EGF and TGF alpha modulate structural and functional differentiation of the mammary gland from pregnant mice in vitro: possible role of the arachidonic acid pathway

Influence of terminal differentiation and PACAP on the cytokine, chemokine, and growth factor secretion of mammary epithelial cells

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide with trophic and cytoprotective effects, has been shown to affect cell survival, proliferation, and also differentiation of various cell types. The high PACAP level in the milk and its changes during lactation suggest a possible effect of PACAP on the differentiation of mammary epithelial cells. Mammary cell differentiation is regulated by hormones, growth factors, cytokines/chemokines, and angiogenic proteins. In this study, differentiation was hormonally induced by lactogenic hormones in confluent cultures of HC11 mouse mammary epithelial cells. We investigated the effect of PACAP on mammary cell differentiation as well as release of cytokines, chemokines, and growth factors. Differentiation was assessed by expression analysis of the milk protein β-casein. Differentiation significantly decreased the secretion of interferon gammainduced protein (IP)-10, "regulated upon activation normal T cell expressed and presumably secreted" (RANTES), insulin-like growth factor-binding protein (IGFBP)-3 and the epidermal growth factor receptor (EGFR) ligands, such as epidermal growth factor (EGF) and amphiregulin (AREG). The changes in the levels of IP-10 and RANTES may be relevant for the alterations in homing of T cells and B cells at different stages of mammary gland development, while the changes of the EGFR ligands may facilitate the switch from proliferative to lactating stage. PACAP did not modulate the expression of β-casein or the activity of hormone-induced pathways as determined by the analysis of phosphorylation of Akt, STAT5, and p38 MAPK. However, PACAP decreased the release of EGF and AREG from non-differentiated cells. This may influence the extracellular signal-related transactivation of EGFR in the non-differentiated mammary epithelium and is considered to have an impact on the modulation of oncogenic EGFR signaling in breast cancer.

Epithelial cell cycling predicts p53 responsiveness to γ-irradiation during post-natal mammary gland development

The tumor suppressor gene, TP53, plays a major role in surveillance and repair of radiation-induced DNA damage. In multiple cell types, including mammary epithelial cells, abrogation of p53 (encoded by Trp53) function is associated with increased tumorigenesis. We examined γ-irradiated BALB/c-Trp53+/+ and -Trp53–/– female mice at five stages of post-natal mammary gland development to determine whether radiation-induced p53 activity is developmentally regulated. Our results show that p53-mediated responses are attenuated in glands from irradiated virgin and lactating mice, as measured by induction of p21/WAF1 (encoded by Cdkn1a) and apoptosis, while irradiated early- and mid-pregnancy glands exhibit robust p53 activity. There is a strong correlation between p53-mediated apoptosis and the degree of cellular proliferation, independent of the level of differentiation. In vivo, proliferation is intimately influenced by steroid hormones. To determine whether steroid hormones directly modulate p53 activity, whole organ cultures of mammary glands were induced to proliferate using estrogen plus progesterone or epidermal growth factor plus transforming growth factor-α and p53 responses to γ-irradiation were measured. Regardless of mitogens used, proliferating mammary epithelial cells show comparable p53 responses to γ-irradiation, including expression of nuclear p53 and p21/WAF1 and increased levels of apoptosis, compared to non-proliferating irradiated control cultures. Our study suggests that differences in radiation-induced p53 activity during post-natal mammary gland development are influenced by the proliferative state of the gland, and may be mediated indirectly by the mitogenic actions of steroid hormones in vivo.

