Specific involvement of caspases in the differentiation of monocytes into macrophages

Caspase-9 has a nonapoptotic function in Xenopus embryonic primitive blood formation

Caspases constitute a family of cysteine proteases centrally involved in programmed cell death, which is an integral part of normal embryonic and fetal development. However, it has become clear that specific caspases also have functions independent of cell death. In order to identify novel apoptotic and nonapoptotic developmental caspase functions, we designed and transgenically integrated novel fluorescent caspase reporter constructs in developing Xenopus embryos and tadpoles. This model organism has an external development, allowing direct and continuous monitoring. These studies uncovered a nonapoptotic role for the initiator caspase-9 in primitive blood formation. Functional experiments further corroborated that caspase-9, but possibly not the executioners caspase-3 and caspase-7, are required for primitive erythropoiesis in the early embryo. These data reveal a novel nonapoptotic function for the initiator caspase-9 and, for the first time, implicate nonapoptotic caspase activity in primitive blood formation.

PGBD5 promotes site-specific oncogenic mutations in human tumors

Genomic rearrangements are a hallmark of human cancers. Here, we identify the piggyBac transposable element derived 5 (PGBD5) gene as encoding an active DNA transposase expressed in the majority of childhood solid tumors, including lethal rhabdoid tumors. Using assembly-based whole-genome DNA sequencing, we found previously undefined genomic rearrangements in human rhabdoid tumors. These rearrangements involved PGBD5-specific signal (PSS) sequences at their breakpoints and recurrently inactivated tumor-suppressor genes. PGBD5 was physically associated with genomic PSS sequences that were also sufficient to mediate PGBD5-induced DNA rearrangements in rhabdoid tumor cells. Ectopic expression of PGBD5 in primary immortalized human cells was sufficient to promote cell transformation in vivo. This activity required specific catalytic residues in the PGBD5 transposase domain as well as end-joining DNA repair and induced structural rearrangements with PSS breakpoints. These results define PGBD5 as an oncogenic mutator and provide a plausible mechanism for site-specific DNA rearrangements in childhood and adult solid tumors.

Caspases and their substrates

, or for pyroptosis, gasdermin D. For the most part, it appears that cleavage events function cooperatively in the cell death process to generate a proteolytic synthetic lethal outcome. In contrast to apoptosis, far less is known about caspase biology in non-apoptotic cellular processes, such as cellular remodeling, including which caspases are activated, the mechanisms of their activation and deactivation, and the key substrate targets. Here we survey the progress made in global identification of caspase substrates using proteomics and the exciting new avenues these studies have opened for understanding the molecular logic of substrate cleavage in apoptotic and non-apoptotic processes.

Evolution of caspase-mediated cell death and differentiation: twins separated at birth

The phenotypic and biochemical similarities between caspase-mediated apoptosis and cellular differentiation are striking. They include such diverse phenomenon as mitochondrial membrane perturbations, cytoskeletal rearrangements and DNA fragmentation. The parallels between the two disparate processes suggest some common ancestry and highlight the paradoxical nature of the death-centric view of caspases. That is, what is the driving selective pressure that sustains death-inducing proteins throughout eukaryotic evolution? Plausibly, caspase function may be rooted in a primordial non-death function, such as cell differentiation, and was co-opted for its role in programmed cell death. This review will delve into the links between caspase-mediated apoptosis and cell differentiation and examine the distinguishing features of these events. More critically, we chronicle the evolutionary origins of caspases and propose that caspases may have held an ancient role in mediating the fidelity of cell division/differentiation through its effects on proteostasis and protein quality control.

Apoptosis‑independent cleavage of RhoGDIβ at Asp19 during PMA‑stimulated differentiation of THP‑1 cells to macrophages

Rho GDP-dissociation inhibitor β (RhoGDIβ), a regulator of the Rho family of proteins, is expressed abundantly in the hematopoietic cell lineage. During apoptosis of hematopoietic cells, RhoGDIβ is cleaved by caspase-3 at Asp19 and this cleaved form (Δ19-RhoGDIβ) has been implicated in the apoptotic pathway. To clarify the role of RhoGDIβ in hematopoietic cells, the present study performed immunoblotting and immunofluorescence staining to examine the expression of RhoGDIβ and ∆19-RhoGDIβ during phorbol 12-myristate 13-acetate (PMA)-stimulated differentiation of human THP-1 monocytic cells to macrophages. During differentiation of the THP-1 cells to macrophages, the expression of RhoGDIβ remained stable; however, the expression of Δ19-RhoGDIβ increased, particularly in well-spreading, non-apoptotic cells, which differentiated into macrophages. These results suggested that Δ19-RhoGDIβ has an apoptosis-independent role in the PMA-induced differentiation of THP-1 cells to macrophages.

Non-apoptotic functions of caspases in myeloid cell differentiation

Subtle caspase activation is associated with the differentiation of several myeloid lineages. A tightly orchestrated dance between caspase-3 activation and the chaperone HSP70 that migrates to the nucleus to protect the master regulator GATA-1 from cleavage transiently occurs in basophilic erythroblasts and may prepare nucleus and organelle expel that occurs at the terminal phase of erythroid differentiation. A spatially restricted activation of caspase-3 occurs in maturing megakaryocytes to promote proplatelet maturation and platelet shedding in the bloodstream. In a situation of acute platelet need, caspase-3 could be activated in response to IL-1α and promote megakaryocyte rupture. In peripheral blood monocytes, colony-stimulating factor-1 provokes the formation of a molecular platform in which caspase-8 is activated, which downregulates nuclear factor-kappa B (NF-κB) activity and activates downstream caspases whose target fragments such as those generated by nucleophosmin (NPM1) cleavage contribute to the generation of resting macrophages. Human monocytes secrete mature IL-1β in response to lipopolysaccharide through an alternative inflammasome activation that involves caspase-8, a pathway that does not lead to cell death. Finally, active caspase-3 is part of the proteases contained in secretory granules of mast cells. Many questions remain on how these proteases are activated in myeloid cell lineages, which target proteins are cleaved, whereas other are protected from proteolysis, the precise role of cleaved proteins in cell differentiation and functions, and the link between these non-apoptotic functions of caspases and the death of these diverse cell types. Better understanding of these functions may generate therapeutic strategies to control cytopenias or modulate myeloid cell functions in various pathological situations.

