Prolactin binds with its cell surface receptor through dimerization, activating various signaling pathways. Prolactin signals through a complicated web of kinases, including Janus kinase 2 (JAK2), Src kinase, phosphatidylinositol 3'-kinase (PI3K), protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). The best-characterized signaling pathway is the JAK2/STAT5 pathway, which has been shown to be critical for prolactin actions in mammary gland development. This pathway plays a central role in cell proliferation, differentiation and cell death. In addition, the ras/raf/MAPK pathway is also activated by prolactin and may be involved in the proliferative effects of the hormone (Freeman, Kanyicska et al. 2000; Gutzman, Miller et al. 2004).
There is substantial scientific evidence that human prolactin is associated with the growth of certain cancers as well as resistance to platinum and taxane therapies (Asai-Sato, Nagashima et al. 2005; Howell, Anderson et al. 2008; LaPensee, Schwemberger et al. 2009; Levina, Nolen et al. 2009). Our preclinical data indicate that the inhibition of the in vivo effects of prolactin on cancer cells (through antagonism with Prolanta™) may have a therapeutic effect on gynecological and other cancers.
Human Prolactin Production and Function. Human prolactin is a single-chain polypeptide of 200 amino acids with a molecular weight of approximately 23,000 Daltons. Prolactin is both a circulating hormone and a cytokine. It is primarily associated with the development and function of the reproductive system, but also may be involved in angiogenesis and regulation of the immune system (Goffin, Binart et al. 2002). Prolactin is mainly synthesized and secreted by lactotroph cells in the anterior pituitary gland, but is also produced at sites outside the pituitary gland, such as mammary gland, ovary, uterus, prostate, lymphocytes, brain, and several types of tumor cells (Sabharwal, Glaser et al. 1992; Freeman, Kanyicska et al. 2000; Levina, Nolen et al. 2009). This extra-pituitary prolactin can act as a tumor growth factor in an autocrine-paracrine fashion, both on the tumor cells that secrete prolactin (autocrine) as well as on nearby cells (paracrine) (Ben-Jonathan,
Liby et al. 2002).
Prolactin increases DNA synthesis (Peyrat, Djiane et al. 1984) and the expression of bcl-2 in breast cancer cells in vitro, and decreases the release of mitochondrial cytochrome C (Peirce and Chen 2004). Prolactin also activates the enzyme, glutathione-S-transferase, that detoxifies platinum drugs, doxorubicin, cyclophosphamide and etoposide, contributing to the resistance of cancer cells to chemotherapies (LaPensee, Schwemberger et al. 2009). Accordingly, the antagonism of prolactin by Prolanta may increase the effectiveness of these chemotherapies (Asai-Sato, Nagashima et al. 2005; Howell, Anderson et al. 2008; LaPensee, Schwemberger et al. 2009), and our own data indicates synergy with taxanes.
Dopamine agonists such as bromocriptine suppress pituitary prolactin production (Utian, Begg et al. 1975). Bromocriptine has been evaluated as an anti-cancer agents in a clinical trial but was not effective, potentially because this agonist has no effect on non-pituitary prolactin production (there are no dopamine receptors on cancer cells) (Bonneterre, Mauriac et al. 1988). Oncolix believes the prolactin receptor must be blocked through an antagonist like Prolanta™ in order to antagonize the activity of extra-pituitary prolactin.
Prior Evidence of Prolactin in Ovarian Cancers. Levina et al have shown that serum levels of prolactin are significantly elevated in women with a strong family history of ovarian cancer. Increased expression of the prolactin receptor has been observed in ovarian and endometrial tumors, signifying the importance of prolactin signaling in malignant conditions (Mor, Visintin et al. 2005; Levina, Nolen et al. 2009). Additionally, prolactin mRNA was detected in ovarian and endometrial tumors, indicating the presence of an autocrine loop. Prolactin potently induced proliferation in several ovarian and endometrial cancer cell lines (Asai-Sato, Nagashima et al. 2005). Prolactin also activated the ras oncogenic signaling in normal ovarian epithelial cells such that the malignantly transformed cells were able to form tumors in immunodeficient mice (Levina, Nolen et al. 2009).
