Suppressor of Cytokine Signaling 3 Inhibits LPS-induced IL-6 Expression in Osteoblasts by Suppressing CCAAT/Enhancer-binding Protein β Activity

Abstract

Suppressor of cytokine signaling 3 (SOCS3) is an important intracellular protein that inhibits cytokine signaling in numerous cell types and has been implicated in several inflammatory diseases. However, the expression and function of SOCS3 in osteoblasts are not known. In this study, we demonstrated that SOCS3 expression was transiently induced by LPS in osteoblasts, and apparently contributed to the inhibition of IL-6 induction by LPS treatment. We found that tyrosine 204 of the SOCS box, the SH2 domain, and the N-terminal kinase inhibitory region (KIR) of SOCS3 were all involved in its IL-6 inhibition. Furthermore, we demonstrated that CCAAT/enhancerbinding protein (C/EBP) ␤ was activated by LPS (increased DNA binding activity), and played a key role in LPS-induced IL-6 expression in osteoblasts. We further provided the evidence that SOCS3 functioned as a negative regulator for LPS response in osteoblasts by suppressing C/EBP␤ DNA binding activity. In addition, tyrosine 204 of the SOCS box, the SH2 domain, and the N-terminal kinase inhibitory region (KIR) of SOCS3 were all required for its C/EBP␤ inhibition. These findings suggest that SOCS3 by interfering with C/EBP␤ activation may have an important regulatory role during bone-associated inflammatory responses. SOCS3 belongs to the family of suppressors of cytokine signaling (SOCS)2 proteins, which is induced by a number of mediators, including LPS, TNF-␣, as well as IL-6 and IL-10 (1-3). SOCS3 has been shown to function as a proinflammatory mediator by suppressing IL-6-gp130 signaling, interfering with its ability to inhibit LPS signaling (4, 5). For example, mice lacking SOCS3 in macrophages and neutrophils are resistant to LPS-induced shock (4). In contrast, accumulating data suggest that SOCS3 may suppress inflammatory responses (6). Thus, the function of SOCS3 during inflammation seems to be dependent on the particular disease model used and cell type studied. Moreover, the precise role of SOCS3 in LPS responses remains enigmatic.The stimulation of Toll-like receptor (TLR) 4 by LPS plays a critical role in innate immune responses in mammals. Although most studies on LPS-induced inflammation and the ensuing tissue destruction have been focused on immune systems, recent studies demonstrate that osteoblasts also express functional TLR4, which may play an important role in the pathogenesis of LPS-mediated bone disorders (2,7,8). For example, LPS stimulates osteoblasts to secrete receptor activator of NF-B ligand (RANKL), IL-6, IL-1, TNF-␣, GM-CSF, and PGE 2 , each of which seems to to be involved in LPS-mediated bone resorption (9). Among these proinflammatory mediators, IL-6 regulation in bone is extremely important for tissue homeostasis. Inappropriate expression of IL-6 has been suggested to have an impact on the increase in bone resorption observed in several bone inflammatory diseases (10, 11). Stimulation of IL-6 mRNA synthesis by LPS in human osteoblasts has been suggested to occur through CD14, p38 MAPK, and MEK (12). Several transcription factors s...

SOCS-3 Regulates Alveolar Bone Loss in Experimental Periodontitis

“…Injections of Escherichia coli LPS in the palatal gingival tissues of first 645332J DRXXX10.1177/0022034516645332Journal of Dental ResearchSOCS-3 Regulates Alveolar Bone Loss research-article2016 molars of rats increased inflammation; induced an upregulation of SOCS-3 expression paralleled by an increase of the proinflammatory cytokines IL-1β, IL-6, and TNF-α; and increased phosphorylation of STAT3 (Chaves de Souza et al 2013). SOCS-3 downregulates additional proinflammatory mediators induced by LPS in osteoblasts, thereby playing a critical role in osteoblast-mediated immune signaling (Yan et al 2010;Gao et al 2013). The analysis of mice lacking genes for SOCS proteins has shed additional light on their critical importance in restraining inflammation and allowing optimal levels of protective immune responses in several inflammatory disorders, including arthritis, atherosclerosis, and cancer (Wong et al 2006;Recio et al 2015;Yu et al 2015).…”

Suppressors of cytokine signaling 3 is essential for FcγR-mediated inflammatory response via enhancing CCAAT/enhancer-binding protein δ transcriptional activity in macrophages

“…A variety of cells express SOCS3, including macrophages, osteoblasts, microglia, and neutrophils [4, 18, 24], and roles of SOCS3 in inflammation have been investigated. However, the effect of SOCS3 on inflammatory responses after FcγR cross-linking remains largely unknown.…”

“…Besides STAT3, SOCS3 could also repress inflammatory responses through other mechanisms. Our recent studies have proved that SOCS3 decreases IL-6 expression in osteoblasts incubated with LPS by inhibiting C/EBPβ DNA binding activity [4]. In contrast to the above reports, SOCS3 could also act as an inflammatory promoter.…”

Impairment of Bilirubin Clearance and Intestinal Interleukin-6 Expression in Bile Duct-Ligated Vitamin D Receptor Null Mice

“…We speculated that while VDR primarily regulates inflammatory responses by suppressing NF-κB activity, VDR deletion may induce compensatory immunoregulatory mechanisms in the intestine. Since suppressor of cytokine signaling (SOCS) 1 and SOCS3 have been reported to inhibit LPS-induced IL-6 production [48], [49], we examined the expression of Socs1 and Socs3 in the intestine. There was no difference in Socs1 expression in sham-operated wild-type, BDL wild-type, sham-operated VDR-null and BDL VDR-null mice, although BDL tended to decrease Socs1 expression in wild-type mice (Fig.…”