Changes in gene expression are central to the control of cellular processes such as proliferation, growth arrest, differentiation, and oncogenic transformation. The research in our laboratory focuses on the involvement of the CCAAT/enhancer binding protein (C/EBP) family of transcription factors in controlling these processes. The C/EBP family is composed of five members: C/EBPalpha, beta, gamma, delta and epsilon. With the exception of C/EBPgamma, these proteins function primarily as positive regulators of gene expression. Our research utilizes mouse models - i.e., genetically engineered mice containing targeted disruptions of individual c/ebp genes and cell lines derived from these knockout animals - to investigate the functional roles of C/EBP proteins in animals and cultured cells. A major focus of our research involves the recently discovered role of C/EBPbeta in oncogenic transformation. In collaboration with Robert Smart (North Carolina State University)and Esta Sterneck (NCI-Frederick) we found that C/EBPbeta-deficient mice are completely resistant to the development of skin tumors induced by carcinogens. The tumors that develop in normal mice contain mutations in the Ras proto-oncogene, suggesting that C/EBPbeta is an essential component of the Ras tumorigenesis pathway. Interestingly, mutant animals displayed significantly increased cell death in the epidermis after carcinogen treatment, indicating that C/EBPbeta is involved in suppressing apoptosis in pre-cancerous cells. We are currently determining whether C/EBPbeta is required for transformation of other cell types including macrophages and fibroblasts and whether it plays a pro-survival role in these cells. If C/EBPbeta activity proves to be required for the survival of many types of cancer cells that contain mutations in the Ras signaling pathway, this transcription factor may be an attractive target for novel anti-tumor drugs. To that end, we are conducting studies aimed at understanding the mechanism by which C/EBPbeta is activated in response to oncogenic Ras signaling. We have identified several sites of phosphorylation on C/EBPbeta that are induced by oncogenic Ras and are investigating how these modifications affect C/EBPbeta activity. Another major area of research involves the roles of C/EBPalpha and epsilon in the growth and differentiation of myeloid cells. We are performing this work as a collaborative study with Jonathan Keller (Laboratory of Molecular Immunoregulation). We are using mutant mice and fetal liver-derived hematopoietic precursor cells to examine how specific C/EBP gene deletions affect the functional maturation of myeloid cells in vitro, as well as the development of leukemias in vivo. Additional work is directed toward determining the dimeric status of C/EBP proteins in cells and investigating how changes in dimerization affect their transcriptional activity. We have found that C/EBPgamma is a preferential heterodimeric partner of the other C/EBP family members in vivo and that C/EBPgamma inhibits C/EBP transactivation function in a manner that requires heterodimerization. Transcriptional repression by C/EBPgamma is cell-specific, and we are currently investigating the molecular basis for its variable inhibitory activity in different cell types. To aid in the studies of C/EBPgamma function, we have obtained C/EBPgamma null mice and are using them to analyze the role of this protein in cell growth, survival, and transformation.
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