morphine , the main component of opium , is perhaps the oldest drug known to man . pure morphine was isolated in 1803 by sertrner ( schmitz 1985 ) , and its structure was elucidated 120  years later . full systemic name of morphine is 7,8-didehydro-4,5-epoxy-17-methyl-(5 , 6)-morphinan-3,6-diol . morphine was found to be a particularly good analgesic and sedative , far more effective than crude opium . morphine was shown to exert its action through opioid receptors (  ,  , and  ) localized in the brain ( pasternak 1993 ; reisine and bell 1993 ; harrison et al . 1998 ; kieffer 1999 ; kieffer and gaveriaux - ruff 2002 ) . however , morphine binds to the -opioid receptor with nearly two orders of magnitude greater affinity compared with the other two opioid receptors ( zadina et al . although morphine acts directly on the central nervous system ( cns ) to relieve pain , its activity on peripheral tissues is responsible for many of the secondary complications . besides its strong analgesic effect , morphine exerts a number of adverse side - effects , including addiction , tolerance , respiratory depression , immunosuppression , and constipation . the lack of equally strong painkillers is the reason that despite the drawbacks mentioned above , morphine is still the most commonly used analgesic for management of severe pain , including cancer pain ( mantyh 2006 ) . administration of morphine to cancer patients gave evidence that apart from its analgesic action , morphine can significantly alter tumor growth . in the last decade , numerous studies employing cancer cell lines and experimental animals have been performed to reveal complex mechanisms by which morphine affects tumor cells . whereas the pharmacology and function of opioids in the cns have been extensively characterized , still little is known about their effect on cancer cells . morphine was reported to increase the proliferation of endothelial and tumor cells ( simon and arbo 1986 ; moon 1988 ; ishikawa et al . on the other hand , morphine and other opioids were also found to promote tumor cell death ( maneckjee et al . 1990 ; yeager and colacchio 1991 ; page et al . 1993 ; hatzoglou et al . 1996 ; sueoka et al . 1996 , 1998 ) . several reviews have covered this important research area from different perspectives ( rasmussen et al . 2002 ; fichna and janecka 2004 ; tegeder and geisslinger 2004 ; chen et al . this review will focus on the latest findings on the influence of morphine on tumor cell proliferation , apoptosis , angiogenesis , and migration . despite extensive research , it is still not well understood whether morphine itself directly modifies the growth of tumor cells . some authors postulate that morphine can promote tumor growth and reduce the survival rate of tumor - bearing animals due to immunosuppression , since the negative effects of morphine and other opioids on the immune system are well established ( odunayo et al . multiple research data indicate that morphine can accelerate or inhibit cancer cell growth in vitro and in vivo by different mechanisms . ( 2003 ) reported that morphine inhibited tumor cell proliferation at concentrations of > 10  m . in nude mice , intermittent injections of morphine decreased the growth of tumors in a rat model of metastasizing colon cancer ( yeager and colacchio 1991 ) . on the other hand , morphine was shown to trigger stimulation of human glioblastoma t98 g cell proliferation ( lazarczyk et al . gupta et al . ( 2002 ) demonstrated that morphine , in clinically relevant doses , promoted tumor neovascularization in a human breast tumor xenograft model in mice , leading to increased tumor progression . the discrepancies in results may be due to the differences in administered doses or / and the mode of administration ( systemic versus localized ) . 2002 ) , showed that tumor suppression occurs after chronic high doses of morphine , while tumor - enhancing effects with morphine occur after a single dose or low daily doses ( zong and pollack 2000 ) . this dual concentration - dependent effect , i.e. , mitogenesis at low and growth inhibition at higher concentrations was observed both in vitro and in vivo . the lack of effects of morphine on the proliferation of colon cancer ht-29 cells ( zagon et al . 2008 ) and on breast cancer mcf-7 cells was also reported ( janecka et al . 