'Did He Who Made the Lamb Make Thee?' New Developments in Treating the 'Fearful Symmetry' of Acute Myeloid Leukemia

Malignant cells must circumvent endogenous cell death pathways to survive and develop into cancers. Acquired cell death resistance also sets up malignant cells to survive anticancer therapies. Acute Myeloid Leukemia (AML) is an aggressive blood cancer characterized by high relapse rate and resistance to cytotoxic therapies. Recent collaborative profiling projects have led to a greater understanding of the 'fearful symmetry' of the genomic landscape of AML, and point to the development of novel potential therapies that can overcome factors linked to chemoresistance. We review here the most recent research in the genetics of AML and how these discoveries have led, or might lead, to therapies that specifically activate cell death pathways to substantially challenge this 'fearful' disease.

Caspase-1 promotes monocyte-macrophage differentiation by repressing PPARγ

Monocyte-to-macrophage differentiation is tightly controlled in vivo, as disruption of the normal differentiation program can lead to diverse disorders. Caspase-1, the first identified member of the caspase family, regulates differentiation in various cell types such as Th17 cells and adipocytes. However, the contribution of caspase-1 in monocyte-macrophage differentiation remains elusive. Here we report that caspase-1 is significantly downregulated in leukemia cells from patients with acute monocytic leukemia. By using the phorbol 12-myristate 13-acetate-induced cell differentiation model, we found that caspase-1 activation was required for the differentiation of human monocytes to macrophages. Further analysis of peroxisome proliferator-activated receptor γ (PPARγ) protein levels revealed that the monocyte-macrophage differentiation program could be divided into two stages. Caspase-1-mediated downregulation of PPARγ was important in the late stage of monocyte-macrophage differentiation; however, PPARγ protein levels had little effect on the early stage differentiation. Accumulation of PPARγ protein by troglitazone treatment potently suppressed the late stage of macrophage differentiation, which might be linked to inhibition of nuclear factor-κB activity. The data provide a plausible mechanistic basis by which caspase-1 promotes the differentiation of macrophages from monocytes.

CCL2/CCR2 signaling pathway in glioblastoma multiforme

Glioblastoma multiform (GBM) is described as one of the most frequent primary brain tumors. These types of malignancies constitute only 15% of all primary brain tumors. Despite, extensive developments on effective therapeutic methods during the 20th century as well as the first decade of the present century (21st), the median survival rate for patients suffering from GBM is only approximately 15 months, even in response to multi-modal therapy. numerous types of reticuloendothelial system cells such as macrophages and microglial cells occupied within both GBM and also normal surrounding tissues. These immune cells acquire an otherwise activated phenotype with potent tumor-tropic functions that contribute to the glioma growth and invasion. The CC chemokine, CCL2 (previously named MCP-1) is of the most important CC chemokines family member involving in regulation of oriented migration and penetrative infiltration of mainly reticuloendothelial system cells specifically monocyte/macrophage phenotypes. Fundamental parts are played by CCL2 and its related receptor (the CCR2) in brain tumors and obviously in migration of monocytes from the bloodstream through the vascular endothelium. Therefore, CCL2/CCR2 axis is required for the routine immunological surveillance of tissues, in accordance with response to inflammation. Briefly, in this review, we have tried our best to collect the latest, straightened and summarize literature reports exist within data base regarding the interaction between microglia/macrophages and CCL2/CCR2 axis in GBM. We aimed to discuss potential application of this chemokine/receptor interaction axis for the expansion of future anti-glioma therapies as well.

Human Cytomegalovirus Stimulates the Synthesis of Select Akt-Dependent Antiapoptotic Proteins during Viral Entry To Promote Survival of Infected Monocytes

Primary peripheral blood monocytes are responsible for the hematogenous dissemination of human cytomegalovirus (HCMV) following a primary infection. To facilitate viral spread, we have previously shown HCMV to extend the short 48-h life span of monocytes. Mechanistically, HCMV upregulated two specific cellular antiapoptotic proteins, myeloid leukemia sequence 1 (Mcl-1) and heat shock protein 27 (HSP27), to block the two proteolytic cleavages necessary for the formation of fully active caspase 3 and the subsequent initiation of apoptosis. We now show that HCMV more robustly upregulated Mcl-1 than normal myeloid growth factors and that Mcl-1 was the only myeloid survival factor to rapidly induce HSP27 prior to the 48-h cell fate checkpoint. We determined that HCMV glycoproteins gB and gH signal through the cellular epidermal growth factor receptor (EGFR) and αvβ3 integrin, respectively, during viral entry in order to drive the increase of Mcl-1 and HSP27 in an Akt-dependent manner. Although Akt is known to regulate protein stability and transcription, we found that gB- and gH-initiated signaling preferentially and cooperatively stimulated the synthesis of Mcl-1 and HSP27 through mTOR-mediated translation. Overall, these data suggest that the unique signaling network generated during the viral entry process stimulates the upregulation of select antiapoptotic proteins allowing for the differentiation of short-lived monocytes into long-lived macrophages, a key step in the viral dissemination strategy.

IMPORTANCE

Human cytomegalovirus (HCMV) infection is endemic within the human population. Although primary infection is generally asymptomatic in immunocompetent individuals, HCMV is a significant cause of morbidity and mortality in the immunocompromised. The multiorgan inflammatory diseases associated with symptomatic HCMV infection are a direct consequence of the monocyte-mediated systemic spread of the virus. In order for peripheral blood monocytes to facilitate viral dissemination, HCMV subverts the short 48-h life span of monocytes by inducing the expression of cellular antiapoptotic proteins Mcl-1 and HSP27. Here, we demonstrate that the rapid and simultaneous upregulation of Mcl-1 and HSP27 is a distinctive feature of HCMV-induced monocyte survival. Moreover, we decipher the signaling pathways activated during viral entry needed for the robust synthesis of Mcl-1 and HSP27. Identifying the virus-specific mechanisms used to upregulate select cellular factors required for the survival of HCMV-infected monocytes is important to the development of new classes of anti-HCMV drugs.

