EBP b , but not C / EBP a , is essential for ductal morphogenesis , lobuloalveolar proliferation , and functional differentiation in the mouse mammary gland

The CCAAT/enhancer binding proteins (C/EBPs) are differentially expressed throughout mammary gland development and interact with binding sites within the promoter of a milk protein gene, b-casein. The specific roles of C/EBPb and C/EBPa in mouse mammary gland development and differentiation have been investigated in mice that carry targeted deletions of these genes. C/EBPb virgin mice exhibited cystic, enlarged mammary ducts with decreased secondary branching. Transplantation of C/EBPb mammary epithelium into the cleared mammary fat pads of nude mice confirmed that this defect in ductal morphogenesis was intrinsic to the epithelium. When treated with estrogen/progesterone (E+P) to simulate pregnancy, C/EBPb mammary glands displayed only limited lobuloalveolar development and ductal side branching. Primary mammary epithelial cells obtained from E+P-treated C/EBPb mice that were cultured on extracellular matrix gels did not functionally differentiate in response to lactogenic hormones despite their organization into three-dimensional structures. Expression of b-casein protein was inhibited 85%–100% and whey acidic protein (WAP) was undetectable. In contrast, no detectable alterations in mammary development or b-casein expression were observed in mammary outgrowths derived from newborn C/EBPa mammary epithelium transplanted into the cleared mammary fat pads of syngeneic hosts. These results demonstrate that C/EBPb, but not C/EBPa, is required for ductal morphogenesis, lobuloalveolar development, and functional differentiation of mammary epithelial cells.