2004 ) . recently , it was demonstrated that the -opioid receptor regulates cancer progression in animal models ( moss and rosow 2008 ; wang et al . the -opioid receptor - knockout mice were shown not to develop significant tumors when injected with lewis lung cancer cells as did the wild - type controls ( mathew et al . 2009 ) . silencing the expression of the -opioid receptor in lewis lung cancer cells inhibited lung metastasis in wild - type mice by about 75% . finally , infusion of the -opioid receptor antagonist , methylnaltrexone , markedly attenuated tumor growth in wild - type mice treated with lewis lung cancer cells by up to 90% ( mathew et al . these experimental data strongly support the hypothesis that the -opioid receptor promotes tumor growth and metastasis . possible mechanisms of morphine action mediated through the opioid receptors are demonstrated in fig .  1 and through the non - opioid pathways in fig .  2 . the summary of in vivo growth - promoting and growth - inhibiting effects of morphine is presented in table  1 . morphine binds to the -opioid receptor and a stimulates mapk / erk pathway , which results in the cell cycle progression ; b activates pi3k / akt pathway mediating anti - apoptotic effects ; c up - regulates upa expression and secretion promoting metastasis ; d transactivates vegf receptors and induces angiogenesis ; e suppresses the function of t lymphocytes , leading to immunosuppressionfig . 2possible pathways , other than through the opioid receptors , by which morphine influences cancer progression and suppressiontable 1summary of in vivo growth - promoting and growth - inhibiting effects of morphinecell line / tumor typein vivo effectsin vivo dose / timesuggested mechanismantagonist / inhibitionref.ehrlich carcinoma injected s.c . in the left thigh of balb / c micestimulation of angiogenesis0.714 mg / kg / day for 7  days ( equivalent to 50  mg per day for a 70  kg human)ustun et al . 2010colon cancer cells injected i.p . in fisher 344 ratsinhibition of tumor growth and metastasis ( significant decrease in the hepatic tumor burden)20 mg / kg / day  s.c . , the day before and for 2  days after colon cancer cell inoculationenhancement of nk cell activity at the time of tumor cell injectionyeager and colacchio 1991mcf-7 , mda - mb231 breast cancer or ht-29 colon cancer cells injected s.c . in nmri - nu / nu miceinhibition of tumor growth ( mcf-7 and mda - mb231 ) no effect on ht-29 tumors1030 mg / kg / day  i.p . for 3  weeks ( stepwise 10 , 20 , and 30  mg / kg / day  i.p . for the first , second , and third week)inhibition of tumor growth through a p53 dependent mechanism ( up - regulation of p53-dependent p21 , bax , 1993walker 256 carcinosarcoma cells injected i.v . in sprague  dawley ratincrease of the number metastases5  mg / kg single dosesuppresion of nk cellsnaloxone itself had no significant influence on the number of metastases . the effect was inhibited by preadministration of naloxonesimon and arbo 1986b16-bl6 melanoma cells injected into hindpaw in c57bl miceinhibition of tumor growth and metastasis ( decreased number of tumor nodules in the lung)5 and 10  mg / kg / day  s.c . for 6  days , starting 16  days after cancer cells injectionpain reduction and blockade of pain signalssasamura et al . 2002colon 26-l5 carcinoma cells injected i.v . in balb / c miceinhibition of tumor metastasis ( reduction of the number of tumor colonies in the lung)10  mg / kg / day  i.p . for 6  days , starting on day 2 after i.v . inoculation of tumor cellsinhibition of adhesion and invasion of cancer cells ( inhibition of mmp-2 and 9 production)harimaya et al . 2002mcf-7 cells injected into the mammary fat pad of nude miceincrease of tumor growth associated with increased angiogenesis ( neovascularization , increased microvessel density , higher vessel number , increased total length , and more vessel branching)0.714  mg / kg mouse / day for first 15  days and then 1.43  mg / kg mouse / day ( equivalent to 50  mg and 100  mg morphine per day , respectively , for a 70  kg human)activation of pi3k / akt and erk pathways , resulting in inhibition of apoptosis and promotion of cell cycle progressionnaloxone itself had no significant effect on angiogenesis . the anti - angiogenic effect of morphine was not inhibited by naloxonegupta et al . 