Dioxin-elicited decrease in cobalamin redirects hepatic propionyl-CoA metabolism to the β–oxidation-like pathway resulting in acrylyl-CoA conjugate accumulation

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant which induces diverse biological and toxic effects, including the reprograming of intermediate metabolism, mediated by the aryl hydrocarbon receptor (AHR). Targeted LC-MS analysis of hepatic extracts from mice gavaged with TCDD every 4 days for 28 days detected an increase in S-(2-carboxyethyl)-L-cysteine, a conjugate produced following the spontaneous reaction between the sulfhydryl group of cysteine and highly reactive acrylyl-CoA, an intermediate in the cobalamin (Cbl)-independent β–oxidation-like metabolism of propionyl-CoA. In addition to repressing genes in both the canonical Cbl-dependent carboxylase and the alternate Cbl-independent β–oxidation-like pathways at 30 µg/kg TCDD, methylmalonyl-CoA mutase (MUT) activity was inhibited at lower doses. Moreover, TCDD decreased serum Cbl levels and hepatic cobalt levels while eliciting negligible effects on gene expression associated with Cbl absorption, transport, trafficking, or the derivatization to 5’-deoxy-adenosylcobalamin (AdoCbl), the required MUT cofactor. In addition to inducing Acod1 that encodes for aconitate decarboxylase 1, the enzyme responsible for the decarboxylation cis-aconitate to itaconate, TCDD also dose-dependently increased itaconate levels in hepatic extracts. MUT inhibition is consistent with itaconate activation to itaconyl-CoA, a MUT suicide inactivator that adducts AdoCbl, that in turn, inhibits MUT activity and reduces Cbl levels. Collectively, these results suggest the decrease in MUT activity is due to Cbl depletion following TCDD treatment that redirected propionyl-CoA metabolism to the alternate Cbl-independent β–oxidation-like pathway. The resulting hepatic accumulation of acrylyl-CoA likely contributes to TCDD-elicited hepatotoxicity and the multi-hit progression of steatosis to steatohepatitis with fibrosis. Beginning on post-natal day (PND) 30, male C57BL/6 mice were repeatedly gavaged at 23° C every 4 days for 28 days with some dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (0.03 - 30 µg/kg TCDD) or sesame oil vehicle (n = 5 per dose) and sacrificed on PND 58 with CO2. Gavaging and tissue collection always occurred between ZT0 and ZT3. Tissues were flash frozen in liquid nitrogen and stored at -80°C. For total RNA extraction using trizol, 100 mg was used per liver sample. Only samples which passed Nanodrop and RIN quality control were used. 150 bp paired-end RNA reads were sequenced using a NovoSeq 6000.