Biomedical subjects

A M Jetten

Publications and source records attributed to A M Jetten.

At least 55 records · Page 3Linked to original sources

PubMed1996

Cloning and regulation of cornifin beta, a new member of the cornifin/spr family. Suppression by retinoic acid receptor-selective retinoids.

In this study, we describe the isolation and characterization of a cDNA clone C12 that encodes a new member of the cornifin/small proline-rich protein (spr) family, which we have named cornifin beta. C12 encodes a 1.1-kilobase pair mRNA and a 24.3-kDa cytosolic protein with a high proline content (19%). Its total amino acid sequence exhibits a 37-66% identity while the first 30 amino acids at the amino terminus are 87% identical to that of members of the cornifin family. At its carboxyl terminus, cornifin beta contains 21 tandem repeats of an octapeptide. Cornifin beta expression is restricted to several squamous epithelia. It is highly expressed in esophagus, tongue, and oral mucosa but, in contrast to cornifin alpha, is not detectable in the epidermis. Both retinoic acid and a retinoid selective for the nuclear retinoic acid receptors were very potent suppressors of cornifin beta expression while an analog selective for the nuclear retinoid X receptors was much less effective, suggesting that a specific retinoid signaling pathway is involved in this suppression. Cornifin beta can function through some of its Gln residues as an amine acceptor in transglutaminase-catalyzed cross-linking reactions. These results indicate that cornifin beta functions as a cross-linked envelope precursor.

Amino Acid Sequence↗

PubMed1996

Localization and expression of cornifin-alpha/SPRR1 in mouse epidermis, anagen hair follicles, and skin neoplasms.

Recently, cornifin-alpha/SPPR1 has been identified as a putative precursor protein of the cornified cell envelope. In this study, the expression and localization of cornifin-alpha/SPPR1 was examined in untreated and tumor promoter-treated mouse skin, hair follicles, and skin neoplasms. Western analysis with antiserum (SQ37A) to a rabbit cornifin-alpha peptide or antiserum (SQ37C) to a human SPRR1 peptide demonstrated a 31-kDa immunoreactive protein in mouse epidermis and Northern analysis revealed the presence of a 1-kb mRNA. Immunohistochemical staining of mouse skin with SQ37A or SQ37C revealed intense and specific staining of the infundibulum, isthmus, and of Henle's layer of the inner root sheath of the lower anagen hair follicle and weak staining of the telogen follicle and the suprabasal layers of the epidermis. Treatment of mouse skin with 12-0-tetradecanoyl-phorbol-13-acetate (TPA) produced a large increase in cornifin-alpha/SPRR1 protein and mRNA. Immunohistochemical localization of cornifin-alpha/SPRR1 in TPA-treated skin indicated that cornifin-alpha/SPRR1 was increased in the suprabasal epidermis but not in the follicle. sn-1,2,-didecanoylglycerol, a model lipid second messenger, produced an increase in cornifin-alpha/SPRR1 protein similar to that of TPA, while mirex, a non-phorbol ester-type promoter had no effect. Topical doses of retinoic acid did not repress TPA-induced cornifin-alpha/SPRR1 expression. Papillomas demonstrated a 10- and 100-fold increase in cornifin-alpha/SPRR1 protein and mRNA, and expression was restricted to suprabasal cells. Squamous cell carcinomas exhibited an intermediate level of cornifin-alpha protein, and expression was restricted to keratinized areas. These data indicate: i) cornifin-alpha/SPRR1 is expressed in mouse skin; ii) cornifin-alpha/SPRR1 is localized to specific areas of the anagen hair follicle with weak staining in the telogen follicle and epidermis; iii) epidermal cornifin-alpha/SPRR1 expression is induced by phorbol ester and sn-1,2-didecanoylglycerol but not mirex, and iv) papillomas and squamous cell carcinomas demonstrate a constitutive increase in cornifin-alpha/SPRR1 in differentiated areas of the neoplasms.

Animals↗

PubMed1996

Gamma-interferon induces an irreversible growth arrest in mid-G1 in mammary epithelial cells which correlates with a block in hyperphosphorylation of retinoblastoma.

