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• W2066742532 abstract "Skin cancer is the most common cancer in the United States. Its major environmental risk factor is UVB radiation in sunlight. In response to UVB damage, epidermal keratinocytes activate a specific repair pathway, i.e. nucleotide excision repair, to remove UVB-induced DNA lesions. However, the regulation of UVB response is not fully understood. Here we show that the long isoform of the nuclear factor erythroid 2-related factor 1 (Nrf1, also called NFE2L1), a cytoprotective transcription factor critical for the expression of multiple antioxidant response element-dependent genes, plays an important role in the response of keratinocytes to UVB. Nrf1 loss sensitized keratinocytes to UVB-induced apoptosis by up-regulating the expression of the proapoptotic Bcl-2 family member Bik through reducing glutathione levels. Knocking down Bik reduced UVB-induced apoptosis in Nrf1-inhibited cells. In UVB-irradiated surviving cells, however, disruption of Nrf1 impaired nucleotide excision repair through suppressing the transcription of xeroderma pigmentosum C (XPC), a factor essential for initiating the global genome nucleotide excision repair by recognizing the DNA lesion and recruiting downstream factors. Nrf1 enhanced XPC expression by increasing glutathione availability but was independent of the transcription repressor of XPC. Adding XPC or glutathione restored the DNA repair capacity in Nrf1-inhibited cells. Finally, we demonstrate that Nrf1 levels are significantly reduced by UVB radiation in mouse skin and are lower in human skin tumors than in normal skin. These results indicate a novel role of Nrf1 in UVB-induced DNA damage repair and suggest Nrf1 as a tumor suppressor in the skin. Skin cancer is the most common cancer in the United States. Its major environmental risk factor is UVB radiation in sunlight. In response to UVB damage, epidermal keratinocytes activate a specific repair pathway, i.e. nucleotide excision repair, to remove UVB-induced DNA lesions. However, the regulation of UVB response is not fully understood. Here we show that the long isoform of the nuclear factor erythroid 2-related factor 1 (Nrf1, also called NFE2L1), a cytoprotective transcription factor critical for the expression of multiple antioxidant response element-dependent genes, plays an important role in the response of keratinocytes to UVB. Nrf1 loss sensitized keratinocytes to UVB-induced apoptosis by up-regulating the expression of the proapoptotic Bcl-2 family member Bik through reducing glutathione levels. Knocking down Bik reduced UVB-induced apoptosis in Nrf1-inhibited cells. In UVB-irradiated surviving cells, however, disruption of Nrf1 impaired nucleotide excision repair through suppressing the transcription of xeroderma pigmentosum C (XPC), a factor essential for initiating the global genome nucleotide excision repair by recognizing the DNA lesion and recruiting downstream factors. Nrf1 enhanced XPC expression by increasing glutathione availability but was independent of the transcription repressor of XPC. Adding XPC or glutathione restored the DNA repair capacity in Nrf1-inhibited cells. Finally, we demonstrate that Nrf1 levels are significantly reduced by UVB radiation in mouse skin and are lower in human skin tumors than in normal skin. These results indicate a novel role of Nrf1 in UVB-induced DNA damage repair and suggest Nrf1 as a tumor suppressor in the skin. Proper maintenance of DNA integrity is vital for the viability and the health of organisms. The genome is under constant threats from endogenous metabolic byproducts and environmental factors that can alter its chemical structure and corrupt its encoded message. The resulting DNA lesions must be