Expression of retinoic acid receptor beta is associated with inhibition of keratinization in human head and neck squamous carcinoma cells

SIRT1 as a potential key regulator for mediating apoptosis in oropharyngeal cancer using cyclophosphamide and all-trans retinoic acid

Although cyclophosphamide (CTX) has been used for recurrent or metastatic head and neck cancers, resistance is usually expected. Thus, we conducted this study to examine the effect of adding all-trans retinoic acid (ATRA) to CTX, to increase efficacy of CTX and reduce the risk of resistance developed. In this study, we investigated the combined effect of ATRA and CTX on the expression of apoptotic and angiogenesis markers in oropharyngeal carcinoma cell line (NO3), and the possible involved mechanisms. ATRA and CTX in combination significantly inhibited the proliferation of NO3 cells. Lower dose of CTX in combination with ATRA exhibited significant cytotoxicity than that of CTX when used alone, implying lower expected toxicity. Results showed that ATRA and CTX modulated oxidative stress; increased NOx and MDA, reduced GSH, and mRNA expression of Cox-2, SIRT1 and AMPK. Apoptosis was induced through elevating mRNA expressions of Bax and PAR-4 and suppressing that of Bcl-xl and Bcl-2, parallel with increased caspases 3 and 9 and decreased VEGF, endothelin-1 and CTGF levels. The primal action of the combined regimen on inflammatory signaling highlights its impact on cell death in NO3 cell line which was mediated by oxidative stress associated with apoptosis and suppression of angiogenesis.

The Endogenous Retinoic Acid Receptor Pathway Is Exploited by Mycobacterium tuberculosis during Infection, Both In Vitro and In Vivo

Retinoic acid (RA) is a fundamental vitamin A metabolite involved in regulating immune responses through the nuclear RA receptor (RAR) and retinoid X receptor. While performing experiments using THP-1 cells as a model for Mycobacterium tuberculosis infection, we observed that serum-supplemented cultures displayed high levels of baseline RAR activation in the presence of live, but not heat-killed, bacteria, suggesting that M. tuberculosis robustly induces the endogenous RAR pathway. Using in vitro and in vivo models, we have further explored the role of endogenous RAR activity in M. tuberculosis infection through pharmacological inhibition of RARs. We found that M. tuberculosis induces classical RA response element genes such as CD38 and DHRS3 in both THP-1 cells and human primary CD14+ monocytes via a RAR-dependent pathway. M. tuberculosis-stimulated RAR activation was observed with conditioned media and required nonproteinaceous factor(s) present in FBS. Importantly, RAR blockade by (4-[(E)-2-[5,5-dimethyl-8-(2-phenylethynyl)-6H-naphthalen-2-yl]ethenyl]benzoic acid), a specific pan-RAR inverse agonist, in a low-dose murine model of tuberculosis significantly reduced SIGLEC-F+CD64+CD11c+high alveolar macrophages in the lungs, which correlated with 2× reduction in tissue mycobacterial burden. These results suggest that the endogenous RAR activation axis contributes to M. tuberculosis infection both in vitro and in vivo and reveal an opportunity for further investigation of new antituberculosis therapies.

Anthelmintic mebendazole enhances cisplatin's effect on suppressing cell proliferation and promotes differentiation of head and neck squamous cell carcinoma (HNSCC)

