Beta 2 adrenergic receptor and mu opioid receptor interact to potentiate the aggressiveness of human breast cancer cell by activating the glycogen synthase kinase 3 signaling

BACKGROUND

Opioid and beta-adrenergic receptors are recently shown to cross talk via formation of receptor heterodimers to control the growth and proliferation of breast cancer cells. However, the underlying cell signaling mechanism remained unclear.

METHODS

To determine the effect of the interaction of the two systems in breast cancer, we employed triple-negative breast cancer cell lines MDA-MB-231 and MDA-MB-468, CRISPR or chemical inhibition or activation of beta-adrenergic receptors (B2AR) and mu-opioid receptors (MOR) gene, and PCR array technology and studied aggressive tumor phenotype and signaling cascades.

RESULTS

We show here that in triple-negative breast cancer cells, the reduction in expression B2AR and MOR by genetic and pharmacological tools leads to a less aggressive phenotype of triple-negative breast cancer cells in vitro and in animal xenografts. Genomic analysis indicates the glycogen synthase kinase 3 (GSK3) pathway as a possible candidate messenger system involved in B2AR and MOR cross talk. GSK3 inactivation in MDA-MB-231 and MDA-MB-468 cells induced similar phenotypic changes as the inhibition of B2AR and/or MOR, while a GSK3 activation by wortmannin reversed the effects of B2AR and/or MOR knockdown on these cells. GSK3 inactivation also prevents B2AR agonist norepinephrine or MOR agonist DAMGO from affecting MDA-MB-231 and MDA-MB-468 cell proliferation.

CONCLUSIONS

These data confirm a role of B2AR and MOR interaction in the control of breast cancer cell growth and identify a possible role of the GSK3 signaling system in mediation of these two receptors' cross talk. Screening for ligands targeting B2AR and MOR interaction and/or the GSK3 system may help to identify novel drugs for the prevention of triple-negative breast cancer cell growth and metastasis.

Diet, Sports, and Psychological Stress as Modulators of Breast Cancer Risk: Focus on OPRM1 Methylation

Background: DNA methylation is influenced by environmental factors and contributes to adverse modification of cancer risk and clinicopathological features.

Methods: A case-control study (402 newly diagnosed cases, 470 controls) was conducted to evaluate the effect of environmental factors and OPRM1 methylation in peripheral blood leukocyte (PBL) DNA on the risk of breast cancer. A case-only study (373 cases) was designed to evaluate the effects of environmental factors on OPRM1 methylation in tumor tissue and the relationship of methylation with clinicopathological features.

Results: We found a significant association between hypermethylation of OPRM1 and the risk of breast cancer (OR = 1.914, 95%CI = 1.357–2.777). OPRM1 hypermethylation in PBL DNA combined with low intake of vegetable, garlic, soybean, poultry, and milk; high pork intake; less regular sports and a high psychological stress index significantly increased the risk of breast cancer. Soybean intake (OR = 0.425, 95%CI: 0.231–0.781) and regular sports (OR = 0.624, 95%CI: 0.399–0.976) were associated with OPRM1 hypermethylation in tumor DNA. OPRM1 hypermethylation in tumor tissue was correlated with estrogen receptor (ER) (OR = 1.945, 95%CI: 1.262–2.996) and progesterone receptor (PR) (OR = 1.611, 95%CI: 1.069–2.427) negative status; in addition, OPRM1 hypermethylation in PBL DNA was associated with human epidermal growth factor receptor 2 (HER-2) negative status (OR = 3.673, 95%CI: 1.411–9.564).

Conclusion: A healthy diet, psychosocial adaptability, and regular sports are very beneficial for breast cancer prevention and progress, especially for OPRM1 hypermethylation carriers. Personalized treatment considering the correlation between OPRM1 hypermethylation and ER and PR status may provide a novel benefit for breast cancer patients.