effectively repaired to prevent loss or incorrect transmission of genetic information as errors can cause developmental abnormalities and tumorigenesis, including skin cancer, the most common cancer in the United States. As major building blocks of the skin, keratinocytes are constantly challenged by ultraviolet (UV) radiation in sunlight. In particular, UVB (280–315 nm) causes DNA damage and is the major environmental risk factor for skin cancer. Each year, more than one million new cases of skin cancer are diagnosed in the United States alone, accounting for 40% of all newly diagnosed cancer cases (1.Bowden G.T. Prevention of non-melanoma skin cancer by targeting ultraviolet-B-light signaling.Nat. Rev. Cancer. 2004; 4: 23-35Crossref PubMed Scopus (335) Google Scholar, 2.Johnson T.M. Dolan O.M. Hamilton T.A. Lu M.C. Swanson N.A. Lowe L. Clinical and histologic trends of melanoma.J. Am. Acad. Dermatol. 1998; 38: 681-686Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). Efficient repair of UVB-induced DNA damage lesions, in particular cyclobutane pyrimidine dimers (CPDs), 2The abbreviations used are:CPDcyclobutane pyrimidine dimerBikBcl-2-interacting killerDDB1DNA damage-binding protein 1DDB2DNA damage-binding protein 2Nrf1nuclear factor erythroid 2-related factor 1, also called NFE2L1SCCsquamous cell carcinomasiBiksiRNA targeting BiksiNCnegative control siRNAXPxeroderma pigmentosumXPCxeroderma pigmentosum group CPARPpoly(ADP-ribose) polymerase-2MTS3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt6-4PP(6-4) photoproduct. but not the (6-4) photoproduct (6-4PP), is essential to prevent skin tumorigenesis (3.Jans J. Schul W. Sert Y.G. Rijksen Y. Rebel H. Eker A.P. Nakajima S. van Steeg H. de Gruijl F.R. Yasui A. Hoeijmakers J.H. van der Horst G.T. Powerful skin cancer protection by a CPD-photolyase transgene.Curr. Biol. 2005; 15: 105-115Abstract Full Text Full Text PDF PubMed Scopus (169) Google Scholar). cyclobutane pyrimidine dimer Bcl-2-interacting killer DNA damage-binding protein 1 DNA damage-binding protein 2 nuclear factor erythroid 2-related factor 1, also called NFE2L1 squamous cell carcinoma siRNA targeting Bik negative control siRNA xeroderma pigmentosum xeroderma pigmentosum group C poly(ADP-ribose) polymerase-2 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt (6-4) photoproduct. To remove UVB-induced CPD and 6-4PP, cells are equipped with a delicate DNA repair system, nucleotide excision repair. Defects in nucleotide excision repair by mutations cause the autosomal recessive xeroderma pigmentosum (XP) and Cockayne syndromes (4.Cleaver J.E. Cancer in xeroderma pigmentosum and related disorders of DNA repair.Nat. Rev. Cancer. 2005; 5: 564-573Crossref PubMed Scopus (339) Google Scholar, 5.Cleaver J.E. Lam E.T. Revet I. Disorders of nucleotide excision repair: the genetic and molecular basis of heterogeneity.Nat. Rev. Genet. 2009; 10: 756-768Crossref PubMed Scopus (306) Google Scholar, 6.Kraemer K.H. Sunlight and skin cancer: another link revealed.Proc. Natl. Acad. Sci. U.S.A. 1997; 94: 11-14Crossref PubMed Scopus (340) Google Scholar, 7.Hoeijmakers J.H. Genome maintenance mechanisms for preventing cancer.Nature. 2001; 411: 366-374Crossref PubMed Scopus (3137) Google Scholar). In particular, XP patients are clinically characterized by cutaneous sensitivity to sunlight and a predisposition to skin cancer. Among the identified genes that correspond to seven XP complementation groups (XPA to XPG), XPC is a key factor in damage recognition in global genome nucleotide excision repair (8.Volker M. Moné M.J. Karmakar P. van Hoffen A. Schul W. Vermeulen W. Hoeijmakers J.H. van Driel R. van Zeeland A.A. Mullenders L.H. Sequential assembly of the nucleotide excision repair factors in vivo.Mol. Cell. 