Novel treatment for triple-negative breast and ovarian cancer: endogenous opioid suppression of women's cancers

Abstract

Breast and ovarian cancers are responsible for more than 500,000 female deaths worldwide each year. Fifteen percent of the 230,000 women in the USA diagnosed with breast cancer in 2013 will have triple-negative breast cancer, a disease with few options for treatment, and a twofold greater mortality risk than other breast cancers. The OGF–OGF receptor pathway is present in these cancers, and regulates cell proliferation during homeostasis and disease. OGF is a tonically active peptide that inhibits DNA synthesis by upregulation of cyclin-dependent inhibitory kinases, without disrupting cell migration, differentiation or apoptosis. OGF receptor is a determinant in the proliferation of triple-negative breast cancer and ovarian cancer, and can be genetically modified to alter neoplastic cell replication in vitro and in nude mice. Preclinical studies warrant the use of OGF alone, or in combination, for treatment of triple-negative breast and ovarian cancer patients.

Keywords::

• cell proliferation
• enkephalins
• opioid growth factor
• ovarian cancer
• triple negative breast cancer

Women's cancers (i.e., breast and ovary) take the lives of more than 500,000 females worldwide each year. Over 200,000 women will be diagnosed with breast cancer in the USA, and more than 39,000 will die of the disease in 2013 [1]. Fifteen percent of these women are diagnosed with triple-negative breast cancer [2]. Breast cancer is successfully treated if the malignancy responds to hormonal treatments that target specific receptors such as the HER receptor-2, estrogen and/or progesterone receptors. However, if these receptors are not present, the cancer is deemed triple-negative breast cancer and the patient has few options for treatment as hormonal therapy is ineffective. Triple-negative breast cancer has a twofold greater mortality risk than other breast cancers, and comparable healthcare costs for these patients are increased 50% [3]. This cancer is particularly aggressive and often metastasizes to sites outside the breast.

Ovarian cancer is the 10th most common cancer among women [1]. More than 22,000 women in the USA will be diagnosed with ovarian cancer in 2013, and it will be the cause of death for 14,000 [1]. Ovarian cancer is considered the deadliest gynecologic cancer because of its fast onset and lack of predictive markers. Current therapies for ovarian cancer include surgery, radiation and/or chemotherapy, but this neoplasm often returns in a more aggressive form.

The financial burden is considerable for inpatient and outpatient care for these cancers. Thus, these neoplasms are major unresolved health issues for women worldwide. There is an unmet medical need to develop treatments that target the underlying biology of tumor growth. By understanding the cellular and molecular pathways involved with the progression of these cancers, targeted therapy can be devised.

OGF–OGF receptor axis & cancer progression

The biological pathway involving endogenous opioids and opioid receptors has garnered much attention over the last decade [4–7]. One opioid peptide, OGF, chemically termed [Met⁵]-enkephalin, interacts with a nuclear-associated OGF receptor (OGFr) to maintain cell proliferation during homeostasis of tissue and organ development, renewal and repair [4]. Disruption of this pathway by disease, genetic mutation or other imbalance, particularly in neoplastic cells and tissues, can exacerbate cancer progression [5]. The OGF–OGFr axis is a tonically active pathway that suppresses DNA synthesis in normal and abnormal cells, and thus cell proliferation, by upregulating cyclin-dependent inhibitory kinases such as p16 and/or p21. In many cases, cancers have mutated cyclin-dependent inhibitory kinases. For example, ovarian cancers appear to only have p21, and not p16, cyclin-dependent inhibitory kinase [6]. OGF is autocrine and paracrine produced, with primary sources being the brain and gastrointestinal tissues. OGF is a product of the prohormone preproenkephalin, and to a lesser extent, pro-opiomelanocortin. The peptide is transported throughout the body in the blood, but is rapidly degraded after secretion. However, production of OGF can be induced by manipulation of the OGF–OGFr axis. Short-term blockade of OGF–OGFr interactions stimulate production of OGF in ovarian cancer cells in culture [8], and in the plasma of nude mice [9].

OGF inhibition of MDA-MB-231 triple-negative breast cancer

Both the OGF peptide and OGFr receptor are present in the triple-negative breast cancer cell line MDA-MB-231, as well as a number of other breast cancer cell lines [7]. The OGF–OGFr axis is present and functional in normal tissues and a wide variety of neoplasia [4–10]. OGF inhibits proliferation in a dose-dependent, receptor-mediated and reversible manner. OGF repressed cell growth by almost 20% relative to controls. Exogenous OGF is the selective peptide to inhibit cell proliferation, and neutralization of endogenous OGF results in stimulated growth of breast cancer cells. The specificity of the receptor was investigated by molecular perturbation of OGFr using siRNA technology. Knockdown of OGFr results in accelerated proliferation of MDA-MB-231 cells, and addition of OGF does not repress DNA synthesis. However, knockdown of classical mu, delta or kappa opioid receptors had no effect on triple-negative breast cancer cell number, and the addition of OGF continued to inhibit cell proliferation. Thus, OGFr is the receptor that mediates the inhibitory action of OGF. In addition to exogenous treatment by OGF, intermittent opioid receptor blockade invoked by short-term exposure to 10⁻⁶ M naltrexone which stimulates production and release of endogenous OGF, also resulted in a 35% reduction in cell number after 72 h in comparison with control cultures [7].

