The transporter "variome": the missing link between gene variants and bile salt transporter function

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

Cholestatic liver disease

Cholestasis develops as a consequence of impaired bile formation and/or bile flow and can be classified as intra- or extrahepatic. Chronic cholestatic diseases are mostly intrahepatic with the exception of primary and secondary sclerosing cholangitis affecting intra- and extrahepatic bile ducts. Recent genome-wide association studies have confirmed major histocompatibility complex associations and discovered multiple susceptibility loci in primary biliary cirrhosis and primary sclerosing cholangitis, providing new insights into disease pathogenesis, which may translate into more precise therapeutic prevention and intervention in the future. Diagnostic steps in cholestatic conditions comprise a thorough patient history, abdominal imaging and distinct serological studies including antimitochondrial antibodies and IgG4 levels; if the diagnosis remains unclear, liver biopsy is warranted. Genetic testing should also be considered, as mutations in the hepatobiliary transporters ATP8B1, ABCB11 and ABCB4 are causative for three different forms of familial intrahepatic cholestasis. Disease severity is dependent on the genotypic variants of these transporters, ranging from mildly elevated liver enzymes in adults to cirrhosis in early childhood. Ligands of nuclear receptors, which represent important regulators of hepatobiliary transporters, and modified bile salts are new promising therapeutic options in cholestatic liver disease and are currently being investigated in clinical trials.