Functional characterization of folate transport proteins in Staten's Seruminstitut rabbit corneal epithelial cell line.

Purpose: The overall objective of this study was to investigate and characterize the expression of folate transport proteins in Staten's Seruminstitut rabbit corneal (SIRC) epithelial cell line. Methods: [(3)H] Folic acid uptake was studied with respect to time, pH, temperature, sodium, and chloride ion dependency. Inhibition studies were conducted with structural analogs, vitamins, and metabolic inhibitors. [(3)H] Folic acid uptake was also determined with varying concentrations of cold folic acid. Uptake kinetics was studied in the presence of various modulators of intracellular regulatory pathways, protein kinases A and C (PKA and PKC), protein tyrosine kinase (PTK), and calcium-calmodulin modulators. Ex vivo corneal permeability studies were carried out with [(3)H] folic acid in the presence and absence of 1 mM cold folic acid. Results: Linear increase in [(3)H] folic acid uptake was observed over 30 min. The process followed saturation kinetics with apparent K(m) of 14.2 +/- 0.2 nM, V(max) of (1.5 +/- 0.1)*10(-5) micro. moles/min/mg protein, and K(d) of (2.1 +/- 0.2)*10(-6) min(-1). The uptake process was found to be dependent on pH, sodium ions, chloride ions, temperature, and energy. Uptake was inhibited in the presence of structural analogs (cold folic acid, methyltetrahydro folate, and methotrexate), but structurally unrelated vitamins did not show any effect. Membrane transport inhibitors SITS, DIDS, probenecid and endocytic inhibitor, colchicine significantly inhibited the [(3)H] folic acid uptake indicating the involvement of receptor/transporter mediated process. PKA, PTK, and Ca(2+)/calmodulin pathways significantly regulate the process. RT-PCR and Western blot analysis confirmed the presence of folate receptor-alpha (FR-alpha) and proton-coupled folate transporter (PCFT). Permeability of [(3)H] folic acid across the rabbit cornea was (1.48 +/- 0.13)*10(-05) cm/sec, and in the presence of cold folic acid it was (1.08 +/- 0.10)*10(-05) cm/sec. Conclusions: This work demonstrated the functional and molecular presence of FR-alpha and PCFT in SIRC epithelial cell line.