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

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Abstract
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, β-casein. The specific roles of C/EBPβ and C/EBPα in mouse mammary gland development and differentiation have been investigated in mice that carry targeted deletions of these genes. C/EBPβ−/− virgin mice exhibited cystic, enlarged mammary ducts with decreased secondary branching. Transplantation of C/EBPβ−/− 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/EBPβ−/− mammary glands displayed only limited lobuloalveolar development and ductal side branching. Primary mammary epithelial cells obtained from E+P-treated C/EBPβ−/−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 β-casein protein was inhibited 85%–100% and whey acidic protein (WAP) was undetectable. In contrast, no detectable alterations in mammary development or β-casein expression were observed in mammary outgrowths derived from newborn C/EBPα−/− mammary epithelium transplanted into the cleared mammary fat pads of syngeneic hosts. These results demonstrate that C/EBPβ, but not C/EBPα, is required for ductal morphogenesis, lobuloalveolar development, and functional differentiation of mammary epithelial cells.