Caspases in T-cell receptor-induced thymocyte apoptosis

IFNγ and iNOS-Mediated Alterations in the Bone Marrow and Thymus and Its Impact on Mycobacterium avium-Induced Thymic Atrophy

Disseminated infection with the high virulence strain of Mycobacterium avium 25291 leads to progressive thymic atrophy. We previously showed that M. avium-induced thymic atrophy results from increased glucocorticoid levels that synergize with nitric oxide (NO) produced by interferon gamma (IFNγ) activated macrophages. Where and how these mediators act is not understood. We hypothesized that IFNγ and NO promote thymic atrophy through their effects on bone marrow (BM) T cell precursors and T cell differentiation in the thymus. We show that M. avium infection cause a reduction in the percentage and number of common lymphoid progenitors (CLP). Additionally, BM precursors from infected mice show an overall impaired ability to reconstitute thymi of RAGKO mice, in part due to IFNγ. Thymi from infected mice present an IFNγ and NO-driven inflammation. When transplanted under the kidney capsule of uninfected mice, thymi from infected mice are unable to sustain T cell differentiation. Finally, we observed increased thymocyte death via apoptosis after infection, independent of both IFNγ and iNOS; and a decrease on active caspase-3 positive thymocytes, which is not observed in the absence of iNOS expression. Together our data suggests that M. avium-induced thymic atrophy results from a combination of defects mediated by IFNγ and NO, including alterations in the BM T cell precursors, the thymic structure and the thymocyte differentiation.

Methoxychlor metabolite HPTE alters viability and differentiation of embryonic thymocytes from C57BL/6 mice

Endocrine-disrupting chemicals (EDC) are widespread in the built and natural environments. Heightened public awareness of their potential danger has led to concern about whether EDC and their metabolites have significant negative biological effects. Studies have shown that EDC like DDT and other organochlorine pesticides, such as methoxychlor (MXC), have adverse effects on immune cells, but no studies have addressed the impact of HPTE, the primary metabolite of MXC. To elucidate the presence and significance of HPTE adverse effects, this study explored the impact of HPTE on a critical window and component of immune system development, embryonic T-cell development. Lesions at this phase of development can lead to lifelong immune dysfunction and increased incidence of immune disease, such as autoimmunity. Embry-onic thymocytes (GD 16-18) from C57BL/6 mice were subjected to an in vitro differentiation culture that mimicked early steps in thymocyte development in the presence of 0.005, 0.05, 0.5, 5, or 50 μM HPTE, or a model endocrine disruptor, DES. The results indicated that compared to the vehicle control, HPTE- and DES-induced death of thymocytes. Annexin-V staining and Caspase 8, markers of programed cell death, revealed that the loss of cells was due at least in part to induction of apoptosis. Moreover, HPTE-induced cell death not only resulted in selective loss of double positive thymocytes, but also loss of developing CD4 intermediate cells (post-double positive partially differentiated thymocyte population). Phenotypic analysis of thymocyte maturation (T-cell receptor, TCR) and TCR ligation (CD5) surface markers revealed that surviving embryonic thymocytes expressed low levels of both. Taken together these data demonstrate that immature embryonic thymocytes are sensitive to HPTE exposure and that HPTE exposure targets thymocyte populations undergoing critical differentiation steps. These findings suggest HPTE may play a pivotal role in MXC exposure-induced immune dysfunction.

Genetic ataxia telangiectasia porcine model phenocopies the multisystemic features of the human disease