Opioids and cancer prognosis: A summary of the clinical evidence

Pain is a common and devastating symptom among cancer patients. It can be caused by the cancer itself or by certain therapies like surgery, radiation or chemotherapy. Opioids are the first line of treatment for moderate to severe cancer-related pain. Opioids alone or in combination with non-opioid analgesics and adjuvant medications are important components for pain management during the perioperative period for cancer patients. Opioids act on the μ-opioid receptor (MOR), which is expressed in cancer cells and non-malignant cells of the tumor microenvironment. Retrospective studies suggest an association between the expression of MOR in cancers and shorter survival. In addition, recent evidence suggests that opium use and prescription opioids can influence clinical oncological outcomes. In this review, we will summarize the clinical evidence regarding the effect of opioid administration and survival in patients with cancer as well as the current evidence involving MOR expression and cancer progression.

The effect of anesthetic technique on µ-opioid receptor expression and immune cell infiltration in breast cancer

BACKGROUND

Clinical histological studies demonstrate that the distribution of natural killer (NK) cells, other immune cells and μ-opioid receptors (MOR) within cancer tissue can predict cancer prognosis. No clinical study has evaluated whether anesthetic technique influences immune cell and MOR expression within human breast cancer.

METHODS

Excised preoperative biopsies and intraoperative breast cancer specimens from 20 patients randomly chosen from patients previously enrolled in an ongoing, prospective, randomized trial (NCT00418457) investigating the effect of anesthetic technique on long-term breast cancer outcome were immunohistochemically stained and microscopically examined by two independent investigators, masked to randomization, to quantify MOR and immune cell infiltration: CD56, CD57 (NK cells), CD4 (T helper cells), CD8 (cytotoxic T cells) and CD68 (macrophages). Patients had been randomized to receive either a propofol-paravertebral anesthetic with continuing analgesia (PPA, n = 10) or balanced general anesthetic with opioid analgesia (GA, n = 10).

RESULTS

There were no differences between the groups in staining intensity in preoperative biopsy specimens. Expression intensity values (median 25-75%) for MOR in intraoperative resected biopsy were higher in GA 8.5 (3-17) versus PPA 1 (0-10), p = 0.04. The numbers of MOR-positive cells were also higher in GA patients. Expression and absolute numbers of CD56, CD57, CD4 and CD68 were similar in resected tumor in both groups.

CONCLUSION

General anesthesia with opioid analgesia increased resected tumor MOR expression compared with propofol-paravertebral anesthetic technique, but the anesthetic technique did not significantly influence the expression of immune cell markers.

Modulation of miR-139-5p on chronic morphine-induced, naloxone-precipitated cAMP overshoot in vitro

Chronic exposure to morphine can produce tolerance, dependence and addiction, but the underlying neurobiological basis is still incompletely understood. c-Jun, as an important component of the activator protein-1 transcription factor, is supposed to take part in regulating gene expression in AC/cAMP/PKA signaling. MicroRNA (miRNA) has emerged as a critical regulator of neuronal functions. Although a number of miRNAs have been reported to regulate the μ-opioid receptor expression, there has been no report about miRNAs to regulate chronic morphine-induced, naloxone-precipitated cAMP overshoot. Our results showed that chronic morphine pretreatment induced naloxone-precipitated cAMP overshoot in concentration- and time-dependent manners in HEK 293/μ cells. Chronic morphine pretreatment alone elevated both c-Jun protein and miR-139-5p expression levels, while dramatically artificial elevation of miR-139-5p inhibited c-Jun at the translational level. Furthermore, dramatically artificial upregulation of intracellular miR-139-5p limited chronic morphine-induced, naloxone-precipitated cAMP overshoot. These findings suggested that miR-139-5p was involved in regulating chronic morphine-induced, naloxone-precipitated cAMP overshoot in a negative feedback manner through its target c-Jun, which extends our understanding of neurobiological mechanisms underlying morphine dependence and addiction.

The standardized Withania somnifera Dunal root extract alters basal and morphine-induced opioid receptor gene expression changes in neuroblastoma cells

Background

Behavioral studies demonstrated that the administration of Withania somnifera Dunal roots extract (WSE), prolongs morphine-elicited analgesia and reduces the development of tolerance to the morphine’s analgesic effect; however, little is known about the underpinning molecular mechanism(s). In order to shed light on this issue in the present paper we explored whether WSE promotes alterations of μ (MOP) and nociceptin (NOP) opioid receptors gene expression in neuroblastoma SH-SY5Y cells.

