Mouse mu opioid receptor gene expression. A 34-base pair cis-acting element inhibits transcription of the mu opioid receptor gene from the distal promoter

Simian Immunodeficiency Virus Impacts MicroRNA‐16 Mediated Post‐Transcriptional Regulation of mu Opioid Receptor in CEM ×174 Cells

Although the mechanism which regulates transcription in the 5′‐UTR of the mu opioid receptor gene (OPRM1) in lymphocytes has been well‐studied, a question remains as to whether there is post‐transcriptional regulation of OPRM1 gene in lymphocytes. In this study, the authors describe both the role played by miRNAs and the impact of SIVmac239 infection on post‐transcriptional regulation of OPRM1 gene in CEM ×174 cells. Our results show that miR‐16 is able to bind the target site in the range of 8699–8719 nt from the stop codon in MOR‐1 mRNA 3′‐UTR and suppress the expression of OPRM1 gene. Mutation of this target site reduces the effect of miR‐16. Morphine (1 µM) inhibits the expression of miR‐16, and this effect is reversed by the antagonist naloxone. Thus, morphine may up‐regulate receptor level by both stimulating OPRM1 gene transcription and stabilizing its mRNA. SIVmac239 infection results in an apparent elevation of miR‐16 and gradual reduction of OPRM1 gene expression. The inverse correlation of elevated miR‐16 and reduced OPRM1 gene expression under viral loading confirmed the effect of SIVmac239 on post‐transcriptional regulation of OPRM1 gene in lymphocytes. The authors conclude that miR‐16 is a primary factor in post‐transcriptional regulation of OPRM1 gene. SIVmac239 upregulates miR‐16 levels and consequently suppresses OPRM1 gene expression. This finding will be helpful for full understanding of the regulatory mechanism of OPRM1 gene in lymphocytes, as well as the synergistic mechanism of HIV infection and morphine addiction in the pathogenesis of AIDS. J. Cell. Biochem. 117: 84–93, 2016. © 2015 Wiley Periodicals, Inc.

Inhibition of c-Jun NH2-terminal kinase stimulates mu opioid receptor expression via p38 MAPK-mediated nuclear NF-κB activation in neuronal and non-neuronal cells

Despite its potential side effects of addiction, tolerance and withdrawal symptoms, morphine is widely used for reducing moderate and severe pain. Previous studies have shown that the analgesic effect of morphine depends on mu opioid receptor (MOR) expression levels, but the regulatory mechanism of MOR is not yet fully understood. Several in vivo and in vitro studies have shown that the c-Jun NH2-terminal kinase (JNK) pathway is closely associated with neuropathic hyperalgesia, which closely resembles the neuroplastic changes observed with morphine antinociceptive tolerance. In this study, we show that inhibition of JNK by SP600125, its inhibitory peptide, or JNK-1 siRNA induced MOR at both mRNA and protein levels in neuronal cells. This increase in MOR expression was reversed by inhibition of the p38 mitogen-activated protein kinase (MAPK) pathway, but not by inhibition of the mitogen-activated protein/extracellular signal-regulated kinase (MEK) pathway. Further experiments using cell signaling inhibitors showed that MOR upregulation by JNK inhibition involved nuclear factor-kappa B (NF-κB). The p38 MAPK dependent phosphorylation of p65 NF-κB subunit in the nucleus was increased by SP600125 treatment. We also observed by chromatin immunoprecipitation (ChIP) analysis that JNK inhibition led to increased bindings of CBP and histone-3 dimethyl K4, and decreased bindings of HDAC-2, MeCP2, and histone-3 trimethyl K9 to the MOR promoter indicating a transcriptional regulation of MOR by JNK inhibition. All these results suggest a regulatory role of the p38 MAPK and NF-κB pathways in MOR gene expression and aid to our better understanding of the MOR gene regulation.

