Retinoids selective for retinoid X receptor response pathways.

Retinoids have a broad spectrum of biological activities and are useful therapeutic agents. Their physiological activities are mediated by two types of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). RARs, as well as several related receptors, require heterodimerization with RXRs for effective DNA binding and function. However, in the presence of 9-cis-retinoic acid, a ligand for both RARs and RXRs, RXRs can also form homodimers. A series of retinoids is reported that selectively activates RXR homodimers but does not affect RAR-RXR heterodimers and thus demonstrates that both retinoid response pathways can be independently activated.

Cytochrome c release and apoptosis induced by mitochondrial targeting of nuclear orphan receptor TR3.

TR3, an immediate-early response gene and an orphan member of the steroid-thyroid hormone-retinoid receptor superfamily of transcription factors, regulates apoptosis through an unknown mechanism. In response to apoptotic stimuli, TR3 translocates from the nucleus to mitochondria to induce cytochrome c release and apoptosis. Mitochondrial targeting of TR3, but not its DNA binding and transactivation, is essential for its proapoptotic effect. Our results reveal a mechanism by which a nuclear transcription factor translocates to mitochondria to initiate apoptosis.

Regulation of Nur77 nuclear export by c-Jun N-terminal kinase and Akt.

Proapoptotic nuclear receptor family member Nur77 translocates from the nucleus to the mitochondria, where it interacts with Bcl-2 to trigger apoptosis. Nur77 translocation is induced by certain apoptotic stimuli, including the synthetic retinoid-related 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN)/CD437 class. In this study, we investigated the molecular mechanism by which AHPN/CD437 analog (E)-4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC) induces Nur77 nuclear export. Our results demonstrate that 3-Cl-AHPC effectively activated Jun N-terminal kinase (JNK), which phosphorylates Nur77. Inhibition of JNK activation by a JNK inhibitor suppressed 3-Cl-AHPC-induced Nur77 nuclear export and apoptosis. In addition, several JNK upstream activators, including the phorbol ester TPA, anisomycin and MAPK kinase kinase-1 (MEKK1), phosphorylated Nur77 and induced its nuclear export. However, Nur77 phosphorylation by JNK, although essential, was not sufficient for inducing Nur77 nuclear export. Induction of Nur77 nuclear export by MEKK1 required a prolonged MEKK1 activation and was attenuated by Akt activation. Expression of constitutively active Akt prevented MEKK1-induced Nur77 nuclear export. Conversely, transfection of dominant-negative Akt or treatment with a phosphatidylinositol 3-kinase (PI3-K) inhibitor accelerated MEKK1-induced Nur77 nuclear export. Furthermore, mutation of an Akt phosphorylation residue Ser351 in Nur77 abolished the effect of Akt or the PI3-K inhibitor. Together, our results demonstrate that both activation of JNK and inhibition of Akt play a role in translocation of Nur77 from the nucleus to the cytoplasm.

Retinoids. Structure-function relationship in normal and leukemic hematopoiesis in vitro.

Retinoids were studied both to identify what skeletal components are important in the modulation of normal and leukemic human myeloid clonal proliferation and differentiation in vitro and to elucidate the mechanism by which retinoids modulate proliferation of hematopoietic cells. Retinoids with a derivatized terminal carboxyl group were significantly less active than all-trans-retinoic acid, and those with the addition of two methyl groups to the cyclohexenyl ring of retinoic acid or substitution of its beta-cyclogeranylidene group with a 1,1,3,3-5-indanyl ring system were markedly more active than all-trans-retinoic acid. Five of the retinoids strongly inhibited clonal growth of the HL-60 and KG-1 human leukemic cell lines (50% inhibition in the range of 3 X 10(-10)-1 X 10(-8) M) and markedly stimulated normal human myeloid colony formation (granulocyte-macrophage colony-forming cells [GM-CFC] 150% stimulation in the range of 3 X 10(-9)-3 X 10(-8) M). Further studies suggested that: Common structural requirements of the retinoids were important in the modulation of both normal and leukemic hematopoiesis. The retinoids were able to inhibit leukemic proliferation without induction of differentiation of the neoplastic cells. Studies on normal human GM-CFC suggested that the retinoids did not act by themselves as a colony-stimulating factor (CSF), or by stimulating accessory cells to produce CSF, but either required earlier progenitor cells to become GM-CFC or enhanced the sensitivity of GM-CFC to the action of CSF.

