Single-strand DNase and poly rAase, activities characteristic of endo-exonu
clease, were co-activated in nuclear fractions of HL-60 cells by caspase-3.
Activation was accompanied by cleavages of large soluble polypeptides (130
-185 kDa) and a 65 kDa inactive chromatin-associated polypeptide related to
the endo-exonuclease of Neurospora crassa as detected on immunoblots. The
major products seen in vitro were a 77 kDa soluble polypeptide and an activ
e chromatin-associated 34 kDa polypeptide. When HL-60 cells were induced to
undergo apoptosis by treating with 50 mu M etoposide (VP-16) for 4 hours,
77 kDa and 40 kDa polypeptides accumulated in nuclear fractions. Chromatin
DNA fragmentation activity was also activated in cytosol and nuclear extrac
t either by pre-treating the cells in vivo with VP-16 or by treating the cy
tosol in vitro with caspase-3 or dATP and cytochrome c. Endo-exonuclease ac
tivated by caspase-3 in cytosol-derived fractions augmented chromatin DNA f
ragmentation activity in vitro. Endo-exonuclease is proposed to act in vivo
in conjunction with the caspase-activated DNase (CAD) to degrade chromatin
DNA during apoptosis of HL-60 cells.