RACK1 identified as the PCBP1-interacting protein with a novel functional role on the regulation of human MOR gene expression

Poly C binding protein 1 (PCBP1) is an expressional regulator of the mu-opioid receptor (MOR) gene. We hypothesized the existence of a PCBP1 co-regulator modifying human MOR gene expression by protein-protein interaction with PCBP1. A human brain cDNA library was screened using the two-hybrid system with PCBP1 as the bait. Receptor for activated protein kinase C (RACK1) protein, containing seven WD domains, was identified. PCBP1-RACK1 interaction was confirmed via in vivo validation using the two-hybrid system, and by co-immunoprecipitation with anti-PCBP1 antibody and human neuronal NMB cell lysate, endogenously expressing PCBP1 and RACK1. Further co-immunoprecipitation suggested that RACK1-PCBP1 interaction occurred in cytosol alone. Single and serial WD domain deletion analyses demonstrated that WD7 of RACK1 is the key domain interacting with PCBP1. RACK1 over-expression resulted in a dose-dependent decrease of MOR promoter activity using p357 plasmid containing human MOR promoter and luciferase reporter gene. Knock-down analysis showed that RACK1 siRNA decreased the endogenous RACK1 mRNA level in NMB, and elevated MOR mRNA level as indicated by RT-PCR. Likewise, a decrease of RACK1 resulted in an increase of MOR proteins, verified by (3) H-diprenorphine binding assay. Collectively, this study reports a novel role of RACK1, physically interacting with PCBP1 and participating in the regulation of human MOR gene expression in neuronal NMB cells.

Transcriptional and epigenetic regulation of opioid receptor genes: present and future

Three opioid receptors (ORs) are known: μ opioid receptors (MORs), δ opioid receptors (DORs), and κ opioid receptors (KORs). Each is encoded by a distinct gene, and the three OR genes share a highly conserved genomic structure and promoter features, including an absence of TATA boxes and sensitivity to extracellular stimuli and epigenetic regulation. However, each of the genes is differentially expressed. Transcriptional regulation engages both basal and regulated transcriptional machineries and employs activating and silencing mechanisms. In retinoic acid-induced neuronal differentiation, the opioid receptor genes undergo drastically different chromatin remodeling processes and display varied patterns of epigenetic marks. Regulation of KOR expression is distinctly complex, and KOR exerts a unique function in neurite extension, indicating that KOR is not simply a pharmacological cousin of MOR and DOR. As the expression of OR proteins is ultimately controlled by extensive posttranscriptional processing, the pharmacological implication of OR gene regulation at the transcriptional level remains to be determined.

Effects of trichostatin A on neuronal mu-opioid receptor gene expression

In this study, we determined the effects of a histone deacetylase (HDAC) inhibitor, trichostatin A (TSA), on neuronal mu-opioid receptor (MOR) gene expression using human neuronal NMB cells, endogenously expressing MOR. Recruitment of two classes of HDAC, HDAC1 and HDAC2, to MOR promoter region in situ was detected via chromatin immunoprecipitation (ChIP) analysis with NMB cells. Functional analysis using the luciferase reporter gene system showed that TSA induced an approximately 3-fold increase of the promoter activity as compared to the vehicle treated group. Mutation analysis demonstrated that TSA response was mediated by both dsDNA (Sp1/Sp3 binding site) and ssDNA (PolyC binding protein1, PCBP, binding site) elements located in mouse MOR proximal core promoter region, further suggesting the functional importance of this cis-element, which shows high sequence homology between human and mouse MOR genes. ChIP analysis further suggested that TSA enhanced the recruitment of Sp1/Sp3 and PCBP to the promoter region, whereas no significant changes of total proteins were observed in response to TSA using Western blot analysis. Moreover, confocal images showed TSA-induced nuclear hot spots of endogenous PCBP in neuronal cells, whereas no obvious nuclear PCBP hotspot was observed in vehicle treated cells. Taken together, these results suggested that TSA enhanced neuronal MOR gene expression at the transcriptional level. RT-PCR analysis further revealed that TSA also decreased the steady-state level of MOR mRNA in a time-dependent manner by enhancing its instability. Thus, data suggest that TSA, an epigenetic regulator, affects neuronal MOR gene expression at both transcriptional and post-transcriptional levels.