Methods

A range of WSE concentrations was preliminarily tested to evaluate their effects on cell viability. Subsequently, the effects of 5 h exposure to WSE (0.25, 0.50 and 1.00 mg/ml), applied alone and in combination with morphine or naloxone, on MOP and NOP mRNA levels were investigated.

Results

Data analysis revealed that morphine decreased MOP and NOP receptor gene expression, whereas naloxone elicited their up-regulation. In addition, pre-treatment with naloxone prevented the morphine-elicited gene expression alterations. Interestingly, WSE was able to: a) alter MOP but not NOP gene expression; b) counteract, at its highest concentration, morphine-induced MOP down-regulation, and c) hamper naloxone-induced MOP and NOP up-regulation.

Conclusion

Present in-vitro data disclose novel evidence about the ability of WSE to influence MOP and NOP opioid receptors gene expression in SH-SY5Y cells. Moreover, our findings suggest that the in-vivo modulation of morphine-mediated analgesia by WSE could be related to the hindering of morphine-elicited opioid receptors down-regulation here observed following WSE pre-treatment at its highest concentration.

Post-Transcriptional Regulation of the Human Mu-Opioid Receptor (MOR) by Morphine-Induced RNA Binding Proteins hnRNP K and PCBP1

Expression of the mu-opioid receptor (MOR) protein is controlled by extensive transcriptional and post-transcriptional processing. MOR gene expression has previously been shown to be altered by a post-transcriptional mechanism involving the MOR mRNA untranslated region (UTR). Here, we demonstrate for the first time the role of heterogeneous nuclear ribonucleic acids (hnRNA)-binding protein (hnRNP) K and poly(C)-binding protein 1 (PCBP1) as post-transcriptional inducers in MOR gene regulation. In the absence of morphine, a significant level of MOR mRNA is sustained in its resting state and partitions in the translationally inactive polysomal fraction. Morphine stimulation activates the downstream targets hnRNP K and PCPB1 and induces partitioning of the MOR mRNA to the translationally active fraction. Using reporter and ligand binding assays, as well as RNA EMSA, we reveal potential RNP binding sites located in the 5'-untranslated region of human MOR mRNA. In addition, we also found that morphine-induced RNPs could regulate MOR expression. Our results establish the role of hnRNP K and PCPB1 in the translational control of morphine-induced MOR expression in human neuroblastoma (NMB) cells as well as cells stably expressing MOR (NMB1). J. Cell. Physiol. 232: 576-584, 2017. © 2016 Wiley Periodicals, Inc.

μ-Opioid receptor gene (OPRM1) polymorphism in patients with breast cancer

Structure-dependent μ-opioid receptor (MOR) activity is an important element in cancer opioid analgesic effectiveness. It is widely accepted that guanine (G) substitution for adenine (A) at OPRM1 gene sequence position 118 changes receptor glycosylation pattern. This is associated with decreased binding ability in both exogenous and endogenous opioids, resulting in increased human pain resistance. The endogenous opioid system's function in body homeostasis maintenance is considered mainly regulatory, so its participation in breast tumor formation and progression is identified herein. We examine the association of the most frequent MOR (A118G) gene polymorphism on breast cancer risk in a Northeastern Polish population by PCR-RFLP comparison of A and G allele frequency at OPRM1 gene A118G polymorphic site in breast cancer-diagnosed patients with healthy control group frequencies. Our results highlight a strong association between G allele presence at μ-opioid receptor A118G and increased breast cancer incidence (OR = 3.3, 95 % CI 2.2-5.0, p < 0.0001) and female gender (OR = 2.0, 95 % CI 1.4-2.9, p = 0.0004). Consequently, OPRM1 G allele presence at that site is a highly significant risk factor in breast cancer development.

MOR1 expression in gastric cancer: a biomarker associated with poor outcome

BACKGROUND

At present, the expression of MOR1 and its function in gastric cancer remains unclear with evidence suggesting that it is to be involved in tumor progression and metastasis. The study was to assess the clinicopathologic relevance and prognostic value of MOR1 expression in gastric cancer.

