Objective: As a response to a published report documenting some expression of folate receptor alpha in human mesothelioma, studies were carried out examining the role of this receptor versus that of the reduced folate carrier in the internalization of folate analogs in this neoplasm.
Methods: Influx measurements of tritiated methotrexate were carried out in 4 mesothelioma cell lines, and 2 additional cell lines were used as comparators. Relative gene-expression analysis for the carrier and receptor gene was done by using real-time reverse transcriptase-polymerase chain reaction in the above-mentioned cell lines and mesothelioma tumor tissues obtained from patients.
Results: Internalization of tritiated methotrexate in mesothelioma cell lines grown at physiologic folate levels was carrier mediated rather than receptor mediated. Influx of this model permeant by these cells exhibited characteristics of carrier-mediated membrane transport and was only minimally reduced by the addition of 5 micromol/L folic acid, a concentration of this natural folate that would have completely inhibited influx by the folate receptor. Gene-expression analysis revealed prominent expression of the folate receptor in some but not all mesothelioma cell lines, and in only one case was expression of this receptor gene greater than expression of the reduced folate carrier gene. Similar analysis of human mesothelioma tumor tissue showed that, with few exceptions, receptor gene expression was substantially less than that for the carrier gene.
Conclusion: In view of the ongoing reduced folate carrier-mediated internalization that occurs in mesothelioma cells, these results would seem to argue against a role for the folate receptor in the internalization and cellular pharmacokinetics of methotrexate and other classic folate analogs in this neoplasm. Identifying the mediated route for internalization of these agents in tumor cells is a prerequisite for improving their structural design.