The Importance of Val386 in Transmembrane Domain 8 for the Activation of OATP1B3 by Epigallocatechin Gallate.

Estrone-3-sulfate (E3S) uptake mediated by organic anion transporting polypeptide 1B3 (OATP1B3) can be activated by epigallocatechin gallate (EGCG). In this study, by using chimeric transporters and site-directed mutagenesis, we found that Val386 in transmembrane domain 8 (TM8) is essential for OATP1B3's activation by EGCG. Kinetic studies showed that the loss of activation of 1B3-TM8 and 1B3-V386F in the presence of EGCG is due to their decreased substrate binding affinity and reduced maximal transport rate. The overall transport efficiencies of OATP1B3, 1B3-TM8, and 1B3-V386F in the absence and presence of EGCG are 8.6 ± 0.7 vs 15.9 ± 1.4 (p < 0.05), 11.2 ± 2.1 vs 2.7 ± 0.3 (p < 0.05), and 10.2 ± 1.0 vs 2.5 ± 0.3 (p < 0.05), respectively. While 1B3-V386F cannot be activated by EGCG, its transport activity for EGCG is also diminished. OATP1B3's activation by EGCG is substrate-dependent as EGCG inhibits OATP1B3-mediated pravastatin uptake. Furthermore, the activation of OATP1B3-mediated E3S uptake by quercetin 3-O-α-l-arabinopyranosyl(1 → 2)-α-l-rhamnopyranoside is not affected by TM8 and V386F. Taken together, the activation of OATP1B3 by small molecules is substrate- and modulator-dependent, and V386 in TM8 plays a critical role in the activation of OATP1B3-mediated E3S uptake by EGCG.