Head and neck squamous cell carcinoma (HNSCC) is one of the most common and aggressive types of human cancers worldwide. Nearly a half of HNSCC patients experience recurrence within five years of treatment and develop resistance to chemotherapy. Thus, there is an urgent clinical need to develop safe and novel anticancer therapies for HNSCC. Here, we investigate the possibility of repurposing the anthelmintic drug mebendazole (MBZ) as an anti-HNSCC agent. Using the two commonly-used human HNSCC lines CAL27 and SCC15, we demonstrate MBZ exerts more potent anti-proliferation activity than cisplatin in human HNSCC cells. MBZ effectively inhibits cell proliferation, cell cycle progression and cell migration, and induces apoptosis of HNSCC cells. Mechanistically, MBZ can modulate the cancer-associated pathways including ELK1/SRF, AP1, STAT1/2, MYC/MAX, although the regulatory outcomes are context-dependent. MBZ also synergizes with cisplatin in suppressing cell proliferation and inducing apoptosis of human HNSCC cells. Furthermore, MBZ is shown to promote the terminal differentiation of CAL27 cells and keratinization of CAL27-derived xenograft tumors. Our results are the first to demonstrate that MBZ may exert its anticancer activity by inhibiting proliferation while promoting differentiation of certain HNSCC cancer cells. It's conceivable the anthelmintic drug MBZ can be repurposed as a safe and effective agent used in combination with other frontline chemotherapy drugs such as cisplatin in HNSCC treatment.

Functional activation of PPARγ in human upper aerodigestive cancer cell lines

Upper aerodigestive cancer is an aggressive malignancy with relatively stagnant long-term survival rates over 20 yr. Recent studies have demonstrated that exploitation of PPARγ pathways may be a novel therapy for cancer and its prevention. We tested whether PPARγ is expressed and inducible in aerodigestive carcinoma cells and whether it is present in human upper aerodigestive tumors. Human oral cancer CA-9-22 and NA cell lines were treated with the PPAR activators eicosatetraynoic acid (ETYA), 15-deoxy-δ- 12,14-prostaglandin J2 (PG-J2), and the thiazolidinedione, ciglitazone, and evaluated for their ability to functionally activate PPARγ luciferase reporter gene constructs. Cellular proliferation and clonogenic potential after PPARγ ligand treatment were also evaluated. Aerodigestive cancer specimens and normal tissues were evaluated for PPARγ expression on gene expression profiling and immunoblotting. Functional activation of PPARγ reporter gene constructs and increases in PPARγ protein were confirmed in the nuclear compartment after PPARγ ligand treatment. Significant decreases in cell proliferation and clonogenic potential resulted from treatment. Lipid accumulation was induced by PPARγ activator treatment. 75% of tumor specimens and 100% of normal control tissues expressed PPARγ RNA, and PPARγ protein was confirmed in 66% of tumor specimens analyzed by immunoblotting. We conclude PPARγ can be functionally activated in upper aerodigestive cancer and that its activation downregulates several features of the neoplastic phenotype. PPARγ expression in human upper aerodigestive tract tumors and normal cells potentially legitimizes it as a novel intervention target in this disease. © 2016 Wiley Periodicals, Inc.

In vivo long-term effects of retinoic acid exposure in utero on induced tumours in adult mouse skin

BACKGROUND

Retinoic acid (RA) and its analogues (retinoids) are promising agents in skin cancer prevention following either topical application or oral administration. However, long-term in vivo effects of RA on chemically induced hyperplastic epidermal foci in adult mouse skin have also been described, casting some doubt with regard to its chemopreventive activity.

HYPOTHESIS/OBJECTIVES

To characterize chemically induced skin tumours and to investigate the in vivo long-term action and preventive effect of RA on adult mouse skin carcinogenesis.

ANIMALS

Fifty-six adult Naval Medical Research Institute mice, exposed (n = 28) or not exposed (n = 28) to RA in utero.

METHODS

Mice were treated with a standard two-stage skin carcinogenesis protocol, which included an initiating application of 7,12-dimethylbenz(a)anthracene followed by promotion with 12-O-tetradecanoylphorbol 13-acetate.

RESULTS

Retinoic acid administered to pregnant mice showed a long-term inhibitory action on cell differentiation and development of chemically induced tumours on the adult skin of their offspring, as well as a stimulatory effect on cell proliferation and expression of an early marker of malignant progression (keratin 13).

CONCLUSIONS AND CLINICAL IMPORTANCE

The results suggest that RA exposure in utero confers long-lasting effects on adult mouse skin carcinogenesis. These include chemopreventive activity (reduced number of tumours), as well as enhancement of squamous papilloma progression, which appears to be due to enhanced keratinocyte proliferation and suppression of epidermal maturation. The clinical significance of these findings is not known for other routes of RA administration at this time.