Ataxia telangiectasia (AT) is a progressive multisystem autosomal recessive disorder caused by mutations in the AT-mutated (ATM) gene. Early onset AT in children is characterized by cerebellar degeneration, leading to motor impairment. Lung disease and cancer are the two most common causes of death in AT patients. Accelerated thymic involution may contribute to the cancer, and recurrent and/or chronic respiratory infections may be a contributing factor to lung disease in AT. AT patients have fertility issues, are highly sensitive to ionizing radiation and they present oculocutaneous telangiectasia. Current treatments only slightly ameliorate disease symptoms; therapy that alters or reverses the course of the disease has not yet been discovered. Previously, we have shown that ATM-/- pigs, a novel model of AT, present with a loss of Purkinje cells, altered cerebellar cytoarchitecture and motor coordination deficits. ATM-/- porcine model not only recapitulates the neurological phenotype, but also other multifaceted clinical features of the human disease. Our current study shows that ATM-/- female pigs are infertile, with anatomical and functional signs of an immature reproductive system. Both male and female ATM-/- pigs show abnormal thymus structure with decreased cell cycle and apoptosis markers in the gland. Moreover, ATM-/- pigs have an altered immune system with decreased CD8+ and increased natural killer and CD4+CD8+ double-positive cells. Nevertheless, ATM-/- pigs manifest a deficient IgG response after a viral infection. Based on the neurological and peripheral phenotypes, the ATM-/- pig is a novel genetic model that may be used for therapeutic assessments and to identify pathomechanisms of this disease.

Diethylstilbestrol (DES)-Induced Fetal Thymic Atrophy in C57BL/6 Mice: Inhibited Thymocyte Differentiation and Increased Apoptotic Cell Death

Treatment of pregnant C57Bl/6 mice with 48 μg/kg diethylstilbestrol (DES) on gestation days (GDs) 14 and 16 resulted in both decreased day 18 fetal thymic cellularity as well as alterations in thymocyte phenotype. Histopathologic examination of GD 18 fetal thymi from DES-exposed dams demonstrated a decrease in thymic size and cellularity and an increase in pyknotic nuclei, indicative of apoptosis, relative to control thymi. Thymic architecture was also altered by DES treatment with a decrease in the distinction between the cortical and medullary regions. Flow cytometric staining of day 18 thymocyte suspensions with the apoptotic marker 7-aminoactinomycin D showed a decrease in thymocyte viability after DES, and a concomitant increase of thymocytes in early apoptosis. When thymocytes were co-identified by CD4 and CD8 cell surface antigen expression, trends toward increased apoptosis were present in the CD4+CD8+and CD4+CD8−subpopulations, and significantly increased apoptosis occurred in the CD4−CD8−and CD4−CD8+subpopulations. These histopathologic and flow cytometric findings support enhanced apoptosis of thymocytes as a contributing factor to fetal thymic atrophy caused by DES.

Lethal and nonlethal murine malarial infections differentially affect apoptosis, proliferation, and CD8 expression on thymic T cells

Although thymic atrophy and apoptosis of the double-positive (DP) T cells have been reported in murine malaria, comparative studies investigating the effect of lethal and nonlethal Plasmodium infections on the thymus are lacking. We assessed the effects of P. yoelii lethal (17XL) and nonlethal (17XNL) infections on thymic T cells. Both strains affected the thymus. 17XL infection induced DP T-cell apoptosis and a selective decrease in surface CD8 expression on developing thymocytes. By contrast, more severe but reversible effects were observed during 17XNL infection. DP T cells underwent apoptosis, and proliferation of both DN and DP cells was affected around peak parasitemia. A transient increase in surface CD8 expression on thymic T cells was also observed. Adult thymic organ culture revealed that soluble serum factors, but not IFN-γ or TNF-α, contributed to the observed effects. Thus, lethal and nonlethal malarial infections led to multiple disparate effects on thymus. These parasite-induced thymic changes are expected to impact the naïve T-cell repertoire and the subsequent control of the immune response against the parasite. Further investigations are required to elucidate the mechanism responsible for these disparate effects, especially the reversible involution of the thymus in case of nonlethal infection.