2001; 8: 213-224Abstract Full Text Full Text PDF PubMed Scopus (650) Google Scholar, 9.Sugasawa K. Ng J.M. Masutani C. Iwai S. van der Spek P.J. Eker A.P. Hanaoka F. Bootsma D. Hoeijmakers J.H. Xeroderma pigmentosum group C protein complex is the initiator of global genome nucleotide excision repair.Mol. Cell. 1998; 2: 223-232Abstract Full Text Full Text PDF PubMed Scopus (742) Google Scholar, 10.Riedl T. Hanaoka F. Egly J.M. The comings and goings of nucleotide excision repair factors on damaged DNA.EMBO J. 2003; 22: 5293-5303Crossref PubMed Scopus (345) Google Scholar). Although the function of XPC has been investigated in great detail, the regulation of XPC remains largely unknown. Nuclear factor erythroid 2-related factor 1 (Nrf1, also called NFE2L1) belongs to the Cap'n' collar basic leucine zipper (CNC-bZIP) family of proteins, which also includes p45 NF-E2, Nrf2, Nrf3, Bach1, and Bach2 (11.Chan J.Y. Han X.L. Kan Y.W. Cloning of Nrf1, an NF-E2-related transcription factor, by genetic selection in yeast.Proc. Natl. Acad. Sci. U.S.A. 1993; 90: 11371-11375Crossref PubMed Scopus (295) Google Scholar, 12.Andrews N.C. Erdjument-Bromage H. Davidson M.B. Tempst P. Orkin S.H. Erythroid transcription factor NF-E2 is a hematopoietic-specific basic leucine zipper protein.Nature. 1993; 362: 722-728Crossref PubMed Scopus (566) Google Scholar, 13.Kobayashi A. Ito E. Toki T. Kogame K. Takahashi S. Igarashi K. Hayashi N. Yamamoto M. Molecular cloning and functional characterization of a new Cap'n' collar family transcription factor Nrf3.J. Biol. Chem. 1999; 274: 6443-6452Abstract Full Text Full Text PDF PubMed Scopus (245) Google Scholar, 14.Moi P. Chan K. Asunis I. Cao A. Kan Y.W. Isolation of NF-E2-related factor 2 (Nrf2), an NF-E2-like basic leucine zipper transcriptional activator that binds to the tandem NF-E2/AP1 repeat of the β-globin locus control region.Proc. Natl. Acad. Sci. U.S.A. 1994; 91: 9926-9930Crossref PubMed Scopus (1226) Google Scholar, 15.Oyake T. Itoh K. Motohashi H. Hayashi N. Hoshino H. Nishizawa M. Yamamoto M. Igarashi K. Bach proteins belong to a novel family of BTB-basic leucine zipper transcription factors that interact with MafK and regulate transcription through the NF-E2 site.Mol. Cell Biol. 1996; 16: 6083-6095Crossref PubMed Scopus (522) Google Scholar, 16.Zhang D.D. Mechanistic studies of the Nrf2-Keap1 signaling pathway.Drug. Metab. Rev. 2006; 38: 769-789Crossref PubMed Scopus (846) Google Scholar, 17.Pi J. Freeman M.L. Yamamoto M. Nrf2 in toxicology and pharmacology: the good, the bad, and the ugly?.Toxicol. Appl. Pharmacol. 2010; 244: 1-3Crossref PubMed Scopus (14) Google Scholar). Nrf1 is widely expressed in mouse and human tissues (11.Chan J.Y. Han X.L. Kan Y.W. Cloning of Nrf1, an NF-E2-related transcription factor, by genetic selection in yeast.Proc. Natl. Acad. Sci. U.S.A. 1993; 90: 11371-11375Crossref PubMed Scopus (295) Google Scholar, 18.Luna L. Johnsen O. Skartlien A.H. Pedeutour F. Turc-Carel C. Prydz H. Kolstø A.B. Molecular cloning of a putative novel human bZIP transcription factor on chromosome 17q22.Genomics. 1994; 22: 553-562Crossref PubMed Scopus (85) Google Scholar, 19.Murphy P. Kolstø A. Expression of the bZIP transcription factor TCF11 and its potential dimerization partners during development.Mech. Dev. 2000; 97: 141-148Crossref PubMed Scopus (42) Google Scholar) and regulates cytoprotective genes through antioxidant-response elements in their promoter regions (20.Venugopal R. Jaiswal A.K. Nrf1 and Nrf2 positively and c-Fos and Fra1 negatively regulate the human antioxidant response element-mediated expression of NAD(P)H:quinone oxidoreductase1 gene.Proc. Natl. Acad. Sci. U.S.A. 1996; 93: 14960-14965Crossref PubMed Scopus (926) Google Scholar, 21.Johnsen O. Murphy