DNA repair initiation induces expression of ribonucleotide reductase in human chronic lymphocytic leukemia cells.

Mammalian ribonucleotide reductase (RR) is a heterodimer enzyme responsible for maintaining levels of deoxynucleotides needed for DNA replication. The M2 subunit of RR is known to increase in tandem with progression of cells into S phase, whereas the M1 subunit is expressed at steady-state. Since the expression level of the M2 subunit increases because the cells need deoxyribonucleoside triphosphates (dNTPs) for replication, it is logical to hypothesize that the same increase will be seen during DNA repair. To test this, we used chronic lymphocytic leukemia (CLL) cells, which are replicationally quiescent and have low endogenous levels of RR and dNTPs. Cyclophosphamide was selected as the DNA damaging agent because of its clinical use in the treatment of CLL. DNA repair, measured by [(3)H]thymidine incorporation after 4 h treatment with 4-hydroperoxycyclophosphamide, increased in a dose-dependent manner at 3, 10 and 50 μM. The induction of DNA repair concomitantly increased the mRNA and protein levels of M2 subunit (median 1.6-fold; range 0.9-5.3). Maximum induction occurred at 10 μM after 4 h and correlated with [(3)H]thymidine incorporation (p = 0.02). In contrast, no change was observed in mRNA or protein levels of M1 subunit. We conclude that RR is regulated not only during replication but also during DNA repair, and in both cases M2 subunit expression is increased.