Aryl-hydrocarbon receptor is an inhibitory regulator of lipid synthesis and of commitment to adipogenesis

Abstract

The aryl-hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological effects of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD). In mouse embryo fibroblasts, TCDD activates expression of multiple genes, including CYP1B1, the predominant cytochrome P450 expressed in these cells. Here, we analyze constitutive functions of the AhR in primary mouse embryo fibroblasts (MEFs) and spontaneously immortalized MEF cell lines derived from wild-type (WT) C57BL/6 mice and also from congenic mice with a targeted disruption of the AhR gene (AhR-/-). After multiple passages, primary MEFs exhibit spontaneous differentiation, growth cessation and senescence. Eventually, colonies of immortalized MEFs arise to provide clonal lines. The senescent phase occurs much earlier for AhR-/- MEFs, while immortalization is substantially delayed. Comparison of AhR-/- and WT MEFs also indicates that constitutive AhR activity is required for basal expression of CYP1B1 and suppresses lipogenesis in subconfluent cultures. Primary WT and AhR-/- MEFs and the corresponding lines undergo adipogenesis when treated at confluence with the appropriate hormonal inducers. Addition of TCDD before or concurrent with hormonal induction suppressed PPAR gamma mRNA and adipogenesis, as measured by lipid accumulation, glycerol phosphate dehydrogenase activity and stearoyl CoA desaturase type 1 mRNA expression. This effect of TCDD treatment was absent in AhR-/- MEFs, establishing the role of AhR in hormone-induced adipogenesis. Such hormonal activation of confluent MEFs and preadipocytes results in a limited proliferative expansion followed by irreversible growth arrest. TCDD-treated MEFs undergo the mitotic expansion but fail to exit the cell cycle. In AhR-/- MEFs, there is no such effect of TCDD. These findings implicate the AhR as a constitutive inhibitor of triglyceride synthesis, and as an early regulator of adipocyte differentiation. AhR interference with cell-cycle arrest in differentiation may be linked to the increased rate of senescence.

Aryl hydrocarbon receptor blocks aging-induced senescence in the liver and fibroblast cells

“…AhR-null mouse embryonic fibroblasts (MEF), on passaging in culture, acquire a senescent phenotype earlier than wild type cells [ 28 ] and have reduced proliferation potential [ 48 ]. Flow cytometry and cytological staining for SA-β-Gal indicated that, upon passage in culture, AhR-null MEFs acquired higher levels of senescence than AhR+/+ MEFs (significant after passage 3, P3) ( Figure 6A – 6C ).…”

“…AhR modulates senescence in different cell types and organs, including mesenchymal embryonic fibroblasts [ 28 ]. Using passaging as a model for aging in culture, we found that MEFs from AhR−/− mice are more susceptible to senescence as determined by higher levels of SA-β-Gal, p16 Ink4a and p21 Cip1 .…”

“…An early report revealed that mouse embryo fibroblasts (MEFs) from AhR−/− mice reached senescence earlier than AhR+/+ cells during adipogenic differentiation [ 28 ]. Later work showed that AhR reduces smoking-induced inflammation in the lung parenchyma through, among other mechanisms, the control of senescence [ 29 ].…”

Aryl Hydrocarbon Receptor: From Homeostasis to Tumor Progression

“…Preliminary studies with HL60 and HEL cell lines showed that the differentiation from monocytes to macrophages with phorbol esters required the transcriptional activation of AHR ( Hayashi et al, 1995 ). Moreover, experiments performed to differentiate AHR +/+ and AHR −/− mouse embryonic fibroblasts (MEFs) to adipocytes revealed that AHR deficiency impairs the differentiation process, suggesting that AHR could be an early regulator of adipogenesis ( Alexander et al, 1998 ). Moreover, the accumulation of TCDD in adipose tissue induces an effect on oxidative stress enzymes in both adipocytes and liver, exacerbating oxidative stress ( Kern et al, 2002 ).…”

Insight into the physiological and pathological roles of the aryl hydrocarbon receptor pathway in glucose homeostasis, insulin resistance, and diabetes development

“…were the first to provide evidence of a link between AhR activation by TCDD and energy balance [ 110 ]. Following this, several studies demonstrated the inhibitory action of TCDD on the synthesis of fatty acids in liver and adipose tissues [ 111 , 112 ], gluconeogenic enzymes such as G6Pase and PEPCK [ 113 ], and adipogenesis [ 114 ]. In this context, it has been reported that mice expressing a high constitutively active form of AhR spontaneously developed hepatic steatosis, characterized by high amounts of fatty acid import, suppressed fatty acid oxidation, and increased oxidation and mobilization of peripheral fat storage [ 115 ].…”