News and Publications

Molecular Genetics of Cancer

The essential events that transform a normal cell into a cancer cell include changes at the levels of transcription and translation. These changes result in an upregulation of growth-promoting genes and proteins and downregulation of growth inhibitory genes and proteins.

Transcriptional Regulation

We work on 3 families of transcription factors, the AP-1 complex with Jun, Fos, and Maf; the Fox (forkhead-winged helix) proteins with Qin and PAX3-FKHR; and the ß-catenin-LEF complex. All transcription factors are potential oncoproteins, and gain of function or ectopic expression can activate this cancer-inducing potential. An understanding of oncogenicity mediated by transcriptional regulation requires the identification of genes that are differentially transcribed in cancer cells. A subset of these target genes is responsible for the oncogenic phenotype of each cell. We continue to search for transformation-relevant targets of oncogenic transcription factors.

We generated extensive lists of genes that are differentially expressed in Jun-transformed chicken and mouse cells. Of particular interest are genes associated with downregulation of the proteins AKAP12 and MARCKS. The upregulation of either AKAP12 or MARCKS in Jun-transformed cells results in reversion of the oncogenic cellular phenotype to near-normal. AKAP12 and MARCKS are coregulated; reexpression of one induces the expression of the other. Downregulation of AKAP12 and of MARCKS is required for oncogenic transformation by Jun, and upregulation is sufficient for reversion. We used this system of reversible transformation to follow the expression pattern of other Jun targets, asking whether reversion also abrogates the differential expression of these targets. Correlation of expression with the normal or neoplastic cellular phenotype marks transformation-relevant targets.

We also constructed artificial oncoproteins that address the AP-1 consensus sequence but are made up entirely by polypeptide components from nonvertebrate sources. One of these artificial oncoproteins contains the dimerization and DNA-binding domain of the yeast protein GCN4 and the transcriptional activation domain of the herpesvirus VP16 protein. This chimeric protein induces oncogenic transformation by forming exclusively homodimers; it does not interact with endogenous proteins of the AP-1 complex. Transformation by the chimeric protein has essential mechanistic features in common with transformation by Jun. Therefore, differentially regulated targets shared by Jun and by the chimeric protein most likely are relevant in oncogenic transformation. The identification of shared targets allows a substantial reduction in the list of genes that must be tested for the function in oncogenesis.

A genetic analysis of Qin-induced transformation suggests that this transcriptional repressor transforms cells by binding to DNA as a multimeric protein complex consisting of several molecules of Qin, of the corepressor TLE, and of histone deacetylase. Overexpression of Qin in developing chicken embryos results in a decrease in the rate of neuroepithelial apoptosis and in a thickening of the neuroepithelium. Ultimately, it produces large outgrowths of the telencephalon and mesencephalon. These results indicate a previously unrecognized role for Fox proteins in the regulation of neural cell apoptosis.

ß-Catenin-LEF is regulated by the Wnt signaling pathway. In human colon cancer, this pathway commonly shows a gain of function, resulting in the deregulation of LEF target genes. We carried out a mutational analysis of ß-catenin and LEF, determining the functions and protein domains that are essential for oncogenic transformation. In ß-catenin, resistance to proteolytic degradation is required plus one of the transactivation domains and the armadillo repeats that mediate interaction with LEF. For LEF, only the DNA-binding domain is required for oncogenic transformation. We are evaluating target proteins important for LEF-induced transformation and plan to continue this search for critical proteins responsible for the oncogenic phenotype.

Translational Regulation

Our work on the oncogenic forms of phosphatidylinositol-3´-kinase and the Akt kinase revealed an important role of protein synthesis in oncogenic transformation. Inhibition of the TOR kinase by rapamycin results in reduced activity of the translation initiation factor 4E and the S6 kinase and effectively interferes with oncogenic transformation induced by phosphatidylinositol-3´-kinase or Akt. Recently, we isolated an RNA-binding protein, YB-1, as a downregulated target of phosphatidylinositol-3´-kinase and of Akt.

Overexpression of YB-1 causes a specific resistance to transformation induced by phosphatidylinositol-3´-kinase or Akt. YB-1 also induces a flat and adherent cellular phenotype and lowers the growth potential of the cells. These activities are lost in YB-1 mutants that no longer bind to mRNA. We are analyzing the effects of YB-1 on mRNA function and on oncogenic transformation.

Novel Small-Molecule Inhibitors of Oncogenic Proteins

In collaboration with D.L. Boger, K.C. Nicolaou, H. Kolb, and K.B. Sharpless, Department of Chemistry, we are screening combinatorial chemical libraries in molecular and cell-based assays for specific inhibitors of Myc, LEF, and Akt. We identified several lead compounds. These compounds are now being optimized, and their mechanism of action is being investigated.

Publications

Aoki, M., Sobek, V., Maslyar, D.J., Hecht, A., Vogt, P.K. Oncogenic transformation by ß-catenin: deletion analysis and characterization of selected target genes. Oncogene 21:6983, 2002.

Aoki, M., Vogt, P.K. Retroviral oncogenes and TOR. Curr. Top. Microbiol. Immunol., in press.

Berg, T., Cohen, S.B., Desharnais, J., Sonderegger, C., Maslyar, D.J., Goldberg, J., Boger, D.L., Vogt, P.K. Small-molecule antagonists of Myc/Max dimerization inhibit Myc-induced transformation of chicken embryo fibroblasts. Proc. Natl. Acad. Sci. U. S. A. 99:3830, 2002.

Blazek, E., Wasmer, S., Kruse, U., Aronheim, A., Aoki, M., Vogt, P.K. Partial oncogenic transformation of chicken embryo fibroblasts by Jun dimerization protein 2, a negative regulator of TRE- and CRE-dependent transcription. Oncogene 22:2151, 2003.

Nishizawa, M., Fu, S.-L., Kataoka, K., Vogt, P.K. Artificial oncoproteins: modified versions of the yeast bZip protein GCN4 induce cellular transformation. Oncogene 22:5987, 2003.

Nishizawa, M., Kataoka, K., Vogt, P.K. MafA has strong cell transforming ability but is a weak transactivator. Oncogene 22:5938, 2003.

Sonderegger, C., Narisawa-Saito, M., Vogt, P.K. The C-terminal region of cellular Qin oligomerizes: correlation with oncogenic transformation and transcriptional repression. Oncogene 22:1908, 2003.

Sonderegger, C., Vogt, P.K. Binding of the corepressor TLE1 to Qin enhances Qin-mediated transformation of chicken embryo fibroblasts. Oncogene 22:1749, 2003.

Vogt, P.K. Fortuitous convergences: the beginnings of JUN. Nat. Rev. Cancer 2:465, 2002.