Role of caspase-8 in thymus function

The thymus is the primary organ responsible for de novo generation of immunocompetent T cells that have a diverse repertoire of antigen recognition. During the developmental process, 98% of thymocytes die by apoptosis. Thus apoptosis is a dominant process in the thymus and occurs through either death by neglect or negative selection or through induction by stress/aging. Caspase activation is an essential part of the general apoptosis mechanism, and data suggest that caspases may have a role in negative selection; however, it seems more probable that caspase-8 activation is involved in death by neglect, particularly in glucocorticoid-induced thymocyte apoptosis. Caspase-8 is active in double-positive (DP) thymocytes in vivo and can be activated in vitro in DP thymocytes by T-cell receptor (TCR) crosslinking to induce apoptosis. Caspase-8 is a proapoptotic member of the caspase family and is considered an initiator caspase, which is activated upon stimulation of a death receptor (e.g., Fas), recruitment of the adaptor molecule FADD, and recruitment and subsequent processing of procaspase-8. The main role of caspase-8 seems to be pro-apoptotic and, in this review, we will discuss about the involvement of caspase-8 in (1) TCR-triggered thymic apoptosis; (2) death receptor-mediated thymic apoptosis; and (3) glucocorticoid-induced thymic apoptosis. Regarding TCR triggering, caspase-8 is active in medullary, semi-mature heat-stable antigen(hi) (HAS(hi) SP) thymocytes as a consequence of strong TCR stimulation. The death receptors Fas, FADD, and FLIP are involved upstream of caspase-8 activation in apoptosis; whereas, Bid and HDAC7 are involved downstream of caspase-8. Finally, caspase-8 is involved in glucocortocoid-induced thymocyte apoptosis through an activation loop with the protein GILZ. GILZ activates caspase-8, promoting GILZ sumoylation and its protection from proteasomal degradation.

The membrane adaptor LAT is proteolytically cleaved following Fas engagement in a tyrosine phosphorylation-dependent fashion

Engagement of the TCR (T-cell receptor) induces tyrosine phosphorylation of the LAT (linker for the activation of T-cells) adaptor, and thereby it recruits several cytosolic mediators for downstream signalling pathways. The Fas protein is essential for T-lymphocyte apoptosis, and following Fas engagement, many proteins are proteolytically cleaved, including several molecules that are important for the transduction of TCR intracellular signals. In the present study, we demonstrate that the adaptor LAT is also subject to a proteolytic cleavage in mature T-lymphocytes and thymocytes in response to Fas engagement, and also on TCR stimulation, and we identify three aspartic acid residues at which LAT is cleaved. Interestingly, these aspartic acid residues are located in proximity to several functionally important tyrosine residues of LAT, raising the possibility that their phosphorylation could modulate LAT cleavage. Consistent with that hypothesis, we show that induction of phosphorylation by pervanadate or H2O2 in Jurkat cells and thymocytes inhibits Fas-mediated cleavage of LAT. Moreover, we show that LAT proteolysis is also enhanced during anergy induction of primary human T-cells, suggesting that LAT cleavage may act as a regulator of TCR-mediated activation of T-cells and not only as a transducer of cell death promoting stimuli.

Immunohistological analysis of cell cycle and apoptosis regulators in thymus

The combined expression patterns of cell cycle and apoptosis regulators have not been analyzed in details in human thymus to the best of our knowledge. Our objective was to provide multiparametric and combined immunohistological information regarding the expression levels and the topographical distribution of major cell cycle and apoptosis regulators in postnatal human thymus. Ki67 and cyclins A, B1, D3 and E were frequently expressed by thymocytes with higher expression in cortical than medullary thymocytes. The expression of cyclin D2 was low in thymocytes. Thymic epithelial cells (TEC) exhibited low expression of Ki67 and cyclins. Bid was frequently expressed by thymocytes, Bcl-xL by cortical thymocytes and Bcl-2 by medullary thymocytes. The expression levels of Bim and survivin in thymocytes were low. The expression levels of Bax and Mcl-1 were higher in medullary than cortical thymocytes and TEC. Bak and Bad were mainly expressed in medullary TEC and Hassall Bodies (HB). c-FLIP and Fas were frequently expressed in TEC and FasL was mainly expressed by medullary TEC and HB. Cleaved caspase-3 was expressed by scattered thymocytes at the cortex and the corticomedullary junction and very rarely at the medulla. The different expression profiles and immunotopographical distribution of cell cycle and apoptosis regulators in thymocytes and TEC indicate that their expression is tightly regulated during thymic cell differentiation and that they are differentially involved in the cell survival/death regulation of thymocytes and TEC. Furthermore, this study indicates decrease of the proliferation and caspase-dependent apoptosis of thymocytes from the cortex to the medulla.

