Du Laboratory

The major objective of the research conducted in our lab is to understand the cellular processes by which healthy cells become cancerous. More specifically, our work focuses on mechanism of apoptosis, DNA damage response, autophagy and energy metabolism. We use a combination of biochemistry, molecular biology, mouse genetics and bioinformatics to achieve our goals. Our long-term objective is to utilize the knowledge for novel therapies to combat cancer and metabolic diseases. Among possible lines of investigations, we chose to focus primarily on BRUCE, an ubiquitin ligase and inhibitor of apoptosis protein (IAP), and Smac/Diablo, a mitochondrial pro-apoptotic protein and an IAP antagonist, because BRUCE and Smac carry a great deal of promise in regulation of apoptosis, cancer, and metabolic diseases, which are outlined below.

Apoptosis

Apoptosis (programmed cell death) is an essential process for eliminating unwanted cells in development and homeostasis. It is executed through precisely orchestrated biochemical and genetic pathways. Dysregulation of apoptosis contributes to development of human diseases including cancer, autoimmune diseases, and neurodegenerative disorders. Our work highlights the central role of mitochondria in apoptosis and provides anti-cancer therapeutic potential. Acquired resistance toward apoptosis is one of the hallmarks of cancer. IAPs are frequently overexpressed in tumors and believed to contribute to resistance to apoptosis. Overcoming IAP inhibition of apoptosis is a challenge to cancer therapy. Our work has elucidated critical roles of a key mitochondrial pro-apoptotic protein and an IAP-antagonist Smac/Diablo (cell, 2000) in removal the inhibition of apoptosis by IAP. Smac sensitizes cancer cells to cell death at least in part through promoting ubiquitination and proteasomal degradation of two endogenous IAPs, cIAP1 and cIAP2 (JBC, 2004). Further, a mitochondrial serine protease Omi/HtrA2, a functional homolog of Smac, can irreversible inactivate IAP by proteolysis (Genes & Development 2003). In a related study on the IAP BRUCE, we identified a pathway that BRUCE regulates p53 and mitochondrial apoptosis in mice (PNAS, 2005). The scientific relevance of the work is reflected by the therapeutic application of Smac-mimicking small molecule compounds in sensitization of cancer cells apoptosis which is under clinical trials by several companies.

DNA damage response

Cross talk of apoptotic proteins with DNA damage response (DDR) is important for suppression of tumorigenesis. DNA damage response is a collective cellular protective mechanism for detecting and repairing DNA lesions to maintain genomic integrity and to prevent the passage of damaged genetic material to daughter cells. We found a novel phosphorylation site at serine residue S139 of caspase-2 that together with its catalytic site C320 suppress tumor development in nude mice by sustaining G2/M checkpoint and inhibiting NF-kappa B activity. We also found that caspase-2 is significant downregulated in a number of malignancies. This work provided the first instance for any caspases to suppress tumorigenesis through maintaining cell cycle checkpoint (JBC, 2012). In another DDR project, we identified a non-IAP function of BRUCE in regulation of the “BRIT1-SWI-SNF” DSB response pathway and genomic stability. BRIT1/MCPH1 is recruited to DNA double-strand breaks (DSBs) by γ-H2AX where it promotes chromatin relaxation by recruiting SWI-SNF chromatin remodeler to facilitate DDR, discovery by other groups. However, regulation of BRIT1 recruitment is not fully understood. We demonstrate that BRIT1 is K63 ubiquitinated in unstimulated cells and that deubiquitination of BRIT1 is a prerequisite for its recruitment to DSB sites by γ-H2AX. Mechanistically, BRUCE acts as a scaffold, bridging the deubiquitinase USP8 and BRIT1 in a complex to coordinate USP8-catalyzed deubiquitination of BRIT1. Loss of BRUCE or USP8 impairs BRIT1 deubiquitination, BRIT1 binding with γ-H2AX, BRIT1 DNA damage foci formation, and chromatin relaxation.Moreover, BRUCE-depleted cells display reduced homologous recombination repair and BRUCE mutant mice exhibit repair defects and genomic instability. These findings identify BRUCE and USP8 as two hitherto uncharacterized critical DDR regulators and uncover a deubiquitination regulation of BRIT1 assembly at damaged chromatin for efficient DDR and genomic stability. Thus, BRUCE represents a novel component in safeguard of genomic stability (PNAS, 2015). Also see the Research focus diagram up in the front of the lab page.

Autophagy

One ongoing research project is to understand how autophagy regulates liver function and how impaired autophagy links to hepatocellular carcinoma. Autophagy is a cellular catabolic process to maintain tissue and energy homeostasis under normal and stress conditions. Our work is aimed to investigate new regulatory mechanisms that govern autophagy in normal liver homeostasis and malignant hepatocellular carcinoma. This work is also expected to provide insights to other liver diseases including nonalcoholic fatty liver diseases (NAFLD). This study is expected to reveal new regulation of liver diseases and provide insight to identify potential diagnostic and prognostic biomarkers for liver disease.