RAR beta 2-mediated growth inhibition in HeLa cells

Retinoic acid regulates cell-shape and -death of E-FABP (FABP5)-immunoreactive septoclasts in the growth plate cartilage of mice

Septoclasts, which are mononuclear and spindle-shaped cells with many processes, have been considered to resorb the transverse septa of the growth plate (GP) cartilage at the chondro-osseous junction (COJ). We previously reported the expression of epidermal-type fatty acid-binding protein (E-FABP, FABP5) and localization of peroxisome proliferator-activated receptor (PPAR)β/δ, which mediates the cell survival or proliferation, in septoclasts. On the other hand, retinoic acid (RA) can bind to E-FABP and is stored abundantly in the GP cartilage. From these information, it is possible to hypothesize that RA in the GP is incorporated into septoclasts during the cartilage resorption and regulates the growth and/or death of septoclasts. To clarify the mechanism of the cartilage resorption induced by RA, we administered an overdose of RA or its precursor vitamin A (VA)-deficient diet to young mice. In mice of both RA excess and VA deficiency, septoclasts decreased in the number and cell size in association with shorter and lesser processes than those in normal mice, suggesting a substantial suppression of resorption by septoclasts in the GP cartilage. Lack of PPARβ/δ-expression, TUNEL reaction, RA receptor (RAR)β, and cellular retinoic acid-binding protein (CRABP)-II were induced in E-FABP-positive septoclasts under RA excess, suggesting the growth arrest/cell-death of septoclasts, whereas cartilage-derived retinoic acid-sensitive protein (CD-RAP) inducing the cell growth arrest or morphological changes was induced in septoclasts under VA deficiency. These results support and do not conflict with our hypothesis, suggesting that endogenous RA in the GP is possibly incorporated in septoclasts and utilized to regulate the activity of septoclasts resorbing the GP cartilage.

Family of microRNA-146 Regulates RARβ in Papillary Thyroid Carcinoma

Retinoic acid is a promising tool in adjuvant cancer therapies, including refractory thyroid cancer, and its biological role is mediated by the retinoic acid receptor beta (RARβ). However, expression of RARβ is lowered in papillary thyroid carcinoma (PTC), contributing to promotion of tumor growth and inefficiency of retinoic acid and radioactive iodine treatment. The causes of aberrant RARB expression are largely unknown. We hypothesized that the culpable mechanisms include the action of microRNAs from the miR-146 family, previously identified as significantly upregulated in PTC tumors. To test this hypothesis, we assessed the expression of RARB as well as miR-146a-5p and miR-146b-5p in 48 PTC tumor/normal tissue pairs by Taqman assay to reveal that the expression of RARB was 3.28-fold decreased, and miR-146b-5p was 28.9-fold increased in PTC tumors. Direct interaction between miRs and RARB was determined in the luciferase assay and further confirmed in cell lines, where overexpression of miR-146a-5p and miR-146b-5p caused a 31% and 33% decrease in endogenous RARB mRNA levels. Inhibition of miR-146a and miR-146b resulted in 62.5% and 45.4% increase of RARB, respectively, and a concomitant decrease in proliferation rates of thyroid cancer cell lines, analyzed in xCELLigence system.We showed that two microRNAs of the miR-146 family directly regulate RARB. Inhibition of miRs resulted in restoration of RARB expression and decreased rates of proliferation of thyroid cancer cells. By restoring RARB levels, microRNA inhibitors may become part of an adjuvant therapy in thyroid cancer patients.

Human papillomavirus type 16 E7 oncoprotein upregulates the retinoic acid receptor-beta expression in cervical cancer cell lines and K14E7 transgenic mice

Persistent infection with high-risk human papillomaviruses is the main etiological factor in cervical cancer (CC). The human papillomavirus type 16 (HPV16) E7 oncoprotein alters several cellular processes, regulating the expression of many genes in order to avoid cell cycle control. Retinoic acid receptor beta (RARB) blocks cell growth, inducing differentiation and apoptosis. This tumor suppressor gene is gradually silenced in late passages of foreskin keratinocytes immortalized with HPV16 and in various tumors, including CC, mainly by epigenetic modifications. We investigated the effect of E7 oncoprotein on RARB gene expression. We found that HPV16 E7 increases RARB mRNA and RAR-beta protein expression both in vitro and in the cervix of young K14E7 transgenic mice. In E7-expressing cells, RARB overexpression is further increased in the presence of the tumor suppressor p53 (TP53) R273C mutant. This effect does not change when either C33-A or E7-expressing C33-A cell line is treated with Trichostatin A, suggesting that E7 enhances RARB expression independently of histone deacetylases inhibition. These findings indicate that RARB overexpression is part of the early molecular events induced by the E7 oncoprotein.

Lidocaine sensitizes the cytotoxicity of cisplatin in breast cancer cells via up-regulation of RARβ2 and RASSF1A demethylation

It has been reported that lidocaine is toxic to various types of cells. And a recent study has confirmed that lidocaine exerts a demethylation effect and regulates the proliferation of human breast cancer cell lines. To recognize a potential anti-tumor effect of lidocaine, we evaluated the DNA demethylation by lidocaine in human breast cancer lines, MCF-7 and MDA-MB-231 cells, and determined the influence of demethylation on the toxicity to these cells of cisplatin, which is a commonly utilized anti-tumor agent for breast cancer. Results demonstrated that lidocaine promoted a significant global genomic demethylation, and particularly in the promoters of tumor suppressive genes (TSGs), RARβ2 and RASSF1A. Further, the lidocaine treatment increased cisplatin-induced apoptosis and enhanced cisplatin-induced cytotoxicity. The combined treatment with both lidocaine and cisplatin promoted a significantly higher level of MCF-7 cell apoptosis than singular lidocaine or cisplatin treatment. Moreover, the abrogation of RARβ2 or RASSF1A expression inhibited such apoptosis. In conclusion, the present study confirms the demethylation effect of lidocaine in breast cancer cells, and found that the demethylation of RARβ2 and RASSF1A sensitized the cytotoxicity of cisplatin in breast cancer cells.

