Effect of the sulfhydryl reagent thimerosal on cytosolic free Ca2+ and membrane potential of thymocytes.

The sulfhydryl reagent thimerosal at concentrations 5-100 microM has been found to induce a variety of changes in ion transport in rat thymocytes. In particular, [Ca2+]i increases about 10-fold from the basal level. The [Ca2+]i response to thimerosal displays a two-stage time course, with the main [Ca2+]i rise during the second stage. Evidence has been obtained for the depletion of intracellular Ca2+ pools in thimerosal-treated cells, however, Ca2+ mobilization from intracellular stores does not contribute significantly into [Ca2+]i rise. Thimerosal elicits permeability not only for Ca2+, but also for Mn2+ and Ni2+, which is Ca(2+)-dependent. We failed to get any evidence on thimerosal-induced inhibition of the plasma membrane Ca(2+)-ATPase. The induction of Ca2+ influx, rather than inhibition of Ca(2+)-ATPase, accounts for the disturbance of [Ca2+]i homeostasis in thimerosal-treated cells. Thimerosal also elicits changes in monovalent ion fluxes resulting in marked depolarization. The latter seems unrelated to the changes in [Ca2+]i and is suggested to be mediated both by increased permeability for Na+ and a decreased one for K+. Thimerosal significantly stimulates AA release from thymocytes. Evidence has been presented that AA metabolite(s), probably, LO product(s), may mediate the changes in the transport of mono- and divalent cations elicited by the sulfhydryl reagent. Prolonged treatment of thymocytes with thimerosal resulted in cell death.

Dependence of thymocyte apoptosis on protein kinase C and phospholipase A2.

The effect of inhibitors and activators of protein kinase C and phospholipase A2 on radiation-induced apoptosis of rat and mouse thymocytes has been studied. It is shown that the apoptosis is prevented by the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperasine dihydrochloride and is potentiated by protein kinase C activator phorbol 12-myristate 13-acetate, calcium ionophore A23187 and concanavalin A. The protein kinase C activators initiate apoptosis in mouse but not in rat thymocytes. The inhibitor of phospholipase A2 prevents apoptosis induced by all the factors. The results obtained indicate that both protein kinase C and phospholipase A2 are involved in the thymocyte apoptosis.

On the role of intracellular concentration of Ca2+ and H+ in thymocyte death after irradiation.

The role of intracellular Ca2+ and H+ concentrations in radiation-induced interphase death of rat thymocytes has been studied. In response to concanavalin A treatment in the Ca(2+)-containing medium, or to the CaCl2 treatment in the Ca(2+)-free medium, the [Ca2+]i rise in irradiated cells was as in the non-treated cells. No changes in the level of [Ca2+]i and pHi were found within 1 h after irradiation of thymocytes with a dose of 6 Gy. 15 microM 5-(N-ethyl-N-isopropyl)-amiloride, an inhibitor of Na+/H+ exchange, did not affect the DNA fragmentation. The fragmentation was prevented by 2-4 microM (1-[bis(4-chlorophenyl)methyl]-3-[2-(2,4-dichlorophenyl)]-2-[(2,4- dichlorophenyl)-methoxy]-ethyl)-1-H-imidazolium chloride, an inhibitor of calmodulin. The above data indicate that triggering of interphase death in irradiated thymocytes is not mediated by changes in either [Ca2+]i or pHi. Such changes seem to be involved in intermediate steps of the interphase death process.

Role of arachidonic acid metabolism in thymocyte apoptosis after irradiation.

The present study demonstrates that DNA fragmentation, nuclear pycnosis and trypan blue staining of irradiated thymocytes is prevented by inhibition of the lipoxygenase pathway of arachidonic acid metabolism and is not affected by cyclooxygenase inhibition. Exposed to irradiation [3H]arachidonic acid-labeled thymocytes release radioactive products to the external medium. The process is blocked by the lipoxygenase inhibitor, nordihydroguaiaretic acid. Thus, it can be concluded that irradiation activates arachidonic acid metabolism and that lipoxygenase metabolites play an important role in thymocyte apoptosis.

The effect of low-dose irradiation on proliferation of mammalian cells in vitro.

The effect of low doses of gamma radiation on proliferation of Raji lymphoma cells and Chinese hamster fibroblasts in vitro has been investigated. It is shown that irradiation in the dose range between 2 and 20 cGy (maximum at 10 cGy) stimulates growth of cultured cells due to the shortening of the lag phase of cell growth, with the duration of the doubling time being unchanged. Preirradiation of growth medium in this dose range also stimulates cell proliferation, though to a lesser extent than irradiation of cells and the medium does. The stimulatory effect manifests itself in an increase in the cell number as well as in the size and number of colonies.

Tissue regeneration as the basic oncogenic factor.

A new hypothesis of oncogenesis has been suggested. The errors of deoxyribonucleic acid (DNA) synthesis are supposed to be the main cause of oncogenic mutations. Since cell division and consequently DNA synthesis are continually needed for the renewal of some tissues in adult organisms, the physiological and reparative regeneration of these tissues (epithelium, blood) is the basic factor of oncogenesis. The exogenous and endogenous agents increase the probability of tumor frequency via increasing the cell renovation rate in tissues and decreasing the DNA synthesis fidelity. This hypothesis is in accord with the experimental and epidemiological data including the results which are not explained reasonably until now, namely, the association of the carcinogenic activity of drugs with their toxicity, the organ and tissue specificity of tumor frequency, etc.

Cross-linking of cell surface receptors as a trigger or cell apoptosis and proliferation.

A hypothesis of the mechanisms by which the protein cross-linking agents trigger apoptosis of lymphoid cells and proliferation of other cell types is proposed. It is assumed that both effects are triggered by aggregation of receptors on cell surface, which results from their cross-linking. This idea is substantiated by the example of one of these agents, ionizing radiation. As in the case of physiological agents, such as, antigens and growth factors, the aggregation of receptors induced by radiation activates receptor protein tyrosine kinases from which the signal is transduced to genes through protein kinase C. The hypothesis is consistent with the relationship between these effects and the PTK-PKC-dependent signal transduction pathway and its activation after irradiation.

Thymocyte proliferation and apoptosis induced by ionizing radiation.

Proliferation and apoptosis of rat and mouse thymocytes caused by ionizing radiation were studied. The percentage of proliferating cells was determined by the method of colchicine metaphases and the apoptosis was estimated as DNA fragmentation. In vitro irradiation with 0.05-0.2 Gy was found to stimulate thymocyte proliferation, the maximum was observed at 0.05 Gy for mouse thymocytes and at 0.1 Gy for rat thymocytes. These doses caused a slight decrease in DNA fragmentation, as compared to control. By raising the radiation dose, proliferation was reduced and DNA fragmentation was increased. The results obtained indicate that low radiation doses stimulate cell proliferation while higher doses trigger apoptosis of thymocytes.