The expression and function of the OGF-OGFr axis - a tonically active negative regulator of growth - in COS cells

The Opioid Growth Factor in Growth Regulation and Immune Responses in Cancer

It has become apparent that endogenous opioids act not only as neurotransmitters and neuromodulators, but have multiple functions in the body. Activation of the opioid system by opiate drugs is associated with a risk of cancer development through direct stimulation of tumor cell proliferation and through immunosuppression. In contrast, the endogenous peptide opioid [Met5]-enkephalin, now commonly referred to as Opioid Growth Factor (OGF), negatively regulates cell proliferation in a wide number of cells during development, homeostasis, and neoplasia. This action is mediated through the opioid growth factor receptor, originally designated the zeta (ζ) opioid receptor. Further, contrary to the traditional notion of opiates as immunosuppressive, endogenous OGF has been shown to possess a number of positive immunomodulatory properties and may provide a beneficial effect in cancer by augmenting the activity of cells involved in both innate and acquired immunity. Taken together, the evidence supports consideration of opioid peptides such as OGF as new strategies for cancer therapy.

Opioid growth factor receptor: Anatomical distribution and receptor colocalization in neurons of the adult mouse brain

The opioid growth factor (OGF) is an endogenous peptide that binds to the nuclear-associated receptor (OGFr), and plays a significant role in the proliferation of developing, renewing, and healing tissues. The receptor is widely expressed in a variety of organs, however its distribution in the brain remains unknown. In this study, we investigated the distribution of OGFr in different brain regions of male heterozygous (-/+ Lepr ^db/J), non -diabetic mice and determined the localization of the receptor in three major brain cell types, astrocytes, microglia, and neurons. Immunofluorescence imaging revealed that the highest number of OGFr was in hippocampal CA3 subregion followed by primary motor cortex, hippocampal CA2, thalamus, caudate and hypothalamus in a descending order. Double immunostaining revealed receptor colocalization with neurons and little or no colocalization in microglia and astrocytes. The highest percentage of OGFr positive neurons was identified in the CA3. Hippocampal CA3 neurons play an important role in memory processing, learning and behavior, and motor cortex neurons are important for muscle movement. However, the significance of the OGFr receptor in these brain regions and its relevance in diseased conditions are not known. Our findings provide a basis for understanding the cellular target and interaction of the OGF- OGFr pathway in neurodegenerative diseases such as Alzheimer's, Parkinson's, and stroke where hippocampus and cortex have an important role. This foundational data may also be useful in drug discovery to modulate OGFr by opioid receptor antagonist in various CNS diseases.

Research progress of opioid growth factor in immune-related diseases and cancer diseases

Methionine enkephalin (MENK) has an important role in both neuroendocrine and immune systems. MENK was known as an opioid growth factor (OGF) for its growth regulatory characteristics. OGF interacts with the OGF receptor (OGFr) to inhibit DNA synthesis by upregulating p16 and/or p21, which delays the cell cycle transition from G0/G1 to S phase, and inhibits cell proliferation. In addition, OGF combines with OGFr in immune cells to exert its immunomodulatory activity and regulate immune function. OGF has been studied as an immunomodulator in a variety of autoimmune diseases, including multiple sclerosis, inflammatory bowel disease, diabetes and viral infections, and has been proven to relieve symptoms of certain diseases in animal and in vitro experiments. Also, OGF and OGFr have various anti-tumor molecular mechanisms. OGF can be used as the primary therapy alone or combined with other drugs to treat tumors. This article summarizes the research progress of OGF in immune-related diseases and cancer diseases.

Long noncoding RNA LINC00673 promotes the proliferation and metastasis of epithelial ovarian cancer by associating with opioid growth factor receptor

Purpose

The long noncoding RNA LINC00673 has emerged as an important regulator of cancer development and progression. However, the clinical significance and biological roles of LINC00673 in epithelial ovarian cancer (EOC) remain unclear. In this study, we aimed to explore the oncogenic roles and underlying molecular mechanisms of LINC00673 in EOC.

Patients and methods

The expression levels of LINC00673 in EOC tissues and cell lines were detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Real-time cellular analysis (RTCA), flow cytometry, and transwell assays were conducted to investigate cell proliferation, apoptosis, migration and invasion in vitro. Subcutaneous transplanted tumors were established to explore the oncogenic role of LINC00673 in vivo. Differentially expressed genes were analyzed using transcriptome sequencing. Protein levels were determined by Western blot assays.

Results

LINC00673 was upregulated in EOC tissues and cell lines compared to their corresponding normal controls. High expression of LINC00673 was associated with advanced International Federation of Gynecology and Obstetrics (FIGO) stage, serous histological subtype, lymph node metastasis and poor prognosis in patients with EOC. LINC00673 was also identified as an independent prognostic factor for EOC. In addition, LINC00673 promoted cell migration, invasion and proliferation and inhibited cell apoptosis in vitro and induced tumor growth in vivo. Mechanistically, opioid growth factor receptor (OGFR) was found to be a potential downstream target gene that mediated the oncogenic effect of LINC00673 in EOC.

