As is widely reported around the world, the expression of HMGB1 protein level was higher in almost all tumors (especially in epithelial tumors) than in healthy tissues. HMGB1 is a cytokine and also a growth factor. Necrosis of tumor cells can release HMGB1, which can cause chronic inflammation in the tumor microenvironment and help tumor cells survive, grow and metastasize [17
]. Primarily two signaling pathways mediate the proinflammatory effects of HMGB1. Importantly, the Ras/MAPK pathway, which is involved in MAPKs phosphorylation and activates NF-kB, induceing inflammation and immune cell migration. NF-κB affects the cell cycle directly leading to the occurrence of cancer [18
]. Previous studies have shown that NF-kB may be involved in promoting tumor metastasis by regulating cytokines such as TNF, IL, MMP-9, and ICAM [19
]. HMGB1 could bind to RAGE, which belongs to the immunoglobulin superfamily, a kind of transmembrane protein, playing an important role in diseases such as diabetes and inflammatory diseases. Wang [21
] has found that HMGB1 may be binding to RAGE on the surface of dendritic cell (DC) cells to regulate TNF-a, IL-2, and IL-6 expression. The MAPK pathway may be one of the major intracellular signal transduction pathways inducing RAGE regulation of gene expression [22
]. The MAPK pathway may also participate in the process of maturation of Tregs induced by inflammation [23
]. However, HMGB1 expression and function may not be the same in certain tumor cells and healthy cells. In the process of tumor progression, HMGB1 has been shown to inhibit dendritic cells in the tumor microenvironment, indicating that HMGB1 may participate in tumor evasion and in the promotion of cancer development [24
]. In one study of a tumor-burdened rat model after thermal injury, the release of excessive HMGB1 after burn injury was found to activate splenic Treg cells. The most likely mechanism was binding to the Treg surface receptor, RAGE, thus suppressing T lymphocytes in favor of shifting from Th1 cells to Th2 cells [14
]. Some studies has found that HMGB1, Tregs, and CD88 on the surface of DC cells were significantly positively correlated in spleen tissues, which indicated that HMGB1 might participate in the process of maturation of spleen DC cells through mediating Tregs after serious burn [14
]. Studies from Dumitriu et al
] and Rovere-Querini et al.
] has found that the conditioned medium which contains HMGB1 may promote the maturation of dendritic cells and promote the proliferation of T lymphocytes. Recent studies have showed that various related TOLL-like receptor (TLRs) could be expressed on the surface of Tregs [27
], and related studies also showed that HMGB1 might bind to RAGE, TLRs to conduct the activation of cells [27
]. Cytokines such as IL-2, IL-6, and IL-15 also participate in mediating the proliferation and immunosuppression of Tregs [27
]. Thus we assume HMGB1 may be binding to the TLRs on Treg surfaces to regulate TLR expression and to activate related signal pathways and then activate NF-κB in the nucleus, leading to the release of different cytokines such as IL-4, IL-10, and IL-2. One study conducted in a tumor-burdened rat model of breast cancer indicated that interactions between Tregs cells and cytokines released from Tregs in this process resulted in T cells shifting from Th1 cells to Th2 cells [14
]. There is also evidence to suggest that HMGB1 could not stimulate nature killer cells (NK cells) induced by Tregs releasing INF-γ when coordinated with IL-2, IL-1, and IL-10 [28
]. Thus we speculate that the inhibition of T lymphocytes after burn injury was not caused directly by HMGB1 stimulation, but, may have been mediated by Tregs. The cytokine microenvironment was crucial in deciding the differentiation of T lymphocytes, and the upregulation of IL-4, and IL-10 played a key role in this process. Whether this kind of shift takes place in cervical cancer is currently unclear. To access the correlation between the expression of HMGB1 and immune regulation, the level of HMGB1 and the expression of Treg-related FOXP3, IL-2, and IL-10 protein were examined in cervical tissues. The present study demonstrated that HMGB1 might be correlated positively with Foxp3 expression and both of them were significantly enhanced in cervical cancer tissues. The lesions were found to be more likely to progress in cervical tissues with positive expressions of both HMGB1 and FOXP3. In contrast, in cervical tissues that were negative for both types of expression, the lesions underwent spontaneous regression. In this way, HMGB1 may suppress the human immune function by up-regulating Tregs, which facilitates infection and the growth of cervical-cancer cells. Enhanced Treg activity has been shown to regulate T lymphocytes cell-mediated immunity in the tumor microenvironment [14
]. The IL-2 cytokine is mainly produced by Thl cells during immune responses, and IL-10 is mainly produced by Th2 cells and Tregs as an anti-inflammatory cytokine. The data shown here suggest that HMGB1 may directly activate Tregs by binding to RAGE, which promotes IL-10 production in cervical cancer or helps to shift Th1 cells to Th2 cells and assist in tumor evasion and development. However, the specific and exact mechanism by which HMGB1 stimulates Tregs and its role in shifting of Th1 cells to Th2 cells in cervical cancer remains obscure. These questions will be addressed in our next research project.
The correlations between HMGB1 expression and clinicopathologic factors were then assessed to determine whether HMGB1 expression could be used as a risk predictor of malignancy and metastasis. These data indicate that HMGB1 expression increased with the advancement of FIGO stage and lymph node cervical cancer metastasis. These data indicate that HMGB1 may be critical for cervical carcinoma progression. However, HMGB1 protein overexpression was not found to be correlated with age, differentiation or histology of cervical cancer in patients. A larger number of cervical cancer samples need to be analyzed to substantiate these results.