In vivo long-term effects of retinoic acid exposure in utero on induced hyperplastic epidermal foci in murine skin

Adult Naval Medical Research Institute (NMRI) mice, after prenatal exposure to retinoic acid (RA), were treated with a standard two-stage skin carcinogenesis regime to characterize hyperplastic epidermal foci that precede the appearance of cutaneous papillomas, and to investigate the in vivo long-term action of RA on adult mouse skin treated with DMBA (7,12 dimethyl benz[a]anthracene) and TPA (12-O-tetradecanoylphorbol 13-acetate). The results demonstrate that RA administered to pregnant mice had a long-term inhibitory action on the cell differentiation and development of hyperplastic lesions occurring prior to cancer on the adult skin of their offspring as well as a stimulatory effect on cell proliferation of these hyperplastic lesions.

Mouse epidermal development: effects of retinoic acid exposure in utero

Epidermal morphogenesis was studied in vivo following prenatal exposure to retinoic acid (RA). In pregnant mice, a single oral dose of RA on day 11.5 of gestation failed to induce histological changes in fetal epidermal development except in epidermal thickness. Epidermal thickness increased from 16.5 days post-coitum (dpc) onwards, and temporal and spatial epidermal modifications in keratins K5 and K14 related to proliferative activity of keratinocytes were observed. An RA effect on cell proliferation was supported by a statistically significant increase in the number of epidermal S-phase cells, containing BrdU-incorporated DNA in RA-exposed mice compared with nonexposed animals. The prolonged in utero action of RA on epidermal proliferative activity in fetuses and newborns suggests a long-term RA effect that may play a role on the development and evolution of diseases in adult skin.

Hypermethylation of the retinoic acid receptor-beta(2) gene in head and neck carcinogenesis

PURPOSE

Retinoic acid receptor-beta(2) (RAR-beta(2)) expression is suppressed in oral premalignant lesions and head and neck squamous cell carcinomas (HNSCCs). This study was conducted to determine whether RAR-beta(2) gene expression in such lesions can be silenced by promoter methylation.

EXPERIMENTAL DESIGN

RAR-beta(2) methylation was analyzed in DNA samples from 22 pairs of primary HNSCC and adjacent normal epithelium, 124 samples of oral leukoplakia, and 18 HNSCC cell lines using methylation-specific PCR. RAR-beta(2) promoter was methylated in 67, 56, and 53% of HNSCC tumors, HNSCC cell lines, and microdissected oral leukoplakia specimens, respectively. RAR-beta(2) hypermethylation was confirmed by sodium bisulfite-PCR combined with restriction enzyme digestion analysis and by random cloning and sequencing of bisulfite-treated DNA isolates.

RESULTS

Significantly higher RAR-beta(2) hypermethylation levels were found in tumor tissue compared with adjacent normal tissue (P = 0.002). RAR-beta(2) methylation in the cell lines was correlated with loss of RAR-beta(2) expression (P = 0.013) and inversely related to the presence of mutated p53 (P = 0.025). The demethylating agent 5-aza-2'-deoxycytidine (5-aza-CdR) restored RAR-beta(2) inducibility by all-trans-retinoic acid (ATRA) in some of the cell lines, which posses a methylated RAR-beta(2) promoter. In some cell lines, this effect was associated with increased growth inhibition after combined treatment with 5-aza-CdR and ATRA.

CONCLUSIONS

RAR-beta(2) silencing by methylation is an early event in head and neck carcinogenesis; 5-Aza-CdR can restore RAR-beta(2) inducibility by ATRA in most cell lines, and the combination of 5-aza-CdR and ATRA is more effective in growth inhibition than single agents.

Anti-tumor effect of vitamin A and D on head and neck squamous cell carcinoma

OBJECTIVES

Vitamin A and D(3) have a very strong differentiation induction effect.

