Regulation of cell cycle progression by forkhead transcription factor FOXO3 through its binding partner DNA replication factor Cdt1 Yiru Zhanga, Yuqian Xinga, Lei Zhanga, Yang Meia, Kazuo Yamamotob, Tak W. Makb,1, and Han Youa,1 aState Key Laboratory of Cellular Stress Biology, School of Life Sciences, Xiamen University, Xiamen, Fujian 361005, China; and bCampbell Family Institute for Breast Cancer Research, Ontario Cancer Institute, University Health Network (UHN), Toronto, Ontario, Canada M5G Contributed by Tak W. Mak, February 24, 2012 (sent for review December 21, 2011) To ensure genome stability, DNA must be replicated once and only Cells expressing the constitutively active form of FOXO3 in the S once during each cell cycle. Cdt1 is tightly regulated to make sure phase display a delay in their progression through the G2 phase that cells do not rereplicate their DNA. Multiple regulatory of the cell cycle. Two targets were identified that may mediate mechanisms operate to ensure degradation of Cdt1 in S phase. the effect of FOXOs at the G2/M boundary: cyclin G2 and However, little is known about the positive regulators of Cdt1 GADD45. Thus, FOXO factors mediate cell cycle arrest at the under physiological conditions. Here we identify FOXO3 as G1/S and G2/M transitions, two checkpoints that are critical in a binding partner of Cdt1. FOXO3 forms a protein complex with the cellular response to stress. Notably, these previous reports Cdt1, which in turn blocks its interaction with DDB1 and PCNA. characterizing the biological functions of FOXO in cell cycle Conversely, FOXO3 depletion facilitated the proteolysis of Cdt1 regulation were largely, if not all, based on overexpression of in unperturbed cells. Intriguingly, FOXO3 deficiency resulted in constitutively active form of FOXO members. In contrast to impaired S-phase entry and reduced cell proliferation. We provide these previous reports, here we provide evidence that depleting data that FOXO3 knockdown mimics Cdt1 down-regulation and FOXO3 reduced G1/S transition and cell proliferation. Cdt1 was affects G1/S transitions. Our results demonstrate a unique role identified as a binding partner of FOXO3. FOXO3 is crucial for of FOXO3 in binding to Cdt1 and maintaining its level required maintaining Cdt1 basal levels. Our data suggest a unique bi- CELL BIOLOGY for cell cycle progression. ological function of FOXO3 in cell cycle progression. Results n eukaryotic cells, DNA replication initiates from thousands of Ireplication origins. Each origin acquires replication compe- Cdt1 Interacts with FOXO3. Our laboratory is interested in identi- tence through the assembly of a prereplication complex (pre- fying binding partners of FOXO3. To this end, epitope-tagging fi RC) occurring in late mitosis and early G1 (1–3). Pre-RCs are strategy and af nity chromatography on M2 (anti-Flag antibody) assembled at the origins of DNA replication through the se- agarose beads was used to isolate protein complexes containing quential loading of the initiation factors ORC, Cdc6, Cdt1, and Flag-tagged FOXO3 from nuclear extracts of HEK293T cells. fi MCM2-7 (4). In S phase, pre-RCs are sequentially acted on by After SDS/PAGE fractionation and silver staining, we identi ed ∼ fi two protein kinases, Cdc7 and Cdk2, which promote recruitment a major protein band of 64 kDa that copuri ed with FOXO3 of proteins required for helicase activation and replisome as- and that mass spectrometry revealed to be Cdt1. We next showed fi sembly, leading to origin unwinding and DNA synthesis. To that puri ed recombinant His-FOXO3 was able to interact with A ensure that no replication origin fires more than once, the as- GST-Cdt1 under cell-free conditions (Fig. 1 ), suggesting a di- sembly of the replication apparatus at origins is tightly regulated rect interaction between FOXO3 and Cdt1. To determine the by the cell cycle machinery. Among the most important of these region of Cdt1 required for FOXO3 association, we generated regulatory mechanisms are the degradation of Cdt1 during S a series of Cdt1 deletion mutants. Mutants lacking the Cdt1 N- – terminal domain [amino acids (aa) 1–282] failed to bind to His- phase and the sequestration of Cdt1 by the geminin protein (5 – 7). Phosphorylation of Cdt1 by Cdk2 promotes its binding to FOXO3, whereas those lacking aa 1 200 exhibited robust in- – teraction with FOXO3 (Fig. 1B). These data suggest that the SCF-Skp2 E3 ubiquitin ligase (8 10), which results in its degra- – dation in S phase. In addition to the Skp2 pathway, PCNA/ middle region (aa 200 282) of Cdt1 mediates FOXO3-Cdt1 as- DDB1/Cul4-dependent signaling was found to degrade Cdt1 sociation. Reciprocal mapping using Flag-FOXO3 deletion con- – structs pinpointed FOXO3 aa 396–455 as a critical region for during S phase via the interaction