolonged Nrf2activation, which will mimic the effect of arsenic in promoting type II diabetes. Therefore, we proposeto: Aim 1: Determine the molecular mechanism by which arsenic blocks autophagosome-lysosome fusion, leading to prolonged Nrf2 activation. Aim 2: Determine if prolonged Nrf2 activation resulting from autophagic dysfunctioninduces metabolic reprogramming in muscle, kidney, pancreas, liver and fat cells. Aim 3: Determine if autophagy dysfunction and prolonged Nrf2 activation phenocopyarsenic in exacerbating insulin resistance, obesity, and diabetic nephropathy using type IIdiabetes models in SNAP29f/f, Nrf2-/-, and SNAP29f/f/Nrf2-/- mice. Impact: A detailed and thorough understanding of autophagy dysfunction and the prolongedactivation of Nrf2 in arsenic-induced metabolic disease will prove extremely valuable in the generationof preventive and therapeutic strategies, as well as in the identification of biomarkers, for thepopulations at risk.