Anesthesia for a Patient with Undiagnosed Myotonic Dystrophy.

ABSTRACT: Myotonic dystrophy (DM) is an autosomal dominant genetic disorder characterized by progressively worsening loss of muscle mass and weakness. Anesthesiologists face challenges in managing these patients due to risks such as prolonged intubation and delayed recovery associated with anesthesia in such conditions. We report a case of a 40-year-old male patient undergoing open total gastrectomy under general anesthesia. After the surgery, we administered sugammadex to reverse neuromuscular blockade and confirmed the patient's spontaneous breathing. We then proceeded to extubate the patient. However, the patient experienced complications such as apnea, desaturation, and mental changes. The patient was re-intubated and transferred to the intensive care unit for ventilator support. He was diagnosed with DM by genetic test later. Poor preoperative assessment or undiagnosed DM in surgical patients can lead to severe complications. Thus, it is important to carefully check preoperative laboratory results, patient history, and physical findings.

The recurrence of phantom limb pain with spinal anesthesia.

The recurrence or exacerbation of phantom limb pain (PLP) induced by spinal anesthesia in patients with amputated limbs is rare, but it can occur in any amputee. A 76-year-old woman with an amputated right knee underwent three left knee surgeries with spinal anesthesia over a period of 6 months. She did not experience PLP in the previous two surgeries but experienced the recurrence of severe PLP after the third surgery for the left knee amputation. It is believed that this third operation caused the patient to experience even more severe psychological stress than the previous two operations. Regional blocks can induce PLP in amputees. In addition, PLP can be triggered and exacerbated by psychological factors. Therefore, we suggest that physicians check the patient's psychological state and provide adequate mental stability when performing surgeries with spinal anesthesia in amputated patients.

Risk Factors Affecting Complications of Access Site in Vascular Intervention through Common Femoral Artery.

BACKGROUNDS: Traditionally, vascular interventions have been performed through the femoral artery. AIMS: The purpose of this study was to evaluate risk factors affecting access-site complications in patients with hepatocellular carcinoma or peripheral arterial disease in lower extremity who underwent vascular intervention by accessing the common femoral artery (CFA). PATIENTS AND METHODS: From December 2015 to November 2018, 287 patients underwent transarterial chemoembolization (TACE) or peripheral vascular intervention with ultrasound (US)-guided CFA access. Standard 18-gauge (G) access was used in 127 patients and Micropuncture® 21-G needles in 160 patients. Most access sites were managed with vascular closure devices and several were managed with manual compression. Within 24 hours after the procedure, all patients underwent US to evaluate the puncture site. RESULTS: Access-site complications occurred in 55 of 287 patients: 34 hematomas (11.9%), 20 pseudoaneurysms (7.0%), and 1 dissection (0.4%). In the crude model, risk factors related to access-site complications were the usage of 18-G needles (OR, 2.18; 95% CI, 1.17-4.07; P = 0.014), smoking (OR, 2.23; 95% CI, 1.16-4.27; P = 0.016), and approach route (OR, 3.23; 95% CI, 1.33-7.82; P = 0.009). Needle size (OR, 2.13; 95% CI, 1.10-4.12; P = 0.025) was the only factor associated with access-site complications in the adjusted model. CONCLUSION: Needle profile was the only factor associated with access-site complications in this study. Therefore, a needle with a smaller profile than an 18-G needle will reduce the incidence of complications at the access site.

Interaction of hepatitis B virus X protein with PARP1 results in inhibition of DNA repair in hepatocellular carcinoma.