In this study, we analyze effects of IFN-gamma on the proliferation of normal human mammary epithelial cells (MECs) and several mammary carcinoma cell lines. We demonstrate that IFN-gamma blocks the proliferation of MECs in a time- and concentration-dependent manner. This growth arrest is irreversible and occurs at a specific stage in the G1 phase of the cell cycle. IFN-gamma caused a rapid (within 12-24 h) down-regulation of cyclin A, c-myc, and cdc2 proteins, as well as a disappearance of hyperphosphorylated forms of the retinoblastoma family proteins, Rb and p130. The synthesis of several other growth control proteins, p53, p21/Waf1, and proliferating cell nuclear antigen, was down-regulated between 24 and 48 h. In MECs synchronized by epidermal growth factor deprivation and released for cell cycle traverse by re-addition of epidermal growth factor to the medium, IFN-gamma was able to block DNA synthesis only if added in the first 6 to 7 h after epidermal growth factor. The block in Rb phosphorylation and cyclin A expression was coordinately regulated during the same narrow window of G1. Several mammary carcinoma cell lines demonstrated resistance to the growth-inhibitory effects of IFN-gamma and did not exhibit down-regulation of cdc2 and cyclin A expression or a change in hyperphosphorylation of Rb when treated with IFN-gamma. Initial studies suggest, in some carcinoma cell lines, that resistance to IFN-gamma may be caused by defects in the IFN-gamma signal transduction pathway (measured by expression of the IFN-gamma-responsive gene GBP), while resistance in others may be due to defects in cell cycle regulatory proteins that are the targets of IFN-gamma action.

Breast↗

PubMed1996

Regulation of retinoid-induced differentiation in embryonal carcinoma PCC4.aza1R cells: effects of retinoid-receptor selective ligands.

Retinoic acid (RA) is a potent inducer of differentiation of embryonal carcinoma PCC4.aza1R cells into mesenchymal stem cells. Induction of Hoxa-1, Hoxa-5, cellular retinoic acid-binding protein (CRABP) I and II, and retinoic acid receptor (RAR)-beta expression occurs early in this multistage program of differentiation. RA is also a potent inducer of these genes in the differentiation-defective mutant PCC4(RA)-1; however, RA is much less effective in the mutant cell line PCC4(RA)-2. The up-regulation of several of these genes by RA is, at least in part, due to increased transcription. It is likely that some of these changes are mediated either directly or indirectly by nuclear retinoid receptors. Previously, we characterized the expression of RARs in PCC4.aza1R and (RA)-1 and (RA)-2 cells. In this study, we show that these cells also express retinoid X receptor (RXR)-alpha, RXR-beta, and RXR-gamma and that RA treatment down-regulates the expression of RXR-gamma. No large differences were found in RXR mRNA expression between parental and mutant cell lines except that PCC4(RA)-1 cells expressed an 8-fold higher level of RXR gamma mRNA than the parental cells. To obtain more insight into the retinoid signaling pathways involved in the regulation of this pathway of differentiation, we examined the action of two retinoid receptor-selective agonists and one antagonist. The RAR-selective retinoid SRI-6751-84 is a very effective inducer of transactivation of beta RARE-tk-LUC, but not of RXRE-tk-CAT, in PCC4.aza1R cells and is a very potent inducer of morphological differentiation and Hoxa-1, Hoxa-5, CRABP II, and RAR-beta expression. In contrast, the RXR-selective retinoid SR11,217, which transactivates the RXRE-tk-CAT effectively, but beta RARE-tk-LUC poorly, is unable to induce differentiation and has little effect on the expression of these early genes. The RAR-alpha-selective antagonist Ro 41-5253, which inhibits RARE-dependent transcriptional activation, has by itself no effect on the differentiation of PCC4.aza1R cells. However, this antagonist is able to block the induction of morphological differentiation by the RAR-selective retinoid as well as the expression of Hoxa-1, Hoxa-5, CRABP II, and RAR-beta. Our data suggest that the activation of RAR signaling pathways is important in initiating the cascade of changes in gene expression that result in the differentiation of PCC4.aza1R into mesenchymal stem cells. In addition, we demonstrate that the two mutant cell lines, PCC4(RA)-1 and PCC4(RA)-2, are defective at different stages of the differentiation program.

Animals↗

PubMed1995

Identification and characterization of a novel squamous cell-associated gene related to PMP22.

