Degradation and Dephosphorylation of Focal Adhesion Kinase During Okadaic Acid-Induced Apoptosis in Human Neuroblastoma Cells

Abstract

Focal adhesion kinase (FAK) prevents apoptosis in many cell types. We have reported that tyrosine residues in FAK are dephosphorylated and FAK is degraded during mannitol-induced apoptosis in human neuroblastoma cells. Several studies suggest that FAK dephosphorylation and degradation are separate events. The current study defines the relationship between FAK dephosphorylation and degradation in neuroblastoma cells using okadaic acid (OA). OA, a serine phosphatase inhibitor, promotes serinetthreonine phosphorylation, which in turn blocks tyrosine phosphorylation. OA induced focal adhesion loss, actin cytoskeleton disorganization, cellular detachment, which corresponded to a loss of FAK Tyr397 phosphorylation. These changes preceded caspase-3 activation, Akt and MAP kinase activity loss, protein ubiquitination, cellular apoptosis. Insulin-like growth factor-I prevented mannitol-induced, but not OA-induced, substrate detachment and FAK Tyr397 dephosphorylation, the effects of OA on FAK Tyr397 phosphorylation were irreversible. The proteolytic degradation of FAK is temporally distinct from its tyrosine dephosphorylation, occurring when apoptotic pathways are already initiated and during a generalized destruction of signaling proteins. Therefore, agents resulting in the dephosphorylation of FAK may be beneficial for therapeutic treatment, irrespective of FAK protein levels, as this may result in apoptosis, which cannot be prevented by growth factor signaling.