Stimulation of TNFα expression by hyperosmotic stress
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Hyperosmotic stress is known to induce apoptotic cell death, an effect previously attributed to seemingly ligand-independent clustering of tumour necrosis factor α (TNFα) receptors. An alternative explanation for the clustering of TNFα receptors may be stimulation of TNFα production, with subsequent autocrine or paracrine stimulation of the receptors. The present study was performed to test for an effect of exposure to hyperosmotic extracellular fluid on cellular TNFα production. In both the macrophage cell line U937 and the B lymphocyte cell line LCL721, an increase of extracellular osmolarity to 500 mosmol/l indeed increased TNFα expression, an effect reversed by the p38 kinase inhibitor SB203580. In both cell types hyperosmotic stress triggered apoptosis, which in U937 cells was significantly inhibited by neutralizing antibodies against TNFα and by SB203580 and was similarly elicited by exogenous addition of TNFα. In contrast, osmotically induced apoptosis of LCL721 cells was only slightly blunted by anti-TNFα antibodies and rather increased by SB203580. In conclusion, through activation of p38 kinase hyperosmotic stress stimulates the expression of TNFα which at least in U937 macrophages may participate in the triggering of subsequent apoptotic cell death. However, the observations in LCL721 cells point to other, TNFα-independent, mechanisms mediating apoptotic cell death following an excessive increase of extracellular osmolarity.
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