Human Cytomegalovirus Promotes Survival of Infected Monocytes via a Distinct Temporal Regulation of Cellular Bcl-2 Family Proteins

Monocytes play a key role in the hematogenous dissemination of human cytomegalovirus (HCMV) to target organ systems. To infect monocytes and reprogram them to deliver infectious virus, HCMV must overcome biological obstacles, including the short life span of monocytes and their antiviral proapoptotic response to infection. We have shown that virally induced upregulation of cellular Mcl-1 promotes early survival of HCMV-infected monocytes, allowing cells to overcome an early apoptotic checkpoint at around 48 h postinfection (hpi). Here, we demonstrate an HCMV-dependent shift from Mcl-1 as the primary antiapoptotic player to the related protein, Bcl-2, later during infection. Bcl-2 was upregulated in HCMV-infected monocytes beginning at 48 hpi. Treatment with the Bcl-2 antagonist ABT-199 only reduced the prosurvival effects of HCMV in target monocytes beginning at 48 hpi, suggesting that Mcl-1 controls survival prior to 48 hpi, while Bcl-2 promotes survival after 48 hpi. Although Bcl-2 was upregulated following viral binding/signaling through cellular integrins (compared to Mcl-1, which is upregulated through binding/activation of epidermal growth factor receptor [EGFR]), it functioned similarly to Mcl-1, adopting the early role of Mcl-1 in preventing caspase-3 cleavage/activation. This distinct, HCMV-induced shift from Mcl-1 to Bcl-2 occurs in response to a cellular upregulation of proapoptotic Bax, as small interfering RNA (siRNA)-mediated knockdown of Bax reduced the upregulation of Bcl-2 in infected monocytes and rescued the cells from the apoptotic effects of Bcl-2 inhibition. Our data demonstrate a distinct survival strategy whereby HCMV induces a biphasic regulation of cellular Bcl-2 proteins to promote host cell survival, leading to viral dissemination and the establishment of persistent HCMV infection.

IMPORTANCE

Hematogenous dissemination of HCMV via infected monocytes is a crucial component of the viral survival strategy and is required for the establishment of persistent infection and for viral spread to additional hosts. Our system of infected primary human blood monocytes provides us with an opportunity to answer specific questions about viral spread and persistence in in vivo-relevant myeloid cells that cannot be addressed with the more traditionally used replication-permissive cells. Our goal in examining the mechanisms whereby HCMV reprograms infected monocytes to promote viral dissemination is to uncover new targets for therapeutic intervention that would disrupt key viral survival and persistence strategies. Because of this important role in maintaining survival of HCMV-infected monocytes, our new data on the role of Bcl-2 regulation during viral infection represents a promising molecular target for mitigating viral spread and persistence.

Proteolytic fragmentation of inositol 1,4,5-trisphosphate receptors: a novel mechanism regulating channel activity?

Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are a family of ubiquitously expressed intracellular Ca(2+) release channels. Regulation of channel activity by Ca(2+) , nucleotides, phosphorylation, protein binding partners and other cellular factors is thought to play a major role in defining the specific spatiotemporal characteristics of intracellular Ca(2+) signals. These properties are, in turn, believed pivotal for the selective and specific physiological activation of Ca(2+) -dependent effectors. IP3 Rs are also substrates for the intracellular cysteine proteases, calpain and caspase. Cleavage of the IP3 R has been proposed to play a role in apoptotic cell death by uncoupling regions important for IP3 binding from the channel domain, leaving an unregulated leaky Ca(2+) pore. Contrary to this hypothesis, we demonstrate following proteolysis that N- and C-termini of IP3 R1 remain associated, presumably through non-covalent interactions. Further, we show that complementary fragments of IP3 R1 assemble into tetrameric structures and retain their ability to be regulated robustly by IP3 . While peptide continuity is clearly not necessary for IP3 -gating of the channel, we propose that cleavage of the IP3 R peptide chain may alter other important regulatory events to modulate channel activity. In this scenario, stimulation of the cleaved IP3 R may support distinct spatiotemporal Ca(2+) signals and activation of specific effectors. Notably, in many adaptive physiological events, the non-apoptotic activities of caspase and calpain are demonstrated to be important, but the substrates of the proteases are poorly defined. We speculate that proteolytic fragmentation may represent a novel form of IP3 R regulation, which plays a role in varied adaptive physiological processes.

A New Triggering Receptor Expressed on Myeloid Cells (TREM) Family Member, TLT-6, is Involved in Activation and Proliferation of Macrophages

The triggering receptor expressed on myeloid cells (TREM) family, which is abundantly expressed in myeloid lineage cells, plays a pivotal role in innate and adaptive immune response. In this study, we aimed to identify a novel receptor expressed on hematopoietic stem cells (HSCs) by using in silico bioinformatics and to characterize the identified receptor. We thus found the TREM-like transcript (TLT)-6, a new member of TREM family. TLT-6 has a single immunoglobulin domain in the extracellular region and a long cytoplasmic region containing 2 immunoreceptor tyrosine-based inhibitory motif-like domains. TLT-6 transcript was expressed in HSCs, monocytes and macrophages. TLT-6 protein was up-regulated on the surface of bone marrow-derived and peritoneal macrophages by lipopolysaccharide stimulation. TLT-6 exerted anti-proliferative effects in macrophages. Our results demonstrate that TLT-6 may regulate the activation and proliferation of macrophages.

Conditional deletion of caspase-8 in macrophages alters macrophage activation in a RIPK-dependent manner

INTRODUCTION

Although caspase-8 is a well-established initiator of apoptosis and suppressor of necroptosis, recent evidence suggests that this enzyme maintains functions beyond its role in cell death. As cells of the innate immune system, and in particular macrophages, are now at the forefront of autoimmune disease pathogenesis, we examined the potential involvement of caspase-8 within this population.

METHODS

Cre (LysM) Casp8 (fl/fl) mice were bred via a cross between Casp8 (fl/fl) mice and Cre (LysM) mice, and RIPK3 (-/-) Cre (LysM) Casp8 (fl/fl) mice were generated to assess the contribution of receptor-interacting serine-threonine kinase (RIPK)3. Immunohistochemical and immunofluorescence analyses were used to examine renal damage. Flow cytometric analysis was employed to characterize splenocyte distribution and activation. Cre (LysM) Casp8 (fl/fl) mice were treated with either Toll-like receptor (TLR) agonists or oral antibiotics to assess their response to TLR activation or TLR agonist removal. Luminex-based assays and enzyme-linked immunosorbent assays were used to measure cytokine/chemokine and immunoglobulin levels in serum and cytokine levels in cell culture studies. In vitro cell culture was used to assess macrophage response to cell death stimuli, TLR activation, and M1/M2 polarization. Data were compared using the Mann-Whitney U test.

