Caspase-mediated Cleavage of p130cas in Etoposide-induced Apoptotic Rat-1 Cells

Abstract

Apoptosis causes characteristic morphological changes in cells, including membrane blebbing, cell detachment from the extracellular matrix, and loss of cell–cell contacts. We investigated the changes in focal adhesion proteins during etoposide-induced apoptosis in Rat-1 cells and found that during apoptosis, p130cas (Crk-associated substrate [Cas]) is cleaved by caspase-3. Sequence analysis showed that Cas contains 10 DXXD consensus sites preferred by caspase-3. We identified two of these sites (DVPD⁴¹⁶G and DSPD⁷⁴⁸G) in vitro, and point mutations substituting the Asp of DVPD⁴¹⁶G and DSPD⁷⁴⁸G with Glu blocked caspase-3-mediated cleavage. Cleavage at DVPD⁴¹⁶G generated a 74-kDa fragment, which was in turn cleaved at DSPD⁷⁴⁸G, yielding 47- and 31-kDa fragments. Immunofluorescence microscopy revealed well-developed focal adhesion sites in control cells that dramatically declined in number in etoposide-treated cells. Cas cleavage correlated temporally with the onset of apoptosis and coincided with the loss of p125FAK (focal adhesion kinase [FAK]) from focal adhesion sites and the attenuation of Cas–paxillin interactions. Considering that Cas associates with FAK, paxillin, and other molecules involved in the integrin signaling pathway, these results suggest that caspase-mediated cleavage of Cas contributes to the disassembly of focal adhesion complexes and interrupts survival signals from the extracellular matrix.