STUDY DESIGN

We examined the anti tumor effect on head and neck squamous cell carcinoma (HNSCC) by treatment with several vitamins having strong differentiation induction effects in vitro.

METHODS

We used KB cell that an oral floor squamous cell carcinoma, vitamins as all-trans retinoic acid (ATRA), 4-[3,5-bis (trimethylsilyl) benzamido] benzoic acid (TAC-101), 1alpha,25(OH)(2)D(3) (calcitriol) and 22-oxa-1,25-(OH)(2)D(3) (OCT). We determined receptors of vitamin A and D(3) using RT-PCR. Furthermore, we investigated the proliferation of tumor cells in concentration dependency using [3H]TdR uptake method, apoptosis and apoptosis related factors using TUNEL method and real-time PCR, cell cycle changes using flow cytometry, changing of the sensitivity of using MTT method, cytokine production and the angiogenesis factor using ELISA, by treatment with these vitamins.

RESULTS

The deficit of RAR-beta was found in the KB cell. Each vitamin suppressed the cell proliferation, induced apoptosis, and cell cycle arrest, upregulated sensitivity of the chemotherapeutics drugs and downregulated several angiogenesis factors and an apoptotic factor; survivin.

CONCLUSIONS

These results support the idea that vitamin A, D(3) and their derivatives are useful for preventing and/or treating patients with HNSCC.

Biological relevance of terpenoids. Overview focusing on mono-, di- and tetraterpenes

The role of fruit and vegetables in human nutrition and public health are taken into account in most nutritional recommendations. Fruit and vegetables contain an abundance of phenolic substances, terpenoids and other natural antioxidants that have been associated with protection from and treatment of chronic diseases such as heart disease or cancer. Terpenoids are a group of substances which occur in nearly every natural food. Their main subclasses discussed as beneficial to maintain and improve health are monoterpenes (like limonene, carvone or carveol), diterpenes (including the retinoids), and tetraterpenes (which include all different carotenoids like alpha- and beta-carotene, lutein, lycopene, zeaxanthine and cryptoxanthine). To be discussed as health promoting or biofunctional, the significant impact of a substance either on human metabolism or on well-defined and appropriate biomarkers must be shown. Based on the latter point, this paper reviews the literature on mono-, di- and tetraterpenes, with special focus on their impact on human health to answer the question of their biofunctionality. Special emphasis will be placed on their different mode of action, e.g. to affect oxidative stress, carcinogenesis and cardiovascular diseases.

Rap1A and rap1B ras-family proteins are prominently expressed in the nucleus of squamous carcinomas: nuclear translocation of GTP-bound active form

We recently showed that rap1 regulates growth and proliferation in normal keratinocytes, which provoked us to investigate its expression and regulation in malignant cells. Rap1 is variably expressed in whole cell lysates of squamous cell carcinoma (SCC) cell lines. Immunoblot analysis of nuclear and cytosolic fractions and immunohistochemistry revealed that in addition to cytoplasmic expression, SCC cells also exhibit prominent punctate rap1 expression in the nucleus. This unexpected nuclear distribution was confirmed by the evaluation of human oral cancer specimens by immunohistochemistry, which showed both nuclear and cytoplasmic localization. Cytoplasmic rap1 expression was observed mostly in large differentiated cells, whereas nuclear localization was found in morphologically less differentiated cells. Quantitative reverse transcriptase polymerase chain reaction and Northern blot analysis showed that both rap1A and rap1B are expressed in SCC cell lines although rap1B signals are more prominent. Transfection with enhanced GFP-tagged constitutively active and inactive forms of rap1B demonstrated that the active GTP-bound form translocates to the nucleus whereas inactive rap1B(GDP) is retained in the cytoplasm, much of which is in a perinuclear distribution. Furthermore, growth factors induce nuclear translocation of rap1 in oral cancer cells. This novel discovery that active, GTP-bound rap1 translocates to the nucleus makes it only the second of over 100 small GTP-binding proteins to be identified in the nucleus, and the striking prominence of rap1 expression in the nucleus of SCC cells suggests that activated rap1 plays a role in the malignant process.

