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Retinoids: Inducers of Tumor/Growth Suppressors

2004; Elsevier BV; Volume: 123; Issue: 6 Linguagem: Inglês

10.1111/j.0022-202x.2004.23533.x

1523-1747

Sunil Nagpal,

Histone Deacetylase Inhibitors Research

all-trans retonoic acid retinoic acid receptors retinoic acid response elements retinoid X receptors The process of carcinogenesis involves genomic disasters that result in inactivation or underexpression of tumor/growth suppressor genes and/or overexpression of oncogenes. Decreased levels of tumor suppressors may result from DNA hypermethylation, mutations, translocation, or transcriptional repression. Therefore, logically, one of the most promising chemopreventive and chemotherapeutic targets appears to be the activation of tumor suppressors. The expression of tumor suppressor genes could be activated by reversal of epigenetic gene silencing by using DNA methyltransferase inhibitors and/or histone deacetylase inhibitors (Zelent et al., 2004Zelent A. Waxman S. Carducci M. Wright J. Zweibel J. Gore S.D. State of the translational science: Summary of Baltimore workshop on gene re-expression as a therapeutic target in cancer. January 2003.Clin Cancer Res. 2004; 10: 4622-4629Crossref PubMed Scopus (18) Google Scholar). Alternatively, the expression of multiple tumor suppressor genes in cancer cells could be induced by ligand-dependent transcription factors, such as nuclear receptors for the active form of vitamin A, all-trans retinoic acid (ATRA) that belong to the superfamily of steroid/thyroid hormone nuclear receptors. Retinoids (ATRA and its synthetic analogs) exert their anti-proliferative and differentiation effects through retinoic acid receptors (RAR) that function as heterodimers with retinoid X receptors (RXR). RXR, a nuclear receptor for 9-cis retinoic acid, is an obligate partner of RAR in mediating retinoid action. Three distinct isotypes of RAR (α, β and γ) and RXR (α, β and γ) are encoded by separate genes. The RXR–RAR heterodimer binds to retinoic acid response elements (RARE) present in the promoter regions of responsive genes. RARE are direct repeat (DR) of 5′-RGKTCA-3′ motifs (where R is A or G and K is G or T) separated by either two (DR-2) or five (DR-5) nucleotides (Nagpal and Chandraratna, 1998Nagpal S. Chandraratna R.A. Vitamin A and regulation of gene expression.Curr Opin Clin Nutr Metab Care. 1998; 1: 341-346Crossref PubMed Scopus (34) Google Scholar). Like other nuclear receptors, the RAR protein is modular in nature and can be divided into three distinct functional regions. The N-terminal region contains a ligand-independent transactivation function, AF-1. The central region contains the DNA-binding domain consisting of two C2–C2 type zinc fingers that target the receptor to the DNA. The C-terminal multifunctional region contains the ligand-binding domain, the heterodimerization domain, and a ligand-dependent transactivation function, AF-2. The RXR–RAR heterodimer modulates the expression of retinoid-responsive genes in two different ways. It either positively regulates the expression of certain genes by binding to the RARE present in their promoter regions or negatively regulates the expression of other genes by antagonizing the enhancer action of transcription factors AP1 or NF-IL6 (Nagpal and Chandraratna, 1998Nagpal S. Chandraratna R.A. Vitamin A and regulation of gene expression.Curr Opin Clin Nutr Metab Care. 1998; 1: 341-346Crossref PubMed Scopus (34) Google Scholar). The expression of a number of tumor/growth suppressor genes is induced by ATRA and its synthetic analogs. These genes include RARβ2, RARγ, RARα2, tazarotene-induced gene 1 (TIG1), TIG2, TIG3, p21/Cip1, p27/Kip1, thioredoxin reductase, interferon-regulatory factor 1, IGFBP3, and tumor suppressor in lung cancer 1. In this issue,Papoutsaki et al., 2004Papoutsaki M. Lanza M. Marinari B. et al.The p73 gene is an antitumoral target of the RARb/g selective retinoid tazarotene.J Invest Dermatol. 