Ribonucleotide reductase (RR) is a multisubunit enzyme responsible for the reduction of ribonucleotides to their corresponding deoxyribonucleotides, which are building blocks for DNA replication and repair. The key role of RR in DNA synthesis and cell growth control has made it an important target for anticancer therapy. Increased RR activity has been associated with malignant transformation and tumor cell growth. Efforts for new RR inhibitors have been made in basic and translational research. In recent years, several RR inhibitors, including Triapine, Gemcitabine, and GTI-2040, have entered clinical trial or application. Furthermore, the discovery of p53R2, a p53-inducible form of the small subunit of RR, raises the interest to develop subunit-specific RR inhibitors for cancer treatment. This review compiles recent studies on (1) the structure, function, and regulation of two forms of RR; (2) the role in tumorigenesis of RR and the effect of RR inhibition in cancer treatment; (3) the classification, mechanisms of action, antitumor activity, and clinical trial and application of new RR inhibitors that have been used in clinical cancer chemotherapy or are being evaluated in clinical trials; (4) novel approaches for future RR inhibitor discovery.

Liver metastases develop in more than half of patients with colorectal cancer (CRC) and are associated with a poor prognosis. The factors influencing liver metastasis of CRC are poorly characterized, but this information is urgently needed. We have now discovered that small extracellular vesicles (sEVs, exosomes) derived from CRC can be specifically targeted to liver tissue and induce liver macrophage polarization toward an interleukin-6 (IL-6)-secreting pro-inflammatory phenotype. More importantly, we found that microRNA-21-5p (miR-21) was highly enriched in CRC-derived sEVs and was essential for creating a liver pro-inflammatory phenotype and liver metastasis of CRC. Silencing either miR-21 in CRC-sEVs or Toll-like receptor 7 (TLR7) in macrophages, to which miR-21 binds, abolished CRC-sEVs' induction of pro-inflammatory macrophages. Furthermore, miR-21 expression in plasma-derived sEVs was positively correlated with liver metastasis in CRC patients. Collectively, our data demonstrate a pivotal role of CRC-sEVs in promoting liver metastasis by inducing an inflammatory pre-metastatic niche through the miR-21-TLR7-IL6 axis. Thus, sEVs-miR-21 represents a potential prognostic marker and therapeutic target for CRC patients with liver metastasis.

Ribonucleotide reductase plays a central role in cell proliferation by supplying deoxyribonucleotide precursors for DNA synthesis and repair. The holoenzyme is a protein tetramer that features two large (hRRM1) and two small (hRRM2 or p53R2) subunits. The small subunit contains a di-iron cluster/tyrosyl radical cofactor that is essential for enzyme activity. Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone, 3-AP) is a new, potent ribonucleotide reductase inhibitor currently in phase II clinical trials for cancer chemotherapy. Ferric chloride readily reacts with Triapine to form an Fe(III)-(3-AP) complex, which is reduced to Fe(II)-(3-AP) by DTT. Spin-trapping experiments with 5,5-dimethyl-1-pyrroline-N-oxide prove that Fe(II)-(3-AP) reduces O 2 to give oxygen reactive species (ROS). In vitro activity assays show that Fe(II)-(3-AP) is a much more potent inhibitor of hRRM2/hRRM1 and p53R2/hRRM1 than Triapine. Electron paramagnetic resonance measurements on frozen solutions of hRRM2 and p53R2 show that their tyrosyl radicals are completely quenched by incubation with Fe(II)-(3-AP). However, the enzyme activity is maintained in protein samples supplemented with catalase alone or in combination with superoxide dismutase. Furthermore, catalase alone or in combination with superoxide dismutase markedly decreases the antiproliferative effect of Triapine in cytotoxicity assays. These results indicate that Triapineinduced inhibition of ribonucleotide reductase is caused by ROS. We suggest that ROS may ultimately be responsible for the pharmacologic effects of Triapine in vivo. [Mol Cancer Ther 2006;5(3):586 -92]

SummaryThis study describes a novel mechanism of the inflammatory cytokine IL-6 induced Fra-1 upregulation through activating STAT3 by phosphorylation and acetylation, and demonstrates that this signaling pathway plays a critical role in promoting epithelial–mesenchymal transition and aggressiveness of colorectal cancer.