Characterization of the CCAAT/enhancer binding protein-beta activator isoforms in normal mammary epithelial cells and breast cancer cells

A member of the C/EBP family of transcription factors, C/EBPβ, has recently proven to be an important player in both growth and differentiation of the epithelial cells in the mammary gland. When the gene for C/EBPβ is disrupted in mice, these mice fail to either develop normal mammary ducts during puberty or pregnancy, or to lactate upon parturition. C/EBPβ can be present in cells in three isoforms: C/EBPβ-1, -2, and -3. These isoforms have the same carboxy terminus, but different N-termini, due to alternative translational initiation at three different initiator codons within the C/EBPβ mRNA. Using a commercially available antibody specific to the C-terminus of C/EBPβ, and a novel antibody specific to the N-terminus of C/EBPβ-1, we have uncovered a striking difference in the forms of C/EBPβ present in normal mammary epithelial cells versus breast cancer cell lines. C/EBPβ-1 is found exclusively in normal mammary epithelial cells, whereas C/EBPβ-2 is found only in dividing cells, both normal and neoplastic. Introduction of C/EBPβ-1 into breast cancer cells markedly reduces the cells' invasive ability in vitro. Furthermore, this study demonstrates that C/EBPβ-1 and not C/EBPβ-2 is modified by the small ubiquitin-like modifiers, SUMO-2 and SUMO-3. The goal of this study is to demonstrate that C/EBPβ-1 and C/EBPβ-2 are differentially expressed in normal cells versus cancer cells and that the proteins have different functions within mammary epithelial cells. Despite the fact that the two proteins differ by only 2–23 amino acids, depending on the species, these transcription factors are markedly distinct.