Increased retinoic acid responsiveness in lung carcinoma cells that are nonresponsive despite the presence of endogenous retinoic acid receptor (RAR) beta by expression of exogenous retinoid receptors retinoid X receptor alpha, RAR alpha, and RAR gamma

Nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs) are thought to mediate most of the effects of retinoids on cell growth and differentiation. Despite expressing abundant levels of RAR beta mRNA, lung adenocarcinoma H1792 cells are resistant to the growth-inhibitory effects of all-trans-retinoic acid, suggesting that they have a defect in retinoid signaling. To determine whether transfection of exogenous receptors can restore retinoid responsiveness, we transiently transfected into H1792 cells coexpression vectors containing cDNAs of cell surface antigen CD7 and either RAR alpha, RAR beta, RAR gamma, or RXR alpha. The cells were then treated with retinoids and incubated with 5'-bromo-2'-deoxyuridine. Cells that express exogenous receptor were identified using antibodies against CD7, and cells that synthesized DNA were identified with anti-5'-bromo-2'-deoxyuridine antibodies using secondary antibodies with red and green fluorescence, respectively. RXR alpha and RAR alpha enhanced growth inhibition by all-trans-retinoic acid or 9-cis-retinoic acid, whereas RAR gamma was less effective, and RAR beta was ineffective. The effects of the transfected receptors were associated with antagonism of activator protein 1 (AP-1) activity. Studies with RXR alpha deletion and point mutants indicated that growth suppression is: (a) dependent on intact DNA-binding and ligand-binding regions but not on the NH2-terminal region, which contains a ligand-independent transactivation function; (b) dependent on RXR homodimer formation and transactivation of RXR response element; and (c) associated with AP-1 antagonism. These results demonstrate that transfected receptors can restore responsiveness to retinoids by antagonizing AP-1 in H1792 cells.     

Expression of nuclear retinoid receptors in normal, premalignant and malignant gastric tissues determined by in situ hybridization.

Retinoids exhibit multiple functions through interaction with nuclear retinoid receptors and have growth-suppressive activity on gastric cancer cells. To better understand the roles of nuclear retinoid receptors during gastric carcinogenesis, we have used in situ hybridization to investigate expression of retinoic acid receptors (RARs) and retinoid x receptors (RXRs) in premalignant and malignant formalin-fixed paraffin-embedded gastric tissues. Histological sections of eight normal, 17 distal normal and nine gastric cancer tissues were hybridized with non-radioactive RNA probes for subtypes of RAR and RXR. Expression of RAR alpha, RAR beta, RAR gamma, RXR alpha and RXR beta was found in most cell types in gastric mucosa tissues from normal individuals as well as in distal normal tissues from cancer patients. Expression of RAR alpha and RAR beta were found in three and seven cancer tissues, respectively, and levels of RXR alpha mRNA were significantly decreased in poorly differentiated cancer tissues. Among the five investigated nuclear retinoid receptors, only expression of RAR alpha mRNA was significantly decreased in intestinal metaplasia, dysplasia and cancer tissues when compared to adjacent normal tissues. In conclusion, normal gastric mucosa expressed both RARs and RXRs, which supports the physiological role of retinoic acid on normal gastric mucosa. The decrease in RAR alpha expression in premalignant and malignant gastric tissues suggests a significant role of RAR alpha during gastric carcinogenesis.     

Retinoid receptor expression and its correlation to retinoid sensitivity in non-M3 acute myeloid leukemia blast cells.

All-trans-retinoic acid (ATRA) has significantly improved the treatment results in acute promyelocytic leukemia (M3). In non-M3 acute myeloid leukemia (AML), the effects are less clear, and there is a pronounced heterogeneity in the sensitivity to the growth-inhibitory effects of retinoids in leukemic cells from different non-M3 AML patients. Retinoids exert their effects through a number of nuclear receptors [retinoic acid receptors (RARs) and retinoid X receptors (RXRs)]. In this study, we determined the expression of RAR alpha, RAR beta, RAR gamma, and RXR alpha by real-time PCR in four cell lines and in blast cells from patients with non-M3 AML before and after ATRA incubation. All four receptors were expressed in cells from all 18 tested patient samples and in four myeloid cell lines. In the majority of the patient samples as well as in the cell lines, there was a pattern of high expression of RAR alpha and RXR alpha and low expression of RAR beta and RAR gamma. There was no correlation between the basal expression of any of the retinoid receptors and sensitivity to ATRA. A 24-h exposure to ATRA increased the expression of RAR alpha, RAR beta, RAR gamma, and RXR alpha in 46%, 77%, 30%, and 38% of the samples, respectively. The mean increase in receptor expression was most pronounced for RAR beta and RXR alpha. There was a significant correlation between an increase in RAR beta expression in response to ATRA and sensitivity to ATRA (P < 0.014). No such correlations were found for RAR alpha, RAR gamma, and RXR alpha. The expression of the monocytoid marker CD14 was significantly correlated with increased expression of RAR alpha (P = 0.03). We conclude that RAR alpha, RAR beta, RAR gamma, and RXR alpha are expressed in non-M3 AML blast cells and that ATRA-induced expression of RAR beta may be a marker for retinoid sensitivity.     

Synergistic cytotoxicity exhibited by combination treatment of selective retinoid ligands with taxol (Paclitaxel).