of Cdt1 with PCNA (11 15). C Recently, APC/CCdh1 was proposed as a third ubiquitin ligase FOXO3-Cdt1 interaction (Fig. 1 ), prompting us to designate regulating Cdt1 degradation (16). Cdt1 is also targeted for this region as the Cdt1-binding domain (CBD). To assess if other degradation after DNA damage to stop licensing of new origins FOXO factors could bind to Cdt1, HEK293T cells were trans- until after DNA repair. Both the SCF-Skp2 complex and the fected with vectors overexpressing Flag-tagged FOXO1, FOXO3, or FOXO4. A Flag-tagged mutant lacking FOXO3 aa 395–455 Cul4-DDB1 complex have been reported to induce degradation Δ of Cdt1 after UV irradiation (17, 18). (FOXO3 CBD) served as a negative control for immunopreci- FOXO transcription factors are critical for the regulation of pitations. Surprisingly, only FOXO3 was able to bring down and cell cycle arrest, cell death, and DNA damage repair. Ample evidence has suggested that FOXO exerts a negative effect on cell cycle progression. In dividing cells, overexpression of the Author contributions: Y.Z., Y.X., and H.Y. designed research; Y.Z., Y.X., L.Z., and Y.M. performed research; Y.Z., Y.X., L.Z., Y.M., and K.Y. contributed new reagents/analytic active form of FOXO family members promotes cell cycle arrest tools; Y.Z., Y.X., L.Z., T.W.M., and H.Y. analyzed data; and T.W.M. and H.Y. wrote at the G1/S boundary. Target genes that mediate FOXO-induced the paper. cell cycle arrest are the Cdk inhibitors p27KIP1 and p21 (in the The authors declare no conflict of interest. β presence of TGF- ), the Rb family member p130, and cyclin D1 Freely available online through the PNAS open access option. and D2. The ectopically expressed active form of FOXO factors 1To whom correspondence may be addressed. E-mail: [email protected] or can cause G1 arrest both by up-regulating cell cycle inhibitors [email protected]. (p21 and p27) and by repressing cell cycle activators (cyclin D1/ This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. D2). FOXO factors also regulate other cell cycle checkpoints. 1073/pnas.1203210109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1203210109 PNAS Early Edition | 1of6 Downloaded by guest on September 30, 2021 Interacon A C DBD with Cdt1 Flag-FOXO3(FL) + 1 140-673 + GST Pull His-FOXO3 2 244-673 + Down 3 140-360 - His-FOXO3 4 140-395 - 5 140-455 + GST-Cdt1 6 140-530 + 7 396-455) - Input GST Ni2+ Resin Pull Down Lysate Flag-FOXO3 FL FL 1 2 3 4 5 6 FL FL 1 2 3 4 5 6 His-Cdt1 - + + + + + + + Lysate Pull Down Flag-FOXO3 FL FL 7 FL FL 7 His-Cdt1 - + + GST FOXO3 FOXO3 His His Interacon IP Flag Input B with FOXO3 D Cdt1(FL) + CBD 1 N 52 + 2 N 105 + FOXO1 FOXO4 3 N 140 + Vector FOXO3 FOXO3 CBD FOXO1 FOXO4 Vector FOXO3 FOXO3 4 N 200 + Cdt1 5 N 282 - * 6 C 168 + Flag Ni2+ Resin Pull Down Lysate E Myc-Cdt1 FL FL 1 2 3 4 5 6 FL FL 1 2 3 4 5 6 Myc-Cdt1 Flag-FOXO3 DAPI Merge His-FOXO3 - + + + + + + + Myc His Fig. 1. Cdt1 is a unique FOXO3-interacting protein. (A) Cdt1 directly interacts with FOXO3. Purified FOXO3 was incubated with GST or GST-Cdt1 coupled to GSH-Sepharose. Cell lysates (“Input”) and the eluates (“GST pull down”) were subjected to SDS/PAGE, followed by immunoblot analysis with the indicated antibodies. (B and C) Mapping of the binding domains for FOXO3 and Cdt1. 293T cells were transfected with the indicated constructs, and lysates were incubated with His-FOXO3 (B) or His-Cdt1 (C). Proteins retained on nickel resin were subjected to Western blotting with the indicated antibodies. (D) Cdt1 interacts specifically with FOXO3. 293T cells transfected with indicated plasmids were subjected to immunoprecipitation with M2 beads, followed by im- munoblotting with anti-Cdt1 and anti-Flag antibodies. The asterisk indicates a nonspecific band. (E) Cdt1 and FOXO3 form a complex in the nucleus. MCF-7 cells cotransfected with Myc-Cdt1 and Flag-FOXO3 were subjected to immunofluorescent staining with anti-Myc (red) and anti-Flag (green) antibodies. For A–E, results shown are representative of at least three independent experiments. stabilize endogenous Cdt1, and Cdt1 did not accumulate in cells We also observed decreased Cdt1 protein in the human lung overexpressing FOXO3ΔCBD, FOXO1, or FOXO4 (Fig. 1D). cancer cell line H1299 and in the osteosarcoma cell line U2OS Interestingly, exogenous FOXO3 had no effect on Cdt1 mRNA upon FOXO3 knockdown (Fig. S2B). Importantly, the degree of levels, and geminin protein was unchanged in cells overexpressing reduction in Cdt1 protein was tightly correlated with FOXO3 FOXO proteins (Fig. S1). To determine the subcellular locali- knockdown efficiency, in that MCF-7 cells infected with various zation of the FOXO3-Cdt1 complex, we cotransfected MCF-7 amounts of FOXO3 shRNA lentiviral soup showed dose-de- cells with vectors expressing Myc-Cdt1 and Flag-FOXO3.