ABSTRACT
Hepatocellular carcinoma (HCC), one of the most deadly diseases all around the world. HBV infection is a causative factor of HCC and closely associated with HCC development. Ribonucleotide reductase (RR) is a key enzyme for cellular DNA synthesis and RR small subunit M2 (RRM2) is highly upregulated in HCC with poor survival rates. We have previously shown that HBV can activate the expression of RRM2 and the activity of RR enzyme for the viral DNA replication in host liver cells. Thus, RRM2 may be an important therapeutic target for HCC and HBV-related HCC. Pterostilbene, a natural plant component, potently inhibited in vitro RR enzyme activity with the IC50 of about 0.62 µM through interacting with RRM2 protein, which was much higher than current RRM2 inhibitory drugs. Pterostilbine inhibited cell proliferation with an MTT IC50 of about 20-40 µM in various HCC cell lines, causing DNA synthesis inhibition, cell cycle arrest at S phase, and accordingly apoptosis. On the other hand, the compound significantly inhibited HBV DNA replication in HBV genome integrated and newly transfected HCC cells, and the EC50 for inhibiting HBV replication was significantly lower than the IC50 for inhibiting HCC proliferation. Notably, pterostilbene possessed a similar inhibitory activity in sorafenib and lamivudine resistant HCC cells. Moreover, the inhibitory effects of pterostilbine against HCC proliferation and HBV replication were significantly reversed by addition of dNTP precursors, suggesting that RR was the intracellular target of the compound. Finally, pterostilbine effectively inhibited HCC xenograft growth with a relatively low toxicity in nude mouse experiments. This study demonstrates that pterostilbene is a novel potent RR inhibitor by targeting RRM2. It can simultaneously inhibit HCC proliferation and HBV replication with a potential new use for treatment of HCC and HBV-related HCC.
ABSTRACT
Ribonucleotide reductase (RR) is a unique enzyme for the reduction of NDPs to dNDPs, the building blocks for DNA synthesis and thus essential for cell proliferation. Pan-cancer profiling studies showed that RRM2, the small subunit M2 of RR, is abnormally overexpressed in multiple types of cancers; however, the underlying regulatory mechanisms in cancers are still unclear. In this study, through searching in cancer-omics databases and immunohistochemistry validation with clinical samples, we showed that the expression of MYBL2, a key oncogenic transcriptional factor, was significantly upregulated correlatively with RRM2 in colorectal cancer (CRC). Ectopic expression and knockdown experiments indicated that MYBL2 was essential for CRC cell proliferation, DNA synthesis, and cell cycle progression in an RRM2-dependent manner. Mechanistically, MYBL2 directly bound to the promoter of RRM2 gene and promoted its transcription during S-phase together with TAF15 and MuvB components. Notably, knockdown of MYBL2 sensitized CRC cells to treatment with MK-1775, a clinical trial drug for inhibition of WEE1, which is involved in a degradation pathway of RRM2. Finally, mouse xenograft experiments showed that the combined suppression of MYBL2 and WEE1 synergistically inhibited CRC growth with a low systemic toxicity in vivo. Therefore, we propose a new regulatory mechanism for RRM2 transcription for CRC proliferation, in which MYBL2 functions by constituting a dynamic S-phase transcription complex following the G1/early S-phase E2Fs complex. Doubly targeting the transcription and degradation machines of RRM2 could produce a synthetic inhibitory effect on RRM2 level with a novel potential for CRC treatment.
