Constitutive STAT1 tyrosine phosphorylation in U937 monocytes overexpressing the TYK2 protein tyrosine kinase does not induce gene transcription.

Janus kinase (JAK) family protein tyrosine kinases are constituents of a signaling path leading to tyrosine phosphorylation and activation of signal transducer and activator of transcription (STAT) family transcription factors. IFN-alpha activates two JAK family protein tyrosine kinases (TYK2 and JAK1) and two STAT family proteins (STAT1 and STAT2). We have generated a line of U937 promonocytes expressing a tyk2 transgene. 12-O-Tetradecanoylphorbol-13-acetate-mediated differentiation into monocytes resulted in transgene induction and both overexpression and constitutive activation of the kinase. TYK2 protein in the transgenic line was found predominantly in a membrane fraction. Coprecipitation experiments demonstrated an association of constitutively tyrosine-phosphorylated TYK2 with the IFN-alpha receptor 1 chain. TYK2 activity led to an IFN-alpha-independent appearance of tyrosine-phosphorylated STAT1 but not STAT2 or JAK1 proteins. Consistent with this, TYK2 activity also caused constitutive activation of the IFN-alpha-responsive transcription factor IFN-alpha activation factor, a dimer of tyrosine-phosphorylated STAT1, but not of the IFN-alpha-responsive transcription factor IFN-stimulated gene factor 3, a heterotrimer of tyrosine-phosphorylated STAT1 and STAT2 in association with a M(r) 48,000 DNA-binding subunit. Expression of STAT1 target genes was not observed in TYK2-overexpressing cells. Our results suggest that in addition to activated TYK2, there is a requirement for additional, IFN-alpha-dependent signals for the phosphorylation of STAT2 and the generation of IFN-stimulated gene factor 3 as well as for the conversion of tyrosine-phosphorylated STAT1 into transcriptionally active IFN-alpha activation factor.