The effect of opiates upon prostatic carcinoma cell growth

Reduction of Prostate Cancer Risk: Role of Frequent Ejaculation-Associated Mechanisms

Prostate cancer (PCa) accounts for roughly 15% of diagnosed cancers among men, with disease incidence increasing worldwide. Age, family history and ethnicity, diet, physical activity, and chemoprevention all play a role in reducing PCa risk. The prostate is an exocrine gland that is characterized by its multi-functionality, being involved in reproductive aspects such as male ejaculation and orgasmic ecstasy, as well as playing key roles in the regulation of local and systemic concentrations of 5α-dihydrotestosterone. The increase in androgen receptors at the ventral prostate is the first elevated response induced by copulation. The regulation of prostate growth and function is mediated by an androgen-dependent mechanism. Binding 5-DHT to androgen receptors (AR) results in the formation of a 5α-DHT:AR complex. The interaction of the 5α-DHT:AR complex with the specific DNA enhancer element of androgen-regulated genes leads to the regulation of androgen-specific target genes to maintain prostate homeostasis. Consequently, ejaculation may play a significant role in the reduction of PCa risk. Thus, frequent ejaculation in the absence of risky sexual behavior is a possible approach for the prevention of PCa. In this review, we provide an insight into possible mechanisms regulating the impact of frequent ejaculation on reducing PCa risk.

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.

Mapping the Evidence for Opioid-Mediated Changes in Malignancy and Chemotherapeutic Efficacy: Protocol for a Scoping Review

BACKGROUND

Numerous reports contend opioids can augment or inhibit malignancy. At present, there is no consensus on the risk or benefit posed by opioids on malignancy or chemotherapeutic activity. Distinguishing the consequences of opioid use from pain and its management is challenging. Additionally, opioid concentration data is often lacking in clinical studies. A scoping review approach inclusive of preclinical and clinical data will improve our understanding of the risk-benefit relationship concerning commonly prescribed opioids and cancer and cancer treatment.

OBJECTIVE

The aim of the study is to map diverse studies spanning from preclinical to clinical regarding opioids with malignancy and its treatment.

METHODS

This scoping review will use the Arksey six stages framework to (1) identify the research question; (2) identify relevant studies; (3) select studies meeting criteria; (4) extract and chart data; (5) collate, summarize, and report results; and (6) conduct expert consultation. An initial pilot study was undertaken to (1) parameterize the extent and scale of existing data for an evidence review, (2) identify key factors to be extracted in systematic charting efforts, and (3) assess opioid concentration as a variable for its relevance to the central hypothesis. Six databases will be searched with no filters: MEDLINE, Embase, CINAHL Complete, Cochrane Library, Biological Sciences Collection, and International Pharmaceutical Abstracts. Trial registries will include ClinicalTrials.gov, Cochrane CENTRAL, International Standard Randomised Controlled Trial Number Registry, European Union Clinical Trials Register, and World Health Organization International Clinical Trials Registry. Eligibility criteria will include preclinical and clinical study data on opioids effects on tumor growth or survival, or alteration on the antineoplastic activity of chemotherapeutics. We will chart data on (1) opioid concentration from human subjects with cancer, yielding a "physiologic range" to better interpret available preclinical data; (2) patterns of opioid exposure with disease and treatment-related patient outcomes; and (3) the influence of opioids on cancer cell survival, as well as opioid-related changes to cancer cell susceptibility for chemotherapeutics.

RESULTS

This scoping review will present results in narrative forms as well as with the use of tables and diagrams. Initiated in February 2021 at the University of Utah, this protocol is anticipated to generate a scoping review by August 2023. The results of the scoping review will be disseminated through scientific conference proceedings and presentations, stakeholder meetings, and by publication in a peer-reviewed journal.

CONCLUSIONS

The findings of this scoping review will provide a comprehensive description of the consequences of prescription opioids on malignancy and its treatment. By incorporating preclinical and clinical data, this scoping review will invite novel comparisons across study types that could inform new basic, translational, and clinical studies regarding risks and benefits of opioid use among patients with cancer.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/38167.

