Regulating Oocyte Maturation: Understanding exactly how ovarian follicles mature to generate fertile eggs is key to many aspects of fertility treatment. When the pituitary surge of luteinizing hormone (LH) binds to its receptor on granulosa cells of preovulatory follicles, a cascade of signaling events triggers granulosa cells to become luteal cells and the oocyte to resume meiosis. Fan et al. (p. 938 ; see the Perspective by Duggavathi and Murphy ), using the mouse as a model system, targeted disruption of the kinases ERK1 and ERK2 selectively in granulosa cells. The kinases were essential in vivo mediators of LH induction of ovulation and luteinization.

A large number of intercellular signaling molecules have been identified that orchestrate female reproductive physiology. However, with the exception of steroid hormone receptors, little information exists about the transcriptional regulators that mediate cellular responses to these signals. The transcription factor C/EBPβ (CCAAT/enhancer-bindingprotein β) is expressed in ovaries and testes, as well as many other tissues of adult mice. Here we show that mice carrying a targeted deletion of the C/EBPβ gene exhibit reproductive defects. Although these animals develop normally and males are fertile, adult females are sterile. Transplantation of normal ovaries into mutant females restored fertility, thus localizing the primary reproductive defect to the ovary proper. In normal ovaries, C/EBPβ mRNA is specifically induced by luteinizing hormone (LH/hCG) in the granulosa layer of preovulatory antral follicles. C/EBPβ-deficient ovaries lack corpora lutea and fail to down-regulate expression of the prostaglandin endoperoxidase synthase 2 and P450 aromatase genes in response to gonadotropins. These findings demonstrate that C/EBPβ is essential for periovulatory granulosa cell differentiation in response to LH. C/EBPβ is thus established as a critical downstream target of G-protein-coupled LH receptor signaling and one of the first transcription factors, other than steroid hormone receptors, known to be required for ovarian follicle development in vivo.

Studies of C/EBPβ-deficient mice have demonstrated a pivotal role for this transcription factor in hematopoiesis, adipogenesis, and ovarian function. Here we show that C/EBPβ is also essential for normal development and function of the mammary gland. Ductal morphogenesis in virgin C/EBPβ-deficient mice was disrupted, with ducts displaying reduced growth and branching. To distinguish whether the effect of C/EBPβ deficiency on mammary epithelium is indirect or cell autonomous, we performed ovarian and mammary gland transplants. Transplants of wild-type ovaries into mutant females partially restored ductal morphogenesis during puberty but failed to support mammopoiesis during pregnancy. At term, mutant mice harboring wild-type ovaries exhibited reduced alveolar proliferation and impaired epithelial cell differentiation, including a complete absence of milk protein expression. Mammary gland transplant experiments demonstrated that development of C/EBPβ-deficient epithelium was defective within a wild-type stroma and host background. Cell proliferation during pregnancy was reduced and differentiation, as measured by the activity of milk protein genes, was inhibited. However, wild-type epithelium developed in a C/EBPβ-deficient stroma. Thus, C/EBPβ plays an essential, cell autonomous role in the proliferation and differentiation of mammary secretory epithelial cells and is required for the activation of milk protein genes.

The basic leucine zipper transcription factor CCAAT/enhancer binding protein-β (C/EBPβ) is expressed in many cell types, including keratinocytes. C/EBPβ activity can be increased by phosphorylation through pathways stimulated by oncogenic Ras, although the biological implications of Ras-C/EBPβ signaling are not currently understood. We report here thatC/EBPβ-nullizygous mice are completely refractory to skin tumor development induced by a variety of carcinogens and carcinogenesis protocols, including 7,12-dimethylbenz[a]anthracene-initiation/12-O-tetradecanoylphorbol 13-acetate promotion, that produce tumors containing oncogenicRasmutations. No significant differences in TPA-induced epidermal keratinocyte proliferation were observed inC/EBPβ-null versus wild-type mice. However, apoptosis was significantly elevated (17-fold) in the epidermal keratinocytes of 7,12-dimethylbenz[a]anthracene-treatedC/EBPβ-null mice compared with wild-type mice. In v-Ha-rastransgenic mice,C/EBPβ deficiency also led to greatly reduced skin tumor multiplicity and size, providing additional evidence for a tumorigenesis pathway linking Ras and C/EBPβ. Oncogenic Ras potently stimulated C/EBPβ to activate a C/EBP-responsive promoter-reporter in keratinocytes and mutating an ERK1/2 phosphorylation site (T188) in C/EBPβ abolished this Ras effect. Finally, we observed that C/EBPβ participates in oncogenic Ras-induced transformation of NIH 3T3 cells. These findings indicate that C/EBPβ has a critical role in Ras-mediated tumorigenesis and cell survival and implicate C/EBPβ as a target for tumor inhibition.

The CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ) is a transcription factor that modulates many biological processes including cell differentiation, motility, growth arrest, proliferation, and cell death. The diversity of C/EBPδ's functions depends in part on the cell type and cellular context and can have opposing outcomes. For example, C/EBPδ promotes inflammatory signaling, but it can also inhibit pro-inflammatory pathways, and in a mouse model of mammary tumorigenesis, C/EBPδ reduces tumor incidence but promotes tumor metastasis. This review highlights the multifaceted nature of C/EBPδ's functions, with an emphasis on pathways that are relevant for cancer and inflammation, and illustrates how C/EBPδ emerged from the shadow of its family members as a fascinating “jack of all trades.” Our current knowledge on C/EBPδ indicates that, rather than being essential for a specific cellular process, C/EBPδ helps to interpret a variety of cues in a cell-type and context-dependent manner, to adjust cellular functions to specific situations. Therefore, insights into the roles and mechanisms of C/EBPδ signaling can lead to a better understanding of how the integration of different signaling pathways dictates normal and pathological cell functions and physiology.