P. Prydz H. Kolsto A.B. Interaction of the CNC-bZIP factor TCF11/LCR-F1/Nrf1 with MafG: binding-site selection and regulation of transcription.Nucleic Acids Res. 1998; 26: 512-520Crossref PubMed Scopus (93) Google Scholar, 22.Nioi P. Hayes J.D. Contribution of NAD(P)H:quinone oxidoreductase 1 to protection against carcinogenesis, and regulation of its gene by the Nrf2 basic region leucine zipper and the arylhydrocarbon receptor basic helix-loop-helix transcription factors.Mutat. Res. 2004; 555: 149-171Crossref PubMed Scopus (306) Google Scholar). Genes transactivated by Nrf1 include those encoding the glutamate-cysteine ligase catalytic and modifier subunits, which together catalyze the rate-limiting step in glutathione (GSH) biosynthesis, as well as (NAD(P)H:quinone oxidoreductase) (NQO1), heme oxygenase 1, ferritin, and metallothionein (23.Chen L. Kwong M. Lu R. Ginzinger D. Lee C. Leung L. Chan J.Y. Nrf1 is critical for redox balance and survival of liver cells during development.Mol. Cell Biol. 2003; 23: 4673-4686Crossref PubMed Scopus (111) Google Scholar, 24.Leung L. Kwong M. Hou S. Lee C. Chan J.Y. Deficiency of the Nrf1 and Nrf2 transcription factors results in early embryonic lethality and severe oxidative stress.J. Biol. Chem. 2003; 278: 48021-48029Abstract Full Text Full Text PDF PubMed Scopus (243) Google Scholar, 25.Yang H. Magilnick N. Lee C. Kalmaz D. Ou X. Chan J.Y. Lu S.C. Nrf1 and Nrf2 regulate rat glutamate-cysteine ligase catalytic subunit transcription indirectly via NF-κB and AP-1.Mol. Cell Biol. 2005; 25: 5933-5946Crossref PubMed Scopus (187) Google Scholar, 26.Ohtsuji M. Katsuoka F. Kobayashi A. Aburatani H. Hayes J.D. Yamamoto M. Nrf1 and Nrf2 play distinct roles in activation of antioxidant response element-dependent genes.J. Biol. Chem. 2008; 283: 33554-33562Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar). Transcriptional activation of antioxidant-response element-driven genes by Nrf1 prevents oxidative stress in vivo (23.Chen L. Kwong M. Lu R. Ginzinger D. Lee C. Leung L. Chan J.Y. Nrf1 is critical for redox balance and survival of liver cells during development.Mol. Cell Biol. 2003; 23: 4673-4686Crossref PubMed Scopus (111) Google Scholar). Nrf1 inhibition increased apoptosis in response to arsenic (27.Zhao R. Hou Y. Xue P. Woods C.G. Fu J. Feng B. Guan D. Sun G. Chan J.Y. Waalkes M.P. Andersen M.E. Pi J. Long isoforms of NRF1 contribute to arsenic-induced antioxidant response in human keratinocytes.Environ. Health Perspect. 2011; 119: 56-62Crossref PubMed Scopus (75) Google Scholar). Although some of the CNC-bZIP factors, particularly Nrf2, have been studied in detail (16.Zhang D.D. Mechanistic studies of the Nrf2-Keap1 signaling pathway.Drug. Metab. Rev. 2006; 38: 769-789Crossref PubMed Scopus (846) Google Scholar, 17.Pi J. Freeman M.L. Yamamoto M. Nrf2 in toxicology and pharmacology: the good, the bad, and the ugly?.Toxicol. Appl. Pharmacol. 2010; 244: 1-3Crossref PubMed Scopus (14) Google Scholar), relatively little is known about the biological functions of Nrf1. In particular, the involvement of Nrf1 in skin cancer and UVB response remains unknown. Here we used mouse skin, squamous cell carcinoma (SCC) from human skin, and the human HaCaT keratinocyte cell line to determine the role of Nrf1 in UVB response. We found that Nrf1 plays an important role in cell survival and DNA repair after UVB irradiation and likely acts as a tumor suppressor in skin carcinogenesis. All human specimens were studied after approval by the University of Chicago Institutional Review Board. Frozen tissues were obtained under consent (Department of Medicine, University of Chicago). Protein lysate was used to determine Nrf1 levels by Western blotting. Human HaCaT keratinocytes were obtained from