In this study, we identify and characterize a novel gene, CL-20, that encodes a 17.8-kDa protein with sequence and structural similarity to the growth arrest-specific gene gas3/peripheral myelin protein gene PMP22. The CL-20 protein exhibits a 43% identity with PMP22. The positions of the four lipophilic domains and the N-glycosylation site of PMP22 are conserved in CL-20, suggesting that it also is an integral membrane glycoprotein. The CL-20 gene is located on human chromosome 12 rather than 17 and encodes a 2.8-kilobase mRNA instead of 1.7-kilobase mRNA. These observations indicate that the CL-20 gene is related to but distinct from PMP22. In contrast to PMP22, CL-20 mRNA and protein are induced during squamous differentiation of rabbit tracheal epithelial cells in vitro, and Northern blot analysis and in situ hybridization demonstrated that CL-20 mRNA is most abundant in squamous epithelia. These results indicate that the high expression of CL-20 is closely correlated with squamous differentiation. The differences in tissue-specific expression and regulation between CL-20 and PMP22 suggest different roles for these two proteins. Retinoids, which inhibit squamous differentiation, repress the induction of CL-20. The retinoic acid receptor-selective retinoid SRI-6751-84 is the most effective in suppressing CL-20, suggesting that the activation of the retinoic acid receptor signaling pathway is important in this suppression.

Amino Acid Sequence↗

PubMed1995

Cloning of the gene encoding the murine orphan receptor TAK1 and cell-type-specific expression in testis.

We have cloned the gene encoding the mouse homologue of the orphan receptor, TAK1, a member of the nuclear receptor superfamily, from a mouse testis cDNA library. The amino acid sequence of mouse TAK1 (mTAK1) is highly homologous to that of human TAK1, with an overall identity of 98%. Northern blot analysis using RNA from different testicular cell types showed that the mTAK1 transcript is predominantly expressed in pachytene spermatocytes at a low level in round spermatids, but not in germ cells at earlier phases of spermatogenesis or in Sertoli cells. Southern analysis using genomic DNA prepared from a panel of hamster/human and mouse/human hybrid cell lines indicated that the TAK1 gene is located on human chromosome 3.

Amino Acid Sequence↗

PubMed1995

The orphan receptor TAK1 acts as a repressor of RAR-, RXR- and T3R-mediated signaling pathways.

Recently, we reported the cloning and characterization of the novel orphan receptor TAK1. In this study, we analyze the interaction of TAK1 with a variety of response elements (RE's) and demonstrate that TAK1 binds effectively to RE's composed of the core motif PuGGTCA configured in direct repeats spaced by one or more nucleotides. TAK1 bound poorly to palindromic or inverted palindromic motifs and was unable to bind to a single core motif, suggesting that a dimeric site is required for binding. Transfection experiments with CV-1 cells revealed that TAK1 is able to repress retinoid- and thyroid-hormone-induced transactivation through a subset of retinoid and thyroid hormone RE's. Our studies indicate that the antagonism of RAR-mediated transactivation does not involve the formation of heterodimers between TAK1 and RAR or RXR but is due to the competition of TAK1 homodimers with RAR-RXR heterodimers and RXR homodimers for binding to RARE and RXRE, respectively. Our results suggest that the orphan receptor TAK1 can be a negative modulator of the regulation of gene expression mediated by retinoid and thyroid hormone signaling pathways.

Animals↗

PubMed1995

Evidence for the involvement of retinoic acid receptor RAR alpha-dependent signaling pathway in the induction of tissue transglutaminase and apoptosis by retinoids.

In this study, we show that all-trans-retinoic acid (RA) is a potent inducer of tissue transglutaminase (TGase II) and apoptosis in the rat tracheobronchial epithelial cell line SPOC-1. We demonstrate that these cells express the retinoid receptors RAR alpha, RAR gamma, and RXR beta. To identify which of these receptors are involved in regulating these processes, we analyzed the effects of several receptor-selective agonists, an antagonist, and a dominant-negative RAR alpha. We show that the RAR-selective retinoid SRI-6751-84 strongly increased TGase II expression at both the protein and mRNA levels, whereas the RXR-selective retinoid SR11217 had little effect. The RAR alpha-selective retinoid Ro40-6055 was also able to induce TGase II, whereas the RAR gamma-selective retinoid CD437 was inactive. The induction of TGase II by the RAR-selective retinoid was completely inhibited by the RAR alpha-antagonist Ro41-5253. Overexpression of a truncated RAR alpha gene with dominant-negative activity also inhibited the induction of TGase II expression. The increase in TGase II is associated with an induction of apoptosis as revealed by DNA fragmentation and the generation of apoptotic cells. We demonstrate that apoptosis is affected by retinoids in a manner similar to TGase II. Our results suggest that the induction of TGase II expression and apoptosis in SPOC-1 cells are mediated through an RAR alpha-dependent signaling pathway.

