Scientific Report 2005

Molecular Biology

Delineation of Oncogenic and Tumor-Suppressing Pathways via Genetic Approaches

P. Sun, Q. Deng, C. Kannemeier, R. Liao, B. Moser

Our major interests are the genetic alterations involved in tumorigenesis and the cellular pathways that must be altered during oncogenic transformation. To this end, we analyzed the behaviors of primary, normal human cells after stable transduction of oncogenes, such as ras and MPM2.

Members of the ras family of oncogenes encode small GTP-binding proteins that transduce growth signals. Constitutive activation of ras often occurs in tumors and contributes to tumor development. In normal cells, activation of ras triggers an antioncogenic response called premature senescence, a stable growth arrest that must be overcome before transformation occurs. We showed that ras induces senescence through sequential activation of 2 MAP kinase pathways. Initially, ras activates the MAP kinase kinase (MEK)–extracellular signal–regulated kinase (ERK) pathway. Sustained activation of MEK-ERK turns on the stress-induced p38 pathway, which subsequently causes senescence.

These results revealed a novel, tumor-suppressing function of p38, in addition to its known roles in inflammation and stress responses. In other studies, we identified additional signaling components, either upstream or downstream of p38, that mediate premature senescence.

To determine how premature senescence is bypassed in tumors, we dissected the functions of an adenovirus-encoded oncoprotein, E1A, that can rescue cells from ras-induced senescence. E1A directly binds to and inhibits the functions of several cellular proteins, such as members of the Rb family, p300/CBP, and p400, that have been implicated in tumor-suppressing pathways. Our results indicated that senescence-bypassing activity resides in the N terminus of E1A and requires binding of both Rb and p300/CBP, but not binding of p400. Although interference with the p16^INK4A/Rb pathway or with p300/CBP functions alone did not result in bypassing of senescence, these 2 types of genetic alterations complemented mutants of E1A with defects in Rb binding and p300/CBP binding, respectively, to rescue cells from ras-induced senescence and lead to cellular transformation. Therefore, genetic alterations that disrupt the p16^INK4A/Rb pathway and those that perturb the p300/CBP functions cooperate to bypass ras-induced senescence. These results indicate that p300 and CBP are integral components of the senescence pathway. Both p300 and CBP have tumor-suppressing functions. The critical role of p300 and CBP in the senescence response has provided a mechanistic basis for the tumor-suppressing function of these proteins.

Another focus of our research is MDM2, an oncogene that can mediate transformation primarily through inactivation of the tumor suppressor protein p53. However, we found that MDM2 confers resistance to a growth-inhibitory cytokine, transforming growth factor β, through a p53-independent mechanism. Currently, we are delineating this p53-independent activity of MDM2, which may play an important role in tumorigenesis.

In other studies, we are systematically searching for genetic alterations that contribute to specific tumor-associated phenotypes, such as drug resistance, cellular immortalization, and metastasis. For these investigations, we are using cDNA expression libraries or libraries of short interfering RNAs.

Publications

de Parseval, A., Chatterji, U., Morris, G., Sun, P., Elder, J.H. Fine mapping of CD134 residues critical for interaction with feline immunodeficiency virus. Nat. Struct. Mol. Biol. 12:60, 2005.

de Parseval, A., Chatterji, U., Sun, P., Elder, J.H. Feline immunodeficiency virus targets activated CD4⁺ T cells by using CD134 as a binding receptor. Proc. Natl. Acad. Sci. U. S. A. 101:13044, 2004.

de Parseval, A., Ngo, S., Sun, P., Elder, J.H. Factors that increase the effective concentration of CXCR4 dictate feline immunodeficiency virus tropism and kinetics of replication. J. Virol. 78:9132, 2004.

Deng, Q., Li, Y., Tedesco, D., Liao, R., Fuhrmann, G., Sun, P. The ability of E1A to rescue ras-induced premature senescence and confer transformation relies on inactivation of both p300/CBP and Rb family proteins. Cancer Res. 65:8298, 2005.