All-trans retinoic acid converts E2F into a transcriptional suppressor and inhibits the growth of normal human bronchial epithelial cells through a retinoic acid receptor- dependent signaling pathway.

Retinoids, including retinol and retinoic acid derivatives, maintain the normal growth and differentiation of human bronchial epithelial (HBE) cells and are under investigation as agents for lung cancer prevention. In this study, we examined the biologic effects of retinoids on normal HBE cells and the molecular mechanisms of retinoid actions. At a dose of 10(-6) M, all-trans retinoic acid (t-RA) suppressed the proliferation of normal HBE cells, which accumulated in the G0 phase. No evidence of programmed cell death was observed. The class of retinoid nuclear receptor that mediated the growth arrest was explored. Normal HBE cell growth was suppressed by a retinoid that selectively activates retinoic acid receptors but not by one that activates retinoid X receptors. The E2F transcription factor has demonstrated a role in G0 entry through transcriptional suppression of genes that induce cell cycle progression. To investigate the role of E2F in retinoid signaling, transient transfection assays were performed using reporter plasmids containing E2F-binding sites. Findings from these experiments suggested that t-RA treatment converted E2F into a transcriptional suppressor. Supporting this possibility, t-RA inhibited the expression of the E2F target genes B-myb, cyclin A, and cyclin E. Further, t-RA increased the levels of nuclear E2F-4, p107, and p130 and enhanced the binding of E2F-4 to p107, which have been associated with the conversion of E2F into a transcriptional suppressor in other cells. These findings point to retinoic acid receptor- and E2F-dependent pathways as potential mediators of retinoid-induced growth arrest in normal HBE cells and have implications for the use of retinoids in clinical trials on the prevention of lung cancer.

The receptor-DNA complex determines the retinoid response: a mechanism for the diversification of the ligand signal.

To obtain insights into the principles governing the complex biological responses to retinoids, we have analyzed the ligand sensitivities of various retinoid receptor-DNA complexes. We find that different retinoid receptor heterodimers show distinct activation patterns with various response elements while a given heterodimer can be activated at different retinoic acid concentrations on different response elements. In vitro binding experiments suggest that the same retinoic acid receptor-retinoid X receptor (RAR-RXR) heterodimer can have different ligand affinities, depending on the response element it is bound to. The differential responses of a particular receptor heterodimer with various retinoic acid responsive elements can be enhanced through the use of conformationally restricted retinoids. RAR- and RXR-selective retinoids can also synergistically activate the receptor heterodimers, indicating that both partners in the heterodimer can contribute to ligand-induced transcriptional activation. However, the relative influence of the RAR or RXR partner is specific for each response element. Together, our data demonstrate that it is the receptor-DNA complex and not the receptor alone that determines the ligand response. This flexibility allows for a highly pleiotropic retinoid response. Furthermore, conformationally restricted retinoids can accentuate the differential responses and exhibit a certain degree of gene selectivity by differentially activating the RAR or RXR component in the context of a given response element.

Inhibition of trans-retinoic acid-resistant human breast cancer cell growth by retinoid X receptor-selective retinoids.

All-trans-retinoic acid (trans-RA) and other retinoids exert anticancer effects through two types of retinoid receptors, the RA receptors (RARs) and retinoid X receptors (RXRs). Previous studies demonstrated that the growth-inhibitory effects of trans-RA and related retinoids are impaired in certain estrogen-independent breast cancer cell lines due to their lower levels of RAR alpha and RARbeta. In this study, we evaluated several synthetic retinoids for their ability to induce growth inhibition and apoptosis in both trans-RA-sensitive and trans-RA-resistant breast cancer cell lines. Our results demonstrate that RXR-selective retinoids, particularly in combination with RAR-selective retinoids, could significantly induce RARbeta and inhibit the growth and induce the apoptosis of trans-RA-resistant, RAR alpha-deficient MDA-MB-231 cells but had low activity against trans-RA-sensitive ZR-75-1 cells that express high levels of RAR alpha. Using gel retardation and transient transfection assays, we found that the effects of RXR-selective retinoids on MDA-MB-231 cells were most likely mediated by RXR-nur77 heterodimers that bound to the RA response element in the RARbeta promoter and activated the RARbeta promoter in response to RXR-selective retinoids. In contrast, growth inhibition by RAR-selective retinoids in trans-RA-sensitive, RAR alpha-expressing cells most probably occurred through RXR-RAR alpha heterodimers that also bound to and activated the RARbeta promoter. In MDA-MB-231 clones stably expressing RAR alpha, both RARbeta induction and growth inhibition by RXR-selective retinoids were suppressed, while the effects of RAR-selective retinoids were enhanced. Together, our results demonstrate that activation of RXR can inhibit the growth of trans-RA-resistant MDA-MB-231 breast cancer cells and suggest that low cellular RAR alpha may regulate the signaling switch from RAR-mediated to RXR-mediated growth inhibition in breast cancer cells.

