Submitted on August 12, 2007
Accepted on September 19, 2007
Phenotypic differences in PFIC2 and BRIC2 correlate with protein stability of mutant Bsep and impaired taurocholate secretion in MDCK II cells
Tatehiro Kagawa1*, Norihito Watanabe1, Kaori Mochizuki1, Asano Numari1, Yoshie Ikeno1, Johbu Itoh1, Hitrotoshi Tanaka2, Irwin m Arias3, and Tetsuya Mine1
1 Department of Gastroenterology, Tokai University School of Medicine, Isehara, Kanagawa, Japan
2 Division of Clinical Immunology, Advanced Clinical Research Center, Institute of Medical Science, University of Tokyo, Tokyo, Japan
3 NICHD and Unit for Cellular Polarity, National Institutes of Health, Bethesda, Maryland, United States; Physiology, Tufts Universtiy School of Medicine, Boston, Massachusetts, United States
* To whom correspondence should be addressed. E-mail: kagawa{at}is.icc.u-tokai.ac.jp.
Progressive familial cholestasis (PFIC) 2 and benign recurrent intrahepatic cholestasis (BRIC) 2 are caused by mutations in the bile salt export pump (BSEP, ABCB11) gene, however, their prognosis differs. PFIC2 progresses to cirrhosis and requires liver transplantation, whereas BRIC2 is clinically benign. To identify molecular mechanism(s) responsible for the phenotypic differences, eight PFIC2 and two BRIC2 mutations were introduced in rat Bsep. Taurocholate transport activity, protein expression and subcellular distribution were studied in a polarized MDCK II monolayer transfected with these mutants. Transport activity was approximately half of the wild type (WT) in BRIC2 mutants (A570T, R1050C), was substantially less in two PFIC2 mutants (D482G, E297G), and was almost abolished in six other PFIC2 mutants (K461E, G982R, R1153C, R1268Q, 3767-3768insC, R1057X). Bsep protein expression levels correlated closely with transport activity, except for R1057X. The half-life of D482G was shorter than that of WT (1.35 h vs. 3.49 h in mature form). BRIC2 mutants and three PFIC mutants (D482G, E297G, R1057X) were predominantly distributed in the apical membrane. The other PFIC2 mutants remained intracellular. R1057X mutant was stably expressed and trafficked to the apical membrane, suggesting that the C-terminal tail is required for transport activity but not for correct targeting. In conclusion, taurocholate transport function was impaired in proportion to rapid degradation of Bsep protein in the mutants which were aligned in the following order; A570T, R1050C > D482G > E297G > K461E, G982R, R1153C, R1268Q, 3767-3768insC, R1057X. These results may explain the phenotypic difference between BRIC2 and PFIC2.
