Organisms as diverse as fungi and humans use G-protein-coupled receptors to control signal transduction pathways responsive to various hormones, neuroregulatory molecules and other sensory stimuli 1. ...Continual stimulation of these receptors often leads to their desensitization, which is mediated in part by the consecutive actions of two families of proteins – the G-protein-coupled receptor kinases, which phosphorylate the agonist-occupied receptors 2, and the arrestin proteins, which subsequently bind to the receptors 3. We now present evidence that a group of proteins – the G0S8/Sst2p family – may be a third class of receptor-desensitizing factors.
The development of cancer requires multiple genetic alterations perturbing
distinct cellular pathways. In human cancers, these alterations often arise
owing to mutations in tumor-suppressor genes ...whose normal function is to either
inhibit the proliferation, apoptosis, or differentiation of cells, or maintain
their genomic integrity. Mouse models for tumor suppressors frequently provide
definitive evidence for the antitumorigenic functions of these genes. In
addition, animal models permit the identification of previously unsuspected
roles of these genes in development and differentiation. The availability of
null and tissue-specific mouse mutants for tumor-suppressor genes has greatly
facilitated our understanding of the mechanisms leading to cancer. In this
review, we describe mouse models for tumor-suppressor genes.
Inactivation of the retinoblastoma (Rb) tumor suppressor in the mouse induces mid-gestational death accompanied by massive apoptosis in certain tissues. Herein, we analyzed the role of the apoptosis ...protease-activating factor Apaf-1, an essential component of the apoptosome, in mediating apoptosis in Rb-deficient mice. Analysis of compound mutant embryos lacking Rb and Apaf-1 revealed that Apaf-1 was absolutely required for apoptosis in the central nervous system and lens. In contrast, apoptosis in the peripheral nervous system and skeletal muscles only partly depended on Apaf-1 function. The dependency on Apaf-1 coincided with the requirement documented previously for E2F1 and p53 in the respective tissues. Loss of Apaf-1 specifically suppressed apoptosis but not the proliferation and differentiation defects in Rb-mutant embryos. We also show that the Apaf1+ but not the Rb+ allele is retained in pituitary tumors arising in Rb+/-:Apaf1+/- double heterozygous mice. Our results indicate that Apaf-1 plays a critical role in apoptosis in a subset of tissues and that both E2F1:p53:Apaf-1-dependent and -independent apoptotic pathways operate downstream of Rb.
At sites of chronic inflammation seen during infections, autoimmunity, graft-vs-host response, and cytokine therapy, endothelial cell injury is known to occur, the exact mechanism of which is ...unknown. In the current study we used IL-2-induced vascular leak syndrome (VLS) as a model to investigate whether cytotoxic lymphocytes use CD44 in mediating endothelial cell injury. Administration of IL-2 to wild-type mice triggered significant VLS in the lungs and liver. In contrast, in CD44 knockout (KO) mice, IL-2-induced VLS was markedly reduced in the lungs and liver. IL-2-treated wild-type and CD44 KO mice had similar levels of perivascular infiltration with lymphocytes in the lungs and liver. This suggested that the decrease in VLS seen in CD44 KO mice was not due to the inability of lymphocytes to migrate to these organs. Ultrastructural studies demonstrated extensive endothelial cell damage in the lungs and liver of IL-2-treated wild-type, but not CD44 KO, mice. Moreover, CD44-KO mice exhibited a marked decrease in IL-2-induced lymphokine-activated killer cell activity. The induction of VLS was dependent on the expression of CD44 on immune cells rather than endothelial cells because adoptive transfer of CD44+, but not CD44- spleen cells along with IL-2 into CD44 KO mice triggered VLS. The IL-2-induced VLS was blocked by administration of F(ab')2 of Abs against CD44. The current study demonstrates that CD44 plays a key role in endothelial cell injury. Blocking CD44 in vivo may offer a novel therapeutic approach to prevent endothelial cell injury by cytotoxic lymphocytes in a variety of clinical disease models.