Although the standard course of treatment for breast cancer is surgery, a number of chemotherapies, along with radiation, have successfully reduced tumor burden in breast cancers, including triple-negative breast cancer. Paclitaxel is a drug of choice for treatment regimens as its mode of action is to stabilize microtubules and prevent mitosis during the cell cycle [11]. Unfortunately, the side effects and anaphylaxis associated with the drug range from the tolerable (e.g., fever, chills, nausea) to the more serious (e.g., dyspnea and hypotension), with some fatal reactions noted if proper procedures and medical supervision are not adequate [12]. Preclinical studies with OGF and paclitaxel treatment of squamous cell carcinoma of the head and neck reveal that combination therapy provides cumulative inhibitory effects on both cells in culture [13] as well as tumors in nude mice [14]. Furthermore, the addition of OGF conferred a protective effect for mice receiving paclitaxel as body weights of mice receiving combination therapy were normal in mice with human head and neck tumors receiving paclitaxel alone [14]. In vitro studies with MDA-MB-231 cells treated with paclitaxel and/or OGF revealed that the addition of OGF to the dosage of paclitaxel utilized in this study did not decrease cell number in culture, but combination therapy did reduce by 60% the amount of cell death in cell cultures [7].

OGF inhibition of ovarian cancer

OGF and its receptor are present in a variety of human ovarian cancer cell lines [15], as well as in biopsy specimens from patients with benign ovarian cysts and malignant tumors [16]. The presence of the peptide and receptor hold promise for positive therapeutic interventions. In vitro analyses of growth following exposure to OGF reveal that peptide treatment reduced cell number in a dose-related, serum-independent, receptor-mediated and reversible manner. Decreases from controls of 51 and 36% were recorded in OVCAR-3 and SKOV-3 cell cultures, respectively, treated with OGF for 72 h. OGF is constitutively expressed and tonically active in a number of ovarian cancer cell lines as neutralization of endogenous peptide results in accelerated cell proliferation [15]. Molecular perturbation of the OGFr alters replication. Overexpression of the receptor enhances the inhibitory effectiveness of OGF, while a loss of OGFr results in lack of response to endogenous or exogenous administration of peptide [17]. The specificity of the OGF–OGFr axis for mediating the homeostatic balance of ovarian cancer cell progression was confirmed when transfected cell lines were xenografted into nude mice. Subcutaneous ovarian cancer tumors in mice have been successfully treated by OGF and combination therapy with cisplatin or paclitaxel.

Mechanisms of action for the OGF–OGFr axis

The mechanism of action for OGF is the inhibition of DNA synthesis which is mediated in the case of both ovarian and triple-negative breast cancer by upregulation of p21 cyclin-dependent inhibitory kinase activity and reductions in cyclin-dependent kinase 2 activity [7,15]. OGF has been shown in a number of human cancers to have no effect on apoptosis, cell migration, adhesion, invasion or differentiation [5], but to retard passage of cells in the G₁/S phase of the cell cycle and thus inhibit DNA synthesis. The number of functional OGF receptors in ovarian cancer has been shown to be significantly less than that reported for normal tissues, or even benign cysts [16]. Defects in OGFr such as post-translational modifications including phosphorylation and/or methylation may limit the number of useful targets for OGF interaction. Preclinical studies have shown that overexpression of OGFr can retard ovarian tumor appearance in nude mice and suppress the growth of the cancer [18].

Clinical studies

Clinical trials with OGF have not been conducted in patients with either ovarian or triple-negative breast cancer, but are warranted. However, in Phase I and Phase II studies using OGF as a treatment of patients who failed other therapies and had unresectable pancreatic cancer, OGF was found to be safe and non-toxic [19,20]. Some patients had reduced tumor growth and resolution of metastases. Analysis of ovarian tissue samples from patients with benign cysts or stage III/IV tumors showed that OGF and OGFr are present in human ovarian surface epithelial cells but depressed 29 and 34% respectively, in benign cysts, and decreased 58 and 48% respectively, in malignant ovarian cancer [16]. More than fivefold fewer OGFr receptors were quantitated in ovarian cancer than in benign cysts suggesting that deficits in the OGF–OGFr axis are amplified as disease progresses.

Conclusion

In conclusion, a novel biological pathway encompassing an endogenous opioid (OGF) and its receptor (OGFr) is present and functioning in both human ovarian and triple-negative breast cancers, although the OGF–OGFr axis may be defective as tumors become more malignant. Compelling studies have suggested that the biological pathway can be manipulated in a variety of ways to inhibit progression of the cancer cells. Exogenous OGF is an effective means of deterring cell proliferation, but changes in receptor expression such as overexpressing OGFr can also be a viable tool for regulating cell replication. Combination therapy with OGF and standard chemotherapies such as paclitaxel or cisplatin has added inhibitory effectiveness and/or protection against the toxicity of the chemotherapeutic agent. Finally, stimulation of the body's own endogenous opioid repository by using low dosages of naltrexone to block the OGF–OGFr interaction for short durations also proved effective for inhibition of tumor progression in the ovarian cancer model. Although the US FDA trials using OGF have not been initiated for ovarian or triple-negative breast cancer, OGF has been deemed non-toxic and safe for use in clinical studies of liver adenocarcinoma or pancreatic cancer [19,20]. Thus, studies to ascertain the efficacy of OGF as a biotherapy are warranted for these women's cancers. Collectively, these findings should provoke pharmaceutical agencies to underwrite clinical trials using endogenous molecules, and to encourage physicians to include biotherapies in their repertoire of care.

Financial & competing interests disclosure

Work described in this editorial was supported by the Paul K. and Anna E. Shockey Family Foundation. IS Zagon and PJ McLaughlin are inventors on intellectual property assigned to Penn State University and recently licensed to TNI Biotech Inc., Bethesda, MD, USA that is related to the use of OGF for treatment of gastrointestinal cancers. The authors are also inventors on a patent related to OGFr protein detection. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.