Ube2l3 gene expression is modulated by activation of the aryl hydrocarbon receptor: implications for p53 ubiquitination.

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a halogenated aromatic hydrocarbon and environmental contaminant, results in several deleterious effects, including fetal malformation and cancer. These effects are mediated by the aryl hydrocarbon receptor (AhR), a ligand-activated receptor that regulates the expression of genes encoding xenobiotic-metabolizing enzymes. Several reports suggest that AhR function is beyond the adaptive chemical response. In the present study, we analyzed and compared gene expression profiles of C57BL/6N wild-type (WT) and Ahr-null mice. DNA microarray and quantitative RT-PCR analyses revealed changes in the expression of genes involved in the ubiquitin-proteasome system (UPS). UPS has an important role in cellular homeostasis control and dysfunction of this pathway has been implicated in the development of several human pathologies. Protein ubiquitination is a multi-step enzymatic process that regulates the stability, function, and/or localization of the modified proteins. This system is highly regulated post-translationally by covalent modifications. However, little information regarding the transcriptional regulation of the genes encoding ubiquitin (Ub) proteins is available. Therefore, we investigated the role of the AhR in modulation of the UPS and regulation of Ube2l3 transcription, an E2 ubiquitin-conjugating enzyme, as well as the effects on p53 degradation. Our results indicate that AhR inactivation decreases on liver proteasome activity, probably due to a down-regulation on the expression of several proteasome subunits. On the other hand, AhR activation increases Ube2l3 mRNA and protein levels by controlling Ube2l3 gene expression, resulting in increased p53 ubiquitination and degradation. In agreement with this, induction of apoptosis was attenuated by the AhR activation.

Aromatic hydrocarbons upregulate glyceraldehyde-3-phosphate dehydrogenase and induce changes in actin cytoskeleton. Role of the aryl hydrocarbon receptor (AhR).

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional enzyme involved in several cellular functions including glycolysis, membrane transport, microtubule assembly, DNA replication and repair, nuclear RNA export, apoptosis, and the detection of nitric oxide stress. Therefore, modifications in the regulatory ability and function of GAPDH may alter cellular homeostasis. We report here that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and beta-naphthoflavone, which are well-known ligands for the aryl hydrocarbon receptor (AhR), increase GAPDH mRNA levels in vivo and in vitro, respectively. These compounds fail to induce GAPDH transcription in an AhR-null mouse model, suggesting that the increase in GAPDH level is dependent upon AhR activation. To analyse the consequences of AhR ligands on GAPDH function, mice were treated with TCDD and the level of liver activity of GAPDH was determined. The results showed that TCDD treatment increased GAPDH activity. On the other hand, treatment of Hepa-1 cells with beta-naphthoflavone leads to an increase in microfilament density when compared to untreated cultures. Collectively, these results suggest that AhR ligands, such as polycyclic hydrocarbons, can modify GAPDH expression and, therefore, have the potential to alter the multiple functions of this enzyme.

Arsenite and its metabolites, MMA(III) and DMA(III), modify CYP3A4, PXR and RXR alpha expression in the small intestine of CYP3A4 transgenic mice.

Arsenic is an environmental pollutant that has been associated with an increased risk for the development of cancer and several other diseases through alterations of cellular homeostasis and hepatic function. Cytochrome P450 (P450) modification may be one of the factors contributing to these disorders. Several reports have established that exposure to arsenite modifies P450 expression by decreasing or increasing mRNA and protein levels. Cytochrome P450 3A4 (CYP3A4), the predominant P450 expressed in the human liver and intestines, which is regulated mainly by the Pregnane X Receptor-Retinoid X Receptor alpha (PXR-RXR alpha) heterodimer, contributes to the metabolism of approximately half the drugs in clinical use today. The present study investigates the effect of sodium arsenite and its metabolites monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)) on CYP3A4, PXR, and RXR alpha expression in the small intestine of CYP3A4 transgenic mice. Sodium arsenite treatment increases mRNA, protein and CYP3A4 activity in a dose-dependent manner. However, the increase in protein expression was not as marked as compared to the increase in mRNA levels. Arsenite treatment induces the accumulation of Ub-protein conjugates, indicating that the activation of this mechanism may explain the differences observed between the mRNA and protein expression of CYP3A4 induction. Treatment with 0.05 mg/kg of DMA(III) induces CYP3A4 in a similar way, while treatment with 0.05 mg/kg of MMA(III) increases mostly mRNA, and to a lesser degree, CYP3A4 activity. Sodium arsenite and both its metabolites increase PXR mRNA, while only DMA(III) induces RXR alpha expression. Overall, these results suggest that sodium arsenite and its metabolites induce CYP3A4 expression by increasing PXR expression in the small intestine of CYP3A4 transgenic mice.