• The Role of Bile Salt Export Pump (BSEP; ABCB11) Gene Repression in Drug-Induced Cholestatic Liver Injury

      Garzel, Brandy N.; Wang, Hongbing, Ph.D. (2017)
      Drug-induced liver injury (DILI) is a persistent matter in the pharmaceutical field, with extensive research being dedicated to understanding the causes, risk factors, and potential biomarkers associated with it. While DILI manifests in a variety of ways and diseases, the most severe form is drug-induced cholestasis. Bile acids are amphipathic molecules responsible for the extraction of lipids from the diet, and while necessary for function, their amphipathic nature can cause toxicity if allowed to accumulate in tissue. Constant flow of bile acids through biliary tissue is facilitated by specific bile transporters, with the bile salt export pump (BSEP; ABCB11) dictating excretion of bile acids from the liver to the gall bladder. Disruption of BSEP function is a major contributor to both inherited and acquired cholestasis. The predominant mechanism of drug-induced cholestasis is direct inhibition of BSEP, however in the following work, the contribution of BSEP repression on BSEP function was investigated. In the following studies, human primary hepatocytes (HPH) were used predominantly as a physiologically relevant system for studying drug transporter function and expression upon drug treatment. The functional transport assay employed was modified from the prototypical inhibition assays primarily used in cholestatic studies, and represents a novel approach to assess the cholestatic potential of BSEP repressors. Additional BSEP repressors were predicted from reported cases of drug-induced cholestasis combined with Bayesian modeling. It was determined from our study, that compared to inhibition alone, BSEP inhibition combined with repression correlated with severe clinical manifestations of DILI. Mechanistically, activation of AMPK was found to repress basal BSEP expression, while known transcriptional regulators FXR and Nrf2 had no effect. Most importantly, under conditions which cause BSEP repression, bile acid efflux was also significantly reduced. Metformin was used as a model drug in our studies, however, a number of drugs associated with cholestasis do not inhibit BSEP directly. Employing a strategy to elucidate additional cholestatic BSEP repressors, tamoxifen was found to potently repress BSEP expression, indicating the potential discovery of additional repressors. Overall, these results confirm BSEP repression represents an understudied mechanism for drug-induced cholestasis.