Signal transducers and activators of transcription (STATs) mediate cell proliferation, differentiation and survival in immune responses, hematopoiesis, neurogenesis and other biological processes. For example, STAT3 has been reported to be constitutively activated in numerous cancer cells. To clarify the molecular mechanisms underlying the STAT activation, we performed yeast two-hybrid screening and identified KAP1/TIF1b as a novel STATbinding partner. KAP1 is a universal corepressor protein for the Kruppel-associated box zinc-finger protein superfamily of transcriptional repressors. We found endogenous KAP1 associated with endogenous STAT3 in vivo. Importantly, small-interfering RNA-mediated reduction of KAP1 expression enhanced interleukin (IL)-6-induced STAT3-dependent transcription and gene expression. Furthermore, reduction of KAP1 expression resulted in the marked accumulation of STAT3 phosphorylated on Ser727 in the nucleus, a modification that regulates its transcriptional activation. These results indicate that KAP1 may serve as a transcriptional regulator of the IL-6/STAT3 signaling pathway.

Signal transducer and activator of transcription 3 (STAT3), which mediates biological actions in many physiological processes, is activated by cytokines and growth factors, and has been reported to be involved in the pathogenesis of various human diseases. Here, we show that treatment of HeLa cells with a histone deacetylase (HDAC) inhibitor, trichostatin A, or small-interfering RNA (siRNA)-mediated repression of HDAC3, enhances phosphorylation of STAT3 at Ser727.Furthermore, dephosphorylation of STAT3 at Ser727 by protein phosphatase 2A (PP2A) was restored by treatment of cells with HDAC3 siRNA. We further found that formation of a complex between STAT3 and PP2A was enhanced in the presence of HDAC3. Importantly, smallinterfering RNA-mediated repression of both HDAC3 and PP2A effectively enhanced leukemia inhibitory factor (LIF)-induced STAT3 activation. These results indicate that HDAC3 may act as a scaffold protein for PP2A to regulate the LIF/STAT3-mediated signaling pathway.

Signal transducers and activators of transcription (STATs) mediate cell proliferation, differentiation, and survival in immune responses, hematopoiesis, neurogenesis, and other biological processes.Recently, we showed that KAP1 is a novel STAT-binding partner that regulates STAT3-mediated transactivation. KAP1 is a universal corepressor protein for the KRAB zinc finger protein superfamily of transcriptional repressors. In this study, we found KAP1-dependent repression of interferon (IFN) /STAT1-mediated signaling. We also demonstrated that endogenous KAP1 associates with endogenous STAT1 in vivo. Importantly, a small-interfering RNA-mediated reduction in KAP1 expression enhanced IFN-induced STAT1-dependent IRF-1 gene expression.These results indicate that KAP1 may act as an endogenous regulator of the IFN/STAT1 signaling pathway.

Krüppel-associated box-associated protein 1 (KAP1) is thought to act mainly as a scaffold for protein complexes, which together silence transcription by triggering the formation of heterochromatin. Using small interfering RNA-mediated KAP1 knockdown, we found that endogenous KAP1 negatively regulated TNF-α–induced IL-6 production in HeLa cells. KAP1 is likely to modulate the binding of NF-κB to the IL-6 promoter because KAP1 knockdown enhanced TNF-α–induced NF-κB-luciferase activity, but not IκBα degradation. Of importance, we found negative regulatory effects of KAP1 on the serine phosphorylation of STAT3, the acetylation of NF-κB/p65 by p300, and the nuclear localization of NF-κB/p65. In addition, KAP1 associated with NF-κB/p65 and inhibited the binding between NF-κB/p65 and p300. Thus, KAP1 is likely to negatively control the acetylation of NF-κB/p65, which is critical for its nuclear retention. Taken together, KAP1 modulated the acetylation of NF-κB/p65 by interfering with the interactions among STAT3, p300, and NF-κB/p65, resulting in reduced IL-6 production after TNF-α stimulation. Our findings that KAP1 directly interacts with transcriptional factors are new, and will inform further research to elucidate KAP1 function.

Sma-and MAD-related protein 3 (Smad3) plays crucial roles in the transforming growth factor-b (TGF-b)-meditaed signaling pathway, which produce a variety of cellular responses, including cell proliferation and differentiation. In our previous study, we demonstrated that protein inhibitor of activated STATy (PIASy) suppresses TGF-b signaling by interacting with and sumoylating Smad3. In the present study, we examined the molecular mechanisms of