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Biomedical subjects

R Liu

Publications and source records attributed to R Liu.

At least 217 records · Page 12Linked to original sources
PubMed1997

Activation of c-Jun N-terminal kinase by human granulocyte macrophage-colony stimulating factor in BA/F3 cells.

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF) induces various signaling events in hematopoietic cells. We reported that there are at least two distinct pathways of hGM-CSF signals, one for activation of proliferation and the other one for activation of c-fos promoter through the MAPK cascade. Activation of other members of the MAPK family, c-Jun N-terminal kinase (JNK) and p38 MAPK under various cellular stress have also been reported. We found that hGM-CSF activates JNK in BA/F3 cells expressing the hGM-CSF receptor (hGMR) and that activation depends on a membrane proximal region including box1 and requires a more membrane distal region of hGMR beta subunit (beta c). There are 8 known tyrosine (tyr) residues in the cytoplasmic region of beta c. Mutant beta c lacking all the tyr residues hardly activates JNK, thereby indicating that the tyr residue(s) is essential for the activation of JNK. Mutation analyses of each tyr residue indicated that none of the tyr residues seems essential for the activation of JNK, indicating multiple tyr residues play a similar function to transduce signals for this activation.

Calcium-Calmodulin-Dependent Protein Kinases↗
PubMed1997

Role of the glucosamine pathway in fat-induced insulin resistance.

To examine whether the hexosamine biosynthetic pathway might play a role in fat-induced insulin resistance, we monitored the effects of prolonged elevations in FFA availability both on skeletal muscle levels of UDP-N-acetyl-hexosamines and on peripheral glucose disposal during 7-h euglycemic-hyperinsulinemic (approximately 500 microU/ml) clamp studies. When the insulin-induced decrease in the plasma FFA levels (to approximately 0.3 mM) was prevented by infusion of a lipid emulsion in 15 conscious rats (plasma FFA approximately 1.4 mM), glucose uptake (5-7 h = 32.5+/-1.7 vs 0-2 h = 45.2+/-2.8 mg/kg per min; P < 0.01) and glycogen synthesis (P < 0.01) were markedly decreased. During lipid infusion, muscle UDP-N-acetyl-glucosamine (UDP-GlcNAc) increased by twofold (to 53.4+/-1.1 at 3 h and to 55.5+/-1.1 nmol/gram at 7 h vs 20.4+/-1.7 at 0 h, P < 0.01) while glucose-6-phosphate (Glc-6-P) levels were increased at 3 h (475+/-49 nmol/gram) and decreased at 7 h (133+/-7 vs 337+/-28 nmol/gram at 0 h, P < 0.01). To discern whether such an increase in the skeletal muscle UDP-GlcNAc concentration could account for the development of insulin resistance, we generated similar increases in muscle UDP-GlcNAc using three alternate experimental approaches. Euglycemic clamps were performed after prolonged hyperglycemia (18 mM, n = 10), or increased availability of either glucosamine (3 micromol/kg per min; n = 10) or uridine (30 micromol/kg per min; n = 4). These conditions all resulted in very similar increases in the skeletal muscle UDP-GlcNAc (to approximately 55 nmol/gram) and markedly impaired glucose uptake and glycogen synthesis. Thus, fat-induced insulin resistance is associated with: (a) decreased skeletal muscle Glc-6-P levels indicating defective transport/phosphorylation of glucose; (b) marked accumulation of the endproducts of the hexosamine biosynthetic pathway preceding the onset of insulin resistance. Most important, the same degree of insulin resistance can be reproduced in the absence of increased FFA availability by a similar increase in skeletal muscle UDP-N-acetyl-hexosamines. In conclusion, our results support the hypothesis that increased FFA availability induces skeletal muscle insulin resistance by increasing the flux of fructose-6-phosphate into the hexosamine pathway.

Animals↗
PubMed1997

Modulation of retinoic acid sensitivity in lung cancer cells through dynamic balance of orphan receptors nur77 and COUP-TF and their heterodimerization.

