Scientific Report 2006

Molecular Biology

Delineating Oncogenic and Tumor-Suppressing Signal Transduction Pathways

P. Sun, Q. Deng, C. Kannemeier, R. Liao, A. Seit-Nebi, N. Yoshizuka

Development of cancer is a result of multiple oncogenic genetic alterations, including activation of oncogenes and inactivation of tumor suppressors. Despite the essential roles of these mutations in tumor formation, normal cells usually respond to these oncogenic changes by initiating tumor-suppressing defense mechanisms such as premature senescence and apoptosis. Our main interests are delineating the signal transduction pathways that mediate these tumor-suppressing responses and determining how oncogenes allow a cell to evade the regulation by these cellular defense mechanisms to cause cancer. Currently, we are focusing on 2 well-known oncogenes: ras and mdm2.

The oncogene ras encodes a family of small GTP-binding proteins that transduce mitogenic signals from extracellular growth factors. Constitutive activation of ras is common in tumors and contributes to tumor development. In normal cells, however, the initial response to ras activation is a stable growth arrest called premature senescence. As a result, the senescence response triggered by ras must be evaded before transformation can occur. 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 studies have 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. We found that 1 of the 4 isoforms of p38 contributes to ras-induced senescence by activating the p53 tumor suppressor protein. In addition, a serine/threonine protein kinase, which is a direct substrate of p38, also plays an essential role in ras-induced senescence. Inactivation of this protein kinase disrupts ras-induced senescence and promotes tumorigenesis both in vitro and in vivo. Our results have confirmed the tumor-suppressing function of the p38 pathway.

To determine how premature senescence is bypassed in tumors, we dissected the functions of an adenovirus-encoded oncoprotein, E1A, that can rescue ras-induced senescence. Our results indicated that bypassing of senescence requires binding of the cellular proteins Rb and p300/CBP by E1A. Although interference with the p16^INK4A/Rb pathway or p300/CBP functions alone did not result in bypassing of senescence, these 2 types of genetic alterations cooperated to rescue cells from ras-induced senescence and lead to cellular transformation. 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. We are delineating this p53-independent activity of MDM2, which may play an important role in tumorigenesis. We have identified several MDM2 domains and activities that are essential for the ability of MDM2 to mediate resistance to the growth factor.

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

Publications

Lin, S., Xiao, R., Sun, P., Xu, X., Fu, X.D. Dephosphorylation-dependent sorting of SR splicing factors during mRNP maturation. Mol. Cell 20:413, 2005.