Hepatitis B virus X protein (HBx) contributes to the development of hepatocellular carcinoma (HCC), probably by regulating activities of many host or viral proteins through protein-protein interactions. In this study, we identified poly(ADP-ribose) polymerase (PARP1), a crucial factor in DNA repair, as an HBx-interacting protein using a proteomics approach. Coimmunoprecipitation and proximity ligation assays confirmed the binding and colocalization of HBx and PARP1 in the nucleus. The carboxyl-terminus of HBx protein bound to the catalytic domain of PARP1, and this binding reduced the enzymatic activity of PARP1 in both in vitro and in vivo assays. HBx interrupted the binding of PARP1 to Sirt6, which catalyzes the mono-ADP-ribosylation required for DNA repair. Consistently, overexpression of HBx inhibited the clearance of γH2AX DNA repair foci generated under oxidative stress in Chang liver cells. Recruitment of the DNA repair complex to the site-specific double-strand breaks was inhibited in the presence of HBx, when measured by laser microirradiation assay and damage-specific chromatin immunoprecipitation assays. Consequently, HBx increased signs of DNA damage such as accumulation of 8-hydroxy-2'-deoxyguanosine and comet formation, which were reversed by overexpression of PARP1 and/or Sirt6. Finally, the interaction between PARP1 and Sirt6 was markedly lower in the livers of HBx-transgenic mice and specimens obtained from HCC patients to compare with the corresponding control. Our data suggest that the physical interaction of HBx and PARP1 accelerates DNA damage by inhibiting recruitment of the DNA repair complex to the damaged DNA sites, which may lead to the onset of hepatocarcinogenesis.

Poly(ADP-ribosyl)ation of p53 induces gene-specific transcriptional repression of MTA1.

The metastasis-associated protein 1 (MTA1) is overexpressed in various human cancers and is closely connected with aggressive phenotypes; however, little is known about the transcriptional regulation of the MTA1 gene. This study identified the MTA1 gene as a target of p53-mediated transrepression. The MTA1 promoter contains two putative p53 response elements (p53REs), which were repressed by the p53-inducing drug 5-fluorouracil (5-FU). Notably, 5-FU treatment decreased MTA1 expression only in p53 wild-type cells. p53 and histone deacetylases 1/2 were recruited, and acetylation of H3K9 was decreased on the promoter region including the p53REs after 5-FU treatment. Proteomics analysis of the p53 repressor complex, which was pulled down by the MTA1 promoter, revealed that the poly(ADP-ribose) polymerase 1 (PARP-1) was part of the complex. Interestingly, p53 was poly(ADP-ribose)ylated by PARP-1, and the p53-mediated transrepression of the MTA1 gene required poly(ADP-ribose)ylation of p53. In summary, we report a novel function for poly(ADP-ribose)ylation of p53 in the gene-specific regulation of the transcriptional mode of p53 on the promoter of MTA1.

Epigenetic control of metastasis-associated protein 1 gene expression by hepatitis B virus X protein during hepatocarcinogenesis.

Expression of metastasis-associated protein 1 (MTA1) gene correlates with the degree of invasion and metastasis in hepatocellular carcinoma (HCC). Expression of MTA1 is induced by hepatitis B virus X protein (HBx); however, little is known about the transcriptional regulation of MTA1 gene expression. Here, we report that the 5'-flanking region of the human MTA1 promoter contains two CpG islands. Transient expression of HBx in Chang liver cells increased the methylation of the CpG island1 from 18 to 49% when measured by bisulfite-modified direct sequencing. Chromatin immunoprecipitation showed that HBx recruited DNA methyltransferase 3a (DNMT3a) and DNMT3b to the CpG island1. In silico analysis of CpG island1 predicted the existence of putative p53-binding sequences. p53 was pulled down by a DNA probe encoding the p53-binding sequences but not by the methylated DNA probe. The mouse MTA1 promoter also contains a CpG island encoding a p53-binding sequence of which p53 binding was decreased in the presence of HBx, and the expression of MTA1 and DNMT3 was increased in the liver of HBx-transgenic mice. Comparison of MTA1 and DNMT3a expression in the human normal liver and HCC specimens produced a significant correlation coefficient >0.5 (r=0.5686, P=0.0001) for DNMT3a, and a marginally significant coefficient (r=0.3162, P=0.0103) for DNMT3b. These data show that HBx induces methylation of CpG island in the MTA1 promoter, which interferes with DNA binding of p53 in the specific DNA region. This result may explain the molecular mechanism responsible for the induction of MTA1 gene expression by HBx.

Transient expression of iron transport proteins in the capillary of the developing rat brain.

