Normal Proliferation and Tumorigenesis but Impaired Pancreatic Function in Mice Lacking the Cell Cycle Regulator Sei1 Pablo J. Fernandez-Marcos1, Cristina Pantoja1, Agueda Gonzalez-Rodriguez2,3, Nicholas Martin4, Juana M. Flores5, Angela M. Valverde2,3, Eiji Hara4, Manuel Serrano 1 * 1 Tumor Suppression Group, Spanish National Cancer Research Centre (CNIO), Madrid, Spain, 2 Institute of Biomedicine Alberto Sols (CSIC/UAM), Madrid, Spain, 3 Centro de Investigacio´ n Biome´dica en Red de Diabetes y Enfermedades Metabo´ licas Asociadas (CIBERDEM), ISCIII, Madrid, Spain, 4 Cancer Institute of the Japanese Foundation for Cancer Research, Tokyo, Japan, 5 Department of Animal Surgery and Medicine, Complutense University of Madrid, Madrid, Spain Abstract Sei1 is a positive regulator of proliferation that promotes the assembly of Cdk4-cyclin D complexes and enhances the transcriptional activity of E2f1. The potential oncogenic role of Sei1 is further suggested by its overexpression in various types of human cancers. To study the role of Sei1, we have generated a mouse line deficient for this gene. Sei1-null fibroblasts did not show abnormalities regarding proliferation or susceptibility to neoplastic transformation, nor did we observe defects on Cdk4 complexes or E2f activity. Sei1-null mice were viable, did not present overt pathologies, had a normal lifespan, and had a normal susceptibility to spontaneous and chemically-induced cancer. Pancreatic insulin- producing cells are known to be particularly sensitive to Cdk4-cyclin D and E2f activities, and we have observed that Sei1 is highly expressed in pancreatic islets compared to other tissues. Interestingly, Sei1-null mice present lower number of islets, decreased b-cell area, impaired insulin secretion, and glucose intolerance. These defects were associated to nuclear accumulation of the cell-cycle inhibitors p21Cip1 and p27Kip1 in islet cells. We conclude that Sei1 plays an important role in pancreatic b-cells, which supports a functional link between Sei1 and the core cell cycle regulators specifically in the context of the pancreas. Citation: Fernandez-Marcos PJ, Pantoja C, Gonzalez-Rodriguez A, Martin N, Flores JM, et al. (2010) Normal Proliferation and Tumorigenesis but Impaired Pancreatic Function in Mice Lacking the Cell Cycle Regulator Sei1. PLoS ONE 5(1): e8744. doi:10.1371/journal.pone.0008744 Editor: Kathrin Maedler, University of Bremen, Germany Received July 8, 2009; Accepted December 4, 2009; Published January 18, 2010 Copyright: 2010 Fernandez-Marcos et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Work in the laboratory of M.S. is funded by the Centro Nacional de Investigaciones Oncol o´gicas (CNIO) and by grants from the Spanish Ministry of Science (SAF and CONSOLIDER), the Regional Government of Madrid (GsSTEM), the European Union (PROTEOMAGE), the European Research Council (ERC), and the ‘‘Marcelino Botin’’ Foundation. Work in the laboratory of A.M.V. was funded by the Ministry of Science (BFU2008-02420) and by the Centro de Investigacio´ n Biome´dica en Red de Diabetes y Enfermedades Metabo´ licas Asociadas (CIBERDEM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: mserrano@cnio.es Introduction In mammalian cells, entry from quiescence into the cell cycle and advance through G1 into S phase are controlled by the activity of the D-type cyclin-dependent kinases Cdk4 and Cdk6 (Cdk4,6/D complexes) [1]. The expression of D-type cyclins in response to extracellular mitogenic signals is tightly regulated at different levels, as well as their binding to Cdk4,6. An important level of regulation is exerted by the Cdk inhibitors (CKIs), which include the Ink4 family (p16Ink4a, p15Ink4b, p18Ink4c and p19Ink4d) and the Cip/Kip family (p21Cip1, p27Kip1 and p57Kip2) [2,3]. Once active, Cdk4,6/D complexes phosphorylate members of the pocket protein family, namely, Rb, p107 and p130. These proteins bind and inhibit transcription factors important for cell cycle progression, most notably the E2f family of transcription factors, whose target genes are necessary for the replication of the DNA during S-phase. Upon phosphorylation, the pocket proteins are released from the E2f transcription factors allowing them to become transcriptionally active [4]. Alterations in the proteins that regulate the early stages of the cell cycle, including Cdk4, D-type cyclins and E2f1, have proven to be important in the development of cancer [1]. At a more physiological level, Cdk4, D-type cyclins and the E2f family have a clear impact in pancreas development and homeostasis. Mice deficient for these proteins display pancreatic abnormalities, mainly characterized by decreased numbers of b-cells which result in insulin resistance and diabetes [5-7]. Together, these observations indicate that b-cells are highly sensitive to the activity of the Cdk4/cyclinD/E2f pathway [8]. Sei1, also known as Sertad1, belongs to the Sertad family of proteins composed by four members named Sertad1-4 [9]. Sei1 binds to and stabilizes Cdk4/D complexes, stimulating their kinase activity and preventing their inhibition by p16Ink4a [10,11]. Of note, CKIs p21Cip1 and p27Kip1 also participate in the assembly and stabilization of Cdk4/D complexes, as well as, in their nuclear translocation [12,13]. However, it is still under debate whether the Cdk4/D complexes associated to p21Cip1 or p27Kip1 are inactive or may retain activity under particular conditions (discussed in [14]). Independent studies on Sei1 and its closest homologue Sertad2 have indicated that these proteins also promote proliferation by binding to the transcription factor E2f1 and by enhancing its transcriptional activity [15]. Together, these studies suggest that Sei1 plays a positive role in cell cycle progression. PLoS ONE | www.plosone.org 1 January 2010 | Volume 5 | Issue 1 | e8744