Expression of the reduced folate carrier SLC19A1 in IEC-6 cells results in two distinct transport activities

Intestinal absorption of folates has been characterized as a facilitative process with a low pH optimum. Studies with intestinal epithelial cells have suggested that this activity is mediated by the reduced folate carrier (RFC1). In this paper, we report on folate transport characteristics in an immortalized rat IEC-6 cell line that was found to exhibit the predominant influx activity for methotrexate (MTX) at pH 5.5 with a low level of activity at pH 7.4. Transfection of this cell line with an RFC1 construct resulted in clones exhibiting increased MTX uptake at both the pHs and high folic acid uptake only at the low pH. For the two clones with the highest level of transport activity, relative MTX influx at the two pHs was reversed. Moreover, the low pH MTX influx activity ([MTX]_e = 0.5 μM) was markedly inhibited by 20 μM folic acid while influx at neutral pH was not. Furthermore, in the presence and absence of glucose at low pH, MTX and folic acid influx activity was inhibited by azide, while MTX influx at pH 7.4 was stimulated by azide in the absence of glucose but was unchanged in the presence of glucose and azide. This was contrasted with the results of transfection of the same RFC1 construct into an L1210 murine leukemia cell line bearing a nonfunctional endogenous carrier. In this case, the activity expressed was only at pH 7.4. These data indicate that RFC1 can exhibit two distinct types of folate transport activities in intestinal cells that must depend on tissue-specific modulators.