Effects of PPARγ Ligands on Leukemia

Peroxisome proliferator-activated receptors (PPARs) and retinoic acid receptors (RARs), members of the nuclear receptor superfamily, are transcription factors that regulate a variety of important cellular functions. PPARs form heterodimers retinoid X receptor (RXR), an obligate heterodimeric partner for other nuclear receptors. Several novel links between retinoid metabolism and PPAR responses have been identified, and activation of PPAR/RXR expression has been shown to increase response to retinoids. PPARγ has emerged as a key regulator of cell growth and survival, whose activity is modulated by a number of synthetic and natural ligands. While clinical trials in cancer patients with thiazolidinediones (TZD) have been disappointing, novel structurally different PPARγ ligands, including triterpenoids, have entered clinical arena as therapeutic agents for epithelial and hematopoietic malignancies. Here we shall review the antitumor advances of PPARγ, alone and in combination with RARα ligands in control of cell proliferation, differentiation, and apoptosis and their potential therapeutic applications in hematological malignancies.

Molecular analysis of plasma DNA for the early detection of lung cancer by quantitative methylation-specific PCR

PURPOSE

Aberrant promoter hypermethylation of tumor suppressor genes is a promising marker for lung cancer detection. We investigated the likelihood of detecting aberrant DNA methylation of tumor suppressor genes in plasma samples of patients with abnormalities of the lung detected upon computed tomography (CT) scan.

EXPERIMENTAL DESIGN

In a small evaluation cohort, four gene promoters (DCC, Kif1a, NISCH, and Rarb) were found to be methylated with increased frequency in samples from cancer patients specifically. We then examined DNA from 93 plasma samples from patients with abnormal findings in the lung detected upon CT scan for aberrant methylation of these four gene promoters by quantitative fluorogenic real-time PCR. The patients were divided into two groups, ground glass opacity (n = 23) and cancerous tumors (n = 70). Plasma DNA from age-matched nodule-free individuals were used as controls (n = 80).

RESULTS

In plasma, 73% of patients with cancerous tumors showed methylation of at least one gene with a specificity of 71% (P = 0.0001). Only 22% patients with ground glass opacity exhibited methylation of at least one gene. When smoking history was taken into account, 72% of cancer patients with no smoking history or those who smoked <20 pack-years showed methylation of at least one gene with 100% specificity (P = 0.05) when compared with matched controls. Among heavy smokers with 20+ pack-years of smoking history, 30% of the control group and 73% of the patients with cancerous tumors showed methylation (P = 0.0001).

CONCLUSIONS

These biomarkers can distinguish between cancerous and noncancerous abnormal CT findings.

Synthetic retinoids: structure-activity relationships

Retinoid signalling pathways are involved in numerous processes in cells, particularly those mediating differentiation and apoptosis. The endogenous ligands that bind to the retinoid receptors, namely all-trans-retinoic acid (ATRA) and 9-cis-retinoic acid, are prone to double-bond isomerisation and to oxidation by metabolic enzymes, which can have significant and deleterious effects on their activities and selectivities. Many of these problems can be overcome through the use of synthetic retinoids, which are often much more stable, as well as being more active. Modification of their molecular structures can result in retinoids that act as antagonists, rather than agonists, or exhibit a large degree of selectivity for particular retinoid-receptor isotypes. Several such selective retinoids are likely to be of value as pharmaceutical agents with reduced toxicities, particularly in cancer therapy, as reagents for controlling cell differentiation, and as tools for elucidating the precise roles that specific retinoid signalling pathways play within cells.

Hepatitis B virus X protein overcomes the growth-inhibitory potential of retinoic acid by downregulating retinoic acid receptor-beta2 expression via DNA methylation

Aberrant promoter methylation of retinoic acid receptor-beta(2) (RAR-beta(2)) is frequently detected in hepatitis B virus (HBV)-positive hepatocellular carcinoma (HCC); however, the mechanism of methylation and its biological significance are unknown. This study showed that HBx, the principal oncogene product of HBV, induced promoter hypermethylation of RAR-beta(2) via upregulation of DNA methyltransferases 1 and 3a, resulting in downregulation of its expression in human HCC cells. In addition, HBx abolished the potential of retinoic acid (RA) to downregulate levels of G(1)-checkpoint regulators including p16, p21 and p27, resulting in activation of E2F1 in the presence of RA. As a consequence, HBx-expressing cells were less susceptible to RA-induced cell growth inhibition compared with control cells. These effects almost completely disappeared when levels of RAR-beta(2) in HBx-expressing cells were restored by treatment with a universal DNA methylation inhibitor, 5-aza-2'-deoxycytidine. As RAR-beta(2) is a major executor of the anti-tumour potential of RA, its epigenetic downregulation by HBx is likely to be an important step during HBV-mediated tumorigenesis.

The role of retinoid deficiency and estrogens as cofactors in cervical cancer

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is involved in cervical cancer (CC), a major cause of cancer mortality worldwide. Infection occurs primarily at the transformation zone (TZ), the most estrogen- and retinoid-sensitive region of the cervix. Development of CC affects a small percentage of HR-HPV-infected women and often takes decades after infection, suggesting that HR-HPV is a necessary but not sufficient cause of CC. Thus, other cofactors are necessary for progression from cervical HR-HPV infection to cancer such as long-term use of hormonal contraceptives, multiparity, smoking, as well as micronutrient depletion and in particular retinoid deficiency, which alters epithelial differentiation, cellular growth and apoptosis of malignant cells. Therefore, early detection of HR-HPV and management of precancerous lesions together with a profound understanding of additional risk factors could be a strategy to avoid this disease. In this review we focus on the synergic effect of estrogens, retinoid deficiency and HR-HPVs in the development of CC. These risk factors may act in concert to induce neoplastic transformation in squamous epithelium of the cervix, setting the stage for secondary genetic or epigenetic events leading to cervical cancer.

Epstein-Barr virus latent membrane protein 1 suppresses the growth-inhibitory effect of retinoic acid by inhibiting retinoic acid receptor-beta2 expression via DNA methylation

Epigenetic alteration through DNA methylation in retinoic acid receptor-beta2 (RAR-beta2) is common in human tumors including nasopharyngeal carcinoma (NPC); however, the mechanism and its biological significance are unknown. Here, we report that the Epstein-Barr virus (EBV) oncogene product, latent membrane protein 1 (LMP1), induces promoter hypermethylation of RAR-beta2 via up-regulation of DNA methyltransferases 1, 3a, and 3b, leading to decrease in RAR-beta2 expression in NPC cells. In addition, LMP1 abolished the potentials of retinoic acid (RA) to down-regulate Cdk2 and Cdk4 and to up-regulate p16, p21, and p27, resulting in activation of E2F1 in the presence of RA. As a consequence, LMP1 could abrogate the growth-inhibitory effect of RA by releasing cell cycle arrest at G1 phase. Considering that RAR-beta2 is a major executor of the anti-tumor potentials of retinoids, its down-regulation by LMP1 might play an important role during EBV-mediated tumorigenesis.