Retinoic acid signaling through PI 3-kinase induces differentiation of human endometrial adenocarcinoma cells

The specific signals required for actin polymerization in response to extracellular factors remain unknown. However, in many cell types, there is a correlation between actin polymerization, activation of phosphatidylinositol 3-kinase (PI 3-kinase), and the production of the second messenger phosphatidylinositol-3,4,5-triphosphate. Increased levels of PI 3-kinase have been detected during cell growth and transformation. However, PI 3-kinase is also activated during differentiation, suggesting that PI 3-kinase and its lipid products also play a role in the regulation of cellular differentiation. The newly characterized CAC-1 cell line established from a poorly differentiated human endometrial adenocarcinoma (Exp. Mol. Pathol. 69 (2000), 175) was used as a model to investigate the role of PI 3-kinase in differentiation induction. CAC-1 cells differentiated upon treatment with pharmacological doses of retinoids (1 micro M of 13-cis or all-trans), evidenced by actin filament reorganization, and cell enlargement. PI 3-kinase staining is primarily localized to perinuclear regions in untreated cells. However, retinoic acid treatment induced PI 3-kinase to relocalize throughout the cytoplasm. Subcellular fractionation and Western blotting confirmed that PI 3-kinase decreased in the particulate fraction, concurrent with retinoid-induced differentiation. Interestingly, pretreatment with the PI 3-kinase inhibitor wortmannin (100 nM) prior to retinoic acid treatment prevented retinoic acid-induced actin reorganization and cell enlargement. To distinuish whether retinoid regulation of PI 3-kinase is mediated through traditional nuclear retinoic acid receptors, the levels of retinoic acid receptor-beta (RAR-beta) protein were evaluated. Retinoid treatment did not alter RAR-beta protein levels compared to controls. These data suggest that PI 3-kinase activity and cytoplasmic relocalization are required for retinoid-induced differentiation of poorly differentiated human endometrial adenocarcinoma cells.

Anticancer effect of retinoic acid via AP-1 activity repression is mediated by retinoic acid receptor alpha and beta in gastric cancer cells

To uncover the mechanisms relating to the anticancer effect of retinoic acids in gastric cancer cells, the mediation of activator protein-1 (AP-1) activity repression by retinoic acid receptors (RARs) was investigated. All-trans retinoic acid (ATRA) inhibited AP-1 activity in BGC-823 cells (RARalpha(+), RARbeta(+)), but not in MKN-45 cells (RARalpha(lo), RARbeta(-)). Transient transfection of RARbeta expression vector into MKN-45 cells significantly resulted in direct repression of AP-1 activity in a receptor concentration-dependent manner, and this could be strengthened by ATRA. Stable transfection of RARbeta into MKN-45 cells directly inhibited cell growth and colony formation, and ATRA also enhanced these effects. Transient transfection of RARalpha into MKN-45 cells however, displayed receptor concentration-dependent AP-1 activity inhibition only in the presence of ATRA. Stable transfection of RARalpha into MKN-45 cells resulted in ATRA-dependent inhibition of cell growth and colony formation. For AP-1 binding activity induced by TPA, the repressive effect of ATRA was only observed in BGC-823 and RARalpha and RARbeta stably transfected MKN-45 cells, but not in intact MKN-45 cells. This indicates the necessity for sufficient cellular RARalpha and/or RARbeta in order for AP-1 activity repression to occur. Deletion of DNA binding domain (DBD) of RARbeta, but not ligand binding domain (LBD), eliminated the anti-AP-1 function of RARbeta. It is therefore concluded that both RARalpha and RARbeta are mediators in the anticancer function of ATRA via AP-1 activity inhibition, and that RARbeta, not RARalpha, can inhibit AP-1 activity to a certain extent directly by itself. Thus DBD, not LBD, is critical for anti-AP-1 activity.