RESULTS

Loss of caspase-8 expression in macrophages promotes onset of a mild systemic inflammatory disease, which is preventable by the deletion of RIPK3. In vitro cell culture studies reveal that caspase-8-deficient macrophages are prone to a caspase-independent death in response to death receptor ligation; yet, caspase-8-deficient macrophages are not predisposed to unchecked survival, as analysis of mixed bone marrow chimeric mice demonstrates that caspase-8 deficiency does not confer preferential expansion of myeloid populations. Loss of caspase-8 in macrophages dictates the response to TLR activation, as injection of TLR ligands upregulates expression of costimulatory CD86 on the Ly6C(high)CD11b(+)F4/80(+) splenic cells, and oral antibiotic treatment to remove microbiota prevents splenomegaly and lymphadenopathy in Cre (LysM) Casp8 (fl/fl) mice. Further, caspase-8-deficient macrophages are hyperresponsive to TLR activation and exhibit aberrant M1 macrophage polarization due to RIPK activity.

CONCLUSIONS

These data demonstrate that caspase-8 functions uniquely in macrophages by controlling the response to TLR activation and macrophage polarization in an RIPK-dependent manner.

The Impact of Neuroimmune Alterations in Autism Spectrum Disorder

Autism spectrum disorder (ASD) involves a complex interplay of both genetic and environmental risk factors, with immune alterations and synaptic connection deficiency in early life. In the past decade, studies of ASD have substantially increased, in both humans and animal models. Immunological imbalance (including autoimmunity) has been proposed as a major etiological component in ASD, taking into account increased levels of pro-inflammatory cytokines observed in postmortem brain from patients, as well as autoantibody production. Also, epidemiological studies have established a correlation of ASD with family history of autoimmune diseases; associations with major histocompatibility complex haplotypes and abnormal levels of immunological markers in the blood. Moreover, the use of animal models to study ASD is providing increasing information on the relationship between the immune system and the pathophysiology of ASD. Herein, we will discuss the accumulating literature for ASD, giving special attention to the relevant aspects of factors that may be related to the neuroimmune interface in the development of ASD, including changes in neuroplasticity.

The SAP motif and C-terminal RS- and RD/E-rich region influences the sub-nuclear localization of Acinus isoforms

Acinus has been reported to function in apoptosis, RNA processing and regulation of gene transcription including RA-dependent transcription. There are three different isoforms of Acinus termed Acinus-L, Acinus-S', and Acinus-S. The isoforms of Acinus differ in their N-terminus while the C-terminus is consistent in all isoforms. The sub-nuclear localization of Acinus-L and Acinus-S' was determined using fluorescence microscopy. Acinus-S' colocalizes with SC35 in nuclear speckles while Acinus-L localizes diffusely throughout the nucleoplasm. RA treatment has little effect on the sub-nuclear localization of Acinus-L and Acinus-S'. The domains/regions necessary for the distinct sub-nuclear localization of Acinus-L and Acinus-S' were identified. The speckled sub-nuclear localization of Acinus-S' is dependent on its C-terminal RS- and RD/E-rich region but is independent of the phosphorylation status of Ser-453 and Ser-604 within this region. The unique N-terminal SAP motif of Acinus-L is responsible for its diffuse localization in the nucleus. Moreover, the sub-nuclear localization of Acinus isoforms is affected by each other, which is determined by the combinatorial effect of the more potent SAP motif of Acinus-L and the C-terminal RS- and RD/E-rich region in all Acinus isoforms. The C-terminal RS- and RD/E-rich region of Acinus mediates the colocalization of Acinus isoforms as well as with its interacting protein RNPS1. In conclusion, the SAP motif is responsible for the difference in the nuclear localization between Acinus-L and Acinus-S'. This difference in the nuclear localization of Acinus-S' and Acinus-L may suggest that these two isoforms have different functional roles.

Apoptosis: role in myeloid cell development

Hematopoiesis is the process that generates blood cells in an organism from the pluripotent stem cells. Hematopoietic stem cells are characterized by their ability to undergo self-renewal and differentiation. The self-renewing ability ensures that these pluripotent cells are not depleted from the bone marrow niche. A proper balance between cell death and cell survival is necessary to maintain a homeostatic condition, hence, apoptosis, or programmed cell death, is an essential step in hematopoiesis. Recent studies, however, have introduced a new aspect to this process, citing the significance of the apoptosis mediator, caspase, in cell development and differentiation. Extensive research has been carried out to study the possible role of caspases and other apoptosis related factors in the developmental processes. This review focuses on the various apoptotic factors involved in the development and differentiation of myeloid lineage cells: erythrocytes, megakaryocytes, and macrophages.

Distinct contribution of protein kinase Cδ and protein kinase Cε in the lifespan and immune response of human blood monocyte subpopulations

Monocytes, key components of the immune system, are a heterogeneous population comprised of classical monocytes (CD16(-) ) and non-classical monocytes (CD16(+) ). Monocytes are short lived and undergo spontaneous apoptosis, unless stimulated. Dysregulation of monocyte numbers contribute to the pathophysiology of inflammatory diseases, yet the contribution of each subset remains poorly characterized. Protein kinase C (PKC) family members are central to monocyte biology; however, their role in regulating lifespan and immune function of CD16(-) and CD16(+) monocytes has not been studied. Here, we evaluated the contribution of PKCδ and PKCε in the lifespan and immune response of both monocyte subsets. We showed that CD16(+) monocytes are more susceptible to spontaneous apoptosis because of the increased caspase-3, -8 and -9 activities accompanied by higher kinase activity of PKCδ. Silencing of PKCδ reduced apoptosis in both CD16(+) and CD16(-) monocytes. CD16(+) monocytes express significantly higher levels of PKCε and produce more tumour necrosis factor-α in CD16(+) compared with CD16(-) monocytes. Silencing of PKCε affected the survival and tumour necrosis factor-α production. These findings demonstrate a complex network with similar topography, yet unique regulatory characteristics controlling lifespan and immune response in each monocyte subset, helping define subset-specific coordination programmes controlling monocyte function.

