The characteristic serological feature of primary biliary cirrhosis (PBC) is the presence of antimitochondrial antibodies (AMAs), and the major proteins recognized by AMAs are subunits of the 2-oxo acid dehydrogenase complexes (2-OADC), including the E2 components of the pyruvate dehydrogenase complex (PDC), the 2-oxo-glutarate dehydrogenase complex (OGDC), the branched-chain 2-oxoacid dehydrogenase complex (BCOADC), the E3 binding protein (E3BP or protein X) and the E1a component of mammalian PDC. Previous work has postulated that either E3BP, or a molecule cross-reactive with the PDC-E2 molecule, is uniquely expressed on the surface of biliary epithelial cells in PBC. To address this issue, we performed in situ hybridization for all of the major 2-OADC components at the mRNA level, including PDC-E2, BCOADC-E2, OGDC-E2, PDC-E1a, BCOADC-E1a, OGDC-E1, and E3BP using 13 PBC and 9 control livers using 7 mitochondrial antisense probes. In both PBC and controls, the expression of all 2-OADC component mRNA studied herein were found in hepatocytes and infiltrating mononuclear cells, without significant differences. Interestingly, however, despite published data on immunohistochemical staining, interlobular bile ducts including the injured bile ducts in PBC were generally negative or only faintly positive, with the exception of 1 bile duct in 1 of 13 cases of PBC and 1 of 9 control liver specimens. Moreover, confocal microscopic examination and image analysis revealed that the mRNA signal intensity of each of the 2-OADC components in the bile ducts of PBC was relatively lower in comparison with control liver diseases. These data suggest that continuous enhanced synthesis of the 2- OADC components is not likely to be occurring in the biliary epithelial cells in PBC, and that an increase of PDC-E2 or E3BP immunoreactivity in PBC is caused by exogenous imported or cross-reactive molecules.
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