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

Recognition of Apoptotic Cells by Viruses and Cytolytic Lymphocytes: Target Selection in the Fog of War

Viruses and cytolytic lymphocytes operate in an environment filled with dying and dead cells, and cell fragments. For viruses, irreversible fusion with doomed cells is suicide. For cytotoxic T lymphocyte and natural killer cells, time and limited lytic resources spent on apoptotic targets is wasteful and may result in death of the host. We make the case that the target membrane cytoskeleton is the best source of information regarding the suitability of potential targets for engagement for both viruses and lytic effector cells, and we present experimental evidence for detection of apoptotic cells by HIV, without loss of infectivity.

Caspase Cleavage of Gelsolin Is an Inductive Cue for Pathologic Cardiac Hypertrophy

Background

Cardiac hypertrophy is an adaptive remodeling event that may improve or diminish contractile performance of the heart. Physiologic and pathologic hypertrophy yield distinct outcomes, yet both are dependent on caspase‐directed proteolysis. This suggests that each form of myocardial growth may derive from a specific caspase cleavage event(s). We examined whether caspase 3 cleavage of the actin capping/severing protein gelsolin is essential for the development of pathologic hypertrophy.

Methods and Results

Caspase targeting of gelsolin was established through protein analysis of hypertrophic cardiomyocytes and mass spectrometry mapping of cleavage sites. Pathologic agonists induced late‐stage caspase‐mediated cleavage of gelsolin. The requirement of caspase‐mediated gelsolin cleavage for hypertrophy induction was evaluated in primary cardiomyocytes by cell size analysis, monitoring of prohypertrophy markers, and measurement of hypertrophy‐related transcription activity. The in vivo impact of caspase‐mediated cleavage was investigated by echo‐guided intramyocardial injection of adenoviral‐expressed gelsolin. Expression of the N‐terminal gelsolin caspase cleavage fragment was necessary and sufficient to cause pathologic remodeling in isolated cardiomyocytes and the intact heart, whereas expression of a noncleavable form prevents cardiac remodeling. Alterations in myocardium structure and function were determined by echocardiography and end‐stage cardiomyocyte cell size analysis. Gelsolin secretion was also monitored for its impact on naïve cells using competitive antibody trapping, demonstrating that hypertrophic agonist stimulation of cardiomyocytes leads to gelsolin secretion, which induces hypertrophy in naïve cells.

Conclusions

These results suggest that cell autonomous caspase cleavage of gelsolin is essential for pathologic hypertrophy and that cardiomyocyte secretion of gelsolin may accelerate this negative remodeling response.

CD56+/CD16- Natural Killer cells expressing the inflammatory protease granzyme A are enriched in synovial fluid from patients with osteoarthritis

OBJECTIVE

Natural killer (NK) cells have been involved in the pathology of different inflammatory and autoimmune disorders. Inflammation is an important regulator of osteoarthritis (OA), but the molecular and cellular mechanisms regulating this process are not well defined.

DESIGN

To understand the role of NK cells in OA, we have compared the phenotype (CD56 subsets and perforin and granzyme expression) and cytotoxic function of NK cells in peripheral blood and synovial fluid from patients with OA undergoing total knee arthroplasty.

RESULTS

In contrast to peripheral blood lymphocytes (PBLs), the majority of NK cells from the synovial fluid were CD56^brightCD16(-) cells. As expected the expression of the cytolytic mediators perforin and granzyme B in CD56^brightCD16(-) cells was low and correlated with a poor cytotoxic potential against K562 sensitive target cells. Surprisingly, this low cytotoxic NK cell subset expressed high levels of granzyme A (a protease recently characterized as a key modulator of inflammation in mouse models) in synovial fluid but not in peripheral blood. The presence of the CD56(+)^brightCD16(-) cells expressing granzyme A correlated with increased levels of pro-inflammatory cytokines in synovial fluid from OA patients.

CONCLUSION

Our results indicate that NK cells from the synovium of patients with OA, which present an immunoregulatory non-cytotoxic phenotype, show different phenotype comparing with NK cells from peripheral blood, especially expressing granzyme A, a pro-inflammatory molecule which may contribute to the establishment of chronic articular inflammation in this type of patients.