Negative selection assay based on stimulation of T cell receptor transgenic thymocytes with peptide-MHC tetramers

Thymocyte negative selection is a requirement for the development of self tolerance. Although it is possible to assay the induction of cell death in thymocytes in vitro using antibody cross-linking, this stimulus is much stronger than the normal range of T cell receptor ligands that could be encountered during normal development. Signaling in thymocytes is finely balanced between positive and negative selection stimuli, where a negative selecting ligand can be only marginally higher affinity than a positive selecting ligand. We have therefore developed an assay for the induction of negative selection that can distinguish such cases, and that is amenable to high-throughput analysis. The assay is based on the induction of activated caspase 3 in thymocytes expressing a defined T cell receptor, after stimulation with MHC-peptide tetramers in vitro for 24 hours or less.

Fas apoptosis inhibitory molecule regulates T cell receptor-mediated apoptosis of thymocytes by modulating Akt activation and Nur77 expression

Fas apoptosis inhibitory molecule (FAIM) has been demonstrated to confer resistance to Fas-induced apoptosis of lymphocytes and hepatocytes in vitro and in vivo. Here, we show that FAIM is up-regulated in thymocytes upon T cell receptor (TCR) engagement and that faim(-/-) thymocytes are highly susceptible to TCR-mediated apoptosis with increased activation of caspase-8 and -9. Furthermore, injection of anti-CD3 antibodies leads to augmented depletion of CD4(+)CD8(+) T cells in the thymus of faim(-/-) mice compared with wild-type control, suggesting that FAIM plays a role in thymocyte apoptosis. Cross-linking of the TCR on faim(-/-) thymocytes leads to an elevated protein level of the orphan nuclear receptor Nur77, which plays a role in thymocyte apoptosis. Interestingly, in the absence of FAIM, there are reduced ubiquitination and degradation of the Nur77 protein. Faim(-/-) thymocytes also exhibit a defective TCR-induced activation of Akt whose activity we now show is required for Nur77 ubiquitination. Further analyses utilizing FAIM-deficient primary thymocytes and FAIM-overexpressing DO-11.10 T cells indicate that FAIM acts upstream of Akt during TCR signaling and influences the localization of Akt to lipid rafts, hence affecting its activation. Taken together, our study defined a TCR-induced FAIM/Akt/Nur77 signaling axis that is critical for modulating the apoptosis of developing thymocytes.

[Immunocytochemical detection of caspase 3 in various diseases of human nasal mucosa]

BACKGROUND

Inflammatory processes play a central role in the pathogenesis of chronic rhinosinusitis (CRS), however, the impact of apoptosis in CRS in unclear. The aim of this study was a comparison of caspase 3 activity, a key enzyme in the apoptosis cascade, in samples of nasal tissue from patients with different types of rhinosinusitis.

MATERIAL AND METHODS

Immunohistochemical analyses were carried out to detect caspase 3 in samples of nasal tissue from patients with CRS and nasal polyps (NP), allergic rhinitis (AR), rhinitis medicamentosa (RM) and atrophic rhinitis.

RESULTS

Evidence of increased apoptosis was found in the epithelium, submucosal glands and blood vessels in NP samples. Samples of RM showed strong caspase 3 activity in the endothelial cells and the lamina muscularis of blood vessels. In atrophic rhinitis the epithelium and subepithelial glands showed increased caspase 3 activity. In samples of patients with allergic rhinitis caspase 3 activity could not be detected.

CONCLUSIONS

Activated apoptosis seems to play an important role in the pathogenesis of different types of rhinosinusitis. Further investigations on the induction of caspase 3 are necessary for the development of new therapeutic strategies to influence the regulation of apoptosis.

