Soluble Liver Antigen/Liver and Pancreas, an Antigen in Autoimmune Hepatitis Patients: Influence on Selenocysteine Synthesis and its Complex with HSP70

摘要

Autoimmune hepatitis (AIH) serum contains autoantibodies against soluble liver antigen/liver and pancreas (SLA/LP), a protein that interacts with the selenocysteine (Sec)-tRNASec. We investigated the influence of AIH serum on Sec production. Two (#3 and #4) of four AIH sera inhibited its synthesis, with serum #4 showing the strongest inhibition. This did not parallel the level of antibodies to SLA/LP (anti-SLA/LP) in sera, because the titer in serum #3 was higher than that in serum #4. Thus, we found that AIH inhibited Sec synthesis but we could not conclude that this inhibition depended on an autoantibody against SLA/LP. To clarify the function of SLA/LP, we prepared an antibody against an SLA/LP peptide. Inhibition of Sec synthesis by this anti-peptide antibody was incomplete and sometimes did not occur. Absorption of autoantibodies in the AIH serum with the SLA/LP protein did not have any effects on Sec synthesis by Sec synthase (SecS) which converts Seryl (Ser)-tRNASec to Sec-tRNASec. We then used SLA/LP instead of active bovine SecS but we could not detect SecS activity in SLA/LP itself. Next, we studied the distribution of SLA/LP in HEK293 cells transfected with an SLA/LP expression vector. Immunofluorescence microscopy detected SLA/LP expression in the cytoplasm, but not the nucleus of HEK293 cells. Higher levels of SLA/LP in the cytosol did not correlate with higher Sec production, which remained identical to mock-transfected cells. Interestingly, we found that SLA/LP formed a complex in the cytosol of HEK293 cells. This complex was purified and the components analyzed using Nano-LC and MS/MS, which we identified as HSP70 and tubulin. Altogether, our data suggest that SLA/LP may play an indirect role to produce Sec in the selenoprotein synthesis machinery.