The diverse function of retinoic acid (RA) is mediated by its nuclear receptors, the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). However, the RA response is often lost in cancer cells that express the receptors. Previously, it was demonstrated that the RA response is regulated by the COUP-TF orphan receptors. Here, we present evidence that nur77, another orphan receptor whose expression is highly induced by phorbol esters and growth factors, is involved in modulation of the RA response. Expression of nur77 enhances ligand-independent transactivation of RA response elements (RAREs) and desensitizes their RA responsiveness. Conversely, expression of COUP-TF sensitizes RA responsiveness of RAREs by repressing their basal transactivation activity. Unlike the effect of COUP-TFs, the function of nur77 does not require direct binding of nur77 to the RAREs, but is through interaction between nur77 and COUP-TFs. The interaction occurs in solution and results in inhibition of COUP-TF RARE binding and transcriptional activity. Unlike other nuclear receptors, a large portion of the carboxy-terminal end of nur77 is not required for its interaction with COUP-TF. In human lung cancer cell lines, COUP-TF is highly expressed in RA-sensitive cell lines while nur77 expression is associated with RA resistance. Stable expression of COUP-TF in nur77-positive, RA-resistant lung cancer cells enhances the inducibility of RARbeta gene expression and growth inhibition by RA. These observations demonstrate that a dynamic equilibrium between orphan receptors nur77 and COUP-TF, through their heterodimerization that regulates COUP-TF RARE binding, is critical for RA responsiveness of human lung cancer cells.

Animals↗
PubMed1997

Transfection and expression of MnSOD cDNA decreases tumor malignancy of human oral squamous carcinoma SCC-25 cells.

Overexpression of human manganese-containing superoxide dismutase (MnSOD) activity has been demonstrated to suppress malignancy in human melanoma and breast carcinoma cells in vitro and in vivo. To study its effects on human oral squamous carcinoma cells, stable transfection and expression of MnSOD in SCC-25 cells have been conducted. The MnSOD-overexpressing cell clones were shown to have approximately two- to five-fold increased MnSOD activity compared to the wild-type parental- or vector control-transfected cell clones, respectively. Plating efficiency with different concentrations of serum was decreased in the high MnSOD activity cell clones. Soft agar assays demonstrated that the clonogenic fractions of high-expressing MnSOD clones were dramatically reduced. When inoculated in nude mice, tumor growth was markedly inhibited in MnSOD overexpressing cell clones compared with the wild-type or vector control transfected cell lines. Thus, gene therapy of human oral cancer by increasing the expression of MnSOD activity in target cells might be used to prevent or reduce human oral tumor malignancy.

Aged↗
PubMed1997

Fluorescence studies on the nucleotide binding domains of the P-glycoprotein multidrug transporter.

One of the major causes of multidrug resistance in human cancers is expression of the P-glycoprotein multidrug transporter, which acts as an efflux pump for a diverse range of natural products, chemotherapeutic drugs, and hydrophobic peptides. In the present study, fluorescence techniques were used to probe the nucleotide binding domains (NBD) of P-glycoprotein. The transporter was labeled at two conserved cysteine residues, one within each NBD, using the thiol-reactive fluor 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (MIANS), and collisional quenching was used to assess solvent accessibility of the bound probe. Acrylamide was a poor quencher, which suggests that MIANS is buried in a relatively inaccessible region of the protein. Iodide ion was a highly effective quencher, whereas Cs+ was not, demonstrating the presence of a positive charge in the region close to the ATP binding site. The fluorescent nucleotide derivative 2'(3')-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) was hydrolysed slowly by P-glycoprotein, with a V(max) approximately 20-fold lower than that for unmodified ATP, and a K(M) of 81 microM. TNP-ATP and TNP-ADP inhibited P-glycoprotein ATPase activity, indicating that they interact with the NBD, whereas TNP-AMP was a very poor inhibitor. When TNP-nucleotides bound to P-glycoprotein, their fluorescence intensity was enhanced in a concentration-dependent manner. Both TNP-ATP and TNP-ADP bound to P-glycoprotein with substantially higher affinity than ATP, with K(d) values of 43 and 36 microM, respectively. Addition of ATP led to only partial displacement of TNP-ATP. Resonance energy transfer was observed between cysteine-bound MIANS and TNP-ATP/ADP, which indicated that the two fluorescent groups are located close to each other within the catalytic site of P-glycoprotein.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

The tissue concentration of UDP-N-acetylglucosamine modulates the stimulatory effect of insulin on skeletal muscle glucose uptake.

