Absorption of folate by Caco-2 cells is not affected by high glucose concentration

The aim of this work was to investigate the effect of high glucose exposure on the absorption of folate by Caco-2 cells. We verified that apical high glucose did not affect the apical uptake of [(3)H]folate. Both different concentrations of glucose (10-45 mM) and different exposure times (10 min-24 h) were tested. Furthermore, apical high glucose (30 mM) did not affect the intracellular steady-state levels of [(3)H]folate, and simultaneous apical and basolateral high glucose (30 mM) did not change the apical-to-basolateral apparent permeability (P(app)) to [(3)H]folate. Both the apical uptake and the steady-state intracellular levels of [(3)H]folate were strongly reduced by 5-methyltetrahydrofolate, methotrexate, SITS (4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid), DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) and indomethacin, but were not affected or only hardly affected by p-aminohippuric acid and fumitremorgin C. Moreover, DIDS and indomethacin significantly reduced (by 50-60%) the apical-to-basolateral P(app) to [(3)H]folate, but [(3)H]folate present in the cells at the end of the experiment was higher in the case of indomethacin. Fumitremorgin C had no effect. The effect of the drugs tested was not changed or only hardly changed by high glucose. In conclusion, absorption of [(3)H]folate is not modulated by either apical or basolateral high glucose exposure in Caco-2 cells. Moreover, our results suggest that the apical uptake of [(3)H]folate by Caco-2 cells involves the Reduced Folate Transporter (but not the Organic Anion Transporter), and that Multidrug Resistance Protein and/or Organic Anion Transporter (but not Breast Cancer Resistance Protein) may mediate apical efflux of [(3)H]folate.