Chromosomal integration of retinoic acid response elements prevents cooperative transcriptional activation by retinoic acid receptor and retinoid X receptor

All-trans-retinoic acid receptors (RAR) and 9-cis-retinoic acid receptors (RXR) are nuclear receptors known to cooperatively activate transcription from retinoid-regulated promoters. By comparing the transactivating properties of RAR and RXR in P19 cells using either plasmid or chromosomal reporter genes containing the mRAR beta 2 gene promoter, we found contrasting patterns of transcriptional regulation in each setting. Cooperativity between RXR and RAR occurred at all times with transiently introduced promoters, but was restricted to a very early stage (<3 h) for chromosomal promoters. This time-dependent loss of cooperativity was specific for chromosomal templates containing two copies of a retinoid-responsive element (RARE) and was not influenced by the spacing between the two RAREs. This loss of cooperativity suggested a delayed acquisition of RAR full transcriptional competence because (i) cooperativity was maintained at RAR ligand subsaturating concentrations, (ii) overexpression of SRC-1 led to loss of cooperativity and even to strong repression of chromosomal templates activity, and (iii) loss of cooperativity was observed when additional cis-acting response elements were activated. Surprisingly, histone deacetylase inhibitors counteracted this loss of cooperativity by repressing partially RAR-mediated activation of chromosomal promoters. Loss of cooperativity was not correlated to local histone hyperacetylation or to alteration of constitutive RNA polymerase II (RNAP) loading at the promoter region. Unexpectedly, RNAP binding to transcribed regions was correlated to the RAR activation state as well as to acetylation levels of histones H3 and H4, suggesting that RAR acts at the mRAR beta promoter by triggering the switch from an RNA elongation-incompetent RNAP form towards an RNA elongation-competent RNAP.

Differential effects of chromosome 3p deletion on the expression of the putative tumor suppressor RAR beta and on retinoid resistance in human squamous carcinoma cells

Retinoids' effects on cell growth and differentiation are mediated by nuclear retinoid receptors, which are ligand-activated transcription enhancing factors. Because the expression of the retinoic acid receptor beta (RARbeta) gene, which is located on chromosome 3p24, is diminished in premalignant and malignant tissues it has been proposed that it acts as a tumor suppressor. To test the hypothesis that RARbeta loss leads to retinoid resistance, we studied several karyotyped head and neck squamous carcinoma (HNSCC) cell lines (UMSCC-17A, -17B, -22A, -22B, and -38) with deletion of one chromosome 3p arm. RARbeta mRNA was neither detected nor induced by retinoic acid in these cells, whereas it was expressed and induced by retinoic acid in two other HNSCC cell lines (1483 and 183) without 3p deletion. Methylation of the RARbeta gene promoter was detected in the 17B and 22B cells that failed to express RARbeta but no methylation was found in 183A cells that did express RARbeta mRNA. Responsiveness of HNSCC cells to several retinoids in assays of growth inhibition and colony formation, was rank ordered as: 22B>1483>38>183>17B. Additionally, retinoid response elements were transactivated in 22B more efficiently than in 17B cells. These results indicate that loss of RARbeta expression does not necessarily lead to loss of growth inhibition by retinoids or to a block of retinoid signaling.

Expression of keratins in normal, immortalized and malignant oral epithelia in organotypic culture

