Met-enkephalin induces cytolytic processes of apoptotic type in K562 human erythroid leukemia cells

Involvement of the Opioid Peptide Family in Cancer Progression

Peptides mediate cancer progression favoring the mitogenesis, migration, and invasion of tumor cells, promoting metastasis and anti-apoptotic mechanisms, and facilitating angiogenesis/lymphangiogenesis. Tumor cells overexpress peptide receptors, crucial targets for developing specific treatments against cancer cells using peptide receptor antagonists and promoting apoptosis in tumor cells. Opioids exert an antitumoral effect, whereas others promote tumor growth and metastasis. This review updates the findings regarding the involvement of opioid peptides (enkephalins, endorphins, and dynorphins) in cancer development. Anticancer therapeutic strategies targeting the opioid peptidergic system and the main research lines to be developed regarding the topic reviewed are suggested. There is much to investigate about opioid peptides and cancer: basic information is scarce, incomplete, or absent in many tumors. This knowledge is crucial since promising anticancer strategies could be developed alone or in combination therapies with chemotherapy/radiotherapy.

Impact of Opioid Use in Hematological Malignancies: Clinical, Immunological and Concomitant Aspects

Opioid agents play a unique role in pain and symptom management for cancer patients. Research shows that opiate use, especially when associated with underlying cancer, has significant effects on hematological parameters. These changes may lead to greater risk for immunosuppression, tumor growth and progression of metastatic processes. The aim of this review is to explore the effects of opiates on various metabolic and biological processes, as well as the hematopoietic system, especially in cancer patients. Our findings demonstrate that the tumor-promoting effects of opiates remain contradictory, as both growth-promoting and anti-tumor effects have been observed. However, available data suggest that opiates can facilitate the proliferation and migration of tumor cells, and understanding of this process on cancer treatment is tremendously important.

Naltrindole inhibits human multiple myeloma cell proliferation in vitro and in a murine xenograft model in vivo

It has been demonstrated previously that immune cell activation and proliferation were sensitive to the effects of naltrindole, a nonpeptidic δ-opioid receptor-selective antagonist; therefore, we hypothesized that human multiple myeloma (MM) would be a valuable model for studying potential antineoplastic properties of naltrindole. [(3)H]naltrindole exhibited saturable, low-affinity binding to intact human MM cells; however, the pharmacological profile of the binding site differed considerably from the properties of δ-, κ-, and μ-opioid receptors, and opioid receptor mRNA was not detected in MM cells by reverse transcriptase-polymerase chain reaction. Naltrindole inhibited the proliferation of cultured human U266 MM cells in a time- and dose-dependent manner with an EC(50) of 16 μM. The naltrindole-induced inhibition of U266 cell proliferation was not blocked by a 10-fold molar excess of naltrexone, a nonselective opioid antagonist. Additive inhibition of MM cell proliferation was observed when using a combination of naltrindole with the histone deacetylase inhibitor sodium valproate, the proteasome inhibitor bortezomib, the glucocorticoid receptor agonist dexamethasone, and the HMG CoA reductase inhibitor simvastatin. Treatment of U266 cells with naltrindole significantly decreased the level of the active, phosphorylated form of the kinases, extracellular signal-regulated kinase and Akt, which may be related to its antiproliferative activity. The antiproliferative activity of naltrindole toward MM cells was maintained in cocultures of MM and bone marrow-derived stromal cells, mimicking the bone marrow microenvironment. In vivo, naltrindole significantly decreased tumor cell volumes in human MM cell xenografts in severe combined immunodeficient mice. We hypothesize that naltrindole inhibits the proliferation of MM cells through a nonopioid receptor-dependent mechanism.

