Signal transducer and activator of transcription 3 (STAT3) is a ubiquitous transcription factor involved in many biological processes, including hematopoiesis, development and immune response. Dysfunctional STAT3 signalling has been reported in many pathophysiological conditions such cancer, chronic inflammation and fibrosis. In the first part of this work, we investigated the functions of the N-terminal domain and GAS-site recognition during IL-6-induced STAT3 signaling. Our results demonstrate the nonessential role of GAS-element recognition for both cytokine-induced and basal nuclear import of STAT3. In turn, deletion of the NTD markedly decreased nuclear accumulation upon IL-6 treatment resulting in a prolonged accumulation of phosphorylated dimers in the cytoplasm, at the same time preserving specific DNA recognition ability of the truncation mutant. Observed defect in nuclear localization could not be explained by flawed importin-α binding. Furthermore, our data indicated mechanistic differences between active and latent nuclear trafficking of STAT3, as well as between STAT1 and STAT3 active nuclear import.In the second part of this thesis, we analyzed the excessive STAT1 activation in STAT3-deficient cells upon IL-6 treatment. Our findings show that STAT3-mediated regulation of IL-6-induced STAT1 signaling depends on STAT3 target gene expression, but not on isolated STAT3 NTD functions. In the third part, we demonstrated that NF-κB and STAT3 have no direct influence on each other canonical signaling pathways. Instead, the expression of NF-κB subunit p65 correlates with total levels of STAT1 and STAT3.In the final part of this work, we characterized a STAT3-YFP knock-in murine model as a potentially powerful tool for the visualization of STAT3 in vivo. Our data show that STAT3-YFP Knock-In mice have been successfully generated and that the YFP fluorescence can be detected by common microscopy techniques. The STAT3-YFP knock-in mice will be a valuable tool for deciphering the function of STAT3 in vivo
