Breast Disease

Studies initially focused on understanding the hormonal regulation of milk protein gene expression have evolved using transgenic and knockout mouse models to help provide new insights into the mechanisms by which hormones regulate proliferation during normal mammary gland development, and how these regulatory mechanisms have deviated in breast cancer progression. During normal mammary development, a non-uniform pattern of expression of the estrogen receptor alpha, and the progesterone and the prolactin receptors in the ductal epithelium is established. All of these receptors apparently are co-expressed in non-proliferating ductal epithelial cells. Local growth factors, which are members of the Wnt, EGF and IGF families and Rank ligand, act as mediators of systemic steroid and lactogenic hormones to stimulate proliferation of adjacent ductal epithelial cells via a paracrine mechanism. During early breast cancer progression a more uniform pattern of steroid receptor expression is observed accompanied by an apparent switch to an autocrine mechanism regulating cell proliferation.

Since the gene encoding BRCA1 was first cloned in 1994, researchers have sought to establish the molecular basis for its linkage to breast and ovarian cancer. As universal functions for this protein have emerged, questions persist concerning how its disruption can elicit cancer in a tissue- and gender-specific manner. Here, we review evidence that a functional interrelationship between BRCA1 and estrogen signaling may be involved in breast tumorigenesis.

Breast cancers often have increased mitogen-activated protein kinase (MAPK) activity; this pathway influences breast cancer cell growth in part by targeting steroid hormone receptors. Activation of p42 and p44 MAPKs increases progesterone receptor (PR) transcriptional activity in the presence of progestins, and triggers their rapid down-regulation by the ubiquitin-proteasome pathway. In turn, progestins increase the expression of type I growth factor receptor tyrosine kinases that feed into MAPK activation. Most recently, progestins have been shown to activate the p42/p44 MAPK module in a progesterone receptor (PR) dependent manner, but independently of their function as transcription factors. Indeed, mechanisms of bi-directional cross-talk between these two pathways are becoming well-documented. In this reveiw we provide an overview of the primary ways in which steroid hormone receptor and growth factor cross-talk occurs, using examples from our work and others with human PR as a model receptor. We highlight the regulation of PR by phosphorylation and the role of intracellular protein kinases as key mediators of PR action. Cross-talk between growth factor and PR-mediated signaling events is an important means by which growth regulatory genes may be coordinately regulated, and may contribute to the growth and development of hormonally responsive normal breast tissue and to breast cancer progression.

Many studies have suggested an association between overexpression of receptor tyrosine kinases (RTKs) from the EGF receptor (or ErbB) family and breast cancer. The orphan RTK ErbB2/HER2/Neu is highly overexpressed in up to 30% of human breast cancer cases as a consequence of gene amplification. ErbB2/HER2/Neu can be activated by simple overexpression, and its signaling is thought to play an important role in initiation and progression of ErbB2-positive breast cancers. In support of this, ErbB2/HER2/Neu-targeted therapy (the Herceptin® antibody) has proven valuable in many of these cases. Other studies also correlate EGF receptor (EGFR) expression with poor prognosis in breast cancer, but follow-up studies suggest that this association is much less robust than with ErbB2, and requires more careful analysis. A particular problem is that EGFR, unlike ErbB2/HER2/Neu, cannot be activated simply by overexpression, its signaling remaining growth factor-dependent under these conditions. It is therefore critical to analyze EGFR signaling activity itself, rather than simply EGFR expression, in order to establish causal links and to identify patients for ErbB-targeted therapies. This distinction between ErbB2/HER2/Neu and EGFR is satisfyingly explained by recent crystallographic studies of ErbB receptor family members that are reviewed here. These structures also provide new insight into how ErbB2/HER2/Neu-targeted and EGFR-targeted therapeutic agents function, and suggest approaches for the development of novel mechanism-based ErbB inhibitors.

The IGF family, composed of ligands, receptors, binding proteins and proteases, is critical during development and in the maintenance of normal tissue homeostasis. This review describes the role of the IGF system in the normal breast but focuses on the changes that occur in breast cancer. It discusses the cross-talk which can occur between the IGF family with other signalling pathways, including estrogen, epidermal growth factor (EGF) and integrins. It also describes various cell and animal models which have been developed to elucidate the function and significance of the IGF family in cancer with a view to developing potential therapeutics.

TGF-β is now recognized as an important factor regulating normal mammary gland development as well as breast cancer. In development, TGF-β regulates branching morphogenesis and differentiation by acting on both epithelial and stromal cells. TGF-β also regulates apoptosis and matrix remodeling during involution at the end of the pregnancy cycle. TGF-β has biphasic effects on tumor progression, acting as a tumor suppressor in early stages of cancer and promoting invasion and metastasis at later stages. Furthermore, TGF-β may play a role in tumor progression through effects on the microenvironment. The tumor promoting effects of TGF-β may provide a therapeutic target for late stage breast cancer via TGF-βantagonists like the soluble receptors recently described. Future experiments will uncover the precise mechanisms of TGF-β action in development and neoplastic disease providing more opportunities for prevention and treatment of breast disease.

Prolactin (PRL), a hormone utilized at both the endocrine and autocrine levels, stimulates breast epithelial growth, differentiation, and motility. Recent data at the cellular, epidemiologic, and genetic levels have implicated a significant role for this hormone in the pathogenesis of human breast cancer. A family of prolactin receptor (PRLr) isoforms mediates the effects of PRL in human tissue. Now numbering six, these isoforms are co-variably expressed to differing degrees in normal versus malignant tissues. Following ligand binding, proximal PRLr signaling is initiated by three tyrosine kinases, namely Jak2, Src, and Tec. Activation of these kinases results in the triggering of multiple signaling networks, many of which are integrated by the Stat5 transcription factor. Both tyrosine and serine phosphorylation regulate Stat5 activity, as does the interaction of this transcription factor with co-activators and -repressors within the nucleus. Recently our lab has discovered that Stat5 activity is also regulated by its direct interaction with the retrotranslocated complex of PRL and the peptidyl prolyl isomerase cyclophilin B. This interaction results in the release of a repressor of Stat5 DNA-binding, the Peptide Inhibitor of Activated Stat 3 (PIAS3). Taken together, these data suggest that the summated genomic and non-genomic signaling actions of the PRL/PRLr complex serve to trigger an orchestrated pattern of gene expression that contributes to mammary development and the pathobiology of breast cancer.