Conclusion

LINC00673 contributes to EOC proliferation and metastasis and may be a promising prognostic biomarker for EOC patients.

Selective opioid growth factor receptor antagonists based on a stilbene isostere

As part of an ongoing drug development effort aimed at selective opioid receptor ligands based on the pawhuskin natural products we have synthesized a small set of amide isosteres. These amides were centered on lead compounds which are selective antagonists for the delta and kappa opioid receptors. The amide isomers revealed here show dramatically different activity from the parent stilbene compounds. Three of the isomers synthesized showed antagonist activity for the opioid growth factor (OGF)/opioid growth factor receptor (OGFR) axis which is involved in cellular and organ growth control. This cellular signaling mechanism is targeted by "low-dose" naltrexone therapy which is being tested clinically for multiple sclerosis, Crohn's disease, cancer, and wound healing disorders. The compounds described here are the first selective small molecule ligands for the OGF/OGFR system and will serve as important leads and probes for further study.

Low-dose naltrexone targets the opioid growth factor-opioid growth factor receptor pathway to inhibit cell proliferation: mechanistic evidence from a tissue culture model

Naltrexone (NTX) is an opioid antagonist that inhibits or accelerates cell proliferation in vivo when utilized in a low (LDN) or high (HDN) dose, respectively. The mechanism of opioid antagonist action on growth is not well understood. We established a tissue culture model of LDN and HDN using short-term and continuous opioid receptor blockade, respectively, in human ovarian cancer cells, and found that the duration of opioid receptor blockade determines cell proliferative response. The alteration of growth by NTX also was detected in cells representative of pancreatic, colorectal and squamous cell carcinomas. The opioid growth factor (OGF; [Met(5)]-enkephalin) and its receptor (OGFr) were responsible for mediating the action of NTX on cell proliferation. NTX upregulated OGF and OGFr at the translational but not at the transcriptional level. The mechanism of inhibition by short-term NTX required p16 and/or p21 cyclin-dependent inhibitory kinases, but was not dependent on cell survival (necrosis, apoptosis). Sequential administration of short-term NTX and OGF had a greater inhibitory effect on cell proliferation than either agent alone. Given the parallels between short-term NTX in vitro and LDN in vivo, we now demonstrate at the molecular level that the OGF-OGFr axis is a common pathway that is essential for the regulation of cell proliferation by NTX.

Internalization of the opioid growth factor, [Met5]-enkephalin, is dependent on clathrin-mediated endocytosis for downregulation of cell proliferation

The opioid growth factor (OGF; [Met(5)]-enkephalin), a constitutively expressed and tonically active inhibitory peptide, interacts with the OGF receptor (OGFr) to form an endogenous growth-regulating pathway in homeostasis. Amplification of OGF-OGFr interfacing in animal and clinical studies depresses development, neoplasia, angiogenesis, and immunity. Disruption of the OGF-OGFr axis accelerates cell proliferation and has been particularly important in wound repair. To investigate how OGF enters cells, OGF was labeled with 5,6-tetramethylrhodamine OGF (RhoOGF) to study its uptake in live cells. African green monkey kidney cells (COS-7) incubated with RhoOGF exhibited a temperature-dependent course of entry, being internalized at 37 degrees C but not at 4 degrees C. RhoOGF was detected in the cytoplasm 15 min after initial exposure, observed in both cytoplasm and nucleus within 30 min, and remained in the cells for as long as 5 h. A 100-fold excess of OGF or the opioid antagonist naltrexone, but not other opioid ligands (some selective for classic opioid receptors), markedly reduced entry of RhoOGF into cells. RhoOGF was functional because DNA synthesis in cells incubated with RhoOGF (10(-5) to 10(-8) M) was decreased 24-36%, and was comparable to cells treated with unlabeled OGF (reductions of 26-39%). OGF internalization was dependent on clathrin-mediated endocytosis, with addition of clathrin siRNA diminishing the uptake of RhoOGF and upregulating DNA synthesis. RhoOGF clathrin-mediated endocytosis was unrelated to endosomal or Golgi pathways. Taken together, these results suggest that OGF enters cells by active transport in a saturable manner that requires clathrin-mediated endocytosis.

The opioid growth factor-opioid growth factor receptor axis regulates cell proliferation of human hepatocellular cancer

Hepatocellular carcinoma (HCC) is the third leading cause of cancer deaths worldwide, with a mortality rate approximating its incidence. Understanding the biology of these tumors, as well as treatment modalities, has been challenging. The opioid growth factor (OGF; [Met(5)]-enkephalin) and the OGF receptor (OGFr) form an endogenous growth-regulating pathway in homeostasis and neoplasia. In this investigation, we examined the relationship of the OGF-OGFr axis in HCC and define its presence, function, and mechanism. Using SK-HEP-1, Hep G2, and Hep 3B human HCC cell lines, we found that OGF and OGFr were present and functional. Exogenous OGF was observed to have a dose-dependent, reversible, and receptor-mediated inhibitory action on cell proliferation. Endogenous OGF was found to be constitutively produced and tonically active on cell replicative activities, with neutralization of this peptide accelerating cell proliferation. Silencing of OGFr using siRNA stimulated cell replication, even when exogenous OGF was added to the cultures, documenting its importance in mediating OGF activity. The mechanism of OGF-OGFr action on cell number was related to inhibition of DNA synthesis and not to apoptotic or necrotic pathways. Both OGF and OGFr were detected in surgical specimens of HCC, and no quantitative differences were recorded in peptide or receptor between pathological and normal specimens. These data are the first to report that the OGF-OGFr system is a native biological regulator of cell proliferation in HCC. The findings may provide important insight in designing treatment strategies for this deadly disease.

