Deregulation of cyclin D1 occurs in numerous human cancers through mutations, alternative splicing, and gene amplification. Although cancer-derived cyclin D1 mutants are potent oncogenes in vitro and in vivo, the mechanisms whereby they contribute to neoplasia are poorly understood. We now provide evidence derived from both mouse models and human cancer-derived cells revealing that nuclear accumulation of catalytically active mutant cyclin D1/CDK4 complexes triggers DNA rereplication, resulting from Cdt1 stabilization, which in turn triggers the DNA damage checkpoint and p53-dependent apoptosis. Loss of p53 through mutations or targeted deletion results in increased genomic instability and neoplastic growth. Collectively, the data presented reveal mechanistic insights into how uncoupling of critical cell cycle regulatory events will perturb DNA replication fidelity, thereby contributing to neoplastic transformation.[Keywords: Cdt1; Cul4; cyclin D1T286A; cyclin D1P287A; rereplication; p53; genomic instability] Supplemental material is available at http://www.genesdev.org. Received June 22, 2007; revised version accepted September 21, 2007. Dysregulation of the pathways that contribute to the normal integration of growth factor signaling and cell division lies at the heart of neoplastic growth. Cyclin D expression, accumulation, and its ability to associate with catalytic partners CDK4/6 are regulated by mitogenic signaling, allowing D cyclins to serve as mediators of growth factor signaling and cell cycle progression (Gladden and Diehl 2005). Cyclin D1 gene expression depends on activation of the small GTP-binding protein, Ras (Marshall 1999). Ras activates canonical MAP kinase signaling (RAF-MEK-ERK); activation of this pathway is both necessary and sufficient for cyclin D1 expression and association with CDK4 (Albanese et al. 1995;Lavoie et al. 1996;Aktas et al. 1997;Cheng et al. 1998). Ras also contributes to the accumulation of the cyclin D1 protein, via a pathway involving PI3K and Akt, which attenuates the GSK3␤ kinase (Franke et al. 1995(Franke et al. , 1997; GSK3␤-dependent phosphorylation of cyclin D1 on Thr286 promotes cyclin D1 proteolysis (Diehl et al. 1998). Cyclin D1 subcellular localization is also regulated during cell cycle progression. Phosphorylation of cyclin D1 on Thr286 at the G1/S-phase boundary promotes CRM1-dependent nuclear export (Alt et al. 2000). Thus, Thr286 phosphorylation coordinates two critical regulatory events. First, it removes the cyclin D1 kinase from the nucleus during S phase, thereby sequestering it from potential substrates. Second, ubiquitin-mediated destruction of cyclin D1 requires cytoplasmic localization, as the D1 E3 ligase is restricted to the cytoplasm (Lin et al. 2006).Cyclin D1 overexpression occurs in carcinomas of the esophagus, head and neck, breast, and colon, as well as certain B-cell lymphomas (Bartkova et al. 1994a(Bartkova et al. ,b, 1995Gillett et al. 1994;Herman et al. 1995;Barnes and Gillett 1998;Hibberts et al. 1999;Hosokawa et al. 1999; BaniHani...

Cyclin D1 levels are maintained at steady state by phosphorylation-dependent nuclear export and polyubiquitination by SCF FBX4-aB crystallin . Inhibition of cyclin D1 proteolysis has been implicated as a causative factor leading to its overexpression in breast and esophageal carcinomas; however, the contribution of stable cyclin D1 to the genesis of such carcinomas has not been evaluated. We therefore generated transgenic mice wherein expression of either wild-type or a stable cyclin D1 allele (D1T286A) is regulated by MMTV-LTR. MMTV-D1T286A mice developed mammary adenocarcinomas at an increased rate relative to MMTV-D1 mice. Similar to human cancers that overexpress cyclin D1, D1T286A tumors were estrogen receptor-positive and exhibited estrogen-dependent growth. Collectively, these results suggest that temporal control of cyclin D1 subcellular localization and proteolysis is critical for maintenance of homeostasis within the mammary epithelium.