Mitochondrial-perturbating agents such as toxic coumponds induce apoptosis. We note that the loss of CD45 expression in the lymphoblastic leukemia cell line HPB-ALL (HPB45.0) leads to an inhibition of nuclear apoptosis. Our hypothesis is that the absence of CD45 disturbs protein function regulated by a proto-oncogene of the Src family playing a significant role in nuclear apoptosis. In this work we explore the importance of a chloride efflux on DNA fragmentation. The role of tyrosine kinase in the function and regulation of the chloride channels was determined. Our results showed a disturbance of ionic homeostasis in CD45 deficient lymphocytes (CD45-) in contrast to normal lymphocytes (CD45+). The phosphorylation levels of the chloride channels are considerably inhibited in CD45-, while the expression levels of these channels are similar in the two types of cells. A hypertonic medium inhibits DNA fragmentation in CD45+ while a hypotonic medium increases DNA fragmentation in CD45-. Thus CD45 plays a significant role in nuclear apoptosis by the regulation of the chloride channels responsible for ionic homeostasis of the cell essential for the DFF40 activation.

CD45 is a transmembrane molecule with phosphatase activity expressed in all nucleated haematopoietic cells and plays a major role in immune cells. It is a protein tyrosine phosphatase that is essential for antigenreceptor-mediated signal transduction by regulating Src family members that initiate TCR signaling. CD45 is being attributed a new emerging role as an apoptosis regulator. Cross-linking of the extracellular portion of the CD45 by monoclonal antibodies and by galectin-1, can induce apoptosis in T and B cells. Interestingly, this phosphatase has also been involved in nuclear apoptosis induced by mitochondrial perturbing agents. Furthermore, it is involved in apoptosis induced by HIV-1. CD45 defect is implicated in various diseases such as severe-combined immunodeficiency disease (SCID), acquired immunodeficiency syndrome (AIDS), lymphoma and multiple myelomas. The understanding of the mechanisms by which CD45 regulates apoptosis would be very useful in disease treatment.

CD45 is a type I transmembrane molecule with phosphatase activity which comprises up to 10% of the cell surface area in nucleated haematopoietic cells. We have previously demonstrated the absence of nuclear apoptosis in CD45-negative T cells after chemical-induced apoptosis. The aim of this study was to characterize the role of CD45 in nuclear apoptosis. In contrast to wild type CD45-positive T cells, the CD45-deficient T cell lines are resistant to the induction of DNA fragmentation and chromatin condensation following tributyltin (TBT) or H2O2 exposure, but not to cycloheximide-induced apoptosis. CD45 transfection in deficient cell lines led to the restoration of chromatin condensation and DNA fragmentation following TBT exposure. In both CD45-positive and negative T cell lines, TBT exposure mediates intracellular calcium mobilization, caspase-3 activation and DFF45 cleavage. Moreover, DNA fragmentation was also induced by TBT in cells deficient in expression of p56lck, ZAP-70 and SHP-1. Subcellular partitioning showed a decrease in nuclear localisation of caspase-3 and DFF40. Together, these results demonstrate for the first time, that CD45 expression plays a key role in internucleosomal DNA fragmentation and chromatin condensation processes during apoptosis. CD45 activity or its substrates' activity, appears to be located downstream of caspase-3 activation and plays a role in retention of DFF40 in the nucleus.