Animals↗

PubMed1995

RTR: a new member of the nuclear receptor superfamily that is highly expressed in murine testis.

We have identified and cloned a novel member of the nuclear receptor superfamily from murine testis, referred to as retinoid receptor-related testis-associated receptor or RTR. Degenerate PCR primers homologous to two conserved regions of the DNA-binding domain of members of this superfamily were employed to identify this gene. The amino-acid sequence of RTR is most closely related to that of the mouse RXRs with an overall identity of 32-34%; the highest similarity (61%) is observed in the DNA-binding domain. Northern blot analysis using RNA from multiple tissues showed that RTR is predominantly expressed in the testis. Northern blot analysis using RNA from different testicular cell types showed that RTR mRNA is not expressed in early germ cells or Sertoli cells but is most abundant in round spermatids. Our observations suggest that this putative transcription factor plays a role in the regulation of gene expression particularly during the post-meiotic phase of spermatogenesis.

Amino Acid Sequence↗

PubMed1995

Retinoic acid nuclear receptor beta inhibits breast carcinoma anchorage independent growth.

Retinoids modulate cellular proliferation and mediate gene function through a series of nuclear receptors. The retinoic acid nuclear receptor beta (RAR beta) plays an important role in the differentiation of a number of cell types. We now demonstrate that RAR beta expression is confined to normal mammary tissue and is not expressed in either immortalized normal or malignant cell lines. Treatment of RAR beta-transfected MDA-MB-231 cells with 1 microM all-trans-retinoic acid (RA) significantly inhibited monolayer growth of the cells which express recombinant RAR beta. RAR beta-expressing MDA-MB-231 cells formed significantly smaller and fewer colonies in soft agar than the mock-transfected cells. Addition of 1 microM RA stimulated colony size and number in the RAR beta-transfected MDA-MB-231 cells. In contrast to the RAR beta-expressing cells, colony formation by the RAR alpha-expressing cells was similar to the mock-transfected controls and the addition of 1 microM RA to the RAR alpha-transfected cells inhibited colony formation. While demonstrating decreased colony formation in agar, RAR beta-expressing MDA-MB-231 cells failed to exhibit decreased growth in SCID mice. Our results show that RAR beta functions as a negative regulator of growth in breast epithelial cells. In addition, the growth of these cells is differentially regulated by RAR alpha and RAR beta which is most likely the result of the modulation of different genes.

Agar↗

PubMed1995

Identification of a transposon-related RNA down-regulated by retinoic acid in embryonal carcinoma and embryonic stem cells.

The differential display polymerase chain reaction was employed to identify changes in mRNA expression during retinoic acid-induced differentiation in embryonal carcinoma PCC4.aza1R cells. In this study, we report on one cDNA, EC1, that was identified by this method. EC-1 encodes a 0.6-kb mRNA that is present in PCC4.aza1R cells and down-regulated by retinoic acid. Sequence analysis revealed that EC-1 exhibits a 47% identity with the early transposon RNA ETn and does not contain a long open reading frame. EC-1 mRNA expression was reduced by 50% after 24 h of treatment with 10 nM retinoic acid and was undetectable after 48 h. Down-regulation of EC-1 mRNA was observed at retinoic acid concentrations as low as 0.1 nM. EC-1 was found to be expressed in several other embryonal carcinoma cell lines as well as in embryonic stem cells but was undetectable in differentiated cell types obtained after RA treatment. Northern blot analysis using RNA from multiple mouse tissues demonstrated that the expression of EC-1 is restricted to the testis. Treatment of PCC4.aza1R cells with an RAR-selective agonist also repressed the expression of EC-1 mRNA while treatment with an RXR-selective agonist reduced EC-1 expression slightly. The RAR alpha-specific antagonist Ro 41-5253 had little effect on the down-regulation of EC-1 by retinoic acid. Our observations indicate that the repression of EC-1 is associated with the induction of differentiation in embryonal carcinoma and embryonic stem cells and involves an RAR-activated signaling pathway.

Animals↗

PubMed1994

ROR gamma: the third member of ROR/RZR orphan receptor subfamily that is highly expressed in skeletal muscle.

In this study, we describe the identification and cloning of a novel member of the nuclear receptor superfamily. This orphan receptor, referred to as ROR gamma, belongs to the ROR/RZR subfamily. The open reading frame of ROR gamma encodes a protein of 560 amino acid residues with a predicted molecular mass of 63 kD. The amino acid sequence of ROR gamma exhibits a 50 and 51% identity with those of ROR alpha/RZR alpha and RZR beta, respectively, whereas the DNA-binding domains were 89% identical. ROR gamma was localized on human chromosome 1. Northern blot analysis using RNA from multiple tissues indicated that ROR gamma is expressed in several tissues but is most highly expressed in skeletal muscle.