The Reed-Sternberg (RS) cells of classical Hodgkin lymphoma (cHL) produce several cytokines, which are thought to account for the unique clinical and pathologic features of this disease. We ...previously identified interleukin (IL)-13 expression as a common feature of cHL and have studied the potential role of this cytokine as an autocrine growth factor for RS cells. IL-13 and the IL-13-specific receptor chain (IL-13R α 1) are frequently expressed in cHL-derived cell lines and in RS cells from biopsies of cHL tissues. In contrast, IL-13 expression in non-Hodgkin lymphoma (NHL) is uncommon. Neutralization of IL-13 in cultures of cHL-derived cell lines HDLM-2 and L-1236 leads to a dose-dependent inhibition of proliferation, and is associated with increased apoptosis in L-1236 cells. IL-13 neutralization also decreased activation of signal transducer and activator of transcription (STAT)6, an important mediator of IL-13 function. Moreover, STAT6 is often activated in RS cells from primary tumor samples, implying that IL-13 signaling is occurring in these cells in vivo. This review will describe the biologic activities of IL-13 in the immune system, and summarize the evidence implicating IL-13 as an autocrine growth factor for RS cells in cHL. Finally, we will discuss the potential influence of IL-13 on the reactive inflammatory infiltrate that is characteristic of cHL.
Interferon regulatory factor (IRF) genes encode a family of DNA-blndlng proteins that are involved in the transcriptional regulation of type-l Interferon and/or interferon-inducible genes. We report ...here the characterization of LSIRF, a new member of the IRF gene family cloned from mouse spleen by the polymerase chain reaction using degenerate primers. LSIRF was found to encode a 51 kDa protein that shares a high degree of amino acid sequence homology in the DNA-binding domain with other IRF family members. LSIRF expression was detectable only In lymphold cells. In contrast to other IRF genes, LSIRF expresslon was not Induced by interferons, but rather by antigen-receptor mediated stimuli such as plant lectins, CD3 or IgM crosslinking. In In vitro DNA binding studies, LSIRF was able to bind to the interferon-stimulated response element (ISRE) of the MHC class I promoter. The expression pattern and DNA binding activities suggest that LSIRF plays a role in ISRE-targeted signal transduction mechanisms specific to lymphoid cells.
Excess exposure of skin to ultraviolet B (UVB) results in the appearance of so-called sunburn cells. Although it has been demonstrated that sunburn cells represent apoptotic keratinocytes, the ...molecular mechanisms for UVB-induced apoptosis in keratinocytes have not been fully elucidated. The cytokine, TNF-alpha, has been shown to induce apoptosis in a variety of cell types. Since UVB induces keratinocytes to release TNF-alpha, we hypothesized that TNF-alpha is involved in UVB-induced apoptosis in keratinocytes. In order to confirm this hypothesis and to further delineate which type of TNF receptor signaling mediates the apoptosis pathway, we performed both in vivo and in vitro experiments using gene-targeted knockout mice lacking either the TNF p55 receptor or the TNF p75 receptor. In the in vivo study, wild-type and mutant mice were exposed to UVB, and apoptotic keratinocytes were detected by examining DNA fragmentation using in situ nick-end labeling. For the in vitro experiments, keratinocytes derived from the wild-type and mutant mice were irradiated with UVB, and the degree of apoptosis was determined by flow cytometry, nick-end labeling of DNA, and a DNA ladder assay. Both in vivo and in vitro studies demonstrated that the deletion of TNF receptor p55 could suppress UVB-induced apoptosis in keratinocytes. Our observations support the notion that TNF-alpha is involved in UVB-induced keratinocyte apoptosis, and demonstrate that p55 receptor signaling plays a pivotal role in this event.
