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 CCAAT/enhancer binding proteins (C/EBPs), a family of transcriptional regulators belonging to the bZIP (leucine zipper-containing) class of DNA-binding proteins. The C/EBP proteins (C/EBPalpha, C/EBPbeta, C/EBPgamma, C/EBPdelta and C/EBPepsilon) are related in their DNA-binding and leucine zipper dimerization specificities. With the exception of C/EBPgamma, these proteins are capable of activating transcription of target genes. Our research is aimed at elucidating the roles of C/EBP proteins in regulating cell growth and differentiation and cellular responses to stress. Recently, we have also begun to investigate the involvement of C/EBP proteins in oncogenic transformation and tumorigenesis. Our primary experimental tools are mutant mice carrying targeted mutations of c/ebp genes, as well as cell lines derived from these knockout animals. Studies performed in collaboration with Dr. Robert Smart (North Carolina State University) showed that C/EBPbeta is involved in regulating terminal differentiation of keratinocytes. Mice lacking C/EBPbeta form a functional epidermis but display altered expression of certain keratinocyte markers and mild skin hyperplasia. Experiments to examine the effect of C/EBPbeta deficiency on skin tumorigenesis are in progress. Related studies include examining oncogenic transformation of C/EBPbeta deficient macrophages and fibroblasts by infecting the cells with transforming retroviruses. We are also conducting collaborative studies with Jonathan Keller (Laboratory of Molecular Immunoregulation) to investigate the roles of C/EBPalpha and epsilon in the growth and differentiation of myeloid cells. 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. Other projects include determining the dimeric status of C/EBP proteins in cells and investigating how changes in dimerization partners affect their transcriptional activity. We have found that C/EBPgamma is a preferential heterodimeric partner of the other C/EBP family members in vivo. 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. C/EBPgamma null mice have been obtained and are being analyzed for defects in cellular physiology and gene expression. We are also interested in understanding the mechanisms that control the activity of C/EBP proteins in response to specific signal transduction pathways. C/EBPbeta can be activated by expression of oncogenic Ha-Ras, and we are exploring the mechanism by which Ras signaling increases C/EBPbeta activity. We have mapped several novel sites of phosphorylation that may regulate the transcriptional activity of C/EBPbeta. In addition, we are using deletion analysis to identify regions of the protein that are required for Ras responsiveness and are examining protein-protein interactions that may mediate the increased transcriptional activity of C/EBPbeta in response to Ras signaling.
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