Opioids in cancer: The κ‑opioid receptor (Review)

The κ‑opioid receptor (KOR) is one of the primary receptors of opioids and serves a vital role in the regulation of pain, anesthesia, addiction and other pathological and physiological processes. KOR is associated with several types of cancer and may influence cancer progression. It has been proposed that KOR may represent a new tumor molecular marker and provide a novel basis for molecular targeted therapies for cancer. However, the association between KOR and cancer remains to be explored comprehensively. The present review introduces KOR and its association with different types of cancer. Improved understanding of KOR may facilitate development of novel antitumor therapies.

Impact of perioperative pain management on cancer recurrence: an ASRA/ESRA special article

Cancer causes considerable suffering and 80% of advanced cancer patients experience moderate to severe pain. Surgical tumor excision remains a cornerstone of primary cancer treatment, but is also recognized as one of the greatest risk factors for metastatic spread. The perioperative period, characterized by the surgical stress response, pharmacologic-induced angiogenesis, and immunomodulation results in a physiologic environment that supports tumor spread and distant reimplantation.In the perioperative period, anesthesiologists may have a brief and uniquewindow of opportunity to modulate the unwanted consequences of the stressresponse on the immune system and minimize residual disease. This reviewdiscusses the current research on analgesic therapies and their impact ondisease progression, followed by an evidence-based evaluation of perioperativepain interventions and medications.

Macrocyclic peptides decrease c-Myc protein levels and reduce prostate cancer cell growth

The oncoprotein c-Myc is often overexpressed in cancer cells, and the stability of this protein has major significance in deciding the fate of a cell. Thus, targeting c-Myc levels is an attractive approach for developing therapeutic agents for cancer treatment. In this study, we report the anti-cancer activity of the macrocyclic peptides [D-Trp]CJ-15,208 (cyclo[Phe-D-Pro-Phe-D-Trp]) and the natural product CJ-15,208 (cyclo[Phe-D-Pro-Phe-Trp]). [D-Trp]CJ-15,208 reduced c-Myc protein levels in prostate cancer cells and decreased cell proliferation with IC50 values ranging from 2.0 to 16 µM in multiple PC cell lines. [D-Trp]CJ-15,208 induced early and late apoptosis in PC-3 cells following 48 hours treatment, and growth arrest in the G2 cell cycle phase following both 24 and 48 hours treatment. Down regulation of c-Myc in PC-3 cells resulted in loss of sensitivity to [D-Trp]CJ-15,208 treatment, while overexpression of c-Myc in HEK-293 cells imparted sensitivity of these cells to [D-Trp]CJ-15,208 treatment. This macrocyclic tetrapeptide also regulated PP2A by reducing the levels of its phosphorylated form which regulates the stability of cellular c-Myc protein. Thus [D-Trp]CJ-15,208 represents a new lead compound for the potential development of an effective treatment of prostate cancer.

Morphine-induced apoptosis in the ventral tegmental area and hippocampus after the development but not extinction of reward-related behaviors in rats

Some data suggest that morphine induces apoptosis in neurons, while other evidences show that morphine could have protective effects against cell death. In this study, we suggested that there is a parallel role of morphine in reward circuitry and apoptosis processing. Therefore, we investigated the effect of morphine on modifications of apoptotic factors in the ventral tegmental area (VTA) and hippocampus (HPC) which are involved in the reward circuitry after the acquisition and extinction periods of conditioned place preference (CPP). In behavioral experiments, different doses of morphine (0.5, 5, and 10 mg/kg) and saline were examined in the CPP paradigm. Conditioning score and locomotor activity were recorded by Ethovision software after acquisition on the post-conditioning day, and days 4 and 8 of extinction periods. In order to investigate the molecular mechanisms in each group, we then dissected the brains and measured the expression of apoptotic factors in the VTA and HPC by western blotting analysis. All of the morphine-treated groups showed an increase of apoptotic factors in these regions during acquisition but not in extinction period. In the HPC, morphine significantly increased the ratio of Bax/Bcl-2, caspases-3, and PARP by the lowest dose (0.5 mg/kg), but, in the VTA, a considerable increase was seen in the dose of 5 mg/kg; promotion of apoptotic factors in the HPC and VTA insinuates that morphine can affect the molecular mechanisms that interfere with apoptosis through different receptors. Our findings suggest that a specific opioid receptor involves in modification of apoptotic factors expression in these areas. It seems that the reduction of cell death in response to high dose of morphine in the VTA and HPC may be due to activation of low affinity opioid receptors which are involved in neuroprotective features of morphine.

