SUMOylation in cell death: SUMO E3 ligase PIAS1 plays a critical role in determining cell survival following UV stress

Abstract

SUMO modification of proteins plays an important role in many cellular processes such as DNA replication, DNA repair, stress response, transcriptional regulation, signal transduction, cell cycle regulation, differentiation and cell death. SUMO modification can have varied effects on its substrates; it may alter subcellular localization, interaction partners or activity of the target proteins. SUMO modifies its substrates by forming an isopeptide bond between its own C-terminal glycine and the ε-amino group of a preferred lysine on the target protein. The modification requires three enzymes, a SUMO activating enzyme E1, a SUMO conjugating enzyme E2, and a SUMO E3 ligase, acting in a cascade. Since all eukaryotes have only one of each of the first two enzymes involved in SUMOylation, the variability in substrate selection and the choice of SUMO paralog is often attributed to a relatively small group of the third enzymes in the SUMOylation pathway, collectively called the SUMO E3 ligases. SUMO modification is also reversible and SUMO can be cleaved off of its substrates by deSUMOylating enzymes. We hypothesized that the variability in SUMOylation substrate selection is guided by the choice of SUMO E3 ligase and each of the five different PIAS isoforms regulates distinct cellular pathways, including response to DNA damage, by SUMOylation of a unique subset of substrates. To verify our hypothesis, we ectopically expressed PIAS E3 ligases in cells and provided various stress stimuli, such as UV irradiation. We then analyzed the response dictated by the expression of each of the PIAS isoforms. We observed that both the ectopic expression of PIAS1 as well as the RNAi-mediated depletion of PIAS1 in cells leads to increased sensitivity of cells to UV irradiation and cell death. This hypersensitivity to UV irradiation is unique to PIAS1 overexpression and is not observed with overexpression of other PIAS isoforms. Each PIAS isoform shows distinct localization in cells and a unique SUMOylation profile. Our results indicate that PIAS1 induced cell death depends on the unique set of proteins it SUMOylates. Domain analysis of PIAS1 implicated the N-terminal SAP domain of PIAS1 in guiding its unique subcellular localization and the determination of substrate specificity. We identified proapoptotic protein Daxx as a mediator of PIAS1 induced apoptosis. Daxx colocalizes with PIAS1 modified SUMO foci in cells and RNAi-mediated depletion of Daxx alleviates UV-induced apoptosis in PIAS1-expresing cells. Daxx is a known transcription co-repressor and mediates its transcription repression activity by interacting with SUMOylated transcription factors, including those required for production of anti-apoptotic proteins. Daxx has two motifs, one at its N-terminus and another at the very C-terminus, called SUMO interacting motifs (SIM), that are known to be important for interaction with SUMOylated proteins. Daxx is proposed to interact with only a subset of SUMOylated proteins; however the basis for this specificity is not clear. Our data suggests that the Daxx C-terminus SIM is essential for interaction with PIAS1-mediated SUMO foci and the resulting cell death. Since cell death is regulated by a fine balance between the levels of pro- and anti-apoptotic proteins in cells, we analyzed the expression of the apoptosis mediators in cells expressing PIAS1. Results from multigene apoptotic reverse-transcription PCR arrays have revealed that the expression of a number of pro- and anti-apoptotic genes is altered in PIAS1-expressing cells. Most of these proteins appear to be targets of the NFκB family of transcription factors. We also observed a direct interaction of the NFκB transcription factor protein RelB with both PIAS1 and Daxx.