The focus of this study was to develop retinoic acid receptor (RAR) RAR alpha/beta selective agonists with anticancer efficacy and reduced toxicity associated with RAR gamma activity. In these studies, we report the identification and characterization of high-affinity RAR alpha/beta selective agonists with limited RAR gamma activity. These compounds inhibited human tumor cell line proliferation with similar efficacy to that observed for a pan-RAR agonist. However, for most tumor cell lines, the efficacy of these compounds was restricted to the micromolar range. To determine whether the RAR alpha/beta selective agonists could be additive or synergistic with existing agents, we investigated the effects of combining RAR alpha/beta selective agonists with various cytotoxic agents. Our results showed that the alpha/beta selective retinoids dramatically lowered the effective dose of Taxol needed to induce cytotoxicity of a wide range of tumor cell lines. This synergy was specific to tubulin-modifying agents and could not be observed with a variety of other cytotoxic agents of diverse function. Examination of pathways common to Taxol and retinoid signaling revealed that this synergy was related in part to effects on Bcl-2 expression/phosphorylation as well as the activity of the c-Jun NH(2)-terminal kinase and activator protein-1. In contrast, the tubulin polymerization induced by Taxol was not further affected by cotreatment with a variety of retinoid receptor ligands. These observations indicate that potent RAR alpha/beta selective agonists may be of therapeutic benefit in combination with Taxol therapy.     

RAR beta2 suppression in head and neck squamous cell carcinoma correlates with site, histology and age

Retinoids as important growth and differentiation regulating agents have a potential role in the chemoprevention of head and neck squamous cell carcinoma (HNSCC). Despite the promising preclinical and early clinical findings, limitations of application are raised by intrinsic resistance acquired during carcinogenesis. Retinoic acid receptor beta2 (RAR beta2) is one of the proximate mediators of retinoid signalling and its expression is often diminished in early stages of head and neck carcinogenesis. One form of retinoid resistance has been associated with the methylation-induced silencing of the RAR beta gene. We studied primary HNSCC samples of different anatomical sites in respect of methylation, expression and allelic loss of RAR beta gene. A strong correlation (p<0.01) was found between hyper-methylation and reduced expression of RAR beta2, however the allelic loss at 3p24, the locus of RAR beta, did not considerably influence its mRNA level. Hypopharynx tumors showed significantly lower hypermethylation (p<0.05) and higher mRNA expression levels of RAR beta2 compared to the tumors located at other sites of the head and neck. We could also provide evidence that poorly differentiated grade 3 tumors had significantly higher RAR beta2 expression and lower methylation levels (p<0.05) than better differentiated grade 1 and grade 2 tumors. In addition, we found a good correlation between the methylation degree of the RAR beta2 promoter and the ages of patients. Collectively, our results suggest that evaluation of several factors such as tumor location, age, histology and methylation state of the RAR beta gene might contribute to the selection of patients for retinoid-based chemoprevention.     

Nicotine modulates the effects of retinoids on growth inhibition and RAR beta expression in lung cancer cells

Epidemiological and animal studies have demonstrated that vitamin A and its natural and synthetic derivatives, retinoids, are effective agents in preventing the development of tobacco-associated cancers. Unfortunately, clinical trials of retinoids on cigarette smokers have shown lack of efficacy in preventing lung cancer. In our study, we investigated the effect of nicotine on the anti-cancer activity of all trans-retinoic acid (trans-RA) in human lung cancer cells. Our results demonstrated that nicotine could abrogate the growth inhibitory effect of trans-RA by suppressing its ability to induce the expression of RA receptor beta (RAR beta), a tumor suppressor. The inhibitory effect of nicotine was accompanied with induction of orphan receptor TR3. Inhibition of TR3 expression by overexpression of TR3 anti-sense RNA in H460 lung cancer cells strongly prevented the suppressive effect of nicotine on trans-RA activity. Treatment with nicotine or the cotransfection of TR3 expression vector inhibited the induction of RAR beta promoter activity by trans-RA in transient transfection assays. The inhibition of RAR beta promoter activity was due to the interaction of TR3 with orphan receptor COUP-TF, resulting in inhibition of COUP-TF DNA binding and transactivation on the RAR beta promoter. Furthermore, we found that nicotine failed to suppress the effect of a retinoid X receptor (RXR)-selective retinoid SR11237 on inducing both growth inhibition and RAR beta promoter activity, due to the ability of SR11237 to activate the RAR beta promoter through the RXR/TR3 heterodimer. Together, our results demonstrate that nicotine suppresses the growth inhibitory effects of trans-RA by inhibiting RAR beta expression through its induction of TR3 expression and suggest that RXR-selective retinoids may be more effective than classical retinoids for preventing and treating tobacco-associated cancers.     

Retinoic acid nuclear receptor α (RARα), a major role in mediating retinoids inhibition of growth in human breast carcinoma cells

Retinoids mediate their actions via RARs (retinoic acid receptors) and RXRs (retinoid X receptors). Each classes of these nuclear retinoid receptor is further subdivided into three species namely α, β and γ. Recent studies demonstrate that ER - positive HBC cell lines are sensitive and ER -negative cell lines are resistant to growth inhibitory effects of retinoic acid (RA). In this study we look at the expresion of RARs and RXRs in 6 HBC cell lines, we found only RARα mRNA level was strong correlated with ER - status. To further investigate the major role of RARα in retinoid - mediated inhibition of growth, we transfected RARα cDNA in two RA - resistant ER - negative HBC cell lines. Analyses of different clonal populations of RARα transfectants from each cell line revealed growth inhibition by retinoids. Our results demonstrates that RARα plays a major role in mediating retinoids inhibition of growth in HBC cells and adequate levels are required for such actions.