The role of opioids in cancer progression.

Abstract

Cancer is the second most common cause of death in the United States. The vast majority of cancer-related deaths occur due to metastatic disease, which may be the result of direct extension, lymphatic spread, or hematogenous spread of the cancer.1,2 As such, our ability to effectively treat cancer is mostly dependent on our capacity to stop the process of metastasis. Even though surgery is the mainstay for cancer treatment for many patients, it has been theorized that surgery may accelerate tumor progression and micrometastasis development.3,4 Given the comments above, the next question is whether or not the perioperative period is a window of opportunity for cancer progression and metastasis. The answer is complicated and deserves special attention, given the fact that many factors including inflammation, immunosuppression, angiogenesis, and the direct effects of analgesics and anesthetics can interact to promote cancer progression.3,4 Opioids deserve special consideration because they are broadly used during the perioperative period and are essential for chronic pain management in patients with cancer. It has been speculated that opioids could promote cancer growth and progression. Laboratory investigations indicate that the mu-opioid receptor (MOR) and opioids stimulate proliferation, migration, and invasion in lung cancer cells.5–7 In humans with lung cancer, high or persistent use of opioids is associated with cancer progression independently from tumor staging.8,9 This review discusses the role of opioids on tumorigenesis and metastasis and how MOR signaling plays a role in immunosuppression, inflammation, and angiogenesis in the context of cancer. We also discuss the clinical evidence of opioid effects on cancer progression. Basic concepts of cancer Before discussing the role of MOR signaling in cancer progression and metastasis, it is vital to understand how tumors develop and progress. Carcinogenesis is the result of genetic and epigenetic changes in which normal cells are transformed and gain features of cancer cells.10 These genomic changes promote the dysregulated division of cancer cells.10 Abnormally functioning genes can be categorized into 3 groups. The first group, called tumor suppressors, encodes proteins that normally inhibit cell division or induce apoptosis; thus, reduced activity of these inhibitors can promote cancer. In many cancers, tumor-suppressor genes are lost or inactivated. As a result, transformed cells proliferate in an uncontrolled manner.11 As an example, tumor protein p53, also known as the “guardian of the genome,” is dysregulated. The TP53 gene is linked to a high risk of developing several malignancies including breast cancer and almost half of all head and neck squamous cell carcinomas (SCCs).12,13 The second group, called proto-oncogenes, represents the opposite side of cell growth control and encodes proteins that stimulate cell division or inhibit apoptosis.14 For instance, RET gene mutations have been associated with nonhereditary cancers such as papillary thyroid carcinoma.15 The third group contains DNA repair genes. These genes produce proteins that are in charge of repairing oncogenic mutations. The BRCA1 gene is one of the most well-known DNA-repair genes. Mutations of this gene are present in breast, ovarian, and prostate cancers.16 Therefore, the complete sequence of events leading to cancer involves both the activation of oncogenes and/or the inactivation of tumor-suppressor genes.11,14 Metastasis is defined as the spread of tumor cells away from the primary tumor. The formation of distant metastasis is a process that dynamically involves a series of complex events that are highly regulated by the genetic and/or epigenetic mechanisms within tumor cells. Those cellular events can be summarized as follows: (1) invasion: tumor cells move into the adjacent extracellular tissue matrix through the secretion proteinases, (2) intravasation: the transendothelial migration of cancer cells into blood vessels to enter the bloodstream, (3) survival in the circulatory system, (4) extravasation: tumor cells exit the bloodstream through the vascular endothelium and into the tissue, and (5) proliferation in distant organs to form micrometastsis first and then macrometastasis.17 Angiogenesis is defined as the formation of new blood vessels. Angiogenesis is essential for tumor progression because it is required for cell survival, migration, and invasion. A central factor underlying tumor angiogenesis is the balance between pro-angiogenic and anti-angiogenic