EVIDENCE FOR THE INVOLVEMENT OF PROTEIN-KINASE-C INHIBITION BY NORATHYRIOL IN THE REDUCTION OF PHORBOL ESTER-INDUCED NEUTROPHIL SUPEROXIDE ANION GENERATION AND AGGREGATION

Norathyriol, a xanthone aglycone, inhibited superoxide anion (O-2(-)) generation and O-2 consumption in phorbol 12-myristate 13-acetate (PMA )-activated rat neutrophils in a concentration-dependent manner. In ad dition, norathyriol inhibited PMA-but enhanced formylmethionyl-leucyl- phenylalanine (fMLP)-induced neutrophil aggregation. Norathyriol suppr essed neutrophil cytosolic protein kinase C as well as rat brain prote in kinase C over the same range of concentrations at which it inhibite d the respiratory burst. Norathyriol did not affect [H-3]phorbol 12,13 -dibutyrate ([H-3]PDB) binding to neutrophil cytosolic protein kinase C, but effectively attenuated trypsin-treated rat brain protein kinase C activity. Moreover, norathyriol was found to be a noncompetitive in hibitor with respect to ATP and peptide substrate (N-terminal acetylat ed, amino acid sequence 4-14 of the myelin basic protein, Ac-MBP-(4-14 )). Unlike staurosporine, norathyriol did not affect porcine heart pro tein kinase A activity. On the immunoblot analysis of protein kinase C subcellular distribution, the PMA-induced translocation of protein ki nase C-beta from the cytosol to the membrane was not affected by norat hyriol. These results show that the inhibition by a plant product, nor athyriol, of PMA-induced respiratory burst and aggregation is, at leas t pastry, attributed to the direct suppression of protein kinase C act ivity through blockade of the catalytic region, but is not due to inte rference with the membrane translocation of protein kinase C during PM A-induced cell activation. (C) 1997 Elsevier Science B.V.