2002madb-100 mammary adenocarcinoma cells injected i.v . in fisher 344 rats , 5  h after surgeryinhibition of surgery - induced increase in metastasis5  mg / kg 3 doses ( pre , post and 5  h post surgery)inhibition of postoperative pain and stress - induced immunosuppressionpage et al . 1993murine ccl-11 sarcoma cells injected into femur of the right leg in c3h / hej micemorphine did not alter tumor burden50 , 25 , 7.5 , 2.5  mg / ml implanted ( s.c . ) for 7  days following injection of ccl-11 cells into the femurking et al . 2007lewis lung carcinoma cells injected s.c . in nude micereduction of tumor cell - induced angiogenesis and tumor growtha continuous slow - release morphine pellet resulting in morphine plasma levels within 250400  ng / mlsuppression of the hypoxia - induced mitochondrial p38 mitogen - activated protein kinase ( mapk ) pathwaythe effect was abolished with naltrexonekoodie et al . 2010tc-1 cells injected s.c . into the right leg of c57bl/6j micedose - dependent increase of tumor growth10  mg / kg  i.p . , twice daily from day 10 before tumor injectionimmunosuppression ( -opioid receptor mediated suppression of t lymphocytes proliferation , promotion of apoptosis of t lymphocytes through bcl-2 and bax apoptosis - related molecules)cheng et al . 2006 possible mechanisms of opioid receptor mediated influence of morphine on tumor growth . morphine binds to the -opioid receptor and a stimulates mapk / erk pathway , which results in the cell cycle progression ; b activates pi3k / akt pathway mediating anti - apoptotic effects ; c up - regulates upa expression and secretion promoting metastasis ; d transactivates vegf receptors and induces angiogenesis ; e suppresses the function of t lymphocytes , leading to immunosuppression possible pathways , other than through the opioid receptors , by which morphine influences cancer progression and suppression summary of in vivo growth - promoting and growth - inhibiting effects of morphine apoptosis is an active process of controlled cell death in the development and maintenance of tissue homeostasis . apoptosis is controlled by two main pathways , the intrinsic or the mitochondrial - mediated pathway ( green and reed 1998 ) and the extrinsic or death receptor - mediated pathway ( ashkenazi and dixit 1999 ) . mitochondrial - mediated apoptosis is controlled by bcl-2 family of proteins ( cory and adams 2002 ; cory et al . the death receptor - mediated pathway is initiated by the ligation of cell death ligands with their death receptors ( sartorius et al . caspase-3 is activated in both apoptotic pathways and plays central role in the execution phase of cell apoptosis ( ashkenazi and dixit 1999 ; hengartner 2000 ; fulda and debatin 2006 ) . apoptosis is usually deregulated in cancer cells , and this deregulation can contribute to uncontrollable proliferation and tumor growth ( hanahan and weinberg 2000 ; hengartner 2000 ; kaufmann and hengartner 2001 ) . morphine was shown to induce the apoptosis of human endothelial cells ( hsiao et al . morphine in high concentrations ( 1010  m ) was demonstrated to induce apoptotic cell death in human tumor cell lines ( kawase et al . in other studies , the pro - apoptotic effect of morphine in human tumor cell lines was observed at clinical concentration ( 10  m ) ( smith et al . for example , morphine produced a higher number of necrotic cells in the mcf-7 breast cancer cell line than in the a549 lung cancer cell line ( hatsukari et al . 2003 ) , it was recently demonstrated that chronic high - dose morphine treatment was able to cause apoptotic cell death of sh - sy5y cells in an opioid receptor - independent manner ( lin et al . a pivotal role in this process was played by c - jun n - terminal kinase ( jnk ) . activation of jnk by morphine led to reactive oxygen species ( ros ) generation and induced cytochrome c release and caspase-9/3 activation through enhancement of expression of the pro - apoptotic protein bim and reduction of expression of the anti - apoptotic protein bcl-2 . the key role of the jnk pathway in morphine - induced mitochondria - dependent apoptosis was further confirmed by the observation that decreased levels of jnk in cells transfected with specific small interfering rna resulted in resistance to the pro - apoptotic effect of morphine . it can be concluded that morphine can induce apoptosis and inhibit the growth of cancer cells by activating different signal pathways . apart from the mitochondrial pathway mentioned above , apoptosis of mcf-7 cells was shown to be mediated by a novel sigma-2 receptor and p53 and caspase - independent pathway ( crawford et al . cne2 human epithelial tumor cell line was reported to undergo apoptosis by the activation of the -opioid receptor via the phospholipase pathway ( diao et al . the growth - inhibitory or apoptosis - inducing effects of morphine might be directly associated with morphine tolerance ( wu et al . 