Molecular determinants of AHPN (CD437)-induced growth arrest and apoptosis in human lung cancer cell lines.

6-[3-(1-Adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN or CD437), originally identified as a retinoic acid receptor gamma-selective retinoid, was previously shown to induce growth inhibition and apoptosis in human breast cancer cells. In this study, we investigated the role of AHPN/CD437 and its mechanism of action in human lung cancer cell lines. Our results demonstrated that AHPN/CD437 effectively inhibited lung cancer cell growth by inducing G0/G1 arrest and apoptosis, a process that is accompanied by rapid induction of c-Jun, nur77, and p21(WAF1/CIP1). In addition, we found that expression of p53 and Bcl-2 was differentially regulated by AHPN/CD437 in different lung cancer cell lines and may play a role in regulating AHPN/CD437-induced apoptotic process. On constitutive expression of the c-JunAla(63,73) protein, a dominant-negative inhibitor of c-Jun, in A549 cells, nur77 expression and apoptosis induction by AHPN/CD437 were impaired, whereas p21(WAF1/CIP1) induction and G0/G1 arrest were not affected. Furthermore, overexpression of antisense nur77 RNA in A549 and H460 lung cancer cell lines largely inhibited AHPN/CD437-induced apoptosis. Thus, expression of c-Jun and nur77 plays a critical role in AHPN/CD437-induced apoptosis. Together, our results reveal a novel pathway for retinoid-induced apoptosis and suggest that AHPN/CD437 or analogs may have a better therapeutic efficacy against lung cancer.

Post-transcriptional regulation of the DNA damage-inducible gadd45 gene in human breast carcinoma cells exposed to a novel retinoid CD437.

The biologically active synthetic retinoid CD437 (6-[3-adamantyl-4-hydroxyphenyl]-2-naphthalene, AHPN) and different human breast carcinoma (HBC) cell lines were used to examine the possible mechanism(s) of gadd45 induction. Northern blot analysis of mRNA isolated from MCF-7, MDA-MB-468 and MDA-MB-231 HBC cell lines demonstrated a progressive increase in the 1.4 kb gadd45 transcript after exposure to 1 microM CD437. Western blot analysis showed increased gadd45 protein levels in MDA-MB-468 HBC cells following exposure to CD437. CD437 increased gadd45 mRNA levels by approximately 20-fold in MDA-MB-468 cells, however, the transcriptional activity was increased approximately 2-3-fold as demonstrated by the human gadd45 promoter-luciferase reporter construct and nuclear run-off assays. Sublines of MDA-MB-468 HBC cells expressing stably integrated GADD45 cDNA fragments were obtained and CD437-dependent induction of GADD45 analyzed. We report that approximately 300 nt located in the 5"-untranslated region (5"-UTR) of gadd45 mRNA are involved in the CD437-dependent 4-fold enhanced stability of gadd45 transcripts. MDA-MB-468 cells were stably transfected with either a plasmid having a CMV promoter-driven rabbit beta-globin gene or plasmids having a CMV promoter-driven chimeric gadd45 5"-UTR-rabbit beta-globin gene, where the entire gadd45 5"-UTR (from +1 to +298) or a 45 bp subfragment of the gadd45 5"-UTR (from +10 to +55) was positioned at the 5"-end of the rabbit beta-globin gene. CD437 was found to up-regulate expression of both the chimeric gadd45 -rabbit beta-globin transcripts, suggesting that cis element(s) involved in the CD437-dependent enhanced stability of gadd45 mRNA are contained in the 45 nt of the 5"-UTR of the gadd45 mRNA.

Cyclin D1 proteolysis: a retinoid chemoprevention signal in normal, immortalized, and transformed human bronchial epithelial cells.

