Title

Regulation of hepatobiliary transporters during drug-induced liver injury

Date of Completion

January 2007

Keywords

Health Sciences, Toxicology

Degree

Ph.D.

Abstract

Drug-induced liver injury impairs hepatobiliary function and results in altered disposition of xenobiotics. Previous studies have demonstrated that gene and protein expression of hepatobiliary transporters is altered in response to chemical-mediated hepatotoxicity. The work presented in this dissertation includes a comprehensive temporal characterization of mRNA and protein expression for mouse uptake and efflux transporters in response to acetaminophen and carbon tetrachloride exposure. In general, a hepatotoxic dose of acetaminophen or carbon tetrachloride decreased mRNA and protein levels of uptake transporters (organic anion transport proteins, Oatps, and sodium-taurocholate co-transporting polypeptide, Ntcp) and increased levels of efflux transporters (multidrug resistance-associated proteins, Mrps) coordinately with detoxification, stress and antioxidant enzymes. Induction of Mrp3 and Mrp4 occurred in centrilobular hepatocytes, and Mrp4 staining localized primarily to proliferating cells. Administration of a second higher dose of acetaminophen to mice when Mrp3 and Mrp4 levels and hepatocellular regeneration were maximal resulted in lower hepatotoxicity or autoprotection. Livers from protected mice demonstrated marked increases not only in hepatocyte proliferation but also Mrp4 protein far beyond those seen after a single dose of acetaminophen. Inhibition of hepatocyte proliferation blocked acetaminophen autoprotection and also diminished induction of Mrp4. It is hypothesized that upregulation of Mrp4 is a protective response to minimize accumulation of potentially toxic chemicals in hepatocytes and may also be a mechanism for paracrine signaling within the liver during recovery from injury. ^ Work in this dissertation also investigates regulatory factors underlying transporter regulation during liver injury. Previous data demonstrate that increased expression of detoxification and antioxidant genes in mice exposed to acetaminophen is mediated by the transcription factor, NFE2-related factor 2, Nrf2. Experiments were designed to investigate whether up-regulation of Mrp mRNA and protein after acetaminophen was dependent on Nrf2 expression. The results showed that induction of Mrp3 and Mrp4 mRNA and protein is absent in Nrf2 knockout mice suggesting that Nrf2 signaling is critical for regulation of these two transporters during APAP toxicity. These findings provide insightful information on the regulation of hepatobiliary transporter expression in mice during drug-induced liver injury. Further studies are necessary to establish the functional consequences of altered transporter levels during hepatotoxicity. ^