Effects of sufentanil on human gastric cancer cell line SGC-7901 in vitro

Sufentanil is a new kind of opioid analgesic and acts on μ opioid receptor. In this study, we aim to investigate the effects of sufentanil on gastric cancer cell line SGC-7901, after being exposed to different concentrations of sufentanil. Gastric cancer cells were exposed to sufentanil for a predetermined time at concentrations of 0, 0.5, 5, 50 and 500 nmol/l, respectively. Cell viability at different time points after exposure to sufentanil was tested by CCK-8 assay. FDA-PI staining was used to observe membrane integrity of gastric cancer SGC7901 cells. The apoptosis of gastric cancer cells was analyzed by Annexin V-FITC/PI Flow Cytometry and the changes of the cell cycle was determined by a detection kit. As a result, cell viability decreased in a dose- and time-dependent manner. Furthermore, with the concentration of sufentanil increased, the proportion of dead and apoptotic SGC-7901 cells increased, and more cells were arrested in G2/M phase. In a word, sufentanil can inhibit the cell viability and induce the apoptosis of gastric cancer SGC-7901 cells in vitro.

The role of morphine in regulation of cancer cell growth

Morphine is considered the "gold standard" for relieving pain and is currently one of the most effective drugs available clinically for the management of severe pain associated with cancer. In addition to its use in the treatment of pain, morphine appears to be important in the regulation of neoplastic tissue. Although morphine acts directly on the central nervous system to relieve pain, its activities on peripheral tissues are responsible for many of the secondary complications. Therefore, understanding the impact, other than pain control, of morphine on cancer treatment is extremely important. The effect of morphine on tumor growth is still contradictory, as both growth-promoting and growth-inhibiting effects have been observed. Accumulating evidence suggests that morphine can affect proliferation and migration of tumor cells as well as angiogenesis. Various signaling pathways have been suggested to be involved in these extra-analgesic effects of morphine. Suppression of immune system by morphine is an additional complication. This review provides an update on the influence of morphine on the growth and migration potential of tumor cells.

Chronic high-dose morphine treatment promotes SH-SY5Y cell apoptosis via c-Jun N-terminal kinase-mediated activation of mitochondria-dependent pathway

Chronic high doses of morphine inhibit the growth of various human cancer cell lines. However, the mechanisms by which such high-dose morphine inhibits cell proliferation and induces cell death are not fully understood. Here we show that c-Jun N-terminal kinase (JNK) plays a pivotal role in high-dose morphine-induced apoptosis of SH-SY5Y cells in a mitochondria-dependent manner. Activation of JNK by morphine led to reactive oxygen species (ROS) generation via the mitochondrial permeability transition pore, because the mPTP inhibitor cyclosporin A significantly inhibited ROS generation. ROS in turn exerted feedback regulation on JNK activation, as shown by the observations that cyclosporin A and the antioxidant N-acetylcysteine significantly inhibited the phosphorylation of JNK induced by morphine. ROS-amplified JNK induced cytochrome c release and caspase-9/3 activation through enhancement of expression of the proapoptotic protein Bim and reduction of expression of the antiapoptotic protein Bcl-2. All of these effects of morphine could be suppressed by the JNK inhibitor SP600125 and N-acetylcysteine. The key role of the JNK pathway in morphine-induced 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 proapoptotic effect of morphine. Thus, the present study clearly shows that morphine-induced apoptosis in SH-SY5Y cells involves JNK-dependent activation of the mitochondrial death pathway, and that ROS signaling exerts positive feedback regulation of JNK activity.

