<p class="first" id="P1">Organic Anion Transporting Polypeptides (OATPs), encoded
by genes of the Solute Carrier
Organic Anion (
<i>SLCO</i>) family, are transmembrane proteins involved in the uptake of various
compounds of
endogenous or exogenous origin. In addition to their physiological roles, OATPs influence
the pharmacokinetics and drug-drug interactions of several clinically relevant compounds.
To examine the function and molecular interactions of human OATPs, including several
poorly characterized family members, we expressed all 11 human OATPs at high levels
in the baculovirus-Sf9 cell system. We measured the temperature- and inhibitor-sensitive
cellular accumulation of sodium fluorescein and fluorescein-methotrexate, two fluorescent
substrates of the OATPs, OATP1B1 and 1B3. OATP1B1 and 1B3 were functional in Sf9 cells,
showing rapid uptake (t
<sub>1/2(fluorescein-methotrexate)</sub> 2.64 and 4.16 min, and t
<sub>1/2(fluorescein)</sub> 6.71 and 5.58 min for OATP1B1 and 1B3, respectively) and
high-affinity transport
(K
<sub>m(fluorescein-methotrexate)</sub> 0.23 and 0.53 μM, and K
<sub>m(fluorescein)</sub> 25.73 and 38.55 μM for OATP1B1 and 1B3, respectively) of
both substrates. We found
that sodium fluorescein is a general substrate of all human OATPs: 1A2, 1B1, 1B3,
1C1, 2A1, 2B1, 3A1, 4A1, 4C1, 5A1 and 6A1, while fluorescein-methotrexate is only
transported by 1B1, 1B3, 1A2 and 2B1. Acidic extracellular pH greatly facilitated
fluorescein uptake by all OATPs, and new molecular interactions were detected (between
OATP2B1 and Imatinib, OATP3A1, 5A1 and 6A1 and estradiol 17-β–D-glucuronide, and OATP1C1
and 4C1 and prostaglandin E2). These studies demonstrate for the first time the applicability
of the insect cell system for the functional expression of the entire human OATP family,
and for drug-OATP interaction screening.
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