A mutation in caspase-9 decreases the expression of BAFFR and ICOS in patients with immunodeficiency and lymphoproliferation

Lymphocyte apoptosis is mainly induced by either death receptor-dependent activation of caspase-8 or mitochondria-dependent activation of caspase-9. Mutations in caspase-8 lead to autoimmunity/lymphoproliferation and immunodeficiency. This work describes a heterozygous H237P mutation in caspase-9 that can lead to similar disorders. H237P mutation was detected in two patients: Pt1 with autoimmunity/lymphoproliferation, severe hypogammaglobulinemia and Pt2 with mild hypogammaglobulinemia and Burkitt lymphoma. Their lymphocytes displayed defective caspase-9 activity and decreased apoptotic and activation responses. Transfection experiments showed that mutant caspase-9 display defective enzyme and proapoptotic activities and a dominant-negative effect on wild-type caspase-9. Ex vivo analysis of the patients' lymphocytes and in vitro transfection experiments showed that the expression of mutant caspase-9 correlated with a downregulation of BAFFR (B-cell-activating factor belonging to the TNF family (BAFF) receptor) in B cells and ICOS (inducible T-cell costimulator) in T cells. Both patients carried a second inherited heterozygous mutation missing in the relatives carrying H237P: Pt1 in the transmembrane activator and CAML interactor (TACI) gene (S144X) and Pt2 in the perforin (PRF1) gene (N252S). Both mutations have been previously associated with immunodeficiencies in homozygosis or compound heterozygosis. Taken together, these data suggest that caspase-9 mutations may predispose to immunodeficiency by cooperating with other genetic factors, possibly by downregulating the expression of BAFFR and ICOS.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Modifying caspase-3 activity by altering allosteric networks

Caspases have several allosteric sites that bind small molecules or peptides. Allosteric regulators are known to affect caspase enzyme activity, in general, by facilitating large conformational changes that convert the active enzyme to a zymogen-like form in which the substrate-binding pocket is disordered. Mutations in presumed allosteric networks also decrease activity, although large structural changes are not observed. Mutation of the central V266 to histidine in the dimer interface of caspase-3 inactivates the enzyme by introducing steric clashes that may ultimately affect positioning of a helix on the protein surface. The helix is thought to connect several residues in the active site to the allosteric dimer interface. In contrast to the effects of small molecule allosteric regulators, the substrate-binding pocket is intact in the mutant, yet the enzyme is inactive. We have examined the putative allosteric network, in particular the role of helix 3, by mutating several residues in the network. We relieved steric clashes in the context of caspase-3(V266H), and we show that activity is restored, particularly when the restorative mutation is close to H266. We also mimicked the V266H mutant by introducing steric clashes elsewhere in the allosteric network, generating several mutants with reduced activity. Overall, the data show that the caspase-3 native ensemble includes the canonical active state as well as an inactive conformation characterized by an intact substrate-binding pocket, but with an altered helix 3. The enzyme activity reflects the relative population of each species in the native ensemble.

Revisiting caspases in sepsis

Sepsis is a life-threatening illness that occurs due to an abnormal host immune network which extends through the initial widespread and overwhelming inflammation, and culminates at the late stage of immunosupression. Recently, interest has been shifted toward therapies aimed at reversing the accompanying periods of immune suppression. Studies in experimental animals and critically ill patients have demonstrated that increased apoptosis of lymphoid organs and some parenchymal tissues contributes to this immune suppression, anergy and organ dysfunction. Immediate to the discoveries of the intracellular proteases, caspases for the induction of apoptosis and inflammation, and their striking roles in sepsis have been focused elaborately in a number of original and review articles. Here we revisited the different aspects of caspases in terms of apoptosis, pyroptosis, necroptosis and inflammation and focused their links in sepsis by reviewing several recent findings. In addition, we have documented striking perspectives which not only rewrite the pathophysiology, but also modernize our understanding for developing novel therapeutics against sepsis.

The Drosophila effector caspase Dcp-1 regulates mitochondrial dynamics and autophagic flux via SesB

Increasing evidence reveals that a subset of proteins participates in both the autophagy and apoptosis pathways, and this intersection is important in normal physiological contexts and in pathological settings. In this paper, we show that the Drosophila effector caspase, Drosophila caspase 1 (Dcp-1), localizes within mitochondria and regulates mitochondrial morphology and autophagic flux. Loss of Dcp-1 led to mitochondrial elongation, increased levels of the mitochondrial adenine nucleotide translocase stress-sensitive B (SesB), increased adenosine triphosphate (ATP), and a reduction in autophagic flux. Moreover, we find that SesB suppresses autophagic flux during midoogenesis, identifying a novel negative regulator of autophagy. Reduced SesB activity or depletion of ATP by oligomycin A could rescue the autophagic defect in Dcp-1 loss-of-function flies, demonstrating that Dcp-1 promotes autophagy by negatively regulating SesB and ATP levels. Furthermore, we find that pro-Dcp-1 interacts with SesB in a nonproteolytic manner to regulate its stability. These data reveal a new mitochondrial-associated molecular link between nonapoptotic caspase function and autophagy regulation in vivo.

The plasticity of inflammatory monocyte responses to the inflamed central nervous system

Over the last three decades it has become increasingly clear that monocytes, originally thought to have fixed, stereotypic responses to foreign stimuli, mediate exquisitely balanced protective and pathogenic roles in disease and immunity. This balance is crucial in core functional organs, such as the central nervous system (CNS), where minor changes in neuronal microenvironments and the production of immune factors can result in significant disease with fatal consequences or permanent neurological sequelae. Viral encephalitis and multiple sclerosis are examples of important human diseases in which the pathogenic contribution of monocytes recruited from the bone marrow plays a critical role in the clinical expression of disease, as they differentiate into macrophage or dendritic cells in the CNS to carry out effector functions. While antigen-specific lymphocyte populations are central to the adaptive immune response in both cases, in viral encephalitis a prominent macrophage infiltration may mediate immunopathological damage, seizure induction, and death. However, the autoimmune response to non-replicating, non-infectious, but abundant, self antigen has a different disease progression, associated with differentiation of significant numbers of infiltrating monocytes into dendritic cells in the CNS. Whilst a predominant presence of macrophages or dendritic cells in the inflamed CNS in viral encephalitis or multiple sclerosis is well described, the way in which the inflamed CNS mobilizes monocytes in the bone marrow to migrate to the CNS and the key drivers that lead to these specific differentiation pathways in vivo are not well understood. Here we review the current understanding of factors facilitating inflammatory monocyte generation, migration and entry into the brain, as well as their differentiation towards macrophages or dendritic cells in viral and autoimmune disease in relation to their respective disease outcomes.