Extracellular regulated kinase (ERK)-dependent regulation of sialomucin complex/rat Muc4 in mammary epithelial cells

Sialomucin complex (SMC, rat Muc4) is a membrane mucin implicated in the protection of epithelia and the metastasis of some tumors. It is a heterodimeric complex, containing a mucin subunit with anti-adhesive activity and a transmembrane subunit with epidermal growth factor-like domains, one of which acts as an intramembrane ligand for ErbB2. Serum, insulin and insulin-like growth factor, but not epidermal growth factor, induce the expression of sialomucin complex in mammary epithelial cells. Induction correlates with sustained, but not transient, activation of extracellular-regulated protein kinase (ERK). MEK inhibitor U0126 blocked the induction, while activated MEK-1 transfected into a rat mammary adenocarcinoma cell line induced a sustained activation of ERK and up-regulated SMC/Muc4 expression. Northern and Western blotting indicated that up-regulation occurred concomitantly at the transcript and protein levels, both of which could be blocked by U0126. These results suggest that expression of SMC/Muc4 in mammary epithelial cells is regulated by selected growth factors through an ERK-dependent pathway at the transcript level.

Mammary gland specific hEGF receptor transgene expression induces neoplasia and inhibits differentiation

The epidermal growth factor receptor (EGFR) is overexpressed in about 48% of human breast cancer tissues. To analyse the role of the EGFR in mammary tumor development we generated transgenic mice expressing the human EGFR under the control of either the MMTV-LTR (MHERc) or the beta-lactoglobulin promoter (BLGHERc). The BLGHERc-transgene was expressed exclusively in the female mammary gland, whereas the MHERc transgene was expressed more promiscuously in other organs, such as ovary, salivary gland and testis. Female virgin and lactating transgenic mice of both strains have impaired mammary gland development. Virgin EGFR transgenic mice developed mammary epithelial hyperplasias, whereas in lactating animals progression to dysplasias and tubular adenocarcinomas was observed. In both strains the number of dysplasias increased after multiple pregnancies. The transgene expression pattern was heterogeneous, but generally restricted to regions of impaired mammary gland development. Highest EGFR transgene expression was observed in adenocarcinomas. By using a whole mount organ culture system to study the differentiation potential of the mammary epithelium, we observed a reduced number of fully developed alveoli and a decrease in whey acidic protein expression. Taken together, EGFR overexpression results in a dramatic effect of impaired mammary gland development in vitro as well as in vivo, reducing the differentiation potential of the mammary epithelium and inducing epithelial cell transformation.

Cripto: a novel epidermal growth factor (EGF)-related peptide in mammary gland development and neoplasia

Growth and morphogenesis in the mammary gland depend on locally derived growth factors such as those in the epidermal growth factor (EGF) superfamily. Cripto-1 (CR-1, human; Cr-1, mouse)--also known as teratocarcinoma-derived growth factor-1--is a novel EGF-related protein that induces branching morphogenesis in mammary epithelial cells both in vitro and in vivo and inhibits the expression of various milk proteins. In the mouse, Cr-1 is expressed in the growing terminal end buds in the virgin mouse mammary gland and expression increases during pregnancy and lactation. Cr-1/CR-1 is overexpressed in mouse and human mammary tumors and inappropriate overexpression of Cr-1 in mouse mammary epithelial cells can lead to the clonal expansion of ductal hyperplasias. Taken together, this evidence suggests that Cr-1/CR-1 performs a role in normal mammary gland development and that it might contribute to the early stages of mouse mammary tumorigenesis and the pathobiology of human breast cancer.

Role of epidermal growth factor (EGF) in oesophageal mucosal integrity

Oesophageal mucosa has well established protective mechanisms, which operate within pre-epithelial, epithelial and post-epithelial compartments. Since refluxed acid and pepsin always act from the luminal side of the mucosa, protective factors like EGF, operating as a part of pre-epithelial defence, are thought to be pivotal in the maintenance of the integrity of the oesophageal mucosa. The significant contribution of salivary EGF to the quality of the oesophageal mucosal barrier has been demonstrated in an experimental setting and in a clinical scenario. Patients with low salivary EGF levels are predisposed to severe oesophageal damage if they develop gastro-oesophageal reflux and are a high-risk group for development of Barrett's oesophagus. Not only the salivary glands but also the human oesophagus has a profound ability to elaborate and release EGF. Some changes in luminal release of EGF during oesophageal mucosal exposure to intraluminal damaging factors imply its role in the oesophageal protective mechanisms. To exert biological effects within the oesophageal mucosal compartment, EGF requires binding to the ligand-binding domain of its receptor. This process results in receptor dimerisation, autophosphorylation and activation of intracellular signal transduction pathways. EGF receptors are localised on the basolateral and luminal aspect of the mucosal cells playing an important role in fast regeneration of oesophageal epithelium through the high mitotic activity of its proliferative zone. An increase in the rate of salivary EGF secretion during masticatory stimulation suggests its potential therapeutic benefit in the treatment of patients with damaged oesophageal mucosa.