To delineate the biochemical mechanism by which increased availability of GlcN impairs insulin action on skeletal muscle glucose uptake, we replenished the uridine pool during GlcN administration. Co-infusion of uridine with GlcN prevented the GlcN-induced fall in skeletal muscle UDP-glucose levels (24.9 +/- 5. 3 versus 10.1 +/- 2.9 nmol/g; p < 0.01) and further increased the skeletal muscle UDP-GlcNAc levels (198.4 +/- 26.3 versus 96.0 +/- 8. 4 nmol/g; p < 0.01). Greater reductions in the rates of glucose infusion ( approximately 53%), glucose uptake ( approximately 43%), and glycogen synthesis ( approximately 60%) were observed with the addition of uridine. Similarly, the infusion of uridine alone markedly increased the skeletal muscle levels of both UDP-glucose (55.2 +/- 14.2 versus 17.8 +/- 6.1 nmol/g; p < 0.01) and UDP-GlcNAc (86.8 +/- 8.8 versus 35.9 +/- 8.4 nmol/g; p < 0.05) and induced marked insulin resistance. The decrease in insulin action on peripheral glucose uptake was highly correlated with the increase in skeletal muscle UDP-GlcNAc levels. Finally, immunoisolation of GLUT4-containing vesicles revealed that the rate of labeled GlcN incorporation was approximately 100-fold greater following GlcN compared with saline infusions (p < 0.01). We suggest that the marked reduction in insulin action induced by GlcN and uridine is mediated by increased accumulation of muscle UDP-N-acetylhexosamines, perhaps via altered glycosylation of protein(s) in GLUT4-containing vesicles.

Animals↗
PubMed1997

Machado-Joseph disease: clinical, molecular, and metabolic characterization in Chinese kindreds.

Machado-Joseph disease, an autosomal dominant multisystem motor degeneration, has been described mainly in people of Portuguese descent. Our report documents the presence of Machado-Joseph disease in the Chinese population, based on the specific molecular marker of a CAG repeat array in the 3' end of the MJD gene. We screened 21 Chinese families with dominant spinocerebellar ataxia. The results showed that Machado-Joseph disease with CAG expansion accounted for 52% of families with autosomal dominant cerebellar ataxia in this series. The clinical characteristics, besides the well-documented cerebellar ataxia, dysarthria, nystagmus, corticospinal dysfunctions, a variable degree of facial muscle fasciculation, and proprioceptive loss, included loss of optokinetic nystagmus and autonomic nervous system dysfunction. The CAG repeat number in the MJD gene ranged from 14 to 39 among normal alleles, and from 63 to 81 among MJD alleles. There was a strong inverse correlation (gamma = -0.77) between number of CAG repeats and age at symptom onset, accounting for 60% of the variance of age at onset. A strong clinical anticipation of age at onset existed in successive generations. Mild instabilities of expanded CAG repeat numbers during meiotic transmission occurred, with no significant difference according to the gender of the transmitting parent. Finally, brain metabolism in Machado-Joseph disease, studied with positron emission tomography, was characterized by significant progressive regional hypometabolism in the occipital cortex, as well as the cerebellar hemispheres, vermis, and brainstem.

Adult↗
PubMed1997

Long-term transgene expression in fetal rat suprachiasmatic nucleus neurografts following ex vivo adenoviral vector-mediated gene transfer.