HOXA5 acts directly downstream of retinoic acid receptor beta and contributes to retinoic acid-induced apoptosis and growth inhibition

The promise of retinoids as chemopreventive agents in breast cancer is based on the differentiation and apoptosis induced upon their binding to the retinoic acid (RA) receptor beta (RARbeta). We have previously shown that HOXA5 induces apoptosis in breast cancer cells. In this study, we investigated whether RA/RARbeta and HOXA5 actions intersect to induce apoptosis and differentiation in breast cancer cells. We found that HOXA5 expression can be induced by RA only in RARbeta-positive breast cancer cells. We have, for the first time, identified the RA response element in HOXA5, which was found to be located in the 3' end of the gene. Chromatin immunoprecipitation assays showed that RARbeta binds directly to this region in vivo. Overexpression of RARbeta strongly enhances RA responsiveness, and knocking down RARbeta expression abolishes RA-mediated induction of HOXA5 expression in breast cancer cells. In addition, there is coordinated loss of both HOXA5 and RARbeta expression during neoplastic transformation and progression in the breast epithelial cell model, MCF10A. Knockdown of HOXA5 expression partially abrogates retinoid-induced apoptosis and promotes cell survival upon RA treatment. These results strongly suggest that HOXA5 acts directly downstream of RARbeta and may contribute to retinoid-induced anticancer and chemopreventive effects.

Retinoic acid receptor beta silences human papillomavirus-18 oncogene expression by induction of de novo methylation and heterochromatinization of the viral control region

Retinoic acid receptor beta2 (RAR beta2) is often down-regulated during the multistep process to cervical cancer. In that way, its inhibitory function on the transcription factor AP-1, indispensable to maintain human papillomavirus (HPV) gene expression is relieved. Using HPV-18 positive HeLa cells as a model system, we show that ectopic expression of RAR beta2 is able to down-regulate HPV-18 transcription by selectively abrogating the binding of AP-1 to the viral regulatory region in a ligand-independent manner. This resulted in down-regulation of the viral mRNAs at the level of initiation of transcription. Decreased oncogene expression was accompanied by a re-induction of cell cycle inhibitory proteins such as p53, p21(CIP1), and p27(KIP) as well as by a cessation of cellular growth. Reduced transcriptional activity as a consequence of AP-1 reduction by selective c-Jun degradation apparently targets the HPV-18 regulatory region for epigenetic modification such as de novo methylation and nucleosomal condensation. This mechanism is otherwise counterbalanced by active and abundant viral transcription in malignant cells, because RAR beta2 itself becomes inactivated during cervical carcinogenesis. Hence, our study shows that the temporal co-existence of a potential repressor and viral oncoproteins is mutually exclusive and provides evidence of a cross-talk between a nuclear receptor, AP-1, and the epigenetic machinery.

Apo-10'-lycopenoic acid inhibits lung cancer cell growth in vitro, and suppresses lung tumorigenesis in the A/J mouse model in vivo

High intake of lycopene has been associated with a lower risk of a variety of cancers including lung cancer. We recently showed that lycopene can be converted to apo-10'-lycopenoids [Hu et al. (2006). J. Biol. Chem., 281, 19327-19338] in mammalian tissues both in vitro and in vivo, raising the question of whether apo-10'-lycopenoids have biological activities against lung carcinogenesis. In the present study, we report that apo-10'-lycopenoic acid inhibited the growth of NHBE normal human bronchial epithelial cells, BEAS-2B-immortalized normal bronchial epithelial cells and A549 non-small cell lung cancer cells. This inhibitory effect of apo-10'-lycopenoic acid was associated with decreased cyclin E, inhibition of cell cycle progression from G(1) to S phase and increased cell cycle regulators p21 and p27 protein levels. In addition, apo-10'-lycopenoic acid transactivated the retinoic acid receptor beta (RARbeta) promoter and induced the expression of RARbeta. We further examined the effect of apo-10'-lycopenoic acid treatment on 4-(N-methyl-N-nitrosamino)-1-(3-pyridal)-1-butanone (NNK)-induced lung tumorigenesis in the A/J mouse model. We found that the lung tumor multiplicity was decreased dose dependently from an average of 16 tumors per mouse in the NNK injection alone group, to an average of 10, 7 and 5 tumors per mouse in groups injected with NNK and supplemented with 10, 40 and 120 mg/kg diet of apo-10'-lycopenoic acid, respectively. These observations demonstrate that apo-10'-lycopenoic acid is a biological active metabolite of lycopene and suggest that apo-10'-lycopenoic acid is a potential chemopreventive agent against lung tumorigenesis.

Promoter hypermethylation of the retinoic acid receptor beta2 gene is frequent in acute myeloid leukaemia and associated with the presence of CBFbeta-MYH11 fusion transcripts

Silencing of the putative tumour suppressor gene retinoic acid receptor beta2 (RARbeta2) caused by aberrant promoter hypermethylation has been identified in several solid tumours. In order to evaluate the extent of RARbeta2 hypermethylation and transcription in acute myeloid leukaemia (AML) at diagnosis, 320 patients were investigated by bisulphite-denaturing gradient gel electrophoresis and mRNA transcription levels were analysed in 61 of these by quantitative real-time polymerase chain reaction. The results were compared with demographic- and molecular data from the patients. While RARbeta2 was unmethylated in 10/10 bone marrow and 7/7 blood samples from healthy individuals, the gene was hypermethylated in 43% of the AML patients. The RARbeta2 degree of promoter methylation differed between and within individuals, and the mRNA transcription levels of the gene varied inter-individually by a factor of 4000. A significant inverse correlation between promoter hypermethylation and gene expression could be established (t-test, P = 0.019). Comparison of methylation data with a series of other molecular alterations in the same patient materials revealed a correlation between hypermethylation of the RARbeta2 promoter and the presence of CBF-MYH11 fusion transcripts (P < 0.01). Our data suggest that RARbeta2 promoter methylation is frequent in AML and may co-operate with the expression of CBF-MYH11 fusion transcripts in leukaemogenesis.