Prior Evidence of Prolactin in Breast Cancers. Evidence supports the hypothesis that prolactin is also intimately involved in the pathogenesis of breast cancer (Clevenger, Chang et al. 1995; Vonderhaar 1998; Vonderhaar 1999; Llovera, Pichard et al. 2000; Llovera, Touraine et al. 2000). Prior studies have demonstrated that the inhibition of the binding of prolactin to the prolactin receptor inhibits breast cancer cell growth (Fuh and Wells 1995; Chen, Ramamoorthy et al. 1999; Chen, Holle et al. 2002).
Asai-Sato, M., Y. Nagashima, et al. (2005). "Prolactin inhibits apoptosis of ovarian carcinoma cells induced by serum starvation or cisplatin treatment." Int J Cancer 115(4): 539-44.
Bartholomeusz, C., D. Rosen, et al. (2008). "PEA-15 induces autophagy in human ovarian cancer cells and is associated with prolonged overall survival." Cancer Res 68(22): 9302-10.
Beck, M. T., S. K. Peirce, et al. (2002). "Regulation of bcl-2 gene expression in human breast cancer cells by prolactin and its antagonist, hPRL-G129R." Oncogene 21(33): 5047-55.
Ben-Jonathan, N., K. Liby, et al. (2002). "Prolactin as an autocrine/paracrine growth factor in human cancer." Trends Endocrinol Metab 13(6): 245-50.
Bernichtein, S., C. Kayser, et al. (2003). "Development of pure prolactin receptor antagonists." J Biol Chem 278(38): 35988-99.
Bonneterre, J., L. Mauriac, et al. (1988). "Tamoxifen plus bromocriptine versus tamoxifen plus placebo in advanced breast cancer: results of a double blind multicentre clinical trial." Eur J Cancer Clin Oncol 24(12): 1851-3.
Chen, N. Y., L. Holle, et al. (2002). "In vivo studies of the anti-tumor effects of a human prolactin antagonist, hPRL-G129R." Int J Oncol 20(4): 813-8.
Chen, W. Y., P. Ramamoorthy, et al. (1999). "A human prolactin antagonist, hPRL-G129R, inhibits breast cancer cell proliferation through induction of apoptosis." Clin Cancer Res 5(11): 3583-93.
Clevenger, C. V., W. P. Chang, et al. (1995). "Expression of prolactin and prolactin receptor in human breast carcinoma. Evidence for an autocrine/paracrine loop." Am J Pathol 146(3): 695-705.
Freedman, L. S. (1982). "Tables of the number of patients required in clinical trials using the logrank test." Stat Med 1(2): 121-9.
Freeman, M. E., B. Kanyicska, et al. (2000). "Prolactin: structure, function, and regulation of secretion." Physiol Rev 80(4): 1523-631.
Fuh, G. and J. A. Wells (1995). "Prolactin receptor antagonists that inhibit the growth of breast cancer cell lines." J Biol Chem 270(22): 13133-7.
Godwin, A. K., A. Meister, et al. (1992). "High resistance to cisplatin in human ovarian cancer cell lines is associated with marked increase of glutathione synthesis." Proc Natl Acad Sci U S A 89(7): 3070-4.
Goffin, V., N. Binart, et al. (2002). "Prolactin: the new biology of an old hormone." Annu Rev Physiol 64: 47-67.
Goffin, V., S. Kinet, et al. (1996). "Antagonistic properties of human prolactin analogs that show paradoxical agonistic activity in the Nb2 bioassay." J Biol Chem 271(28): 16573-9.
Gutzman, J. H., K. K. Miller, et al. (2004). "Endogenous human prolactin and not exogenous human prolactin induces estrogen receptor alpha and prolactin receptor expression and increases estrogen responsiveness in breast cancer cells." J Steroid Biochem Mol Biol 88(1): 69-77.
