Possible involvement of caspase-like family in maintenance of cytoskeleton integrity.

Caspases, a family of cysteine proteases, are the key effector proteins of apoptosis. These proteases cleave cellular proteins and are responsible for the destruction of the cell body during apoptosis. They are also involved in the activation of other proteins, such as cytokines. In this study, we demonstrate a novel function for these proteases. Z-Asp-CH2-DCB (Z-Asp), a general caspase inhibitor, blocked cell spreading on collagen-coated plates in a dose-dependent manner but did not affect cell viability. Caspase 3-like activity but not caspase 1-like activity was detected in adherent cells on both collagen-coated and poly-L-lysine-coated plates but not in suspended cells. The caspase 3-like activity was significantly inhibited by Z-Asp. However, only Z-Asp, not specific caspase inhibitors (Z-DEVD for caspase 3, Z-YVAD for caspase 1), was effective in the suppression of cell spreading. The inhibitory effect of Z-Asp was blocked by a phosphokinase C activator, PMA, and a Rho activator, lysophosphatidic acid (LPA), while neither a Rac activator, bradykinin, nor a Cdc42 activator, sphingosine-1 -phosphate, was effective. Immunoprecipitation demonstrated that Z-Asp downregulated the expression of focal adhesion kinase (FAK) protein, downstream of Rho signaling, in adherent cells. Our results suggest that not caspase 1 or 3 but another yet unknown caspase(s) plays an important role in the maintenance of cytoskeleton integrity via FAK protein expression, implying a new function for caspases.