Diammonium glycyrrhizinate, a component of traditional Chinese medicine Gan-Cao, prevents murine T-cell-mediated fulminant hepatitis in IL-10- and IL-6-dependent manners
Diammonium glycyrrhizinate (DG) is extracted and purified from traditional Chinese medicinal liquorices (Gan-Cao). Based upon the clinical practice, the liquorices exert an important function in the treatment of hepatitis because of its anti-inflammatory effect. Another study has demonstrated that DG was also efficacious in experimental ulcerative colitis in rats [9]. The aim of the present study was to characterize the role of DG in preventing liver from the ConA-induced hepatitis. We found a critical role of IL-10 secreted by liver MNCs upon DG administration.
ConA is a T-lymphocyte mitogen in vitro and in vivo that leads to lymphocyte proliferation and acute liver injury in mice [1] and [2]. ConA-induced hepatitis represents an experimental model for fulminant hepatitis in mice with no other organs being affected [3]. NKT cells, conventional T cells and macrophages appear to play crucial roles in the ConA-induced hepatitis through several different mechanisms such as those involving the Fas/FasL system, perforin/granzyme system, and IFN-γ, IL-4 and TNF-α-mediated system.
As a research from Shangdong University, liver histological studies were used to determine the protective effect of DG on ConA-induced apoptosis of hepatocytes. As shown in Fig. 2A, examination of liver pathology showed that injection with ConA caused obvious spotted necrosis in the saline/ConA group, whereas mice pretreated with DG showed minor damage. The fate of liver cells was examined in different groups by TUNEL assays as shown in Fig. 2B. Only faint and rare TUNEL positive staining in the liver of DG/ConA-pretreatment mice was observed, and never in the untreated control mice.
Diammonium glycyrrhizinate (DG) is extracted and purified from a traditional Chinese medicinal herb liquorices (Gan-Cao). Gan-Cao has been used for the treatment of hepatitis for hundreds of years because of its anti-inflammatory effect, resistance to biologic oxidation and membranous protection. In order to clarify the mechanism of DG in hepatitis treatment, ConA-induced mice hepatitis model that was considered as an experimental model of autoimmune fulminant hepatitis was used in this study [2]. Data from this study suggested that DG pretreatment could inhibit ConA-induced elevation of ALT level, liver necrosis, hepatocyte apoptosis and even mortality. On the other hand, we observed that pretreatment of DG markedly decreased the total numbers of NKT cells and T cells in liver, and these cells are extensively recognized as effectors cells to mediate liver injury. Also DG pretreatment dramatically increased both IL-6 and IL-10 serum levels in ConA-treated mice, with IL-10 level increased by a big margin.
Interleukin-6 (IL-6) is a pleiotropic cytokine and was capable of protecting liver against ConA-induced injury by decreasing TNF-α production [24]. It has also been proven that IL-6 prevents ConA-induced hepatitis via inhibition of NKT cells [21]. In our experiments, compared with the saline/ConA treatment group, an apparent increase in the expression of IL-6 was detected in the DG/ConA treatment group in early phase after ConA injection. As the general properties of cytokine, endogenous cytokine IL-6 can exert a broad protective effect even at a very low level. In another research, an injection of IL-6 can prevent ConA-induced hepatitis via induction of the anti-apoptotic protein Bcl-XL and suppression of IFN-γ signaling [22]. This finding was confirmed in the present study, whereby treatment with DG apparently decreased ConA-induced apoptosis of hepatocytes.
Interleukin-10 (IL-10) is a multifunctional regulatory cytokine with diverse effects on most hemopoietic cells [10]. The major function of IL-10 appears to reduce pro-inflammatory responses. At the same time, IL-10 could efficiently inhibit T-cell proliferation and cytokine responses as a suppressor cytokine [23]. In this study, DG pretreatment up-regulated IL-10 secretion from hepatic lymphocytes, and diminished the numbers of NKT cells and T cells in liver. As all known, ConA-induced fulminant hepatitis in mice has been shown to be mediated by activated T cells [11], and their subset NKT cells, as recently reported [6]. The mechanism by which DG inhibits recruitment and function of NKT cells and T cells is not clear. It has been reported that Tr1 (IL-10 secreting T regulatory type 1) cells regulate immune responses by secreting the immunosuppressive cytokines IL-10 and. Whether DG-induced IL-10 secretion is involved in the immunosuppression of NKT cells and T cells through the Tr1 cells remains unknown and needs further investigation.
In conclusion, these results suggest that the protection effect of DG pretreatment is, at least partly, correlated to the enhanced production of IL-10 as well as IL-6. DG may possibly protect the liver from injury via direct protection of hepatocytes through IL-6-dependent way and indirect inhibition of T-cell-mediated liver injury through IL-10-dependent manner. Since IL-10 is also involved in a variety of autoimmune diseases, such as DVHD, rheumatoid arthritis, allergy and asthmaand it will be interesting to examine whether DG also have effects in these diseases via an IL-10- or IL-6-dependent way.
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