Roles of RRM2 and RRM2B in pyrimidine stress responses and differentiation of acute myeloid leukemia cells.

Differentiation therapy offers a promising approach in acute myeloid leukemia (AML) by overcoming the developmental block that maintains leukemic blasts. Increasing evidence indicates that DNA replication stress can promote differentiation rather than cytotoxicity; however, the metabolic mechanisms linking replication stress to differentiation remain poorly defined. Here, we investigated how perturbations in nucleotide metabolism regulate replication stress-driven differentiation. Using metabolomic and functional analyses in AML cell lines, we show that agents inducing differentiation through replication stress, including 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr), dihydroorotate dehydrogenase (DHODH) inhibition, and low-dose cytarabine, converge on disruption of nucleotide pool balance. Low-dose AICAr induced a pyrimidine-purine imbalance, S phase arrest, and enhanced differentiation, whereas high-dose reduced these effects. Although brequinar and cytarabine altered nucleotide metabolism through distinct mechanisms, differentiation induced by all agents was abolished by supplementation with high levels of ribo- and deoxyribonucleosides, confirming that nucleotide imbalance is a central driver. We further identify ribonucleotide reductase (RNR) as a critical modulator of this process. Replication stress induced context-dependent regulation of RNR subunits, with RRM2 upregulated in p53-mutant U937 cells and the p53-responsive RRM2B isoform predominating in p53-wild-type MOLM-13 cells. Consistent with these differences, RRM2 depletion enhanced differentiation in U937 cells without affecting viability but impaired differentiation and survival in MOLM-13 cells. These findings position nucleotide metabolism as a key regulator of AML differentiation and suggest that combining RNR-targeted and checkpoint-modulating strategies could optimize therapeutic responses.