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Vol. 55, Issue 4, 761-769, April 1999
Department of Oncology, Section of Biochemical Pharmacology,
University Hospital Vrije Universiteit, Amsterdam, The Netherlands
(G.J., I.K., P.N., G.J.P.);
Department of Biochemistry and Molecular
Biology, Medical University of South Carolina, Charleston, South
Carolina (M.A.B., D.G.P.); and
Department of Biology, The Technion,
Israel Institute of Technology, Haifa, Israel (H.B., Y.G.A.)
Chinese hamster ovary PyrR100 cells display more than
1000-fold resistance to pyrimethamine (Pyr), a lipophilic antifolate
inhibitor of dihydrofolate reductase. PyrR100 cells had
wild-type DHFR activity, lost folate exporter activity, and had a
4-fold increased activity of a low pH folic acid transporter. Here we
report on the marked alterations identified in PyrR100
cells compared with parental cells: 1) ~100-fold decreased folic acid
growth requirement; 2) a 25-fold higher glucose growth requirement in
Pyr-containing medium; 3) a 2.5- to 4.1-fold increase in
folylpolyglutamate synthetase activity; 4) a 3-fold increase in the
accumulation of [3H]folic acid and a 3-fold expansion of
the intracellular folate pools; 5) a 4-fold increase in the activity of
the lysosomal marker
-hexoseaminidase, suggesting an increased
lysosome number/PyrR100 cell; and 6) a small reduction in
the steady-state accumulation of [3H]Pyr and no evidence
of catabolism or modification of cellular [3H]Pyr.
Consequently, PyrR100 cells were markedly resistant to the
lipophilic antifolates trimetrexate (40-fold) and AG377 (30-fold) and
to the polyglutamatable antifolates 5,10-Dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF) (26-fold) and AG2034 (14-fold). Resistance to these drugs was reversed in
PyrR100 cells transferred into folate-depleted medium. In
conclusion, these multiple resistance factors collectively result in a
prominent increase in folate accumulation, an expansion of the
intracellular folylpolyglutamate pool, and abolishment of the cytotoxic
activity of polyglutamatable and lipophilic antifolates. The role of
increased lysosome number per cell in sequestration of hydrophobic weak base drugs such as Pyr is also discussed as a novel mechanism of drug resistance.
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