Mutant H-ras over-expression inhibits a random apoptotic nuclease in myeloid leukemia cells.

Cell suicide, or apoptosis, is now recognized as an essential regulatory step in such diverse developmental processes as embryogenesis, thymocyte restriction, and hematopoiesis. One of the major features of apoptosis is the activation of an endogenous nuclease that cleaves DNA into nucleosomal fragments. Little is known about the activation or specificity of the apoptotic endonuclease. In this study, we investigated signalling pathways and the specificity of the apoptotic nuclease. We found that forced over-expression of activated H-ras inhibited activation of the apoptotic endonuclease. Since a high percentage of myelodysplasias and leukemias have mutations that activate ras, this finding lends insight into how ras might be leukemogenic. In addition, the phorbol ester TPA and a cyclic AMP analogue also slowed activation of this endonuclease. Interestingly, protein synthesis inhibition stimulated the endonuclease activity. In addition, by cloning and sequencing apoptotic fragments we found that the apoptotic nuclease has no sequence specificity. Thus, the apoptotic nuclease inhibited by H-ras over-expression was random in nature.