Stimulation of Na+/Cl--coupled opioid peptide transport system in SK-N-SH cells by L-kyotorphin, an endogenous substrate for H+-coupled peptide transporter PEPT2

We have recently identified a Na+/Cl--coupled transport system in mammalian cells for endogenous and synthetic opioid peptides. This transport system does not transport dipeptides/tripeptides, but is stimulated by these small peptides. Here we investigated the influence of L-kyotorphin (L-Tyr-L-Arg), an endogenous dipeptide with opioid activity, on this transport system. The activity of the transport system, measured in SK-N-SH cells (a human neuronal cell line) with deltorphin II as a model substrate, was stimulated approximately 2.5-fold by L-kyotorphin, with half-maximal stimulation occurring at approximately 100 microM. The stimulation was associated primarily with an increase in the affinity for deltorphin II. The stimulation caused by L-kyotorphin was stereospecific; L-Tyr-D-Arg (D-kyotorphin) had minimal effect. The influence of L-kyotorphin was observed also in a different cell line which expressed the opioid peptide transport system. While L-kyotorphin is a stimulator of opioid peptide transport, it is a transportable substrate for the H+-coupled peptide transporter PEPT2, which is expressed widely in the brain. Since the activity of the opioid peptide transport system is modulated by extracellular L-kyotorphin and since PEPT2 is an important determinant of extracellular L-kyotorphin in the brain, the expression/activity of PEPT2 may be a critical factor in the modulation of opioidergic neurotransmission in vivo.

Morphine inhibits doxorubicin-induced reactive oxygen species generation and nuclear factor kappaB transcriptional activation in neuroblastoma SH-SY5Y cells

Morphine is recommended as a first-line opioid analgesic in the pain management of cancer patients. Accumulating evidence shows that morphine has anti-apoptotic activity, but its impact on the therapeutic applications of antineoplastic drugs is not well known. The present study was undertaken to test the hypothesis that morphine might antagonize the pro-apoptotic activity of DOX (doxorubicin), a commonly used antitumour drug for the treatment of neuroblastoma, in cultured SH-SY5Y cells. In the present study we demonstrated that morphine suppressed DOX-induced inhibition of cell proliferation and programmed cell death in a concentration-dependent, and naloxone as well as pertussis toxin-irreversible, manner. Further studies showed that morphine inhibited ROS (reactive oxygen species) generation, and prevented DOX-mediated caspase-3 activation, cytochrome c release and changes of Bax and Bcl-2 protein expression. The antioxidant NAC (N-acetylcysteine) also showed the same effects as morphine on DOX-induced ROS generation, caspase-3 activation and cytochrome c release and changes in Bax (Bcl-2-associated X protein) and Bcl-2 protein expression. Additionally, morphine was found to suppress DOX-induced NF-kappaB (nuclear factor kappaB) transcriptional activation via a reduction of IkappaBalpha (inhibitor of nuclear factor kappaB) degradation. These present findings support the hypothesis that morphine can inhibit DOX-induced neuroblastoma cell apoptosis by the inhibition of ROS generation and mitochondrial cytochrome c release, as well as by blockade of NF-kappaB transcriptional activation, and suggests that morphine might have an impact on the antitumour efficiency of DOX.

