Wang Xinjiang Lab

Department of Pharmacology & Therapeutics                                                                

Roswell Park Cancer Institute

Elm and Carlton Streets

Buffalo, NY USA 14263
Tel: 716-845-7652
Fax: 716-845-8857
E-mail: xinjiang.wang@roswellpark.org

 

Education
Ph.D., Weizmann Institute of Science, Rehovot, Israel
Post-Doctoral, Memorial Sloan-Kettering Cancer Center, New York

Decades of intensive investigation concluded that tumorigenesis constitutes a progressive and multi-step selection process that eventually allows cancer cells to acquire at least six capabilities. These include self-sufficiency in growth signals, insensitivity to growth-inhibitory signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, and tissue invasion and metastasis (Hanahan D and Weinberg RA, Cell, 100(1):57-70, 2000). The gene mutations under cancer selection fall into two categories: dominant gain of function mutations in oncogenes and recessive loss of function mutations in tumor suppressor genes. Genetic changes in oncogenes such as Myc, Ras and Bcl-2 give rise to super active or elevated oncoprotein levels endowing cells with the capabilities described above. In contrast, genetic alterations in tumor suppressor genes disable their cancer suppressing function such as the cases of p53 and PTEN. Due to their potent biological effects, the activities of both oncogenic proteins and tumor suppressors themselves are under tight regulation in normal cells. Multiple layers of control form a delicate regulatory network for each major player in a pathway. Yet, the critical regulatory components of each pathway are also found under selection in favor of cancer development. For example, p53 protein abundance is negatively regulated by its own target gene product Mdm2, a RING finger E3 ligase, to ensure normal cell growth. However, when Mdm2 is overexpressed as found in many types of human cancers, it is sufficient to nullify the p53 function thus bypassing the requirement of a critical hit on tumor suppressor p53 gene directly. Therefore it is very important to uncover potential regulators in a given pathway that a particular tumor suppressor and/or an oncogenic protein operate in. This will be essential for a better understanding of tumorigenesis and improve cancer drug design.

 

Our laboratory is interested in identification and characterization of positive or negative regulators for major tumor suppressors. This endeavor is aimed to understanding of biochemical regulation of tumor suppressors in cells and the progress thereof will lead to discovery of novel drug targets for cancer drug development. The present focus of this lab is the regulation of tumor suppressor PTEN and p53 by their E3 ligases. E3 ligases are the substrate specificity determining components of the ubiquitin conjugation system. Covalent conjugation of ubiquitin moieties to a protein tags the protein either for proteasomal degradation or protein translocation through protein sorting system or protein complex assembly. Therefore, protein ubiquitination is an important type of modification regulating protein abundance, localization and function. The specificity of an E3 ligase can be very high but not absolute, for example, an E3 ligase can have more than one substrate while one protein substrate can have several E3 ligases. In human genome, we have more than 500 different E3 ligases in total. In fact, ubiquitination is involved in numerous aspects of cellular processes including cell cycle control, cell signaling, immune response and stress response and so on.  Inherited or somatic aberration of this system causes diseases including cancer.

 

PTEN is a lipid phosphatase and acts as a major antagonist of PI3K pathway by reducing phosphatidylinositol triphosphate PI(3,4,5)P3 levels. PTEN has been shown to play a fundamental role in regulating cell growth, migration and many other cellular processes. The PTEN gene is frequently mutated in many cancer types including the familial cancer syndrome, Cowden Disease. This places PTEN as a second most frequently mutated cancer gene after p53. Although PTEN is a well established as a negative regulator for PI-3K pathway, the regulation of PTEN itself remains unclear. To better understand how PTEN is regulated in cells, it’s a critical step to find out what other proteins physically or functionally interact with PTEN. It’s equally important to know what types of post-translational modification that PTEN undergoes in cells in response to growth signals and what enzymes mediate those types of modifications.  By looking into these aspects of PTEN, we can identify more components that regulate the output of PTEN/PI-3K pathway, which can be potentially important drug targets.

 

One of such research strategy is to use biochemical assays that PTEN is included in test tubes and identify functional regulators of PTEN through biochemical purification from crude cell lysate. Using this approach, we recently identified NEDD4-1 as the first E3 ligase for tumor suppressor PTEN (Wang et al, Cell. 2007, 128 (1):129-39). Characterization of this E3 ligse revealed that PTEN regulation by ubiquitination is rather complicated. It appears that polyubiquitination of PTEN by this enzyme promotes PTEN degradation while monoubiquitination of PTEN promotes PTEN nuclear transport. The former negatively regulates PTEN stability while the latter strengthens PTEN’s nuclear function (Trotman, et alCell. 2007, 128(1):141-56). Interestingly, through more detailed characterization, we have demonstrated that the C-terminal region of PTEN protein plays a critical role in antagonizing NEDD4-1 interaction with PTEN and the subsequent PTEN ubiquitnation and degradation (Wang, et al, Biochem J. 2008, 414 (2):221-9). This explains why cancer cells frequently select PTEN frameshift mutation causing truncation at its C-terminal region. It’s because loss of PTEN C-terminus renders PTEN instablity. Apart from the self stabilizing effect conferred by PTEN C-terminus, other mechanisms underlying PTEN stabilization also exist in cells. For instance, Rak, a recently reported PTEN interacting protein can bind to and protect PTEN from NEDD4-1 mediated polyubiquitination, therefore acts as a positive regulator of PTEN protein stability (Yim EK, et al, Cancer Cell. 2009 15(4):304-14). Actually, loss of Rak in cancer cells results in loss of PTEN expression in the same time due to decreased PTEN protein stability. Therefore these regulatory mechanisms via PTEN interacting proteins determine PTEN as a relative stable protein by default in the presence of decent levels of NEDD4-1 enzymes in cells. So, it’s not surprising that the picture of PTEN regulation by ubiquitination is emerging in a way different from that of p53. That is, p53 is known to undergo regulated stabilization in signaling to genotoxic and oncogenic stress while PTEN might undergo regulated degradation in certain physiological cues. More such PTEN interacting proteins await to be identified in years ahead that may regulate PTEN stability in a cell-type dependent or stimulus-dependent manner. Another important research direction may be the regulation of NEDD4-1 activity toward PTEN via post-translational modification of NEDD4-1. It remains plausible that additional PTEN E3 ligases exist as indicated by Nedd4-1 knockout MEFs (Cao XR, et al, Sci Signal. 2008 Sep 23;1(38):ra5). 

 

Another direction of this lab is to investigate the mechanisms underlying p53 regulation by its E3 ligase Mdm2. p53 is such a fundamental tumor suppressor whose functional inactivation occurs through multiple mechanisms in many cancer types, one of which is overexpression of Mdm2 RING Finger E3 ligase, the major p53 negative regulator that targets p53 for degradation and transcriptional inactivation. Understanding of how Mdm2 cooperates with MdmX in p53 regulation and how Mdm2 E3 ligase activity is regulated will provide insights into developing better inhibitors against this oncogenic E3 activity. These inhibitors will be able to unleash wild type p53 in cancer cells as novel cancer therapeutics.

 

Without being limited to E3 ligases for PTEN and p53, this lab will be interested in identifying cancer relevant E3 ligases through biochemical purification and genetic screen using shRNA library. We believe that research in this direction will lead to drug target identification among E3 ligases thus paves ways to novel cancer drug development.