Caspase-mediated apoptosis in AK-5 tumor cells: a cell-free study using peptide inhibitors and antisense strategy.

An in vitro system has been employed to study the apoptotic mechanisms in the AK-5 tumor which is a spontaneously regressing rat histiocytoma. Cytosolic extracts of tumor cells primed for apoptosis using dexamethasone and immune serum from tumor-regressing animals were able to induce apoptosis in intact nuclei and reproduce the classical morphological and biochemical features typical of apoptotic cells. The cleavage of lamin A and PARP to signature fragments by these extracts and the inhibition of the same using peptide inhibitors signify the pivotal role of ICE and ICE-related proteases in apoptosis. Lamin A cleavage was insensitive to YVAD but PARP cleavage was blocked by both YVAD and DEVD. Cell extracts derived from cells overexpressing the Bcl-2 gene and Nedd-2 antisense gene, respectively, failed to induce apoptosis in exogenously added nuclei, suggesting that Bcl-2 gene product is downregulating a key event in apoptotic cascade. The study also demonstrates the coherent action of different ICE-related proteases in apoptosis and their functional redundancy. This system may prove useful for analyzing complex molecular mechanisms underlying apoptosis in tumor cells.