Cancer biology and hormesis: human tumor cell lines commonly display hormetic (biphasic) dose responses

This article assesses the nature of the dose-response relationship of human tumor cell lines with a wide range of agents including antineoplastics, toxic substances (i.e., environmental pollutants), nonneoplastic drugs, endogenous agonists, and phyto-compounds. Hormetic-like biphasic dose responses were commonly reported and demonstrated in 136 tumor cell lines from over 30 tissue types for over 120 different agents. Quantitative features of these hormetic dose responses were similar, regardless of tumor cell line or agent tested. That is, the magnitude of the responses was generally modest, with maximum stimulatory responses typically not greater than twice the control, while the width of the stimulatory concentration range was usually less than 100-fold. Particular attention was directed to possible molecular mechanisms of the biphasic nature of the dose response, as well as clinical implications in which a low concentration of chemotherapeutic agent may stimulate tumor cell proliferation. Finally, these findings further support the conclusion that hormetic dose responses are broadly generalizable, being independent of biological model, endpoint measured, and stressor agent, and represent a basic feature of biological responsiveness to chemical and physical stressors.

Leumorphin has an anti-apoptotic effect by activating epidermal growth factor receptor kinase in rat pheochromocytoma PC12 cells

Endogenous opioid peptides, found in the central and peripheral nervous systems, perform neuromodulatory roles, and display a wide range of functional and pharmacological properties in vitro and in vivo. In this study, we investigated the effects of prodynorphin gene products on intracellular signaling events and cell survival in rat pheochromocytoma PC12 cells. Leumorphin, but not other prodynorphin gene products including dynorphin A, beta-neoendorphin and rimorphin (dynorphin B), increased cell viability in PC12 cells. The cytoprotective effect of leumorphin was dependent on the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, but was insensitive to both naloxone, a general antagonist of the opioid receptor, and nor-binaltorphimine, a specific antagonist of the kappa opioid receptor. Moreover, a competition-binding assay clearly revealed that leumorphin had another binding site(s) in addition to that for the kappa opioid receptor. Interestingly, leumorphin induced activation of the epidermal growth factor receptor via a Src-dependent mechanism, which was proved to be responsible for the increased survival response. Flow cytometric and microscopic analysis showed that leumorphin rescued cells from serum deprivation-induced apoptosis. Collectively, we suggest that leumorphin prevents apoptosis via epidermal growth factor receptor-mediated activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, which occur independent of the kappa opioid receptor.

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.

Kappa-opioid receptor binding varies inversely with tumor grade in human gliomas

BACKGROUND

Opioid agonists can inhibit cell proliferation in various neural tumor cell lines, including rat gliomas. Because opioid antimitogenic effects are mediated by opioid receptors, it was of interest to the authors to determine opioid receptor levels in human brain tumors.

METHODS

Specimens obtained at craniotomy from 30 patients with glioma and nonneoplastic brain disorders were evaluated for their kappa-opioid receptor binding. Kd and Bmax values were estimated from homologous competition binding curves with the kappa1-selective radioligand [3H]U69,593.

RESULTS

Receptor binding density was greatest in nonneoplastic brain tissue, less in Grade 2 and 3 astrocytoma, and least in glioblastoma multiforme.

CONCLUSIONS

These results suggest that opioid receptor-based stratification of grade may have clinical utility in distinguishing glioblastoma multiforme from lower grade astrocytomas, and thereby may facilitate diagnosis and treatment.

Characterization of the aminopeptidase activity of epidermal leukotriene A4 hydrolase against the opioid dynorphin fragment 1-7

