Ce. Tomkins et al., APOPTOSIS IS INDUCED IN POSTMITOTIC RAT SYMPATHETIC NEURONS BY ARABINOSIDES AND TOPOISOMERASE-II INHIBITORS IN THE PRESENCE OF NGF, Journal of Cell Science, 107, 1994, pp. 1499-1507
Sympathetic neurons depend on nerve growth factor (NGF) for their surv
ival and die by apoptosis when NGF is withdrawn, despite their post-mi
totic state. Martin et al. (1990, J. Neurosci. 10, 184-193) showed tha
t cytosine arabinoside, but no other arabinofuranosyl nucleoside, coul
d induce cell death in the presence of NGF and they suggested that it
may block a critical step in the NGF-signalling pathway. We show that
cytosine arabinoside is not the only nucleoside capable of inducing ap
optosis in sympathetic neurons in the presence of NGF. In newly isolat
ed neurons from P0 rat pups cultured in the presence of NGF, all the a
rabinose nucleosides (adenine, cytosine, guanine. and thymine) induce
apoptosis at 10 mu M when combined with 5-fluorodeoxyuridine treatment
. Because 1-beta-arabinofuranosylcytosine is associated with double-st
rand breaks and chromosomal abberrations, we examined whether topoisom
erase II inhibitors, which also cause double-strand breaks by stabilis
ing the enzyme-DNA 'cleavable complex', were capable of promoting apop
tosis in these neurons. Although P0 rat neurons are strictly postmitot
ic, topoisomerase II inhibitors teniposide and mitoxantrone induced th
em to die by apoptosis in the presence of NGF with the same apparent t
ime-course as arabinose treatment or NGF withdrawal. By contrast, ICRF
193, a catalytic inhibitor of topoisomerase II, reduced the extent of
apoptosis induced by mitoxantrone or teniposide by 80% if added simul
taneously with the latter but by 2 hours it had no rescue effect, sugg
esting that topoisomerase II is highly active in these neurons. ICRF 1
93 also partially reduced the induction of fluorodeoxyuridine-dependen
t apoptosis by the arabinose nucleosides. These data suggest that indu
ction of double-strand breaks in the DNA or NGF withdrawal activate si
milar apoptotic programmes in sympathetic neurons. We propose that tre
atment with FdUr results in dUTP incorporation in the neurons, that th
e presence of arabinose nucleosides causes disruption of the repair sy
stem that involves topoisomerase II and this causes double-strand brea
ks and thus apoptosis.