1999 ; mao et al . 2002 ) or receptor desensitization , as assessed by a lack of morphine - stimulated gtp - ase activity at concentrations that inhibit tumor growth ( tegeder et al . drugs that prevented the development of morphine tolerance in rats also prevented cell death ( mao et al . 2002 ) . this close association between apoptosis and receptor desensitization suggests that receptor internalization may be the key event in initiating opioid - evoked cell death . on the other hand , evidence of the anti - apoptotic activity of it was shown that morphine can antagonize the pro - apoptotic activity of a well - known anti - tumor drug , doxorubicin , in neuroblastoma sh - sy5y cells ( lin et al . 2007 ) . the effect could not be reversed by naloxone , indicating a non - opioid receptor - mediated signaling pathway . further studies showed that morphine attenuated doxorubicin - induced apoptosis by the inhibition of ros generation and mitochondrial cytochrom c release , as well as by blockade of nuclear factor b ( nf-b ) transcriptional activation . nf-b is a ubiquitous nuclear transcription factor that plays a major regulatory role in apoptosis and inflammation . ( 2003 ) demonstrated that in sh - sy5y neuroblastoma cells , morphine ( 1010  m ) promoted cell survival after serum deprivation without inducing cell proliferation , and this effect was fully reversed by naloxone . it was shown that in neuronal cells , -opioid agonists do not directly induce apoptosis , but are able to activate the phosphatidylinositol 3-kinase / akt ( pi3k / akt ) signal transduction pathway , thus leading to cell survival . successful tumor growth depends on many aspects , of which the most important are proliferation of tumor cells and angiogenesis or the formation of new blood vessels which are necessary for metastasis ( carmeliet and jain 2000 ) . to date , very few studies have investigated the effect of morphine on tumor cell - induced angiogenesis . gupta et al . ( 2002 ) showed that at clinically relevant concentrations , morphine stimulated human microvascular endothelial cell proliferation and angiogenesis in vitro , and that in vivo these effects translated into enhanced tumor neovascularization in the mcf-7 breast cancer model . in addition , this group demonstrated that morphine promoted activation of vascular endothelial growth factor ( vegf ) receptor and increased metastasis and reduced survival in animal model of hormone - dependent breast cancer ( chen et al . however , the opioid receptor antagonist , naloxone , did not inhibit the pro - angiogenic activity of morphine , indicating that the effect was not mediated by the typical opioid receptors . on the other hand , singleton and moss ( 2010 ) demonstrated that -opioids ( morphine ) transactivate the vegf receptors and promote angiogenesis . the -opioid receptor antagonist methylnaltrexone blocks opioid - induced angiogenesis in cultured human endothelial cells . it was postulated that the pro - angiogenic activity of morphine is connected with the stimulation of mitogen - activated protein kinase ( mapk ) signaling pathway via g protein - coupled receptors and nitric oxide ( no ) . chronic morphine treatment increased the levels of nitric oxide synthase ( nos ) , no , and cyclooxygenase-2 ( cox-2 ) in mouse kidney ( stefano et al . 1995 ; arerangaiah et al . 1993 ) , and activated cox-2 in turn increased prostaglandin e2 production ( salvemini et al . two weeks of chronic morphine treatment of experimental mice with highly invasive sck breast cancer model stimulated cox-2 , prostaglandin e2 , and angiogenesis , which was accompanied by increased tumor weight , increased metastasis , and reduced survival . it was postulated that products of cox-2 , such as prostaglandin e2 produced in tumor cells , act on the tumor endothelium and promote angiogenesis . notably , morphine induced a much stronger expression of cox-2 in tumor cells than in endothelial cells . co - administration of cox-2 inhibitor , celecoxib , prevented morphine - induced promotion of angiogenesis and tumor growth . therefore , morphine - induced tumor growth may in part be due to the up - regulation of cox-2 and prostaglandin e2-mediated stimulation of angiogenesis . it was also shown that morphine did not have any effect on mcf-7 human breast cancer cell proliferation in vitro ( farooqui et al . 