Elucidating epigenetic mechanisms of cellular cadmium toxicity

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Abstract Cadmium (Cd) is a widespread environmental pollutant that affects millions of individuals worldwide. Cd exposure in humans occurs most often through Cd’s many industrial applications, or consumption of contaminated food. Due to its extremely extended biological half-life, Cd persists for decades in tissues, primarily in the liver and kidneys. Cd exerts numerous deleterious effects, including bone, reproductive, neurodevelopmental, and pulmonary toxicities, and carcinogenesis. The kidneys are the major target of Cd toxicity, particularly the proximal tubular epithelial cells, injury to which hampers tubular reabsorption. Despite the many sequelae associated with Cd exposure in humans, specific molecular mechanisms of Cd toxicity are poorly understood, and no specific therapies exist to mitigate the effects of Cd exposure. Via unbiased high- throughput screening, we identified a previously unknown ability of multiple chemically distinct histone deacetylase inhibitors (HDACi) and Bromodomain and Extra-Terminal motif inhibitors (BETi) to rescue acute cellular Cd toxicity. The long-term goal of these studies is to elucidate novel aspects of the cellular and molecular mechanisms of Cd toxicity. The objectives of this application are: i) to evaluate changes in gene expression and chromatin acetylation (Ac) occurring in response to Cd exposure in cultured cells and the kidney, and their potential rescue by HDACi or BETi treatment; and ii) to test the ability of HDACi and BETi to mitigate Cd induced nephrotoxicity. The central hypothesis of this application is that the interplay between histone Ac and mitochondrial metabolism represents a key functional target of Cd toxicity in mammalian cells. The rationale for this application is our novel foundational data demonstrating that Cd exposure induces a reduction in mitochondrial function and histone Ac. Crucially, treatment with HDACi and BETi rescue Cd-induced defects in mitochondrial respiration, metabolite levels, and cell viability. These findings implying that histone Ac and mitochondrial function are important functional cellular targets of Cd. The work will take place in the context of two Specific Aims. First, via RNA-seq and chromatin profiling, functionally important genes and pathways targeted by Cd, and rescued by an HDACi and a BETi, will be identified in cultured fibroblasts. Second, rescue of Cd-induced nephrotoxicity by an HDACi and a BETi will be evaluated in vivo in mice. We will compare the gene expression signatures of Cd-exposed kidney and fibroblasts, to identify core gene expression programs driving Cd toxicity, particularly focusing on those rescued by HDACi and BETi. The approach is innovative, in that mechanistic links between epigenetic alterations induced by Cd and its biological effects have yet to be conclusively established, and the use of small molecules directed against histone Ac or epigenetic perturbations more generally as treatments for Cd toxicity has not previously been described. The work is significant, since there is an unmet need for improved, mechanism-treatments for Cd toxicity. These studies may establish histone Ac as a novel therapeutic target for Cd toxicity.