Molecular Pathways of Programmed C ell Death And Viral C

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Abstract

Internal death programs control the number and types of cells in the body. Diseases can result from inefficient cell death or from inappropriate or excessive death such as is caused by the human immunodeficiency virus (HIV) during AIDS or the SAR-CoV virus during SARS. In this project we are taking a multifaceted approach to studying molecular mechanisms of both apoptotic and nonapoptotic death programs in lymphocytes as well as other cell types. A major focus of our investigations are death-inducing cell surface receptors in the tumor necrosis factor receptor (TNFR) superfamily such as TNFR1 and CD95/Fas/APO-1. Both receptors play an important role in stimulating both apoptotic and nonapoptotic death of cells principally in immune processes. interestingly, both receptors can have effects beside death such as the induction of transcription factors. We are trying to understand how these receptors stimulate the intracellular machinery that causes cell death in preference to other cellular outcomes. We have focused much of our efforts on the activation of a protease called caspase-8 which regulates the death program. We have characterized two death programs that emanate from TNFR1 and the Fas receptor, one which is caspase-8 dependent and has an apoptotic morphology and the other which is caspase-8 independent and involves necrosis. Interestingly, the latter death program is only observed when caspase-8 is inhibited. The regulation and molecular pathways of these two forms of lymphocyte death are distinct. In addition, we have discovered that inhibition of caspase-8 in non-lymphoid cells can lead to another form of cell death exhibiting particular cytoplasmic membrane structures called autophagy. although initially controversial, several labs have now shown that this form of death is particularly important for the demise of tumor cells by chemotherapeutic agents. We have now shown that the mechanism of autophagic death program is selective degradation of catalase which leads to a marked overaccumulation of reactive oxygen species leading to cellular damageand death. [unreadable] In parallel, we are exploring how the regulation of cellular death programs may play a role in cytopathicity associated with the virus infections in AIDS and SARS. In particular, a critical effect in the onset of AIDS following infection with HIV is the death of T lymphocytes caused by the virus. We have found that this death process is necrotic rather than apoptotic and have now identified two viral gene products, vif and vpr, that are involved in this process. In order to study this process rigorously we have constructed a mathematical model to analyze cell death in tissue culture during HIV infection. Remarkably, both of these cytotoxic gene products cause says cycle arrest at the boundary of the G2 and M phases. The mathematical model reveals that the principal cause of cell loss is cell death rather than cell cycle arrest. We are using molecular genetic approaches to determine if cell cycle arrest actually causes cell death and how this might come about. In contrast to HIV, the human coronavirus that causes SARS, SARS-CoV, causes necrotic cell death that does not involve cell cycle arrest. We have found that cell death can be traced to a novel open reading frame, termed ORF 3b, that is present in SARS-CoV but not other less pathological human coronaviruses. The cellular effect of ORF3b is to cause a dramatic reorganization of the Golgi apparatus which has lethal effects on the cell. We are now trying to established a molecular pathway entrained to ORF3b that causes this cytopathic effect.