Thapsigargin-induced calcium entry and apoptotic death of neutrophils are blocked by activation of protein kinase C

Intracellular free calcium ([Ca2+](i)) concentration, free oxygen radical ( FOR) production, DNA breakdown, and plasma membrane phosphorylation were st udied in human neutrophils activated with thapsigargin and phorbol myrisate acetate (PMA). Thapsigargin produced a rapid and sustained rise of [Ca2+]( i), activated the endonuclease, and caused the breakdown of the neutrophil' s DNA with a half-time close to 6 h. The protein kinase C activator PMA fai led to inhibit the initial rise of [Ca2+](i), but inhibited the second phas e of thapsigargin-induced calcium transient and completely blocked the acti vation of the endonuclease induced by thapsigargin. Thapsigargin induced a minor and delayed production of FOR, whereas PMA caused an abrupt and susta ined FOR production that was enhanced by thapsigargin. Two plasma membrane proteins close to 50 and 64 kD were phosphorylated in PMA-activated neutrop hils. These results suggest that the nonphosphorylated form of the membrane protein permits basal and thapsigargin-induced calcium entry. Phosphorylat ion by PMA of plasma membrane protein inhibits calcium uptake in both resti ng and thapsigargin-activated neutrophils and contributes to the block of t he activation of the apoptotic endonuclease. Copyright (C) 2001 S. Karger A G, Basel.