ABCG5 and ABCG8 Are Involved in Vitamin K Transport.

ATP-binding cassette protein G5 (ABCG5)/ABCG8 heterodimer exports cholesterol from cells, while Niemann-Pick C1-like 1 (NPC1L1) imports cholesterol and vitamin K. We examined whether ABCG5/ABCG8 transports vitamin K similar to NPC1L1. Since high concentrations of vitamin K show cytotoxicity, the cytoprotective effects of ABCG5/ABCG8 were examined. BHK cells expressing ABCG5/ABCG8 were more resistant to vitamin K cytotoxicity than control cells, suggesting that ABCG5/ABCG8 transports vitamin K out of cells. The addition of vitamin K reversed the effects of ABCG5/ABCG8, suggesting that vitamin K competitively inhibits the transport of vitamin K. To examine the transport of vitamin K by ABCG5/ABCG8, vitamin K levels in the medium and cells were measured. Vitamin K levels in cells expressing ABCG5/ABCG8 were lower than those in control cells, while vitamin K efflux increased in cells expressing ABCG5/ABCG8. Furthermore, the biliary vitamin K concentration in Abcg5/Abcg8-deficient mice was lower than that in wild-type mice, although serum vitamin K levels were not affected by the presence of Abcg5/Abcg8. These findings suggest that ABCG5 and ABCG8 are involved in the transport of sterols and vitamin K. ABCG5/ABCG8 and NPC1L1 might play important roles in the regulation of vitamin K absorption and excretion.