Apoptotic and anti-proliferative effects of all-trans retinoic acid. Adenine nucleotide translocase sensitizes HeLa cells to all-trans retinoic acid

We examined the apoptotic and anti-proliferative effects of all-trans retinoic acid (atRA) in HeLa cells. Our results demonstrated that HeLa cells were more sensitive to the anti-proliferative effects of atRA than to its apoptotic effects. Furthermore, we demonstrated that caspase inhibition attenuates cell death but does not alter the atRA-dependent reduction in cell proliferation, which suggests that atRA-induced apoptosis is independent of the arrest in cell proliferation. To check whether ANT proteins mediated these atRA effects, we transiently transfected cells with expression vectors encoding for individual ANT (adenine nucleotide translocase 1-3). Our results revealed that ANT1 and ANT3 over-expressing HeLa cells increased their atRA sensitivity. Thus, our results not only demonstrate the different functional activities of ANT isoforms, but also contribute to a better understanding of the properties of atRA as an anti-tumoral agent used in cancer therapy.

Response of human rhabdomyosarcoma cell lines to retinoic acid: relationship with induction of differentiation and retinoic acid sensitivity

The ability of retinoids to induce growth inhibition associated with differentiation of diverse cell types makes them potent anti-cancer agents. We examined the effect of retinoic acid (RA) in cell lines derived from rhabdomyosarcoma (RMS), a malignant soft-tissue tumor committed to the myogenic lineage, but arrested prior to terminal differentiation. We showed that several RMS derived cell lines, including RD human rhabdomyosarcoma cells, are resistant to the growth-inhibitory and differentiation effects of RA. We established that this RA-resistance correlates with reduced expression and activity of RA-receptors in RD cells. We stably expressed either RARalpha, RARbeta, RARgamma, or RXRalpha expression vector into RD cells and found that only RARbeta or RARgamma induced a significant RA growth arrest without promoting differentiation indicating that changes in the amounts of RARs and RXRs are not sufficient to determine the RA myogenic response of rhabdomyosarcoma cells. Activation of RD cell differentiation by ectopic MRF4 expression enhanced RA-receptor activity and led to RA induction of differentiation. These studies demonstrate that RA-resistance of RD cells is linked to their lack of differentiation and suggest that the differentiation-promoting activity of RA requires factors other than RAR-RXR heterodimers.

A retinoic acid receptor beta/gamma-selective prodrug (tazarotene) plus a retinoid X receptor ligand induces extracellular signal-regulated kinase activation, retinoblastoma hypophosphorylation, G0 arrest, and cell differentiation

Retinoic acid receptor (RAR)beta is perceived to function as a tumor suppressor gene in various contexts where its absence is associated with tumorigenicity and its presence causes cell cycle arrest. Tazarotene is a prodrug selective for RARbeta/gamma, thereby motivating interest in determining whether tazarotene might activate putative tumor suppressor activity. Using HL-60 human myeloblastic leukemia cells, a cell line that undergoes G0 cell cycle arrest and myeloid differentiation in response to retinoic acid (RA), tazarotene failed to cause extracellular signal-regulated kinase (ERK) activation, a requirement for retinoic acid (RA)-induced G0 arrest and differentiation; retinoblastoma (RB) hypophosphorylation, another characteristic of RA-induced G0 arrest and cell differentiation; G0 arrest; or differentiation into mature myeloid cells. However, when used in combination with a retinoid X receptor (RXR)-selective ligand, tazarotene caused ERK activation, RB tumor suppressor protein hypophosphorylation, G0 arrest, and myeloid differentiation. The kinetics of G0 arrest and differentiation was similar to that of RA. Dose-response studies showed that diminishing tazarotene progressively diminished both induced cell differentiation and G0 arrest, where the doses for cellular effects were consistent with the transcriptional transactivation data. For either tazarotene or an RARalpha-selective ligand, diminishing the coadministered RXR-selective ligand diminished both induced differentiation and G0 arrest. Tazarotene could propel either early or late portions of the period leading to differentiation and G0 arrest and was interchangeable with an RARalpha-selective ligand. Tazarotene used with RXR-selective ligand may thus be a useful antineoplastic agent in differentiation induction therapy as exemplified by the prototypical RA treatment of acute promyelocytic leukemia.

RARbeta2 is a candidate tumor suppressor gene in myelofibrosis with myeloid metaplasia

Myelofibrosis with myeloid metaplasia (MMM) is a clonal stem-cell disorder that leads to ineffective hematopoiesis, bone marrow fibrosis, and extramedullary hematopoiesis. The molecular mechanisms underlying the development of this myeloproliferative syndrome are currently unknown. In order to identify tumor suppressor genes that may be involved in the disease process, we performed an analysis for loss of heterozygosity (LOH) in CD34+ cells from 29 patients with MMM. We observed a frequency of allelic loss on chromosomal arm 3p in 24% of cases. Detailed mapping of 3p revealed a distinct region of deletion at 3p24. Among the genes known to map within this region is the retinoic acid receptor-beta (RARbeta2) gene. To determine whether RARbeta2 gene activity is diminished in this disease, we analysed its expression in CD34+ cells from 17 patients with MMM using quantitative PCR. Our results indicate that expression of RARbeta2 is significantly decreased in 100% of patient samples compared to that in CD34+ cells from 10 normal individuals. Since allelic loss at 3p24 occurs in <25% of patients, we investigated the contribution of epigenetic modifications to RARbeta2 inactivity. Using methylation-specific PCR, we found hypermethylation of RARbeta2 in 16 of 18 patients (89%), while the methylated form of the gene was absent in CD34+ cells from nine normal individuals. Our results suggest that RARbeta2 acts as a tumor suppressor gene in MMM and that epigenetic changes are the most significant determinants of RARbeta2 gene activity in these patients.

Ectopic expression of nonliganded retinoic acid receptor beta abrogates AP-1 activity by selective degradation of c-Jun in cervical carcinoma cells

Expression of the nuclear retinoic acid receptor beta2 (RARbeta2) gene is often disturbed in cervical carcinoma cells. One important mechanism by which RARbeta2 can exert growth inhibitory function is based on its ability to repress the AP-1 transcription factor in a ligand-dependent manner. Because less is known about the biological effects of RARbeta in the absence of ligand, the corresponding cDNA was stably introduced into HPV18-positive HeLa cervical carcinoma cells. In the present study we describe a novel mechanism by which AP-1 becomes inactivated. Constitutive expression of nonliganded RARbeta abrogated both AP-1 binding affinity and activity by a selective degradation of the c-Jun protein as major dimerization partner, without substitution by other members of the Jun family. Blockage of the proteasomal pathway completely rescued c-Jun and reconstituted the AP-1 function. Moreover, HeLa RARbeta2 clones treated either with tumor necrosis factor-alpha or transfected with a constitutive active upstream mitogen-activated protein kinase (MEKK1Delta) also resulted in c-Jun phosphorylation and restoration of AP-1 affinity and functionality similar to that found in nontransfected parental HeLa cells. These data revealed an important cross-talk between trans-repression of AP-1 and nonliganded RARbeta in human papillomavirus-positive cells. Because AP-1 activity was not irreversibly disturbed, but could be switched on through activation of the Jun N-terminal kinase pathway, a model for the transient activation of AP-1 even in the presence of RARbeta as repressor is suggested.