Delay in apoptosome formation attenuates apoptosis in mouse embryonic stem cell differentiation

Differentiation is an inseparable process of development in multicellular organisms. Mouse embryonic stem cells (mESCs) represent a valuable research tool to conduct in vitro studies of cell differentiation. Apoptosis as a well known cell death mechanism shows some common features with cell differentiation, which has caused a number of ambiguities in the field. The research question here is how cells could differentiate these two processes from each other. We have investigated the role of the mitochondrial apoptotic pathway and cell energy level during differentiation of mESCs into the cardiomyocytes and their apoptosis. p53 expression, cytochrome c release, apoptosome formation, and caspase-3/7 activation are observed upon induction of both apoptosis and differentiation. However, remarkable differences are detected in time of cytochrome c appearance, apoptosome formation, and caspase activity upon induction of both processes. In apoptosis, apoptosome formation and caspase activity were observed rapidly following the cytochrome c release. Unlike apoptosis, the release of cytochrome c upon differentiation took more time, and the maximum caspase activity was also postponed for 24 h. This delay suggests that there is a regulatory mechanism during differentiation of mESCs into cardiomyocytes. The highest ATP content of cells was observed immediately after cytochrome c release 6 h after apoptosis induction and then decreased, but it was gradually increased up to 48 h after differentiation. These observations suggest that a delay in the release of cytochrome c or delay in ATP increase attenuate apoptosome formation, and caspase activation thereby discriminates apoptosis from differentiation in mESCs.

New roles for old enzymes: killer caspases as the engine of cell behavior changes

It has become increasingly clear that caspases, far from being merely cell death effectors, have a much wider range of functions within the cell. These functions are as diverse as signal transduction and cytoskeletal remodeling, and caspases are now known to have an essential role in cell proliferation, migration, and differentiation. There is also evidence that apoptotic cells themselves can direct the behavior of nearby cells through the caspase-dependent secretion of paracrine signaling factors. In some processes, including the differentiation of skeletal muscle myoblasts, both caspase activation in differentiating cells as well as signaling from apoptotic cells has been reported. Here, we review the non-apoptotic outcomes of caspase activity in a range of different model systems and attempt to integrate this knowledge.

Olive (Olea europaea) leaf extract induces apoptosis and monocyte/macrophage differentiation in human chronic myelogenous leukemia K562 cells: insight into the underlying mechanism

Differentiation therapy is an attractive approach aiming at reversing malignancy and reactivating endogenous differentiation programs in cancer cells. Olive leaf extract, known for its antioxidant activity, has been demonstrated to induce apoptosis in several cancer cells. However, its differentiation inducing properties and the mechanisms involved are still poorly understood. In this study, we investigated the effect of Chemlali Olive Leaf Extract (COLE) for its potential differentiation inducing effect on multipotent leukemia K562 cells. Results showed that COLE inhibits K562 cells proliferation and arrests the cell cycle at G0/G1, and then at G2/M phase over treatment time. Further analysis revealed that COLE induces apoptosis and differentiation of K562 cells toward the monocyte lineage. Microarray analysis was conducted to investigate the underlying mechanism of COLE differentiation inducing effect. The differentially expressed genes such as IFI16, EGR1, NFYA, FOXP1, CXCL2, CXCL3, and CXCL8 confirmed the commitment of K562 cells to the monocyte/macrophage lineage. Thus our results provide evidence that, in addition to apoptosis, induction of differentiation is one of the possible therapeutic effects of olive leaf in cancer cells.

Apoptosis induction in duck tissues during duck hepatitis A virus type 1 infection

To investigate the role of apoptosis in duck viral hepatitis pathogenesis, 4- and 21-d-old ducks were inoculated with duck hepatitis A virus serotype 1 and killed at 2, 6, 12, 24, and 48 h postinfection. TdT-mediated dUTP nick-end labeling was used to detect apoptosis cells. Expression profiles of apoptosis-related genes including caspase-3, -8, -9, and Bcl-2 in spleen, bursa of Fabricius, liver, and the quantity of virus in blood were examined using real-time PCR. The TdT-mediated dUTP nick-end labeling analysis indicated there was a significant difference of apoptotic cells between treatments and controls. The same difference also appeared in virus amount variation in blood during infection. Gene expression analysis revealed that the apoptosis-related gene expression profile was different in the 2 groups, and also different between various organs. This study suggested that apoptosis may play an important role in duck hepatitis A virus serotype 1 infection, and apoptosis suppression might facilitate virus multiplication, resulting in the highest virus concentration in the host.

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.

HCMV reprogramming of infected monocyte survival and differentiation: a Goldilocks phenomenon

The wide range of disease pathologies seen in multiple organ sites associated with human cytomegalovirus (HCMV) infection results from the systemic hematogenous dissemination of the virus, which is mediated predominately by infected monocytes. In addition to their role in viral spread, infected monocytes are also known to play a key role in viral latency and life-long persistence. However, in order to utilize infected monocytes for viral spread and persistence, HCMV must overcome a number of monocyte biological hurdles, including their naturally short lifespan and their inability to support viral gene expression and replication. Our laboratory has shown that HCMV is able to manipulate the biology of infected monocytes in order to overcome these biological hurdles by inducing the survival and differentiation of infected monocytes into long-lived macrophages capable of supporting viral gene expression and replication. In this current review, we describe the unique aspects of how HCMV promotes monocyte survival and differentiation by inducing a “finely-tuned” macrophage cell type following infection. Specifically, we describe the induction of a uniquely polarized macrophage subset from infected monocytes, which we argue is the ideal cellular environment for the initiation of viral gene expression and replication and, ultimately, viral spread and persistence within the infected host.

