TCDD-elicited effects on liver, serum, and adipose lipid composition in C57BL/6 mice

Abstract

The aryl hydrocarbon receptor (AhR) mediates alterations in hepatic lipid composition elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to further investigate the effects of TCDD, liver, serum, and gonadal white adipose tissue (gWAT) fatty acid methyl esters (FAMEs) and lipids were examined in fasted 4-week-old female mice orally gavaged with 30 µg/kg TCDD at 24, 72, and 168 h postdose. Mean hepatic FAME levels increased (236.7 µmol/g in controls compared with 392.2 µmol/g in TCDD treated) with minimal changes in gWAT and serum. In the liver, TCDD decreased saturated fatty acids (SFAs 16:0, 18:0, 20:0, and 22:0) and increased monounsaturated fatty acids (MUFAs 16:1n7, 18:1n9, and 20:1n9). Hepatic polyunsaturated fatty acids (PUFAs) 20:2n6, 20:3n6, 18:3n3, and 22:5n3 also increased, whereas 20:4n6 and 22:6n3 levels decreased. gWAT PUFAs 20:2n6 and 20:3n6 exhibited modest increases, whereas serum 18:0 decreased and 18:1n9 increased. Serum analyses also identified a ~25% decrease in total cholesterol (CHOL), low-density lipoprotein (LDL), and high-density lipoprotein following TCDD treatment. The decrease in serum CHOL was consistent with the induction of hepatic reverse CHOL transport genes Lcat (2.0-fold), Apoa1 (1.7-fold), and Ldlr (3.6-fold), and the repression of CHOL biosynthesis genes Hmgcs1 (-2.1-fold) and Hmgcr (-2.3-fold). In addition, TCDD decreased serum Apob100 (4.4-fold) and Apob48 (2.2-fold) protein levels, suggesting serum lipid clearance and decreased hepatic efflux. Collectively, the TCDD-elicited decreases in serum lipid levels are consistent with AhR-mediated enhancement of dietary fat distribution to the liver.

FIG. 1.

TFA composition in (A) liver (µmol/g), (B) gWAT, aka peripheral fat pad (µmol/g), and (C) serum (nmol/g) at 168h post 30 µg/kg TCDD or sesame oil vehicle dose. TFAs were extracted by Folch method and quantified by GC-MS. *p < 0.05 for TCDD compared with vehicle. Bars represent mean + SEM. Liver, n = 4; adipose and serum, n = 5.

FIG. 2.

Serum CHOL and Apob100 and Apob48 levels in mice treated with 30 µg/kg TCDD or sesame oil vehicle. (A) Effect of TCDD on total CHOL, LDL, and HDL levels at 168h postdose. Serum lipids (n = 8) were measured by commercial assay (WAKO Diagnostics) and are presented as mg/dl. (B) Serum Apob100 and Apob48 protein levels detected by Western blot at 72h (n = 3). (C–D) Densitometry (determined with ImageJ) identified a TCDD-dependent 4.4- and 2.2-fold decrease in Apob100 (C) and Apob48 (D) band intensity, respectively. *p < 0.05 for TCDD compared with vehicle. Bars represent mean + SEM.

FIG. 3.

Differential expression of (A–E) hepatic CHOL metabolism genes and (F–J) gWAT lipid metabolism and transport genes in mice orally gavaged with 30 µg/kg TCDD or sesame oil vehicle for 24h. Total mRNA levels were normalized to the geometric mean of Hprt, Actb, and Gapdh. Genes are indicated by official gene symbols. *Represents p < 0.05 for TCDD compared with vehicle. Bars represent mean + SEM, n = 5 biological replicates.