Professor N. Fusenig. HaCaT cells were transduced with MISSION lentiviral particles for short hairpin RNAs targeting Nrf1 to knock down the expression of the long isoform NRF1 gene (NRF1-KD; 3Throughout this article, the designation NRF1-KD indicates Nrf1 knockdown HaCaT cells, and the designation Scr indicates HaCaT cells transduced with scrambled negative control short hairpin RNA. SHVRS_NM_003204; Sigma) or scrambled nontarget negative control (Scr; SHC002V; Sigma) and then cultured in medium containing 1.0 μg/ml puromycin. The selected cells were cultured in medium without puromycin. Scr and NRF1-KD cells were maintained in a monolayer culture in 95% air, 5% CO2 at 37 °C in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 100 units/ml penicillin, and 100 mg/ml streptomycin (Invitrogen). All animal procedures have been approved by the University of Chicago Institutional Animal Care and Use Committee. B6 mice were shaved and 2 days later exposed to UVB (400, 750, and 1000 mJ/cm2) dorsally or sham-irradiated, three times a week for 1 week. Mouse skin was snap-frozen at 72 h after the final irradiation for immunoblotting analysis. Mice were housed no more than five animals per cage. For UVB treatment, a Stratalinker 2400 equipped with 312-nm UVB bulbs (Stratagene, La Jolla, CA) (UVC 0%, UVB 51%, and UVA 49%) was used. The UVB exposure was performed in PBS after washing the cells with PBS twice to avoid the photosensitization effect of components in culture medium on the cells. Our UVB radiation was monitored every other week to measure the exposure output and dose. Cells were transfected with negative control siRNA (siNC) or siRNA (ON-TARGETplus SMARTpool, Dharmacon) targeting Bik (siBik) (5 μg), using an Amaxa Nucleofector according to the manufacturer's instructions for HaCaT cells. For overexpression of XPC, cells were transfected with a vector expressing human XPC (OriGene, 1 μg) using an Amaxa Nucleofector according to the manufacturer's instructions for HaCaT cells. Empty vector was used as a transfection control. Briefly, HaCaT cells were resuspended, transfected with siRNAs or plasmids using the Amaxa Nucleofector II device, and then treated with or without UVB at 72 h after transfection. Protein concentrations were determined using the BCA assay (Pierce). Equal amounts of protein were subjected to electrophoresis. Western blotting was performed in triplicate using antibodies including Nrf1, DNA damage-binding protein 1 (DDB1) and 2 (DDB2), XPC, HR23B, PARP, β-actin (equal loading control), and GAPDH (Santa Cruz Biotechnology) and Bcl-2 family proteins (Cell Signaling Technology and Epitomics) according to the recommendation of the manufacturer. The blots shown were representative of three replicates. β-Actin blots were done by stripping each blot. In Figs. 2A and 4A, the β-actin shown is a representative of each stripping. Approximate molecular mass is marked in each blot.FIGURE 4Nrf1 promotes UVB-induced DNA damage repair through regulating XPC. A, immunoblot analysis of Nrf1, DDB1, DDB2, XPC, HR23B, and β-actin in Scr and NRF1-KD HaCaT cells. B, real-time PCR analysis of the XPC mRNA levels. C, reporter assay of XPC promoter in Scr and NRF1-KD HaCaT cells. a.u., arbitrary units. D, reporter assay of wild-type (WT) and E2F site mutant (Mut) XPC promoter in Scr and NRF1-KD HaCaT cells. E, immunoblot analysis of XPC and GAPDH in Scr, NRF1-KD, and XPC-transfected NRF1-KD HaCaT cells. F and H, slot blot analysis of the levels of CPD (F) and 6-4PP (H) in NRF1-KD HaCaT cells transfected with empty vector (EV) or XPC at 0, 6, and 24 h after UVB (20 mJ/cm2). G and I, quantification of percentage of CPD repair (G) from F and 6-4PP repair (I) from H. *, p < 0.05, significant differences between NRF1-KD and NRF1-KD/XPC groups. Error bars in panels B, C, D, G, and I indicate S.E.