Post-transcriptional regulation of mouse mu opioid receptor (MOR1) via its 3' untranslated region: a role for microRNA23b

Expression of the mu opioid receptor (MOR1) protein is regulated temporally and spatially. Although transcription of its gene has been studied extensively, regulation of MOR1 protein production at the level of translation is poorly understood. Using reporter assays, we found that the MOR1 3'-untranslated region (UTR) represses reporter expression at the post-transcriptional level. Suppression by the 3'-UTR of MOR1 is mediated through decreased mRNA association with polysomes, which requires microRNA23b (miRNA23b), a specific miRNA that is expressed in mouse brain and NS20Y mouse neuroblastoma cells. miRNA23b interacts with the MOR1 3'-UTR via a K box motif. By knocking down endogenous miRNA23b in NS20Y cells, we confirmed that miRNA23b inhibits MOR1 protein expression in vivo. This is the first study reporting a translationally repressive role for the MOR1 3'-UTR. We propose a mechanism in which miRNA23b blocks the association of MOR1 mRNA with polysomes, thereby arresting its translation and suppressing the production of MOR1 protein.

A proteomics approach for identification of single strand DNA-binding proteins involved in transcriptional regulation of mouse mu opioid receptor gene

The pharmacological actions of morphine and morphine-like drugs such as heroin are mediated primarily through the μ opioid receptor. Previously a single strand DNA element of the mouse μ opioid receptor gene (Oprm1) proximal promoter was found to be important for regulating Oprm1 in neuronal cells. To identify proteins binding to the single strand DNA element as potential regulators for Oprm1, affinity column chromatography with the single strand DNA element was performed using neuroblastoma NS20Y cells followed by two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry. We identified five poly(C)-binding proteins: heterogeneous nuclear ribonucleoprotein (hnRNP) K, α-complex proteins (αCP) αCP1, αCP2, αCP2-KL, and αCP3. Binding of these proteins to the single strand DNA element of Oprm1 was sequence-specific as confirmed by supershift assays. In cotransfection studies, hnRNP K, αCP1, αCP2, and αCP2-KL activated the Oprm1 promoter activity, whereas αCP3 acted as a repressor. Ectopic expression of hnRNP K, αCP1, αCP2, and αCP2-KL also led to activation of the endogenous Oprm1 transcripts, and αCP3 repressed endogenous Oprm1 transcripts. We demonstrate novel roles as transcriptional regulators in Oprm1 regulation for hnRNP K and αCP binding to the single strand DNA element.

Novel function of the poly(C)-binding protein alpha CP3 as a transcriptional repressor of the mu opioid receptor gene

The alpha-complex proteins (alphaCP) are generally known as RNA-binding proteins that interact in a sequence-specific fashion with single-stranded poly(C). These proteins are mainly involved in various post-transcriptional regulations (e.g., mRNA stabilization or translational activation/silencing). Here we report a novel function of alphaCP3, a member of the alphaCP family. alphaCP3 bound to the double-stranded poly(C) element essential for the mu opioid receptor (MOR) promoter and repressed the promoter activity at the transcriptional level. We identified alphaCP3 using affinity column chromatography containing the double-stranded poly(C) element and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. AlphaCP3 binding to the poly(C) sequence of the MOR gene was sequence specific, as confirmed by the supershift assay. In cotransfection studies, alphaCP3 repressed the MOR promoter only when the poly(C) sequence was intact. Ectopic expression of alphaCP3 led to repression of the endogenous MOR transcripts in NS20Y cells. When alphaCP3 was disrupted using small interfering RNA (siRNA) in NS20Y cells, the transcription of the endogenous target MOR gene was increased significantly. Our data suggest that alphaCP3 can function as a repressor of MOR transcription dependent on the MOR poly(C) sequence. We demonstrate for the first time a role of alphaCP3 as a transcriptional repressor in MOR gene regulation.

