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Dihydrofolate reductase (DHFR) regenerates tetrahydrofolate from dihydrofolate in the presence of NADPH. Tetrahydrofolate is the coenzyme for thymidilate synthetase in the biosynthesis of thymidine and is also critical for the synthesis of amino acids and purines. DHFR is a protein of 186 amino acids that is highly conserved among different organisms. DHFR levels change during the cell cycle, with the highest content during the G1/S transition. Expression of DHFR tightly correlates with the turnover rate of its mRNA. Because dividing cells require a continuous supply of thymidine, DHFR has been a target for anticancer drugs. The folic acid antagonists aminopterin and amethopterin (methotrexate), widely used in cancer treatments, inhibit DHFR. However, over long treatments, treated cells often amplify the DHFR gene, producing drug-resistant cells.