signals, which are the result of the dynamic interaction between cancer cells and the tumor microenvironment.18 Inflammation also plays a pivotal role in every aspect of tumor development and subsequent metastasis, and affects the immune function and responses to anti-cancer therapies.19 Both inflammation and angiogenesis are aggravated by increased production of cytokines, growth factors, and prostaglandins during the perioperative period. In addition, immune surveillance, which is the ability of the immune system to identify and eliminate tumor cells, is impaired during the perioperative period; the origin of this impairment is multifactorial and includes surgically induced stress, inflammation, analgesics, and anesthetics.20–22 Altogether, genetic instability leads to transformation of normal cells into cancer cells. Inflammation, immunosuppression, and angiogenesis facilitate tumor proliferation, invasion, and metastasis. These 3 factors are commonly seen during and after surgery and have been related to the administration of opioids perioperatively. Direct effects of opioids and MOR signaling in tumorigenesis and cancer progression MOR mediates the actions of opioids such as morphine. The MOR is a 7-transmembrane domain receptor, which couples to intracellular signaling molecules by activating G protein. It is well known that MOR is expressed in the peripheral and central nervous system. In neurons, activation of MOR initiates numerous complex intracellular signals that lead to analgesia.16,17 In other cell lines including lymphocyte, macrophages, and tumor cells, MOR signaling can promote immunosuppression, inflammation, angiogenesis, direct cancer cell proliferation, and ultimately metastasis (Fig. 1).Figure 1: Outside their activity on cancer cells, opioids can promote tumor growth by causing immunosuppression, inflammation, and angiogenesis. NK indicates natural killer.MOR is upregulated in human non–small cell lung cancer (NSCLC) cells and in metastatic nodes.23 Several signaling pathways have been implicated in the pro-tumoral effects of MOR agonists (Fig. 2A). Lennon et al6 indicated that MOR regulated growth factor-induced EGF receptor signaling (Src, PI3K, Akt, and STAT3 activation), which plays a major role in cell proliferation and migration of lung cancer cells.7 Similarly, a study carried out by Gonzalez-Nunez et al24 reported that a low dose of morphine (10 nM) promotes cell division in the neuroblastoma cell line, whereas higher doses (1 μM) inhibit cell proliferation. However, naloxone, an opioid antagonist, did not fully reverse the effect of morphine, suggesting that the observed effects after morphine administration were partially mediated by MOR activation.24 Gene activity is also modulated by opioids. In vitro studies by Ecimovic and colleagues showed that morphine (50 to 100 ng/mL) increased the expression of the NET1 gene in 2 different breast cancer cell lines. In comparison with vehicle treatment, morphine-induced expression of NET1 led to an increased migration of those cells.25 Chemotherapy agents have the objective of triggering cancer cell death directly by promoting apoptosis or indirectly by modulating the microenvironment. Experimental studies have indicated that opioids (ie, morphine, fentanyl, methadone, and tramadol) can counteract the cytotoxic effects of cisplatin in HELA, lymphoma, bladder, nasopharyngeal, and lung cancer cells.26–30 As an example, morphine (1 mg/kg every 2 d) promoted tumor growth in CNE-2 (nasopharyngeal cancer) in nude mice by inhibiting the effect of cisplatin (3 mg/kg every 2 d).26 Similar studies showed that methylnaltrexone and naltrexone potentiated the in vitro and in vivo cytotoxic effects of cisplatin and 5-fluorouracil in non–small cell lung, breast, and ovarian cancer cells.31,32 Contrary to the literature above, studies have also shown that MOR agonists inhibit tumor growth. For instance, high concentrations of morphine (>10 μM) inhibited breast cancer cell proliferation through a p53-dependent mechanism.33 Other described mechanisms implicated in the anti-cancer effects of MOR signaling include (i) upregulation of caspase 2, 3, 9, and 10, (ii) decreased expression of anti-apoptotic Bcl-2, and (iii) downregulation of Bcl-2, p53, and NFκB pathways (Fig. 2B).33–35 Finally, Friesen et al36,37 demonstrated that methadone sensitizes glioblastoma and leukemia cells to the effect of doxorubicin.Figure 2: A, MOR signaling activates multiple signaling pathways, which include proliferation, survival, angiogenesis, metastasis, and inflammation. Binding of MOR agonists (opioids) triggers the phosphorylation of SRC-Gab-1-PI3K/SRC-mTOR/SRC-MAPK-ERK1/β-Arrestin-MAPK-RhoA and COX-2, which will result in translocation into the nucleus. Opioids can also promote metastasis by acting on MOR by the transactivation of EGFR-SRC-Gab-1-PI3K/ERK-AKT/STAT3 signaling pathways. To accomplish this pathway, they produce metalloproteinase such as MMP-2 and MMP-9. Angiogenic effects of opioids are mediated by their binding to MOR, which then triggers Src and MAPK phosphorylation, increases the expression of RhoA and COX-2, and promotes the VEGF activation. Opioids can directly or indirectly activate TLR4. Activation of this receptor promotes the release of pro-inflammatory cytokines such as IL-1 and NFκB. Their activation allows the migration of macrophages from the circulatory system to the tumor microenvironment and stimulates the secretion of IL-1, IL-6, and TNF-α and MMP-2/9. B, MOR signaling can activate programmed cell death such as apoptosis and necrosis. The anti-cancer effects of MOR agonists are related to a decrease in the production of MMP-2 and MMP-9 release and inhibition of the expression of adhesion molecules such as ICAM-1, VCAM, and Selectin-E. In cancer cells, MOR signaling induces upregulation of caspase 2, 3, and 9/ PI3K-Bad pathways, which results in cell death. BAD indicates BCL-2-associated agonist of cell death; COX-2, cyclooxygenase-2; EGFR, epidermal growth factor receptor; ICAM-1, Intercellular Adhesion Molecule 1; IL-1, interleukin 1; IL-6, interleukin-6; MMP-2, matrix metalloproteinase 2; MMP-9, matrix metalloproteinase 9; MAPK, mitogen-activated protein kinase; MOR, mu-opioid receptor; TLR4, toll like receptor 4; TNF-α, tumor necrosis factor alpha; VCAM-1, vascular cell adhesion protein 1; VEGF, vascular endothelial growth factor.The different and opposite effects of MOR signaling and opioids on cancer growth and progression appear to be linked to the type of MOR agonists investigated, duration and dosage of exposure, cell line studied, and animal model used. Opioids, MOR signaling, and angiogenesis Increasing evidence suggests that opioids are involved in angiogenesis, a process that is critical to cancer progression. Opioids stimulate angiogenesis by at least 2 different mechanisms; the first one is dependent on MOR signaling and involves Src-mediated VEGF receptor transactivation and activation of STAT3 (Fig. 2A).38 This knowledge is based on several observations showing that a short exposure of pulmonary endothelial cells to a sub-clinical dose (100 nM) of morphine stimulates MOR proliferation and migration, and that morphine’s effects on angiogenesis are blocked by clinically relevant concentrations of the peripheral MOR antagonist, methylnaltrexone. 39–41 In addition, Gupta et al39 reported that clinically relevant concentrations of morphine caused endothelial cell proliferation and tube formation. The second mechanism by which opioids can promote angiogenesis is MOR independent and involves the upregulation of cyclooxygenase-2 and/or prostaglandin E2.42 Some reports have shown that opioids inhibit angiogenesis. Balasubramanian et al43 reported that low doses of morphine inhibit, in vitro, the secretion of VEGF induced by hypoxia in endothelial cells. Similarly, previous studies carried out by Koodie et al44 showed that the expression of VEGF in tumors is altered in the presence of morphine, leading to decreased angiogenesis, tumor cell apoptosis, and reduced growth. In vivo experiments conducted by the same group of investigators showed that when mice were administered 75 mg of slow-release morphine over 21 days, morphine reduced leukocyte migration and angiogenesis and tumor growth compared with a placebo.45 Together, these results suggest that MOR signaling can have opposite effects on angiogenesis. The underlying processes of opioid induction of angiogenesis include direct stimulation of cells of the tumor microenvironment (ie, fibroblasts, immune cells, and endothelial cells) and tumor cells. Opioids and MOR signaling modulate inflammation and immunity Today, the causal relationship between inflammation and cancer is widely accepted.46 The inflammation process is characterized by marked activation of inflammatory cells, dysregulated release of inflammatory molecules, and the formation of oncogenic products such as nitric oxide, cytokines interleukin (IL)-1β, IL-2, IL-6, and tumor necrosis factor-α (TNF-α), and growth factor, and chemokines. These inflammatory molecules make the tumor microenvironment more conducive to tumorigenesis, possibly by inducing neoplastic mutation, resistance