Effects of expression of the 10-formyltetrahydrofolate metabolizing enzyme ALDH1L1 on pyrimidine nucleotide synthesis and the salvage pathway

Proliferating cells, such as tumor cells, require nucleotides for DNA replication. Mammalian cells are equipped with de novo purine and pyrimidine nucleotide biosynthesis pathways, and a salvage pathway that recycles purine bases, to supply nucleotides for the same. To avoid imbalance in intracellular nucleotide levels, de novo nucleotide biosynthesis pathway is regulated by feedback mechanisms, such as synthase inhibition by nucleotide products. Recently, we reported that the aldehyde dehydrogenase 1 family member L1 (ALDH1L1) consumes 10-formyltetrahydrofolate (10-fTHF), which is utilized by the de novo purine nucleotide synthesis, and results in the accumulation of 5-aminoimidazole-4-carboxamide ribonucleotide (ZMP) during purine biosynthesis. Given that ZMP inhibits pyrimidine nucleotide synthesis, in the present study, we examined the effects of ZMP using brequinar, a dihydroorotate dehydrogenase inhibitor. ALDH1L1-mediated ZMP accumulation was unaffected by brequinar, and no effect was observed when brequinar was combined with 5-aminoimidazole-4-carboxamide riboside (AICAr), a nucleoside of ZMP. Furthermore, we examined involvement in the salvage pathway, because attenuation of de novo purine nucleotide synthesis may require a supply of purine nucleotides from the salvage pathway. The guanine analog, 6-thioguanine, and the 2'-deoxycytidine analog, cytarabine, were used in the assessment of dependency on the salvage pathway. We found that neither ALDH1L1-mediated ZMP accumulation nor the presence of AICAr affected the salvage pathway. Collectively, these results suggest that these purine and pyrimidine analogs can be useful for the treatment of tumor cells, regardless of ALDH1L1 expression.