THE ABILITY TO CLEAVE 28S RIBOSOMAL-RNA DURING APOPTOSIS IS A CELL-TYPE DEPENDENT TRAIT UNRELATED TO DNA FRAGMENTATION

Discrete cleavages within 28S rRNA divergent domains have previously b een found to coincide with DNA fragmentation during apoptosis. Here we show that rRNA and DNA cleavages can occur independently in apoptotic cells, i.e. that the previously observed correlation is likely to be coincidental, in HL-60 cells, apoptosis with massive DNA fragmentation could be induced without any signs of rRNA cleavage. The opposite sit uation; rRNA cleavage without concomitant internucleosomal DNA fragmen tation, was found in okadaic acid-treated Molt-4 cells. Other leukemia cell lines underwent apoptosis either without (K562 and Molt-3) or wi th (U937) both forms of polynucelotide cleavage. In K562 cells transfe cted with a temperature-sensitive p53 mutant, internucleosomal DNA fra gmentation but not 28S rRNA cleavage was inducible by wild-type p53 ex pression. The absence of apoptotic rRNA cleavage in some cell types su ggests that this phenomenon is tightly regulated and unrelated to DNA fragmentation or a presumed scheme for general macromolecular degradat ion in apoptotic cells.