Amino Acid Sequence↗

PubMed1994

Retinoid-resistant estrogen receptor-negative human breast carcinoma cells transfected with retinoic acid receptor-alpha acquire sensitivity to growth inhibition by retinoids.

Retinoids mediate their actions via RARs (retinoic acid receptors) and RXRs (retinoid X receptors). Each class of these nuclear retinoid receptors is further subdivided into three species, namely alpha, beta, and gamma. Recent studies demonstrate that estrogen receptor (ER)-positive human breast carcinoma (HBC) cell lines and tumor samples exhibit significantly higher levels of RAR alpha than their ER-negative counterparts. ER-positive HBC cell lines are sensitive to, and ER-negative cell lines are resistant to, growth inhibitory effects of retinoic acid (RA). We previously demonstrated that the expression of functional ERs in an established ER-negative cell line resulted in higher levels of RAR alpha and sensitivity to growth inhibition by RA. To further investigate the major role of RAR alpha in retinoid-mediated inhibition of growth, we transfected RAR alpha cDNA in two RA-resistant ER-negative HBC cell lines. Analyses of different clonal populations of RAR alpha transfectants from each cell line revealed growth inhibition by retinoids. Utilizing RAR- and RXR-class selective retinoids, we further demonstrated that only the RAR alpha-selective retinoids mediated the growth inhibition in these cells, while the RXR-selective retinoids were biologically inert. We thus provide evidence that the molecular mechanisms of retinoid inhibition of HBC proliferation predominantly involve RAR alpha.

Base Sequence↗

PubMed1994

Control of growth regulatory and differentiation-specific genes in human epidermal keratinocytes by interferon gamma. Antagonism by retinoic acid and transforming growth factor beta 1.

Interferon gamma (IFN-gamma) is a potent inducer of squamous differentiation in normal human epidermal keratinocytes. This induction is characterized by a > or = 95% decrease in the mRNA level of two growth regulatory genes, cdc2 and E2F-1, and a 7-15-fold increase in the expression of two squamous cell-specific genes, transglutaminase type I and cornifin. In contrast to the decrease in cdc2 and E2F-1 expression, the increase in transglutaminase type I and cornifin mRNAs by IFN-gamma occurs after a lagtime of more than 12 h. These results are consistent with the hypothesis that in normal human epidermal keratinocyte cells irreversible growth arrest precedes the expression of the squamous-differentiated phenotype. The action of IFN-gamma on the expression of squamous cell-specific genes is antagonized by retinoic acid and transforming growth factor beta 1. Both factors are potent suppressors of the induction of transglutaminase type I and cornifin; however, they do not prevent the commitment to irreversible growth arrest. Several squamous cell carcinoma cell lines do not show a detectable decrease in cdc2 or increase in transglutaminase type I mRNA levels after IFN-gamma treatment and appear to be altered in their control of squamous differentiation.

CDC2 Protein Kinase↗

PubMed1994

Differential responsiveness of human bronchial epithelial cells, lung carcinoma cells, and bronchial fibroblasts to interferon-gamma in vitro.

The present study examines interferon-gamma (IFN gamma)-induced changes in the expression of immunomodulatory genes, proliferation-associated genes, and squamous-specific genes in primary cultures of human bronchial epithelial cells and fibroblasts. IFN gamma induced the expression of guanylate binding protein (GBP or p67) and the MHC class II antigen, HLADR alpha, in both epithelial cells and fibroblasts. In contrast, the expression of complement component C3 was induced in bronchial epithelial cells but not in fibroblasts. Similarly, IFN gamma induced growth arrest (EC50 approximately 50 U/ml) only in bronchial epithelial cells. This growth arrest was accompanied by a down-regulation of cdc2, E2F-1, and p53 mRNA levels and was associated with expression of the squamous-specific marker genes, transglutaminase type I and cornifin. These findings are consistent with IFN gamma inducing squamous differentiation in bronchial epithelial cells. In contrast, several lung carcinoma cell lines did not respond to IFN gamma with respect to the down-regulation of proliferation-associated genes or the induction of squamous-specific genes. However, GBP expression was induced in all the cell lines in response to IFN gamma. The present study demonstrates that cultured human bronchial epithelial cells are sensitive to the immunomodulatory, growth-inhibitory, and differentiation-inducing properties of IFN gamma. In contrast, several lung carcinoma cell lines are insensitive to the growth-inhibitory and differentiation-inducing actions of IFN gamma, suggesting they may have acquired defects in certain IFN gamma signaling pathways. Although the growth of human bronchial fibroblasts is not altered, expression of certain immunomodulatory genes is induced by IFN gamma.(ABSTRACT TRUNCATED AT 250 WORDS)

Bronchi↗

PubMed1994

TAK1: molecular cloning and characterization of a new member of the nuclear receptor superfamily.