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Twenty-four hours after UVB irradiation, cell viability was measured using the MTS assay (CellTiter 96 AQueous One Solution proliferation assay, Promega) and monitored at 492 nm using a TECAN Infinite M200 plate reader according to the manufacturer's instructions. Sub-G1 flow cytometric analysis was performed to determine apoptosis. Briefly, cells were fixed and stained with propidium iodide. The percentage of cells at the sub-G1 phase was quantified by flow cytometry. For the promoter reporter assay, the plasmid mixtures, containing 1 μg of XPC promoter luciferase construct (XPC-Luc in pGL3 vector, kindly provided by Dr. Pradip Raychaudhuri at the University of Illinois at Chicago) and 0.025 μg of pRL-TK (Promega, used as a transfection efficiency control), were transfected with FuGENE 6 transfection reagent (Roche Applied Science) according to the manufacturer's protocol. The luciferase activity was measured with a TD-20/20 luminometer (Turner BioSystems) and normalized with the values of pRL-TK luciferase activity using a Dual-Luciferase Reporter (DLR) assay kit (Promega). Real-time PCR analysis was performed using an ABI7300 (Applied Biosystems, Foster City, CA) with 96-well plates with the SYBR® Green PCR master mix (Applied Biosystems). The primers for XPC (HP207915) and Bik (HP205127) were obtained from OriGene. Slot blot assays of CPD and 6-4PP were performed as described previously (28.Maeda T. Chua P.P. Chong M.T. Sim A.B. Nikaido O. Tron V.A. Nucleotide excision repair genes are up-regulated by low-dose artificial ultraviolet B: evidence of a photoprotective SOS response?.J. Invest. Dermatol. 2001; 117: 1490-1497Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar). Briefly, mouse skin or cells were collected at different time points after UVB, and DNA was isolated using a QIAamp DNA mini kit (Qiagen, Valencia, CA). The DNA concentration was calculated from the absorbance at 260 nm using NanoDrop 1000 (Thermofisher). The CPD and 6-4PP in DNA were quantified by slot blot (Bio-Rad) with monoclonal antibodies (TDM-2 for CPD and 64 M-2 for 6-4PP, COSMO BIO Co., Koto-Ku, Tokyo, Japan) (28.Maeda T. Chua P.P. Chong M.T. Sim A.B. Nikaido O. Tron V.A. Nucleotide excision repair genes are up-regulated by low-dose artificial ultraviolet B: evidence of a photoprotective SOS response?.J. Invest. Dermatol. 2001; 117: 1490-1497Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar). The chemiluminescence was detected with a Carestream imaging station (Carestream). For examining repair kinetics, the percentage of repair was calculated by comparing the optical density at the indicated time with that of the corresponding absorbance at time 0, when there was no opportunity for repair and 100% of CPDs (or 6-4PPs) were present after UVB. Cells were treated with or without UVB radiation and then incubated with 5-(and-6)-chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate (2 μm, Invitrogen) for 30 min at 37 °C. Cells were washed with PBS three times, and oxidative stress was analyzed by flow cytometry. Statistical analyses were performed using Prism 5 (GraphPad Software, San Diego, CA). Data were expressed as the mean of three independent experiments and analyzed by Student's t test. A p value of less than 0.05 was considered statistically significant (supplemental Materials and Methods). To determine the role of Nrf1 in apoptosis, we first assessed cell viability in parental and Nrf1-inhibited HaCaT cells after sham or after UVB irradiation. UVB irradiation decreased cell viability in scrambled control (Scr) cells at 30 mJ/cm2 but not 20 mJ/cm2 (Fig. 1A). Nrf1 knockdown significantly increased