Functional involvement of Daxx in gp130-mediated cell growth and survival in BaF3 cells

Development and Validation of a Reporter-Cell-Line-Based Bioassay for Therapeutic Soluble gp130-Fc

Soluble glycoprotein 130 kDa (sgp130)-Fc fusion protein, an innovative therapeutic bio-macromolecular drug specifically targeting IL-6 trans-signaling, proved to have good potential for application in the treatment of chronic inflammatory diseases. A simple and quick bioassay for sgp130-Fc was developed in this study. First, a stable reporter cell line was obtained by transfecting CHO-K1 cells with a sis-inducible element (SIE)-driving luciferase reporter gene (CHO/SIE-Luc). Sgp130-Fc could inhibit the expression of luciferase induced by IL-6/sIL-6Rα complex, and the dose–response curve fitted the four-parameter logistic model, with 50% inhibitive concentration (IC₅₀) being about 500 ng/mL and detection range between 40 and 5000 ng/mL. Both the intra-assay and inter-assay coefficient of variation (CV) were below 10.0%, and the accuracy estimates ranged from 94.1% to 106.2%. The assay indicated a good linearity (R² = 0.99) in the range of 50% to 150% of optimized initial concentration. No significant difference was found between the test results of new assay and BAF3/gp130 proliferation assay (unpaired t test, p = 0.4960, n = 6). The dose-response effect and copy number of the luciferase gene was basically unchanged after long-term culture (up to passage 60), demonstrating the stability of CHO/SIE-Luc cells. These results suggested that the new reporter assay was suited to routine potency determination of therapeutic sgp130-Fc.

A New STAT3-binding Partner, ARL3, Enhances the Phosphorylation and Nuclear Accumulation of STAT3

Signal transducer and activator of transcription 3 (STAT3) is involved in cell proliferation, differentiation, and cell survival during immune responses, hematopoiesis, neurogenesis, and other biological processes. STAT3 activity is regulated by a variety of mechanisms, including phosphorylation and nuclear translocation. To clarify the molecular mechanisms underlying the regulation of STAT3 activity, we performed yeast two-hybrid screening. We identified ARL3 (ADP-ribosylation factor-like 3) as a novel STAT3-binding partner. ARL3 recognizes the DNA-binding domain as well as the C-terminal region of STAT3 in vivo, and their binding was the strongest when both proteins were activated. Importantly, small interfering RNA-mediated reduction of endogenous ARL3 expression decreased IL-6-induced tyrosine phosphorylation, nuclear accumulation, and transcriptional activity of STAT3. These results indicate that ARL3 interacts with STAT3 and regulates the transcriptional activation of STAT3 by influencing its nuclear accumulation of STAT3.

Signal transducer and activator of transcription 3 regulation by novel binding partners

Signal transducers and activators of transcription (STATs) mediate essential signals for various biological processes, including immune responses, hematopoiesis, and neurogenesis. STAT3, for example, is involved in the pathogenesis of various human diseases, including cancers, autoimmune and inflammatory disorders. STAT3 activation is therefore tightly regulated at multiple levels to prevent these pathological conditions. A number of proteins have been reported to associate with STAT3 and regulate its activity. These STAT3-interacting proteins function to modulate STAT3-mediated signaling at various steps and mediate the crosstalk of STAT3 with other cellular signaling pathways. This article reviews the roles of novel STAT3 binding partners such as DAXX, zipper-interacting protein kinase, Krüppel-associated box-associated protein 1, Y14, PDZ and LIM domain 2 and signal transducing adaptor protein-2, in the regulation of STAT3-mediated signaling.

HIC1 interacts with and modulates the activity of STAT3

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene, expression of which is frequently suppressed in human cancers. Very little is known about the molecular basis of HIC1 in antagonizing oncogenic pathways. Here, we report that HIC1 forms complexes with the signal transducers and activators of transcription 3 (STAT3) and attenuates STAT3-mediated transcription. STAT3 was identified as a HIC1-interacting protein by affinity capture and followed by mass spectrometry analysis. Overexpression or depletion of HIC1 resulted in decreased or increased levels of interleukin-6 (IL-6)/oncostatin M (OSM)-induced STAT3-mediated reporter activity and expression of target genes such as VEGF and c-Myc, respectively. Furthermore, HIC1 suppressing the VEGF and c-Myc promoter activity and the colony formation of MDA-MB 231 cells were STAT3-dependent. Further studies showed that HIC1 interacts with the DNA binding domain of STAT3 and suppresses the binding of STAT3 to its target gene promoters. Domain mapping study revealed that HIC1 C-terminal domain binds to STAT3. HIC1 mutant defective in STAT3 interaction reduced its repressive effect on STAT3 DNA binding activity, the reporter activity and gene expression of the VEGF and c-Myc genes, and cell growth in MDA-MB 231 cells. Altogether, our findings not only provide a novel role of HIC1 in antagonizing STAT3-mediated activation of VEGF and c-Myc gene expression and cell growth, but also elucidate a molecular basis underlying the inhibitory effect of HIC1 on STAT3 transcriptional potential.