BACKGROUND: Retinoids (derivatives of vitamin A) are reported to reduce the occurrence of some second primary cancers, including aerodigestive tract tumors. In contrast, beta-carotene does not reduce the occurrence of primary aerodigestive tract cancers. Mechanisms explaining these effective retinoid and ineffective carotenoid chemoprevention results are poorly defined. Recently, the all-trans-retinoic acid (RA)-induced proteolysis of cyclin D1 that leads to the arrest of cells in G1 phase of the cell cycle was described in human bronchial epithelial cells and is a promising candidate for such a mechanism. In this study, we have investigated this proteolysis as a common signal used by carotenoids or receptor-selective and receptor-nonselective retinoids. METHODS: We treated cultured normal human bronchial epithelial cells, immortalized human bronchial epithelial cells (BEAS-2B), and transformed human bronchial epithelial cells (BEAS-2BNNK) with receptor-selective or receptor-nonselective retinoids or with carotenoids and studied the effects on cell proliferation by means of tritiated thymidine incorporation and on cyclin D1 expression by means of immunoblot analysis. We also examined whether calpain inhibitor I, an inhibitor of the 26S proteasome degradation pathway, affected the decline (i.e., proteolysis) of cyclin D1. RESULTS: Receptor-nonselective retinoids were superior to the carotenoids studied in mediating the decline in cyclin D1 expression and in suppressing the growth of bronchial epithelial cells. Retinoids that activated retinoic acid receptor beta or retinoid X receptor pathways preferentially led to a decrease in the amount of cyclin D1 protein and a corresponding decline in growth. The retinoid-mediated degradation of cyclin D1 was blocked by cotreatment with calpain inhibitor I. CONCLUSIONS: Retinoid-dependent cyclin D1 proteolysis is a common chemoprevention signal in normal and neoplastic human bronchial epithelial cells. In contrast, carotenoids did not affect cyclin D1 expression. Thus, the degradation of cyclin D1 is a candidate intermediate marker for effective retinoid-mediated cancer chemoprevention in the aerodigestive tract.

Inhibition by retinoids of anthralin-induced mouse epidermal ornithine decarboxylase activity and anthralin-promoted skin tumor formation.

The retinoids all-trans-retinoic acid, 13-cis-retinoic acid, 4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1E- propen-1-yl]benzoic acid, 6-[1-(4-carboxyphenyl)-1E-propen-2-yl]-3,4-dihydro-4,4-dimethyl-2H -1-benzothiopyran, and 6-(5,6,7,8,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)- 2-naphthalenecarboxylic acid inhibited the induction of ornithine decarboxylase in CD-1 mouse epidermis treated with the weak tumor promoter anthralin (444 nmol). Enzyme activity reached maximal levels 48 h after the application of the promoter. This activity was most effectively inhibited when retinoids were applied to the epidermis 24 h after the promoter. These retinoids also inhibited the appearance of papillomas in mouse epidermis in the two-stage tumorigenesis model using 7,12-dimethylbenz(a)anthracene (200 nmol) as the initiator and anthralin (444 nmol) as the promoter during the 32-week period of promotion. Comparison of the doses of retinoids required to inhibit anthralin-induced ornithine decarboxylase by 50% and those required to inhibit anthralin-induced tumor promotion by 50% demonstrated that these values correlated.

Inhibition of tumor promoter-induced transformation by retinoids that transrepress AP-1 without transactivating retinoic acid response element.

Both retinoic acid (RA) treatment and dominant-negative c-Jun mutant expression effectively inhibit phorbol ester-induced AP-1 activity and induced neoplastic transformation in mouse epidermal JB6 cells. However, both reagents also target non-AP-1 molecules in addition. Because liganded retinoic acid receptors interact with and transactivate RA response elements (RAREs) on DNA, as well as interact with Jun protein to block AP-1 activity, the question arises as to which of these two activities of retinoids is responsible for antitumor-promoting activity. To address this question we generated JB6 promotion-sensitive (P+) cell lines that are stably transfected with a construct containing the collagenase promoter bearing one AP-1-binding site that drives a luciferase reporter gene. The stable collagenase-luciferase-transfected cell lines showed 1.5-3.5-fold enhanced AP-1 activity when treated with 12-0-tetradecanoyl-phorbol-13-acetate (TPA). Up to 90% of TPA-induced AP-1 activity was blocked by retinoids SR11238, SR11302, or trans-RA, but not by retinoid SR11235. Of these retinoids, only RA and SR11235 were able to transactivate RARE-dependent gene expression. Transrepression of TPA-induced AP-1 and transactivation of RARE by RA, SR11238, and SR11302 were concentration dependent at 10(-10) to 10(-6) M retinoid. When tested for activity in inhibiting tumor promoter-induced transformation in JB6 P+ cells, the retinoids specific for AP-1 transrepression were inhibitory, whereas SR11235, which only activated RARE, showed little effect. We thus conclude that the AP-1-blocking activity of retinoids is likely to be responsible for the antitumor-promoting activity. This result, together with the observation that dominant-negative Jun blocks transformation, argues for a requirement of induced AP-1 in the tumor promoter-induced transformation process.

