Student Research: Chistopher Schapp

, Environmental Toxicology (Tox), 2017
Faculty Advisor: Terrance J. Kavanagh

Plasticity of Antioxidant Defense Pathways in Response to Aging and Cadmium in Glutathione-Deficient Mice


Abstract

The ability to maintain cellular homeostasis during aging is an important determinant of organismal health throughout the lifespan. Understanding key pathways that govern homeostatic responses to endogenous and exogenous stressors is an important goal in aging research. The Nrf2 (nuclear factor erythroid 2-related factor 2; NFE2L2)/Keap1 (Kelch-like erythroid cell-derived protein with CNC homology [ECH]-associated protein 1) signaling pathway is one of the most important cellular defense and survival pathways, and has been shown to interact with a myriad of other signaling pathways, including those controlling xenobiotic metabolism, cellular growth, proliferation and nutrient sensing. Nrf2 can modulate the expression of genes involved in gluconeogenesis, fatty acid β-oxidation and lipogenesis, and can enhance (and be enhanced by) the effects of caloric restriction, a regimen that is known to promote longevity in many species. In this regard, Nrf2 and its orthologs have been shown to decrease aging associated pathology and increase longevity in a number of organisms. Among the most important Nrf2 regulated genes are glutamate cysteine ligase catalytic (GCLC) and modifier (GCLM) subunits, which are critical for the synthesis of the antioxidant tripeptide glutathione (GSH), a free radical scavenger and cofactor for many antioxidant enzymes. Gclm null mice are a model of chronic GSH deficiency, and have modified expression of a number of antioxidant genes, including those regulated by Nrf2, presumably to compensate for their low GSH levels. Moreover, because Gclm null mice appear to have chronic Nrf2 activation, they exhibit resistance to some but not all environmental stressors, including ozone, diesel exhaust, and CdSe/ZnS quantum dot nanoparticles. Importantly, Gclm null mice are approximately 20% leaner than Gclm wild-type mice, appear resistant to high fat diet-induced weight gain, are more glucose-tolerant and insulin-sensitive than Gclm wild-type mice, and may be resistant to the development of Type II diabetes mellitus (T2DM), a disease of major public health importance associated with aging. While these resistant phenotypes have been shown to occur in younger (10 week old) Gclm null mice, it is unclear if they persist with aging. Because Nrf2 signaling may be attenuated with aging, the resistance of young Gclm null mice against metabolic and environmental stress may not persist with age. Being both GSH deficient and Nrf2 inefficient, older Gclm null mice may show greater susceptibility to exogenous stressors, such as cadmium. This model toxicant has been chosen because Cd has been associated with the development of T2DM in several species and is known to accumulate in pancreatic β-cells, causing their dysfunction (i.e. impaired insulin release), in addition to its more canonical role as a oxidative stress-inducing heavy metal. In this work, we addressed whether aging compromises the favorable phenotype of increased Nrf2 activity and improved glucose homeostasis observed in younger Gclm null mice, and if so, whether such compromise was due to attenuation of Nrf2 signaling with aging, which has been reported by others to occur in flies, mice, and rats. The main thrust of this work was to confirm existing data on the differences in Nrf2 activity and glucose homeostasis between young Gclm wild-type and null mice, and to add to these data by examining the effects of aging and cadmium exposure on these phenotypes. In response to aging and acute cadmium administration, we found Gclm null mice maintain improved glutathione redox homeostasis, increased Nrf2 target gene expression and inducibility, and improved parameters of glucose homeostasis, relative to Gclm wild-type mice. These unexpected findings support the notion that chronic adaption to severely compromised glutathione levels, primarily via upregulation of Nrf2 stress responses, persists throughout the lifespan. This stands in contrast to the prevailing literature suggesting that compensatory cytoprotective responses (particularly antioxidant defense) decline with age. This work supports the notion that pharmacologic interventions to upregulate the Nrf2 pathway could improve late-life insulin sensitivity, glucose homeostasis and promote an improved healthspan.