2006 ) but stimulated mcf-7 breast tumor growth by promoting angiogenesis ( gupta et al . that means that morphine by itself did not modulate tumor cell growth in culture , but due to up - regulation of cox-2 in tumor cells , stimulated angiogenesis in vivo , resulting in increased tumor growth and metastasis . it is well known that when the solid tumor grows , new tumor cells are localized further away from their vascular supply , and low oxygen tensions or hypoxia stimulate tumor cells to secrete pro - angiogenic factors ( folkman and d'amore 1996 ) . a potent pro - angiogenic factor secreted by hypoxic tumor cells within developing solid tumor is vascular endothelial growth factor ( vegf ) . this factor promotes new blood vessel formation to sustain tumor growth and initiates endothelial cell proliferation and migration ( brekken and thorpe 2001 ; ferrara 2004 ) . ( 2001 ) reported that morphine inhibited hypoxia - induced vegf secretion in rat cardiomyocytes and human umbilical vein endothelial cells and therefore attenuated the ability of hypoxic tumor cells to induce angiogenesis . most recently , the same effect was observed in a murine lewis lung carcinoma tumor model ( koodie et al . morphine was administered to mice at the clinically relevant analgesic doses by implantation of continuous slow - release pallets . morphine plasma level within 250400  ng / ml was sufficient to significantly reduce tumor cell - induced angiogenesis and tumor growth . the effect of morphine was abolished by concomitant administration of opioid antagonist , naltrexone , and also in the -opioid receptor knockout mice , supporting the involvement of classical opioid receptors in the process . the authors also demonstrated that the inhibitory effect of morphine was mediated through the suppression of the hypoxia - induced mitochondrial p38 mapk pathway . much earlier , it was shown that high concentrations of morphine ( 10  mg / ml of plasma ) inhibited angiogenesis in the chick chorioallantoic membrane assay ( pasi et al . 1991 ) . however , morphine is cytotoxic to endothelial cells at such high concentrations ( gupta et al . 2002 ) , so the observed effect could have been non - specific . in wound healing experiments in mice , systemic administration of high - dose morphine resulted in impared mobilization of endothelial progenitor cells and angiogenesis , delaying the healing process . consistent with the in vivo experiment , in cultured endothelial cells , morphine reduced capillary tube formation in a concentration - dependent manner ( lam et al . a major feature of cancer cells is their ability to migrate and settle in surrounding or distant tissues . most cancer deaths and the main cause of failure in cancer treatment is not due to growth of the primary tumor but results from its intensive spread ( metastasis ) to secondary sites ( engbring and kleinman 2003 ; widel and widel 2006 ) . the critical step of cancer dissemination is migration of cancer cells through the extracellular matrix ( ecm ) . indispensable in this process is activation of urokinase plasminogen activator system , which includes a serine proteinase , urokinase plasminogen activator ( upa ) , two inhibitors , pai-1 and pai-2 , and the membrane - linked receptor ( upar ) ( duffy and duggan 2004 ) , plasmin and matrix metalloproteinases ( mmps ) ( mignatti and rifkin 2000 ) capable of degrading the ecm . it was shown that upa , pai-1 , and upar levels are up - regulated in most types of cancers ( shapiro et al . ( 2009 ) showed that morphine caused a markedly increased secretion of upa in mcf-7 breast cancer cells , which correlated well with up - regulation of upa and upar mrna levels . naloxone reversed morphine - induced up - regulation of upa and upa mrna levels confirming the involvement of the opioid receptors in this process . ( 2008 ) who demonstrated that morphine stimulated