The mammary gland is a branched epithelial organ comprised of myoepithelial,
ductal and alveolar cells that are derived from resident stem and progenitor cells. The
progression from mammary gland stem cell(s) to the differentiated mammary gland
cell types is poorly understood. Here, I describe the identification and
characterization of two luminal progenitor cell populations in the mouse mammary
gland, and investigate the role of the transcription factor C/EBPβ in their
development. In Chapter 2, I describe the isolation of two luminal progenitor cell
populations (Sca1+ and Sca1- luminal cells) and show that they are differentially
primed in their gene expression towards ductal and alveolar cell fates, respectively.
Furthermore, I show that in vivo genetic priming affects the in vitro differentiation
potential of Sca1+ and Sca1- luminal cells. In Chapter 3, I show that C/EBPβ is
required for the appropriate specification of ductal and alveolar lineages, and in its
absence, alveolar lineage priming is lost, and ductal lineage priming is up-regulated
in both Sca1+ and Sca1- cells. Preliminary data also shows that in addition to severe
proliferation defects, the changes in in vivo lineage priming in Cebpb-/- mice also
affect the in vitro differentiation potential of Cebpb-/- Sca1+ and Sca1- luminal
progenitors. Lastly, in Chapter 4, I describe the genome-wide binding characteristics
of C/EBPβ in Sca1+, Sca1- and P16.5 alveolar cells. These experiments reveal that
genome-wide C/EBPβ occupancy is correlated with alveolar cells fate, and that
C/EBPβ target genes perform distinct cellular functions in alveolar cells (Sca1- cells
and P16.5). Furthermore, I show that Elf5 is directly regulated by C/EBPβ, and posit
that direct regulation of Elf5 by C/EBPβ may be one mechanism through which
C/EBPβ exerts its alveolar cell fate programming