Free radical scavenger edaravone suppresses x-ray-induced apoptosis through p53 inhibition in MOLT-4 cells

Edaravone, a clinical drug used widely for the treatment of acute cerebral infarction, is reported to scavenge free radicals. In the present study, we investigated the radioprotective effect of edaravone on X-ray-induced apoptosis in MOLT-4 cells. Apoptosis was determined by the dye exclusion test, Annexin V binding assay, cleavage of caspase, and DNA fragmentation. We found that edaravone significantly suppressed the X-ray-induced apoptosis. The amount of intracellular ROS production was determined by the chloromethyl-2',7'-dichlorodihydro-fluorescein diacetate system. We found that the intracellular ROS production by X-irradiation was completely suppressed by the addition of edaravone. The accumulation and phosphorylation of p53 and the expression of p21(WAF1), a target protein of p53, which were induced by X-irradiation, were also suppressed by adding edaravone. We conclude that the free radical scavenger edaravone suppresses X-ray-induced apoptosis in MOLT-4 cells by inhibiting p53.

Vhlh gene deletion induces Hif-1-mediated cell death in thymocytes

The von Hippel-Lindau gene product (pVHL) targets the alpha subunit of basic helix-loop-helix transcription factor hypoxia-inducible factor (HIF) for proteasomal degradation. Inactivation of pVhl in the mouse germ line results in embryonic lethality, indicating that tight control of Hif-mediated adaptive responses to hypoxia is required for normal development and tissue function. In order to investigate the role of pVhl in T-cell development, we generated mice with thymocyte-specific inactivation of Vhlh resulting in constitutive transcriptional activity of Hif-1, as well as mice with thymocyte-specific repression of Hif-1 in a wild-type and Vhlh-deficient background. Thymi from Vhlh-deficient mice were small due to a severe reduction in the total number of CD4/CD8-double-positive thymocytes which was associated with increased apoptosis in vivo and in vitro. Increased apoptosis was a result of enhanced caspase 8 activity, while Bcl-2 and Bcl-XL transgene expression had little effect on this phenotype. Inactivation of Hif-1 in Vhlh-deficient thymocytes restored thymic cellularity as well as thymocyte viability in vitro. Our data suggest that tight regulation of Hif-1 via pVhl is required for normal thymocyte development and viability and that an increase in Hif-1 transcriptional activity enhances caspase 8-mediated apoptosis in thymocytes.

Involvement of MKK6 in TCRalphabeta(int)CD69lo: a target population for apoptotic cell death in thymocytes

By analyzing real-time caspase activity and DNA fragmentation in live thymocytes, we found that apoptosis occurs predominantly in a TCRalphabeta(int)/hiCD69lo population. The number of caspase-active cells and DNA-fragmented cells in MKK6-deficient mice, which were originally generated in our laboratory by gene targeting, was decreased in the TCRalphabeta(int)CD69lo population but not in the TCRalphabetahiCD69lo population. The percentage of caspase-active cells in the H-Y-specific TCRint population was more clearly decreased in male MKK6-deficient H-Y TCR-transgenic mice. Furthermore, the absolute number of TCRhiCD4loCD8lo cells, which are developmentally next to TCRintCD4hiCD8hi cells, was increased in MKK6-deficient H-Y TCR-transgenic mice. Deletion of TCRalphabeta(int)CD4hiCD8hi cells by injecting antigenic lymphocytic chorio-meningitis virus (LCMV) peptide into LCMV-specific TCR-transgenic mice was incomplete in MKK6-deficient mice. Cellular death of TCRalphabeta(int) fetal thymocytes induced by adding an antigenic peptide into an in vitro fetal thymic organ culture system was also diminished in MKK6-deficient TCR-transgenic thymi. These results indicate that MKK6 plays a role in the developing thymocytes, especially in the population of TCRalphabeta(int)CD69lo cells, which possibly undergo negative selection.

Involvement of caspace-3 in the cleavage of terminal transferase

To investigate the in vivo role of caspase-3 in Terminal Transferase metabolism DMSO-treated RPMI-8402, a human pre-T cell line was used. In DMSO treated samples (3)H-dGTP incorporation and TdT phosphorylation occurs after 4 hours of treatment. After 8 hours cells undergo TdT proteolysis in addition to its inactivation. The cleavage of TdT into 32- and 58-KDa proteolytic fragments occurred simultaneously with the activation of Caspase-3, but preceded changes associated with the apoptotic process described after 48 hours of treatment. The Caspase-3 peptide inhibitor V, used as a specific inhibitor, prevented TdT proteolysis prolonging its activity and rescued cells from apoptosis. Our experiments suggest that TdT is a nuclear substrate for Caspase-3, the main apoptotic effector protease in many cell types, and that the cleavage of TdT represents a primary step in a signal cascade leading to pre-T cell apoptosis.

