Behring Institute Mitteilungen最新文献

The tumor suppressor gene p53 plays a major role in the protection of cells from DNA damage. Activation of the protein in response to irradiation or genotoxic agents, and possibly by other signals, results in growth arrest at the G1 phase of the cell cycle or in apoptosis. While it has been shown that the ability of p53 to function as a sequence-specific transcriptional activator is necessary for the induction of growth arrest, the mechanism of p53-mediated apoptosis is not clear yet. In the present report we summarize the results obtained by several groups concerning p53-mediated apoptotic pathway. We suggest that p53 may induce apoptosis via a complex network of interacting pathways, which may be transcriptionally dependent or independent, depending on external signals and on the cellular context. Whatever the mechanisms are, the outcome-cell death by apoptosis-is a key function of the tumor suppressor p53.

It is generally believed that the avidity of the T cell receptor for self antigen/MHC determines the fate of a thymocyte. However, it is not understood how the thymocyte distinguishes a survival signal (positive selection) from a death signal (negative selection). Recent studies from our laboratory have explored the role that thymus-produced glucocorticoids may play in influencing thymocyte development. It appears that glucocorticoids are important and necessary at several points during thymocyte differentiation and that they may regulate antigen-specific T cell development.

The protein product of the proto-oncogene bcl-2, originally discovered by virtue of its chromosomal translocation in human follicular centre B cell lymphoma, is a physiological inhibitor of programmed cell death, apoptosis. Initial studies in transgenic mice overexpressing Bcl-2 in B or T lymphocytes demonstrated that Bcl-2 can potently antagonise cell death induced by multiple independent signal transduction routes and can contribute to oncogenesis, particularly in combination with other oncogenes, like c-myc, that promote cell proliferation. Further investigations using crosses between bcl-2 transgenic mice and T cell receptor or immunoglobulin transgenic mice or mutant mice deficient in proper antigen receptor gene rearrangement demonstrated that Bcl-2 can only block death of cells that failed to receive a positive stimulus, "death by neglect', but not activation induced apoptosis. Collectively, these results provide evidence that distinct signalling pathways for apoptosis converge upon a common effector machinery where Bcl-2 acts as an antagonist, but that there also exists a mechanism that can either bypass the Bcl-2 checkpoint or override its protective function. These experimental data are reviewed here and discussed in context of current knowledge of lymphocyte differentiation, tumorigenesis and cell death regulation.

In mice deficient in the Fas/FasL apoptotic signaling pathways, T-dependent antibody responses to conventional environmental antigens are mechanistically distinct from the antibody responses to self-antigens. The latter appear to be initiated in the T cell rich inner PALS of the spleen, where the majority of antibody producing cells are located early in the disease process. The data are consistent with the premise that the inner PALS, and not germinal centers, is the major site of FasL regulation of B cell activity.

Fas is a cell-surface protein belonging to the tumor necrosis factor (TNF) receptor family, whereas the Fas ligand (FasL) is a member of the TNF family. FasL binds to Fas, which results in target cell apoptosis. A family of cysteine proteases is sequentially activated to proceed the Fas-induced apoptosis, whereas Bcl-2 inhibits the process. FasL is expressed in activated T cells and natural killer (NK) cells, and works as an effector of these cytotoxic cells to remove the cells infected by virus, or cancer cells. The Fas system is also involved in peripheral clonal deletion, and/or the activation-induced suicide of T cells to down-regulate the immune reaction. Mouse mutations of lymphoproliferation (lpr) and generalized lymphoproliferative disease (gld), which cause lymphadenopathy and splenomegaly, and accelerate autoimmune disease, are loss-of-function mutations in the Fas and FasL genes, respectively. Moreover, the Fas-null mice established by gene targeting showed hyperplasia in the liver, suggesting that the Fas system is involved in turn-over of senescent hepatocytes.

We have studied apoptosis in lymph node and peripheral blood mononuclear cells (PBMCs) from HIV-infected children and adults and SIV-infected rhesus macaques. In lymph nodes, we found that apoptosis and productive infection occurred only rarely in the same cell. There was, however, a direct correlation between the numbers of apoptotic and productively-infected cells. In HIV-infected children, we found a direct correlation between disease severity and percentage CD4+ T cell apoptosis (p = 0.001). Both CD4+ and CD8+ T cell apoptosis were directly related to CD4+ T cell depletion (p = 0.006 and p = 0.01, respectively). In addition, we found a trend towards diminished CD4+ T cell apoptosis on anti-retroviral agents. These findings suggest that apoptosis of uninfected cells may be important in HIV pathogenesis and that measurement of apoptosis may be a useful marker of disease activity.

Cytotoxic drugs currently remain as the basis for the chemotherapy of metastatic cancer. Why they fail to kill sufficient tumour cells in the major human solid cancers, such as the carcinomas, is suggested in this review to be due to the inherent inability of these cells to engage apoptosis after drug-induced damage. As a paradigm for drug resistant cancers, the resistance of bladder carcinoma cell lines to DNA damaging drugs is described here in terms of their response to the topoisomerase II poison etoposide. 60%-70% of bladder carcinomas have mutant p53; this can prevent the detection of and response to DNA damage. In vitro studies with a bladder carcinoma cell line containing a wild type p53 showed that it underwent a G1 checkpoint after etoposide, potentially allowing DNA damage repair, as well as apoptosis. In lines with mutant or non-functional p53 there is no checkpoint and no apoptosis. All lines showed constitutive expression of bcl-2 and bcl-XL (the suppressors of apoptosis) with low and non-inducible levels of bax (a promoter of apoptosis). Taken together, this menu of gene expression is more favourable to survival than apoptosis after the imposition of drug-induced DNA damage and may contribute to their inherent drug resistance.

Activation induced cell death (AICD) plays a critical role in eliminating autoimmune cells and limiting inflammation after activation. The two major signaling molecules for AICD are the Fas and TNF-R pathways of apoptosis. Defective Fas apoptosis in lpr/lpr mice results in a compensatory increase in TNF-R/TNF-mediated apoptosis. TNF/TNF-R has been shown to be a compensatory pathway of apoptosis in T cells and macrophages of lpr/lpr mice. Therefore, early production of TNF/TNF-R limit an immune response by inducing AICD in the absence of an intact Fas/Fas ligand apoptosis pathway. However, increased TNF production in lpr mice also lead to increased susceptibility to septic shock and autoimmune disease such as arthritis. Therefore TNF production during an inflammatory response can downmodulate this response, but this also results in the failure to downmodulate TNF production leading to septic shock and arthritis. A second pathway of AICD is mediated by Nur77 after T cell stimulation through the CD3 molecule. Mice with defective Nur77 signaling undergo AICD using the Fas-Fas ligand pathway to eliminate autoreactive T cells. A third defect of AICD is observed in HCP-mutant me/me (motheaten) mice which develop autoimmune disease related to defective Fas apoptosis signaling. Therefore, multiple interactive pathways play a role in limiting development of autoimmunity.