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