Ck. Kumar et al., MECHANISM AND REGULATION OF RIBOFLAVIN UPTAKE BY HUMAN RENAL PROXIMALTUBULE EPITHELIAL-CELL LINE HK-2, American journal of physiology. Renal, fluid and electrolyte physiology, 43(1), 1998, pp. 104-110
Riboflavin (RF), a water-soluble vitamin, is essential for normal cell
ular functions, growth, and development. Normal RF body homeostasis de
pends on intestinal absorption and recovery of the filtered vitamin in
renal tubules. The mechanism and cellular regulation of the RF renal
reabsorption process, especially in the human situation, are poorly un
derstood. The aim of this study was therefore to address these issues,
using a recently established human normal renal epithelial cell line,
HK-2, as a model. Uptake of RF by HK-2 cells was found to be 1) linea
r with time for 5 min of incubation and occurring with minimal metabol
ic alterations, 2) temperature dependent, 3) Na+ independent, 4) satur
able as a function of concentration [apparent Michaelis constant (K-m)
of 0.67 +/- 0.21 mu M and maximal velocity (V-max) of 10.05 +/- 0.87
pmol.mg protein(-1).3 min(-1)], 5) inhibited by structural analogs and
anion transport inhibitors, and Bi energy dependent. Protein kinase C
-, protein kinase A-, and protein tyrosine kinase-mediated pathways we
re found to have no role in regulating RF uptake. On the other hand, a
Ca2+/calmodulin-mediated pathway appeared to play a role in the regul
ation of RF uptake by HK-2 cells via an effect on the V-max, as well a
s on the apparent K-m of the RF uptake process. The uptake process of
RF was also found to be adaptively regulated by the level of the subst
rate in the growth medium, with the effect being mediated through chan
ges in the apparent K-m and the V-max of the uptake process. These res
ults demonstrate that RF uptake by the human-derived renal epithelial
cell line HK-2 is via a carrier-mediated system that is temperature an
d energy dependent and appears to be under the regulation of a Ca2+/ca
lmodulin-mediated pathways and substrate level in the growth medium.