Evidence of increased apoptosis and reduced proliferation in basal cell carcinomas treated with tazarotene

A preliminary clinical experience suggested tazarotene, a new acetylenic retinoid, as an effective alternative topical treatment of basal cell carcinomas (BCC). The mechanisms of action of this synthetic retinoid, however, have not been yet clarified. In this work we assessed the in vivo effects of daily application of tazarotene for 24 wk, on 30 small superficial and nodular BCC, and the in vitro effects of tazarotene on immortalized basal and squamous tumor epidermal cells. Cellular proliferation, apoptosis and changes in expression of retinol and retinoic acid receptors (RAR), p53, bcl-2, and bax were studied by immunohistochemistry, western blotting and PCR. Overall, 76.7% of treated tumors showed >50% regression. Complete healing was observed in 46.7% of all treated BCC, without recurrences at 2-y observation. Regression was associated with reduced proliferation and increased apoptosis, demonstrated by Ki-67- and TdT-mediated dUTP-biotin nick-end labelling-positive nuclear staining, and with enhanced RAR-beta and bax expression, with RAR-alpha and -gamma expression unchanged. In vitro, tazarotene induced a concentration-dependent increase of RAR-beta and bax associated with a greater rate of apoptosis and growth inhibition in basaloid tumor cells compared with squamous tumor cells. Our studies provide convincing evidence that tazarotene induces BCC regression possibly by synergistic RAR-beta-dependent anti-proliferative and pro-apoptotic pathways.

Downregulation of retinoic acid receptor-β2 expression is linked to aberrant methylation in esophageal squamous cell carcinoma cell lines

AIM: To study the role of hypermethylation in the loss of retinoic acid receptor β₂ (RARβ₂) in esophageal squamous cell carcinoma (ESCC).

METHODS: The role of hypermethylation in RARβ₂ gene silencing in 6 ESCC cell lines was determined by methylation-specific PCR (MSP), and its methylation status was compared with RARβ₂ mRNA expression by RT-PCR. The MSP results were confirmed by bisulfite sequencing of RARβ₂ promoter regions.

RESULTS: Methylation was detected in 4 of the 6 cell lines, and the expression of RARβ₂ was markedly downregulated in 3 of the 4 methylated cell lines. The expression of RARβ₂ was restored in one RARβ₂ -downregulated cell line with the partial demethylation of promoter region of RARβ₂ after 5-aza-2’-deoxycytidine (5-aza-dc) treatment.

CONCLUSION: The methylation of the 5’ region may play an important role in the downregulation of RARβ₂ in some ESCC cell lines, suggesting that multiple mechanisms contribute to the loss of RARβ₂ expression in ESCC cell lines. This study may have clinical applications for treatment and prevention of ESCC.

Acyclic retinoid activates retinoic acid receptor β and induces transcriptional activation of p21CIP1 in HepG2 human hepatoma cells

Acyclic retinoid (ACR), a novel synthetic retinoid, has recently been demonstrated by us to inhibit the in vitro growth of human hepatoma cells, and this effect was associated with decreased expression of cell cycle-related molecules. These results, taken together with previous in vitro and clinical studies with ACR, suggest that this agent may be useful in the chemoprevention and therapy of hepatoma and possibly other human malignancies. In the present study, we further examined the molecular effects of ACR on the HepG2 human hepatoma cell line, focusing on the expression of nuclear retinoid receptors and the cell cycle inhibitor protein p21CIP1. Reverse transcription-PCR assays and Western blot analyses indicated that these cells express retinoic acid receptors (RARs) α, β, and γ, retinoid X receptors (RXRs) α and β, and peroxisome proliferator-activated receptors (PPAR) γ mRNA. Treatment with ACR caused a rapid induction within 3 h of RARβ mRNA and the related protein, but there was no significant change in the levels of the mRNA or proteins for RARs α and γ, RXRs α and β, and PPARγ. There was also a rapid increase in p21CIP1 mRNA and protein in HepG2 cells treated with ACR, and this induction occurred via a p53-independent mechanism. In transient transfection reporter assays, we cotransfected the retinoic acid response element-chloramphenicol acetyltransferase (CAT) reporter gene into HepG2 cells together with a RARβ expression vector. RARβ expression markedly stimulated CAT activity (up to about 4-fold) after the addition of ACR. However, CAT activity in the presence of ACR was only about 2-fold higher than that in the absence of ACR, when cells were cotransfected with RARs α and γ or RXRα. These findings suggest that the growth inhibitory effects of ACR are mediated at least in part through RARβ and that both RARβ and p21CIP1 play critical roles in the molecular mechanisms of growth inhibition induced by ACR.

Retinoids in cancer therapy and chemoprevention: promise meets resistance

Retinoids (natural and synthetic derivatives of vitamin A) signal potent differentiation and growth-suppressive effects in diverse normal, premalignant, and malignant cells. A strong rationale exists for the use of retinoids in cancer treatment and chemoprevention based on preclinical, epidemiological, and early clinical findings. Despite the success of all-trans-retinoic acid (RA)-based differentiation therapy in acute promyelocytic leukemia (APL), the broad promise of retinoids in the clinic has not yet been realized. In addition to the expected limited activity of any single therapeutic agent, translation of retinoid activities from the laboratory to the clinic has met with intrinsic or acquired retinoid resistance. Evidence suggests that solid tumors develop intrinsic resistance to retinoids during carcinogenesis. In contrast, relapse of APL is often associated with acquired resistance to retinoid maturation induction. This review discusses what is known about retinoid resistance mechanisms in cancer therapy and chemoprevention. Strategies to overcome this resistance will be discussed, including combination therapy with other differentiation-inducing, cytotoxic or chromatin-remodeling agents, as well as the use of receptor-selective and nonclassical retinoids. Opportunities exist in the post-genomic era to bypass resistance to classical retinoids by identifying target genes and associated pathways that directly mediate the antineoplastic effects of retinoids. In this regard, the retinoids are useful pharmacological tools to reveal important pathways targeted in cancer therapy and chemoprevention.