cell death in cells exposed to UVB at both doses (Fig. 1A). Flow cytometric analysis indicated that Nrf1 knockdown significantly increased UVB-induced apoptosis in both serum-starved and nonstarved HaCaT cells (Fig. 1, B–E). Immunoblot analysis showed that Nrf1 knockdown increased PARP cleavage in UVB-irradiated cells (Fig. 1F), further supporting the conclusion that Nrf1 inhibition sensitized cells to UVB-induced apoptosis. These data indicate that Nrf1 is essential for cell survival upon UVB irradiation. To determine the mechanism of the antiapoptotic function of Nrf1, we analyzed the effect of Nrf1 inhibition on the expression of the key apoptosis-regulating Bcl-2 family members. Nrf1 inhibition had no effect on the antiapoptotic proteins Mcl-1, Bcl-XL, or Bcl-2 or the proapoptotic proteins Bim, Bax, Bad, or Bid (Fig. 2A). However, Nrf1 knockdown increased the expression of the proapoptotic protein Bik (Fig. 2A). Real-time PCR analysis showed that Nrf1 knockdown increased the mRNA levels of Bik (Fig. 2B). To determine whether Bik up-regulation played an active role in apoptosis sensitized by Nrf1 inhibition, we used siRNA targeting Bik to inhibit Bik (Fig. 2C). As compared with NRF1-KD cells transfected with siNC, cells transfected with siBik significantly reduced UVB-induced and basal apoptosis (Fig. 2D), indicating that Nrf1 promotes cell survival, at least partially through suppressing Bik expression. To determine the mechanism of Bik regulation by Nrf1, we analyzed the role of oxidative stress. Knockdown of Nrf1 in HaCaT cells (supplemental Fig. S1A) increased cellular oxidative stress (supplemental Fig. S1B), whereas it had no effect on the mitochondrial redox status (supplemental Fig. S1C). Nrf1 knockdown also increased oxidative stress in UVB-irradiated cells, similar to that in sham-irradiated cells (supplemental Fig. S1D). These data indicate that Nrf1 is essential for maintaining redox homeostasis in HaCaT cells prior to and after UVB damage. Furthermore, Nrf1 knockdown significantly reduced GSH levels (supplemental Fig. S2; p < 0.05, Student's t test). Treating Nrf1 knockdown cells with a cell-permeable form of GSH, GSH-monoethyl ester, increased intracellular GSH levels (supplemental Fig. S2; p < 0.05, Student's t test). In HaCaT cells, hydrogen peroxide (H2O2) increased oxidative stress (Fig. 2E) and Bik protein levels (Fig. 2F). Treatment of Nrf1-inhibited cells with glutathione (GSH) reduced Bik protein levels (Fig. 2G). These data demonstrated that Nrf1 suppresses Bik expression through maintaining glutathione levels. To further determine the role of Nrf1 in the UVB response, we investigated the function of Nrf1 in DNA repair in surviving cells as these cells are the potential sources for cancer formation. As compared with Scr cells, repair of UVB-induced CPDs was significantly reduced in Nrf1 knockdown cells (Fig. 3, A and B; p < 0.05, Student's t test). In contrast, it had no effect on the repair of 6-4PPs (Fig. 3, C and D). To determine the mechanism by which Nrf1 regulates DNA repair, we analyzed the difference in levels of proteins specifically involved in UVB-induced DNA repair, including DDB1, DDB2, HR23B, and XPC (8.Volker M. Moné M.J. Karmakar P. van Hoffen A. Schul W. Vermeulen W. Hoeijmakers J.H. van Driel R. van Zeeland A.A. Mullenders L.H. Sequential assembly of the nucleotide excision repair factors in vivo.Mol. Cell. 2001; 8: 213-224Abstract Full Text Full Text PDF PubMed Scopus (650) Google Scholar, 9.Sugasawa K. Ng J.M. Masutani C. Iwai S. van der Spek P.J. Eker A.P. Hanaoka F. Bootsma D. Hoeijmakers J.H. Xeroderma pigmentosum group C protein complex is the initiator of global genome nucleotide excision repair.Mol. Cell. 1998; 2: 223-232Abstract Full Text Full Text PDF PubMed