2004; 123: 1162-1168Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar, very elegantly demonstrate that tazarotene, an RARβ/γ selective synthetic retinoid that has shown efficacy in basal cell carcinoma (Orlandi et al., 2004Orlandi A. Bianchi L. Costanzo A. Campione E. Giusto Spagnoli L. Chimenti S. Evidence of increased apoptosis and reduced proliferation in basal cell carcinomas treated with tazarotene.J Invest Dermatol. 2004; 122: 1037-1041Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar), induces the expression of the proapoptotic isoform TAp73β of tumor suppressor p73 in immortalized keratinocytes. Interestingly, tazarotene treatment of keratinocytes also resulted in the suppression of the p73 anti-apoptotic truncated isoform ΔNp73. Paradoxically, p73 gene encodes for both full-length (TA) tumor suppressor proteins (p73α, β, γ etc.), and dominant negative, anti-apoptotic truncated proteins (ΔNp73α and β) that lack the N-terminal transactivation domain present in TAp73 (Moll and Slade, 2004Moll U.M. Slade N. p63 and p73: Roles in development and tumor formation.Mol Cancer Res. 2004; 2: 371-386PubMed Google Scholar). ΔNp73 proteins can block the function of full-length p53, p63, and p73 wild-type proteins (Moll and Slade, 2004Moll U.M. Slade N. p63 and p73: Roles in development and tumor formation.Mol Cancer Res. 2004; 2: 371-386PubMed Google Scholar). Since p73 encodes both tumor suppressor (TA) and oncogenic (ΔNp73) isoforms, the relative ratio of TAp73 and ΔNp73 appears to be crucial for dictating cell apoptosis or survival. ΔNp73 is in fact overexpressed in breast cancer, ovarian cancer, vulval cancer, neuroblastomas, nasopharyngeal cancer, and bladder cancer (Melino et al., 2002Melino G. De Laurenzi V. Vousden K.H. p73: Friend or foe in tumorigenesis.Nat Rev Cancer. 2002; 2: 605-615Crossref PubMed Scopus (502) Google Scholar;Stiewe and Putzer, 2002Stiewe T. Putzer B.M. Role of p73 in malignancy: Tumor suppressor or oncogene?.Cell Death Differ. 2002; 9: 237-245Crossref PubMed Scopus (142) Google Scholar;Moll and Slade, 2004Moll U.M. Slade N. p63 and p73: Roles in development and tumor formation.Mol Cancer Res. 2004; 2: 371-386PubMed Google Scholar). Decreased expression of p73 has been reported in hematological malignancies (Moll and Slade, 2004Moll U.M. Slade N. p63 and p73: Roles in development and tumor formation.Mol Cancer Res. 2004; 2: 371-386PubMed Google Scholar). Further, specific inactivation of TAp73 leads to chemoresistance and increased expression of ΔNp73 can inhibit chemotherapy-mediated cell death (Moll and Slade, 2004Moll U.M. Slade N. p63 and p73: Roles in development and tumor formation.Mol Cancer Res. 2004; 2: 371-386PubMed Google Scholar). In other words, TAp73:ΔNp73 ratio is also important for chemotherapy outcome of tumor cells. Therefore, therapeutic modulation of TAp73:ΔNp73 ratio might prove beneficial in cancers. Agents that either increase TAp73 or decrease ΔNp73 expression have the potential to become anti-cancer agents either as stand-alone or combination therapies with other therapeutic agents. Observation by Papoutsaki et al, not only provides a plausible mechanism of action of retinoid-mediated growth inhibition but also an opportunity to develop novel molecular assays for the identification of small molecule RAR modulators for the treatment of cancers. They clearly demonstrate that the P1 promoter of p73 gene contains multiple RARE-like motifs that impart positive retinoid responsiveness to P1p73 promoter and result in increased levels of TAp73. P1p73 RARE need, however, to be characterized by genetic and biochemical studies. In the P2 promoter, Papoutsaki et al, have identified a canonical AP1 motif, and promoter deletion studies identify it to be responsible for negative regulation of ΔNp73 expression by tazarotene. Retinoid-mediated antagonism of AP1 action that has been described for other AP1-responsive genes (Nicholson et al., 1990Nicholson R. Mader S. Nagpal S. Leid M. Rochette-Egly C. Chambon P. Negative regulation of the rat stromelysin gene promoter by retinoic acid is mediated by an AP1 binding site.EMBO J. 1990; 9: 4443-4454Crossref