Serine phosphorylation of p60 tumor necrosis factor receptor by PKC-delta in TNF-alpha-activated neutrophils

Tumor necrosis factor-alpha (TNF-alpha) triggers degranulation and oxygen radical release in adherent neutrophils. The p60TNF receptor (p60TNFR) is responsible for proinflammatory signaling, and protein kinase C (PKC) is a candidate for the regulation of p60TNFR. Both TNF-alpha and the PKC-activator phorbol 12-myristate 13-acetate triggered phosphorylation of p60TNFR. Receptor phosphorylation was on both serine and threonine but not on tyrosine residues. The PKC-delta isotype is a candidate enzyme for serine phosphorylation of p60TNFR. Staurosporine and the PKC-delta inhibitor rottlerin inhibited TNF-alpha-triggered serine but not threonine phosphorylation. Serine phosphorylation was associated with receptor desensitization, as inhibition of PKC resulted in enhanced degranulation (elastase release). After neutrophil activation, PKC-delta was the only PKC isotype that associated with p60TNFR within the correct time frame for receptor phosphorylation. In vitro, only PKC-delta, but not the alpha-, betaI-, betaII-, or zeta-isotypes, was competent to phosphorylate the receptor, indicating that p60TNFR is a direct substrate for PKC-delta. These findings suggest a selective role for PKC-delta in negative regulation of the p60TNFR and of TNF-alpha-induced signaling.