METHODS

Real-time quantitative RT-PCR and immunohistochemical staining were used to detect MOR1 expression in primary gastric cancerous surgical specimens and adjacent nontumorous tissues.

RESULTS

High MOR1 expression was detected in cancerous tumor compared with their adjacent nontumorous tissues. In addition, the chi-square test revealed that high MOR1 expression was significantly correlated with depth of invasion (p = 0.006), lymph node metastasis (p = 0.001), distant metastasis (p = 0.017), and TNM staging (p = 0.027). Moreover, Kaplan-Meier analysis revealed a significant association between MOR1 expression and overall survival. High expression of MOR1 was identified as an independent and significant predictor gene of reduced postoperative survival.

CONCLUSION

We conclude that MOR1 expression may be a useful biomarker for better prediction of the clinical outcome and management of gastric cancer patients.

Association of mu-opioid receptor expression with lymph node metastasis in esophageal squamous cell carcinoma

The mu-opioid receptor (MOR), a membrane-bound G protein-coupled receptor, is the main target for opioids in the nervous system. MOR1 has been found in several types of cancer cells and reported to be involved in tumor progression and metastasis. However, the expression and clinical significance of MOR1 in esophageal squamous cell carcinoma (ESCC) remain unclear. In our study, the expression of MOR1 was confirmed in ESCC cell lines (KYSE180, KYSE150, and EC109) by Western blot. MOR1 was also detected on tissue microarrays of ESCC samples in 239 cases using immunohistochemical staining. We found that MOR1 was mainly located in the cytoplasm and occasionally occurred in the membrane or nucleus of ESCC cells. Moreover, results indicated that MOR1 expression in the cytoplasm was associated with lymph node metastasis (R = 0.164, P = 0.008, Kendall's tau-b-test). No more associations were found between MOR1 expression status and other clinical parameters. However, no statistical significant differences were found between MOR1 expression in the cytoplasm, nucleus/membrane, and the overall survival of ESCC patients (P = 0.848; P = 0.167; P = 0.428, respectively, log-rank test). Our results suggest that the cytoplasmic MOR1 may be a high-risk factor for lymph node metastasis of ESCC patients. We also hypothesize that MOR1 agonists used in ESCC patients should be prudent, and opioid receptor antagonists may be novel therapeutic drugs for ESCC patients.

Morphine and fentanyl differently affect MOP and NOP gene expression in human neuroblastoma SH-SY5Y cells

Morphine is widely used for the treatment of severe acute and chronic pain, but long-term therapy rapidly leads to tolerance. Morphine effects are mediated by μ opioid receptor (MOP) activation as well as for fentanyl that, in contrast to morphine, induces less tolerance to analgesia. The mechanisms underlying opioid tolerance involve complex processes, such as MOP desensitization, internalization, and/or changes of gene expression. The development of morphine tolerance also involves adaptive changes of the anti-opioid nociceptin/orphanin FQ-nociceptin receptor system, as suggested by the reduction of morphine tolerance in nociceptin opioid receptor (NOP) knockout mice. The aim of the present study was to investigate the MOP and NOP gene expression in the SH-SY5Y cells following morphine and fentanyl exposure. Results showed that cell exposure to 10 μM morphine for 5 h induced a significant decrease of MOP and NOP gene expression and that the MOP downregulation was reverted by the pretreatment with naloxone. Conversely, SH-SY5Y cells exposed to 0.1 and 1 μM fentanyl for 5 and 72 h showed a significant MOP upregulation, also reverted by naloxone pretreatment. Fentanyl induced no changes of NOP gene expression. The present findings showed a different effect by morphine and fentanyl on MOP mRNA levels that contributes to define the role of MOP gene expression changes in the mechanisms underlying the tolerance. Morphine also triggers an altered NOP-related signaling confirming that the nociceptin/orphanin FQ-nociceptin receptor system also plays a significant role in the development of morphine tolerance.

Nitric oxide and histone deacetylases modulate cocaine-induced mu-opioid receptor levels in PC12 cells

Background

Cocaine exposure has been reported to alter central μ-opioid receptor (MOR) expression in vivo. The present study employed an in vitro cellular model to explore possible mechanisms that may be involved in this action of cocaine.

