6 research outputs found
The role of the mitochondrion in organotin-induced T-cell apoptosis
Apoptosis plays a central role in the development and the function of the
immune system. The cell death program can, however, be inappropriately
activated or suppressed under pathological conditions. Several toxins
have been shown to interfere with intra-thymic processes, thereby
contributing to autoimmune disorders. In this study, we wanted to
investigate the interaction between organotin compounds and the induction
of various cell death pathways in human T lymphocytes.
A critical mechanism in apoptosis is the activation of the pro-caspases.
Here we show that tributyltin (TBT)- and triphenyltin (TPI)- induced
apoptosis was associated with type 11 caspase activation in human Jurkat
T cells. High concentrations of the compounds were able to interact with
vicinal thiol groups in cellular proteins, providing a possible molecular
mechanism by which the organotins inhibit the caspase activity, thereby
directing the cells towards necrotic death instead.
Recent studies have pointed out the impact of the mitochondria in the
initiation of apoptosis and the release of cytochrome c has been shown to
be an essential step in the activation of the caspases. We show that
TBT-induced apoptosis was associated with an early release of cytochrome
c, concomitant with the loss of mitochondrial membrane potential
([delta][psi]m), indicative of mitochondrial permeability transition
(NTT). These events were temporally related to the activation of the
caspases. The mitochondrial events could be blocked with Bongkrekic acid,
a substance binding to the mitochondrial adenine nucleotide translocator
(ANT), thereby inhibiting MPT. Moreover, we could modulate the mode of
cell death by manipulating intracellular ATP levels prior to
TBT-exposure. During ATP-maintaining conditions, TBT-treated cells were
typically apoptotic but, during ATP-limiting conditions, the cells were
converted to deletion by necrotic processes.
We could also show that the TBT-induced loss of [delta][psi]m, and MPT,
are not directly coupled to the TBT-induced rise in cytosolic Ca2+ and,
from this, we hypothesize that TBT might specifically interact with
mitochondrial ANT, thereby inducing MPT. However, Ca2+ is a prerequisite
for caspase activation and apoptosis, but at a step down-stream of the
mitochondrion. Finally, we wanted to study how these potent immunotoxins
affect mature human peripheral T-lymphocytes and we have shown that TBT
is, indeed, a potent inducer of apoptosis in these cells, at even lower
concentrations than required with Jurkat cells. Heterogeneity in the
response was seen, with the CD8+ subpopulation being more resistant than
the CD4+ cells towards TBT-induced apoptosis.
Peripheral blood T lymphocytes, stimulated with anti-CD3, exhibit a
time-dependent induction of type Il caspase activity in the control
cells, in the absence of any apoptotic morphology. No further increase in
caspase activation or induction of apoptosis results after TBT-treatment
in these stimulated T-cells. Instead, the cells were directed towards
necrotic deletion. In conclusion, the essence of our data is that the
mitochondrion seems to be the primary site for the onset of TBT-induced
apoptosis in human lymphocytes. TBT interacts rapidly with the
mitochondrion and induces MPT and releases cytochrome c that participates
in the type II caspase activation. These mitochondrial events are not a
consequence of elevated cytosolic Ca2+ levels. Instead, we hypothesis
that TBT interacts directly with a component of the pore transition
complex. However, elevated cytosolic Ca2+ levels required for correct
caspase activation in the overall mechanism of T-cell apoptosis, but at a
level downstream from the mitochondrion
Tributyltin-Induced Apoptosis Requires Glycolytic Adenosine Trisphosphate Production
The toxicity of tributyltin chloride (TBT) involves Ca2+ overload, cytoskeletal damage, and mitochondrial failure leading to cell death by apoptosis or necrosis. Here, we examined whether the intracellular ATP level modulates the mode of cell death after exposure to TBT. When Jurkat cells were energized by the mitochondrial substrate, pyruvate, low concentrations of TBT (1-2 ĂM) triggered an immediate depletion of intracellular ATP followed by necrotic death. When ATP levels were maintained by the addition of glucose, the mode of cell death was typically apoptotic. Glycolytic ATP production was required for apoptosis at two distinct steps. First, maintenance of adequate ATP levels accelerated the decrease of mitochondrial membrane potential, and the release of the intermembrane proteins adenylate kinase and cytochrome c from mitochondria. A possible role of the adenine nucleotide exchanger in this first ATPdependent step is suggested by experiments performed with the specific inhibitor, bongkrekic acid. This substance delayed cytochrome c release in a manner similar to that caused by ATP depletion. Second, caspase activation following cytochrome c release was only observed in ATPcontaining cells. Bcl-2 had only a minor effect on TBT-triggered caspase activation or cell death. We conclude that intracellular ATP concentrations control the mode of cell death in TBTtreated Jurkat cells at both the mitochondrial and caspase activation levels