|Overview of Research Interests
Molecular pathways controlling cell survival and cell death: Identification of novel oncogenes and tumor suppressors:
A homeostatic balance between cell survival and cell death is a key mechanism to prevent the development of cancer. Several molecular pathways have been attributed to regulate the cell survival and cell death pathways. Among various molecular pathways, we are currently interested in exploring PI3-Kinase/Akt-PTEN pathway (Maddika et al. Mol Cell Biol. 2009, J. Cell Sci. 2008, Oncogene 2008, Cell Prolif. 2007), which is one of the most studied cell survival and cell death regulatory pathways in human cancers. We are focused on identifying and characterizing new components (both oncogenes and tumor suppressors) in PI3-Kinase/Akt pathway by using a proteomics based approach. We have recently identified several novel interacting proteins for PTEN tumor suppressor (Maddika and Chen, unpublished). We are now actively pursuing their physiological function and the role of these newly identified components in various human cancers. In our future studies, utilizing various mass spectrometric approaches, we plan to isolate the protein complexes associated with several other cellular pathways implicated to control cell death and cell survival. Through these studies we anticipate to identify several oncogenes and tumor suppressors, which will not only allow us to better understand cancer etiology, but more importantly they will provide ideal targets for the development of anti-cancer therapies.
Role of post-translational modifications in cellular signal transduction pathways:
Post-translational modification is a renowned way to regulate protein functions in diverse biological processes such as cell proliferation, growth, metabolism, differentiation, cell cycle and apoptosis. So far, protein phosphorylation regulated by concerted action of kinases and phosphatases, is the best studied post-translational modification and has been implicated in every possible cellular process. We have a rich library of nearly 250 active human kinases available in our lab, all of which are conveniently cloned in a Gateway recombination system. Utilizing this array of kinases, we started to systematically investigate how different kinases function in various cellular processes (Maddika S and Chen J, Nature Cell Biol. 2009). The knowledge gained from these studies will immensely help us in understanding the molecular basis of human diseases, in particular cancer.
Apart from phosphorylation, several other types of post-translational modifications such as methylation, acetylation and a more recently emerging ubiquitination also play a role in regulation of various cellular functions. Ubiquitination is an ATP-dependent, highly ordered multistep enzymatic process which results in the covalent attachment of ubiquitin to the substrate. Ubiquitin linked to the substrates serves as a molecular tag that marks proteins for either degradation by proteosome dependent pathway or destined to function in wide variety of functions in a proteosome independent manner. The human ubiquitin system comprises hundreds of different enzymes including approximately 500 ubiquitin ligases, ~50 E2s, and nearly 100 deubiquitinating enzymes. Together, these factors target thousands of human proteins for degradation or mediate other cellular functions.
One of the key questions in the ubiquitin field is to identify various substrates for particular E3 and how different protein substrates are recognized by specific E3-ligase complex. Also, it is puzzling how different E3-ligases assemble into a specific complex in mediating a particular cellular function and how E3-complexes are assembled and disassembled during various cellular processes. We are particularly interested in the assembly and the disassembly of E3-ligase complexes during cell cycle progression. For a long time, the only E3 ligase known to regulate proper cell cycle transition was a 13-subunit containing APC/C complex. Recently, we identified a Kinase based HECT-domain E3 ligase complex (DYRK2-EDVP complex), (Maddika S and Chen J, Nature Cell Biol. 2009) and a RING type E3 ligase (Maddika and Chen, JBC 2009), which also play a crucial role in mitotic progression. Further, my lab is also interested in exploring the proteosome independent functions of ubiquitin pathway.
Collectively, we seek to discover new components of the ubiquitin system and uncover basic knowledge about how their activities are regulated, what their substrates are, and how they contribute to diverse cellular functions. Given the fact that several RING-finger E3s and HECT-domain E3s been classified as either tumor suppressors or oncogenes, along with the reported genetic alterations, abnormal expression or dysfunction of various ubiquitin components often accompanied by the occurrence of cancer and other human disorders, we foresee that a better understanding of the basic biology of the ubiquitination process will provide insight into human diseases and we hope that this insight leads to new therapies.