Prolactin Drives Estrogen Receptor-α-Dependent Ductal Expansion and Synergizes with Transforming Growth Factor-α to Induce Mammary Tumors in Males

Abstract

Male breast cancer is rare and has been the focus of limited research. Although the etiology is unclear, conditions increasing circulating prolactin (PRL), as well as estrogen, increase the risk of tumorigenesis.We modeled exposure to elevated PRL in transgenic mice, using the mammary-selective, estrogen-insensitive promoter neu-related lipocalin (NRL), to drive PRL expression. Male NRL-PRL mice did not develop mammary tumors. However, in cooperation with the well-characterized oncogene transforming growth factor-␣ (TGF-␣), PRL induced mammary tumors in 100% of male bitransgenic mice. Similar to disease in human males, these tumors expressed variable levels of estrogen receptor-␣ (ER-␣) and androgen receptors. However, carcinogenesis was not responsive to testicular steroids because castration did not alter latency to tumor development or tumor ER-␣ expression. Interestingly, both NRL-TGF-␣/PRL and NRL-PRL males demonstrated increased ductal development, which occurred during puberty, similar to female mice. This outgrowth was diminished in NRL-PRL males treated with ICI 182,780, suggesting that PRL enhances ER-mediated growth. Treatment of MCF-7-derived cells with PRL increased phosphorylation of ER-␣ at residues implicated in unliganded ER-␣ activity. Together, these studies suggest that PRL expands the pool of cells susceptible to tumorigenesis, which is then facilitated by PRL and TGF-␣ cross talk. Activation of ER-␣ is one mechanism by which PRL may contribute to breast cancer and points to other therapeutic strategies for male patients. An estimated 1690 new cases of male breast cancer will be diagnosed in the United States, and 460 men will die as a result of the disease this year. 1 Because it is a rare disease, male breast cancer has been the focus of very limited research. Unlike breast cancer in women, the incidence in men is rising 1.1% annually, and men are more likely to have advanced disease and poorer survival compared to women. 2 Conditions that elevate the ratio of circulating estrogens and androgens and increase the risk for this disease include Klinefelter's syndrome, liver cirrhosis, estrogen therapy for prostate cancer, obesity, and testicular abnormalities. 3,4 Similar to the postmenopausal disease in women, 85 to 91% of male breast cancers are estrogen receptor-␣ (ER-␣)-positive and respond to tamoxifen therapy, although the survival advantage for ER-␣-positive tumors is not as evident in males as females. 5 Androgen receptor (AR) is also expressed in a significant number (39 to 80%) of tumors. 6,7 Hyperprolactinemia is also a significant risk factor. 3,8 At least one-third of a consecutive series of male breast cancer patients had elevated serum levels of prolactin (PRL), which also correlated with the size of the primary tumor. 9 Drug treatments associated with PRL elevation and prolactinomas are also significantly linked to increased breast cancer risk. 10 In a retrospective study, one in three men with breast cancer had detectable PRL receptor in their tumors. 11 To study the effect o...

Complex prolactin crosstalk in breast cancer: New therapeutic implications

“…In MCF-7 cells, PRL and estrogen augment AP-1 activity through increased phosphorylation of p38, ERK1/2, and c-Fos (Gutzman et al, 2005). Additionally, PRL induces phosphorylation of ERα at S118 and S167, likely through MAPK and PI3K/AKT pathways (Glaros et al, 2006;Arendt and Schuler, 2008b). Phosphorylation of multiple serine residues can enhance ERα transcriptional activity even in the absence of estrogen, which may be further augmented by phosphorylation of coactivators by these same kinases (Likhite et al, 2006; Weigel and Moore, 2007; Gonzalez et al, 2009).…”

Modeling Prolactin Actions in Breast Cancer In Vivo: Insights from the NRL-PRL Mouse

“…Indeed, PRL in this model cooperates with all of the oncogenes that we have examined (TGFα, absence of p53, estrogen, as well as elevated β-catenin ( APC min+/- ) ([56,72,76,101,108], O'Leary and Schuler, ms in prep).…”

“…Although PRL initiates phosphorylation of ERα in the AF-1 domain (Ser118, Ser167) [101] and induces recruitment of ERα to target genes [102] in breast cancer cells, the ability of this mechanism to exert physiologically significant effects in vivo has not been clear. In order to examine this directly, we utilized the ERα(G525L) mouse, which expresses a mutant ERα that is refractory to 17β-estradiol, but can be activated by kinases downstream of growth factors and cytokines [103].…”

“…Notably, the potent crosstalk of local transgenic PRL and TGFα also resulted in mammary tumors in male mice, which did not occur in the single transgenic animals [101]. Although breast cancer is relatively rare in men compared to women, the incidence is rising and the disease is less responsive to treatment [116-118].…”

Alternatives to animal experimentation for hormonal compounds research

“…(f) The systems used for the in vitro and ex vivo analysis of NR-ICs (mainly estrogens and androgens) are generally composed of cells derived from reproductive tissues. Recent knowledge of the widespread distribution of NRs, in particular steroid receptors, in all tissues of an organism and their involvement in several diseases [2,18,30,50,55] make the available systems inadequate for assessing the effects of NR-ICs on the whole physiology. Moreover, the tissue levels of several NRs change with age (i.e.…”