Identification and functional significance of polymorphisms in the μ-opioid receptor gene (Oprm) promoter of C57BL/6 and DBA/2 mice

C57BL/6J and DBA/2J mice demonstrate differences in morphine preference when tested in a two-bottle choice paradigm. Quantitative trait loci (QTL) mapping suggested the proximal region of chromosome 10 was responsible for 41% of the observed genetic variance. The mu-opioid receptor (MOR) gene (Oprm) maps to this region and is a prime candidate for explaining the QTL. We hypothesized that variations in Oprm between these strains are responsible for differences in morphine preference. We identify five single nucleotide polymorphisms (SNPs) in the Oprm promoter; three within or near putative transcription factor binding sites. Promoter fragments were amplified from genomic DNA by polymerase chain reaction (PCR) and subcloned into luciferase reporter vectors. A significant difference in basal Oprm promoter activity was seen with C57BL/6 and DBA/2 approximately 1675 constructs in MOR-positive BE(2)-C cells, but not in MOR-negative Neuro-2a cells. In BE(2)-C cells, average DBA/2 approximately 1675 construct activity was 1.3-2.0x greater than average C57BL/6 activity suggesting that the SNPs might alter MOR expression in these two mouse strains. Significant differences in promoter activities between the two cell lines suggest that cell-type-specific transcription factors are involved. No significant differences in construct activity were found between untreated and morphine-treated BE(2)-C or Neuro-2a cells, suggesting that morphine does not regulate transcription of Oprm.

Transcriptional regulation of mouse mu opioid receptor gene: Sp3 isoforms (M1, M2) function as repressors in neuronal cells to regulate the mu opioid receptor gene

The 5'-flanking region of the mouse mu opioid receptor (MOR) gene has two promoters, referred to as distal and proximal. MOR mRNA is predominantly initiated by the proximal promoter. Previously, several important cis-elements and trans-factors have been shown to play a functional role in the proximal promoter of the MOR gene. In this study, we defined another functional, negative regulatory element located in the -219- to -189-base pair (translational start site designed as +1) region of the proximal promoter. It is designated as the Sp binding sequence for its sequence homology to the consensus Sp binding element. Mutation of the Sp binding element led to a 100% increase of MOR promoter activity in MOR-positive cells (NMB cells), confirming the negative role of the Sp binding sequence. Surprisingly, electrophoretic mobility shift analysis and chromatin immunoprecipitation assays revealed that Sp3 and its isoforms (M1 and M2) were specifically bound to the Sp binding sequence. In cotransfection assays of Drosophila melanogaster SL2 cells using cDNA encoding Sp1, Sp3, and the M1 and M2 isoforms of Sp3, the M1 and M2 isoforms trans-repressed the MOR promoter, whereas Sp1 and Sp3 trans-activated the MOR promoter. Significantly, ectopic expression of the M1 and M2 isoforms of Sp3 led to repression of the endogenous MOR gene transcripts in NMB cells. These results suggest that the binding of the M1 and M2 isoforms of the Sp3 transcription factor to the Sp binding sequence may play a role in mouse MOR gene expression.

Insights into the receptor transcription and signaling: implications in opioid tolerance and dependence

Drug addiction has great social and economical implications. In order to resolve this problem, the molecular and cellular basis for drug addiction must be elucidated. For the past three decades, our research has focused on elucidating the molecular mechanisms behind morphine tolerance and dependence. Although there are many working hypotheses, it is our premise that cellular modulation of the receptor signaling, either via transcriptional or post-translational control of the receptor, is the basis for morphine tolerance and dependence. Thus, in the current review, we will summarize our recent work on the transcriptional and post-translational control of the opioid receptor, with special emphasis on the mu-opioid receptor, which is demonstrated to mediate the in vivo functions of morphine.

