In this study, we assessed the hepatoprotective effects of AhR antagonism on hepatic IR injury in rats. Rats treated with TMF at 10 minutes after ischemia had lower RE values and serum ALT and AST levels than did untreated rats at three hours after reperfusion. After 24 hours of reperfusion, TMF-treated rats had lower RE values, ΔT1 values, serum ALT levels, and necrotic area percentage than did untreated rats. In addition, the expression of the apoptosis-related proteins Bax and cleaved caspase-3 was lower in the TMF-treated rats than in the untreated rats. These results suggest that inhibiting AhR activation after ischemia ameliorates hepatic IR injury.
Under normal conditions, AhR remains inactive in the cytoplasm and is activated by endogenous ligands, such as L-Kyn10, or exogenous ligands, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin.14 Recently, L-Kyn was found to accumulate in the brain during ischemia and induce the activation of AhR, exacerbating neuronal damage even after reperfusion.10 Because a significant amount of L-Kyn is produced by tryptophan metabolism under TDO, brain-like phenomena due to ischemia are likely to occur in the liver as well.13 However, to the best of our knowledge, there have yet to be studies on this. In this study, we investigated, for the first time, whether AhR antagonism after ischemia affects liver damage due to IR. Our results showed that AhR antagonism inhibited the progression of apoptosis and necrosis. This suggests that L-Kyn may accumulate owing to ischemia in the liver, as in the brain, and activate AhR, possibly leading to hepatic IR injury. However, in the present study, the degree of L-Kyn accumulation and the inhibition of AhR activation by the regulation of L-Kyn accumulation were not studied. Due to the limited results of this study, additional research is needed in the future.
The present study showed the inhibitory effect of apoptosis in animals administrated with AhR antagonists after ischemia. However, it has not been elucidated which pathways downstream of AhR mediate hepatic IR injury. In the brain, several studies have reported that AhR inhibition reduces neuronal cell death and neurotoxicity.15,16 In particular, cAMP response element binding protein (CREB) signaling has been reported as one of the downstream pathways caused by AhR activation.10 The report suggests that AhR activation due to brain ischemia regresses CREB protein-dependent signaling and exacerbates brain damage. However, it is considered that AhR likely works with various sub-mechanisms. Therefore, further studies of sub-mechanisms, including alterations of CREB signaling induced by AhR activation in the liver, are needed.
Changes in serum ALT and AST levels in our study were used to evaluate changes in hepatic injury caused by IR. The serum ALT and AST levels are considered the representative markers of hepatocellular injury or necrosis.17,18 Hepatocellular injury or necrosis causes ALT and AST leakage from hepatocytes into the blood, and elevated ALT and AST in serum suggest liver damage.19 In this study, all rats with IR had higher serum ALT and AST levels at 3 and 24 hours after reperfusion than did the sham-operated rats. On the other hand, the TMF-treated rats had lower serum ALT and AST levels at 3 and 24 hours after reperfusion than did the untreated rats. These results indicate that TMF administration after ischemia contributes to the alleviation of IR-induced hepatocellular injury or necrosis. Furthermore, histological analysis also revealed a similar pattern of differences in the necrotic area. However, the mechanisms involved in the association between AhR activation and necrosis in this study have not been elucidated. Further research on this is needed.
We analyzed changes in liver function due to IR using Gd-EOB-DTPA, a paramagnetic hepatobiliary MR contrast agent. This was based on several reports that Gd-EOB-DTPA-enhanced MRI is suitable for evaluating liver function due to its organic anion transporting polypeptide (OATP1B1/B3)-dependent hepatocyte-specific uptake and paramagnetic properties.20,21 In particular, several reports have been demonstrated the effectiveness of liver function assessment using the SI-based indices of T1-WIs and the T1 relaxometry.22,23 In this study, as a result of analyzing RE and LMR values reported to be highly correlated with liver function among SI-based indices,24 RE values were decreased in TMF-treated rats at 3 and 24 hours after reperfusion compared to those of untreated rats. These results suggested that liver function was improved in the TMF-treated group. Based on T1 relaxometry, we analyzed T1-post and ΔT1 values.22,24 At 24 hours after reperfusion, the TMF-treated rats had decreased ΔT1 values compared to those of untreated rats. These results indicated that liver function was improved in the TMF-treated rats.
There were two limitations in this study. First, we only tested a single dose of TMF in our analysis (5 mg/kg of TMF) that had been suggested previously by Cuartero et al. and Kwon et al. .10,12 The concentration of TMF in the blood may be different depending on the concentration of TMF administered; thus, the effect on hepatic IR injury may also vary. Further studies are needed to find the optimal concentration to reduce hepatic IR injury. Second, we only observed that AhR expression at 24 hours after reperfusion was significantly higher in the rats with hepatic IR injury than in the sham-operated and TMF-treated rats. This means that an increased expression of AhR may be one of the factors that play an important role in the progression of hepatic IR injury. However, in this study, the change in AhR expression was observed only 24 hours after reperfusion, and it was not determined how the change in AhR expression changed or how long it lasted after reperfusion. Additional studies are needed to find the optimal timing of TMF administration by tracking changes in AhR expression after reperfusion.
Our findings demonstrated that post-ischemia administration of AhR antagonists has hepatoprotective effects that ameliorate hepatic IR injury. We propose that adequate AhR antagonist activity is a potential therapeutic approach for hepatic I/R injury. Nevertheless, further studies are needed to elucidate the mechanisms underlying the hepatoprotective effect to assess potential clinical applications of AhR antagonist administration.