upa secretion in ht-29 colon cancer cells . the mmps constitute a family of zinc - dependent endopeptidases , whose primary function is remodeling of components in the ecm ( engbring and kleinman 2003 ; widel and widel 2006 ; jespersen et al . , it seems that mmp-2 ( gelatinase a ) and mmp-9 ( gelatinase b ) play a crucial role in cancer invasion because of their ability to degrade type iv collagen , a major component of basement membranes ( widel and widel 2006 ) . the activity of mmps is strictly regulated at the transcription and translation levels by endogenous inhibitors , including 2-macroglobulin and tissue inhibitors of metalloproteinases ( timps ) . several studies have shown that mmp expression and activity were increased in several tumor types , especially breast and lung cancer . the up - regulated levels of mmps correlated well with tumor stage , increased invasion , and potential metastasis ( lynch and matrisian 2002 ; widel and widel 2006 ) . morphine was found to significantly reduce experimental lung metastasis and invasion of colon 26-l5 cells by inhibiting adhesion and migration of these cells to ecm ( harimaya et al . wehi 164 , activity of mmp-2 was shown to be under control of no system and was attenuated by low doses of morphine in a non - opioid receptor - related manner ( shariftabrizi et al . morphine inhibited expression and secretion of mmp-2 and -9 in time- and concentration - dependent manner , and the process was not mediated by opioid receptors but was also under the control of the no system ( gach et al . disorders in the immune system can result in disease , including inflammatory diseases and cancer . the expression of the -opioid receptor gene in neuronal cells is regulated by cytokines , released from the cells of the immune system . on the other hand , -opioid receptor is also expressed in the cells of immune system , such as lymphocytes and macrophages ( brner et al . 2007 ) . this points at the importance of interactions between opioid and immune system ( kraus 2009 ) . the fact that morphine can inhibit numerous immune cell functions and cause immunosuppression is well known ( peterson et al . 1993 ; sacerdote et al . 1997 ; eisenstein and hilburger 1998 ) . in particular , morphine decreases the activity of natural killer cells which play a major role in the rejection of tumors ( sacerdote et al . 1997 ) and increases the susceptibility of animals and humans to bacterial and viral infections ( yeager et al . 1995 ; risdahl et al . 1998 ; macfarlane et al . it was also shown that morphine inhibits production of pro - inflammatory cytokines ( e.g. , tnf , il-6 ) in monocytes ( bonnet et al . 2008 ) , modulates the t helper cell balance by inducing up - regulation of an anti - inflammatory il-4 mrna ( roy et al . 2005 ) and produces immunosuppression by inhibition of transcription of il-2 in activated lymphocytes t ( brner et al . 2009 ) . however , future research is necessary to provide clearer understanding of the cellular and molecular targets of morphine action within the immune system . for more than two decades , researchers have been trying to document the effects that morphine exerts on tumor cells . it has been recognized that morphine can affect tumor growth by either directly acting on the tumor cells or by directly acting on the endothelial cells or on the cns - mediated secretion of growth factors that may alter tumor microenvironment . however , the results obtained both in in vitro and in vivo studies are conflicting . on one hand , morphine was shown to induce tumor growth , inhibit apoptosis , promote angiogenesis and migration of tumor cells , but on the other hand , pro - apoptotic and anti - angiogenic properties of morphine were also demonstrated . performed studies were very heterogenic due to the different doses of morphine applied , and the effects may vary when the experiments were performed in vivo or on isolated cells . the dose and route of administration of morphine might be critical factors that need to be taken into consideration in clinical settings . the final answer to the question whether morphine is an inhibitor of tumor growth or whether it promotes cancer is still to be sought . the effects of morphine can also depend on a cancer type , since different cancer cells can overexpress certain enzymes which are possible morphine targets .