α-Thrombin Inhibits Signal Transducers and Activators of Transcription 3 Signaling by Interleukin-6, Leukemia Inhibitory Factor, and Ciliary Neurotrophic Factor in CCL39 Cells

We recently demonstrated that, in rat aortic smooth muscle cells, α-thrombin stimulated Stat3/SIF-A (signal transducers and activators of transcription 3/sis-inducing factor-A) activity [G. J. Bhatet al.(1997)Hypertension29(Pt. 2), 356–360]. In the present study, we observed that exposure of CCL39 cells (a Chinese hamster lung fibroblast cell line) to α-thrombin resulted in a time-dependent decrease in basal SIF-A activity. We hypothesized that the decrease in basal SIF-A was due to the initiation of an inhibitory pathway, following α-thrombin exposure. To test this hypothesis, we determined if α-thrombin would inhibit Stat3 and SIF-A activation by interleukin-6 (IL-6), leukemia inhibitory factor (LIF), and ciliary neurotrophic factor (CNTF). In support of this hypothesis, α-thrombin inhibited the Stat3/SIF-A response induced by all the above cytokines. The inhibition by α-thrombin was concentration dependent, was sensitive to hirudin, and was mimicked by the thrombin receptor agonist peptide. The inhibition did not require the activation of phorbol 12-myristate 13-acetate-sensitive isoforms of protein kinase C and was reversed by pretreatment with the mitogen-activated protein kinase kinase 1 (MAPKK1 or MEK1) inhibitor PD98059. Inhibitory cross talk between α-thrombin and IL-6 was also observed in MRC-5 cells, a fibroblast cell line derived from human lung tissue. Thus, we identify a novel α-thrombin inhibitory pathway which, acting through a MAPKK1-dependent mechanism, blocks IL-6-, LIF-, and CNTF-induced Stat3/SIF-A activation. This inhibitory cross talk may provide an important regulatory function to modulate gene transcription by these cytokines, during immune and inflammatory responses.