Our laboratory studies cancer cell stress response with three main focuses: 1) Mechanism of proteotoxicity (the cytotoxic effect of misfolded proteins); 2) Regulation of protein and energy homeostasis by PI3 kinases; 3) Mechanism for the tumor promoting function of misfolded protein stress. We use mostly biochemical, molecular biological, and cell biological techniques, as well as mouse models and clinical samples.
1. Proteotoxic stress and signaling
Inhibition of the protein degradation is an emerging anti-cancer strategy. The proteasome inhibitor bortezmib has been approved by FDA for treatment of multiple myeloma, and is being trialed in numerous cancers. However, the molecular mechanisms underlying proteotoxicity remain elusive. We recently reported that proteasome inhibitors can induce an intracellular aggregation and activation of caspase-8 and subsequent apoptosis. This caspase-8 activation is mediated by its association with a ubiquitin-binding protein SQSTM1/p62 and an autophagy-related protein microtubule-associated protein light chain 3 (LC3). These findings prompt us to hypothesize that there exists a previously uncharacterized cell death pathway involving protein aggregate formation and intracellular activation of caspase-8. Along this direction, our additional preliminary results show that p62 itself undergoes ubiquitination. This novel modification of p62 may play a critical function in mediating aggregate formation and apoptosis. To understand the molecular mechanisms underlying the anti-tumor effect of proteasome inhibitors, we are currently pursuing three specific research aims: 1) Identify and characterize the caspase-8 containing protein aggregates induced by proteasomal inhibition. 2) Study how p62 and LC3 regulate aggregate formation and caspase-8 activation. 3) Study whether LC3 can sensitize tumors to proteasome inhibition in animal tumor models.
2. Phosphatidylinositol 3-kinases and autophagy
The phosphatidylinositol 3-kinases (PI3Ks) are lipid kinases that phosphorylate the 3’-hydroxyl group of phosphatidylinositol (PIs) and phosphoinositides. Based on substrate specificity and sequence homology, PI3Ks are grouped into three classes: Class I, Class II, and Class III. In vivo, Class I PI3Ks are believed to preferentially phosphorylate PtdIns(4,5)P2 to generate PI(3,4,5)P3, a pivotal signaling molecule that activates multiple downstream signaling cascades, including the Akt/TOR pathway. Class III PI3K is composed of a sole member, Vps34, that converts PtdIns to PI(3)P. An important cellular process controlled by PI3Ks is autophagy, which is involved in many physiological and pathological conditions. The direct role of PI3Ks, especially that of the Class IA PI3Ks, in autophagy remains unclear. In this project, we study the molecular mechanisms underlying the autophagy-promoting function of p110β, and to explore its biological roles. We also use tissue-specific Vps34 knockout mice to study Vps34 and its interplay with p110β in regulating autophagy.
3. Squamous cell carcinoma antigen (SCCA) in tumorigenesis
Squamous cell carcinoma antigens (SCCAs) are members of the Serpin family of endogenous serine and cysteine protease inhibitors. Elevated expression of SCCA has been found to associate with poorly differentiated and advanced squamous cell carcinomas of the uterine cervix, lung, head and neck, esophagus, liver, and breast. However, despite its relevance with human cancer, the biological function of SCCA remains largely unclear. We are currently studying how SCCA functions to promote tumorigenesis and can be targeted for therapy. We hypothesize that 1) SCCA on one hand promotes tumorigenesis by protecting cells against lysosomal injury, on the other hand it sensitizes cells to proteotoxic stress by blocking protein degradation machinery; and 2) SCCA promotes tumorigenesis by preventing autophago-lysosomal turnover, which leads to a chronic unfolded protein response that triggers a tumor-promoting signaling pathway. We are currently studying how SCCA regulates the above molecular functions, and have developed a LoxP-Stop-LosP conditional transgenic mouse strain to study the effect of SCCA on tumorigenesis and anti-cancer therapy in vivo.