to apoptosis, and environmental changes such as stimulation of angiogenesis.46 Opioids can indirectly promote cancer dissemination through the modulation of the inflammatory response. The pro-inflammatory effects of opioids are elicited not only by acting on MOR located in cancer cells but also by the direct effects on leukocytes and malignant cells expressing nonopioid receptors such as the toll-like (TLR) and bradykinin receptors.47 For instance, Gupta and colleagues measured high concentrations of IL-6, granulocyte-monocyte colony-stimulating factor, and CCL5 in the supernatant of breast cancer tumor lysates from animals receiving morphine chronically (1.5 mg/kg during 7 wk).48 The same group of investigators showed that cyclooxygenase-2 inhibition with celecoxib prevented the production of prostaglandin E2 in breast cancer tumors.42 In contrast, more recent studies have disputed the inflammatory effects of opioids and indicated that the activity of TLR4 was not increased in rodents and patients receiving opioids perioperatively.49,50 Finally, opioids inhibited the production and release of metalloproteinases 2 and 9 and downregulated the expression of adhesion molecules (ie, E-selectin, ICAM-1, and VCAM-1).51,52 Matrix metalloproteinases and adhesion molecules actively participate in tumor growth and metastasis. The immune system plays a major role in the surveillance against tumors. The immune system is continuously monitoring for the appearance of precancerous and/or cancerous cells, and the majority of these atypical cells are recognized and eliminated before they grow into tumors. It was previously believed that most opioids suppress the immune system. For instance, diacetylmorphine (heroin) administration in an in vivo study was shown to decrease the production of IL-1β, IL-2, TNF-α, and IFN gamma (IFN-γ).53 However, heroin stimulated the release of anti-inflammatory cytokines such as TGF-β1 and IL-10.53 Furthermore, morphine has been implicated in the immune modulation of T helper cell balance with facilitation of T helper cell differentiation toward a Th2 state.54 As an example, Roy et al55 showed that treatment of peripheral blood mononuclear cells with morphine (30 to 100 ng/mL) decreased IL-2 and IFN-γ and increased IL-4 and IL-5. These changes in cytokines were abolished in MOR knockout mice.55 In line with this observation, Borner et al56 reported that morphine and β-endorphin (an endogenous MOR agonist) suppressed the transcription of IL-2 in activated human T lymphocytes and the activation of different transcription factors such as NFAT, AP-1, and NF-kB. These effects were mediated by MOR.56 Natural killer (NK) cells play an important role in tumor immune surveillance. NK cells can kill tumor cells and activate other immune cells against malignant cells. After activation, NK cells release cytokines such as IFN-γ and TNF-α, which then modulate the function of macrophage and dendritic cells to enhance the immune surveillance. Laboratory investigations indicate that stimulation of MOR signaling decreases NK cell cytotoxicity and migration.57,58In vivo animal studies show that the absence of MOR is associated with a higher number of intratumoral NK cells than in wild-type mice.58 Franchi et al59 showed that the administration of morphine (10 mg/kg) or fentanyl (0.1 mg/kg) to rodents decreased NK activity and augmented tumor metastasis, whereas buprenorphine (0.1 mg/kg) did not produce the same effects on NK cells. Similarly, Beilin and colleagues investigated the effect of morphine (30 mg/kg), fentanyl (0.3 mg/kg), and sufentanil (0.06 mg/kg) on NK cell activity in rats. All 3 drugs inhibited NK cell cytotoxicity; the effect was blocked by the opioid antagonist naltrexone.60 Not surprisingly, the immune-suppressive and tumor-promoting effects depend on the experimental conditions. Page et al61,62 observed that short-term administration of opioids did not result in promotion of tumor growth in animal models of surgery-induced breast cancer metastasis. In humans, morphine (10 mg) and high doses of fentanyl (75 mcg/kg) cause suppression of NK cytotoxicity compared with low doses of fentanyl (1 mcg/kg), remifentanil (0.02 μg/kg/min), or tramadol (100 mg).63–65 B lymphocytes are involved in humoral immunity mainly by producing antibodies and memory cells. Previous in vivo studies have shown that opioids inhibit antibody secretion and B cell number by activation of MOR.66,67 Taken together, the literature indicates that at high concentrations, opioids significantly inhibit the immune system, whereas low doses can promote