Figure 1.

TCDD-elicited repression of SAM biosynthesis and methyltransferase gene expression. (a) Schematic pathway depicting enzymes (open rectangle) and metabolites (open circle) associated with SAM biosynthesis and utilization by methyltransferases (MT). (b) Hepatic levels of SAM and SAH were determined by LC–MS/MS (mean ± s.e.m., n = 5–6) at 8 and 28 days of repeated TCDD exposure and (c) hepatic gene expression of genes involved in the biosynthesis, regulation, and utilization of SAM and SAH were assessed at 8 and 28 days by RT-qPCR or RNA-seq, respectively (n = 8). (d) Fold change for hepatic MAT1A and GNMT protein levels after 28 days measured by the WES capillary electrophoresis system (mean ± s.e.m., n = 4). (e) Hepatic gene expression associated with SAM metabolism was determined by RNA-seq for a time-course after a bolus dose of 30 µg/kg TCDD (n = 5). For the heatmaps, the median effective dose (ED₅₀) and benchmark dose lower limit (BMDL) and relative transcript count (rel. count, ) are denoted. The red/blue color scale represents the log₂(fold change) for differential gene expression. Orange represents the presence of putative dioxin response elements (pDREs). AhR enrichment peaks (FDR ≤ 0.05) are denoted by light green. pDREs found within AHR ChIP-seq enrichment peaks are denoted by garnet. Asterisks (*) denote p < 0.05 determined by one-way ANOVA with a Dunnett’s post-hoc test. Pound signs (#) denote posterior probabilities P1(t) ≥ 0.80 compared to vehicle. Official gene name and symbol, and metabolite abbreviations: Comt catechol-O-methyltransferase, Gamt guanidinoacetate N-methyltransferase, Gnmt glycine N-methyltransferase, Inmt indolethylamine N-transferase, Mat1a, Mat2a S-adenosylmethionine synthase isoform 1a or 2a, Nnmt nicotinamide N-methyltransferase, Pemt phosphatidylethanolamine N-methyltransferase, Sardh sarcosine dehydrogenase, SAM S-adenosylmethionine, SAH S-adenosylhomocysteine.