Dual regulation of SPI1/PU.1 transcription factor by heat shock factor 1 (HSF1) during macrophage differentiation of monocytes

In addition to their cytoprotective role in stressful conditions, heat shock proteins (HSPs) are involved in specific differentiation pathways, for example, we have identified a role for HSP90 in macrophage differentiation of human peripheral blood monocytes that are exposed to macrophage colony-stimulating factor (M-CSF). Here, we show that deletion of the main transcription factor involved in heat shock gene regulation, heat shock factor 1 (HSF1), affects M-CSF-driven differentiation of mouse bone marrow cells. HSF1 transiently accumulates in the nucleus of human monocytes undergoing macrophage differentiation, including M-CSF-treated peripheral blood monocytes and phorbol ester-treated THP1 cells. We demonstrate that HSF1 has a dual effect on SPI1/PU.1, a transcription factor essential for macrophage differentiation and whose deregulation can lead to the development of leukemias and lymphomas. Firstly, HSF1 regulates SPI1/PU.1 gene expression through its binding to a heat shock element within the intron 2 of this gene. Furthermore, downregulation or inhibition of HSF1 impaired both SPI1/PU.1-targeted gene transcription and macrophage differentiation. Secondly, HSF1 induces the expression of HSP70 that interacts with SPI1/PU.1 to protect the transcription factor from proteasomal degradation. Taken together, HSF1 appears as a fine-tuning regulator of SPI1/PU.1 expression at the transcriptional and post-translational levels during macrophage differentiation of monocytes.

Tango7 directs cellular remodeling by the Drosophila apoptosome

It is now well appreciated that the apoptosome, which governs caspase-dependent cell death, also drives nonapoptotic caspase activation to remodel cells. However, the determinants that specify whether the apoptosome acts to kill or remodel have yet to be identified. Here we report that Tango7 collaborates with the Drosophila apoptosome to drive a caspase-dependent remodeling process needed to resolve individual sperm from a syncytium. In these cells, Tango7 is required for caspase activity and localizes to the active apoptosome compartment via its C terminus. Furthermore, Tango7 directly stimulates the activity of this complex in vitro. We propose that Tango7 specifies the Drosophila apoptosome as an effector of cellular remodeling.

Cell death proteins: an evolutionary role in cellular adaptation before the advent of apoptosis

Programmed cell death (PCD) or apoptosis is a broadly conserved phenomenon in metazoans, whereby activation of canonical signal pathways induces an ordered dismantling and death of a cell. Paradoxically, the constituent proteins and pathways of PCD (most notably the metacaspase/caspase protease mediated signal pathways) have been demonstrated to retain non-death functions across all phyla including yeast, nematodes, drosophila, and mammals. The ancient conservation of both death and non-death functions of PCD proteins raises an interesting evolutionary conundrum: was the primordial intent of these factors to induce cell death or to regulate other cellular adaptations? Here, we propose the hypothesis that apoptotic behavior of PCD proteins evolved or were co-opted from core non-death functions.

Early phosphoproteomic changes in the mouse spleen during deoxynivalenol-induced ribotoxic stress

The trichothecene mycotoxin deoxynivalenol (DON) targets the innate immune system and is of public health significance because of its frequent presence in human and animal food. DON-induced proinflammatory gene expression and apoptosis in the lymphoid tissue have been associated with a ribotoxic stress response (RSR) that involves rapid phosphorylation of mitogen-activated protein kinases (MAPKs). To better understand the relationship between protein phosphorylation and DON's immunotoxic effects, stable isotope dimethyl labeling-based proteomics in conjunction with titanium dioxide chromatography was employed to quantitatively profile the immediate (≤ 30min) phosphoproteome changes in the spleens of mice orally exposed to 5mg/kg body weight DON. A total of 90 phosphoproteins indicative of novel phosphorylation events were significantly modulated by DON. In addition to critical branches and scaffolds of MAPK signaling being affected, DON exposure also altered phosphorylation of proteins that mediate phosphatidylinositol 3-kinase/AKT pathways. Gene ontology analysis revealed that DON exposure affected biological processes such as cytoskeleton organization, regulation of apoptosis, and lymphocyte activation and development, which likely contribute to immune dysregulation associated with DON-induced RSR. Consistent with these findings, DON impacted phosphorylation of proteins within diverse immune cell populations, including monocytes, macrophages, T cells, B cells, dendritic cells, and mast cells. Fuzzy c-means clustering analysis further indicated that DON evoked several distinctive temporal profiles of regulated phosphopeptides. Overall, the findings from this investigation can serve as a template for future focused exploration and modeling of cellular responses associated with the immunotoxicity evoked by DON and other ribotoxins.

Role of mTOR, Bad, and Survivin in RasGAP Fragment N-Mediated Cell Protection

Partial cleavage of p120 RasGAP by caspase-3 in stressed cells generates an N-terminal fragment, called fragment N, which activates an anti-apoptotic Akt-dependent survival response. Akt regulates several effectors but which of these mediate fragment N-dependent cell protection has not been defined yet. Here we have investigated the role of mTORC1, Bad, and survivin in the capacity of fragment N to protect cells from apoptosis. Neither rapamycin, an inhibitor of mTORC1, nor silencing of raptor, a subunit of the mTORC1 complex, altered the ability of fragment N from inhibiting cisplatin- and Fas ligand-induced death. Cells lacking Bad, despite displaying a stronger resistance to apoptosis, were still protected by fragment N against cisplatin-induced death. Fragment N was also able to protect cells from Fas ligand-induced death in conditions where Bad plays no role in apoptosis regulation. Fragment N expression in cells did neither modulate survivin mRNA nor its protein expression. Moreover, the expression of cytoplasmic survivin, known to exert anti-apoptotic actions in cells, still occurred in UV-B-irradiated epidermis of mouse expressing a caspase-3-resistant RasGAP mutant that cannot produce fragment N. Additionally, survivin function in cell cycle progression was not affected by fragment N. These results indicate that, taken individually, mTOR, Bad, or Survivin are not required for fragment N to protect cells from cell death. We conclude that downstream targets of Akt other than mTORC1, Bad, or survivin mediate fragment N-induced protection or that several Akt effectors can compensate for each other to induce the pro-survival fragment N-dependent response.

[HSP70, an erythropoiesis regulator that determines the fate of erythroblasts between death and differentiation]

Erythropoiesis is finely regulated by two major cytokines, stem cell factor (SCF) and erythropoietin (Epo). Decrease levels of Epo result in caspase activation and erythroid progenitors apoptosis. However, normal erythroid cell maturation requests caspase activation and cleavage of various caspase substrates, except the erythroid transcription factor GATA-1, that is protected by interaction with the chaperone HSP70 in the nucleus. Therefore, molecular abnormalities associated with decrease of HSP70 expression in the nucleus may result in ineffective erythropoiesis characterized by apoptosis and impaired maturation of erythroid precursors. These findings open new potential targeted therapies for erythroid disorders.