Relationship between binding affinities to cellular retinoic acid-binding protein and biological potency of a new series of retinoids.

Binding affinities of a new and unusual series of retinoic acid analogues to cellular retinoic acid-binding protein, a possible mediator of their biological function in the control of differentiation and tumorigenesis, and to serum albumin, their plasma transport protein, were determined. Also, biological activity of these retinoids in the reversal of keratinization in hamster tracheal organ cultures was assessed and compared with their binding affinities. Analogues that possessed high biological activity showed high binding efficiency to cellular retinoic acid-binding protein. Those that were biologically less active were poor binders to the binding protein. Three retinoids, 4657-57, 3920-59, and 4445-75, which showed 90 to 100% binding efficiency of that of retinoic acid for cellular retinoic acid-binding protein expressed high biological activity detectable in the range of 10(-10) M as against 10(-11) M for retinoic acid. The correlation noticed in these two activities not only enhances the confidence in the two assay procedures but also paves the way for design and development of potential chemopreventive agents. No apparent differences were observed in the binding affinities of the retinoids to binding proteins of a normal tissue or a tumor tissue. No correlation existed between the binding affinities of these retinoids to serum albumin and their biological activity. Structure-activity relationships of the retinoids in relation to their binding affinities and biological activities have been discussed.

Identification of retinoids with nuclear receptor subtype-selective activities.

Retinoic acid (RA) and its synthetic analogues, retinoids, have shown promising results in the prevention of epithelial carcinogenesis and in the treatment of acute promyelocytic leukemia and various proliferative skin disorders. Retinoid action on gene regulation is mediated by three distinct nuclear retinoic acid receptor subtypes, RA receptors alpha, beta, and gamma. The existence of multiple RA receptors has raised the possibility that receptor subtype-specific retinoids with reduced side effects can be developed. To analyze the activity of retinoids at the molecular level, we used a receptor activation assay. RA and 22 retinoids were compared on the three receptor subtypes. We found the alpha receptor to be least sensitive to activation by RA and the gamma receptor to be most sensitive. Compared with RA, one of the retinoids showed increased activity for the alpha and beta receptors. Three retinoids revealed no gene activation activity and showed no antagonistic effects when assayed in the presence of RA. Surprisingly, several of the retinoids were efficient activators of the beta and gamma receptors but poor activators or nonactivators of the alpha receptor. Our data demonstrate that the three RA receptor subtypes have differential ligand activation specificities and that the design of receptor subtype-selective retinoids is possible.

Enhanced efficacy of combinations of retinoic acid- and retinoid X receptor-selective retinoids and alpha-interferon in inhibition of cervical carcinoma cell proliferation.

Retinoic acid receptors and retinoid X receptors form heterodimers, bind to retinoic acid response elements, and transactivate the transcription of retinoid-responsive genes. Two synthetic retinoids [4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-anthracenyl)benzoic acid (TTAB) and 6-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-2-naphthale n ecarboxylic acid (TTNN)], which preferentially bind retinoic acid receptors, inhibited the proliferation of cervical carcinoma ME180 cells by 50% at 0.2 nM and 0.2 microM, respectively. In contrast, two other retinoids [2-(4-carboxyphenyl)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)-1,3-dithiane (SR11203) and 4-(2-methyl-1-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)propenyl)benzoic acid (SR11217)], which preferentially bind retinoic X receptors, inhibited growth by only 12 and 18% at 1 microM, respectively. The combination of suboptimal concentrations of TTAB (0.1 nM) or TTNN (10 nM) with each of the retinoic X receptor-selective retinoids at 1 microM showed more than additive effects on cell proliferation, especially with SR11217. Further increases in proliferation inhibition were observed when IFN-alpha (100 units/ml) was added to these retinoid combinations. Activation of transcription of a reporter gene linked 3' to the retinoic acid receptor beta retinoic acid response element in transiently transfected cells also exhibited additive effects when the cells were treated with combinations of TTAB or TTNN with SR11217. This additive activation of transcription may be the reason why the combination of retinoids is more effective than each retinoid alone. The results also suggest that the use of combinations of retinoids and IFN-alpha may lead to enhanced antitumor effects.

