Mitochondria in eosinophils: Functional role in apoptosis but not respiration

Abstract

In most eukaryotic cells, mitochondria use the respiratory chain to produce a proton gradient, which is then harnessed for the synthesis of ATP. Recently, mitochondrial roles in regulation of apoptosis have been discovered in many cell types. Eosinophils (Eos) die by apoptosis, but the presence and function of mitochondria in Eos are unknown. This study found that Eos contain mitochondria in small numbers, as shown by labeling with membrane potential-sensitive dyes and in situ PCR for a mitochondrial gene. Eos generate mitochondrial membrane potential from hydrolysis of ATP rather than from respiration, as shown by mitochondrial respiratory inhibitors and mitochondrial uncouplers. The mitochondria provide insignificant respiration but can induce apoptosis, as shown by using the mitochondrial F1F0-ATPase inhibitor oligomycin and translocation of cytochrome c. Thus during differentiation of Eos, although respiration is lost, the other central role of mitochondria, the induction of apoptosis, is retained.

Functional characterization of mitochondria in neutrophils: a role restricted to apoptosis

“…It has been hypothesized that eosinophils, close 'relatives' of neutrophils, which also rely on glycolysis for their energy demands, use ATP transported from the cytoplasm into the mitochondria to maintain Dc m . 4 However, this hypothesis seems not to be applicable to neutrophils, because the inhibitor of the mitochondrial adenine translocator, bongkrekic acid, which blocks the ATP transport to the mitochondrial matrix, had no effect on Dc m in neutrophils (not shown). Similarly, inhibition of glycolysis by sodium iodoacetate did not influence Dc m in neutrophils (during a 90 min incubation), despite a 99% depletion of ATP (not shown).…”

“…Increased oxidant injury due to loss of the protective function of MnSOD has been proposed to play a role in glucocorticoid-induced apoptosis of eosinophils, 34 which are believed to have a defective mitochondrial respiration like neutrophils. 4 Neutrophil mitochondria lacking cytochrome c and having no complex IV activity do, however, create Dc m (Figure 3), although the underlying mechanism remains unclear. It has been hypothesized that eosinophils, close 'relatives' of neutrophils, which also rely on glycolysis for their energy demands, use ATP transported from the cytoplasm into the mitochondria to maintain Dc m .…”

“…4 Measurement of enzymatic activity of GDH, LDH and fumarase Cells were lysed in PBS containing 1% Triton-X100 (v/v; T-X100; Sigma, St. Louis, MO, USA), at a concentration of 10-100 Â 10 6 /ml for 10-15 min on ice. After a 5-min centrifugation at 22 000 Â g, the supernatants were collected, and protein concentration was determined by a BCA protein assay kit (Pierce, Rockford, IL, USA).…”

“…2 Moreover, electron microscopy studies identified hardly any mitochondria in neutrophils (see the references in Fossati et al 3 ), and mitochondrial respiration was found to be very low in these cells. 4 However, mitochondria have been recently visualized in neutrophils as a tubular network, by means of specific fluorescent dyes. 3,5 Furthermore, in neutrophils undergoing spontaneous or induced apoptosis, the mitochondria form clusters to which the proapoptotic Bax protein also is localized.…”

Caspase activation in the absence of mitochondrial changes in granulocyte apoptosis

“…Recently, it was reported that eosinophils contain a small number of mitochondria-like structures, lacking functional respiration, but with a suggested role to participate in the apoptotic process [17]. Tributyltin (TBT) is a potent inducer of caspase activation and apoptosis in human leukaemia T cell lines and human peripheral T lymphocytes [18][19][20][21][22].…”