Differential Regulation of Hepatic UDP‐glucuronosyltransferase (UGT) 1A1 by Toll‐like Receptors during Irinotecan‐induced Steatosis

Abstract

Irinotecan (CAMPTOSAR®) has been widely used in preoperative chemotherapy for colorectal cancer liver metastases. However, irinotecan‐induced steatosis and steatohepatitis increases mortality of liver surgery. Up to date the mechanism underlying this hepatoxicity remains largely unknown. It is known that in human irinotecan is hydrolyzed to SN‐38, a reactive metabolite, by carboxylesterases in liver. SN‐38 is then rapidly detoxified by enzyme, UDP‐glucuronosyltransferase (UGT1A1) to generate its glucuronide form. Our preliminary study indicated that irinotecan‐induced steatosis is associated with activation of Toll‐like receptors (TLRs) signaling. Previously we reported that activation of TLRs decreased multiple drug metabolism enzymes in liver, including UGT1A1. Hereby, we hypothesize that TLRs are involved in the regulation of irinotecan detoxification. TLR2wt(C57BL/6J), TLR2−/−( B6.129‐Tlr2tm1Kir/J), TLR4wt(C3HeB/FeJ) and TLR4mutant (C3H/HeJ) mice (all male, 5‐weeks old) were treated with 50mg/kg irinotecan via intraperitoneal injection once daily for nine days. Wildtype mice were sacrificed on day 2 and day 9, while TLR‐deficient mice were sacrificed on day 9. Blood and liver were collected for further analysis. We found that in C57BL/6J mice, after two days irinotecan did not cause any changes in liver. However, after nine days constitutive injection in wildtype mice, irinotecan increased proinflammatory cytokines and reduced ugt1a1 expression. Also, it decreased UGT1A1 activity in liver. Compared to wildtype mice, deficiency of either TLR2 or TLR4 signaling alleviated irinotecan‐induced proinflammatory response. However, histology evidences showed that only in TLR2−/− mice but not TLR4mutant mice, irinotecan‐induced intracellular vacuoles in liver became less and smaller than in wildtype mice. Additionally, ugt1a1 downregulation by irinotecan was attenuated in TLR2−/− mice. Pharmacokinetics study indicated that knockout of TLR2 significantly decreased the systemic exposure of SN‐38, the toxic metabolite of irinotecan; no such effect was observed in TLR4mutant mice. Based on the results, we confirm that TLRs are involved in irinotecan metabolism and associated steatosis. Although both TLR2 and TLR4 share the same adaptors to regulate similar downstream pathways, TLR2 probably plays more important role in reducing ugt1a1 during irinotecan‐induced steatosis. Hereby, we conclude that TLR2 is preferentially activated during irinotecan hepatotoxicity and such activation suppress irinotecan detoxification. We characterize a novel feature of TLRs signaling in irinotecan‐induced liver injury. It provides valuable information to address drug safety issues for public health.