Do all programmed cell deaths occur via apoptosis?

Ubiquitin pathway involvement in human lymphocyte gamma-irradiation-induced apoptosis

Apoptosis (the classical type of programmed cell death) can be triggered in many cell types by widely diverse stimuli. gamma rays, at low doses, can induce apoptosis in vitro in interphase human lymphocytes. In this type of apoptosis induction, activated gene expression is necessary for the fulfillment of the death program. In this report, we present evidence for a relationship between ubiquitin gene expression or ubiquitination and gamma-irradiation-mediated apoptosis in normal circulating human lymphocytes. Using in vitro nuclear transcription assays (run-on), Northern (RNA) blot analysis, immunolocalization studies, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis after immunoprecipitation, we demonstrate that (i) the ubiquitin mRNA level is increased as a consequence of the activation of ubiquitin gene transcription 15 to 90 min after initiation of apoptosis; (ii) specifically in apoptotic cells, and not in all irradiated cells, nuclear proteins are highly ubiquitinated; and (iii) ubiquitin sequence-specific antisense oligonucleotide inhibition results in a decreased level of ubiquitinated nuclear proteins and considerably diminishes the proportion of cells exhibiting the apoptotic death pattern. Each of these results might be explained by different modifications occurring in irradiated cells. Their convergence strongly suggests that the ubiquitin gene is one of the genes with induced activity in the apoptotic death program and that ubiquitination of nuclear proteins might be involved in chromatin disorganization and oligonucleosomal fragmentation, which are among the key events occurring in apoptosis.

Pro-oxidants and mitochondrial Ca2+: their relationship to apoptosis and oncogenesis

Apoptosis is a physiological process for active cell removal. One of its hallmarks is an increased cytosolic Ca2+ content. Several genes involved in apoptosis control have been identified, but their mode of action is not understood in detail. Apoptosis may relate to oncogenesis, in that some malignant tumors may grow because genes engaged in apoptosis control are altered. L929 cells overexpressing the proto-oncogene bcl-2 have an increased mitochondrial membrane potential (delta psi), as have many carcinoma cells. bcl-2 protects L929 cells against apoptosis caused by pro-oxidant-induced mitochondrial Ca2+ 'cycling' and increased cytosolic Ca2+ levels. Nerve growth factor, which induces catalase, and inhibitors of mitochondrial Ca2+ release also prevent apoptosis. It is suggested that a pro-oxidant-induced Ca2+ release from mitochondria, followed by Ca2+ cycling and ATP depletion, is a common basic event during apoptosis. Accordingly, maintenance of delta psi stabilizes mitochondria, thereby prevents apoptosis, and may confer increased growth potential to cells.

HIV induces thymus depletion in vivo

Human immunodeficiency virus (HIV) disease is typified by declining CD4+ T lymphocyte counts in the peripheral circulation, a loss which may be secondary to accelerated destruction, to suppressed differentiation, and/or to sequestration of circulating cells into tissue spaces. As it is hard to distinguish between these possibilities in human subjects, the pathogenic mechanisms associated with HIV infection are unclear. In particular, little is known about the events that occur within infected lymphoid organs in which most CD4 T lymphocytes mature and function. To obtain a better description of HIV pathogenesis in vivo, we have implanted human haematolymphoid organs into the immunodeficient SCID mouse to create the SCID-hu mouse. We have previously shown that these organ systems promote long-term multilineage human haematopoiesis and are permissive for infection with HIV. Here we report that human thymopoiesis is suppressed by HIV infection, thereby precluding regeneration of the peripheral T-cell compartment.