The Aryl Hydrocarbon Receptor Binds to E2F1 and Inhibits E2F1-induced Apoptosis

Abstract

Cellular stress by DNA damage induces checkpoint kinase-2 (CHK2)-mediated phosphorylation and stabilization of the E2F1 transcription factor, leading to induction of apoptosis by activation of a subset of proapoptotic E2F1 target genes, including Apaf1 and p73. This report characterizes an interaction between the aryl hydrocarbon (Ah) receptor (AHR), a ligand-activated transcription factor, and E2F1 that results in the attenuation of E2F1-mediated apoptosis. In Ahr ؊/؊ fibroblasts stably transfected with a doxycycline-regulated AHR expression vector, inhibition of AHR expression causes a significant elevation of oxidative stress, ␥H2A.X histone phosphorylation, and E2F1-dependent apoptosis, which can be blocked by small interfering RNA-mediated knockdown of E2F1 expression. In contrast, ligand-dependent AHR activation protects these cells from etoposide-induced cell death. In cells expressing both proteins, AHR and E2F1 interact independently of the retinoblastoma protein (RB), because AHR and E2F1 coimmunoprecipitate from extracts of RBnegative cells. Additionally, chromatin immunoprecipitation assays indicate that AHR and E2F1 bind to the Apaf1 promoter at a region containing a consensus E2F1 binding site but no AHR binding sites. AHR activation represses Apaf1 and TAp73 mRNA induction by a constitutively active CHK2 expression vector. Furthermore, AHR overexpression blocks the transcriptional induction of Apaf1 and p73 and the accumulation of sub-G 0 /G 1 cells resulting from ectopic overexpression of E2F1. These results point to a proproliferative, antiapoptotic function of the Ah receptor that likely plays a role in tumor progression. INTRODUCTIONMembers of the E2F family of transcription factors are critical regulators of the G 1 /S phase transition of the cell cycle, during which their transcriptional activity is generally controlled through interaction with retinoblastoma (RB) family proteins. In addition, E2F proteins have functions beyond the G 1 /S phase transition that impact cell proliferation in a variety of ways (Dimova and Dyson, 2005). The E2F family consists of six extensively characterized and three less wellstudied members. E2F1, -2, and -3a are potent activators of transcription, bind exclusively to RB-p105, and are cyclically expressed during the cell cycle. E2F3b and -4, which can interact with RB-p107 and -p130, and E2F5, which binds only to p130, are poor transcriptional activators, and they function mainly as repressors through their recruitment of RB proteins to E2F-regulated promoters (Dyson, 1998;Nevins, 1998;DeGregori and Johnson, 2006). In general, the E2Fs with transactivator activity promote cell cycle progression, whereas the E2Fs with transrepressor activity function in cell cycle exit and differentiation (Dimova and Dyson, 2005). E2F6-8 are distinct from the other E2F members, lacking the transactivation and RB-binding domains, and they repress transcription in an RB-independent manner (Frolov and Dyson, 2004;DeGregori and Johnson, 2006).The best-characterized function of E2F ...

Evidence for a novel anti-apoptotic pathway in human keratinocytes involving the aryl hydrocarbon receptor, E2F1, and checkpoint kinase 1

“…34 Studies on murine embryonic fibroblasts indicated that the AHR inhibits E2F1-induced apoptosis, probably due to a physical interaction of both transcription factors. 35 However, the observed downregulation of E2F1 protein expression (Figure 2a) contradicts an involvement of a putative AHR/E2F1 protein interaction in enhancing the sensitivity of NCTC-shAHR KC to UVB-induced apoptosis.…”

Integrative Bioinformatics Analysis Reveals New Prognostic Biomarkers of Clear Cell Renal Cell Carcinoma

“…In contrast, AHR null mice have hyperplastic phenomena (hyperkeratosis of the skin, hyperproliferation of hair follicles, hyperplasia in the gastric pylorus, hyperproliferation of portal blood vessels) and develop liver and lung adenocarcinoma (20 ). Active AHR expression led to growth inhibition and apoptosis, and caused cell-cycle arrest (21,22 ). It was also reported that AHR activation has a promigratory effect in epithelial cells and enhances cancer invasion in urothelial cancer (23 ).…”

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“…The samples were digested with proteinase K at 45°C for 2 hr. DNA was purified using a QIAquick PCR purification Kit (Qiagen), eluted in double-distilled water, and an aliquot was used for analysis by RT-PCR using specific primers covering the region between kbp −2.9 and +0.6 of the mouse Cyp1a1 promoter (Marlowe et al, 2008).…”