Tumorigenesis and Neoplastic Progression Prolactin–Growth Factor Crosstalk Reduces Mammary Estrogen Responsiveness Despite Elevated ER Expression

Most breast cancers that occur in women express estrogen receptor (ER ). However, a large subset of these cancers either does not initially respond to antiestrogen therapy or develops resistance to such treatment modalities. One postulated mechanism of this failure is signaling cross talk between hormones and local growth factors. To examine these complex interactions in vivo , we assessed the effects of estrogen on transforming growth factor (TGF )and prolactin (PRL)-induced mammary tumorigenesis in transgenic mice. Both PRL and estrogen reduced the latency of TGF -induced oncogenesis, resulting in tumors that were variably ER -positive, but were progesterone receptor-negative. However, despite elevated ER levels in NRL-PRL/TGF glands, tumor latency was not reduced with increasing estrogen levels, nor increased after ovariectomy. Furthermore, PRL and TGF in combination blocked the mitogenic effects of estrogen, dramatically reduced progesterone receptor levels, and diminished ER down-regulation in response to circulating estrogen levels, in contrast to the other genotypes. Notably, however, ductal morphology remained responsive to estrogen, indicating that TGF and PRL in combination can inhibit some, but not all , estrogenic signals. Both in vitro and in vivo , PRL and TGF cooperatively enhanced Akt phosphorylation, which is associated with endocrine resistance in human disease. These findings provide insight into the interactions of PRL with growth factors during mammary oncogenesis and suggest combinatorial approaches that may result in improved therapeutic efficacy. (Am J Pathol 2009, 174:1065–1074; DOI: 10.2353/ajpath.2009.080719) Strong epidemiological and experimental evidence has substantiated the importance of estrogen in the etiology of breast cancer. Increased exposure raises the risk of this disease, and conversely, ovariectomy can be used to prevent cancer in women with familial susceptibility, and treat premenopausal disease. The proliferative effects of estrogen appear to be mediated through estrogen receptor (ER ), which is used both as a prognostic indicator and predictor of response to endocrine therapies. Although 50% to 70% of breast cancers express ER , a significant subset do not respond to anti-estrogen therapy, and 40% of those that initially respond eventually relapse despite the continued presence of ER . One postulated mechanism of this resistance is through enhanced cross talk among local growth factors. However, the contributions of potential participants are not well understood. Prolactin (PRL) plays a critical role in mammary alveologenesis and lactation. Although its role in carcinogenesis has been controversial, recent large prospective studies have linked circulating PRL levels to preand postmenopausal breast cancer. PRL itself is also synthesized within the mammary gland, permitting autocrine/paracrine action, and its production is higher in tumors than in normal tissue. PRL increases the responsiveness of multiple tissues, including the mammary gland, to ovarian hormones, and elevates ER levels in murine mammary epithelial cells. In breast cancer cells, overexpression of PRL increases ER expression, as well as estrogen responsiveness, and PRL and estrogen cooperatively sustain ERK1/2 phosphorylation and enhance activating protein-1 (AP-1) activity, which