Iron is essential for normal brain function and its uptake in the developing rat brain peaks during the first two weeks after birth, prior to the formation of the blood­brain barrier (BBB). The first step of iron transport from the blood to the brain is transferrin receptor (TfR)-mediated endocytosis in the capillary endothelial cells. However, the subsequent step from the endothelium into interstitium has not been fully described. The goal of this study was to examine the expression of iron transport proteins by immunodetection and RT­PCR in the developing rat brain. Tf and TfR are transiently expressed in perivascular NG2+ cells of the capillary wall during the early postnatal weeks in the rat brain. However, MTP-1 and hephaestin were expressed in endothelial cells, but not in the NG2+ perivascular cells. Immunoblot analysis for these iron transfer proteins in the developing brain generally confirmed the immunochemical findings. Furthermore, the expression of Tf and TfR in the blood vessels precedes its expression in oligodendrocytes, the main iron-storing cells in the vertebrate brain. RT­PCR analysis for the primary culture of endothelial cells and pericytes revealed that Tf and TfR were highly expressed in the pericytes while MTP-1 and hephaestin were expressed in the endothelial cells. The specific expression of Tf and TfR in brain perivascular cells and MTP-1 and hephaestin in endothelial cells suggest the possibility that trafficking of elemental iron through perivascular cells may be instrumental in the distribution of iron in the developing central nervous system.

Hepatitis B virus X protein induces the expression of MTA1 and HDAC1, which enhances hypoxia signaling in hepatocellular carcinoma cells.

Expression level of metastasis-associated protein 1 (MTA1) is closely related to tumor growth and metastasis in various cancers. Although increased expression level of MTA1 was observed in hepatocellular carcinoma (HCC), role of MTA1 complex containing histone deacetylase (HDAC) in hepatitis B virus (HBV)-associated hepatocarcinogenesis has not been studied. Here, we demonstrated that HBx strongly induced the expression of MTA1 and HDAC1 genes at transcription level. MTA1 and HDAC1/2 physically associated with hypoxia-inducible factor-1 alpha (HIF-1 alpha) in vivo in the presence of HBx, which was abolished by knockdown of MTA1 by short interfering RNA (siRNA). HBx induced deacetylation of the oxygen-dependent degradation domain of HIF-1 alpha, which was accompanied with dissociation of prolyl hydroxylases and von Hippel-Lindau tumor suppressor from HIF-1 alpha. These results indicate that HBx-induced deacetylation is important for proteasomal degradation of HIF-1 alpha. Further, we observed that protein levels of MTA1 and HDAC1 were increased in the liver of HBx-transgenic mice. Also, there was a higher expression of HDAC1 in HCC than in the adjacent non-tumorous cirrhotic nodules in 10 out of 12 human HBV-associated HCC specimens. Together, our data indicate a positive cross talk between HBx and the MTA1/HDAC complex in stabilizing HIF-1 alpha, which may play a critical role in angiogenesis and metastasis of HBV-associated HCC.

6-Mercaptopurine, an activator of Nur77, enhances transcriptional activity of HIF-1alpha resulting in new vessel formation.

Hypoxia-inducible factor-1alpha (HIF-1alpha) plays a central role in oxygen homeostasis. Previously, we reported that the orphan nuclear receptor Nur77 functions in stabilizing HIF-1alpha. Here, we demonstrate that 6-mercaptopurine (6-MP), an activator of the NR4A family members, enhances transcriptional activity of HIF-1. 6-MP enhanced the protein-level of HIF-1alpha as well as vascular endothelial growth factor (VEGF) in a dose- and time-dependent manner. The induction of HIF-1alpha was abolished by the transfection of either a dominant-negative Nur77 mutant or si-Nur77, indicating a critical role of Nur77 in the 6-MP action. The HIF-1alpha protein level remained up to 60 min in the presence of 6-MP when de novo protein synthesis was blocked by cycloheximide, suggesting that 6-MP induces stabilization of the HIF-1alpha protein. The fact that 6-MP decreased the association of HIF-1alpha with von Hippel-Lindau protein and the acetylation of HIF-1alpha, may explain how 6-MP induced stability of HIF-1alpha. Further, 6-MP induced the transactivation function of HIF-1alpha by recruiting co-activator cyclic-AMP-response-element-binding protein. Finally, 6-MP enhanced the expression of HIF-1alpha and VEGF, and the formation of capillary tubes in human umbilical vascular endothelial cells. Together, our results provide a new insight for 6-MP action in the stabilization of HIF-1alpha and imply a potential application of 6-MP in hypoxia-associated human vascular diseases.

Hepatitis B virus X protein induced expression of the Nur77 gene.