Methods

To assess the effects of cocaine on MOR levels, two treatment regimens were tested in PC12 cells: single continuous or multiple intermittent. MOR protein levels were assessed by western blot analysis and quantitative PCR was used to determine relative MOR mRNA expression levels. To evaluate the role of nitric oxide (NO) and histone acetylation in cocaine-induced MOR expression, cells were pre-treated with the NO synthase inhibitor N^ω-nitro-L-arginine methylester (L-NAME) or the non-selective histone acetyltransferase inhibitor curcumin.

Results

Both cocaine treatment regimens significantly increased MOR protein levels and protein stability, but only multiple intermittent treatments increased MOR mRNA levels as well as c-fos mRNA levels and activator protein 1 binding activity. Both regimens increased NO production, and pre-treatment with L-NAME prevented cocaine-induced increases in MOR protein and mRNA levels. Single and multiple cocaine treatment regimens inhibited histone deacetylase activity, and pre-treatment with curcumin prevented cocaine-induced up-regulation of MOR protein expression.

Conclusions

In the PC12 cell model, both NO and histone deacetylase activity regulate cocaine-induced MOR expression at both the transcriptional and post-transcriptional levels. Based on these novel findings, it is hypothesized that epigenetic mechanisms are implicated in cocaine’s action on MOR expression in neurons.

Low-dose naltrexone suppresses ovarian cancer and exhibits enhanced inhibition in combination with cisplatin

Ovarian cancer is the leading cause of death from gynecological malignancies. Although initial therapeutic modalities are successful, 65% of these women relapse with only palliative treatments available thereafter. Endogenous opioids repress the proliferation of human ovarian cancer cells in vitro, and do so in a receptor-mediated manner. The present study examined whether modulation of opioid systems by the opioid antagonist naltrexone (NTX), alone or in combination with standard of care therapies (taxol/paclitaxel, cisplatin), alters human ovarian cancer cell proliferation in tissue culture and tumor progression in mice. Administration of NTX for six hours every two days, but not continuously, reduced DNA synthesis and cell replication from vehicle-treated controls in tissue culture. Moreover, brief exposure to NTX in combination with taxol or cisplatin had an enhanced anticancer action. Mice with established ovarian tumors and treated with a low dosage of NTX (LDN), which invokes a short period of opioid receptor blockade, repressed tumor progression in a non-toxic fashion by reducing DNA synthesis and angiogenesis but not altering cell survival. The combination of LDN with cisplatin, but not taxol, resulted in an additive inhibitory effect on tumorigenesis with enhanced depression of DNA synthesis and angiogenesis. LDN combined with cisplatin alleviated the toxicity (e.g. weight loss) associated with cisplatin. LDN treatment upregulated the expression of the opioid growth factor (OGF, chemical term ([Met5]-enkephalin) and its receptor, OGFr. Previous tissue culture studies have reported that OGF is the only opioid peptide with antiproliferative activity on ovarian cancer cells, with OGF action mediated by OGFr. Thus, the common denominator of intermittent opioid receptor blockade by short-term NTX or LDN on ovarian cancer proliferation and tumorigenesis recorded herein appears to be related to the OGF–OGFr axis. These preclinical data may offer a non-toxic and efficacious pathway-related treatment that can benefit patients with ovarian cancer.

Chronic high-fat diet drives postnatal epigenetic regulation of μ-opioid receptor in the brain

