Substrate- and pH-specific antifolate transport mediated by organic anion-transporting polypeptide 2B1 (OATP2B1-SLCO2B1).
Human organic anion-transporting polypeptide (OATP) 2B1 (OATP-B; SLCO2B1) is expressed in the apical membrane of the small intestine and the hepatocyte basolateral membrane and transports structurally diverse organic anions with a wide spectrum of pH sensitivities. This article describes highly pH-dependent OATP2B1-mediated antifolate transport and compares this property with that of sulfobromophthalein (BSP), a preferred OATP2B1 substrate. At pH 5.5 and low substrate concentrations (~2.5 μM), only [(3)H]pemetrexed influx [in contrast to methotrexate (MTX), folic acid, and reduced folates] could be detected in OATP2B1-transfected HeLa R1-11 cells that lack endogenous folate-specific transporters. Influx was optimal at pH 4.5 to 5.5, falling precipitously with an increase in pH >6.0; BSP influx was independent of pH. Influx of both substrates at low pH was markedly inhibited by the proton ionophore 4-(trifluoromethoxy)phenylhydrazone; BSP influx was also suppressed at pH 7.4. At 300 μM MTX, influx was one-third that of pemetrexed; influx of folic acid, (6S)5-methyltetrahydrofolate, or (6S)5-formyltetrahydrofolate was not detected. There were similar findings in OATP2B1-expressing Xenopus laevis oocytes. The pemetrexed influx K(m) was ~300 μM; the raltitrexed influx K(i) was ~70 μM at pH 5.5. Stable expression of OAPT2B1 in HeLa R1-11 cells resulted in substantial raltitrexed, but modest pemetrexed, growth inhibition consistent with their affinities for this carrier. Hence, OATP2B1 represents a low-affinity transport route for antifolates (relative affinities: raltitrexed > pemetrexed > MTX) at low pH. In contrast, the high affinity of this transporter for BSP relative to antifolates seems to be intrinsic to its binding site and independent of the proton concentration.
Experimental and Clinical Pharmacology Unit, Centro di Riferimento Oncologico, National Cancer Institute, Aviano, Italy.
Michele
M
Visentin
Min-Hwang
MH
Chang
Michael F
MF
Romero
Rongbao
R
Zhao
I David
ID
Goldman
D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D005493:Folic Acid Antagonists / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism; D006147:Guanine / Q000031:analogs & derivatives / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D009865:Oocytes; D027361:Organic Anion Transporters / Q000378:metabolism*; D000068437:Pemetrexed; D013379:Substrate Specificity; D013448:Sulfobromophthalein; D014982:Xenopus laevis
D005493:Folic Acid Antagonists; D005971:Glutamates; D007202:Indicators and Reagents; D027361:Organic Anion Transporters; C450859:SLCO2B1 protein, human; D000068437:Pemetrexed; D013448:Sulfobromophthalein; D006147:Guanine