Reduced Folate Carrier Gene Silencing in Multiple Antifolate-resistant Tumor Cell Lines Is Due to a Simultaneous Loss of Function of Multiple Transcription Factors but Not Promoter Methylation

Abstract

The human reduced folate carrier (hRFC) is the major uptake route for antifolates used in cancer chemotherapy. Here we explored the molecular basis for the decrease or loss of hRFC gene expression in seventeen tumor cell lines with resistance to multiple antifolates due to impaired antifolate transport. We studied the role of various cis-acting elements including CRE/AP-1-like element and GC-box in hRFC promoters A and B, respectively, as well as AP-2, Mzf-1 and E-box that are contained within or near four tandemly repeated sequences upstream of promoter A. Decreased or abolished binding either to [32P]GC-box, Mzf-1, AP-1, E-box, or CRE oligonucleotides was detected in ∼50-80% of antifolate-resistant cell lines. Strikingly, ∼80% of the cell lines displayed a simultaneously decreased binding to three or more of these hRFC promoter elements, whereas normal AP-2 binding was retained. The possible contribution of promoter methylation to hRFC gene silencing was also explored. None of the antifolate-resistant cell lines, except for MDA-MB-231 cells, showed hRFC promoter methylation; consistently, MDA-MB-231 was the only cell line that retained binding to all six cis-acting elements. Western blot analysis demonstrated decreased expression of transcriptional activators (pCREB-1, pATF-1, USF-1, c-Fos, c-Jun, Sp1, and Sp3) and/or increased expression of repressors (short Sp3 isoforms), whereas normal AP2α levels were retained. Transient expression of the relevant transcription factors restored, at least partially, both promoter binding and hRFC gene expression. This is the first report that transcriptional silencing of the hRFC gene in multiple tumor cell lines with resistance to various novel antifolates is a result of a simultaneous loss of function of multiple transcription factors but not promoter methylation.