Using polymerase chain reaction and two degenerate primers whose designs were based on the two best conserved regions of the DNA-binding domain of the nuclear receptor superfamily, we identified and cloned a novel orphan receptor, named TAK1. The open reading frame of TAK1 encodes a protein of 596 amino acid residues. Based on the modular structure and the presence of a DNA-binding domain containing two zinc fingers TAK1 belongs to the steroid/thyroid hormone receptor superfamily. The amino acid sequence of TAK1 is most closely related to the orphan receptor TR2-11. Their overall sequence homology is 64%, with the highest similarity (82%) being observed in the DNA-binding domain. Northern blot analysis using RNA from multiple human tissues showed that a 9.4 kilobase TAK1 transcript was expressed ubiquitously and that the presence of a 2.8 kilobase mRNA was largely restricted to the testis. In situ hybridization using sections of rat and mouse testes and Northern blot analysis using RNA from testes of rats at various ages revealed that TAK1 is most abundantly expressed in spermatocytes whereas little expression was observed in other germ cells or somatic cells. In situ hybridization using other mouse and rat tissues revealed cell type-specific expression of TAK1 in several tissues. Our observations suggest a role for this putative transcription factor in the regulation of gene expression in specific cell types. In the testis, TAK1 appears to control gene expression during spermatogenesis, particularly during the meiotic phase.

Amino Acid Sequence↗

PubMed1993

Regulation of proliferation-specific and differentiation-specific genes during senescence of human epidermal keratinocyte and mammary epithelial cells.

The expression of proliferation-associated genes, cdc2 and E2F-1 and squamous-specific genes transglutaminase type I and cornifin were examined in senescing human epidermal keratinocytes. Cultured keratinocytes underwent 34 population doublings before senescing. Senescence in keratinocytes was characterised by reduced thymidine incorporation, a change in morphology and the inability of cells to undergo mitosis. This was accompanied by downregulation of cdc2 and E2F-1 mRNA's. In addition, senescing keratinocytes started to express genes such as cornifin, that are specific for squamous differentiation. These changes were similar to those observed in keratinocytes induced to differentiate with phorbol ester or by confluence. E2F-1, cdc2 and cornifin were similarly altered in senescing human mammary epithelial cells. Our data suggest that events regulating senescence may also be linked to squamous differentiation.

Base Sequence↗

PubMed1993

Estrogen receptor-negative breast cancer cells transfected with the estrogen receptor exhibit increased RAR alpha gene expression and sensitivity to growth inhibition by retinoic acid.

We and others have shown previously that retinoic acid (RA) selectively inhibits the growth of estrogen receptor (ER)-positive human breast carcinoma (HBC) cells and ER-negative cells are refractory to RA inhibition of growth. The ER-negative cells inherently express lower levels of RAR alpha and retinoic acid response element (RARE)-mediated RA-induced CAT activity. In this study we report that when ER-negative MDA-MB-231 cells were transfected with the ER gene they not only expressed higher levels of RAR alpha and RARE-mediated RA-induced CAT gene expression, but their growth was not inhibited by RA. Estrogen enhanced RAR alpha gene expression not only in established ER-positive cell lines but also in ER-transfected MDA-MB-231 cells. The estrogen effect appears to be direct and at the gene transcription level since it did not alter the stability of RAR alpha mRNA and cycloheximide failed to block estrogen-mediated enhancement of RAR alpha gene expression. Our data strongly suggest that ER-mediated enhancement of RAR alpha levels plays an important role in RA inhibition of HBC growth. In addition, we also report here that HBC cells appear to express a unique isoform(s) of RAR alpha which was detected only when the full-length RAR alpha cDNA was used as a probe; the RAR alpha 1 and RAR alpha 2 specific probes failed to hybridize with the HBC specific RAR alpha message.

Base Sequence↗