Novel Therapeutic Approaches to Regulate Human Dihydrofolate Reductase Activity and Expression

Emine      Ercikan Abali; Hilal      Celikkaya; Yi-Ching      Hsieh; Debabrata      Banerjee; Joseph      R. Bertino

Abstract

Dihydrofolate reductase (DHFR) enzyme catalyzes the reduction of dihydrofolate to tetrahydrofolate using NADPH as a cofactor to restore the reduced folate pools for reactions requiring one carbon transfer. Folates are required for de novo synthesis of purines and thymidylate, as well as glycine, methionine and serine. For this reason, DHFR has been an important target for chemotherapy for many diseases including cancer. Cell death ensues after the inhibition of DHFR due to the inhibition of nucleotide synthesis and DNA replication.

Methotrexate (MTX), an analog of dihydrofolate is a tight binding inhibitor of DHFR. Since its analog aminopterin’s success in the clinic in the treatment of acute lymphocytic leukemia almost 65 years ago, MTX has also been used to treat non-Hodgkin’s lymphoma, osteosarcoma, choriocarcinoma, head and neck, and breast cancer. However, the development of side effects and both intrinsic and acquired drug resistance to MTX and other antifolates are the main clinical limitations. Myelosuppression and stomatitis are due to cell death of not only cancer cells but also rapidly dividing normal cells such as bone marrow cells and epithelial cells of the gut, respectively. Due to these limitations, a detailed understanding of DHFR at every level has been undertaken such as structure-functional analysis, mechanisms of action, transcriptional and translation regulation of DHFR to develop more effective antifolates. In this paper, we review novel therapeutic approaches to regulate DHFR activity and its expression to overcome resistance or toxicity.

Keywords: Dihydrofolate reductase, thymidylate synthase, mTOR, E2F, NADPH, methotrexate, antifolates, folate, rapamycin, NADPH binding site, translation, transcription, degradation, regulation, expression, activity, synergy, stability, combination studies