A single point mutation in Drosophila dihydrofolate reductase confers methotrexate resistance to a transgenic CHO cell line

Selection for methotrexate resistance in mammalian cells bearing a Drosophila dihydrofolate reductase transgene: Methotrexate resistance in transgenic mammalian cells

Antifolates, such as methotrexate (MTX), are the treatment of choice for numerous cancers. MTX inhibits dihydrofolate reductase (DHFR), which is essential for cell growth and proliferation. Mammalian cells can acquire resistance to antifolate treatment through a variety of mechanisms but decreased antifolate titers due to changes in drug efflux or influx, or alternatively, the amplification of the DHFR gene are the most commonly acquired resistance mechanisms. In Drosophila, however, a resistant phenotype has only been observed to occur by mutation resulting in a MTX-resistant DHFR. It is unclear if differences in gene structure and/or genome organization between Drosophila and mammals contribute to the observed differences in acquired drug resistance. To investigate if gene structure is involved, Drosophila Dhfr cDNA was transfected into a line of CHO cells that do not express endogenous DHFR. These transgenic cells, together with wild-type CHO cells, were selected for 19 months for resistance to increasing concentrations of MTX, from 50- to 200-fold over the initial concentration. Since Drosophila Dhfr appears to have been amplified several fold in the selected transgenic mammalian cells, a difference in genome organization may contribute to the mechanism of MTX resistance.

Transcriptional regulation of aldo-keto reductase 1C1 in HT29 human colon cancer cells resistant to methotrexate: role in the cell cycle and apoptosis

While studying differentially expressed genes between sensitive and 10(-5)M Methotrexate (MTX) resistant HT29 human colon cancer cells, we identified some members of the aldo-keto reductase (AKR) superfamily. The study was followed with the member AKR1C1 (EC 1.1.1.213), validating its increase in mRNA and protein levels in MTX resistant cells. The genomic content for AKR1C1 remained unchanged between sensitive and resistant cells, thereby excluding a mechanism of AKR1C1 gene amplification. Thus, we cloned the AKR1C1 human promoter and performed luciferase experiments that revealed a transcriptional regulation of the gene in the resistant cells. Computational studies showed a putative binding site for the transcription factor Sp1. The co-transfection of Sp1 or Sp3 with different constructs of AKR1C1 promoter deletions, including and excluding the proximal GC-box, demonstrated a key role for these factors in regulating AKR1C1 transcriptional activity. Gel-shift assays revealed an increase in Sp1 and Sp3 binding in resistant compared to sensitive cells, without differences in Sp1 protein levels. Dephosphorylation of the extracts coincided with a decrease in Sp1 binding, which is consistent with a process of regulation of Sp1 by phosphorylation. We also investigated the possible relationship between AKR1C1 expression and MTX action. Overexpression of AKR1C1 counteracted the S-phase accumulation of cells and apoptosis caused by MTX treatment. This suggests a role of AKR1C1 in cell proliferation. Finally, overexpression of AKR1C1 in MTX sensitive HT29 cells conferred resistance to the chemotherapeutic agent and silencing of AKR1C1 by means of iRNA technology sensitized the cells to MTX.

Transgenic rescue of methotrexate-induced teratogenicity in Drosophila melanogaster

The folic acid analog methotrexate (MTX), a competitive inhibitor of dihydrofolate reductase (DHFR), is used to treat a variety of cancers and autoimmune disorders. However, MTX also causes a wide range of toxic effects in healthy cells and is an established teratogen. Efforts to "rescue" the defects caused by MTX by administering a folate analog or by transgenic expression of a DHFR with an altered affinity for MTX have been attempted in a variety of mammals but limited protection was conferred. As a result, our understanding of the effect of MTX at the molecular genetic level remains incomplete and, in addition, continued mammalian sacrifice is not ideal. Due to the similarity of teratogenic effects produced by MTX in Drosophila melanogaster these insects were transformed with DHFR alleles to determine if rescue could be achieved. The resulting "MTX-resistant" flies were subsequently used to investigate changes in gene expression in response to MTX using semiquantitative reverse transcription PCR. The majority (12/14) of key transcripts that were affected in MTX-exposed females including transcripts involved in cell cycle, defense response, and transport were "rescued" in the "MTX-resistant" transgenic flies. These studies illustrate the utility of this invertebrate model for the investigation of molecular effects of MTX-induced teratogenicity, MTX-resistant DHFRs for gene therapy techniques, and teratogenic protection.

Drosophila dihydrofolate reductase mutations confer antifolate resistance to mammalian cells

Antifolates, such as methotrexate, are used to inhibit dihydrofolate reductase (DHFR), an enzyme essential for the biosynthesis of thymidylate, purines, and several amino acids. DHFR sequences corresponding to mutations found in a methotrexate resistant Drosophila S3 cell line (L30Q), a methotrexate resistant fly population (K31P, Q134K), as well as predicted in silico (L22R) were expressed in Chinese Hamster Ovary (CHO) cells. The L30Q and L22R DHFRs both conferred resistance to methotrexate. L22R DHFR provided approximately 200-fold resistance to methotrexate when compared to wild-type Drosophila DHFR allowing CHO(L22R) cells to divide in 10 microM methotrexate, a level of resistance not previously observed in any mammalian system. Constructs using this substitution in combination with other Drosophila DHFR specific residues would make excellent candidates for gene therapy and genetic markers in the treatment of certain human disorders.