Overexpression of Tau Proteins Antagonizes Amyloid-beta-Potentiated Apoptosis Through Mitochondria-Caspase-3 Pathway in N2a Cells

It has been a puzzle why the tangle-bearing neurons in Alzheimer's disease (AD) brain do not die preferentially of apoptosis even though they are actually challenged by multiple proapoptotic factors. Recently, we have reported that phosphorylation of tau can antagonize apoptosis induced by exogenous apoptotic inducers. Amyloid-beta (A beta), a recognized endogenous proapoptotic factor, is significantly increased in the AD brains, however, it is not known whether tau could abate the A beta-potentiated apoptosis. Here, we observed that the cells bearing high level of A beta were more vulnerable than the controls to H2O2-induced apoptosis, and this effect of A beta was associated with decrease of Bcl-2, elevation of Bax and cytosolic cytochrome-c, as well as activation of caspase-3, suggesting that A beta could potentiate the oxidant-induced cell apoptosis with involvement of mitochondria-caspase-3 pathway. More importantly, we also found that expression of tau that became hyperphosphorylated could reduce the A beta-potentiated apoptosis with simultaneous preservation of Bcl-2 and suppression of Bax, cytosolic cytochrome-c, and caspase-3 activity, implying that overexpression of tau that became hyperphosphorylated can attenuate the A beta-potentiated cell apoptosis through mitochondria-caspase-3 pathway. These findings provide an explanation of the chronic nature of neurodegeneration of neurons with neurofibrillary pathology of abnormal hyperphosphorylated tau in AD and related tauopathies.