Opioid system dysregulation has been observed in both genetic and high-fat diet (HFD)-induced models of obesity. An understanding of the molecular mechanisms of MOR transcriptional regulation, particularly within an in vivo context, is lacking. Using a diet-induced model of obesity (DIO), mice were fed a high-fat diet (60% calories from fat) from weaning to >18 weeks of age. Compared with mice fed the control diet, DIO mice had a decreased preference for sucrose. MOR mRNA expression was decreased in reward-related circuitry (ventral tegmental area (VTA), nucleus accumbens (NAc), and prefrontal cortex (PFC)) but not the hypothalamus, important in the homeostatic regulation of feeding. DNA methylation is an epigenetic modification that links environmental exposures to altered gene expression. We found a significant increase in DNA methylation in the MOR promoter region within the reward-related brain regions. Methyl CpG-binding protein 2 (MeCP2) can bind methylated DNA and repress transcription, and DIO mice showed increased binding of MeCP2 to the MOR promoter in reward-related regions of the brain. Finally, using ChIP assays we examined H3K9 methylation (inactive chromatin) and H3 acetylation (active chromatin) within the MOR promoter region and found increased H3K9 methylation and decreased H3 acetylation. These data are the first to identify DNA methylation, MeCP2 recruitment, and chromatin remodeling as mechanisms leading to transcriptional repression of MOR in the brains of mice fed a high-fat diet.

Effects of weaning and syndyphalin-33 on expression of melanocortinergic appetite-regulating genes in swine

Syndyphalin-33 (SD-33) increases feed intake in sheep and recently weaned pigs. To assess the effects of SD-33 on hypothalamic gene expression, hypothalami were collected from unweaned pigs (n=19; 21±3 d of age) on day 0. Remaining pigs received an intramuscular injection of 0.5 μmole/kg SD-33 (SD) or saline (VEH) and weaned into individual pens. On days 1, 4, and 7 after weaning, hypothalami were collected from subsets of pigs (n=8 or 9) within each treatment group. Expression of μ-opioid receptor (MOR) was less in SD pigs than in VEH pigs on day 1 and day 4, suggesting down-regulation of the receptor by SD-33. Expression of hypothalamic melanocortin 4 receptor (MC4R) at 1 d after weaning was increased in VEH pigs (but not SD pigs) relative to levels before weaning. Expression of AGRP was not significantly altered by weaning or treatment at 1 d after weaning. At 4 d after weaning, expression of AGRP was greater in SD pigs than in VEH pigs, but at day 7 expression was less in SD pigs than in VEH pigs. A strong positive correlation was noted between expression levels of MOR and MC4R across treatment and time. Treatment with SD-33 appeared to partially abrogate the effects of weaning on expression of two key appetite-regulating genes within 24 h. Effects of SD-33 appear to be mediated at least in part by the μ-opioid receptor and include actions on the melanocortinergic pathway.

The influence of opioids on matrix metalloproteinase-2 and -9 secretion and mRNA levels in MCF-7 breast cancer cell line

Matrix metalloproteinases (MMPs) are proteolytic enzymes involved in degradation of extracellular matrix, a process that initiates uncontrolled spread of proliferating cancer cells and therefore plays a crucial role in cancer invasion and metastasis. Compounds able to modulate MMP activity may become important tools in cancer research. In the present study we examined the effect of two μ-selective opioids, morphine and endomorphin-2 (EM-2) on the production of MMP-2 and MMP-9 in MCF-7 cells. We report that both opioids time- and concentration-dependently inhibited the expression and secretion of these MMPs. The observed effect was not reversed by naloxone (Nal). Further experiments showed that morphine and EM-2 decreased endothelial nitric oxide synthase (eNOS) mRNA level and nitric oxide (NO) secretion in MCF-7 cells. These findings indicate that attenuation of MMP secretion by opioids was not mediated by opioid receptors but was under the control of nitric oxide system.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The influence of opioids on urokinase plasminogen activator on protein and mRNA level in MCF-7 breast cancer cell line

Urokinase plasminogen activator plays a key role in tumor-associated processes, increasing cancer cell invasion and metastasis, and is therefore used as a marker in cancer prognosis. In this study, we have determined the effect of mu-opioid receptor agonists and antagonists on the urokinase plasminogen activator secretion in MCF-7 cell line. It was shown that mu-opioid receptor agonists, such as morphine and endomorphins, greatly stimulate urokinase plasminogen activator secretion, while naloxone and MOR-selective antagonists elicit the opposite effect. The same tendency was observed also on the urokinase plasminogen activator mRNA level. However, neither agonists nor antagonists had any effect on proliferation of MCF-7 cells. The findings reported in this study may be useful in designing further experiments aimed at elucidating the role of the opioid system in cancer cells.