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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Mechanism of nicotinic acid transport in human liver cells: experiments with HepG2 cells and primary hepatocytes

¹Veterans Affairs Medical Center, Long Beach; and ²Department of Medicine and Department of Physiology and Biophysics, University of California College of Medicine, Irvine, California

Submitted 6 September 2007 ; accepted in final form 5 October 2007

ABSTRACT

This study reports on the functional expression of a specific, high-affinity carrier-mediated mechanism for the transport of niacin (nicotinic acid) in human liver cells. Both human-derived liver HepG2 cells and human primary hepatocytes were used as models in these investigations. The initial rate of transport of nicotinic acid into HepG2 cells was found to be acidic pH, temperature, and energy dependent; it was, however, Na⁺ independent in nature. Evidence for the existence of a carrier-mediated system that is specific for [³H]nicotinic acid transport was found and included the following: 1) saturability as a function of concentration with an apparent K_m of 0.73 ± 0.16 µM and V_max of 25.02 ± 1.45 pmol·mg protein^–1·3 min^–1, 2) cis-inhibition by unlabeled nicotinic acid and nicotinamide but not by unrelated organic anions (lactate, acetate, butyrate, succinate, citrate, and valproate), and 3) trans-stimulation of [³H]nicotinic acid efflux by unlabeled nicotinic acid. Transport of the vitamin into human primary hepatocytes occurs similarly via an acidic pH-dependent and specific carrier-mediated process. Inhibitors of the Ca²⁺-calmodulin-mediated pathway (but not modulators of the PKC-, PKA-, and protein tyrosine kinase-mediated pathways) inhibited nicotinic acid transport into both HepG2 cells and human primary hepatocytes. Maintenance of HepG2 cells (for 48 h) in growth medium oversupplemented with nicotinic acid (or nicotinamide) did not affect the subsequent transport of [³H]nicotinic acid into HepG2 cells. These results show, for the first time, the existence of a specific and regulated membrane carrier-mediated system for nicotinic acid transport in human liver cells.

niacin; transport mechanism

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