Yersinia effector protein (YopO)-mediated phosphorylation of host gelsolin causes calcium-independent activation leading to disruption of actin dynamics

Pathogenic Yersinia bacteria cause a range of human diseases. To modulate and evade host immune systems, these yersiniae inject effector proteins into host macrophages. One such protein, the serine/threonine kinase YopO (YpkA in Yersinia pestis), uses monomeric actin as bait to recruit and phosphorylate host actin polymerization-regulating proteins, including the actin-severing protein gelsolin, to disrupt actin filaments and thus impair phagocytosis. However, the YopO phosphorylation sites on gelsolin and the consequences of YopO-mediated phosphorylation on actin remodeling have yet to be established. Here we determined the effects of YopO-mediated phosphorylation on gelsolin and identified its phosphorylation sites by mass spectrometry. YopO phosphorylated gelsolin in the linker region between gelsolin homology domains G3 and G4, which, in the absence of calcium, are compacted but adopt an open conformation in the presence of calcium, enabling actin binding and severing. Using phosphomimetic and phosphodeletion gelsolin mutants, we found that YopO-mediated phosphorylation partially mimics calcium-dependent activation of gelsolin, potentially contributing to a reduction in filamentous actin and altered actin dynamics in phagocytic cells. In summary, this work represents the first report of the functional outcome of serine/threonine phosphorylation in gelsolin regulation and provides critical insight into how YopO disrupts normal gelsolin function to alter host actin dynamics and thus cripple phagocytosis.

Epitelial-to-mesenchimal transition and invasion are upmodulated by tumor-expressed granzyme B and inhibited by docosahexaenoic acid in human colorectal cancer cells

BACKGROUND

Granzyme B (GrB) is a serine protease, traditionally known as expressed by cytotoxic lymphocytes to induce target cell apoptosis. However, it is emerging that GrB, being also produced by a variety of normal and neoplastic cells and potentially acting on multiple targets, might represent a powerful regulator of a wide range of fundamental biological processes. We have previously shown that GrB is expressed in urothelial carcinoma tissues and its expression is associated to both pathological tumor spreading and EMT. We have also shown that docosahexaenoic acid (DHA), a dietary ω-3 polyunsaturated fatty acid with anti-tumor activity, while inhibiting urothelial and pancreatic carcinoma cell invasion also inhibited their GrB expression in vitro. In this study, we characterized a panel of colorectal carcinoma (CRC) cells, with different invasive capabilities, for GrB expression and for the contribution of GrB to their EMT and invasive phenotype. In addition, we investigated the effect of DHA on CRC cell-associated GrB expression, EMT and invasion.

METHODS

The expression levels of GrB and EMT-related markers were evaluated by Western blotting. GrB knockdown was performed by Stealth RNAi small interfering RNA silencing and ectopic GrB expression by transfection of human GrB vector. Cell invasion was determined by the BioCoat Matrigel invasion chamber test.

RESULTS

GrB was produced in 57.1% CRC cell lines and 100% CRC-derived Cancer Stem Cells. Although GrB was constitutive expressed in both invasive and noninvasive CRC cells, GrB depletion in invasive CRC cells downmodulated their invasion in vitro, suggesting a contribution of GrB to CRC invasiveness. GrB loss or gain of function downmodulated or upmodulated EMT, respectively, according to the analysis of cancer cell expression of three EMT biomarkers (Snail1, E-cadherin, N-cadherin). Moreover, TGF-β1-driven EMT was associated to the enhancement of GrB expression in CRC cell lines, and GrB depletion led to downmodulation of TGF-β1-driven EMT. In addition, DHA inhibited GrB expression, EMT and invasion in CRC cells in vitro.

CONCLUSIONS

These findings present a novel role for GrB as upmodulator of EMT in CRC cells. Moreover, these results support the use of DHA, a dietary compound without toxic effects, as adjuvant in CRC therapy.

Pro-Apoptotic Activity of 4-Isopropyl-2-(1-Phenylethyl) Aniline Isolated from Cordyceps bassiana

Cordyceps species including Cordyceps bassiana are a notable anti-cancer dietary supplement. Previously, we identified several compounds with anti-cancer activity from the butanol fraction (Cb-BF) of Cordyceps bassiana. To expand the structural value of Cb-BF-derived anti-cancer drugs, we employed various chemical moieties to produce a novel Cb-BF-derived chemical derivative, KTH-13-amine-monophenyl [4-isopropyl-2-(1-phenylethyl) aniline (KTH-13-AMP)], which we tested for anti-cancer activity. KTH-13-AMP suppressed the proliferation of MDA-MB-231, HeLa, and C6 glioma cells. KTH-13-AMP also dose-dependently induced morphological changes in C6 glioma cells and time-dependently increased the level of early apoptotic cells stained with annexin V-FITC. Furthermore, the levels of the active full-length forms of caspase-3 and caspase-9 were increased. In contrast, the levels of total forms of caspases-3, caspase-8, caspase-9, and Bcl-2 were decreased in KTH-13-AMP treated-cells. We also confirmed that the phosphorylation of STAT3, Src, and PI3K/p85, which is linked to cell survival, was diminished by treatment with KTH-13-AMP. Therefore, these results strongly suggest that this compound can be used to guide the development of an anti-cancer drug or serve as a lead compound in forming another strong anti-proliferative agent.

