Despite some expression of folate receptor alpha in human mesothelioma cells, internalization of methotrexate is predominantly carrier mediated

Our plain-English summary of Despite some expression of folate receptor alpha in human mesothelioma cells, internalization of methotrexate is predominantly carrier mediated, by N.Z. Khokhar, A.F. Lam, V.W. Rusch, et al (The Journal of Thoracic and Cardiovascular Surgery, 2002).

The study

• Authors: N.Z. Khokhar, A.F. Lam, V.W. Rusch, F.M. Sirotnak
• Journal: The Journal of Thoracic and Cardiovascular Surgery
• Published: 2002;123(5):862-8
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

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.

In plain English

This laboratory study at Memorial Sloan-Kettering Cancer Center in New York asked how the cancer drug methotrexate enters mesothelioma cells. It did not study asbestos exposure. The authors compared two possible routes: a cell-surface protein, folate receptor alpha, and a transporter, the reduced folate carrier. They measured uptake of radioactively labelled methotrexate in four human mesothelioma cell lines, using leukemia, ovarian cancer and prostate cancer cell lines as comparators. They also measured expression of both genes (their messenger RNA levels) in these cells and in mesothelioma tumors removed from patients at surgery. Adding 5 micromol/L folic acid, which blocks the receptor route, had only a minimal effect on uptake. A twentyfold higher concentration did reduce uptake, which the authors said was consistent with the carrier route. Only one mesothelioma cell line (MESO 10) had receptor gene expression equal to or higher than carrier gene expression. In the tumor samples, receptor gene expression was, with few exceptions, much lower than carrier gene expression. The authors concluded the results seemed to argue against a role for the receptor in methotrexate uptake in mesothelioma.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory study of cultured human tumor cell lines (drug uptake measurements and gene-expression analysis), plus gene-expression analysis of surgically removed mesothelioma tumor tissue
• Who was studied: Four human mesothelioma cell lines (VAMT-1, JMN, MESO 9, MESO 10). Comparator lines were CCRF-CEM (acute lymphoblastic leukemia) and SKOV-3 (ovarian cancer), and TSU-PR1 (prostate cancer) also appears in the uptake table. Mesothelioma tumor specimens came from consecutive patients undergoing surgery at Memorial Sloan-Kettering Cancer Center, New York. The number of patients and the dates are not stated.
• Exposure: Not an asbestos exposure study. Cells were exposed in the laboratory to radioactive (tritiated) methotrexate, with or without added folic acid, to test which uptake route operated.
• Main results:
  • In the mesothelioma cell lines, 5 micromol/L folic acid (which blocks the folate receptor route) had only a minimal effect on methotrexate uptake over time.
  • A much higher concentration (100, in the same units) of folic acid did inhibit uptake. The authors said this was consistent with a role for the reduced folate carrier.
  • Among the cell lines, only MESO 10 showed folate receptor gene expression equal to or higher than reduced folate carrier gene expression.
  • Methotrexate uptake broadly tracked carrier gene expression. It was lowest in MESO 9 and MESO 10 and highest in VAMT-1. In the table, VAMT-1 influx was 8.8 without folic acid and 8.4 with 5 micromol/L folic acid; MESO-9 influx was 2.4 and 2.2 (pmol/min per 10 million cells, average of 3 experiments).
  • In patients' mesothelioma tumor specimens, expression of both genes varied widely. With few exceptions, folate receptor gene expression was very substantially lower than carrier gene expression.
  • Gene expression did not differ appreciably between epithelioid and mixed (epithelioid-sarcomatoid) tumors. Only one pure sarcomatoid sample was included.
• The authors' conclusion: The authors concluded that, because carrier-mediated uptake is ongoing in mesothelioma cells, the results seem to argue against a role for the folate receptor in the uptake of methotrexate and other classic folate analogs in this tumor.
• Limitations the authors state:
  • The authors noted that measured gene expression may not reflect functional carrier levels, because differences in how the carrier's mRNA is translated could also matter.
  • The authors stated that prospective clinical-trial studies are needed to establish whether carrier gene expression is relevant to clinical response.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Neoplasm.

Related studies

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• Reduced folate carrier and folylpolyglutamate synthetase, but not thymidylate synthase predict survival in pemetrexed-treated patients suffering from malignant pleural mesothelioma — F. Mairinger, C. Vollbrecht, et al. (2013)
• Expression of the Wilms' tumor gene WT1 in human malignant mesothelioma cell lines and relationship to platelet-derived growth factor A and insulin-like growth factor 2 expression — A.W. Langerak, K.A. Williamson, et al. (1995)
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