Activation of human neutrophils by the plant lectin Viscum album agglutinin-I: modulation of de novo protein synthesis and evidence that caspases are involved in induction of apoptosis

The plant lectin Viscum album agglutinin‐I (VAA‐I) was recently found to modulate protein synthesis and to induce apoptosis in various cells of immune origin. We found that VAA‐I induces de novo protein synthesis of metabolically 35S‐labeled human neutrophils when used at low concentrations (98% of cells at 500 and 1000 ng/mL. VAA‐I was also found to reverse the delaying effect of GM‐CSF on neutrophil apoptosis and to inhibit GM‐CSF‐inducedde novo protein synthesis. In contrast to GM‐CSF, VAA‐I does not induce tyrosine phosphorylation by itself and does not alter the GM‐CSF‐induced response. Among the inhibitors used, genistein, pertussis toxin, staurosporine, H7, Calphostin C, manoalide, BpB, quinacrine HA‐1077, and z‐VAD‐FMK, only the latter (inhibitor of caspases‐1, ‐3, ‐4, and ‐7) was found to inhibit VAA‐I‐induced neutrophil apoptosis as the percentage of apoptotic cells decrease from 98 ± 1.3 to 54 ± 3.2% (n =4). Furthermore, we confirm that caspases are involved in VAA‐I‐induced neutrophil apoptosis as we have observed the fragmentation of the cytoskeletal gelsolin protein that is known to be caspase‐3‐dependent. Such degradation was reversed by the z‐VAD‐FMK inhibitor. We conclude that induction of neutrophil apoptosis by VAA‐I is a caspase‐dependent mechanism that does not involve tyrosine phosphorylation events, G‐proteins, PKCs, and PLA2. In addition, we conclude that at least caspase‐3 is involved. Correlation between VAA‐I‐induced neutrophil apoptosis and VAA‐I‐induced inhibition of de novo protein synthesis is discussed.