Hepatitis B virus (HBV) X protein (HBx) plays an essential role in development of HBV-associated hepatocellular carcinoma (HCC). Recently, we reported that HBx induces Fas Ligand (FasL) expression, which may help HCC cells to evade host-immune surveillance. The aim of this study was to investigate the role of HBx in expression of Nur77, an orphan nuclear receptor implicated in the upregulation of FasL. When Chang X-34 expressing HBx under the control of a doxycycline-inducible promoter was examined, induction of Nur77 was observed following HBx expression. Blocking of Nur77 function by introduction of an antisense or a dominant negative mutant Nur77 significantly inhibited the induction of FasL, indicating that Nur77 plays critical roles in FasL expression. Further, a high-level expression of transcripts and DNA binding of Nur77 were observed in the HBV-integrated cell lines established from HCC patients that express HBx. These results suggested that Nur77 may contribute to leading the HBx-induced Fas/FasL signaling pathway which eliminates invading Fas-expressing lymphocytes.

LH induces orphan nuclear receptor Nur77 gene expression in testicular Leydig cells.

The orphan nuclear receptor Nur77 (NR4A1) is a member of the nuclear receptor superfamily and plays an important role in the regulation of genes involved in steroidogenesis and cell death. Northern blot analysis revealed that the expression of Nur77 mRNA was increased after puberty in mouse testis, and hCG treatment of peripubertal animals induced this gene expression in the testis. Moreover, LH treatment induced a transient increase in Nur77 mRNA, and this induction was LH dose dependent in mouse Leydig tumor cell line, K28. Western blot analysis showed that LH transiently induced Nur77 protein. The protein kinase inhibitor H-89, bisindolymaleimide I, and wortmannin strongly inhibited this inductive effect of LH on Nur77 gene expression. Transient transfection assay demonstrated that LH significantly increased the Nur77 promoter-driven luciferase reporter activity in a dose-dependent manner, and LH also increased the activity of a luciferase reporter gene driven by a promoter containing multi copies of a Nur77-responsive element. Moreover, EMSA showed that Nur77 DNA-binding activity was increased in response to LH. Finally, overexpression of dominant negative Nur77 reduced LH-mediated progesterone biosynthesis. Taken together, these results demonstrate that LH induces Nur77 gene expression, and Nur77 may play an important role in the LH-mediated steroidogenesis in Leydig cells.

Activation of NF-kappaB determines the sensitivity of human colon cancer cells to TNFalpha-induced apoptosis.

Tumor necrosis factor alpha (TNFalpha) generates a potent cytotoxic effect, however many cancer cells are resistant to TNFalpha-mediated killing and the cause of the differential sensitivity remains to be elucidated. In this study, we demonstrated that TNFalpha induced cell death in four different human colon cancer cell lines. The degree of cytotoxic effect was different in each cell line, in that HCT-15 was relatively sensitive, while DLD-1, HT-29 and WiDr were relatively resistant. TNFalpha induced apoptotic changes such as morphological changes, DNA fragmentation and activation of caspase-3 in HCT-15, but to a lesser degree in the others. Transcriptional expression of TNFR1(p55), as well as that of FLICE, Fas, FADD, DR3, FAF, TRADD, and RIP was similar in these cell lines, indicating that the susceptibility to TNFalpha-induced apoptosis may not be determined by the constitutive expression level of these factors. Interestingly, the cytotoxic effect of TNFalpha was well correlated with the DNA binding activity of NF-kappaB in the colon cancer cell lines. Further, the overexpression of a non-phosphorylated mutant form of IkappaBalpha enhanced the cytotoxicity of TNFalpha in the resistant cell line, DLD-1, indicating that NF-kappaB activity may determine the sensitivity of colon cancer cells to TNFalpha-induced apoptosis. Thus, our results indicate that modulation of NF-kappaB activity may provide a useful tool to sensitize colon cancer cells to TNFalpha treatment.

Retinoic acid and its receptors repress the expression and transactivation functions of Nur77: a possible mechanism for the inhibition of apoptosis by retinoic acid.

