The potential role of suppressors of cytokine signaling in the attenuation of inflammatory reaction and alveolar bone loss associated with apical periodontitis

Abstract

Inflammatory cytokines contribute to periapical tissue destruction. Their activity is potentially regulated by suppressors of cytokine signaling (SOCS), which downregulate signal transduction as part of an inhibitory feedback loop. We investigated the expression of the cytokines tumor necrosis factor alpha (TNF-alpha); interleukin (IL)-10 and RANKL; and SOCS-1, -2, and -3 by real-time polymerase chain reaction in 57 periapical granulomas and 38 healthy periapical tissues. Periapical granulomas exhibited significantly higher SOCS-1, -2, and -3, TNF-alpha, IL-10, and RANKL messenger RNA levels when compared with healthy controls. Significant positive correlations were found between SOCS1 and IL-10 and between SOCS3 and IL-10. Significant inverse correlations were observed between SOCS1 and TNF-alpha, SOCS1 and RANKL, and SOCS3 and TNF-alpha. Increased SOCS-1, -2, and -3 messenger RNA levels in periapical granulomas may be related to the downregulation of inflammatory cytokines in these lesions; therefore, SOCS molecules may play a role in the dynamics of periapical granulomas development.

Figure 1. The mechanisms of SOCS (suppressors of cytokine signaling) action in inflammatory cells and osteoclast precursors

A schematic representation of mechanisms of SOCS (suppressors of cytokine signaling) action in inflammatory cells and osteoclast precursors. A) The inflammatory signaling triggered by LPS or cytokines such as TNF-α and IL-1β (1) may involve several signaling intermediates such as JAK/STAT, TRAF6 (2) and transcription factors such as NFkB (3), which initiate specific mRNA transcription (mRNA^a, mRNA^b) at the nucleus (4). The mRNA is translated into proteins, which will exert their biological roles after secretion (5). Inflammatory signaling also induces the expression of SOCS genes (mRNA^socs)(6), which is increased by IL-10 (7). The conversion of SOCS into non-secreted proteins (8) allows their intracellular action, which consists in the inhibition of inflammatory signaling (9), dampening therefore the inflammatory effects of LPS and cytokines. B) The interaction of RANKL with the receptor RANK (1) expressed by osteoclast precursors leads to the intracellular signaling that involves TRAF6 (2) and the transcription factor NFkB (3), which leads to osteoclast differentiation and activation, and initiate specific mRNA transcription (mRNA^a, mRNA^b) at the nucleus (4). Activated osteoclasts produce enzymes such as Cathepsin K and MMPs (5), involved in bone resorption. The expression of SOCS (mRNA^socs)(6), whose inducing factors in osteoclast precursors remain to be identified (7), may impair osteoclast differentiation and activation through the inhibition/degradation of TRAF6 and NFkB (8).

Figure 2. Quantitative expression of SOCS, TNF-α, IL-10 and RANKL in periapical granulomas

Total RNA was extracted from periapical granulomas (G, N=57) and periodontal ligament control samples (C, N=38), and levels of SOCS1, SOCS2, SOCS3, TNF-α, IL-10 and RANKL mRNA were measured quantitatively by RealTimePCR SYBR-Green system. The results are presented as expression of the individual mRNAs, with normalization to housekeeping gene using the Ct method. * Statistically significant difference between C and G (p<0.001).

Figure 3. Correlations between the expression of SOCS and cytokines in periapical granulomas and with the radiographic diameter of the lesions

Total RNA was extracted from periapical granulomas (N=57)(A), of SOCS1, SOCS2, SOCS3, TNF-α, IL-10 and RANKL mRNA were measured quantitatively by RealTimePCR SYBR-Green system. Linear regression analysis was used to test the correlations between the levels of SOCS and cytokines expression, and also between SOCS and the radiographic diameter of the lesions. Values of p and r² are identified in the graphs.