PubMed Scopus (316) Google Scholar) may explain the inhibition of ΔNp73 expression by tazarotene. Therefore, apart from c-fos, c-myc, and ornithine decarboxylase (Nagpal and Chandraratna, 1998Nagpal S. Chandraratna R.A. Vitamin A and regulation of gene expression.Curr Opin Clin Nutr Metab Care. 1998; 1: 341-346Crossref PubMed Scopus (34) Google Scholar), ΔNp73 is another oncogene whose expression is negatively regulated by an RAR ligand. In clinic, topical treatment of basal cell carcinomas with tazarotene gel (0.1%) resulted in tumor regression in 77% and complete healing in 47% of all treated lesions. Tumor responses were accompanied by increased apoptosis as well as induction of RARβ2 and bax in treated basal cell carcinomas (Orlandi et al., 2004Orlandi A. Bianchi L. Costanzo A. Campione E. Giusto Spagnoli L. Chimenti S. Evidence of increased apoptosis and reduced proliferation in basal cell carcinomas treated with tazarotene.J Invest Dermatol. 2004; 122: 1037-1041Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar). Using RNAi technology, authors demonstrate that the treatment of transformed keratinocytes with siRNA specific for ΔNp73 resulted in increased basal and retinoid-mediated induction of the bax promoter reporter expression and apoptosis of HaCat keratinocytes. On the other hand downregulation of all p73 forms abolished retinoid-dependent expression of the bax reporter expression and HaCat apoptosis. These results indicate that the retinoid-mediated apoptosis of HaCat cells is critically dependent upon retinoid-mediated regulation of TAp73:ΔNp73 ratio. Although retinoids have shown immense translational potential because of their activities in vitro and in vivo, their use in clinic has resulted in limited responses. Part of the problem is that newer generation of retinoids that are receptor and/or function selective have not been tried in clinic. Additionally, most of the retinoids studied in clinic were stand-alone therapies and not geared toward identifying an ideal therapeutic regimen where they are used in combination with other anti-cancer or disease-modifying agents. Recent developments in the understanding of gene regulation by nuclear receptors and chromatin organization have increased scientific interest in the identification of agents that modulate gene expression via chromatin re-organization. Retinoids fit the profile of these agents since they can induce the expression of a number of tumor/growth suppressor genes, which otherwise are transcriptionally silent in cancer cells. A number of tumor/growth suppressor genes (e.g., TIG1, RARβ, etc.), however, are also epigenetically silenced because of DNA hypermethylation in their promoter regions (Youssef et al., 2004aYoussef E.M. Chen X.Q. Higuchi E. Kondo Y. Garcia-Manero G. Lotan R. Issa J.P. Hypermethylation and silencing of the putative tumor suppressor Tazarotene-induced gene 1 in human cancers.Cancer Res. 2004; 64: 2411-2417Crossref PubMed Scopus (74) Google Scholar). Since loss of RARβ has been linked to retinoid resistance and RARβ is a tumor suppressor as well as an intracellular effector of retinoid action, a therapy involving a combination of retinoids and histone methyltransferase or histone deacetylase inhibitors may show synergistic efficacy in cancers. As a proof of concept, reversal of transcriptional silencing of RARβ2 gene and increased growth inhibition has been observed in primary head and neck squamous cell carcinoma after treatment of tumor cells with a combination of ATRA and a demethylating agent, 5-aza-2′-deoxycytidine (Youssef et al., 2004bYoussef E.M. Lotan D. Issa J.P. et al.Hypermethylation of the retinoic acid receptor-beta(2) gene in head and neck carcinogenesis.Clin Cancer Res. 2004; 10: 1733-1742Crossref PubMed Scopus (116) Google Scholar). Use of ATRA in acute promyelocytic leukemia has been a dream therapy and in my personal view, now we have an opportunity to extend that dream to other cancers by combining newer generation of receptor selective retinoids with chromatin-modifying enzyme inhibitors.