Our Science – Johnson Website

Peter F. Johnson, Ph.D.

Basic Research Laboratory Head, Eukaryotic Transcriptional Regulation Section Senior Investigator Building 539, Room 122 NCI-Frederick Frederick, MD 21702-1201 Phone:   301-846-1627 Fax:   301-846-5991 E-Mail:   johnsope@mail.nih.gov

Biography

Dr. Peter Johnson received his Ph.D. from the Department of Biology, University of California-San Diego, in 1983 and was a postdoctoral fellow at the Fred Hutchinson Cancer Research Center and the Carnegie Institution from 1984 to 1988 with Dr. Steven McKnight. Dr. Johnson was corecipient of the 1988 to 1989 AAAS Newcomb-Cleveland Prize for a paper describing the leucine zipper motif in bZIP DNA-binding proteins. In 1989, he joined the ABL-Basic Research Program as leader of the Eukaryotic Transcriptional Regulation Group and was appointed section head in 1998. In 1999, Dr. Johnson joined the Center for Cancer Research, NCI.

Research

Function and Regulation of the C/EBP Family of Transcription Factors

Changes in gene transcription underlie many biological processes, including cell growth, differentiation, and tumorigenesis. Our laboratory is investigating the C/EBP (CCAAT/enhancer-binding protein) family of transcription factors and their roles in regulating these physiological processes. The C/EBP family is composed of six related members that belong to the basic-leucine zipper (bZIP) class of DNA-binding proteins. Our research focuses primarily on C/EBPbeta and its involvement in controlling growth and survival of normal and transformed cells, as well as understanding the mechanisms that regulate C/EBPbeta expression and activity.

We have used C/EBPbeta knockout mice and cells derived from these animals to establish an essential function for C/EBPbeta in oncogenic transformation. In collaboration with Dr. Robert Smart (North Carolina State U), we found that C/EBPbeta null mice are completely resistant to the development of skin tumors when subjected to a DMBA/TPA carcinogenesis protocol. The skin tumors that develop in normal animals contain mutations in the Ras protooncogene. Since C/EBPbeta can be post-translationally activated by the Ras signal transduction pathway, we postulate that activation of C/EBPbeta by oncogenic Ras is a necessary event in the development of skin tumor cells. Additional evidence suggests that C/EBPbeta may function in transformed keratinocytes to suppress apoptosis, thus explaining its requirement for skin tumorigenesis. Low levels of C/EBPbeta overexpression stimulate Ras-induced transformation of NIH 3T3 cells and a dominant negative C/EBPbeta protein blocks transformation. These findings demonstrate that C/EBPbeta is an important regulator of neoplastic transformation elicited by the Ras oncogene.

Recently, our laboratory found that bone marrow-derived macrophages infected with a transforming virus carrying the Myc and Raf oncogenes also require C/EBPbeta to become transformed. Macrophages derived from C/EBPbeta knockout mice show a complete block to transformation. We determined that the transformation defect involves the mutant cells' inability to grow and survive in the absence of exogenous hematopoietic growth factors such as M-CSF. Using microarray screening, we identified the gene for insulin-like growth factor I (IGF-I) as a transcriptional target of C/EBPbeta in transformed macrophages. IGF-I acts as an autocrine growth and survival factor in these cells and allows them to become 'self-sufficient' (i.e., independent of exogenous growth factors). In support of this notion, IGF-I-deficient bone marrow cells are also resistant to Myc/Raf transformation and require an exogenous growth factor. We are currently investigating whether C/EBPbeta plays a similar role in regulating IGF-I expression in other kinds of tumor cells and whether it controls serum IGF-I levels, which are known to contribute to the progression and malignancy of many cancers by promoting tumor cell growth and survival.

Because C/EBPbeta is a downstream target of the Ras-Raf pathway we wish to understand the mechanistic basis of its regulation, including identifying post-translational modifications that mediate its functional activation by Ras signaling and elucidating the effector pathways involved. We found that the DNA-binding activity of C/EBPbeta is intrinsically inhibited (auto-repressed) but can be activated by Ras signaling. Using deletion and site-directed mutagenesis, we mapped specific sequences outside the DNA-binding domain that mediate auto-repression. In addition, we have identified several phosphoacceptor sites that are induced by Ras signaling, two of which are cell cycle-regulated and appear to be targets for cyclin dependent kinases (Cdks). Mutation of either of these phosphoacceptor residues to alanine does not affect DNA-binding but blocks the ability of C/EBPbeta to facilitate NIH 3T3 cell transformation. Thus, cell cycle dependent phosphorylation of C/EBPbeta is a critical event in Ras-induced transformation. We are currently investigating other Ras-induced modifications that appear to mediate de-repression of C/EBPbeta DNA-binding activity and we plan to test whether these sites are also important for the 'pro-oncogenic' function of C/EBPbeta.

Our collaborators include Jonathan Keller, SAIC-Frederick; Italo Mocchetti, Georgetown University; Richard Schwartz, Michigan State University; Carl Barrett, NCI; Steve Hursting, NCI; Robert Smart, North Carolina State University; Terry Copeland, NCI-Frederick; and Dhananjaya Kalvakolanu, University of Maryland

This page was last updated on 1/27/2009.