Induction of senescence-associated growth inhibitors in the tumor-suppressive function of retinoids

Retinoids, physiological regulators of cell growth and differentiation, are used in the treatment or chemoprevention of several malignant diseases. This class of compounds can induce growth arrest or apoptosis in tumor cells. Permanent growth arrest of retinoid-treated cells is often assumed to result from retinoid-induced differentiation. Recent studies in breast carcinoma and neuroblastoma cells demonstrated that retinoids can stop tumor cell growth through the program of senescence rather than differentiation. Retinoid-induced tumor suppression is associated with the induction of multiple intracellular and secreted growth-inhibitory proteins. Most of these proteins were also found to be upregulated in senescent cells. The induction of senescence-associated growth inhibitors appears to be an indirect effect of retinoids. Elucidation of the mechanisms responsible for the induction of growth-inhibitory genes in retinoid-treated cells should help in developing agents that would mimic the antiproliferative effect of retinoids in retinoid-insensitive cancers.

Downstream codons in the retinoic acid receptor beta -2 and beta -4 mRNAs initiate translation of a protein isoform that disrupts retinoid-activated transcription

Retinoic acid receptors (RARs) are essential for the differentiation and maintenance of normal epithelium. In studies of RARs in breast cancer, there are striking differences in the expression of certain protein isoforms of the RARbeta gene between cells derived from normal human mammary glands and those derived from breast tumors. While the protein isoforms RARbeta2 and RARbeta4 consist of the longest open reading frames of the RARbeta2 and RARbeta4 mRNAs, respectively, we find that a fraction of scanning ribosomes bypass these upstream RARbeta2 and RARbeta4 protein start codons and initiate translation downstream. This downstream translation initiation site is identical in the RARbeta2 and RARbeta4 transcripts and generates a third RARbeta protein isoform, here termed RARbeta' (formerly human RARbeta4). RARbeta' lacks protein domains found in the N terminus of RARbeta2 and RARbeta4, including one of two zinc fingers required for DNA binding. However, RARbeta' retains the ability to heterodimerize with RXRalpha and interact with transcription cofactors. In reporter gene assays, RARbeta' repressed retinoic acid-activated transcription of co-transfected RARbeta2, RARbeta4, and RARalpha. This repression required the presence of acidic amino acids within the AF2 domain. These findings demonstrate an antagonistic role for RARbeta' in signaling by retinoic acid.

Structural basis for isotype selectivity of the human retinoic acid nuclear receptor

The human retinoic acid receptor (hRAR) belongs to the family of nuclear receptors that regulate transcription in a ligand-dependent way. The isotypes RARalpha,beta and gamma are distinct pharmacological targets for retinoids that are involved in the treatment of various skin diseases and cancers, in particular breast cancer and acute promyelocytic leukemia. Therefore, synthetic retinoids have been developed aiming at isotype selectivity and reduced side-effects. We report the crystal structures of three complexes of the hRARgamma ligand-binding domain (LBD) bound to agonist retinoids that possess selectivity either for RARgamma (BMS184394) or for RARbeta/gamma (CD564), or that are potent for all RAR-isotypes (panagonist BMS181156). The high resolution data (1.3-1. 5 A) provide a description at the atomic level of the ligand pocket revealing the molecular determinants for the different degrees of ligand selectivity. The comparison of the complexes of the chemically closely related retinoids BMS184394 and CD564 shows that the side-chain of Met272 adopts different conformations depending on the presence of a hydrogen bond between its sulfur atom and the ligand. This accounts for their different isotype selectivity. On the other hand, the difference between the pan- and the RARbeta, gamma-selective agonist is probably due to a steric discrimination at the level of the 2-naphthoic acid moiety of CD564. Based on this study, we propose a model for a complex with the RARgamma-specific agonist CD666 that shows the possible applications for structure-based drug design of RAR isotype-selective retinoids.

Retinoic acid upregulates beta(1)-integrin in vascular smooth muscle cells and alters adhesion to fibronectin

Retinoic acid has an established physiological role in differentiation, development, and cellular growth. This study investigated the action of all-trans retinoic acid (ATRA) on vascular integrins, cell-surface receptors that control growth and remodeling of blood vessels. The beta(1)-integrin subunit mRNA and protein was induced after treatment with ATRA in two different rat vascular smooth muscle cell lines. To relate this result to the in vivo state, the aortas from adult rats fed with therapeutic doses of ATRA were examined for beta(1)-integrin protein. A significant upregulation of the integrin subunit was observed in vivo. To assess if this increase contributed to physiological changes in cellular function, cells treated with ATRA were tested for alterations in adhesion to extracellular matrix proteins. The cells exposed to the retinoid were seen to adhere more strongly to fibronectin, via the beta(1)-integrin. These results showed that modulation of vascular integrins by ATRA in adult rats contributes to functional changes that can cause remodeling of blood vessels.

Evidence that retinoic acid receptor beta induction by retinoids is important for tumor cell growth inhibition

Retinoic acid receptor beta (RARbeta) is thought to be involved in suppressing cell growth and tumorigenicity. Many premalignant and malignant cells exhibit a reduced RARbeta expression. However, in some of these cells (e.g. H157 human squamous cell carcinoma cells), RARbeta can be induced by retinoids (e.g. all-trans-retinoic acid, ATRA) because its promoter contains a retinoic acid response element. To examine the hypothesis that RARbeta induction is important for inhibition of cell proliferation by retinoids, we blocked ATRA-induced RARbeta expression in H157 cells using a retroviral vector harboring multiple copies of antisense RARbeta2 sequences. Antisense RARbeta-transfected cells showed not only decreased expression of ATRA-induced RARbeta protein but also reduced ATRA-induced RARE binding activity and transactivation. Importantly, all antisense RARbeta transfectants of H157 cells were less responsive than vector-transfected cells to the growth inhibitory effects of the retinoids ATRA and Ch55 in vitro. These results demonstrate that RARbeta induction may play an important role in mediating growth inhibitory effects of retinoids in cancer cells.

