The roles of transglutaminase II and its substrates in arsenic trioxide-induced cellular stress responses in Cos-1 cells
Date Issued
2009
Date
2009
Author(s)
Chen, Ting-Ching
Abstract
Arsenic trioxide is an environmental carcinogen which causes diseases in humans. However, it has often been used as an effective treatment for acute promyelocytic leukemia and has the potential to provide a cure for solid malignant tumors. Arsenic trioxide induces cellular toxicity by increasing the level of reactive oxygen species which target DNA, proteins, lipids and mitochondria, resulting in apoptosis. Arsenic trioxide has also been shown to induce ER stress and the formation of stress granules. Using the proteomic approach, we have previously identified the substrates of transglutaminase II (TG2) which include many known components of stress granules. To determine whether TG2 could catalyze the formation of stress granules, we used arsenic trioxide for inducing cellular stress to search for the functional relationships between stress granules and TG2. Using immunofluorescence staining, we found that some substrates of TG2 are localized in stress granules. These include APG-1, Tom34 and PDIA4, all of which are novel components of stress granules. We therefore propose that TG2 could assist the formation of stress granules. However, in vivo and in vitro transamidation experiments showed that arsenic trioxide decreased the catalytic activity of TG2 by reacting with its thiol group, ruling out the possibility of the catalytic role of TG2 in the formation of stress granules. The results suggest that the decreased activity of TG2 could explain for the therapeutic mechanisms of arsenic trioxide in treating PML. Additionally, when Cos-1 cells were treated with arsenic trioxide, TG2 seemed to co-localize with the structure of mitochondria, surrounding the nucleus and the microtubule-organizing center (MTOC). We also detected an increase in the expression of Mitofusin-1 and Mitofusin-2, suggesting the formation of mitochondrial fusion. This phenomenon coincides with increased reactive oxygen species, decreased mitochondrial membrane potential. Furthermore, we also detected apoptosis through the mitochondrial release of cytochrome c. The results provide a new strategy to study the relationship between apoptosis and mitochondrial fusion. Co-immunoprecipitation experiments also demonstrated a strong binding affinity between TG2 and Grp78, implying that Grp78 could be a chaperone for TG2 under conditions of oxidative stress. These observations are interesting, but at this point, we know little about the roles played by TG2 in the formation of mitochondrial fusion.
Subjects
stress granule
reactive oxygen species
mitochondrial membrane potential
mitochondrial fusion
transglutaminase II
arsenic trioxide
SDGs
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