Little is known about the molecular mechanisms and transcriptional regulation that govern T cell selection processes and the differentiation of CD4
+ and CD8
+ T cells. Mice lacking the interferon ...regulatory transcription factor-1 (IRF-1) have reduced numbers of mature CD8
+ cells within the thymus and peripheral lymphatic organs. Here we show that positive and negative T cell selection of two MHC class I–restricted TCRαβ transgenes, H-Y and P14, are impaired in IRF-1
−/− mice. The absence of IRF-1 resulted in decreased expression of LMP2, TAP1, and MHC class I on thymic stromal cells. Despite decreased MHC class I expression on IRF-1
−/− thymic stromal cells, the defect in CD8
+ T cells development did not reside in the thymic environment, and IRF-1
−/− stromal cells can fully support development of CD8
+ thymocytes in in vivo bone marrow chimeras and in vitro reaggregation cultures. Moreover, IRF-1
−/− thymocytes displayed impaired TCR-mediated signal transduction, and the induction of negative selection in TCR Tg thymocytes from IRF-1
−/− mice required a 1000-fold increase in selecting peptide. We also provide evidence that IRF-1 is mainly expressed in mature, but not immature, thymocytes and that expression of IRF-1 in immature thymocytes is induced after peptide-specific TCR activation. These results indicate that IRF-1 regulates gene expression in developing thymocytes required for lineage commitment and selection of CD8
+ thymocytes.
Milton's epic poem Paradise lost supplies a colourful metaphor for the immune system and its responses to pathogens. With the role of Satan played by pathogens seeking to destroy the paradise of ...human health, GOD intervenes and imposes order out of chaos. In this context, GOD means 'generation of diversity': the capacity of the innate and specific immune responses to recognize and eliminate a universe of pathogens. Thus, the immune system can be thought of as an entity that self-assembles the elements required to combat bodily invasion and injury. In so doing, it brings to bear the power of specific recognition: the ability to distinguish self from non-self, and the threatening from the benign. This ability to define and protect self is evolutionarily very old. Self-recognition and biochemical and barrier defences can be detected in primitive organisms, and elements of these mechanisms are built upon in an orderly way to establish the mammalian immune system. Innate immune responses depend on the use of a limited number of germline-encoded receptors to recognize conserved molecular patterns that occur on the surfaces of a broad range of pathogens. The B and T lymphocytes of the specific immune response use complex gene-rearrangement machinery to generate a diversity of antigen receptors capable of recognizing any pathogen in the universe. Binding to receptors on both innate and specific immune-system cells triggers intricate intracellular signalling pathways that lead to new gene transcription and effector-cell activation. And yet, regulation is imposed on these responses so that Paradise is not lost to the turning of the immune system onto self-tissues, the spectre of autoimmunity. Lymphocyte activation requires multiple signals and intercellular interactions. Mechanisms exist to establish tolerance to self by the selection and elimination of cells recognizing self-antigens. Immune system cell populations are reduced by programmed cell death once the pathogen threat is resolved. Once Paradise has been regained, memory cells remain in the body to sharply reduce the impact of a second exposure to a pathogen. Vaccination programs take advantage of this capacity of the human immune system for immunological memory, sparing millions the suffering associated with disease scourges. Thus does the order of the immune response spring from the disorder of pathogen attacks, and thus is Paradise preserved.
Phosphatase and tensin homologue (PTEN) deleted on chromosome 10 is a dual-specific phosphatase and a potent antagonist of the phosphoinositide 3-kinase signaling pathway. Although first discovered ...as a tumor suppressor, emerging evidence supports PTEN as a potential therapeutic target for diabetes. PTEN deletion in β cells leads to increased β cell mass and protection from streptozotocin-induced diabetes. Importantly, PTEN deletion does not lead to tumor formation in β cells. To further assess the potential tumorigenic role of PTEN, we tested the biological role of PTEN in the context of activation of the proto-oncogene c-Myc. We generated and characterized β cell-specific PTEN knock-out mice expressing an inducible c-Myc transgene in β cells. Surprisingly, we found that PTEN loss did not confer protection from the overwhelming apoptosis and diabetes development seen with c-Myc activation. Importantly, despite the combined effect of the loss of a tumor suppressor and activation of an oncogene in β cells, there was no evidence of tumor development with sustained c-Myc activation.
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