Phosphorylation of secreted forms of human β₂-interferon/hepatocyte stimulating factor/interleukin-6

"β₂-Interferon/hepatocyte stimulating factor/interleukin-6" (IFN-β₂) has emerged as a major mediator of the plasma protein response to tissue injury (the acute phase response) in addition to its numerous effects on cells of the immune system. Human fibroblasts and monocytes induced with tumor necrosis factor, interleukin-1, bacterial lipopolysaccharide (endotoxin) or virus infection secrete multiple forms of differentially glycosylated IFN-β₂ polypeptides: at least a doublet of molecular mass approximately 25 kD and a triplet of mass approximately 30 kD. We report that immunoprecipitation analyses of medium from [³²P]orthophosphate- labeled cultures of induced fibroblasts carried out using a rabbit polyclonal antibody to recombinant E. coli-derived human IFN-β₂ reveal that the secreted gp23-25 and gp28-30 forms of IFN-β₂ are phosphorylated. IFN-β₂ gp23-25 secreted by induced monocytes is phosphorylated whereas the monocytic gp28-30 is poorly labeled with [³²P]orthophosphate suggesting tissue-specific differences in IFN-β₂ phosphorylation. Phosphoamino acid analyses indicate that all of the detected phosphate is in phosphoserine residues. Furthermore, IFN-β₂ can be completely dephosphorylated by alkaline phosphatase (E.C. No. 3.1.3.1); thus all of the phosphate label is in readily accesible sites. These observations suggest the possibility that differential phosphorylation of IFN-β₂ forms may be a mechanism to modulate its functions in a tissue-specific manner.