Stimulation of TNF alpha expression by hyperosmotic stress

Hyperosmotic stress is known to induce apoptotic cell death, an effect previously attributed to seemingly ligand-independent clustering of tumour necrosis factor alpha (TNFalpha) receptors. An alternative explanation for the clustering of TNFalpha receptors may be stimulation of TNFalpha 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 TNFalpha 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 TNFalpha 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 TNFalpha and by SB203580 and was similarly elicited by exogenous addition of TNFalpha. In contrast, osmotically induced apoptosis of LCL721 cells was only slightly blunted by anti-TNFalpha antibodies and rather increased by SB203580. In conclusion, through activation of p38 kinase hyperosmotic stress stimulates the expression of TNFalpha 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, TNFalpha-independent, mechanisms mediating apoptotic cell death following an excessive increase of extracellular osmolarity.