Arylhydrocarbon receptor (AhR) is involved in negative regulation of adipose differentiation in 3T3-L1 cells: AhR inhibits adipose differentiation independently of dioxin.

The arylhydrocarbon receptor (AhR) is the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Although a physiological ligand for the AhR has yet to be identified, and the precise physiological roles of the AhR are unknown, it may play important roles not only in the regulation of xenobiotic metabolism but also in the maintenance of homeostatic functions. We have previously reported that the level of AhR protein decreased with ongoing adipose differentiation in 3T3-L1 cells. Studies using a TCDD-resistant clone of 3T3-L1 cells suggested that the AhR may be involved in the negative regulation of adipose differentiation. To confirm this hypothesis, 3T3-L1 fibroblast cells were stably transfected with a vector expressing high levels of full-length sense AhR mRNA, antisense AhR mRNA or a control vector. Comparison of the differentiation potency of these clones with that of control cells showed that overexpression of the AhR suppressed morphological differentiation, as well as induction of adipocyte-related genes, whereas decreased expression of the AhR induced much greater morphological differentiation and expression of adipocyte-related genes. Activation of PPARgamma2 with ligands such as troglitazone, ciglitazone and indomethacin restored the ability of the AhR-overexpressing cells to differentiate. The cells overexpressing the AhR exhibited the higher p42/p44 MAP kinase activity compared with the control cells. Treatment with PD98059 or U0126 also abrogated the inhibitory action of the AhR on adipogenesis. We also present data showing that activation of the AhR slowed clonal expansion. During clonal expansion, the AhR inhibited the pRB phosphorylation and the downregulation of p107 expression. Taken together, these results strongly suggest that AhR is a negative regulator of adipose differentiation in 3T3 L1 cells.