Passive diffusion of naltrexone into human and animal cells and upregulation of cell proliferation

Naltrexone (NTX) is a potent opioid antagonist that promotes cell proliferation by upregulating DNA synthesis through displacement of the tonically active inhibitory peptide, opioid growth factor (OGF) from its receptor (OGFr). To investigate how NTX enters cells, NTX was fluorescently labeled [1-( N)-fluoresceinyl NTX thiosemicarbazone; FNTX] to study its uptake by living cultured cells. When human head and neck squamous cell carcinoma cell line (SCC-1) was incubated with FNTX for as little as 1 min, cells displayed nuclear and cytoplasmic staining of FNTX as determined by fluorescent deconvolution microscopy, with enrichment of fluorescent signal in the nucleus and nucleolus. The same temporal-spatial distribution of FNTX was detected in a human pancreatic cancer cell line (MIA PaCa-2), African green monkey kidney cell line (COS-7), and human mesenchymal stem cells (hMSCs). FNTX remained in cells for as long as 48 h. FNTX was internalized in SCC-1 cells when incubation occurred at 4°C, with the signal being comparable to that recorded at 37°C. A 100-fold excess of NTX or a variety of other opioid ligands did not alter the temporal-spatial distribution of FNTX. Neither fluorescein-labeled dextran nor fluorescein alone entered the cells. To study the effect of FNTX on DNA synthesis, cells incubated with FNTX at concentrations ranging from 10−5 to 10−8 M had a 5-bromo-2′-deoxyuridine index that was 39–82% greater than for vehicle-treated cells and was comparable to that of unlabeled NTX (37–70%). Taken together, these results suggested that NTX enters cells by passive diffusion in a nonsaturable manner.

Opioid growth factor-opioid growth factor receptor axis is a physiological determinant of cell proliferation in diverse human cancers

The opioid growth factor (OGF) regulates cell proliferation of human cancer cells through the cyclin-dependent kinase inhibitory pathway, with mediation of this action by the OGF receptor (OGFr). The ubiquity of the OGF-OGFr axis in human cancer is unknown. We used 31 human cancer cell lines, representative of more than 90% of neoplasias occurring in humans, and found that OGF and OGFr were detected in the cytoplasm and nucleus by immunohistochemistry. The addition of OGF to cultures depressed cell number up to 41%, whereas naltrexone (NTX) increased cell proliferation by up to 44%, a total of 85% in the modulating capacity for the OGF-OGFr axis. Neutralization of OGF by specific antibodies led to a marked increase in cell number. Knockdown of OGFr by OGFr-siRNA resulted in a significant increase in the number of cells, even in the face of the addition of exogenous OGF. The cultures to which NTX was added and subjected to OGFr-siRNA were similar to those with OGF-siRNA alone. The OGF-OGFr axis, a physiological determinant of cell-proliferative activity, is a ubiquitous feature of human cancer cells. The identification of this native biological system in neoplasia may be important in understanding the pathophysiology of neoplasia, and in designing treatment modalities that utilize the body's own chemistry.

Cell proliferation of human ovarian cancer is regulated by the opioid growth factor-opioid growth factor receptor axis

Ovarian cancer is the leading cause of death from gynecological malignancies. Understanding the biology of these tumors, as well as treatment modalities, has been challenging. The opioid growth factor (OGF; [Met(5)]-enkephalin) and the OGF receptor (OGFr) form an endogenous growth-regulating pathway in homeostasis and neoplasia. In this investigation, we examined the relationship of the OGF-OGFr axis to ovarian cancer, and defined its presence, function, and mechanisms. Using OVCAR-3 and SKOV-3 ovarian cancer cell lines, we found that OGF and OGFr were present and functional. Exogenous OGF was observed to have a dose-dependent, serum-independent, reversible, and receptor-mediated inhibitory action on cell proliferation that was dependent on RNA and protein synthesis. The repressive effect of OGF on cell proliferation also was observed in SW626, CAOV-3, and HEY ovarian cancer cell lines. Endogenous OGF was found to be constitutively produced and tonically active on cell replicative activities, with neutralization of this peptide accelerating cell proliferation. Silencing of OGFr using siRNA technology stimulated cell replication, documenting its integral role. The mechanism of OGF-OGFr action on DNA synthesis was related to the cyclin-dependent kinase inhibitory pathway because knockdown of p16 or p21 in OVCAR-3 cells, and p21 in SKOV-3 cells, eliminated OGF's inhibitory effect on growth. These data are the first to report that the OGF-OGFr system is a native biological regulator of cell proliferation in human ovarian cancer. This information will be important in designing treatment strategies for this deadly disease.