Importance of poly(ADP-ribose) polymerase and its cleavage in apoptosis - Lesson from an uncleavable mutant

We have studied the apoptotic response of poly(ADP-ribose) polymerase (PARP )-/- cells to different inducers and the consequences of the expression of an uncleavable mutant of PARP on the apoptotic process, The absence of PARP drastically increases the sensitivity of primary bone marrow PARP-/- cells to apoptosis induced by an alkylating agent but not by a topoisomerase I i nhibitor CPT-11 or by interleukin-3 removal. cDNA of wild type or of an unc leavable PARP mutant (D214A-PARP) has been introduced into PARP-/- fibrobla sts, which were exposed to anti-CD95 or an alkylating agent to induce apopt osis. The expression of D214A-PARP results in a significant delay of cell d eath upon CD95 stimulation, Morphological analysis shows a retarded cell sh rinkage and nuclear condensation. Upon treatment with an alkylating agent, expression of wild-type PARP cDNA into PARP-deficient mouse embryonic fibro blasts results in the restoration of the cell viability, and the D214A-PARP mutant had no further effect on cell recovery. In conclusion, PARP-/- cell s are extremely sensitive to apoptosis induced by triggers (like alkylating agents), which activates the base excision repair pathway of DNA, and the cleavage of PARP during apoptosis facilitates cellular disassembly and ensu res the completion and irreversibility of the process.