Hyperosmotic stress-induced apoptosis and tau phosphorylation in human neuroblastoma cells

A characteristic hallmark of Alzheimer's disease brain is the presence of hyperphosphorylated tau; however, the mechanisms responsible for the aberrant tau phosphorylation are unknown. Recently, it has been shown that apoptotic-like processes may be involved in some of the neuronal loss in Alzheimer's disease. In consideration of these findings, the relationship between tau phosphorylation and apoptosis was examined in human neuroblastoma SH-SY5Y cells that were subjected to hyperosmotic stress. In this model caspase 3 activity, which served as an indicator of apoptosis, was increased by 30 min of osmotic stress and remained elevated through 4 hr. Hyperosmotic stress also resulted in a robust increase in tau phosphorylation at both Ser/Pro and non-Ser/Pro sites. Phosphorylation of Ser262/356 (12E8) and Ser396/404 (PHF-1) increased by 5 min and remained elevated for at least 1 hr. In contrast, phosphorylation within the Tau-1 epitope did not increase (as evidenced by decreased immunoreactivity) until 30 min after treatment but remained elevated for a much greater period of time. Treatment with insulin-like growth factor-1 delayed but did not prevent apoptotic cell death induced by osmotic stress and attenuated the increase in phosphorylation at the Tau-1 epitope. Li(+), an inhibitor of glycogen synthase kinase 3 beta, had no effect on osmotic stress-induced caspase activation, but reduced phosphorylation at the Tau-1 epitope. Complete inhibition of osmotic stress-induced caspase activation with DEVD-CHO had no effect on the increases in tau phosphorylation. The results of these studies demonstrate that tau phosphorylation is increased at the specific epitopes during apoptosis. However, the changes in tau phosphorylation likely do not significantly impact the apoptotic process but rather occur concurrently as a result of inappropriate activation of specific protein kinases. Nonetheless, there is increasing evidence of a dysregulation of protein kinases that occurs in Alzheimer's disease brain that may be part of the events of apoptosis, which could contribute to aberrant increases in tau phosphorylation.