intracellular pH, matrix alkalinization, and cytosolic acidification. This could be the
result of a disturbance of the mitochondrial respiration or of the ATPase. Oligomycin,
an inhibitor of the F0F1-ATPase, binds to the F 0 subunit and inhibits the transport
of H+ ions, effectively shutting off the ATPase (Tzagoloff, 1970). Oligomycin was
shown to block apoptosis induced by etoposide and dexamethasone, which involves
the Bax pathway (Eguchi et al., 1997; Leist et al., 1997). Another study showed that
oligomycin inhibits both acidification of the cytosol and the release of cytochrome c
(Matsuyama et al., 2000). Changes in intracellular pH are not detected in death-
receptor apoptosis. Oligomycin has no effect on apoptosis triggered by anti-Fas
antibodies, indicating that the effect is not a general inhibition of apoptosis, but
specific to the Bax-dependent pathway. When Bax was expressed in wild-type yeast,
cytosolic acidification, matrix alkalinization, and cell death were seen, but no effects
were detected in F0F1-ATPase/H+ pump-deficient yeast (Matsuyama et al., 2000).
In one study, calphostin c was shown to induce Bax translocation, homocomplex
formation, cytochrome c release, and decrease of the mitochondrial membrane
potential (Ikemoto et al., 2000). Oligomycin inhibited Bax homodimerization and
the decrease in membrane potential. In a study of Bax toxicity to yeast, it was shown
that mutations of the mitochondrial proteins required for oxidative phosphorylation,
which make the cells respiratory incompetent, decreased Bax toxicity (Harris et al.,
2000). However, mutations in mitochondrial proteins unrelated to the oxidative
phosphorylation machinery did not affect Bax toxicity. Taken together, these results
suggest that Bax and perhaps other multidomain proteins, in addition to their
permeabilizing effect on the outer mitochondrial membrane, also have effects on
components of the inner membrane, interfering with the respiration and/or ATP
production. Whether these effects are reversible remains unclear. This is an important
issue if we want to modulate apoptosis in diseases. Would inhibition of downstream
caspases rescue the cells if severe irreversible damage has been inflicted on the
mitochondrial respiration at an early point in the apoptotic signaling cascade? In a
recent study, neurons and HeLa cells were exposed to apoptotic stimuli in the presence
of BAF, a broad-spectrum caspase inhibitor (Xue et al., 2001). Although the caspase
inhibitor ensured shortterm survival, mitochondria were selectively eliminated from
the cells without any apparent effects on other intracellular structures. Cells deprived
of mitochondria were irreversibly committed to death. In neurons, elimination of
MITOCHONDRIA IN APOPTOSIS INDUCTION 139mitochondrial membrane. Activation of the Bax pathway induces changes in
mitochondria was completely prevented by expression of Bcl-2, which works
upstream of the mitochondria.11.
Conclusion
During the last few years, we have gained an enormous amount of information about
apoptosis and in particular the mitochondrial apoptosis pathway. But much still
remains unclear. The central role of the mitochondria and some mitochondrial
proteins, particularly cytochrome c, is now fairly well understood. That proteins of