Transcriptional Regulation of Mouse μ Opioid Receptor Gene by PU.1

We previously reported that the 34-bp cis-acting element of the mouse micro opioid receptor (MOR) gene represses transcription of the MOR gene from the distal promoter. Using a yeast one-hybrid screen to identify potential transcription factors of the MOR promoter, we have identified PU.1 as one of the candidate genes. PU.1 is a member of the ets family of transcription factors, expressed predominantly in hematopoietic cells and microglia of brain. PU.1 plays an essential role in the development of both lymphoid and myeloid lineages. Opioids exert neuromodulatory as well as immunomodulatory effects, which are transduced by MOR. Moreover, MOR-deficient mice exhibit increased proliferation of hematopoietic cells, suggesting a possible link between the opioid system and hematopoietic development. The PU.1 protein binds to the 34-bp element of the MOR gene in a sequence-specific manner confirmed by electrophoretic mobility shift assay and supershift assays. We have also determined endogenous PU.1 interactions with the 34-bp element of MOR promoter by chromatin immunoprecipitation assays. In co-transfection studies PU.1 represses MOR promoter reporter constructs through its PU.1 binding site. When the PU.1 gene is disrupted as in PU.1 knock-out mice and using small interfering RNA-based strategy in RAW264.7 cells, the transcription of the endogenous target MOR gene is increased significantly. This increase is probably mediated through modification of the chromatin structure, as suggested by the reversal of the PU.1-mediated repression of MOR promoter activity after trichostatin A treatment in neuroblastoma NMB cells. Our results suggest that PU.1 may be an important regulator of the MOR gene, particularly in brain and immune cells.

Transcriptional regulation of mu opioid receptor gene by cAMP pathway

The utility of morphine for the treatment of chronic pain is hindered by the development of tolerance. Fentanyl has been shown to be a potent analgesic with a lower propensity to produce tolerance and physical dependence in the clinical setting. Previous finding has shown that fentanyl induces mu opioid receptor gene expression in PC-12 cells (Brain Res 859:217-223, 2000). In this report, we aim to identify the molecular mechanism of mu-opioid receptor (MOR) gene regulation by fentanyl. We demonstrated that the 4.7-kilobase MOR promoter could be induced by fentanyl in PC-12 cells, and we defined a partial cAMP response element (CRE) located at -106/-111 in 5'-untranslated region of the MOR gene. In electrophoretic mobility shift assay, cAMP response element-binding protein (CREB) was found in the protein-DNA complex formed on the CRE box. CREB was phosphorylated after forskolin induction, and both CREB and CREB-binding protein (CBP) binding to the endogenous MOR promoter was increased by forskolin in chromatin immunoprecipitation assay. The functional role of CREB in the induction of MOR gene was further elucidated by an experiment in which a dominant-negative mutant CREB, CREB-S133A, abolished the forskolin-mediated MOR induction. Moreover, we found that this CRE box is conserved in mouse, rat, and human MOR gene, implying physiological relevance in different species. Collectively, this study demonstrated that fentanyl-triggered MOR gene induction was mediated by the sequential activation of CREB and the binding of CREB and CBP to MOR promoter, thus provides direct evidence for lower propensity of fentanyl to produce tolerance.

Mouse mu opioid receptor distal promoter transcriptional regulation by SOX proteins

We have identified transcription factors that bind to specific sequences in 5'-distal promoter regulatory sequences of the mouse mu opioid receptor (mor) promoter using the yeast one-hybrid system. The sequence between -746 and -707 in mor distal promoter was used as the bait because it acts as a functional promoter element and binds several DNA-binding proteins. From an adult mouse brain cDNA library, five cDNA clones encoding three Sox gene family (Sry like high mobility group (HMG) box gene) transcriptional factors, mSOX18, mSOX21, and mSOX6, were isolated. Electrophoretic mobility shift assays confirmed the presence of a binding site for SOX proteins in the -731/-725 region. Additionally, we have also established that the flanking regions outside the core Sox-binding site play an essential role in high affinity binding. DNase I footprint analysis indicates that proteins from mouse brain interact with the Sox-binding site within the mor distal promoter. Finally, we demonstrated that overexpression of mSOX18 and/or mSOX21 was able to up-regulate mouse mor distal promoter activity in mor-expressing neuronal cells (NMB). These data indicate that SOX proteins might contribute to the transcriptional activity of the mor gene and suggest that mu opioid receptor could mediate some of the developmental processes in which SOX proteins are included.

