Estradiol 17-b d-glucuronide is a high-affinity substrate for the
organic anion transporter "oatp".
Kanai, Naoaki, Run Lu, Yi Bao, Allan W. Wolkoff, Mary Vore, Victor L.
Schuster.
Renal Division, Departments of Medicine, Physiology &
Biophysics; Liver Research Center; Albert Einstein College of
Medicine, Bronx, N.Y. 10461, Department of Pharmacology, University
of Kentucky College of Medicine, Lexington, KY
APStracts 2:0166F, 1995.
Although substantial evidence indicates that estradiol-17 b (E2) is
conjugated to the glucuronide in the kidney and then excreted by a
direct tubular secretory route, and that the liver transports E2
glucuronides via carrier-mediated mechanisms, the transporters
involved in these processes have not been identified. The organic
anion transporting polypeptide "oatp" has a number of known
substrates, including bromosulfophthalein (BSP) and taurocholic acid
(TCA) (Jacquemin et al, PNAS 91:133, 1994). In a companion paper, we
determined that steroid hormones represent a class of hormones that
interacts strongly with oatp when the latter is transiently expressed
in vitro. Here we studied more extensively steroids and steroid anion
conjugates as candidate oatp substrates. In HeLa cell monolayers
transfected with a full-length oatp cDNA, 3H-estradiol 17 b-D
glucuronide (E2-17GA) was transported with a signal-to-noise ratio of
15:1 over that of monolayers transfected with a control plasmid. The
affinity of oatp for 3H-E2-17G was significantly higher than that for
TCA (Km 3 mM vs 27 mM, respectively). In contrast to E2-17G,
unconjugated estradiol (E2) was not significantly transported by
oatp. Several unconjugated steroids and anionic steroid conjugates
were tested for their ability to compete with tracer E2-17G for oatp
-mediated transport. Conjugation at the 17- or 3-position with the
anion of a strong acid (sulfate) resulted in a greater degree of
inhibition of tracer E2-17G transport than did conjugation at the 17-
or 3-position with an uncharged group (acetate), suggesting that the
strength of the negative charge at these positions is an important
determinant of the affinity of a given steroid conjugate for oatp. We
conclude that the preferred substrates for oatp are steroids with a
strong 17- or 3-position anionic group. Since steroid
sulfotransferases and glucuronosyltransferases are expressed in the
proximal tubule, as is oatp, the transporter may serve as an apical
exit pathway for steroids following their conjugation within the
tubule cell.
Received 22 February 1995; accepted in final form 21 August 1995.
APS Manuscript Number F62-5.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 31 October 95