The role of the mitochondrion in organotin-induced T-cell apoptosis

Abstract

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.