The ErbB signaling network in embryogenesis and oncogenesis: signal diversification through combinatorial ligand-receptor interactions

Ligand-induced activation of receptor tyrosine kinases (RTK) results in the initiation of diverse cellular pathways, including proliferation, differentiation and cell migration. The ErbB family of RTKs represents a model for signal diversification through the formation of homo- and heterodimeric receptor complexes. Each dimeric receptor complex will initiate a distinct signaling pathway by recruiting a different set of Src homology 2- (SH2-) containing effector proteins. Further complexity is added due to the existence of an oncogenic receptor that enhances and stabilizes dimerization but has no ligand (ErbB-2), and a receptor that can recruit novel SH-2-containing proteins, but is itself devoid of kinase activity (ErbB-3). The resulting signaling network has important implications for embryonic development and malignant transformation.

Sequential requirement of hepatocyte growth factor and neuregulin in the morphogenesis and differentiation of the mammary gland

We have examined the role of two mesenchymal ligands of epithelial tyrosine kinase receptors in mouse mammary gland morphogenesis. In organ cultures of mammary glands, hepatocyte growth factor (HGF, scatter factor) promoted branching of the ductal trees but inhibited the production of secretory proteins. Neuregulin (NRG, neu differentiation factor) stimulated lobulo-alveolar budding and the production of milk proteins. These functional effects are paralleled by the expression of the two factors in vivo: HGF is produced in mesenchymal cells during ductal branching in the virgin animal; NRG is expressed in the mesenchyme during lobulo-alveolar development at pregnancy. The receptors of HGF and NRG (c-met, c-erbB3, and c-erbB4), which are expressed in the epithelial cells, are not regulated. In organ culture, branching morphogenesis and lobulo-alveolar differentiation of the mammary gland could be abolished by blocking expression of endogenous HGF and NRG by the respective antisense oligonucleotides; in antisense oligonucleotide-treated glands, morphogenesis could again be induced by the addition of recombinant HGF and NRG. We thus show that two major postnatal morphogenic periods of mammary gland development are dependent on sequential mesenchymal-epithelial interactions mediated by HGF and NRG.

Histochemical localization of heart-type fatty-acid binding protein in human and murine tissues

Cellular fatty acid-binding proteins (FABP) are a highly conserved family of proteins consisting of several subtypes, among them the mammary-derived growth inhibitor (MDGI) which is quite homologous to or even identical with the heart-type FABP (H-FABP). The FABPs and MDGI have been suggested to be involved in intracellular fatty acid metabolism and trafficking. Recently, evidence for growth and differentiation regulating properties of MDGI and H-FABP was provided. Using four affinity-purified polyclonal antibodies against bovine and human antigen preparations, the cellular localization of MDGI/H-FABP in human and mouse tissues and organs was studied. The antibodies were weakly cross-reactive with adipose tissue extracts known to lack H-FABP, but failed to react by Western blot analysis with liver-type FABP (L-FABP) and intestinal-type FABP (I-FABP). MDGI/H-FABP protein was mainly detected in myocardium, skeletal and smooth muscle fibres, lipid and/or steroid synthesising cells (adrenals, Leydig cells, sebaceous glands, lactating mammary gland) and terminally differentiated epithelia of the respiratory, intestinal and urogenital tracts. The results provide evidence that expression of H-FABP is associated with an irreversibly postmitotic and terminally differentiated status of cells. Since all the antisera employed showed spatially identical and qualitatively equal immunostaining, it is suggested that human, bovine and mouse MDGI/H-FABP proteins share highly homologous epitopes.