STAT3 Participates in Transcriptional Activation of the C-reactive Protein Gene by Interleukin-6

Abstract

beta-Thymosins are the currently favored candidates for maintaining the large actin monomer (G-actin) pool in living cells. To determine if beta-thymosin behaves like a simple G-actin buffering agent in the complex environment of a cell, we overexpressed thymosin beta10 (Tbeta 10) in NIH3T3 cells and determined the effect on the monomer/polymer equilibrium. Tbeta 10 is the predominant beta-thymosin isoform in the NIH3T3 cell line, and it is present in approximately equal molar ratio to profilin and cofilin/actin depolymerizing factor, two other well characterized actin monomer binding proteins. Clonal cell lines that overexpressed three times more Tbeta 10 had 23-33% more polymerized actin than control cells, and the filaments appeared thicker after staining with fluorescent phalloidin. There was no change in total actin, profilin, and cofilin/actin depolymerizing factor content. The overexpressing cells were more motile; they spread faster and had higher chemotactic and wound healing activity. Assuming that there is no compensatory inactivation of the other classes of monomer binding proteins, our paradoxical observation can be accounted for quantitatively by a parallel in vitro study (Carlier, M.-F., Didry, D., Erk, I., Lepault, J., Van Troys, L., Vanderkekove, J., Perelroizen, I., Yin, H. L., Doi, Y., and Pantaloni, D., (1996) J. Biol. Chem. 271, 9231-9239). beta-Thymosin at levels comparable with that found in the overexpressing cells binds actin filaments and decreases the critical concentration (C(c)) for actin polymerization. This reduces the monomer buffering ability of beta-thymosin, so that above a certain threshold an incremental increase in thymosin does not lead to a corresponding increase in G-actin. Furthermore, the decrease in C(c) reduces the buffering capacity of the other actin monomer binding proteins. As a consequence, an increase in beta-thymosin does not necessarily result in a proportionate increase in actin monomer content in a complex environment containing other actin monomer binding proteins. The outcome depends on the level of beta-thymosin expression relative to the composition of the other actin monomer binding protein. Our results suggest that beta-thymosins are not simple actin buffering proteins and that their biphasic action may have physiological significance.

A Nuclear Receptor Corepressor–Dependent Pathway Mediates Suppression of Cytokine-Induced C-Reactive Protein Gene Expression by Liver X Receptor

“…siRNA-mediated depletion of LXR␣/␤ abolished the inhibitory effect of T0901317 on CRP gene expression, demonstrating a receptor-dependent mechanism. However, analysis of the CRP promoter did not reveal the presence of any putative LXR-responsive elements, indicating that the inhibition of cytokine-induced CRP transcription by LXR ligands is indirect through inhibition of binding of other transcription 19,[21][22][23] To localize the regions important for LXR-dependent repression, we used a series of 5Ј-deletion constructs. We identified a region in the CRP promoter between Ϫ256 and Ϫ125 that is essential for LXR ligand-mediated inhibition of cytokine-induced transcriptional activity.…”

“…18 The transcription factors STAT3 (Signal Transducer and Activator of Transcription 3), nuclear factor B and members of the C/EBP family (CCAAT box/ Enhancer-Binding Protein) participate in cytokine-induced transcriptional activation of the CRP gene. 19,20 In addition, hepatocyte nuclear factor-1 (HNF-1), HNF-3, and Oct-1 also play important roles in the regulation of CRP expression. [21][22][23] The liver X receptors ␣ (LXR␣) and LXR␤ (also known as NR1H3 and NR1H2, respectively) are members of the nuclear hormone receptor superfamily and have been suggested as potential targets for therapeutic intervention in human cardiovascular and metabolic disease.…”

Modulation of hepatic acute phase gene expression by epidermal growth factor and src protein tyrosine kinases in murine and human hepatic cells

“…Gene induction was measured with the following chloramphenicol acetyl transferase (CAT) reporter gene constructs: pHPX(5xIL-6RE)-CAT (containing 5 tandem copies of the STAT3-specific IL-6RE of the rat hemopexin [HPX] gene in pCAT 50 55 ). The 123-bp promoter constructs with site-directed mutations of the binding sites for STAT3 (STAT3mut) and/or CAAT/enhancer binding protein (C/EBP) (C/EBPmut) were also included as described.…”

“…The 123-bp promoter constructs with site-directed mutations of the binding sites for STAT3 (STAT3mut) and/or CAAT/enhancer binding protein (C/EBP) (C/EBPmut) were also included as described. 55 pTIMP-1-CAT containing the AP-1-responsive human tissue inhibitor of metalloproteinase (TIMP)-1 promoter (Ϫ62 to ϩ47) was provided by Dr. C. Richards, McMaster University, Hamilton, Ontario, Canada. 56 Cells and Transfection.…”

“…Previous analyses of several APP gene promoters have indicated the presence of STAT binding sites and their inducibility by STAT3. 39,40,50,52,55 At present, no APP gene has been identified that is significantly inducible by STAT1. Hence, in the following experiments we limited our attention to STAT3.…”