Correlation of retinoid binding affinity to retinoic acid receptor alpha with retinoid inhibition of growth of estrogen receptor-positive MCF-7 mammary carcinoma cells.

Both anchorage-dependent growth and anchorage-independent growth of the estrogen receptor-positive mammary carcinoma cell line MCF-7 are inhibited by all-trans-retinoic acid. This cell line has nuclear retinoic acid receptors (RARs) alpha and gamma. The natural retinoids all-trans-retinoic acid and 9-cis-retinoic acid and a series of 12 conformationally restricted retinoids, which showed a range of binding selectivities for these receptors and had either agonist or antagonist activity for gene transcriptional activation by the RARs, were evaluated for their abilities to inhibit anchorage-dependent (adherent) and anchorage-independent (clonal) growth of MCF-7 cells. Correlation analyses were performed to relate growth inhibition by these retinoids with their binding affinity to RAR alpha or RAR gamma. Inhibition of anchorage-dependent growth in culture after 7 days of retinoid treatment correlated with binding to RAR alpha (n = 14; P < or = 0.001) and not to RAR gamma (n = 14; P > 0.1). Both the RAR alpha-selective retinoid agonists and the two RAR antagonists that were evaluated inhibited adherent cell growth. The RAR gamma-selective agonists had very low growth inhibitory activity (< 10%) at concentrations as high as 12.5 microM. These results suggest that RAR alpha is the retinoid receptor involved in the inhibition of adherent cell growth by retinoids and that transcriptional activation by this receptor on a RAR response element does not appear to be required for this process to occur. For this series of retinoids, inhibition of anchorage-independent growth after 21 days of retinoid treatment only correlated (n = 12; P < or = 0.005) with binding affinity to RAR alpha for the retinoid agonists, although the RAR gamma-selective retinoids displayed weak activity. The RAR antagonists were very poor inhibitors of growth. These results suggest that activation of gene transcription by RAR alpha appears to be required for inhibition of anchorage-independent growth by retinoids in this estrogen receptor-positive mammary carcinoma cell line.

S-phase arrest and apoptosis induced in normal mammary epithelial cells by a novel retinoid.

The addition of all-trans-retinoic acid has been found to mediate a G1 cell cycle phase arrest but not apoptosis in normal mammary epithelial cells. We have now found that addition of the novel retinoid 6-[3-(1-adamantyl)]-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437), which appears to function through a pathway independent of retinoic acid nuclear receptors, results in an S-phase arrest that is preceded by a 4-fold elevation in the levels of the cyclin-cyclin dependent kinase (cdk) inhibitor p21WAF1/CIP1. Failure to inhibit E2F-1 activation of genes through its phosphorylation by the cyclin cdk2 kinase has been shown to result in S-phase arrest and apoptosis in a number of cell types. Although exposure of the normal mammary cells to CD437 does not result in modulation of cyclin A or cdk2 levels, an increase in E2F-1 levels and a marked inhibition of cyclin A/cdk2 kinase activity are observed. Exposure to CD437 results in enhanced E2F-1 binding to its DNA consensus sequences and transcriptional activity during S phase. We hypothesize that this enhanced E2F-1 transcriptional activity results in S-phase arrest and subsequent apoptosis that has been observed in other systems.

Regulation of the human retinoic acid receptor alpha gene in the estrogen receptor negative human breast carcinoma cell lines SKBR-3 and MDA-MB-435.