Secretory lysosomes of mouse mast cells store and exocytose active caspase-3 in a strictly granzyme B dependent manner

In this study, we report that cytoplasmic granules from in vivo and in vitro derived mouse mast cells (MCs) contain active granzyme B (gzmB) and caspase-3, which is consistent with recent findings. Studying WT and gzmB-deficient mice, we observed that BM-derived MCs (BMMCs) from both strains contain cytosolic pro-caspase-3, but only WT BMMCs expressed active caspase-3 limited to their secretory lysosomes. Confocal microscopy revealed colocalization of active caspase-3 and gzmB in these cytoplasmic granules. The combined data demonstrate that the generation and storage of active caspase-3 is gzmB-dependent. The finding that BMMCs secrete caspase-3 and gzmB after Ag stimulation suggests that both proteases contribute to extracellular MC-mediated proteolytic events. Although the extracellular function of MC-derived caspase-3 remains unclear, we show that BMMC-secreted caspase-3 cleaves IL-33, a cytokine that contributes to the development of asthma and arthritis. We also show that an in vitro propagated cytolytic T-lymphocyte line constitutively expresses gzmB together with active caspase-3, suggesting a novel interaction of these proteases in the execution of multiple innate and adaptive immune responses.

A functional genomics screen identifies PCAF and ADA3 as regulators of human granzyme B-mediated apoptosis and Bid cleavage

The human lymphocyte toxins granzyme B (hGrzB) and perforin cooperatively induce apoptosis of virus-infected or transformed cells: perforin pores enable entry of the serine protease hGrzB into the cytosol, where it processes Bid to selectively activate the intrinsic apoptosis pathway. Truncated Bid (tBid) induces Bax/Bak-dependent mitochondrial outer membrane permeability and the release of cytochrome c and Smac/Diablo. To identify cellular proteins that regulate perforin/hGrzB-mediated Bid cleavage and subsequent apoptosis, we performed a gene-knockdown (KD) screen using a lentiviral pool of short hairpin RNAs embedded within a miR30 backbone (shRNAmiR). We transduced HeLa cells with a lentiviral pool expressing shRNAmiRs that target 1213 genes known to be involved in cell death signaling and selected cells with acquired resistance to perforin/hGrzB-mediated apoptosis. Twenty-two shRNAmiRs were identified in the positive-selection screen including two, PCAF and ADA3, whose gene products are known to reside in the same epigenetic regulatory complexes. Small interfering (si)RNA-mediated gene-KD of PCAF or ADA3 also conferred resistance to perforin/hGrzB-mediated apoptosis providing independent validation of the screen results. Mechanistically, PCAF and ADA3 exerted their pro-apoptotic effect upstream of mitochondrial membrane permeabilization, as indicated by reduced cytochrome c release in PCAF-KD cells exposed to perforin/hGrzB. While overall levels of Bid were unaltered, perforin/hGrzB-mediated cleavage of Bid was reduced in PCAF-KD or ADA3-KD cells. We discovered that PCAF-KD or ADA3-KD resulted in reduced expression of PACS2, a protein implicated in Bid trafficking to mitochondria and importantly, targeted PACS2-KD phenocopied the effect of PCAF-KD or ADA3-KD. We conclude that PCAF and ADA3 regulate Bid processing via PACS2, to modulate the mitochondrial cell death pathway in response to hGrzB.

Crosstalk between apoptosis and autophagy: molecular mechanisms and therapeutic strategies in cancer

Both apoptosis and autophagy are highly conserved processes that besides their role in the maintenance of the organismal and cellular homeostasis serve as a main target of tumor therapeutics. Although their important roles in the modulation of tumor therapeutic strategies have been widely reported, the molecular actions of both apoptosis and autophagy are counteracted by cancer protective mechanisms. While apoptosis is a tightly regulated process that is implicated in the removal of damaged or unwanted cells, autophagy is a cellular catabolic pathway that is involved in lysosomal degradation and recycling of proteins and organelles, and thereby is considered an important survival/protective mechanism for cancer cells in response to metabolic stress or chemotherapy. Although the relationship between autophagy and cell death is very complicated and has not been characterized in detail, the molecular mechanisms that control this relationship are considered to be a relevant target for the development of a therapeutic strategy for tumor treatment. In this review, we focus on the molecular mechanisms of apoptosis, autophagy, and those of the crosstalk between apoptosis and autophagy in order to provide insight into the molecular mechanisms that may be essential for the balance between cell survival and death as well as their role as targets for the development of novel therapeutic approaches.

