Activation of poly(ADP-ribose)polymerase in apoptotic human cells

We have studied the effect of the chemotherapeutic drug VP-16 (etoposide) on the metabolism of HeLa cells by analysing different cellular parameters considered as markers of apoptosis. Typical features such as chromatin condensation and internucleosomal DNA cleavage are visible in HeLa cells exposed to VP-16. We investigated whether the appearance of small-sized DNA fragments could regulate the ADP-ribosylation process. To this purpose, we have analysed, by means of the activity gel technique; the structural and catalytical properties of poly(ADP-ribose)polymerase. In extracts from cells where etoposide-induced DNA fragmentation occurred, we have shown that the label of the autoribosylated form of the enzyme is greatly increased even if the amount of the protein remains constant. This phenomenon is completely abolished in cells preincubated with poly(ADP-ribose)polymerase inhibitor, 3-aminobenzamide. After VP-16 administration, we have observed that the level of NAD is not heavily decreased. It is widely agreed that zinc exerts an inhibitory effect on the endonuclease(s) responsible for the fragmentation of DNA during apoptosis. After incubation of cells with zinc/VP-16 we have found the occurrence of apoptotic parameters even in the absence of internucleosomal DNA cleavage. The inhibition of DNA fragmentation prevents the activation of poly(ADP-ribose)polymerase activity. These results indicate that the activation of the enzyme towards the automodification reaction is strictly dependent on the appearance of DNA internucleosomal fragments and could represent a way to control enzyme activity.