Six family genes--structure and function as transcription factors and their roles in development

The members of the Six gene family were identified as homologues of Drosophila sine oculis which is essential for compound-eye formation. The Six proteins are characterized by the Six domain and the Six-type homeodomain, both of which are essential for specific DNA binding and for cooperative interactions with Eya proteins. Mammals possess six Six genes which can be subdivided into three subclasses, and mutations of Six genes have been identified in human genetic disorders. Characterization of Six genes from various animal phyla revealed the antiquity of this gene family and roles of its members in several different developmental contexts. Some members retain conserved roles as components of the Pax-Six-Eya-Dach regulatory network, which may have been established in the common ancestor of all bilaterians as a toolbox controlling cell proliferation and cell movement during embryogenesis. Gene duplications and cis-regulatory changes may have provided a basis for diverse functions of Six genes in different animal lineages.     

Analysis of inhibitor of apoptosis protein family expression during mammary gland development

Background  

Inhibitors-of-Apoptosis-Proteins (IAPs) are an evolutionarily conserved family of proteins capable of regulating several facets of apoptosis. IAPs are frequently dysregulated in cancer, but their role in the regulation of apoptosis during developmental processes is not fully understood. Here we examined the expression of IAPs during the post-natal development of the mouse mammary gland, which is a tissue that exhibits a profound induction of apoptosis during involution.     

Dual roles for macrophages in ovarian cycle-associated development and remodelling of the mammary gland epithelium

Each ovarian cycle, the mammary gland epithelium rotates through a sequence of hormonally regulated cell proliferation, differentiation and apoptosis. These studies investigate the role of macrophages in this cellular turnover. Macrophage populations and their spatial distribution were found to fluctuate across the cycle. The number of macrophages was highest at diestrus, and the greatest number of macrophages in direct contact with epithelial cells occurred at proestrus. The physiological necessity of macrophages in mammary gland morphogenesis during the estrous cycle was demonstrated in Cd11b-Dtr transgenic mice. Ovariectomised mice were treated with estradiol and progesterone to stimulate alveolar development, and with the progesterone receptor antagonist mifepristone to induce regression of the newly formed alveolar buds. Macrophage depletion during alveolar development resulted in a reduction in both ductal epithelial cell proliferation and the number of alveolar buds. Macrophage depletion during alveolar regression resulted in an increased number of branch points and an accumulation of TUNEL-positive cells. These studies show that macrophages have two roles in the cellular turnover of epithelial cells in the cycling mammary gland; following ovulation, they promote the development of alveolar buds in preparation for possible pregnancy, and they remodel the tissue back to its basic architecture in preparation for a new estrous cycle.     

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.     

ErbB4 signaling in the mammary gland is required for lobuloalveolar development and Stat5 activation during lactation.

Signaling by members of the epidermal growth factor receptor family plays an important role in breast development and breast cancer. Earlier work suggested that one of these receptors, ErbB4, is coupled to unique responses in this tissue. To determine the function of ErbB4 signaling in the normal mouse mammary gland, we inactivated ErbB4 signaling by expressing a COOH terminally deleted dominant-negative allele of ErbB4 (ErbB4DeltaIC) as a transgene in the mammary gland. Despite the expression of ErbB4DeltaIC from puberty through later stages of mammary development, an ErbB4DeltaIC-specific phenotype was not observed until mid-lactation. At 12-d postpartum, lobuloalveoli expressing ErbB4DeltaIC protein were condensed and lacked normal lumenal lactation products. In these lobuloalveoli, beta-casein mRNA, detected by in situ hybridization, was normal. However, whey acidic protein mRNA was reduced, and alpha-lactalbumin mRNA was undetectable. Stat5 expression was detected by immunohistochemistry in ErbB4DeltaIC-expressing tissue. However, Stat5 was not phosphorylated at Y694 and was, therefore, probably inactive. When expressed transiently in 293T cells, ErbB4 induced phosphorylation of Stat5. This phosphorylation required an intact Stat5 SH2 domain. In summary, our results demonstrate that ErbB4 signaling is necessary for mammary terminal differentiation and Stat5 activation at mid-lactation.