My laboratory studies the functions of the ubiquitin system, with special emphasis on the endoplasmic reticulum-associated protein degradation (ERAD). Ubiquitin regulates cellular functions through ubiquitination, a process that provides eukaryotic cells with a means to fine-tune both protein function and levels. Ubiquitination affects myriad proteins and potentially impacts all cellular processes. Our research has high relevance to human health, because there is growing evidence that dysfunction of the ubiquitin system and ERAD are intimately involved in the pathogenesis of many human diseases, including cancer, diabetes, cystic fibrosis, heart disease, and neurodegenerative diseases.

gp78 and ER-associated degradation
ERAD safeguards the secretory pathway through eliminating misfolded and unassembled proteins from the ER. It also regulates many physiological processes through regulated-degradation of key proteins. For example, ERAD is essential for sterol-regulated degradation of HMG CoA reductase, a rate-limiting enzyme in cholesterol synthesis. Accelerated degradation of the reductase is one of the several strategies cells use to limit the production of cholesterol. We have identified gp78 as the first ER-resident ubiquitin ligase involved in ubiquitination of unwanted proteins during ERAD. Recently, work from my laboratory has provided novel insights into the molecular mechanism of gp78-mediated ERAD. We were the first group to uncover the mechanism by which gp78-mediated ubiquitination is coupled with p97/VCP-driven retrotranslocation of misfolded proteins from the ER to the cytosol via gp78-p97/VCP interaction (Figure 1, middle).

We have discovered a novel p97/VCP-Interacting Motif (VIM) in gp78 and the Small p97/VCP-Interacting Protein (SVIP). Identification of VIM led to elucidation of a Ufd1-independent mechanism of p97/VCP-mediated retrotranslocation (Figure 1, middle), which challenges the current doctrine in the ERAD field that retrotranslocation of proteins from the ER requires p97/VCP and its cofactors Ufd1 and Npl4 heterodimer. However, we also demonstrated that p97/VCP-Ufd1-Npl4 complex is required for coupling retrotranslocation with ubiquitination catalyzed by other unknown E3s (Figure 1, right, UN: Ufd1-Npl4 dimer) More recently, we have identified SVIP as the first endogenous inhibitor of gp78-mediated ERAD (Figure 1, left). In short, our studies on gp78 have enriched the ERAD field. For example, gp78-p97/VCP interaction has now been recognized as an important mechanism for the regulated degradation of HMG CoA reductase. In addition, gp78 has been reported as a tumor metastasis-promoting ubiquitin ligase through degrading KAI1, a metastasis suppressor protein, from the ER. Currently, we are working on several related projects: 1) identification of novel players in the ERAD pathway, such as ubiquitin ligases and retrotranslocation factor, using a combination of biochemical, cell biological, and proteomic approaches; 2) testing the hypothesis that SVIP plays a pivotal role in balancing degradation vs. trafficking of proteins in the secretory pathway; 3) identification of protein quality control ubiquitin ligases in Golgi and plasma membrane.

Dysfunction of ERAD and ER stress
ERAD process is a common target of many pathogenic insults, such as oxidative stress, hypoxia, certain genetic mutations, and any factor that inhibits the proteasomal degradation pathway. Failure to remove unwanted proteins by ERAD causes ER stress leading to the unfolded protein response (UPR) (see Figure 2). UPR activates both adaptive and apoptotic pathways, which contribute differently to disease pathogenesis. The apoptotic pathway leads to cell death in many diseases, such as diabetes and many neurodegenerative diseases, whereas the adaptive pathway promotes cell survival, as observed in tumors.

To further understand the functional mechanisms of UPR, we identified 12 commonly UPR-upregulated genes by expression microarray analysis. We have characterized one of the genes, namely ARMET. We found that Armet is a secreted protein with multiple functions. Our results suggest that Armet may be involved in tumor growth and survival under hypoxia, in neuroprotection in neurodegenerative diseases, and in tissue hypertrophy in many other diseases. Further efforts are directed to understand how Armet protein is regulated and how Armet is involved in normal and pathological processes. Specifically, we are testing the hypothesis that Armet is a cytoprotective factor involved in the pathogenesis of cancer and neurodegenerative diseases.

Stem cell research and ubiquitin ligase-targeted drug screening
Breakthroughs in our understanding of the ubiquitin system provide enormous potentials for drug discovery. Many human diseases are associated with disturbances of this system. The first drug, VelcadeTM, that targets the step of proteasomal degradation, have been approved bythe FDA for the treatment of patients with multiple myeloma. Studies have directly linked several ubiquitin ligases to diseases, for example, mdm2 and cancer. Therefore, our lab is interested in establishing ubiquitin ligase activity assays for high throughput screening for drugs targeting disease-causing ubiquitin ligases. In addition, we are exploring a new avenue for controlling stem cell differentiation. To achieve this goal, we will identify the ubiquitin ligase(s) for Oct4, an essential transcription factor for regulating stem cell differentiation. Oct-4 ubiquitin ligase-targeting drugs may prove useful to control stem cell differentiation.