The induction and activation of STAT1 by all-trans-retinoic acid are mediated by RAR beta signaling pathways in breast cancer cells

Retinoic acid receptor-beta (RAR beta) and signal transducer and activator of transcription 1 (STAT1) are important mediators of the antiproliferative and apoptotic actions of retinoids and cytokines/growth factors, respectively. Expression of both RAR beta and STAT1 is lost in most breast cancer cell lines but it can be induced by retinoids in estrogen receptor-positive cells. We investigated a possible functional connection between these two mediators and present evidence supporting RAR beta as a tumor suppressor. First, by using different receptor-selective retinoids, we demonstrated that RAR beta induction in MCF-7 cells by all-trans-retinoic acid (atRA) was associated with the activation of STAT1 gene transcription. The direct involvement of RAR beta in atRA-induced STAT1 gene activation was further demonstrated by showing that transfection with an anti-sense RAR beta construct blocked atRA-induced STAT1 expression in MCF-7 cells whereas introduction of a sense-RAR beta construct resulted in STAT1 induction by atRA in MDA-MB 231 cells. In addition, we showed that STAT1 was phosphorylated/activated under atRA treatment of MCF-7 cells; this process required the involvement of RAR beta and protein synthesis. STAT1 phosphorylation/activation was accompanied by increased tyrosine kinase activity that was not due to the activation of JAK1, JAK2 or Tyk 2, suggesting the possible involvement of an unidentified tyrosine kinase.

The targeted disruption of both alleles of RARbeta(2) in F9 cells results in the loss of retinoic acid-associated growth arrest

F9 teratocarcinoma cell lines, carrying one or two disrupted alleles of the RARbeta(2) gene, were generated by homologous recombination to study the role of RARbeta(2) in mediating the effects of retinoids on cell growth and differentiation. Retinoic acid (RA) does not induce growth arrest of the RARbeta(2)-/- cells, whereas the F9 WT and RARbeta(2)+/- heterozygote lines undergo RA-induced growth arrest. The RARbeta(2)+/- lines also exhibit a faster cell cycle transit time in the absence of RA. The RARbeta(2)-/- stem cells exhibit an altered morphology when compared with the F9 WT parent line, and after RA treatment, the RARbeta(2)-/- cells do not exhibit a fully differentiated cell morphology. As compared with F9 WT cells, the RARbeta-/- cells exhibited a markedly lower induction of several early RA-responsive genes and no induction of laminin B1, a late response gene. The induction of RA metabolism in the F9 RARbeta(2)-/- cells following differentiation was not impaired. The research presented here, and prior research suggest that RARbeta is required for RA-induced growth arrest in a variety of cell types and that RARbeta also functions in mediating late responses to RA. These findings are significant in view of the reduced expression of RARbeta transcripts in a number of different types of human carcinomas.

Signal relay by retinoic acid receptors alpha and beta in the retinoic acid-induced expression of insulin-like growth factor-binding protein-3 in breast cancer cells

Neither retinoic acid receptor-beta (RARbeta) nor insulin-like growth factor-binding protein-3 (IGFBP-3) is expressed in breast cancer cell line MCF-7. The expression of both proteins can be induced in response to all-trans-retinoic acid (atRA). By using an RARalpha-selective antagonist (Ro 41-5253), we demonstrated that RARbeta expression was induced by atRA through an RARalpha-dependent signaling pathway and that RARbeta induction was correlated with IGFBP-3 induction. However, MCF-7 cells transfected with sense RARbeta cDNA expressed IGFBP-3 even in the presence of the RARalpha-selective antagonist Ro 41-5253. On the other hand, antisense RARbeta cDNA transfection of MCF-7 cells blocked atRA-induced IGFBP-3 expression, indicating that RARbeta is directly involved in the mediation of IGFBP-3 induction by atRA. Induction of IGFBP-3 expression by atRA occurs at the transcriptional level, as measured by nuclear run-on assays. Finally, we showed that atRA-induced IGFBP-3 is functionally active in modulating the growth-promoting effect of IGF-I. These experiments indicate that RARalpha and RARbeta, both individually and together, are important in mammary gland homeostasis and breast cancer development. By linking IGFBP-3 to RARbeta, our experiments define the signal intersection between the retinoid and IGF systems in cell growth regulation and explain why loss of RARbeta might be critical in breast cancer carcinogenesis/progression.

IFI60/ISG60/IFIT4, a new member of the human IFI54/IFIT2 family of interferon-stimulated genes

We report the cloning and sequencing of a full-length cDNA encoding a new member of the human IFI54 (HGMW-approved symbol IFIT2) gene family, designated IFI60 (HGMW-approved symbol IFIT4). The upstream regulatory region of IFI60 shows conservation in structure with that of the IFI54 and IFI56 (HGMW-approved symbol IFIT1) genes, each containing two interferon-stimulated response elements upstream of a conserved TATA box. We have established a partial gene map of the IFI54 gene family by analysis of YAC library clones. All four members of the human family are clustered together at chromosome 10q23.3. It is proposed that the four members of the IFI54 gene family evolved by a series of duplication events from a common gene of origin.

Repression of transactivation of the retinoic acid receptor beta2 promoter in human breast cancer cells

In previous studies, we have shown that the expression of retinoic acid receptor beta2 (RARbeta2) is altered in certain breast cancer cell lines. To investigate the mechanism responsible for this change, we studied in detail the RARbeta2 promoter in cell lines which demonstrated altered expression and compared these results to cell lines in which RARbeta2 was expressed normally. Direct DNA sequencing failed to identify alterations in the sequences of the known response elements in the cell lines manifesting altered expression patterns. By contrast, electrophoretic mobility shift studies of the proteins binding to these response elements demonstrated striking differences in the cells in which expression was altered, when compared to patterns seen in normal cells. Moreover, transient transfection studies using constructs of the RARbeta2 promoter demonstrated an absence of transactivation in the lines in which the expression of this gene was altered. These data suggest that the mechanism responsible for loss of induction of RARbeta2 in breast tumor cells is, at least in part, transcriptional repression.