Keratins have been extensively studied in tissues and cultured keratinocytes but limited information is available on epithelia reconstructed in vitro. The aim of this study was to examine keratin expression in organotypic epithelia with normal (NOK), immortalized (SVpgC2a) and malignant (SqCC/Y1) human buccal cells. Organotypic epithelia were derived from 10 days of culture at the air-liquid interface of collagen gels containing human oral fibroblasts using a standardized serum-free medium. Sections were stained immunohistochemically with selected mono-specific antibodies to a range of keratins. Organotypic epithelia showed sharp differences in keratin expression and distribution. K4/K13, K1/K10, K6/K16 were variably expressed in NOK and SqCC/Y1 but were not detected in SVpgC2a. K5 was expressed in all organotypic epithelia but K14 was absent in SVpgC2a. K7 and K8 showed variable expression while K18 was expressed uniformly in all epithelia. K19 was expressed consistently in NOK and K20 was distributed heterogeneously in SVpgC2a. Overall, organotypic cultures of normal keratinocytes express many of the same keratins as buccal mucosa. Further, the loss of keratins in SVpgC2a and their retention in SqCC/Y1 have several features in common with the respective keratin profile of oral epithelial dysplasia and well-differentiated oral squamous cell carcinoma. Although qualitative and quantitative differences exist compared to keratin expression in vivo, these cell lines in organotypic culture may serve in studies of the multi-step progression of oral cancer.

Expression of retinoic acid receptor gamma correlates with retinoic acid sensitivity and metabolism in head and neck squamous cell carcinoma cell lines

Retinoids, analogues of vitamin A, can reverse premalignant lesions and prevent second primary tumors in patients with head and neck squamous cell carcinoma (HNSCC). The effects of retinoids are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which act as ligand-activated transcription factors. The regulation of cell growth, differentiation and retinoid metabolism in normal, premalignant and malignant cells by retinoids is thought to be a result of their effects on gene expression. We investigated mRNA expression of RARs (alpha, beta, and gamma) and RXR-beta by means of RNase protection and related this to retinoic acid (RA)-induced growth inhibition and RA turnover in four HNSCC cell lines (UM-SCC-14C, UM-SCC-22A, UM-SCC-35 and VU-SCC-OE). An RA-resistant subline of UM-SCC-35 was generated by exposure to increasing concentrations of RA for 8 months (designated UM-SCC-35R). RA turnover was determined on the basis of decreasing RA levels in the cells and culture medium after exposure to 1 microM RA. We found that RAR-gamma mRNA expression was strongly correlated with RA-induced growth inhibition (p = 0.016, R = 0.92) and RA turnover (p = 0.041, R = 0.86). RAR-beta transcript levels were reduced in three of five cell lines compared with normal mucosa, and these did not correlate with RA-induced growth inhibition and RA turnover. Expression of RAR-alpha and RXR-beta was not substantially altered in any of the cell lines. These findings suggest that in HNSCC cell lines RAR-gamma is the most important retinoid receptor for regulation of RA turnover rate and RA-induced growth inhibition.

Orphan receptor COUP-TF is required for induction of retinoic acid receptor beta, growth inhibition, and apoptosis by retinoic acid in cancer cells

Retinoic acid receptor beta (RARbeta) plays a critical role in mediating the anticancer effects of retinoids. Expression of RARbeta is highly induced by retinoic acid (RA) through a RA response element (betaRARE) that is activated by heterodimers of RARs and retinoid X receptors (RXRs). However, RARbeta induction is often lost in cancer cells despite expression of RARs and RXRs. In this study, we provide evidence that orphan receptor COUP-TF is required for induction of RARbeta expression, growth inhibition, and apoptosis by RA in cancer cells. Expression of COUP-TF correlates with RARbeta induction in a variety of cancer cell lines. In addition, stable expression of COUP-TF in COUP-TF-negative cancer cells restores induction of RARbeta expression, growth inhibition, and apoptosis by RA, whereas inhibition of COUP-TF by expression of COUP-TF antisense RNA represses the RA effects. In a transient transfection assay, COUP-TF strongly induced transcriptional activity of the RARbeta promoter in a RA- and RARalpha-dependent manner. By mutation analysis, we demonstrate that the effect of COUP-TF requires its binding to a DR-8 element present in the RARbeta promoter. The binding of COUP-TF to the DR-8 element synergistically increases the RA-dependent RARalpha transactivation function by enhancing the interaction of RARalpha with its coactivator CREB binding protein. These results demonstrate that COUP-TF, by serving as an accessory protein for RARalpha to induce RARbeta expression, plays a critical role in regulating the anticancer activities of retinoids.