Identification of keratins 18, 19 and heat-shock protein 90 beta as candidate substrates of proteolysis during ionizing radiation-induced apoptosis of estrogen-receptor negative breast tumor cells.

Induction of apoptosis in the estrogen-receptor negative MDA-MB-468 breast tumor cells has been demonstrated by treatments with cytotoxic agents and growth factors. Using these breast tumor cells, we studied ionizing radiation-induced apoptosis. 2D-PAGE of apoptotic cells indicated keratins 18, 19 and heat-shock protein 90 as candidate substrates of apoptosis-associated proteolysis. At the same time, a motif search revealed possible cleavage-sites in keratins 18, 19 (VEVD) and hsp-90 (DEED) that would yield polypeptides of molecular sizes observed experimentally by immunoblotting with specific antisera. This study provides evidence that the insoluble network of intermediate filament proteins of epithelial cells (keratins), and the associated proteins (heat-shock protein 90) constitute targets of caspase-mediated proteolysis during apoptosis triggered by ionizing radiation.