Ubiquitin dependent proteolysis is activated in apoptotic fibroblasts in culture

The ubiquitin mediated pathway constitutes an early response in cultured ce lls where apoptosis, assessed by internucleosomal specific DNA fragmentatio n, was induced by serum withdrawal. Data demonstrate that nuclear ubiquitin proteolytic system, but not cytoplasmic, is activated. This activation is paralleled by a substantial chromatin de-condensation. We suggest that chro matin relaxation is causative of the fragmentation since it exposes the DNA to nucleolytic attack. Finally, maintenance of homeostasis and induction o f apoptosis seem to undergo a parallel contemporary pathway with a possible mutual feedback.