Article

Arsenic-induced malignant transformation of human keratinocytes: Involvement of Nrf2

Laboratory of Comparative Carcinogenesis, NCI at NIEHS, NIH, Research Triangle Park, NC 27709, USA.
Free Radical Biology and Medicine (Impact Factor: 5.71). 07/2008; 45(5):651-8. DOI: 10.1016/j.freeradbiomed.2008.05.020
Source: PubMed

ABSTRACT Arsenic is a well-known human skin carcinogen but the underlying mechanisms of carcinogenesis are unclear. Transcription factor Nrf2-mediated antioxidant response represents a critical cellular defense mechanism, and emerging data suggest that constitutive activation of Nrf2 contributes to malignant phenotype. In the present study when an immortalized, nontumorigenic human keratinocyte cell line (HaCaT) was continuously exposed to an environmentally relevant level of inorganic arsenite (100 nM) for 28 weeks, malignant transformation occurred as evidenced by the formation of highly aggressive squamous cell carcinoma after inoculation into nude mice. To investigate the mechanisms involved, a broad array of biomarkers for transformation were assessed in these arsenic-transformed cells (termed As-TM). In addition to increased secretion of matrix metalloproteinase-9 (MMP-9), a set of markers for squamous differentiation and skin keratinization, including keratin-1, keratin-10, involucrin, and loricrin, were significantly elevated in As-TM cells. Furthermore, As-TM cells showed increased intracellular glutathione and elevated expression of Nrf2 and its target genes, as well as generalized apoptotic resistance. In contrast to increased basal Nrf2 activity in As-TM cells, a diminished Nrf2-mediated antioxidant response induced by acute exposure to high doses of arsenite or tert-butyl hydroxyquinone occurred. The findings that multiple biomarkers for malignant transformation observed in As-TM cells, including MMP-9 and cytokeratins, are potentially regulated by Nrf2 suggest that constitutive Nrf2 activation may be involved in arsenic carcinogenesis of skin. The weakened Nrf2 activation in response to oxidative stressors observed in As-TM cells, coupled with acquired apoptotic resistance, would potentially have increased the likelihood of transmittable oxidative DNA damage and fixation of mutational/DNA damage events.

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    • "However, other transcription factors involved in the regulation of TGIF cannot be ruled out. For example, Nrf2 is involved in arsenic-induced oxidative stress and cellular transformation (Pi et al., 2008; Zhao et al., 2012; Sumi et al., 2013). Furthermore, Nrf2 can regulate several genes that are involved in the cytoprotective response against oxidative stress (Itoh et al., 2010). "
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    ABSTRACT: Arsenic trioxide (ATO) is a multi-target drug approved by the Food and Drug Administration as the first-line chemotherapeutic agent for the treatment of acute promyelocytic leukemia. In addition, several clinical trials are being conducted with arsenic-based drugs for the treatment of other hematological malignancies and solid tumors. However, ATO's modest clinical efficacy on some cancers, and potential toxic effects on humans have been reported. Determining how best to reduce these adverse effects while increasing its therapeutic efficacy is obviously a critical issue. Previously, we demonstrated that the JNK-induced complex formation of phosphorylated c-Jun and TG-interacting factor (TGIF) antagonizes ERK-induced cyclin-dependent kinase inhibitor CDKN1A (p21(WAF1/CIP1)) expression and resultant apoptosis in response to ATO in A431 cells. Surprisingly, at low-concentrations (0.1~0.2 μM), ATO increased cellular proliferation, migration and invasion, involving TGIF expression, however, at high-concentrations (5~20 μM), ATO induced cell apoptosis. Using a promoter analysis, TGIF was transcriptionally regulated by ATO at the FOXO3A binding site (-1486 to -1479 bp) via the c-Src/EGFR/AKT pathway. Stable overexpression of TGIF promoted advancing the cell cycle into S phase, and attenuated 20 μM ATO-induced apoptosis. Furthermore, blockage of the AKT pathway enhanced ATO-induced CDKN1A expression and resultant apoptosis in cancer cells, but overexpression of AKT1 inhibited CDKN1A expression. Therefore, we suggest that TGIF is transcriptionally regulated by the c-Src/EGFR/AKT pathway, which plays a role as a negative regulator in antagonizing ATO-induced CDKN1A expression and resultant apoptosis. Suppression of these antagonistic effects might be a promising therapeutic strategy toward improving clinical efficacy of ATO. Copyright © 2015. Published by Elsevier Inc.
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    • "Involvement of NRF2 in the arsenic-induced malignant transformation of keratinocytes has also been suggested [Pi et al., 2008]. In contrast to NF-jB, however, the activation of the NRF2 pathway provides protection against the effects of arsenic trioxide induced oxidative stress in keratinocytes [Pi et al., 2008] and immortalized human bladder urothelial cells [Wang et al., 2007a]. "
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    • "Very recently it was shown that long isoforms of NRF1 are involved in arsenic-induced apoptosis in human keratinocytes (Zhao et al., 2011), which are the major target tissues of arsenic toxicity as the classical symptoms induced by arsenic exposure are all dermatological in nature. Long term exposure to arsenic at a dose comparable to in vivo exposure, to an immortalized, nontumorigenic human keratinocyte cell line (HaCaT) was found to greatly increase the stability of nuclear protein kinase B (PKB), an antiapoptotic molecule (Pi et al., 2005) and also to activate transcription factor Nrf2 constitutively, which in turn led to marked apoptotic resistance and thus enhancing the possibility of arsenic involvement in skin carcinogenesis (Pi et al., 2008). In fact, it has been demonstrated using an interaction analysis algorithm that several of the top 25 arsenic-interacting genes belong to the cell cycle, cell proliferation and apoptotic pathway (including BCL2, CCND1, CDKN2A, VEGFA, CDNK1A, GADD45A, BAX) and many of them interact with several arsenic compounds (Davis et al., 2008). "
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