Death-inducing receptors and apoptotic changes in lymphocytes of patients with heart transplant vasculopathy

The specific role of lymphocyte apoptosis and transplant-associated atherosclerosis is not well understood. The aim of our study was to investigate the impact of T cell apoptotic pathways in patients with heart transplant vasculopathy. Amongst 40 patients with cardiac heart failure class IV who have undergone heart transplantation, 20 recipients with transplant-associated coronary artery disease (TACAD) and 20 with non-TACAD were investigated one year postoperative. Expression of CD95 and CD45RO, and annexin V binding were measured by FACS. Soluble CD95, sCD95 ligand (sCD95L), tumour necrosis factor receptor type 1 (sTNFR1), and histones were measured in the sera by ELISA. The percentage of cells expressing CD3 and CD4 was significantly reduced in TACAD as well as in non-TACAD patients as compared with control volunteers. Interestingly, the proportion of CD19+ (B cells) and CD56+ (NK) cells was increased in TACAD groups (versus non-TACAD; P < 0.01, and P < 0.001, respectively). In contrast to sCD95, the expression of CD95 (APO-1/Fas) and CD45RO (memory T cells), and sCD95L were significantly increased in non-TACAD and TACAD patients. T cell activation via CD95 with consecutive apoptosis was increased in both groups. The concentration of sTNFR1, IL-10 and histones was significantly elevated in sera from TACAD than non-TACAD patients, and in both groups than in healthy controls. These observations indicate that the allograft may induce a pronounced susceptibility of CD4+ T cells to undergo apoptosis and antibody-driven activation-induced cell death. This data may suggest a paradox immune response similar to that seen in patients with autoimmune diseases.

Novel protein kinase C delta isoform insensitive to caspase-3

Protein kinase C delta (PKC delta) plays a key regulatory role in a variety of cellular functions, including apoptosis, as well as cell growth and differentiation. We previously reported that apoptosis was induced by pretreatment with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), an inhibitor of PKC, in mouse thymocytes. In the present study, we showed that a novel PKC delta isoform (PKC deltaII) was transiently expressed when thymocytes were pretreated with H-7. The analysis of the cDNA encoding PKC deltaII indicated that a 78 bp fragment was inserted into the caspase-3 sensitive site of the original PKC delta (PKC deltaI), presumably by alternative splicing. The PKC deltaII expressed in COS-1 cells was one product with a molecular mass of 81 kDa and with kinase activity similar to that of PKC deltaI. The expressed PKC deltaI protein (78 kDa) was in part cleaved into a 38 kDa fragment in vivo and in vitro, but the PKC deltaII protein was not. Cleavage of the PKC deltaI protein was inhibited by a specific inhibitor of caspase-3, indicating that PKC deltaII is insensitive to caspase-3. The PKC deltaII was highly expressed in the testis and ovary, and at a lower level in the thymocytes, brain and kidney, whereas PKC deltaI was detected in most tissues, suggesting that the function of PKC deltaII is different from that of PKC deltaI.

Impaired T cell proliferation, increased soluble death-inducing receptors and activation-induced T cell death in patients undergoing haemodialysis