Nur77 (NGFI-B) is an orphan nuclear receptor that has been implicated in activation-induced T-cell apoptosis. Retinoids, potent immune modulators, were shown to inhibit the activation-induced apoptosis of immature thymocytes and T-cell hybridomas. To illustrate the mechanism of the inhibition, we examined the effects of retinoic acid (RA) on the expression and transactivation functions of Nur77 in the human peripheral blood mononuclear cells and the human T-cell leukemia, Jurkat. All-trans-RA remarkably repressed the DNA binding and transcriptional induction of Nur77. Among the two potential trans-acting factors that activate Nur77 gene promoter, i.e., AP-1 and related serum response factor (RSRF), all-trans-RA repressed DNA binding and reporter gene activity of AP-1 but not that of RSRF, suggesting that the inhibition may be mediated through AP-1. We also demonstrated a posttranscriptional regulation of Nur77 function by retinoid receptors by showing that transactivation activity of Nur77 was significantly inhibited by cotransfection of RARalpha or RXRalpha. Nur77 bound RARalpha or RXRalpha in both yeast and mammalian two-hybrid tests, suggesting that direct protein-protein interaction between these receptors may mediate the inhibition. Taken all together, we demonstrated that RA repressed Nur77 function through multiple mechanisms that may provide the basis for RA inhibition on the apoptosis of activated T-lymphocytes.

Differential effects of retinoic acid on growth and apoptosis in human colon cancer cell lines associated with the induction of retinoic acid receptor beta.

Retinoids are well known as potential chemopreventive and chemotherapeutic agents against a variety of human cancers. Here, we report that retinoic acid (RA) induced differential growth inhibition in human colon cancer cell lines: while DLD-1, HT-29, and WiDr were relatively resistant, HCT-15 and Colo201 were relatively sensitive. All-trans-retinoic acid caused morphological and biochemical changes such as membrane shrinkage, chromatin condensation, and DNA cleavage, which are typical features of cells undergoing apoptosis in sensitive cell lines. Although retinoic acid receptor (RAR)alpha, beta, gamma and retinoid X receptor alpha were expressed in all cell lines examined, a significant induction of RARbeta by all-trans-RA was observed only in sensitive cell lines, suggesting important roles of RARbeta in RA sensitivity. When a vector containing the RARbeta gene was introduced into a relatively resistant cell line, DLD-1, the cells acquired RA sensitivity. Further, we found that the RARbeta transfectants of DLD-1 expressed an enhanced level of c-Myc and Bax proteins, which may result in the increased susceptibility of the cells to all-trans-RA-induced apoptosis. In summary, our data demonstrated that RA induced growth inhibition and apoptosis in human colon cancer cells and that the induction of RAR3 may mediate the retinoid action.

Apoptosis in human hepatoma cell lines by chemotherapeutic drugs via Fas-dependent and Fas-independent pathways.

Many chemotherapeutic drugs have been found to exert their mode of action via induction of apoptosis in cancer cells. The mechanisms involved in this process are not clear. Recent studies have shown that the Fas/Fas ligand (FasL) system is a key factor controlling apoptotic cell death. In the present study, the involvement of Fas in chemotherapeutic drug-induced apoptosis in hepatoma cell lines was investigated. Five different human hepatoma cell lines, Hep G2, Hep G2.2.15, Hep 3B, SK-Hep-1, and PLC/PRF/5, were used. It was found that they expressed different levels of Fas. However, all five cell lines were susceptible to apoptosis when treated with chemotherapeutic drugs such as 5-fluorouracil (5-FU) or cisplatin. In Hep G2 that constitutively expressed Fas, 5-FU or cisplatin treatment caused an increase in the expression of Fas before the formation of oligonucleosomal DNA fragments, a typical feature of apoptosis. However, in Hep 3B, where Fas is undetectable, apoptosis could also be induced by 5-FU or cisplatin without induction of Fas. The agonistic anti-Fas antibody (CH-11) was capable of inducing apoptosis by itself and promoted drug-induced apoptosis in Hep G2 but not in Hep 3B. The antagonistic anti-Fas antibody (ZB4) inhibited drug-induced apoptosis in Hep G2. Our results suggest that apoptosis can be induced in hepatoma cell lines via both Fas-dependent and Fas-independent pathways.

Differential modulation of transcriptional activity of oestrogen receptors by direct protein-protein interactions with retinoid receptors.

