The au-rich mRNA binding protein tristetraprolin (TTP) contributes to lactation maintenance in the mouse mammary glan

GODDIO MV; GATTELLI A; PEREZ CUERVO L; TOCCI J; MOUTINHO L; VEGGETI M; HYNES N; MEISS R; KORDON E

San Carlos de Bariloche

Simposio; Third South American Spring Symposium in Signal Transduction and Molecular Medicine (SISTAM); 2015

Mammary gland development is a complex, multistep process. From pregnancy to lactation, lobulo-alveolar growth is followed by the complete differentiation of mammary epithelium, which allows the production and secretion of milk proteins. At weaning, a rapid switch from survival to death signaling occurs, leading to involution, which involves extensive remodeling and an innate immune response in the tissue. Control of mRNA half-life plays a central role in normal development and disease: Several pathological conditions, such as inflammation and cancer, tightly correlate with deregulation in mRNA stability of pro-inflammatory genes. The mRNA half-life of these genes appears to be regulated by mRNA-binding proteins that interact with AU-rich elements (AREs) in the 3´-untranslated region of mRNAs. One of these RNA-binding proteins is tristetraprolin (TTP), which induces degradation of mRNA coding for pro-inflammatory cytokines and invasiveness-associated proteins. We have previously shown that TTP expression is lower in mammary tumors than in normal cells, and that this protein is specifically induced during lactation. To determine TTP role during mammary gland development, we analyzed mammary gland implants from TTP-KO mice in virgin and pregnant wild-type hosts. Our results show that TTP loss affects ductal development in virgin females, although mammary growth and differentiation in pregnant hosts proceeded unabated. Similarly, TTP silencing in HC11 cells did not prevent formation of acini or milk protein expression in culture. To determine whether TTP might play a relevant role during lactation, WAP-Cre recombinase transgenic mice, in which the enzyme is expressed uniquely in the lactating mammary tissue, were crossed with lox-P TTP transgenic animals.  Our results show that pups fed by bi-transgenic females were smaller at weaning. In addition, morphological and immuno-histochemical analysis of conditional TTP-KO mammary glands showed lower content of milk and clear signs of involution at mid-lactation. For example, abundant apoptotic epithelial cells, high levels of cleft caspase-3 and phosphorylated STAT3 positive cells were detected at day 15, after delivery. In addition, we found high expression of inflammatory cytokines, as LIF, IL6 and TNFα by qRT-PCR. These observations indicate that TTP significantly contributes to lactation maintenance. Expression of this protein prevents development of spontaneous pro-inflammatory events that might trigger early involution of the lactating mammary gland. Since the pro-inflammatory scenario in which mammary involution occurs has been associated to a window of increased risk for breast cancer development, TTP expression during breastfeeding might also play a relevant role in breast cancer suppression.