Haemodialysis is a widespread option for end-stage renal disease (ESRD). Long-term success of dialysis is, however, limited by a high rate of serious bacterial and viral infections. We compared T cell functions in ESRD patients undergoing haemodialysis (n = 20), or were not dialysed and received conventional medical treatment (n = 20). Healthy volunteers (n = 15) served as controls. The T cell phenotype was examined by immunofluorescence using fluorochrome-labelled monoclonal antibodies and FACS analysis. The concentration of soluble CD95/Fas and of tumour necrosis factor-alpha receptor type 1 (sTNFR1) in the sera was quantified by ELISA. Activation-induced programmed T cell death was triggered by anti-CD3/CD28 antibodies and measured by 7-AAD staining. All immunological tests were performed at least 1 month after dialysis initiation. T cell proliferation in response to phytohaemagglutinin or anti-CD3 monoclonal antibodies was moderately diminished in non-dialysed patients and markedly reduced in haemodialysis patients compared to healthy controls (P < 0.01 and P < 0.001, respectively). In a mixed lymphocyte culture the proliferative response of T cells from dialysed patients was significantly diminished (P < 0.001). T cells of both non-dialysed and dialysed patients have augmented CD95/Fas and CD45RO expression, increased sCD95/Fas and sTNFR1 release and spontaneously undergo apoptosis. Culture of T cells from haemodialysis patients with anti-CD3/CD28 antibodies increased the proportion of CD4(+) T cells committing activation-induced cell death by a mean 7.5-fold compared to T-helper cells from non-dialysed patients (P < 0.001). Renal failure and initiation of haemodialysis results in a reduced proliferative T cell response, an aberrant state of T cell activation and heightened susceptibility of CD4(+) T cells to activation-induced cell death.

Critical role of calcium overloading in cadmium-induced apoptosis in mouse thymocytes

Cadmium (Cd) is a well-known environmental carcinogen and immunotoxin. Currently the direct cytotoxic effects of Cd on thymocytes are largely unexplored. The main objective of the present study was to investigate the apoptogenic property of Cd and the mechanisms involved, using primary cultured mouse thymocytes as a model. Cd-induced apoptosis in thymocytes was studied by TdT-mediated dUTP nick end-labeling assay and DNA gel electrophoresis. The results showed that Cd was able to cause apoptosis in mouse thymocytes in a time- and dose-dependent manner. Moreover, Cd exposure led to a rapid and sustained intracellular calcium (Ca2+) elevation, followed by caspase-3 activation and PARP cleavage, all of which preceded the characteristic DNA fragmentation. BAPTA-AM, a specific intracellular Ca2+ chelator, abolished Cd-induced Ca2+ overloading and subsequently inhibited caspase-3 activation, PARP cleavage, and apoptosis. It is believed that intracellular Ca2+ elevation may trigger caspase-3 activation either through mitochondria or through activation of Ca2+-dependent protease in Cd-treated thymocytes. Results from this study thus provide new information for a better understanding of the immunotoxic and immunomodulatory effects of Cd.

Hormonal regulation of physiological cell turnover and apoptosis

Physiological cell turnover plays an important role in maintaining normal tissue function and architecture. This is achieved by the dynamic balance of cellular regeneration and elimination, occurring periodically in tissues such as the uterus and mammary gland, or at constant rates in tissues such as the gastrointestinal tract and adipose tissue. Apoptosis has been identified as the prevalent mode of physiological cell loss in most tissues. Cell turnover is precisely regulated by the interplay of various endocrine and paracrine factors, which modulate tissue and cell-specific responses on proliferation and apoptosis, either directly, or by altering expression and function of key cell proliferative and/or death genes. Although recent studies have provided significant information on specific tissue systems, a clearly defined pathway that mediates cell turnover has not yet emerged for any tissue. Several similarities exist among the various tissues with regard to the intermediates that regulate tissue homeostatis, enabling a better understanding of the general mechanisms involved in the process. Here we review the mechanisms by which hormonal and cytokine factors mediate cell turnover in various tissues, emphasizing common themes and tissue-specific differences.

Thymocyte apoptosis

Apoptosis is the fate of most thymocytes. Many molecules participate in the decision of whether a thymocyte is to live or to die, including cell surface receptors, such as the T cell receptor for antigen, Notch-1, and costimulatory receptors, ligand-regulated nuclear transcription factors such as the glucocorticoid receptor, signaling, and effector proteases, and direct regulators of the apoptotic machinery such IAPs. In this review we discuss recent data concerning these molecules and pathways and their implication for understanding the mechanisms underlying thymocyte death, survival, and the generation of inmmunocompetent T cells.