Howell, S. J., E. Anderson, et al. (2008). "Prolactin receptor antagonism reduces the clonogenic capacity of breast cancer cells and potentiates doxorubicin and paclitaxel cytotoxicity." Breast Cancer Res 10(4): R68.
Kinet, S., S. Bernichtein, et al. (1999). "Biological properties of human prolactin analogs depend not only on global hormone affinity, but also on the relative affinities of both receptor binding sites." J Biol Chem 274(37): 26033-43.
LaPensee, E. W., S. J. Schwemberger, et al. (2009). "Prolactin confers resistance against cisplatin in breast cancer cells by activating glutathione-S-transferase." Carcinogenesis 30(8): 1298-304.
Levina, V. V., B. Nolen, et al. (2009). "Biological significance of prolactin in gynecologic cancers." Cancer Res 69(12): 5226-33.
Levine, B., S. Sinha, et al. (2008). "Bcl-2 family members: dual regulators of apoptosis and autophagy." Autophagy 4(5): 600-6.
Llovera, M., C. Pichard, et al. (2000). "Human prolactin (hPRL) antagonists inhibit hPRL-activated signaling pathways involved in breast cancer cell proliferation." Oncogene 19(41): 4695-705.
Llovera, M., P. Touraine, et al. (2000). "Involvement of prolactin in breast cancer: redefining the molecular targets." Exp Gerontol 35(1): 41-51.
Lum, J. J., R. J. DeBerardinis, et al. (2005). "Autophagy in metazoans: cell survival in the land of plenty." Nat Rev Mol Cell Biol 6(6): 439-48.
Meng, J., C. H. Tsai-Morris, et al. (2004). "Human prolactin receptor variants in breast cancer: low ratio of short forms to the long-form human prolactin receptor associated with mammary carcinoma." Cancer Res 64(16): 5677-82.
Mor, G., I. Visintin, et al. (2005). "Serum protein markers for early detection of ovarian cancer." Proc Natl Acad Sci U S A 102(21): 7677-82.
NCCN (2012). NCCN Clinical Practice Guidelines in Oncology - Ovarian Cancer Version 2.2012, National Comprehensive Cancer Network.
Peirce, S. K. and W. Y. Chen (2004). "Human prolactin and its antagonist, hPRL-G129R, regulate bax and bcl-2 gene expression in human breast cancer cells and transgenic mice." Oncogene 23(6): 1248-55.
Peyrat, J. P., J. Djiane, et al. (1984). "Stimulation of DNA synthesis by prolactin in human breast tumor explants. Relation to prolactin receptors." Anticancer Res 4(4-5): 257-61.
Reuwer, A. Q., P. Nowak-Sliwinska, et al. "Functional consequences of prolactin signaling in endothelial cells: a potential link with angiogenesis in pathophysiology?" J Cell Mol Med.
Sabharwal, P., R. Glaser, et al. (1992). "Prolactin synthesized and secreted by human peripheral blood mononuclear cells: an autocrine growth factor for lymphoproliferation." Proc Natl Acad Sci U S A 89(16): 7713-6.
Tan, D., K. E. Chen, et al. "Prolactin increases survival and migration of ovarian cancer cells: importance of prolactin receptor type and therapeutic potential of S179D and G129R receptor antagonists." Cancer Lett 310(1): 101-8.
Utian, W. H., G. Begg, et al. (1975). "Effect of bromocriptine and chlorotrianisene on inhibition of lactation and serum prolactin. A comparative double-blind study." Br J Obstet Gynaecol 82(9): 755-9.
Vonderhaar, B. K. (1998). "Prolactin: the forgotten hormone of human breast cancer." Pharmacol Ther 79(2): 169-78.
Vonderhaar, B. K. (1999). "Prolactin involvement in breast cancer." Endocr Relat Cancer 6(3): 389-404.
White, E. (2008). "Autophagic cell death unraveled: Pharmacological inhibition of apoptosis and autophagy enables necrosis." Autophagy 4(4): 399-401.