Differential expression and biological activity of retinoic acid-induced TGFbeta isoforms in embryonic palate mesenchymal cells

The effect of retinoic acid (RA) on TGF-beta mRNA expression and protein production in murine embryonic palate mesenchymal (MEPM) cells was examined by Northern blotting and TGF-beta bioassay in association with TGF-beta isoform-specific neutralizing antibodies. Heat or acid activation was used to distinguish between latent and active TGF-beta protein released into the culture medium. RA had little or no effect on TGF-beta1 mRNA expression and protein production. In contrast, RA increased TGF-beta2 and beta3 protein released into the culture medium, the protein being mostly in an inactive or latent form. The amount of active TGF-beta released was increased relative to the total increase in TGF-beta released, suggesting that RA treatment stimulated activation of latent TGF-beta. RA also increased TGF-beta2 mRNA expression; we have previously shown that RA upregulates TGF-beta3 mRNA in these cells. RA and TGF-beta individually inhibited 3H-thymidine incorporation into MEPM cell DNA, while, when administered simultaneously, they inhibited proliferative activity to a greater extent. Heat- or acid-activated conditioned medium (CM) from MEPM cells treated with RA was able to inhibit 3H-thymidine incorporation into MEPM cell DNA to an extent greater than seen with RA treatment alone. Coincubation of heat-activated CM from RA-treated MEPM cells with pan-specific or TGF-beta2 or beta3-specific neutralizing antibodies partially relieved the inhibitory effect on 3H-thymidine incorporation, suggesting that this proliferative response was due to RA-induced TGF-beta. Simultaneous treatment with RA and TGF-beta also stimulated gycosaminoglycan (GAG) synthesis to an extent greater than that seen with TGF-beta treatment alone, this despite the ability of RA to inhibit GAG synthesis. These data demonstrate a role for RA and RA-induced TGF-beta in the regulation of palate cell proliferation and GAG synthesis and suggest a role for TGF-beta in retinoid-induced cleft palate.

Expression of nuclear retinoic acid receptor and retinoid X receptor mRNA in the cornea and conjunctiva

PURPOSE

The effects of retinoic acid in cells are mediated by the nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Although vitamin A is essential for the normal development and maintenance of the ocular surface, the RARs and RXRs have not been studied in cornea and conjunctiva. The purpose of this study was to identify the mRNA for these receptors in corneal and conjunctival cells in culture and to determine whether all-trans retinoic acid is able to induce expression of RAR mRNA.

METHODS

Total RNA was extracted from cultured rabbit corneal stroma and conjunctival fibroblasts and rabbit corneal epithelial cells. RNA was analyzed by Northern blotting using the cDNA probes for RAR alpha, RAR beta, RAR gamma, RXR alpha, RXR beta and RXR gamma mRNA. To investigate induction of retinoid receptors, cells were exposed to 10(-6) M all-trans retinoic acid for 2-48 h before preparation of RNA. Effects of retinoic acid on cell proliferation were also investigated.

RESULTS

RAR alpha mRNA transcripts (3.7 kb), RAR beta mRNA transcripts (3.3 kb) and RAR gamma mRNA transcripts (3.3 kb) are expressed by all the cell types studied, as are the RXR alpha mRNA transcripts (5.0 kb) and RXR beta mRNA transcripts (3.3 kb). RXR gamma mRNA is not detectable in corneal and conjunctival cells. All-trans retinoic acid induced RAR beta mRNA expression in corneal and conjunctival fibroblasts. Increased mRNA levels were detectable after 4-8 h and peaked by 24 h. RAR beta mRNA was not induced by retinoic acid in corneal epithelial cells. Retinoic acid also inhibited proliferation of conjunctival and corneal fibroblasts but had no effect on growth of corneal epithelial cells.

CONCLUSIONS

The expression of RARs and RXRs in the cornea and conjunctiva is similar to that reported in other tissues. The identification of these receptors may lead to a better understanding of gene transcription pathways in the cornea and conjunctiva and of the mechanisms that control keratinization, differentiation and proliferation of the cells of these tissues. The data suggest a relationship between the induction of RAR beta mRNA expression and inhibition of cell proliferation by retinoic acid.

Arsenic trioxide as an inducer of apoptosis and loss of PML/RAR alpha protein in acute promyelocytic leukemia cells

BACKGROUND

Retinoids, which are derivatives of vitamin A, induce differentiation of acute promyelocytic leukemia (APL) cells in vitro and in patients. However, APL cells develop resistance to retinoic acid treatment. Arsenic trioxide (As2O3) can induce clinical remission in patients with APL, including those who have relapsed after retinoic acid treatment, by inducing apoptosis (programmed cell death) of the leukemia cells. In this study, we investigated the molecular mechanisms by which As2O3 induces apoptosis in retinoic acid-sensitive NB4 APL cells, in retinoic acid-resistant derivatives of these cells, and in fresh leukemia cells from patients.

METHODS

Apoptosis was assessed by means of DNA fragmentation analyses, TUNEL assays (i.e., deoxyuridine triphosphate labeling of DNA nicks with terminal deoxynucleotidyl transferase), and flow cytometry. Expression of the PML/RAR alpha fusion protein in leukemia cells was assessed by means of western blotting, ligand binding, and immunohistochemistry. Northern blotting and ribonuclease protection assays were used to evaluate changes in gene expression in response to retinoic acid and As2O3 treatment.

RESULTS AND CONCLUSIONS

As2O3 induces apoptosis without differentiation in retinoic acid-sensitive and retinoic acid-resistant APL cells at concentrations that are achievable in patients. As2O3 induces loss of the PML/RAR alpha fusion protein in NB4 cells, in retinoic-acid resistant cells derived from them, in fresh APL cells from patients, and in non-APL cells transfected to express this protein. As2O3 and retinoic acid induce different patterns of gene regulation, and they inhibit the phenotypes induced by each other. Understanding the molecular basis of these differences in the effects of As2O3 and retinoic acid may guide the clinical use of arsenic compounds and provide insights into the management of leukemias that do not respond to retinoic acid.

Expression of retinoid X receptors in human dermal fibroblasts

Retinoic acid (RA) is known to exert profound effects on growth and differentiation in human dermal fibroblasts. In the observations presented here, we examined the regulation of expression of members of the RXR multigene family in human dermal fibroblasts. We showed that the messenger RNAs for both RXR alpha and RXR beta are expressed in human fibroblasts, but that the messenger RNA for RXR gamma is not detectable in these cells. Electrophoretic mobility shift studies of binding to the beta 2RARE in human dermal fibroblasts demonstrated that a single complex binds to beta 2RARE in the absence of RA. Stimulating cells with all-trans RA induced a second complex. An antibody to the RXR beta protein supershifted both complexes, while an antibody to the RXR alpha S/B protein had no effect on the binding. These data demonstrate that RXR beta plays an important role in retinoid-regulated signal transduction pathways in human dermal fibroblasts and the regulation of expression of the RXR gene family is different from that of the RAR gene family.