S3) confirms these phenotypes. not CycG1, was abnormal in various cancer cell lines. KaplanMeier curves based on TCGA data disclosed that head and neck cancer patients with reduced CycG2 expression possess poorer clinical prognoses. Taken together, our data suggest that reduced CycG2 expression could be useful as a novel prognostic marker of cancer. Cyclin G1 (CycG1), a member from the cyclin family1, recruits protein phosphatase 2A (PP2A) to its focuses on by interacting with the PP2A_B subunit and regulating PP2A function2. Expression of CycG1, which is a transcriptional target from the tumor suppressor p53, is induced during the DNA damage response (DDR)3. In turn, CycG1 regulates the stability of p53 protein via dephosphorylation of MDM2, the ubiquitin ligase for p53, at T216 (MDM2-pT216), thus constituting a negative feedback system that attenuates p53 activity4, 5. At the initial stage of DNA damage, CycG1 interacts directly with MDM2 and encourages formation from the ARF/MDM2 complex, but the CycG1MDM2 complex dissociates from ARF and consequently captures B6. Thus, the main role of CycG1 is to mediate or regulate the function of p53 in the nucleolus6, 7. Although CycG1-deficient (Ccng1/) mice are given birth to and fully developed normally, they are more CDKN2A sensitive to -radiation than wild-type (WT) mice3. In some cells, CycG1 exerts positive effects on cell growth: exogenously overexpressed CycG1 encourages cancer cell proliferation3, 5, 8, whereas antisense-mediated down-regulation of CycG1 inhibits cell growth9, 10, 11. In other cells, however , CycG1 negatively influences proliferation: in various systems, overexpression of CycG1 can induce G1-phase arrest7, 12, G2/M arrest13, or apoptotic cell death14, 15. In cancer patients, overexpression from the CycG1 gene (CCNG1) is frequently observed in osteosarcoma16, breast and prostate cancer17, colorectal cancer18, and hepatocellular carcinoma19. CycG1 is the target of the liver-specific micro-RNA miR-12220, 21; in human hepatoma cells, alcohol facilitates hepatitis C computer virus RNA replication by upregulating miR-122 expression and thereby inhibiting production of cyclin G122. Notably, overexpression of CycG1 encourages the epithelialmesenchymal transition (EMT) and metastasis by enhancing AKT activation, leading to stabilization of SNAIL, a critical mediator of the EMT23. Cyclin G2 (CycG2), a CycG1 homolog24, also recruits the PP2A holoenzyme via association with B25. CycG2 targets Chk2-pT68 for dephosphorylation26by recruiting the PP2A complex to DNA repair foci, and CycG2-deficient cells exhibit delayed dephosphorylation of Chk2-pT6827. CycG2 also cooperates with B to dephosphorylate H2AX, another DNA repair element, as exhibited by the observation that dephosphorylation of H2AX-pS139 is delayed in CycG2-deficient or siRNA-mediated B knockdown cells27. Ectopic expression of CycG2 inhibits cell growth by inducing cell-cycle arrest25, 28, 29, 30, 31. CycG2 expression is regulated by p63, a homolog of p53, which contributes to G2/M arrest during the DDR32. CycG2 expression correlates with cell-cycle inhibition and is upregulated in response to diverse growth inhibitory stimuli including hypoxia and endoplasmic reticulum stress; however , it is repressed by mitogenic signals33, 34, 35. Nodal signaling promotes transcription of the human being CycG2 gene (CCNG2) by upregulating the forkhead transcription factor FoxO3a31, 36. Moreover, CCNG2is a primary target of estrogen receptor in MCF-7 cells37. The CycG2 mRNA level is high at G0 phase, declines because cells enter the AS-605240 cell cycle, and raises again from mid-S to early G2 phase38. Notably, CycG2 expression is down-regulated in several cancers, including thyroid and oral cancers29, 39. By contrast, upon AS-605240 antibody-mediated inhibition of HER2 signaling, CycG2 is upregulated and translocates to the nucleus30. Based on these observations, CycG1 and CycG2 may serve as prognostic biomarkers and therapeutic targets. However , the precise functions of CycG1 and CycG2 in cancer cells are not fully comprehended. In this study, we sought to investigate the distinct roles of CycG1 and CycG2 by generating mice deficient in CycG1 (G1KO) or CycG2 (G2KO), as well as double knockout (DKO) mice deficient in both CycG1 and CycG2. We found only one abnormal phenotype in the knockout mice, i. e., reduced incidence of diethylnitrosamine (DEN)-induced hepatocarcinogenesis. G2KO-derived mouse embryonic fibroblasts (MEFs), but not G1KO or DKO MEFs, were resistant to DNA damage caused by camptothecin and ionizing radiation (IR), and underwent G2/M arrest. We discuss the origins of those phenotypes, and propose that reduced CycG2 expression may serve as a book prognostic marker of cancer. == Results == == Generation ofCcng1/Ccng2/DKO mice == AS-605240 To understand the physiological role of CycG1 and CycG2 in more fine detail, we generatedCcng1/Ccng2/double knockout (DKO) mice. Previously, we generatedCcng1/(G1KO).