Inhibition of tissue transglutaminase promotes Aβ-induced apoptosis in SH-SY5Y cells.

AIM: Tissue transglutaminase (tTG) catalyzes proteins, including β-amyloid (Aβ), to cross-link as a γ-glutamyl-ε-lysine structure isopeptide, which is highly resistant to proteolysis. Thus, tTG plays an important role in protein accumulation in Alzheimer's disease (AD). In the present study, we examined the effect of an irreversible tTG inhibitor, NTU283, on Aβ mimic-induced AD pathogenesis in SH-SY5Y cells.

METHODS: Western blot and in-cell Western analyses were used to detect tTG and isopeptide (representing the enzyme activity of tTG) protein levels. Moreover, Hoechst and PI co-staining was performed, and caspase-3 and caspase-7 activities and the Bax/Bcl-2 ratio were determined to evaluate the effects of NTU283 on apoptosis.

RESULTS: The results confirmed that tTG activity was inhibited by NTU283 20-500 μmol/L in a concentration-dependent manner in SH-SY5Y cells. Contrary to our expectations, however, the isopeptide bonds were increased when cells were co-treated with Aβ and NTU283. In addition, NTU283 alone did not induce apoptosis in SH-SY5Y cells. However, when co-applied with Aβ, NTU283 promoted rather than inhibited Aβ-induced apoptosis. Consistent with the apoptotic rate, pretreating cells with different concentrations of NTU283 and Aβ significantly increased the activities of caspase-3 and caspase-7 as well as the ratio of Bax/Bcl-2.

CONCLUSION: Irreversible inhibition of tTG activity did not block but rather promoted Aβ-induced apoptosis, which indicated that tTG has complex functions in AD pathogenesis.