Growth-inhibitory effects of 5,10-dideazatetrahydrofolic acid on variant murine L1210 and human CCRF-CEM leukemia cells with different membrane-transport characteristics for (anti)folate compounds

Abstract

5,10-Dideazatetrahydrofolic acid (DDATHF) is a potent inhibitor of glycinamide ribonucleotide transformylase, one of the folate-dependent key enzymes in de novo purine biosynthesis. The present report demonstrates that multiple membrane-transport routes may be involved in the cellular uptake of DDATHF. These routes include the classic reduced folate carrier and a membrane-associated folate-binding protein (mFBP). The role of an mFBP in the uptake of DDATHF was suggested from observations that (a) the mFBP showed a very high binding affinity for DDATHF, (b) murine and human leukemia cells expressing an mFBP were highly sensitive to growth inhibition by DDATHF, and (c) protection against this growth inhibition could be achieved using folic acid rather than reduced folate compounds.

A Structurally Altered Human Reduced Folate Carrier with Increased Folic Acid Transport Mediates a Novel Mechanism of Antifolate Resistance

“…We noted that folic acid rather than purines/pyrimidines in undialyzed fetal calf serum was essential for cell growth of CEM/MTX cells (58). This was illustrated by the fact that growth of CEM/MTX cells was unaffected in medium containing 2.3 M folic acid, 10% dialyzed FCS, whereas cessation of cell growth was observed when CEM/MTX cells were transferred to medium containing 2 nM folic acid and 10% undialyzed FCS (data not shown).…”

Resistance to antifolates

“…On the other hand, CCRF-CEM human leukemia cells that lack RFC but have increased FR-a expression have markedly diminished transport of MTX relative to wild-type cells and are highly resistant to the drug ( Van der Veer et al, 1989). In the case of folic acid with high affinity for FRs but low affinity for RFC, the level of FR expression is a major determinant of the rate of transport and the receptor is upregulated by growth in media with low folate levels (Henderson et al, 1988;Jansen et al, 1989). Likewise, growth inhibition by antifolates with high affinity for FRs and low affinity for RFC is markedly increased by overexpression of receptor even when RFC function is absent (Henderson and Strauss, 1990a;Jansen et al, 1990).…”

“…In the case of folic acid with high affinity for FRs but low affinity for RFC, the level of FR expression is a major determinant of the rate of transport and the receptor is upregulated by growth in media with low folate levels (Henderson et al, 1988;Jansen et al, 1989). Likewise, growth inhibition by antifolates with high affinity for FRs and low affinity for RFC is markedly increased by overexpression of receptor even when RFC function is absent (Henderson and Strauss, 1990a;Jansen et al, 1990). The impact of FR expression on the activity of antifolates is influenced by the folate present and its concentration in vitro or in vivo and the relative affinities of the folate and antifolate for this transporter.…”

Complete Mapping of Divergent Amino Acids Responsible for Differential Ligand Binding of Folate Receptors α and β

“…has the ability to mediate physiologic folate uptake and to transport novel antifolate drugs and folate conjugates (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). The receptor is a current major focus as a tumor target for multiple experimental approaches in cancer therapy.…”

“…The receptor is a current major focus as a tumor target for multiple experimental approaches in cancer therapy. Several studies have shown that FR, when expressed at high levels, could offer the preferred uptake route of novel classes of antifolate drugs that target glycineamide ribonucleotide formyltransferase and thymidylate synthase (3)(4)(5)(6). Taking advantage of the non-destructive nature of FR-mediated internalization of folate-coupled macromolecules (7,8), cytotoxins such as momordin, pseudomonas exotoxin, and maytansinoids were shown to produce selective killing in FR-rich cells (9 -12).…”