Peptidomics: a logical sequel to proteomics

Rapid progress of separation techniques as well as methods of structural analysis provided conditions in the past decade for total screening of complex biologic mixtures for any given class of biomolecules. The present review updates the reader with the modern state of peptidomics, a chapter of chemical biology that deals with structure and biologic properties of sets of peptides present in biologic tissues, cells or fluids. Scope and limitations of currently employed experimental techniques are considered and the main results are outlined. Considerable attention will be afforded to the biologic role of peptides formed in vivo by proteolysis of nonspecialized precursor proteins with other well-defined functions. In conclusion, the connection is discussed between peptidomics and the much more mature and still closely related field of proteomics.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Endomorphins, endogenous opioid peptides, induce apoptosis in human leukemia HL-60 cells

Opioids play a role in the apoptosis machinery. We studied the induction of apoptosis in endomorphin 1 (EM1) and endomorphin 2 (EM2), 2 newly isolated endogenous µ-opioid receptor agonists. These endomorphins were able to reduce the viability of cultured HL-60 cells. The antiproliferative properties of endomorphins appeared to be attributable to their induction of apoptotic cell death as determined by ultrastructural change, internucleosomal DNA fragmentation, and increased proportion of the subdiploid cell population. To elucidate molecular events in the apoptosis, protein expressions of Bcl-2, Bax, Fas, and FasL were measured by western blotting using specific antibodies in HL-60 cells. The level of Bcl-2 indicated down-regulation, but the Bax, Fas, and FasL expression showed up-regulation as compared with the untreated control cells. These data support the idea that endomorphins induce apoptosis in HL-60 cells through the activation of the Bcl-2–Bax and the Fas–FasL pathway. We suggest that endomorphins may play an important role in the regulation of tumor cell death.Key words: endomorphins, HL-60 cell, apoptosis, Bcl-2, Fas.

Methionine enkephalin suppresses metabolic activity of a leukemic cell line (NALM-1) and enhances CD10 expression

NALM-1 cells (a cell line derived from human pre-B leukemia) were exposed to the opioid pentapeptide methionine-enkephalin (Met-enkephalin) and/or to thiorphan, an inhibitor of the enzyme that degrades the enkephalins (membrane endopeptidase EC 3.4.24.11, CALLA, the CD10 marker). Metabolic and proliferative activity was assessed after 6, 24 and 48 h in microplates using a colorimetric assay with vital dye MTT. CD10 expression was determined by means of semi-quantitative RT-PCR. Exposure to the Met-enkephalin at concentrations of 10(-8)-10(-6) M for 6 h reduced the MTT-activity, and after 24 and 48 h the suppression waned. Thiorphan (5 x 10(-6) M) abrogated the suppressive effect of the enkephalin, and after 6 h converted suppression into stimulation. Met-enkephalin (10(-6) M) increased and thiorphan (2.5 x 10(-6)-10(-6) M) decreased expression of CD10 at the RNA level. Suppression of the MTT uptake was attributed to the products of Met-enkephalin degradation caused by the enzymatic activity of CD10.

The effect of opioid agonists of delta-class DSLET, mu-class DAMGO, kappa-class U-69593 and an opioid antagonist, naloxone, on MTT activity of NALM-1 leukemic cells

The effects of synthetic agonists of delta-, mu-, kappa-opioid classes were studied on the proliferation of NALM-1 leukemic cells, using the MTT-test. Delta-opioid DSLET and mu-opioid DAMGO mildly and transiently decreased, in higher concentrations, the MTT-activity of NALM-1 cells after 6 h of treatment. The kappa-opioid agonist U-69593 mildly suppressed proliferation of NALM-1 cells after 48 h of treatment. Naloxone, an opioid receptor antagonist, mildly and transiently diminished MTT-activity of NALM-1 cells after 6 h of treatment. Treatment with opioid agonists, DAMGO, DSLET, U-69593, and an opioid antagonist naloxone for 6, 24, and 48 h, did not trigger DNA fragmentation, which was considered as a possible mechanism of action.

Family of hemorphins: co-relations between amino acid sequences and effects in cell cultures

Hemorphins, i.e. endogenous fragments of beta-globin chain segment (32-41) LVVYPWTQRY(F) suppress the growth of transformed murine fibroblasts L929 cell culture, the effect is due to cytotoxicity and inhibition of cell proliferation. The contribution of cytotoxicity depends on the presence of Leu(32): VV-hemorphins, except VV-hemorphin-4, exhibit cytotoxicity significantly higher than respective LVV-hemorphins. Decrease of cell number induced by hemorphins depend on the extent of N- and C-terminal degradation of hemorphins: VV-hemorphins in most cases are more active than LVV-, V-hemorphins, and hemorphins. In the group of VV-hemorphins the activity of VV-hemorphin-5 (valorphin) is significantly higher than of VV-hemorphin-7, VV-hemorphin-6, and VV-hemorphin-4, meaning that the presence of C-terminal Gln is important for suppressing of cell number. The amino acid sequence VVYPWTQ corresponding to valorphin was identified as important for manifestation of the both cytotoxic and antiproliferative effects.

