C EBP-BETA, BUT NOT C/EBP-ALPHA, IS ESSENTIAL FOR DUCTAL MORPHOGENESIS, LOBULOALVEOLAR PROLIFERATION, AND FUNCTIONAL-DIFFERENTIATION IN THEMOUSE MAMMARY-GLAND/

The CCAAT/enhancer binding proteins (C/EBPs) are differentially expres sed throughout mammary gland development and interact with binding sit es within the promoter of a milk protein gene, beta-casein. The specif ic roles of C/EBP beta and C/EBP alpha in mouse mammary gland developm ent and differentiation have been investigated in mice that carry targ eted deletions of these genes. C/EBP beta(-/-) virgin mice exhibited c ystic, enlarged mammary ducts with decreased secondary branching. Tran splantation of C/EBP beta(-/-) mammary epithelium into the cleared mam mary fat pads of nude mice confirmed that this defect in ductal morpho genesis was intrinsic to the epithelium. When treated with estrogen/pr ogesterone (E+P) to simulate pregnancy, C/EBP beta(-/-) mammary glands displayed only limited lobuloalveolar development and ductal side bra nching. Primary mammary epithelial cells obtained from E+P-treated C/E BP beta(-/-) mice that were cultured on extracellular matrix gels did not functionally differentiate in response to lactogenic hormones desp ite their organization into three-dimensional structures. Expression o f beta-casein protein was inhibited 85%-100% and whey acidic protein ( WAP) was undetectable. In contrast, no detectable alterations in mamma ry development or beta-casein expression were observed in mammary outg rowths derived from newborn C/EBP alpha(-/-) mammary epithelium transp lanted into the cleared mammary fat pads of syngeneic hosts. These res ults demonstrate that C/EBP beta, but not C/EBP alpha, is required for ductal morphogenesis, lobuloalveolar development, and functional diff erentiation of mammary epithelial cells.