We also show that MDM2-mediated H3K4 methylation is checkpoint-dependent, as it can be inhibited by caffeine (Physique 8C). Our data signify that a compromised checkpoint response would allow an increase in untimely origin firing by MDM2. cells. Consistently, inhibition of cyclin-dependent kinases, known to activate DNA replication origins during firing, inhibits MDM2-mediated induction of chk1 phosphorylation indicating the requirement of this activity in MDM2-mediated chk1 phosphorylation. Our data reveal a novel pathway, defended by the intra-S-phase checkpoint, by which MDM2 induces unscheduled origin firing and accelerates S-phase access of cells in the absence of p53. INTRODUCTION Although deregulation of DNA replication is usually a crucial event in oncogenesis, whether or how oncogenes modulate DNA replication has not been investigated in depth. Several oncogenes that overexpress in malignancy cells often induce a growth-suppressive effect in non-transformed cells (1,2), which is considered to be a fail-safe response to suppress uncontrolled cell proliferation. Oncogenic Ras is known to induce a DNA damage repair response (3); Raf-1 induces cell cycle arrest and senescence (4); and MYC is known to trigger DNA damage and checkpoint response (5,6). Furthermore, indicators of oncogene-induced senescence are frequently observed in pre-malignant lesions of humans and animal tumors (7). Even though mechanism of Ras-induced DNA hyper-replication leading to senescence has been reported, whether or how other oncogenes influence DNA replication to elicit DNA damage response is largely unknown. The human homolog of the mouse double minute2 (gene or overexpression of MDM2 in transgenic mice induces tumorigenesis (8,9). The oncoprotein is usually often overexpressed in human sarcomas and carcinomas in the presence or absence of wild-type E3330 (WT) p53 (10,11). MDM2 interacts with the transactivation domain name of p53 inactivating its function (12,13). MDM2 is also an E3 ubiquitin ligase, and is known to degrade p53 (14). Although MDM2 interacts actually and functionally to several growth suppressors, such as the retinoblastoma susceptibility gene product (Rb) and p14, its p53-inactivating and degrading function is usually thought to be the primary cause of oncogenesis (10,13). However, malignancy cells with p53 mutation often overexpress MDM2, and the significance of E3330 MDM2 amplification or overexpression in human tumors lacking WT p53 is not obvious (11,15,16). Despite its oncogenic function, elevation of MDM2 expression induces G1-arrest in the presence or absence of WT p53 (17C19). Removal of the growth inhibitory domains of MDM2 rescues its tumorigenic potential (17). Furthermore, in apparently normal cells, such as early passage mouse embryo fibroblasts or limited passage human lung fibroblast (such as WI38), MDM2 inhibits expression of cyclin A (20). Genetic defects, such as absence of WT p53, the cyclin-dependent kinase inhibitor p16 or the transcription factor BRG1, that deregulate the timely expression of cyclin A, also abrogate the ability of MDM2 to inhibit expression of cyclin A expression, but not its ability to induce G1 arrest (17,20), suggesting that MDM2 expression restricts an event downstream of cyclin A expression, and prospects us to E3330 investigate how MDM2 controls initiation of DNA replication. Initiation of DNA replication takes place at DNA replication origins, recognized by loading of pre-replication complex during late mitosis and G1 phase, a process known as licensing. During G1/S transition and at different times E3330 during the S phase, replication initiation factors are recruited to only a portion of licensed origins forming pre-initiation complex, activating the minichromosome maintenance proteins E3330 2C7 (MCM2C7) helicases and assembly of other replication factors and inducing local DNA unwinding. Origin firing is activated by cyclin-dependent kinases complexed with cyclins E and A and by cdc7/DBf4 (21). In mammalian cells, replication origin firing is regulated by checkpoint kinases, ATR and chk1, during normal unperturbed S phase in response to the single-stranded DNA uncovered at replication forks (22C24). In this statement, we present evidence to show that elevated expression of Rabbit Polyclonal to Cytochrome c Oxidase 7A2 MDM2 in cells lacking p53 elevates endogenous levels of cyclins D2 and A and hastens S-phase access and activation of chk1 phosphorylation. Activation of chk1 phosphorylation is known to stabilize a histone methyl transferase coded by the mixed lineage lymphoma (MLL) gene, which methylates lysine 4 of histone H3 (H3k4) at late replication origins.