Leptin attenuates lipopolysaccharide-induced apoptosis of thymocytes partially via down-regulation of cPLA2 and p38 MAPK activation

Leptin, a 16-kDa protein that is mainly secreted by adipocytes, plays a protective role in many cell types. It has been shown that leptin acts in the central and peripheral immune system to protect thymocytes. Cytosolic phospholipase A(2) (cPLA(2)) is an enzyme that can specifically initiate the release of arachidonic acid (AA) to produce eicosanoids, which regulate inflammation and immune responses. Our previous work has shown that leptin is important to prevent apoptosis of thymocytes. However, the role of cPLA(2) is still unclear, and the precise mechanism also remains to be elucidated. In this work, we demonstrated that leptin inhibited the LPS-induced toxicity and apoptosis of thymocytes. Western blot and RT-PCR showed that leptin led to a reduction of cPLA(2) activity and mRNA level, as well as caspase-3 cleavage. Moreover, we found that leptin could decrease the activation of p38 MAPK. Accordingly, we pre-treated apoptotic thymocytes with the p38 MAPK inhibitor, SB203580 and observed an effect similar to the leptin alone treated group. SB203580 also suppressed expression of cPLA(2) and cleavage of caspase-3. Based on these results, we suggest that leptin could attenuate LPS-induced apoptotic injury in mouse thymocyte cells, mainly through the p38/cPLA(2) signalling pathway. The study of the regulatory role of leptin in LPS-induced thymocyte apoptosis can help to explain the role of leptin in the immune system and may provide a novel treatment option in cases of severe trauma, infection, shock, organ failure and autoimmune disease caused by thymic atrophy.

Interleukin-1R signaling is essential for induction of proapoptotic CD8 T cells, viral clearance, and pathology during lymphocytic choriomeningitis virus infection in mice

The T cell granule exocytosis pathway is essential to control hepatotropic lymphocytic choriomeningitis virus strain WE (LCMV-WE) but also contributes to the observed pathology in mice. Although effective antiviral T cell immunity and development of viral hepatitis are strictly dependent on perforin and granzymes, the molecular basis underlying induction of functionally competent virus-immune T cells, including participation of the innate immune system, is far from being resolved. We demonstrate here that LCMV-immune T cells of interleukin-1 receptor (IL-1R)-deficient mice readily express transcripts for perforin and granzymes but only translate perforin, resulting in the lack of proapoptotic potential in vitro. LCMV is not cleared in IL-1R-deficient mice, and yet the infected mice develop neither splenomegaly nor hepatitis. These results demonstrate that IL-1R signaling is central to the induction of proapoptotic CD8 T cell immunity, including viral clearance and associated tissue injuries in LCMV infection.

The proapoptotic Bcl-2 family member Bim plays a central role during the development of virus-induced hepatitis

The proapoptotic Bcl-2 homolog Bim was shown to control the apoptosis of both T cells and hepatocytes. This dual role of Bim might be particularly relevant for the development of viral hepatitis, in which both the sensitivity of hepatocytes to apoptosis stimuli and the persistence of cytotoxic T cells are essential factors for the outcome of the disease. The relevance of Bim in regulating survival of cytotoxic T cells or induction of hepatocyte death has only been investigated in separate systems, and their relative contributions to the pathogenesis of T cell-mediated hepatitis remain unclear. Using the highly dynamic model system of lymphocytic choriomeningitis virus-mediated hepatitis and bone marrow chimeras, we found that Bim has a dual role in the development of lymphocytic choriomeningitis virus-induced, T cell-mediated hepatitis. Although the absence of Bim in parenchymal cells led to markedly attenuated liver damage, loss of Bim in the lymphoid compartment moderately enhanced hepatitis. However, when both effects were combined in Bim(-/-) mice, the effect of Bim deficiency in the lymphoid compartment was overcompensated for by the reduced sensitivity of Bim(-/-) hepatocytes to T cell-induced apoptosis, resulting in the protection of Bim(-/-) mice from hepatitis.