Differential promoter usage of mouse mu-opioid receptor gene during development

Previously, we demonstrated that mouse mu-opioid receptor (MOR) gene expression is regulated by both distal and proximal promoters, with the latter playing a major role in controlling MOR transcription in the adult mouse brain. Here, we report studies of the relative usages of the mouse MOR dual promoters during murine development. We used the reverse transcription-polymerase chain reaction (RT-PCR) method, which gave results similar to those using binding assays or in situ hybridization. However, due to the greater sensitivity of RT-PCR method, we were able to detect the emergence of MOR as early as at embryonic day 8.5 (E8.5). We found that both proximal and distal promoters were active at E8.5. The proximal promoter initiated approximately two-thirds of total MOR transcripts at E8.5, with the distal promoter directing transcription of the remaining one-third. This is the greatest relative contribution of the distal promoter to MOR transcription we have observed during any time in development. Thereafter, the percentage of transcripts directed by the distal promoter gradually declined, and remained at a low but detectable level (approximately 5% of total MOR transcripts) throughout development and adulthood. Conversely, a progressive increase of the contribution of the proximal promoter to MOR transcription was observed during development, reaching its maximum in the adult. In summary, our results demonstrated the pivotal role of the proximal promoter in directing MOR transcription during murine development.

Opioid receptor polymorphisms and opioid abuse

The sequencing of the human genome is only the first step. The next step is to determine the function of these genes and in particular, how alterations in specific genes lead to major human disorders. Many laboratories are now focusing on identifying and characterizing single nucleotide polymorphisms (SNPs), to determine which correlate in frequency with certain population groups who may be particularly susceptible to certain diseases. The mu opioid receptor (MOR), which mediates the clinically important analgesic effects of drugs like morphine as well as the euphoria sought by heroin abusers, exhibits several dozen polymorphisms. Several of these are associated with altered receptor function and individuals at risk for drug abuse.

Transcriptional regulation of the human mu opioid receptor (hMOR) gene: evidence of positive and negative cis-acting elements in the proximal promoter and presence of a distal promoter

The mu opioid receptor (MOR), the primary binding site for morphine, is an important target for treating pain and drug addiction. The MOR gene is tightly regulated at the level of transcription, and potential polymorphisms in its 5' regulatory region can cause individual variation in MOR gene expression, nociception, and opiate responses. To study the 5' regulatory region of the human MOR gene (hMOR), we further investigated our previous finding of two regulatory regions and have localized a 40-bp positive cis-acting element and a 35-bp negative cis-acting element that regulate hMOR transcription in SK-N-SH cells. Electromobility shift assays and methylation interference assay with the 40-bp probe suggested that protein contacts were made with the core recognition sequence GCC (-510 to -508). The 35-bp sequence (-694 to -660) was the hMOR homolog of the mMOR negative regulatory element, and it suppressed proximal promoter activity of the hMOR gene. Additionally, the presence of an hMOR distal promoter was confirmed using RT-PCR. However, the activity of the distal promoter construct (-2325 to -777) was weak compared with the activity of the proximal promoter construct (-776 to -212).

Transcriptional modulation of mouse mu-opioid receptor distal promoter activity by Sox18

Previously, we reported the presence of dual promoters, referred to as distal (DP) and proximal, with a negative regulatory element between them in the mouse mu-opioid receptor (mor) gene. Here we have identified a positive regulatory element influencing mor DP transcription, which contains multiple consensus binding motifs for Sox factors (sex-determining Sry-like high mobility group box-containing genes). In gel supershift assays, the Sox family member Sox18 bound directly to the multiple Sox consensus binding motifs of the mor DP enhancer. Overexpression of Sox18 cDNA increased luciferase activity regulated by the mor DP, and did so in a Sox18 concentration-dependent manner. In contrast, overexpression of another Sox member, Sox5, triggered no such trans-activation of mor DP-driven luciferase activity or DNA-protein binding activity. These results suggest that Sox18 directly and specifically stimulates mor gene expression, by trans-activating the mor DP enhancer.

Molecular mechanisms and regulation of opioid receptor signaling

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.