The signaling pathway for IFN-γ-mediated induction of ICAM-1 expression was further studied in human NCI-H292 epithelial cells. The Tyr701 phosphorylation of signal transducer and activator of transcription 1 (STAT1) induced by interferon-γ (IFN-γ) and 12-O-tetradecanoylphorbol 13-acetate (TPA) was inhibited by the protein kinase C (PKC) inhibitor staurosporine, the tyrosine kinase inhibitor herbimycin, or the Src kinase inhibitor PP2. An association between c-Src and STAT1 was increased by IFN-γ and TPA, indicating the direct phosphorylation of STAT1 by PKC-dependent c-Src activation. Tyrosine phosphorylation of Janus kinases (JAK) 1/2 was induced by IFN-γ but not by TPA. In addition, ICAM-1 promoter activity induced by IFN-γ, but not that induced by TPA, was inhibited by the dominant-negative JAK1 and JAK2 mutants. IFN-γ-induced tyrosine phosphorylation of phospholipase C (PLC)-γ was inhibited by AG 490 (a JAK inhibitor), and the association between JAK1/2 and PLC-γ was increased after IFN-γ treatment, indicating the activation of PLC-γ via JAK1/ 2 phosphorylation. ICAM-1 promoter activities induced by the overexpression of wild-type JAK1- and PLC-γ2 were blocked by the PLCγ2 mutant or the dominant-negative PKCα (Lys→Arg), c-Src (Lys→Met), or STAT1 (Y701M) mutants, but not by dominant-negative STAT3 (DN) mutants. These results confirmed that IFN-γ activated PLC-γ via JAK1/2 phosphorylation to induce PKC, c-Src, STAT1 activation, and ICAM-1 expression. The association between JAK1/2 and STAT1 was increased by IFN-γ but not by TPA. It was inhibited by AG 490 but not by U73122, indicating the possible involvement of the JAK1/2-STAT1 pathway. All the results show that IFN-γ induces ICAM-1 expression by two different pathways in NCI-H292 epithelial cells. One is the JAK1/2-dependent PLC-γ pathway inducing the activations of PKCα, c-Src, and STAT1, and the other is the direct activation of STAT1 by JAK1/2.