Estradiol-mediated enhancement of retinoic acid receptor alpha (RARalpha) expression in the estrogen receptor (ER)-positive human breast carcinoma (HBC) cells results in their sensitivity to RA-mediated growth inhibition (A. K. Rishi et al., Cancer Res., 55: 4999-5006, 1995). Most ER-negative HBCs are known to express lower levels of RARalpha and are resistant to RA-mediated inhibition of growth. We show that ER-negative SKBR-3 and MDA-MB-435 HBCs express approximately 2-fold higher levels of RARalpha isoform 1 mRNA when compared to the ER-negative MDA-MB-231 and MDA-MB-468 HBCs. SKBR-3 cells are sensitive to growth inhibition by RA, and by using RARalpha-selective synthetic retinoids, we demonstrate that the antiproliferative effects of RA in the SKBR-3 cell line are accomplished, in part, via activation of RARalpha. Both MDA-MB-231 and MDA-MB-468 HBCs are not growth inhibited by RA or any of the retinoids tested. Transient transfection experiments using a 5.0-kb RARalpha promoter fragment fused to the luciferase reporter gene showed 2-3-fold higher transcriptional activation in SKBR-3 cells when compared to MDA-MB-468 cells. We report identification of a 72-bp fragment of RARalpha promoter that contains unique cis elements responsible for mediating an estradiol-independent 2.5-fold enhancement of RARalpha gene expression in SKBR-3 and MDA-MB-435 cells.

Posttranscriptional regulation of p21WAF1/CIP1 expression in human breast carcinoma cells.

p21WAF1/CIP1 plays a major role in the induction of G1 arrest following DNA damage. Although p21WAF1/CIP1 expression is regulated by the tumor suppressor p53, induction of p21WAF1/CIP1 expression through p53-independent pathways has been described in numerous cell types. In this report, we describe the mechanism by which the retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) induces p21WAF1/CIP1 in breast carcinoma cells possessing either a wild-type (MCF-7 cells) or mutated (MDA-MB-468 cells) p53. Exposure of MDA-MB-468 cells to this retinoid results in an approximately 10-fold increase in p21WAF1/CIP1 mRNA levels, whereas less than a 2-fold increase in p21WAF1/CIP1 gene transcription was observed as indicated by transient transfection experiments utilizing a p21WAF1/CIP1 promoter firefly luciferase reporter gene construct and nuclear run-off studies. We found similar results in the MCF-7 cells (Z-M. Shao et al., Oncogene, 11: 493-504, 1995). We have now found that while enhancing p21WAF1/CIP1 gene transcription minimally, this retinoid increases p21WAF1/CIP1 mRNA stability by 3-fold in both cell types. We also demonstrate that approximately 1.5 kb of the 3' untranslated region causes enhanced instability of p21WAF1/CIP1 mRNA. The retinoid-dependent increase in p21WAF1/CIP1 mRNA stability is accompanied by an increase in p21WAF1/CIP1 protein expression, as indicated by Western blot experiments utilizing anti-p21WAF1/CIP1 monoclonal antibody. This increase in p21WAF1/CIP1 is subsequently followed by the onset of programmed cell death in both cell types. Thus, CD437 is a novel retinoid which enhances p21WAF1/CIP1 mRNA levels through stabilization of the message regardless of the p53 status of the cell.

Apoptosis induction in cancer cells by a novel analogue of 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid lacking retinoid receptor transcriptional activation activity.

The retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN) is reported to have anticancer activity in vivo. Induction of cell cycle arrest and apoptosis in cancer cell lines refractory to standard retinoids suggests a retinoid-independent mechanism of action for AHPN. Conformational studies suggested that binding of AHPN does not induce an unusual conformation in retinoic acid receptor (RAR) gamma. The 3-chloro AHPN analogue MM11453 inhibited the growth of both retinoid-resistant (HL-60R leukemia, MDA-MB-231 breast, and H292 lung) and retinoid-sensitive (MCF-7 breast, LNCaP prostate, and H460 lung) cancer cell lines by inducing apoptosis at similar concentrations. Before apoptosis, MM11453 induced transcription factor TR3 expression and loss of mitochondrial membrane potential characteristic of apoptosis. MM11453 lacked the ability to significantly activate RARs and retinoid X receptor alpha to initiate (TREpal)(2)-tk-CAT reporter transcription. These results, differential proteolysis-sensitivity assays, and glutathione S-transferase-pulldown experiments demonstrate that, unlike AHPN or the natural or standard synthetic retinoids, MM11453 does not behave as a RAR or retinoid X receptor alpha transcriptional agonist. These studies strongly suggest that AHPN exerts its cell cycle arrest and apoptotic activity by a signaling pathway independent of retinoid receptor activation.