TY - JOUR
T1 - The human organic anion transport protein SLC21A6 is not sufficient for bilirubin transport
AU - Wang, Pijun
AU - Kim, Richard B.
AU - Chowdhury, J. Roy
AU - Wolkoff, Allan W.
PY - 2003/6/6
Y1 - 2003/6/6
N2 - A recent study (Cui, Y., Konig, J., Leier, I., Buchholz, U., and Keppler, D. (2001) J. Biol. Chem. 276, 9626-9630) suggests that human OATP2 (SLC21A6), also known as OATP-C and LST1, mediates hepatic bilirubin transport. Because of methodologic concerns, this study was designed to examine this issue using a bilirubin transport assay that was validated in overnight cultured rat hepatocytes. These studies showed that cultured rat hepatocytes transported bilirubin with kinetics virtually identical to the transport of sulfobromophthalein. This assay was then used to quantify bilirubin transport by HeLa cells that had been stably transfected with OATP2 under regulation of a metallothionein promoter. Immunoblot analysis revealed abundant expression of OATP2 after incubation of cells for 48 h in zinc, whereas uninduced cells had no expression of this protein. In OATP2-expressing (zinc-induced) HeLa cells at 37 °C, the uptake of [35S]sulfobromophthalein was substantial (51.6 ± 16.5 pmol/15 min/mg protein, n = 5) with little cell-associated ligand in non-expressing (uninduced) cells (0.54 ± 0.16 pmol/15 min/mg protein, n = 5, p < 0.002). In contrast, there was no difference (p > 0.2) in cell-associated [3H]bilirubin in induced (OATP2-expressing) as compared with uninduced cells (11.25 ± 3.02 pmol/15 min/mg protein versus 9.15 ± 1.68 pmol/min/mg protein, respectively, n = 5) We obtained similar results in OATP2-transfected HEK293 cells that were used in the original report. The existence of a bilirubin transporter has been an important field of investigation for many years. Although the current study indicates that a role for OATP2 in hepatocyte bilirubin transport is unlikely, it provides new and sensitive tools that can be adapted to examine the function of putative bilirubin transporters in the future.
AB - A recent study (Cui, Y., Konig, J., Leier, I., Buchholz, U., and Keppler, D. (2001) J. Biol. Chem. 276, 9626-9630) suggests that human OATP2 (SLC21A6), also known as OATP-C and LST1, mediates hepatic bilirubin transport. Because of methodologic concerns, this study was designed to examine this issue using a bilirubin transport assay that was validated in overnight cultured rat hepatocytes. These studies showed that cultured rat hepatocytes transported bilirubin with kinetics virtually identical to the transport of sulfobromophthalein. This assay was then used to quantify bilirubin transport by HeLa cells that had been stably transfected with OATP2 under regulation of a metallothionein promoter. Immunoblot analysis revealed abundant expression of OATP2 after incubation of cells for 48 h in zinc, whereas uninduced cells had no expression of this protein. In OATP2-expressing (zinc-induced) HeLa cells at 37 °C, the uptake of [35S]sulfobromophthalein was substantial (51.6 ± 16.5 pmol/15 min/mg protein, n = 5) with little cell-associated ligand in non-expressing (uninduced) cells (0.54 ± 0.16 pmol/15 min/mg protein, n = 5, p < 0.002). In contrast, there was no difference (p > 0.2) in cell-associated [3H]bilirubin in induced (OATP2-expressing) as compared with uninduced cells (11.25 ± 3.02 pmol/15 min/mg protein versus 9.15 ± 1.68 pmol/min/mg protein, respectively, n = 5) We obtained similar results in OATP2-transfected HEK293 cells that were used in the original report. The existence of a bilirubin transporter has been an important field of investigation for many years. Although the current study indicates that a role for OATP2 in hepatocyte bilirubin transport is unlikely, it provides new and sensitive tools that can be adapted to examine the function of putative bilirubin transporters in the future.
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U2 - 10.1074/jbc.M301100200
DO - 10.1074/jbc.M301100200
M3 - Article
C2 - 12670950
AN - SCOPUS:0038081033
SN - 0021-9258
VL - 278
SP - 20695
EP - 20699
JO - Journal of Biological Chemistry
JF - Journal of Biological Chemistry
IS - 23
ER -