The effect of hyperthermia on the phosphoinositide signalling system of tumour cells
The effect of heat on the phosphoinositide signalling pathway was investigated in CHO-K1 cells and WRK-1 cells. Heat caused a decrease in 1,2-diacylglycerol (1,2- DAG) levels but did not have any effect on monoacylglycerol (MAG) levels in both cell types. On the other hand, an increase in triacylglycerol (TAG) level was observed in both cell lines. This heat-induced decrease in 1,2-DAG level in WRK-1 cells was not due to an increase in turnover rate of 1,2-DAG to phosphatidic acid (PA) since the decrease in 1,2-DAG was not affected when cells were heated in the presence of die DAG kinase inhibitor, dioctanoylethylene glycol (diC(_8)EG). The increase in TAG level may be due to a rapid, he at-induced increase in TAG synthesis from 1,2-DAG, thus leading to decreased levels of 1,2-DAG. Heat also led to an increase in inositol bisphosphate (InsP(_2)) and inositol trisphosphate (InsP(_3)) but not inositol monophosphate (InsP(_1)) or higher inositol phosphate (InsP(_4/5/6)) levels in WRK-1 cells. The increase in InsP(_2) and InsP(_3) was both temperature and heating time-dependent. A transient increase in InsP(_3) was observed at 11 min, and did not require extracellular calcium nor did it depend on the heat-induced increase in cytosolic free calcium ([Ca(^2+)]i). The magnitude of the heat- induced increase in InsP(_3) was comparable to that obtained upon incubation in AIF(_4). Stimulation of WRK-1 cells with vasopressin at 45ºC distorted the pattern of inositol phosphate metabolism. However, the vasopressin-sensitive phosphoinositide signalling pathway remained intact after a severe heat shock, sufficient to lead to the death of greater than 95% of the cells. Heat also led to an increase in [Ca(^2+)]i in WRK-1 cells which came primarily (solely?) from calcium influx from the extracellular medium. This influx was unlikely to occur through voltage-gated calcium channels because calcium channel blockers, such as La(^3+) and nifedipine, did not inhibit the heat-induced elevation in [Ca(^2+)]i. This heat-induced increase in [Ca(^2+)]i may have a protective role in hyperthermic cell death.