Ex vivo gene transfer to fetal suprachiasmatic nucleus (SCN)-containing solid piece neurografts was explored using a first-generation prototype adenoviral vector containing the reporter gene LacZ (Ad-LacZ). Transgene expression was examined at different intervals following grafting in the IIIrd ventricle of rat brain and was compared to that of explant cultures. Large numbers of beta-galactosidase-positive cells were observed 8 days postgrafting. The number of stained cells had decreased considerably at 21 days but transduced cells were still present at 70 days. In vitro culturing of infected SCN tissue revealed high expression up to 21 days, indicating that the in vivo and in vitro fates of Ad-LacZ-infected cells were different. The main reason for this difference appeared to be cell loss by necrosis in the initial phase after transplantation, a phenomenon not related to the infection with Ad-LacZ since it similarly occurred in control grafts. In vivo inflammatory responses, observed after immunostaining for macrophages and T-lymphocytes, were also comparable in control and Ad-LacZ-treated transplants, except that cytotoxic T-cells were observed in the Ad-LacZ-treated transplants and not in controls. The recruitment of these cells was, however, minor and primarily observed at 8 days postgrafting, indicating that a major immunological rejection of the transduced graft did not occur. In both control and Ad-LacZ-infected transplants similar survival and intraimplant neuritic growth of SCN cells were visible. Ex vivo gene transfer of solid piece fetal SCN grafts with adenoviral vectors therefore appeared to be a nontoxic long-term gene-introducing procedure. This would in principle enable the local production of neurotrophic factors within the transplant and has the potential to improve functional SCN neurografting.

Adenoviridae↗
PubMed1997

Diet, apoptosis, and carcinogenesis.

It is known that long-term withdrawal of choline from the diet induces hepatocellular carcinomas in animal models in the absence of known carcinogens. We hypothesize that a choline deficient diet (CD) alters the balance of cell growth and cell death in hepatocytes and thus promotes the survival of clones of cells capable of malignant transformation. When grown in CD medium (5 microM or 0 microM choline) CWSV-1 rat hepatocytes immortalized with SV40 large T-antigen underwent p53-independent apoptosis (terminal dUTP end-labeling of fragmented DNA; laddering of DNA in agarose gel). CWSV-1 cells which were adapted to survive in 5 microM choline acquired resistance to CD-induced apoptosis and were able to form hepatocellular carcinomas in nude mice. These adapted CWSV-1 cells express higher amounts of both the 32 kDa membrane-bound and 6 kDa mature form of TGF alpha compared to cells made acutely CD. Control (70 microM choline) and adapted cells, but not acutely deficient hepatocytes, could be induced to undergo apoptosis by neutralization of secreted TGF alpha. Protein tyrosine phosphorylation is known to protect against apoptosis. We found decreased EGF receptor tyrosine phosphorylation in acutely choline deficient CWSV-1 cells. TGF beta 1 is an important growth-regulator in the liver. CWSV-1 cells express TGF beta 1 receptors and this peptide induced cell detachment and death in control and acutely deficient cells. Hepatocytes adapted to survive in low choline were also resistant to TGF beta 1, although TGF beta 1 receptors and protein could be detected in the cytoplasm of these cells. The non-essential nutrient choline is important in maintaining plasma membrane structure and function, and in intracellular signaling. Our results indicate that acute withdrawal of choline induces p53-independent programmed cell death in hepatocytes, whereas cells adapted to survive in low choline are resistant to this form of apoptosis, as well as to cell death induced by TGF beta 1. Our results also suggest that CD may induce alterations (mutations?) in growth factor signaling pathways which may enhance cell survival and malignant transformation.

Animals↗
PubMed1997

Effect of dexamethasone on Glu-IR and GABA-IR neurons of hippocampus in rats.

Dexamethasone (Dex) was injected into the rat lateral ventricle and the changes of glutamate-immunoreactivity (Glu-IR) and gamma-aminobulyric acid immunoreactivity (GABA-IR) neurons in the hippocampus were immunocytochemically examined 2 h after injection. The results showed that Glu-IR neurons increased and GABA-IR neurons did not show marked change. The mechanism remains to be further studied.

Animals↗
PubMed1997

Adenoviral vector-mediated gene transfer and neurotransplantation: possibilities and limitations in grafting of the fetal rat suprachiasmatic nucleus.

