Functional characterization and expression of folate receptor-α in T47D human breast cancer cells

Abstract

Purpose: The objective of this study was to investigate the functional and molecular expression of a carrier mediated system responsible for folate uptake in breast cancer (BC) (T47D) cells and to delineate the mechanism of intracellular regulation of this transport system. Materials and Methods: [ 3 H]-folic acid uptake was studied in T47D cells with respect to time, pH, temperature, sodium and chloride ion dependency. Inhibition studies were conducted in the presence structural analogs, vitamins, metabolic and membrane transport 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; calcium-calmodulin, protein kinases A and C (PKA and PKC) and protein tyrosine kinase (PTK). Molecular evidence was studied by qualitative and quantitative polymerase chain reaction (PCR) and Western blot analysis. Results: Linear increase in [ 3 H]-folic acid uptake was observed over 30 min. The process followed saturation kinetics with an apparent K m of 11.05 nM, V max of 1.54 * 10−8 μmoles/min/mg proteins and K d of 9.71 * 10−6 /min for folic acid. 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 such as SITC, DIDS, probenecid and endocytic inhibitor colchicine significantly inhibited the [ 3 H]-folic acid uptake process. PKA, PTK and Ca 2+ /calmodulin pathways positively regulate the uptake process. Reverse transcriptase polymerase chain reaction (RT PCR) analysis had shown mRNA expression of folate receptor (FR)-α at 407 bp. Quantitative polymerase chain reaction analysis showed significantly higher FR-α mRNA levels in T47D cells compared to MCF-7 cells and Western blot analysis confirmed the FR-α protein expression at 37 kDa. Conclusions: This work demonstrated the functional characterization and molecular presence of FR-α in the T47D cell line. The high expression of FRs in T47D human breast carcinoma cells supports their validity as molecular therapeutic targets in BC.