Singleminded-2s (Sim2s) promotes delayed Involution of the Mouse Mammary Gland through suppression of Stat3 and NFkB. Mus musculus

Post-lactational involution of the mammary gland provides a unique model to study breast cancer susceptibility and metastasis. We have shown that the short isoform of Singleminded-2 (Sim2s), a bHLH/PAS transcription factor, plays a role in promoting lactogenic differentiation, as well as in maintaining mammary epithelial differentiation and malignancy. Sim2s is dynamically expressed during mammary gland development, with expression peaking during lactation, and decreasing in early involution. To determine the role of Sim2s in involution, we used transgenic mice expressing Sim2s under the mouse mammary tumor virus (MMTV-Sim2s) promoter. Over-expression of Sim2s in the mouse mammary gland resulted in delayed involution, indicated by a lower proportion of cleaved caspase-3 positive cells and slower re-establishment of the mammary fat pad. Immunohistochemical and quantitative RNA analysis showed a decrease in apoptotic markers and inflammatory response genes, and an increase in anti-apoptotic genes, which were accompanied by inhibition of signal transducer and activator of transcription 3 (Stat3) activity. Microarray analysis confirmed that genes in the Stat3 signaling pathway were repressed by Sim2s expression, along with NFκB and other key pathways involved in mammary gland development. Multiparous MMTV-Sim2s females displayed a more differentiated phenotype compared to wild-type controls, characterized by enhanced β-casein expression and alveolar structures. Together, these results suggest a role for Sim2s in the normal involuting gland, and identify potential down-stream pathways regulated by Sim2s. Overall design: Gene expression was measured in 6 mammary gland samples using CodeLink Mouse Whole Genome microarrays. 3 samples were from wild-type mice harvested at 72 hours post weaning and 3 samples were MMTV-Sim2s mice harvested at 72 hours post weaning.