Scopus (742) Google Scholar, 10.Riedl T. Hanaoka F. Egly J.M. The comings and goings of nucleotide excision repair factors on damaged DNA.EMBO J. 2003; 22: 5293-5303Crossref PubMed Scopus (345) Google Scholar). Nrf1 inhibition had no effect on DDB1, DDB2, or HR23B, whereas it decreased XPC levels (Fig. 4A). Real-time PCR analysis and the promoter reporter assay indicated that Nrf1 knockdown suppressed XPC mRNA levels and its transcription (Fig. 4, B and C). However, it seems that the transcription repressor complex E2F4/p130 had no role in the regulation of XPC by Nrf1 as Nrf1 inhibition decreased nuclear localization of both E2F4 and p130 (supplemental Fig. S3), and mutation of the E2F binding site failed to restore XPC transcription in Nrf1-inhibited cells (Fig. 4D). To determine whether XPC plays an important role, we overexpressed XPC in Nrf1-inhibited cells (Fig. 4E). An increase in XPC levels significantly increased CPD repair (Fig. 4, F and G; p < 0.05, Student's t test), whereas it did not affect 6-4PP repair (Fig. 4, H and I). Our results demonstrate that Nrf1 promotes UVB-induced DNA damage repair through regulating XPC transcription. To determine the mechanism by which Nrf1 regulates XPC, we investigated the role of redox status and GSH availability. Increasing GSH levels in Nrf1-inhibited cells increased XPC protein levels (Fig. 5A). Furthermore, it also significantly increased CPD repair (Fig. 5, B and C; p < 0.05, Student's t test), but had no effect on 6-4PP repair (Fig. 5, D and E). Taken together, these data indicate that a proper GSH level is essential for Nrf1-promoted XPC expression. To determine the involvement of Nrf1 in skin cancer, we analyzed the difference in Nrf1 protein levels between normal human skin and human skin SCC and between sham and UVB-irradiated mouse skin. Nrf1 was expressed in two of the three normal human skin samples, whereas its expression was lost in all three SCCs (Fig. 6A), suggesting that Nrf1 acts as a tumor suppressor. In mouse skin, repeated UVB radiation down-regulated Nrf1 protein levels (Fig. 6B), implying that Nrf1 is a molecular target of UVB in vivo during skin tumorigenesis. In this study, we have identified the transcription factor Nrf1 as a critical player in UVB response, including apoptosis and DNA repair, in epidermal keratinocytes. As compared with normal skin, Nrf1 was down-regulated in human SCC. UVB radiation reduced Nrf1 levels in mouse skin. Nrf1 inhibition sensitized keratinocytes to UVB-induced apoptosis through oxidative stress-dependent Bik up-regulation. Furthermore, Nrf1 is required for UVB-induced DNA damage repair through GSH-dependent regulation of XPC expression. Our findings indicate that Nrf1 is likely a tumor suppressor in skin cancer development. In keratinocytes, Nrf1 is a vital factor to promote cell survival. In both untreated and UVB-irradiated cells, Nrf1 knockdown particularly induced the expression of the proapoptotic protein Bik but had no effect on the other members of the Bcl-2 family members. Containing a conserved BH3 domain, Bik binds and regulates the antiapoptotic Bcl-2 proteins Mcl-1 and Bxl-XL to induce apoptosis (29.Shimazu T. Degenhardt K. Nur-E-Kamal A. Zhang J. Yoshida T. Zhang Y. Mathew R. W" @default.
• W2066742532 created "2016-06-24" @default.
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• W2066742532 date "2012-05-01" @default.
• W2066742532 modified "2023-09-26" @default.
• W2066742532 title "Nrf1 CNC-bZIP Protein Promotes Cell Survival and Nucleotide Excision Repair through Maintaining Glutathione Homeostasis" @default.
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• W2066742532 doi "https://doi.org/10.1074/jbc.m112.363614" @default.
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