Heterozygous huntingtin promotes cadmium neurotoxicity and neurodegeneration in striatal cells via altered metal transport and protein kinase C delta dependent oxidative stress and apoptosis signaling mechanisms

Abstract

Huntington's disease (HD) is functionally linked to environmental factors including cigarette use and dyshomeostasis in the levels of metals. Interestingly, one of the most abundant heavy metals in cigarettes is cadmium (Cd), which also accumulates in the striatum and causes neurotoxicity upon exposure. Thus, we hypothesized that heterozygous huntingtin (HTT), responsible for the majority of cases of HD in patients, in combination with Cd exposure would cause neurotoxicity and neurodegeneration via increased intracellular accumulation of Cd and activation of oxidative stress signaling mechanisms in a mouse striatal cell line model of HD. We report that heterozygous HTT striatal cells are significantly more susceptible to Cd-induced cytotoxicity as compared to wild-type HTT cells upon exposure for 48 h. The heterozygous HTT and Cd-induced cytotoxicity led to a NADPH oxidase (NOX) mediated oxidative stress that was attenuated by exogenous antioxidants and a NOX inhibitor, apocynin. Heterozygous HTT coupled with Cd exposure caused increased expression of protein kinase C δ (PKCδ) and other key oxidative stress proteins levels, enhanced the activation of caspase-9 and caspase-3 mediated apoptosis, and blocked the overexpression of extracellular signal-regulated kinase (ERK). We observed significantly greater intracellular accumulation of Cd and reduced expression of divalent metal transporter 1 (DMT1) protein in the heterozygous HTT striatal cells upon Cd exposure. Treatment with zinc, manganese, and iron as well as exogenous antioxidants significantly attenuated the Cd-induced cytotoxicity. Collectively, these results demonstrate that heterozygous HTT exhibits greater neurotoxic properties when coupled with Cd exposure to cause cell death via caspase mediated apoptosis, altered metal transport, and modulation of ERK and PKCδ dependent oxidative signaling mechanisms.

Keywords: Antioxidants; Apoptosis; Cadmium neurotoxicity; ERK; Huntington's disease; Metal transport; Neurodegeneration; Oxidative stress; Protein kinase C delta.