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

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 [(3)H]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 microM 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 [(3)H]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 Ca(2+)-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 [(3)H]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.

Comparative genomics and functional analysis of the NiaP family uncover nicotinate transporters from bacteria, plants, and mammals

“…While there is evidence for nicotinate uptake in bacteria (Neujahr and Varga 1966;McPheat and Wardlaw 1982;Rowe et al 1985), fungi (Llorente and Dujon 2000), plants (Zheng et al 2005), and mammals (Takanaga et al 1996;Nabokina et al 2005;Shimada et al 2006;Said et al 2007), only two nicotinate transporters have so far been characterized biochemically: TNA1 from yeast (Llorente and Dujon 2000;Ma et al 2009) and SMCT1 from mammals (Gopal et al 2005(Gopal et al , 2007. Two lines of evidence show that mammals have at least one other nicotinate transporter.…”

Multivalent niacin nanoconjugates for delivery to cytoplasmic lipid droplets

“…Similarly, we synthesized dendrimer containing only the polyethylene glycol arms (13), and a miktoarm carrier containing two PEG chains and a niacin molecule (15). Azide functionalized PEG-750 was first synthesized using a literature procedure [38], and it was then clicked with 1,3,5-triethynylbenzene core using conditions described earlier to give compound 13 (Scheme 4).…”

“…In the context of liver cells (hepatocytes), a designed DDS may provide specificity by carrying niacin directly to its target, DGAT2, localized on LDs. The mechanism of transport of niacin in hepatocytes was studied and shown to be regulated by specific membrane carriers systems [15]. Niacin, covalently attached to the nanocarrier by an ester bond, can only be released when the bond is cleaved off by cellular esterases.…”

A intrigante bioquímica da niacina - uma revisão crítica

“…As coenzimas NAD + e NADP + sofrem hidrólise enzimática por NAD glico-hidrolases intestinais, sendo a niacinamida e o ácido niacínico absorvidos posteriormente. 5,10,11 Também foi sugerido o envolvimento do sistema Ca 2+ /calmodulina no transporte de niacina em células hepáticas. 11 A niacinamida é a forma predominante no sangue, sendo oriunda da hidrólise das coenzimas no fígado e no intestino delgado.…”

“…5,10,11 Também foi sugerido o envolvimento do sistema Ca 2+ /calmodulina no transporte de niacina em células hepáticas. 11 A niacinamida é a forma predominante no sangue, sendo oriunda da hidrólise das coenzimas no fígado e no intestino delgado. Esses órgãos têm uma grande quantidade de NAD glicohidrolases.…”