Ca2+ influx shutdown in neutrophils induced by Fas (CD95) cross‐linking

Abstract

Summary: In neutrophils, as in most other cell types, Ca²⁺ signalling is important for a number of cellular activities. Although inositol(1,4,5)trisphosphate‐mediated release of Ca²⁺ from intracellular stores is a necessary prelude, it is the Ca²⁺ influx that is responsible for many of the neutrophil responses. We report here that although elevations of cytosolic Ca²⁺ do not accompany Fas‐mediated apoptosis in neutrophils, the Ca²⁺ influx component of the response to N‐formyl‐methionyl‐leucyl‐phenylalanine (FMLP) becomes selectively inactived as the neutrophils progress towards accelerated apoptosis induced by Fas (CD95) cross‐linking. After 4 hr incubation at 37°, untreated neutrophils display an exaggerated Ca²⁺ influx phase in response to FMLP. This was absent in neutrophils that had been Fas‐activated at the same time. No Ca²⁺ influx component was demonstrable by the removal of extracellular Ca²⁺ or by Ca²⁺ channel blockade with Ni²⁺ and no Mn²⁺ influx was detectable. The defect could not be attributed to a decrease in receptor sensitivity, receptor coupling or receptor number because the release of stored Ca²⁺ remained constant during incubation and was unaffected by Fas activation. Ca²⁺ influx became uncoupled from store release before detectable gross morphological changes or phosphatidyl serine externalization and was also insensitive to caspase 3 and 8 inhibitors. These results suggest a mechanism other than caspase‐mediated proteolytic damage to components important for Ca²⁺ influx.