Glucocorticoid-induced thymocyte apoptosis in wall lizard Hemidactylus flaviviridis

Real-time imaging of viable-apoptotic switch in GSNO-induced mouse thymocyte apoptosis

Many scientists are focusing on the apoptotic-necrotic or the necrotic-apoptotic switch and its mechanism, but little attention has been paid to the viable-apoptotic switch. Most of the techniques and methods used for detecting apoptosis are performed on fixed samples, yielding static information of specific time points. We have studied the viable-apoptotic switch in S-nitrosoglutathione (GSNO)-induced mouse thymocyte apoptosis in real-time by means of a novel technique, intensified charge coupled device (ICCD)-based real-time fluorescence micro-imaging, coupled with Annexin V-FITC labeling for phosphatidylserine (PS) translocation in cell membrane. We have successfully recorded the initiating time points (mostly at 2 h) of the viable-apoptotic switch in GSNO-initiated apoptosis, as well as shown the real-time differences between living and apoptotic thymocytes. These findings suggest that NO is also a switch molecule for the conversion from viable to apoptotic cell. Thymocytes cotreated by N-G-monomethyl-L-arginine acetate salt (L-NMMA) provide further evidence for this suggestion, as well as for the suggestion that L-NMMA prolongs the early stage of thymocyte apoptosis rather than strongly blocks it.

Cellular mechanism of estrogen-induced thymic involution in wall lizard: caspase-dependent action

The present study, for the first time in an ectothermic vertebrate, demonstrates the cellular mechanism of estrogen-induced thymic involution. Ovariectomy in lizards during the preparatory phase of the reproductive cycle resulted in distinct differentiation of cortico-medullary regions and increase in cellularity, especially in the cortical region. The ovariectomy-induced changes were reversed following administration of 17-estradiol (E2), suggesting a primary role of E2 in causing thymic atrophy. To understand the cellular mechanism of E2-induced thymic atrophy, in vitro effect of E2 was investigated on thymocyte proliferation and apoptosis. E2 decreased the uptake of tritiated thymidine (3H-TdR) by thymocytes in a dose-dependent manner, suggesting that estrogen directly inhibits the thymocyte proliferation. Unlike proliferation, E2 did not have any direct effect on thymocyte apoptosis, as evident by DNA gel electrophoretic, flow cytometric or fluorescence microscopic studies. However, in the presence of thymic epithelial cell-rich stromal components (TEC), E2 treatment at low or high concentrations resulted in depolarization of plasma membrane, DNA fragmentation and decrease in DNA content. This suggests that E2 indirectly, through TEC-secreted factors, controls thymocyte apoptosis. Similar result was observed following fluorescence microscopy. The indirect effect of E2 was further ascertained with the findings that E2-pretreated TEC-conditioned medium accelerated the thymocyte apoptosis. Nevertheless, exposure of thymocytes to E2 was seen to be inevitable for the apoptotic action of TEC-secreted paracrine factors. In the presence of TEC, a positive reaction for caspase-3, -7 and -9 and enzyme substrate, poly(ADP-ribose) polymerase (PARP) in response to E2 suggests the caspase-dependent thymocyte apoptosis in the wall lizard Hemidactylus flaviviridis. Further, E2 was shown to act through genomic pathway, since the receptor antagonist tamoxifen and transcription/translation inhibitors blocked its apoptotic action. Interestingly, the apoptotic effect of E2 was effectively decreased by progesterone.

Mechanism of androgen-induced thymic atrophy in the wall lizard, Hemidactylus flaviviridis: an in vitro study

The present in vitro study demonstrates the effect of androgen on thymocyte apoptosis leading to thymic atrophy in the wall lizard, Hemidactylus flaviviridis. Thymocytes collected from castrated lizards were incubated with varying concentrations of dihydrotestosterone (DHT) to observe its effect on proliferation and apoptosis. DHT treatment reduced the tritiated thymidine incorporation in thymocytes, suggesting that androgen directly inhibits thymocyte proliferation. It also caused apoptosis of thymocytes effectively at 10(-7)M. However, the increased apoptotic action of DHT was indirectly mediated through thymic epithelial cell-rich stromal cell components (TEC). This observation was reaffirmed by in vitro incubation of thymocytes with DHT-pretreated TEC-conditioned medium. However, the DHT-induced TEC-secreted apoptotic factors could induce thymocyte DNA fragmentation only when DHT was added to the conditioned medium. It implies that DHT priming of thymocytes is required for the apoptotic effect of DHT-induced TEC-secreted factor. DHT-induced thymocyte apoptosis was found to be caspase-dependent since it activated the initiator (caspase-9) and effector caspases (caspases-3 and -7) as well as cleaved the enzyme substrate poly(ADP-ribose) polymerase (PARP). Further, the apoptotic effect of DHT was routed through its classical receptors, as non-steroidal antiandrogen flutamide blocked the DHT-induced thymocyte apoptosis. The inhibition of apoptosis by transcription/translation inhibitors further substantiates the genomic pathway of DHT action. It can be concluded that DHT, in addition to inhibiting thymocyte proliferation directly, accelerates caspase-dependent apoptotic process in thymocytes indirectly through TEC via a genomic pathway. Nevertheless, the priming of thymocytes with DHT is required for the apoptotic effect of TEC-secreted factor.

IgH-2 cells: a reptilian model for apoptotic studies

Regulation of proper cell number in tissues depends upon a balance between cell proliferation and cell death. The process of apoptosis has thus far been studied in a variety of multicellular organisms from humans to higher plants. In order to broaden our perspective and identify another metazoan system with which to deepen our understanding of the function and evolution of the apoptotic machinery, we have characterized cell death in a reptilian cell line. We show that the death of IgH-2 Iguana (Iguana iguana) heart cells [Clark, H.F., Cohen, M.M., Karzon, D.T., 1970. Characterization of reptilian cell lines established at incubation temperatures of 23 to 36 degrees. Proc. Soc. Exp. Biol. Med. 133, 1039-1047.] is, in response to DNA damaging agents, accompanied by classic morphological changes of apoptosis including detachment from the substrate, cell shrinkage, nuclear pyknosis and externalization of the plasma membrane phospholipid phosphatidylserine. Our biochemical studies show that the death of IgH-2 cells is accompanied by internucleosomal DNA fragmentation and activation of caspases. Our studies with the pan-caspase inhibitor zVAD.fmk implicate caspases in the apoptotic process we observe. This work represents the first detailed molecular and biochemical analysis of apoptosis in cells of an organism of class Reptilia and establishes IgH-2 cells as a suitable model system with which to investigate the phenomenon of caspase dependent apoptosis and the apoptotic machinery in a reptilian model.