Received for publication June 13, 2008.
Revised November 6, 2008.
Accepted for publication November 10, 2008.

Functional characterization of multidrug and toxin extrusion protein 1 as a facilitative transporter for fluoroquinolones

Abstract

Many fluoroquinolones are mainly eliminated by urinary excretion, in which tubular secretion by carrier-mediated transport systems has been suggested to be involved. In the present study, we examined the possibility that multidrug and toxin extrusion protein 1 (MATE1), which is abundantly expressed in the kidney, might be involved in that, using rat MATE1 (rMATE1) expressed in MDCKII cells. It was found that rMATE1 can transport fluoroquinolones such as ciprofloxacin, enoxacin, gatifloxacin, levofloxacin, norfloxacin (NFX), pazufloxacin and tosufloxacin. Although rMATE1 has been known as an apical organic cation/H⁺ antiporter, detailed investigation of rMATE1-mediated uptake of NFX has revealed that it is not sensitive to intracellular acidification by treatments using NH₄Cl or nigericin, suggesting that transmembrane proton gradient is not involved in its transport as a driving force. However, it was dependent on extracellular pH, being greatest at pH 7.0 and smaller at both acidic and basic pH in agreement with the profile of zwitterionization of NFX. The basal-to-apical transcellular transport of NFX in rMATE1-expressing MDCKII cells was greater than that in mock cells and insensitive to acidification of the apical medium, demonstrating proton-gradient-independent functionality of rMATE1 in NFX efflux. Finally, rMATE1-mediated NFX uptake at pH 7.4 was saturable with the Michaelis constant of 55.3 µM and inhibited by cationic compounds, such as tetraethylammonium and cimetidine. These results suggest that rMATE1 mediates the transport of NFX by a facilitative manner. MATE1 may play a key role in the renal tubular secretion of fluoroquinolones.

Key words: MATE, antiporter, efflux, fluoroquinolones, secretion, transporter