Antiapoptotic and cytotoxic properties of delta opioid peptide [D-Ala(2),D-Leu(5)]enkephalin in PC12 cells

The delta opioid peptide [D-Ala(2),D-Leu(5)]enkephalin (DADLE) has been shown to promote organ survival and to protect against methamphetamine-induced neurodegeneration. However, the cellular mechanisms of these actions of DADLE are not totally clear. We examined the action of DADLE in serum-deprived pheochromocytoma cells (PC12) and found that DADLE protected against cell death in those cells. However, the dose-response curves of the protective effects of DADLE are U-shaped as judged by three biochemical or morphological assays: the LDH release, the DNA laddering, and the apoptotic nuclei. It was found that femtomolar to picomolar concentrations of DADLE are antiapoptotic, whereas micormolar concentrations of DADLE are cytotoxic in PC12 cells. The protective effect of DADLE could be attenuated by a selective delta2 opioid antagonist and the cytotoxic action of DADLE was reduced by a selective mu opioid receptor antagonist. The treatment of cells with PD98059, a selective inhibitor of ERK kinase (MEK), or the transfection of cells with a dominant interfering form of MEK (MEK-KA97) blocked both the protective effect of DADLE and the ERK phosphorylation induced by DADLE. Cytotoxic concentrations of DADLE, on the other hand, caused an increase of Fas-ligand (FasL) in PC12 cells that was attenuated by a selective mu antagonist. Our results suggest, therefore, that endogenous opioid peptides may, at low concentrations, promote cell survival via the MEK-ERK pathway perhaps through delta2 opioid receptors, whereas they may kill cells at high concentrations via the activation of FasL through an as-yet unknown mechanism involving mu opioid receptors.

LVV- and VV-hemorphins: comparative levels in rat tissues

Screening of hemorphins in extracts of rat lung, brain, heart and spleen was carried out. The threshold for detection of hemorphins was 0.01 nmol for spleen and 0.05 nmol for other tissues. Both the content and the composition of hemorphins differed significantly in the tissues analyzed. Heart and lung extracts were rich in these peptides, the content of the most abundant components reaching 16-44 nmol/g of tissue. In contrast, spleen and brain contained much lower amounts of hemorphins, i.e. about 0.3-2.6 nmol/g of tissue. The most represented hemorphin in lung, heart and brain was VV-hemorphin-5, while the content of other members of the hemorphin family depended significantly on the tissue analyzed: lung extract was also rich in LVV-hemorphin-5, heart contained similar amounts of LVV-hemorphin-7 and LVV-hemorphin-5 and brain of LVV-hemorphin-6. In contrast, the hemorphin family in spleen was represented mainly by C-terminally shortened VV-hemorphins, i.e. VV-hemorphin-4 and VV-hemorphin-3. The levels of hemorphins in all cases were sufficient to activate the opioid receptors of the respective tissues.

A novel system of peptidergic regulation

Systematic analysis of structure and biological activity of peptide components of tissue extracts and biological fluids allows us to formulate a novel concept of a peptidergic regulatory system, complementary to the conventional regulatory systems (i.e. nervous, endocrine and paracrine systems). According to that concept, the proteolytic degradation of tissue proteins carried out by a specific and regulated system of tissue-specific enzymes and protein substrates gives rise to a large group of peptides, which we define as tissue-specific peptide pool. As a result, functional proteins provide their proteolytically derived fragments for maintaining tissue homeostasis.

Tumor cell cytolysis mediated by valorphin, an opioid-like fragment of hemoglobin beta-chain

Valorphin, an endogenous opioid-like hemoglobin fragment, is cytotoxic for L929 and K562 tumor cells in 10(-7)-10(-13) M concentration range. Because cytolytic effects induced by valorphin in K562 cells are inhibited by naloxone, opioid receptors should be involved in induction of valorphin-mediated tumor cell death. Three distinct cytolytic processes, differing in the onset time and the development time, take place with K562 cells within 10-18 h of incubation with valorphin. All three processes are not associated with apoptotic mechanism of cell death.