Several studies have reported on the use of primary neural cells transduced by adenoviral vectors as donor cells in neurotransplantation. In the present investigation, we examined whether adenoviral vector-mediated gene transfer could be used to introduce and express a foreign gene in solid neural transplants of fetal suprachiasmatic nucleus (SCN) tissue. A recombinant adenoviral vector containing the reporter gene LacZ encoding for beta-galactosidase (Ad-LacZ) was used in order to establish the optimal procedure for ex vivo gene transfer. Expression of beta-galactosidase was dependent on the duration of the infection and on the vector concentration. Infection for a short period (< 4 h) with a high concentration of Ad-LacZ (3.4 x 10(9) pfu/ml), or for 18 h with a lower vector concentration (2 x 10(8) pfu/ml), resulted in expression of beta-galactosidase in a large number of neurons and glial cells up to 21 days in vitro. When infected fetal SCN tissue was implanted in the third ventricle of adult Wistar rats, expression was high after 8 days. After 21 days, the number of beta-galactosidase expressing cells had clearly declined, but expression remained present for at least 70 days. The method described in this paper might be applicable to introduce trophic factor genes in SCN grafts in order to support graft survival and to stimulate neurite outgrowth.

Adenoviridae↗
PubMed1997

Effects of tetramethylpyrazine on the release of PGI2 and TXA2 in the hypoxic isolated rat heart.

We examined the effect on 6-Keto-PGF1a and TXB2 (the stable metabolites of PGI2 and TXA2) outflow of pretreatment with tetramethylpyrazine in the hypoxic isolated rat heart. In control hearts, 6-Keto-PGF1a was increased from 185 +/- 46 pg/ml of baseline value to 335 +/- 76 pg/ml after 2 min of hypoxia and TXB2 increased from 136 +/- 28 pg/ml of baseline value to 230 +/- 43 pg/ml at 20 min of hypoxia and 252 +/- 32 pg/ml after 5 min of reoxygenation. Pretreatment with tetramethylpyrazine increased the 6-Keto-PGF1a concentration to 266 +/- 51 pg/ml (143% of control heart), 471 +/- 89 pg/ml (150% of control heart) and 332 +/- 47 pg/ml (195% of control heart) at 15 min of normoxia, 2 min of hypoxia and 5 min of reoxygenation, respectively (p < 0.05 vs control). On the other hand, tetramethylpyrazine diminished the release of TXB2 to 78 +/- 21 pg/ml (174% of control heart), 160 +/- 30 pg/ml (144% of control heart), and 196 +/- 23 pg/ml (128% of control heart) at 15 min of normoxia, 20 min of hypoxia and 5 min of reoxygenation, respectively (p < 0.05 vs control). These data show that pretreatment with tetramethylpyrazine enhances PGI2 outflow and attenuates release of TXA2 in the rat heart during normoxia, hypoxia and reoxygenation.

6-Ketoprostaglandin F1 alpha↗
PubMed1997

Mechanism of action of gemfibrozil on HDL metabolism and atherosclerosis in WHHL rabbits.

We investigated the mechanism of action of gemfibrozil on high-density lipoproteins (HDL) and apolipoprotein (apo) A-I metabolism and atherogenesis in homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits, an animal model of familial hypercholesterolemia and HDL deficiency. Two-month-old WHHL rabbits were fed either a normal control diet or a diet containing 0.5% gemfibrozil for 12 months. In vivo apo A-I kinetics, the fractional rate of cholesterol esterification in HDL (FER[HDL]), which reflects the reactivity of HDL to lecithin:cholesterol acyltransferase, and a morphometrical analysis of atherosclerotic lesions in the descending thoracic aorta, were examined. At 12 months, the mean levels of serum total cholesterol, LDL cholesterol (LDL-C), and HDL cholesterol (HDL-C) in both groups had decreased to approximately 53%, 57%, and 87% of the initial levels (at 0 month), respectively, which is characteristic of homozygous WHHL rabbits of the physiologic influence of aging, and no differences in the levels of serum LDL-C, HDL-C, and triglycerides were found between the two groups. Rabbits treated with gemfibrozil exhibited a decreased FER(HDL) (38% of the controls, P = 0.039). Gemfibrozil induced a significant increase in the total mass of apo A-I (1.7-fold, P < 0.05) and in the rate of apo A-I synthesis (1.6-fold, P < 0.05). The atherosclerotic intimal area was positively correlated with serum LDL-C (P = 0.02) in both groups, but gemfibrozil did not affect the atherosclerotic intimal area. These results indicate that 12 months of treatment with gemfibrozil did not protect against atherosclerosis despite a significant increase in apo A-I synthesis and enhanced HDL function through FER(HDL). It is possible that both the qualitative and quantitative improvement in HDL by gemfibrozil cannot overcome the massive and long-term exposure of the vascular wall to LDL in these animals.

