The Cleavage of Nuclear DNA into High Molecular Weight DNA Fragments Occurs Not Only during Apoptosis but Also Accompanies Changes in Functional Activity of the Nonapoptotic Cells

Abstract

In this paper we demonstrate that apoptosis in primary culture of murine thymocytes and in continuously growing human cells is associated with the progressive disintegration of nuclear DNA into high molecular weight (HMW)–DNA fragments of about 50–150 kb. We also show that the formation of similarly sized HMW–DNA fragments takes place in the same cells in the absence of apoptotic inducers. Unlike an apoptotic fragmentation of nuclear DNA, the formation of HMW–DNA fragments in nonapoptotic cells is rapidly induced, has no correlation with the cell death, and is not associated with the development of oligonucleosomal “ladder” or apoptotic changes in nuclear morphology. The disintegration of DNA into HMW-fragments is also observed in nuclei isolated from healthy, nonapoptosizing tissues of various eukaryotes. We show that the formation of HMW–DNA fragments in the absence of apoptotic inducers is strongly dependent on the ionic detergents, is responsive to the topoisomerase II-specific poison, teniposide, and is completely reversible under conditions that favor topoisomerase II-dependent rejoining reaction. Also, we demonstrate that the formation of HMW–DNA fragments in continuously growing cell lines caused either by serum deprivation or monolayer establishment is of a transient nature and rapidly reverses to the control level following serum addition or dilution of monolayer. The results suggest that the cleavage of nuclear DNA into HMW–DNA fragments is associated not only with apoptosis but also accompanies changes in functional activity of nonapoptotic cells.