Tissue transglutaminase differentially modulates apoptosis in a stimuli‐dependent manner

Abstract

Tissue transglutaminase is a unique member of the transglutaminase family as it not only catalyzes a transamidating reaction, but also binds and hydrolyzes GTP and ATP. Tissue transglutaminase has been reported to be pro-apoptotic, however, conclusive evidence is still lacking. To elucidate the role of tissue transglutaminase in the apoptotic process human neuroblastoma SH-SY5Y cells were stably transfected with vector only (SH/pcDNA), wild-type tissue transglutaminase (SH/tTG) and tissue transglutaminase that has no transamidating activity but retains its other functions (SH/C277S). In these studies three different apoptotic stimuli were used osmotic stress, staurosporine treatment and heat shock to delineate the role of tissue transglutaminase as a transamidating enzyme in the apoptotic process. In SH/tTG cells, osmotic stress and staurosporine treatments resulted in significantly greater caspase-3 activation and apoptotic nuclear changes then in SH/pcDNA or SH/C277S cells. This potentiation of apoptosis in SH/tTG cells was concomitant with a significant increase in the in situ transamidating activity of tissue transglutaminase. However, in the heat shock paradigm, which did not result in any increase in the transamidating activity in SH/tTG cells, there was a significant attenuation of caspase-3 activity, LDH release and apoptotic chromatin condensation in SH/tTG and SH/C277S cells compared with SH/pcDNA cells. These findings indicate for the first time that the effect of tissue transglutaminase on the apoptotic process is highly dependent on the type of the stimuli and how the transamidating activity of the enzyme is affected. Tissue transglutaminase facilitates apoptosis in response to stressors that result in an increase in the transamidating activity of the enzyme. However, when the stressors do not result in an increase in the transamidating activity of tissue transglutaminase, than tissue transglutaminase can ameliorate the apoptotic response through a mechanism that is independent of its transamidating function. Further, neither the phosphatidylinositol-3-kinase pathway nor the extracellular-regulated kinase pathway is downstream of the modulatory effects of wild-type tissue transglutaminase or C277S-tissue transglutaminase in the apoptotic cascade.

Intracellular Localization and Activity State of Tissue Transglutaminase Differentially Impacts Cell Death

“…In this study we demonstrate that when tTG is localized mostly in the cytosol, it facilitates apoptosis. Further, and in agreement with a previous study (15), we found that the transamidating activity of tTG is necessary for its pro-apoptotic activity in the cytosol. Both membrane and nuclear-localized wild-type tTG did not affect apoptosis, even though thapsigargin treatment resulted in a significant increase in the transamidating activity of these constructs.…”

“…However recent studies have provided evidence that tTG can also attenuate apoptosis (14 -16). Interestingly, it has been recently demonstrated that tTG differentially modulates apoptosis in a stimuli-dependent manner (15). This study demonstrated that when the stressor increased the transamidating activity of the enzyme, tTG was pro-apoptotic.…”

“…This study demonstrated that when the stressor increased the transamidating activity of the enzyme, tTG was pro-apoptotic. However, tTG ameliorated apoptosis when the stressor did not result in an increase in the transamidating activity (15). These findings suggest that the effects of tTG on apoptosis are complex and likely due in part to its multifunctional character.…”

“…Several studies have suggested that tTG is pro-apoptotic (21,22) while other studies have shown that tTG attenuates apoptosis (14 -16). Interestingly, it has been shown that the effects of tTG on apoptosis may depend on the type of apoptotic inducer and how the stress impacts the activity of tTG (15). To investigate how the localization and activity of tTG affects apoptosis, HEK293 cells were transiently transfected with the different tTG constructs and subsequently treated with thapsigargin (or vehicle only) prior to measuring caspase activity as an indicator of apoptosis.…”

“…Although Rb was degraded 24 h after treatment with thapsigargin, the results of these studies revealed the extent of Rb degradation was not different between NLS-tTG (0.58 Ϯ 0.15, n ϭ 3 experiments) and NLS-C277S (0.69 Ϯ 0.21, n ϭ 3 experiments)-transfected cells, indicating that NLS-C277S is not attenuating apoptosis by increasing the stability of Rb. DISCUSSION Tissue TG has been shown to be both pro-and anti-apoptotic (15,16,21). Indeed tTG is increased both in conditions such as neurodegenerative disorders where there is increased cell death in specific brain regions (51)(52)(53), and in some tumors which are characterized by a lack of apoptosis (54,55).…”