TNFα-induced lysosomal membrane permeability is downstream of MOMP and triggered by caspase-mediated NDUFS1 cleavage and ROS formation.

When NF-κB activation or protein synthesis is inhibited, tumor necrosis factor alpha (TNFα) can induce apoptosis through Bax- and Bak-mediated mitochondrial outer membrane permeabilization (MOMP) leading to caspase-3 activation. Additionally, previous studies have implicated lysosomal membrane permeability (LMP) and formation of reactive oxygen species (ROS) as early steps of TNFα-induced apoptosis. However, how these two events connect to MOMP and caspase-3 activation has been largely debated. Here, we present the novel finding that LMP induced by the addition of TNFα plus cycloheximide (CHX), the release of lysosomal cathepsins and ROS formation do not occur upstream but downstream of MOMP and require the caspase-3-mediated cleavage of the p75 NDUFS1 subunit of respiratory complex I. Both a caspase non-cleavable p75 mutant and the mitochondrially localized antioxidant MitoQ prevent LMP mediated by TNFα plus CHX and partially interfere with apoptosis induction. Moreover, LMP is completely blocked in cells deficient in both Bax and Bak, Apaf-1, caspase-9 or both caspase-3 and -7. Thus, after MOMP, active caspase-3 exerts a feedback action on complex I to produce ROS. ROS then provoke LMP, cathepsin release and further caspase activation to amplify TNFα apoptosis signaling.

Granzyme B-induced and caspase 3-dependent cleavage of gelsolin by mouse cytotoxic T cells modifies cytoskeleton dynamics.

Granule-associated perforin and granzymes (gzms) are key effector molecules of cytotoxic T lymphocytes (Tc cells) and natural killer cells and play a critical role in the control of intracellular pathogens and cancer. Based on the notion that many gzms, including A, B, C, K, H, and M exhibit cytotoxic activity in vitro, all gzms are believed to serve a similar function in vivo. However, more recent evidence supports the concept that gzms are not unidimensional but, rather, possess non-cytotoxic potential, including stimulation of pro-inflammatory cytokines and anti-viral activities. The present study shows that isolated mouse gzmB cleaves the actin-severing mouse protein, cytoplasmic gelsolin (c-gelsolin) in vitro. However, when delivered to intact target cells by ex vivo immune Tc cells, gzmB mediates c-gelsolin proteolysis via activation of caspases 3/7. The NH(2)-terminal c-gelsolin fragment generated by either gzmB or caspase 3 in vitro constitutively severs actin filaments without destroying the target cells. The observation that gzmB secreted by Tc cells initiates a caspase cascade that disintegrates the actin cytoskeleton in target cells suggests that this intracellular process may contribute to anti-viral host defense.

Non-caspase proteases: triggers or amplifiers of apoptosis?

Caspases are the most important effectors of apoptosis, the major form of programmed cell death (PCD) in multicellular organisms. This is best reflected by the appearance of serious development defects in mice deficient for caspase-8, -9, and -3. Meanwhile, caspase-independent PCD, mediated by other proteases or signaling components has been described in numerous publications. Although we do not doubt that such cell death exists, we propose that it has evolved later during evolution and is most likely not designed to execute, but to amplify and speed-up caspase-dependent cell death. This review shall provide evidence for such a concept.

Improved HSC reconstitution and protection from inflammatory stress and chemotherapy in mice lacking granzyme B.

The serine protease granzyme B (GzmB) is stored in the granules of cytotoxic T and NK cells and facilitates immune-mediated destruction of virus-infected cells. In this study, we use genetic tools to report novel roles for GzmB as an important regulator of hematopoietic stem cell (HSC) function in response to stress. HSCs lacking the GzmB gene show improved bone marrow (BM) reconstitution associated with increased HSC proliferation and mitochondrial activity. In addition, recipients deficient in GzmB support superior engraftment of wild-type HSCs compared with hosts with normal BM niches. Stimulation of mice with lipopolysaccharide strongly induced GzmB protein expression in HSCs, which was mediated by the TLR4-TRIF-p65 NF-κB pathway. This is associated with increased cell death and GzmB secretion into the BM environment, suggesting an extracellular role of GzmB in modulating HSC niches. Moreover, treatment with the chemotherapeutic agent 5-fluorouracil (5-FU) also induces GzmB production in HSCs. In this situation GzmB is not secreted, but instead causes cell-autonomous apoptosis. Accordingly, GzmB-deficient mice are more resistant to serial 5-FU treatments. Collectively, these results identify GzmB as a negative regulator of HSC function that is induced by stress and chemotherapy in both HSCs and their niches. Blockade of GzmB production may help to improve hematopoiesis in various situations of BM stress.

Role of caspases and non-caspase proteases in cell death.

Undoubtedly, caspases are the major driving force for apoptosis execution and mechanisms of their activation and inhibition have been largely unveiled. Recent progress has been made with regard to the exact intracellular ordering of caspases, monitoring their activities in vivo and unveiling their substrate degradomes. Moreover, non-caspase proteases seem to assist caspases in the completion of the death execution program. Here we will consider some very recent data dealing with these aspects. We will also provide novel insights into the mechanisms that dictate apoptotic variability within a cell population.