The role of ARTS in cell death and cancer
All cells in our body have the ability to self-destruct by activating an intrinsic cell suicide program called “Apoptosis”. In this way, the accumulation of damaged and potentially dangerous cells, such as tumor cells, is greatly limited. Unfortunately, cancer cells can acquire resistance towards cell death by inactivating cell suicide pathways, thereby gaining the ability for “unlimited” life and propagation. However, the ability to undergo apoptosis is not completely lost in cancer cells and most currently used anti-cancer therapies (chemotherapy, radiation) act by activation this apoptosis death pathway in cancer cells.
Role of ARTS in initiating mitochondrial cell death
Our lab focuses on investigating the molecular mechanisms of cell death, and how abnormal regulation of this process contributes to human diseases. We cloned and identified a mitochondrial pro-apoptotic protein termed ARTS (Sept4_i2) that is derived from the Sept4 septin gene by differential splicing (Larisch et al., Nature Cell Biol. 2000). Over-expression of ARTS is sufficient to promote apoptosis in many cultured tumor cells. Conversely, the reduction of endogenous ARTS protein by anti-sense or shRNA methods can protect cells against various pro-apoptotic insults. This indicates that ARTS is important for induction of apoptosis in a wide variety of apoptosis paradigms. A central step for the execution of apoptosis is the activation of caspases, a family of enzymes that when activated bring about the demise of the cell. Caspases are regulated by both activators and inhibitors, such as IAPs (Inhibitor of Apoptosis Proteins). The best characterized member of this family is X-IAP (X-linked IAP) (Gottfried et al.,EMBO Journal 2004). ARTS is located at the mitochondrial outer membrane (MOM) in living cells, but in response to pro-apoptotic stimuli it translocates to the cytosol. This permits ARTS to bind and inhibit XIAP, thereby initiating caspase activation and apoptosis.
ARTS acts upstream of MOMP and is required for the proper initiation time of this process
An important step in mitochondrial apoptosis is the release of Cytochrome C, and SMAC/Diablo, which reside in the intermembrane space (IMS) of mitochondria. This requires opening of pores at the mitochondrial outer membrane (MOM), a process termed MOMP (Mitochondrial Outer Membrane Permeabilization). We have shown that ARTS initiates caspase activation upstream of MOMP. ARTS is localized at the outer membrane of mitochondria (MOM) and is bound to Bcl-2. Following induction of apoptosis, ARTS rapidly translocates to the cytosol in a caspase-independent manner, where it binds XIAP. The translocation of ARTS from mitochondria occurs within minutes following apoptotic stimuli and precedes MOMP and the release of cytoC and SMAC/Diablo seen hours after induction of apoptosis. Moreover, knockdown of ARTS strongly inhibits the release of SMAC and cytoC, suggesting that ARTS is required for the proper timing of MOMP and the release of these proteins (Edison et al. Cell Death and Diff., 2012).
ARTS acts as an antagonist of both major anti-apoptotic proteins XIAP and Bcl-2
Upon induction of apoptosis ARTS and Bcl-2 accumulate in the cytosol, minutes following induction of apoptosis. ARTS binds directly to XIAP and promotes its auto-ubiquitilation and degradation by the proteasome. Binding of ARTS to XIAP allows de-repression of active caspases bound to XIAP. These caspases can now cleave substrates such as Bid (to truncated Bid-tBid) which is known to induce MOMP and the release of Cytochrome c and Smac/Diablo. In addition, ARTS binds directly to the BH3 domain in Bcl-2 enabling the formation of a ternary complex. Thus, ARTS serves as an adaptor protein that brings XIAP containing an E3-ligase activity in close proximity to Bcl-2. This induces the Ubiquitylation of lysine 17 (K17) in Bcl-2 and the degradation of Bcl-2 by the proteasome (Edison et al., Cell Reports 2017).
ARTS functions as a tumor suppressor protein
ARTS expression is frequently lost in Acute Lymphoblastic Leukemia (ALL), in lymphoma patients and in Hepatocellular carcinoma patients (Elhasid et al., Oncogene 2004, Garcia-Fernandez et al, Genes &Development 2010, and unpublished results), indicating that ARTS functions as a tumor-suppressor protein in human cancers. Furthermore, Sept4/ARTS-null mice exhibit spontaneous tumors and show accelerated tumor development in an Eu-Myc background. In addition, Sept4/ARTS-null mice display increased numbers of stem and progenitor cells, elevated XIAP and Bcl-2 proteins and increased resistance to cell death. Taken together, these results firmly establish the physiological role of ARTS as an XIAP and Bcl-2 antagonist and a tumor suppressor protein.
Our lab currently focuses on investigating the following projects:
- Investigating molecular mechanism of ARTS-induced apoptosis. Our goal is to determine how the binding of ARTS to various apoptotic regulators lead to caspase activation and cell death. In particular, we are focusing on the role of ARTS as an adaptor which enables the interaction between various E3-ligases to their substrates culminating in apoptosis.
- Role of ARTS as a tumor suppressor in a wide variety of cancer models, including hematopoietic cancers, breast cancer, melanoma etc.
- Identifying and developing small-molecule ARTS- mimetics that selectively kill cancer cells and could serve as anti-cancer drugs.