| Cancer is a common disease that hazards human health. Since ribonucleotide reductase (RR) is the only enzyme responsible for the reduction of ribonucleotides to their corresponding deoxyribonucleotides (dNDP), which are precursors for DNA repair and replication, RR has become one of the most important targets for cancer therapy. Clarifying RR function and its regulation mechanism will take an important impact on research fields of carcinogenesis, cancer prevention and cancer therapy.With development of molecular biology, more and more RR regulation pathways have been revealed. However, most of studies focus on elucidating regulatory rules of one subunit of RR. Now we know, the functional RR holoenzyme (form: a2p2) consists of two large subunits (hRRMl) and two small subunits (hRRM2 and p53R2). No RR activity will be shown with any subunit alone. Furthermore, overexpression of human ribonucleotide reductase subunit M2 (HRRM2) has been shown to well related to Gemcitabine resistance, but the regulatory mechanisms of functional RR holoenzyme and Gemcitabine resistance are still not clear.Lots of studies indicate that RR subunits are separated with each other at quiescent cells. There is no RR function until large and small subunits specifically bind together. Genotoxins activate RR, which would involve in DNA repair and maintain genomic fidelity. On the otherhand, in response to genotoxins, eukaryotes activate pathways of cell cycle checkpoint to prevent genomic instability. So far, it is known that p53 is one of the most important checkpoint proteins, and p21 is down stream target of p53. Following DNA damage, quite a number of inhibitions of cell growth are induced by p53-dependent p21 activation. Both p21 and p27 interact with, and inhibit, cyclin-cdk complexes, resulting in retarding S-phase entry from G1 for repairing damaged DNA or initiating apoptosis. One mechanism identified for inhibition of DNA repair or replication by p21 is depletion of dNTP pools. Recently, one research group found that p53R2 is one of the most important participators to carry out p53's repair function in the damage response. Our pilot study showed that RR subunits p53R2 and hRRM2 bind with p53 protein. These results indicate p53 check-point pathway correlates tightly with RR function.Like other proteins, one of the important ways to regulate RR function is to alter RR subunits' localizations. In the field of RR research, there are two opposing opinions about where deoxyribonucleotide synthesis. On the 20th century eighty years, Reddy and Pardee et al. brought forward "replitase" model, suggesting deoxyribonucleotides are synthesized by ribonucleotide reductase in nucleus. However, subsequent immunocytochemical studies of both hRRMl and hRRM2 showing localization in the cytoplasm of mammalian cells led other groups to argue against the "replitase" model. Additional evidence has recently shown that the hRRM2 homologue, p53R2, was found in nucleus after DNA damage, but that p53R2' s binding partner hRRMl remained in the cytoplasm of resting cells. These findings indicated that the RR subunits might traffic between cytoplasm and nucleus in response to DNA damage.Based on above studies, we hypothesized that checkpoint proteins such as p53, p21 and p27 might post-transcriptionally regulate RR through inducing RR subunits binding and separating with each other or altering their cellular localization.Moreover, it is well known that cooperation between gene transcription, protein translation and modification after translation is required for one protein to display normal function. RR is subject to not only post-transcriptional regulation, but also transcriptional regulation. As a classic small subunit of RR, hRRM2 has been well studied and some transcriptional mechanisms have been recognized. However, the mechanism of hRRM2 transcription related to Gemcitabineresistance is still not clear. Gemcitabine is a nucleoside dCTP analogue. The hRRM2 expression level has been reported to be much higher in selected Gemcitabin...
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