angiogenesis and inflammation. Clinical studies of MOR signaling in oncological patients The effect of opioids on cancer progression and metastasis in humans has been recently investigated by different groups (Table 1). Zylla and colleagues, who included 593 patients with metastatic prostate cancer, published one of the first studies. That study indicated that 5 mg/d of oral morphine increments was associated with reduced progression-free survival and overall survival.74 Sufentanil administration was also an independent risk factor of reduced survival in men undergoing prostatectomy.68 Recent retrospective studies showed a negative association between perioperative opioid consumption and survival in patients with NSCLC and laryngeal SCC.9,69,75,76 Furthermore, the negative association was also found in patients with NSCLC consuming opioids persistently after surgery.8 Similar to metastatic prostate cancer patients, Zylla et al77 reported that patients with advanced NSCLC were more likely to have a shorter survival if they were taking opioids. Finally, Patino and colleagues investigated the association between the dosage of opioids administered intraoperatively to 268 patients undergoing oral cancer surgery and their recurrence-free and overall survival. The authors concluded that high requirements of opioids intraoperatively were an independent predictor of reduced survival.71Table 1: Impact of perioperative opioid use on cancer progression.Other studies have not proven a deleterious association between preoperative, intraoperative, and postoperative opioid consumption and cancer outcomes for adenocarcinoma of the colon and esophagus, leukemia, and abdominal pediatric malignancies.70,72,78,79 Two large registries and one small retrospective study found that opioids were not linked to cancer progression in women with breast cancer.80–82 Finally, a recent study by Du and colleagues reported that large intraoperative requirements of opioids were associated with longer survival in patients with esophageal SCC.83 Recent investigations have also been focused on the impact of the OPRM1 polymorphism on cancer-related outcomes. The most common polymorphism of the MOR receptor, A118G (guanine substitution for adenine at the OPRM1 gene sequence), affects the activity of MOR after binding of its agonists. A study that included a total of 2039 women examined the association between each the A118G polymorphisms and survival. Those with one or more copies of the G allele had shorter cancer-specific mortality.84 However, the association was limited to invasive tumors.84 Cieślińska et al85 found that women with the G allele had a significant risk of developing breast cancer. This finding was supported by findings from Oh and colleagues, who showed that white, but not Korean, women with the G allele had a higher risk of breast cancer. Wang and colleagues included 260 esophageal cancer patients and 291 controls and compared the GG genotype versus the AA genotype. The author reported that compared with the GG genotype, the AA genotype was associated with a higher risk of esophageal SCC in Chinese patients.86 In a similar study, Xu et al87 found that the presence of the A allele of A118G in whole-blood specimens increased the risk of esophageal SCC. Finally, the expression of MOR has shown promising results as a prognosis biomarker in human cancers. In patients with gastric and metastatic prostate cancer, high levels of MOR expression were associated with a higher risk of progression and reduced survival.74,88 Similarly, a moderate to strong cytoplasmic expression of MOR predicted the presence of metastatic lymph nodes in patients with esophageal SCC.89 Conclusions The impact of MOR signaling on tumor development, growth, and progression is still uncertain. MOR can promote tumor growth by either directly signaling on survival pathways of tumor cells or by indirectly acting through increased vascular permeability, promotion of angiogenesis, immunosuppression, and inflammation. However, pro-apoptotic and anti-angiogenic properties of MOR have also been demonstrated. Unfortunately, the experimental data are conflicting, given the differences in dose, opioids agonist, type of cancer cells, and experimental settings. The results from retrospective studies are mixed and there are not yet any conclusive prospective studies or meta-analysis that would indicate the need to change clinical practice. Prospective, randomized trials of opioids to treat cancer-related pain are essential to determine causality between opioids and recurrence-free survival and overall survival.