Caspase-3 cleaves hnRNP K in erythroid differentiation

Post-transcriptional control of gene expression is crucial for the control of cellular differentiation. Erythroid precursor cells loose their organelles in a timely controlled manner during terminal maturation to functional erythrocytes. Extrusion of the nucleus precedes the release of young reticulocytes into the blood stream. The degradation of mitochondria is initiated by reticulocyte 15-lipoxygenase (r15-LOX) in mature reticulocytes. At that terminal stage the release of r15-LOX mRNA from its translational silenced state induces the synthesis of r15-LOX. Heterogeneous nuclear ribonucleoprotein K (hnRNP K) is a key regulator of r15-LOX mRNA translation. HnRNP K that binds to the differentiation control element (DICE) in the 3′ untranslated region (UTR) inhibits r15-LOX mRNA translation initiation. During erythroid cell maturation, activation of r15-LOX mRNA translation is mediated by post-translational modifications of hnRNP K and a decrease of the hnRNP K level. To further elucidate its function in the post-transcriptional control of gene expression, we investigated hnRNP K degradation employing an inducible erythroid cell system that recapitulates both nuclear extrusion and the timely controlled degradation of mitochondria, mediated by the activation of r15-LOX synthesis. Interestingly, we detected a specific N-terminal cleavage intermediate of hnRNP K lacking DICE-binding activity that appeared during erythroid differentiation and puromycin-induced apoptosis. Employing mass spectrometry and enzymatic analyses, we identified Caspase-3 as the enzyme that cleaves hnRNP K specifically. In vitro studies revealed that cleavage by Caspase-3 at amino acids (aa) D334-G335 removes the C-terminal hnRNP K homology (KH) domain 3 that confers binding of hnRNP K to the DICE. Our data suggest that the processing of hnRNP K by Caspase-3 provides a save-lock mechanism for its timely release from the r15-LOX mRNA silencing complex and activation of r15-LOX mRNA synthesis in erythroid cell differentiation.

Ebola virus does not block apoptotic signaling pathways

Since viruses rely on functional cellular machinery for efficient propagation, apoptosis is an important mechanism to fight viral infections. In this study, we sought to determine the mechanism of cell death caused by Ebola virus (EBOV) infection by assaying for multiple stages of apoptosis and hallmarks of necrosis. Our data indicate that EBOV does not induce apoptosis in infected cells but rather leads to a nonapoptotic form of cell death. Ultrastructural analysis confirmed necrotic cell death of EBOV-infected cells. To investigate if EBOV blocks the induction of apoptosis, infected cells were treated with different apoptosis-inducing agents. Surprisingly, EBOV-infected cells remained sensitive to apoptosis induced by external stimuli. Neither receptor- nor mitochondrion-mediated apoptosis signaling was inhibited in EBOV infection. Although double-stranded RNA (dsRNA)-induced activation of protein kinase R (PKR) was blocked in EBOV-infected cells, induction of apoptosis mediated by dsRNA was not suppressed. When EBOV-infected cells were treated with dsRNA-dependent caspase recruiter (dsCARE), an antiviral protein that selectively induces apoptosis in cells containing dsRNA, virus titers were strongly reduced. These data show that the inability of EBOV to block apoptotic pathways may open up new strategies toward the development of antiviral therapeutics.

Caspases playing in the field of neuroinflammation: old and new players

Neuroinflammation is a complex immune response against the harmful effects of diverse stimuli within the central nervous system. Caspases are a family of intracellular cysteine proteases that mediate proteolytic events indispensable for transduction of signaling pathway-controlling biological phenomena such as apoptosis and inflammation. To date, 14 players have been identified in mammals. For many years, caspases were simply divided into 'apoptotic' and 'proinflammatory' caspases and this classification remains useful to some extent. However, increasing evidence indicates that many of these so-called apoptotic caspases also exert nonapoptotic functions. In addition, the role of certain members of the supposed inflammatory caspases in the inflammatory process per se has also been discussed. In this review, we highlight the role for 'apoptotic' and 'proinflammatory' caspases in the regulation of the inflammation response with a special focus on the central nervous system.

ICAD deficiency in human colon cancer and predisposition to colon tumorigenesis: linkage to apoptosis resistance and genomic instability

We previously showed that DNA fragmentation factor, which comprises a caspase-3-activated DNase (CAD) and its inhibitor (ICAD), may influence the rate of cell death by generating PARP-1-activating DNA breaks. Here we tested the hypothesis that ICAD-deficient colon epithelial cells exhibiting resistance to death stimuli may accumulate additional genetic modifications, leading to a tumorigenic phenotype. We show that ICAD deficiency may be associated with colon malignancy in humans. Indeed, an examination of ICAD expression using immunohistochemistry in an array of both colon cancer and normal tissues revealed that ICAD expression levels were severely compromised in the cancerous tissues. Upon DNA damage caused by a low dose of irradiation, ICAD cells acquire a tumorigenic phenotype. Colon epithelial cells derived from ICAD mice showed a significant resistance to death induced by the colon carcinogen dimethylhydrazine in vitro and in mice. Such resistance was associated with a decrease in PARP-1 activation. In an animal model of dimethylhydrazine-induced colon tumorigenesis, ICAD^−/− mice developed significantly higher numbers of tumors with markedly larger sizes than the wild-type counterparts. Interestingly, the phenotype of the ICAD^−/− mice was not associated with a significant increase in the precancerous aberrant crypt foci suggesting a potential link to tumor progression rather than initiation. More importantly, ICAD deficiency was associated with severe genomic instability as assessed by array comparative genomic hybridization. Such genomic instability consisted most prominently of amplifications but with sizable deletions as compared to the wild-type counterparts affecting several cancer-related genes including RAF-1, GSN, LMO3, and Fzd6 independently of p53. Altogether, our results present a viable case for the involvement of ICAD deficiency in colon carcinogenesis and show that apoptosis and genomic instability may comprise the means by which such deficiency may contribute to the process of increasing susceptibility to carcinogen-induced tumorigenesis.