This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. MicroRNAs (miRNAs) are a class of endogenous, non-coding, single-stranded molecules which are crucial post-transcriptional regulators of gene expression. Traditionally the miRNA function in post-transcriptional gene silencing. An estimated 30-60% of the genome is regulated by miRNA-mediated silencing, but recent studies described that miRNA can also mediate upregulation of mRNA translation. Previous studies have shown that miRNAs participate in a wide range of biological functions and play important roles in various human diseases including cancer. While miRNAs important to most somatic stem cells have been identified, the role that miRNAs play in normal mammary gland stem and progenitor cells has yet to be fully investigated. Recently, miR146b has been reported to be involved in inhibition of cancer cell migration and invasion, and suppression of breast cancer metastasis. However, the physiological role of miR146b in normal mammary gland development has not been determined. The objective of this application is to discover the role of miR146b in the survival and self-renewal of mammary alveolar progenitor cells and identify its downstream molecular targets. Our central hypothesis is that miR146b facilitates alveolar progenitor survival and self-renewal during pregnancy through activation of Stat-5 and inhibition of Stat-3. Stat-5 plays an essential role in alveolar cell survival and self-renewal during pregnancy and Stat-3 plays a crucial role in mammary alveolar death during involution. We will achieve these experimental goals through the following specific aims: Specific aim 1: To demonstrate that miR146b promotes survival and self-renewal in mammary alveolar progenitor cells, this will be achieved by in vitro manipulation of the expression of miR146b in multipotent, ductal and alveolar progenitor cells derived from a mammary epithelial cell line and in alveolar progenitor cells derived from female Balb/C virgin and pregnant mice. Specific aim 2: Elucidate the role of miR146b in suppressing Stat-3 and activation of Stat-5. Luciferase reporter and CHIP assays will be used to confirm miR146b regulation of these two genes.The long-term goal of our research is to elucidate the role of microRNAs in mammary gland stem and progenitor cell survival and self-renewal.