SnoN regulation and function during tissue morphogenesis. Mammary gland development takes place in distinct steps during puberty, pregnancy and lactation, and is a complex process that involves lineage-specific differentiation of mammary progenitor populations into ductal and alveolar progenitors

INTRODUCTION SnoN (Ski novel protein; Skil – Mouse Genome Informatics) is a member of the Ski family of proteins that contains both prooncogenic and anti-oncogenic activities in human cancer (Deheuninck and Luo, 2009; Jahchan and Luo, 2010; Jahchan et al., 2010; Pan et al., 2009). It is a critical regulator of transforming growth factor- (TGF-) signaling (Deheuninck and Luo, 2009; Jahchan and Luo, 2010; Luo, 2004). TGF-, signaling through the Smad proteins, is a potent inhibitor of epithelial cell proliferation and acts to suppress tumor development at the early stages of carcinogenesis (Bierie and Moses, 2006; Chen and Wang, 2009; Heldin et al., 2009; Tian and Schiemann, 2009). SnoN interacts with Smad2, Smad3 and Smad4 in both the cytoplasm and nucleus to repress their ability to activate TGF- target genes, thereby blocking the cytostatic response of TGF- (Deheuninck and Luo, 2009; Jahchan and Luo, 2010; Krakowski et al., 2005). This ability of SnoN to antagonize the tumor suppressor activity of TGF- is likely to be responsible for its pro-oncogenic activity. In addition, SnoN is also a potent activator of p53 (Trp53 – Mouse Genome Informatics). SnoN expression is significantly upregulated in response to cellular stress, and this high level of SnoN can induce stabilization and activation of p53, leading to increased senescence and apoptosis (Pan et al., 2009). This ability might account for the anti-tumorigenic activity of SnoN (Deheuninck and Luo, 2009; Jahchan and Luo, 2010; Pan et al., 2009). Most previous studies have focused on the function of SnoN in cancer cells, but little is known about its physiological function in normal adult epithelial cells. SnoN is found to be ubiquitously expressed in all adult tissues at a low level (Nomura et al., 1999; Pearson-White and Crittenden, 1997). SnoN expression can be induced by TGF-, upon tissue injury and in response to a variety of cellular stress signals (Deheuninck and Luo, 2009; Jahchan and Luo, 2010), suggesting that it might play a role in coordinating various cellular processes in response to environmental cues. In our studies, we employed the mammary gland as a model system to determine SnoN regulation and function during tissue morphogenesis. Mammary gland development takes place in distinct steps during puberty, pregnancy and lactation, and is a complex process that involves lineage-specific differentiation of mammary progenitor populations into ductal and alveolar progenitors (Bruno and Smith, 2011). We have shown that SnoN is detected in the luminal epithelial cells lining the ducts and epithelial cells of lobuli and terminal ducts, and its expression is low in the virgin gland, but is sharply upregulated at late pregnancy, peaking by around day 18.5 of pregnancy and lasting until day 1-2 of lactation, before dropping dramatically afterwards (Jahchan et al., 2010). Incidentally, this peak SnoN expression correlates with the profound structural and functional changes in the mammary alveoli (Neville et al., 2002; Oakes et al., 2006) that involve significant expansion of the alveolar luminal space and secretory activation, allowing milk production to take place (Anderson et al., 2007; Richert et al., 2000). A successful secretory activation also requires the closure of tight junctions (GonzálezMariscal et al., 2008; Itoh and Bissell, 2003; Nguyen and Neville, 1998; Nguyen et al., 2001), which is necessary to establish epithelial cell contacts and maintain cell polarity (Feigin and Muthuswamy, 2009; Inman and Bissell, 2010; Knust, 2000; Vasioukhin and Fuchs, 2001), permitting vectorial secretion. The expression pattern of SnoN suggests that it might play an important role in the differentiation of the secretory alveolar epithelial cells, and that pregnancy hormones could regulate its expression. Mammary alveologenesis and lactogenesis are controlled by the coordinated actions of several cytokines, in particular TGF- and prolactin. Prolactin expression increases during pregnancy and remains elevated during lactation (Freeman et al., 2000; Oakes et al., 2008), when it plays a key role in promoting lactogenesis and milk secretion (Brisken et al., 1999; Goffin et al., 1999; Kelly et 1Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA. 2Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.