A role for Ptdlns(4,5)P2 and PIP5Kα in regulating stress-induced apoptosis

The phosphoinositide phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P-2) is essential for many cellular processes and is linked to the etiology of numerous human diseases [1-4]. PtdIns(4,5)P-2 has been indirectly implicated as a negative regulator of apoptosis [5-9]; however, it is unclear if apoptotic stimuli negatively regulate PtdIns(4,5)P-2 levels in vivo. Here, we show that two apoptotic-stress stimuli, hydrogen peroxide (H2O2) and UV irradiation, cause PtdIns(4,5)P-2 depletion during programmed cell death independently of and prior to caspase activation. Depletion of PtdIns(4,5)P-2 is essential for apoptosis because maintenance of PtdIns(4,5)P-2 levels by overexpression of PIP5K alpha rescues cells from H2O2-induced apoptosis. PIP5K alpha expression promotes both basal and sustained ERK1/2 activation after H2O2 treatment, and importantly, pharmacological inhibition of ERK1/2 signaling blocks PIP5K alpha-mediated cell survival. H2O2 induces tyrosine phosphorylation and translocation of PIP5K alpha away from its substrate at the plasma membrane, and both are dependent upon the activity of c-src family kinases. Furthermore, constitutively active c-src enhances tyrosine phosphorylation of PIP5K alpha in vivo and is sufficient for the translocation of PIP5K alpha away from the plasma membrane. These observations demonstrate that certain apoptotic stimuli initiate an essential signaling pathway during cell death, and this pathway leads to caspase-independent downregulation of PIP5K alpha and its product PtdIns(4,5)P-2.