Control of oestradiol-responsive gene regulation by oestrogen receptors (ERs) may involve complex cross-talk with retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Recently, we have shown that ERalpha directly interacts with RARalpha and RXRalpha through their ligand binding domains (LBDs). In the present work, we extend these results by showing that ERbeta binds similarly to RARalpha and RXRalpha but not to the glucocorticoid receptor, as demonstrated by the yeast two-hybrid tests and glutathione S-transferase pull-down assays. These direct interactions were also demonstrated in gel-shift assays, in which the oestrogen response element (ERE) binding by ERalpha was enhanced by the RXRalpha LBD but was abolished by the RARalpha LBD. In addition, we showed that RARalpha and RXRalpha bound the ERE as efficiently as ERalpha, suggesting that competition for DNA binding may affect the transactivation function of the ER. In transient transfection experiments, co-expression of RARalpha or RXRalpha, along with ERalpha or ERbeta, revealed differential modulation of the ERE-dependent transactivation, which was distinct from the results when each receptor alone was co-transfected. Importantly, when the LBD of RARalpha was co-expressed with ERalpha, transactivation of ERalpha on the ERE was repressed as efficiently as when wild-type RARalpha was co-expressed. Furthermore, liganded RARalpha or unliganded RXRalpha enhanced the ERalpha transactivation, suggesting the formation of transcriptionally active heterodimer complexes between the ER and retinoid receptors. Taken together, these results suggest that direct protein-protein interactions may play major roles in the determination of the biological consequences of cross-talk between ERs and RARalpha or RXRalpha.

Estrogen receptor, a common interaction partner for a subset of nuclear receptors.

Nuclear receptors regulate transcription by binding to specific DNA response elements as homodimers or heterodimers. Herein, the yeast and mammalian two-hybrid tests as well as glutathione-S-transferase pull-down assays were exploited to demonstrate that estrogen receptor (ER) directly binds to a subset of nuclear receptors through protein-protein interactions between ligand-binding domains. These receptors include hepatocyte nuclear factor 4, thyroid hormone receptor (TR), retinoic acid receptor (RAR), ERbeta, and retinoid X receptor (RXR). In yeast cells, a LexA fusion protein to the human ER ligand-binding domain (LexA/ER-LBD) was an inert transactivator of a LacZ reporter gene controlled by upstream LexA-binding sites. However, LexA/ER-LBD differentially modulated the LacZ reporter gene expression when coexpressed with native TRs, RARs, or RXRs. Similarly, cotransfection of these receptors in CV1 cells up- or down-regulated transactivations by ER. From these results, we propose that ER is a common interaction partner for a subset of receptors, and these interactions should mediate novel signaling pathways in vivo.

Regulation of RAR beta expression by RAR- and RXR-selective retinoids in human lung cancer cell lines: effect on growth inhibition and apoptosis induction.

Retinoids regulate the growth and differentiation of human tracheobronchial epithelial cells. In this study, we investigated the effects of all-trans-retinoic acid (trans-RA) and receptor class-selective retinoids on the growth and apoptosis of human lung cancer cell lines. Trans-RA significantly inhibited the growth of Calu-6 and H460 cells, accompanied by induction of RA receptor (RAR) beta expression. In contrast, it had little effect on the growth of H292, SK-MES-1 and H661 lung cancer cell lines, in which RAR beta expression was not induced. Stable expression of RAR beta in RAR beta-negative, trans-RA-resistant SK-MES-1 and H661 lung cancer cells led to recovery of trans-RA-induced growth inhibition, which occurred, however, only at low serum concentration. Using fluorescent microscopy and the terminal deoxyribonucleotidyl transferase (TdT) assay, we demonstrated that induction of apoptosis by trans-RA contributed to its growth-inhibitory effect in trans-RA-sensitive lung cancer cell lines. Analysis of RAR-selective and retinoid X receptor (RXR)-selective retionoids showed that activation of both RARs and RXRs could induce growth inhibition in trans-RA-sensitive lung cancer cells. Also, an additive synergistic effect on growth inhibition and RAR beta induction was observed when cells were treated with combinations of RAR-selective and RXR-selective retinoids. Together, our results show that expression of RAR beta plays a role in mediating retinoid response in lung cancer cells and that activation of RARs or RXRs contributes to induction of RAR beta, growth inhibition and apoptosis by retinoids.

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.