Riboflavin uptake by the human-derived liver cells Hep G2: Mechanism and regulation

The water-soluble vitamin riboflavin (RF) plays a critical role in many metabolic reactions, and thus, is essential for normal cellular functions and growth. The liver plays a central role in normal RF metabolism and is the site of maximal utilization of the vitamin. The mechanism of liver uptake of RF has been studied in animals, but no information is available describing the mechanism of the vitamin uptake in the human situation and its cellular regulation. In this study, we used the human-derived liver cells Hep G2 as an in vitro model system to address these issues. Uptake of RF by Hep G2 cells was found to be temperature and energy-dependent but Na⁺-independent in nature. Uptake seemed to involve a carrier-mediated process as indicated by the saturation as a function of substrate concentration (apparent K(m) 0.41 ± 0.08 μM), and by the ability of the structural analogs lumiflavin and lumichrome to inhibit the uptake process [inhibition constant (K(i)) of 1.84 and 6.32 μM, respectively]. RF uptake was energy dependent, and was inhibited by the -SH group blocker p-chloromercuriphenylsulfonate (pCMPS) (K(i) of 0.10 mM). Specific modulators of intracellular protein kinase A (PKA)-, protein kinase C (PKC)-, and protein tyrosine kinase (PTK)-mediated pathways did not affect RF uptake by Hep G2 cells. On the other hand, specific inhibitors of Ca²⁺/calmodulin-mediated pathway significantly inhibited the uptake process; this effect seemed to be mediated through a decrease in the V(max) of the substrate uptake process. Maintaining Hep G2 cells in a RF-deficient growth medium was associated with a significant up- regulation in the substrate uptake; this effect was specific for RF and was mediated mainly by means of an increase in the V(max) of the uptake process. These results describe, for the first time, the mechanism and cellular regulation of RF uptake by a human-derived liver cellular preparation, and shows the involvement of a carrier-mediated system in the uptake process. Furthermore, the uptake process seems to be regulated by an intracellular Ca²⁺/calmodulin-mediated pathway and by extracellular substrate levels.