Leukotriene A4 hydrolase is a bifunctional cytosolic enzyme, which both hydrolyses leukotriene A4 (LTA4) into leukotriene B4 (LTB4) and exerts aminopeptidase activity against opioid peptides. In the present study we have investigated whether the peptides angiotensin I and II, bradykinin, kallidine, histamine, dynorphin fragment 1-7 and substance P can act as substrates for epidermal and neutrophil LTA4 hydrolase. Among the tested substrates, dynorphin fragment 1-7 was found to be the best substrate for the enzyme. The aminopeptidase activity of epidermal and neutrophil LTA4 hydrolase against dynorphin fragment 1-7 was further characterized. The enzyme was purified from human epidermis and human neutrophils by anion exchange chromatography (Q-Sepharose) and affinity chromatography on a column with the LTA4 hydrolase inhibitor bestatin coupled to AH-Sepharose. The incubation of the dynorphin fragment 1-7 with LTA4 hydrolase resulted in the formation of tyrosine. The presence of the N-terminal amino acid tyrosine is essential for the interaction of opioids with their receptors, and this finding indicates that the LTA4 hydrolase can inactivate dynorphin fragment 1-7. After the two purification steps no other aminopeptidases acting at the N-terminal tyrosine of dynorphin fragment 1-7 was present in the preparation. This was demonstrated by the abolishment of the degradation at the N-terminal end of dynorphin fragment 1-7 when preincubating the enzyme preparation with LTA4 before the incubation with the dynorphin fragment 1-7. The abolishment of the aminopeptidase activity shows that activation of the hydrolase part of the enzyme, with conversion of LTA4 into the potent proinflammatory compound LTB4, results in an inhibition of the aminopeptidase activity of the enzyme. As a result, the catabolism of dynorphin fragment 1-7 and probably of other opioid peptides is inhibited, resulting in sustained biological effects of these opioids. This phenomenon may be important for the maintenance of inflammation in skin conditions, such as psoriasis and atopic dermatitis, in which LTB4 is formed.

Expression of opioid peptides in cells and stroma of human breast cancer and adenofibromas

The expression of beta-endorphin, Met-enkephalin and Leu-enkephalin was studied in 63 malignant and benign human breast tumors using immunohistochemical methods. Among invasive ductal carcinomas, 93% were positive for beta-endorphin, 87% for Leu-enkephalin and 90% for Met-enkephalin, in both the tumor stroma and the cell bodies. Enkephalin was predominant in cells, whereas endorphin was predominant in stroma. Nearly the same distribution was found in adenofibromas. In pericancerous normal tissue, neuropeptides were predominantly expressed in the stroma. Although the neuropeptide expression is not cancer-specific, it could be cancer-related, since the results suggest that the neuropeptide expression could reflect the host response to cancer cells and not only the cancer cell activity. The possibility of a direct action of the nervous system on stroma reaction and then on cancer cells is discussed.

Isolation of a growth and mitosis inhibitory peptide from mouse liver

The isolation of a liver peptide that inhibits the growth, mitosis rate and thymidine incorporation in regenerating liver is described. The peptide has the structure Pyroglu-gln-gly-ser-asn, and the deamidated forms are also active. The peptide probably belongs to a class of growth inhibitors with a high degree of tissue specificity. Two such peptides have previously been isolated from the epidermis (Reichelt et al. 1987) and from colonic tissue (Skraastad et al. 1987).

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Further studies on the biological characteristics of an endogenous colon mitosis inhibitor: comparison with some structurally related peptides

Previous work indicates that the colonic epithelial cell proliferation in mice is reversibly inhibited by the tripeptide pGlu-His-GlyOH found in aqueous extracts of the intestine. In the present study we examined the possible tissue specificity of the colon mitosis inhibitor. The mitotic rate in the small intestine, epidermis and forestomach in mice was registered after a single i.p. injection of the tripeptide. A significantly reduced rate of cell renewal was found at 18 h in the epidermis whereas no inhibition was observed in the forestomach or ileal epithelium. To investigate whether the amino acid sequence of the tripeptide is essential for the inhibitory effect, three structurally related bioactive peptides were tested and compared to the effect of CMI. CMI showed a bell-shaped dose-response relationship as previously shown, whereas the mitotic rate was not reduced in the colonic epithelium after treatment with either an epidermal mitosis inhibitory pentapeptide, or the dipeptide pGlu-GlyOH, or an analogue of luteinizing hormone-releasing hormone. The efficacy of the tripeptide was dependent on the basal rate of cell renewal in the colonic epithelium. When the tripeptide was given at the circadian nadir of cell proliferation a delayed reduction of proliferative activity was observed at 6 h after treatment, whereas treatment when the rate of cell proliferation was at its circadian zenith gave an immediate mitotic inhibition.