Animals↗
PubMed1997

Molecular analysis of human cardiac myosin-cross-reactive B- and T-cell epitopes of the group A streptococcal M5 protein.

The group A streptococcal M protein is an important virulence determinant eliciting protective and autoimmune responses against the streptococcus and cardiac myosin, respectively. In this report, the major human cardiac myosin-cross-reactive T-cell epitopes of M5 protein are identified and localized to myosin-like repeats within the M5 molecule. BALB/c mice were immunized with human cardiac myosin, and the dominant myosin-cross-reactive T-cell epitopes of M5 protein were identified with a panel of 23 overlapping peptides spanning the A, B, and C repeat regions of M5 protein. Human cardiac myosin-cross-reactive T-cell epitopes of M5 protein were localized to several sequences in the M5 peptides NT4 (GLKTENEGLKTENEGLKTE), NT5 (KKEHEAENDKLKQQRDTL), B1B2 (VKDKIAKEQENKETIGTL), B2 (TIGTLKKILDETVKDKIA), B3A (IGTLKKILDETVKDKLAK), and C3 (KGLRRDLDASREAKKQ). The NT4 repeated sequence LKTEN was highly homologous with a site conserved in cardiac myosins, the B repeat region peptides were 47% homologous to human cardiac myosin amino acid sequence, and the C3 sequence RRDL was identical to a highly conserved site in skeletal and cardiac myosins. Immunization of BALB/c mice with each of the overlapping M5 peptides revealed myosin-cross-reactive B-cell epitopes throughout the A and C repeat regions and one major epitope in the B repeat region containing the previously reported Gln-Lys-Ser-Lys-Gln (QKSKQ) epitope. The data suggest that the M5 peptides elicited higher antibody titers to cardiac myosin than to skeletal myosin and that several sites in the A and B repeat regions of M5 protein induced myocardial inflammatory infiltrates.

Amino Acid Sequence↗
PubMed1997

A PCR assay for identification of Enterococcus faecium.

Enterococcus faecium has recently emerged as a serious nosocomial pathogen. The prevalence and severity of enterococcal infections, the mortality rate from such infections, and the antibiotic resistance of enterococci are often species dependent. Since conventional biochemical methods fail to differentiate E. faecium from certain newly described enterococcal species, a PCR-based assay was developed for the rapid identification of E. faecium.

Enterococcus faecium↗
PubMed1997

Macrophage tropism of human immunodeficiency virus type 1 and utilization of the CC-CKR5 coreceptor.

The recent identification of the CC-CKR5 beta chemokine receptor as a major cofactor for entry of macrophage-tropic isolates of human immunodeficiency virus type 1 (HIV-1) raises the question of whether macrophage tropism is determined by utilization of this chemokine receptor. We observe that in addition to macrophage-tropic isolates of clades A, B, and E, macrophage-tropic isolates of clade F also utilize the CC-CKR5 molecule for entry. However, using single-round replication-competent reporter viruses carrying the envelope genes of T-cell line-tropic or macrophage-tropic phenotypic recombinant and mutant HIV-1 strains in infection of stable cell lines that coexpress the CD4 and chemokine receptors, we were unable to establish a strict correlation between macrophage tropism and utilization of the CC-CKR5 chemokine receptor. This latter finding suggests that a cofactor other than CC-CKR5 serves to determine entry into primary macrophages.

Amino Acid Sequence↗
PubMed1997

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.

Antineoplastic Agents↗