SLC35B1 significantly contributes to the uptake of UDPGA into the endoplasmic reticulum for glucuronidation catalyzed by UDP-glucuronosyltransferases.

The transport of UDP-glucuronic acid (UDPGA), a co-substrate of UDP-glucuronosyltransferase (UGT), to the intraluminal side of the endoplasmic reticulum (ER) is an essential step in the glucuronidation of exogenous and endogenous compounds. According to a previous study, the expression of recombinant SLC35B1, SLC35B4, or SLC35D1, nucleotide sugar transporters, in V79 cells has the potential to transport UDPGA into the lumen of microsomes. The purpose of this study is to examine whether the transport of UDPGA by these transporters substantially affects UGT activity. Since the knockdown of UDP-glucose 6-dehydrogenase, a synthetase of UDPGA, in HEK293 cells stably expressing UGT1A1 (HEK/UGT1A1 cells) resulted in a significant decrease in 4-methylumbelliferone (4-MU) glucuronosyltransferase activity, supplementation of a sufficient amount of UDPGA is required for UGT activity. By performing qRT-PCR using cDNA samples from 21 human liver samples, we observed levels of the SLC35B1 and SLC35D1 mRNAs that were 15- and 14-fold higher, respectively, than the levels of the SLC35B4 mRNA, and SLC35B1 showed the largest (37-fold) interindividual variability. Interestingly, 4-MU glucuronosyltransferase activity was significantly decreased upon the knockdown of SLC35B1 in HEK/UGT1A1 cells, and this phenomenon was also observed in HepaRG cells. Using siRNAs targeting 23 different SLC35 subfamilies, the knockdown of SLC35B1 and SLC35E3 decreased 4-MU glucuronosyltransferase activity in HEK/UGT1A1 cells. However, the 4-MU glucuronosyltransferase activity was not altered by SLC35E3 knockdown in HepaRG cells, suggesting that SLC35B1 was the main transporter of UDPGA into the ER in the human liver. In conclusion, SLC35B1 is a key modulator of UGT activity by transporting UDPGA to the intraluminal side of the ER.