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Nutrient Metabolism

Choline Uptake in Human Intestinal Caco-2 Cells Is Carrier-Mediated

Department of Pharmaceutical Sciences, School of Pharmacy and Pharmaceutical Sciences, University at Buffalo, The State University of New York, Amherst, NY 14260

³To whom correspondence should be addressed. E-mail: memorris{at}buffalo.edu.

The objective of the current investigation was to examine the transport characteristics of choline, an endogenous quaternary ammonium compound, into human intestinal Caco-2 cells; the transport of choline has not been characterized in human intestine. The cellular accumulation of choline was independent of an inwardly directed Na⁺ gradient and demonstrated temperature dependence and saturability. Using the initial uptake rates, choline accumulation was best characterized by a Michaelis-Menten equation and a diffusion component with a K_m and V_max of 110 ± 3 µmol/L and 2800 ± 250 pmol/(mg protein · 10 min), respectively. Choline uptake was significantly inhibited by an excess of choline itself and by hemicholinium-3, a structural analog of choline. However other hydrophilic organic cations, such as tetraethylammonium (TEA) and N-methylnicotinamide (NMN), did not affect choline uptake in Caco-2 cells. Additionally, two typical p-glycoprotein substrates, daunomycin and verapamil, both inhibited choline accumulation. However the opposite was not true: choline did not inhibit DNM accumulation in Caco-2 cells. These results indicate the presence of a carrier-mediated transport system for choline in Caco-2 cells. The substrate specificity of this carrier is unlike that seen in the rat intestinal epithelium, and the human transport protein is distinct from those for TEA and NMN. P-glycoprotein substrates may inhibit choline uptake through specific or nonspecific interactions with the choline transporter.

KEY WORDS: • Caco-2 • human intestine • transport • choline

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