Alterations in focal adhesion and cytoskeletal proteins during apoptosis.

Ceramide induces cell rounding and subsequent apoptotic cell death in trigeminal neurinoma 476-16 cells. A protein tyrosine phosphatase inhibitor, orthovanadate, inhibits cell rounding and subsequent apoptotic death, while a serine/threonine phosphatase inhibitor, calyculin A, stimulates cell rounding but inhibits apoptosis (reference 11). In an attempt to determine critical cellular changes associated with cell rounding during the induction of apoptosis, focal adhesion and cytoskeletal proteins in apoptotic round cells induced by ceramide were examined by immunoblotting and compared with those of non-apoptotic round cells and adherent cells. As compared with adherent cells, tyrosine phosphorylation of a group of proteins between 110-125 KDa, including p125 focal adhesion kinase (FAK) is reduced in the apoptotic round cells as well as in non-apoptotic round cells induced by calyculin A and metaphase cells in mitosis. However, a concerted decrease of vinculin, paxillin and FAK, preceding the changes of whole cellular proteins, is seen in the apoptotic round cells but not in the non-apoptotic round cells. The inhibition of ceramide-induced apoptosis by orthovanadate is accompanied by a prevention of such a decrease in focal adhesion proteins. It thus appears that these focal adhesion proteins are degraded during the cell rounding occurring during apoptosis. Proteolysis of focal adhesion components may not only irreversibly disrupt cell adhesion but also impede transduction of growth and survival signals, and may play a critical role in the initiation and execution of apoptosis.