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Bimodal degradation of MLL by SCFSkp2 and APCCdc20 assures cell cycle execution: A critical regulatory circuit lost in leukemogenic MLL fusions

Molecular Oncology, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Genes & Development (Impact Factor: 12.64). 11/2007; 21(19):2385-98. DOI: 10.1101/gad.1574507
Source: PubMed

ABSTRACT Human chromosome 11q23 translocations disrupting MLL result in poor prognostic leukemias. It fuses the common MLL N-terminal approximately 1400 amino acids in-frame with >60 different partners without shared characteristics. In addition to the well-characterized activity of MLL in maintaining Hox gene expression, our recent studies established an MLL-E2F axis in orchestrating core cell cycle gene expression including Cyclins. Here, we demonstrate a biphasic expression of MLL conferred by defined windows of degradation mediated by specialized cell cycle E3 ligases. Specifically, SCF(Skp2) and APC(Cdc20) mark MLL for degradation at S phase and late M phase, respectively. Abolished peak expression of MLL incurs corresponding defects in G1/S transition and M-phase progression. Conversely, overexpression of MLL blocks S-phase progression. Remarkably, MLL degradation initiates at its N-terminal approximately 1400 amino acids, and tested prevalent MLL fusions are resistant to degradation. Thus, impaired degradation of MLL fusions likely constitutes the universal mechanism underlying all MLL leukemias. Our data conclude an essential post-translational regulation of MLL by the cell cycle ubiquitin/proteasome system (UPS) assures the temporal necessity of MLL in coordinating cell cycle progression.

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    • "Previous studies including ours have indicated that MLL participates in the progression of S phase (22–24,26,30). MLL complexes with Host cell factor-1 (HCF-1) to interact with E2F1 and activate S-phase genes (23). "
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    ABSTRACT: MLL, the trithorax ortholog, is a well-characterized histone 3 lysine 4 methyltransferase that is crucial for proper regulation of the Hox genes during embryonic development. Chromosomal translocations, disrupting the Mll gene, lead to aggressive leukemia with poor prognosis. However, the functions of MLL in cellular processes like cell-cycle regulation are not well studied. Here we show that the MLL has a regulatory role during multiple phases of the cell cycle. RNAi-mediated knockdown reveals that MLL regulates S-phase progression and, proper segregation and cytokinesis during M phase. Using deletions and mutations, we narrow the cell-cycle regulatory role to the C subunit of MLL. Our analysis reveals that the transactivation domain and not the SET domain is important for the S-phase function of MLL. Surprisingly, disruption of MLL–WRAD interaction is sufficient to disrupt proper mitotic progression. These mitotic functions of WRAD are independent of SET domain of MLL and, therefore, define a new role of WRAD in subset of MLL functions. Finally, we address the overlapping and unique roles of the different SET family members in the cell cycle.
    Nucleic Acids Research 05/2014; 42(12). DOI:10.1093/nar/gku458 · 9.11 Impact Factor
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    • "The gene codes for a precursor protein, which is processed to generate a heterodimer, MLLN320/C180. MLL deficiency causes S-phase checkpoint dysfunction (43). MLL contributes to crucial cellular functions by methylating H3K4 by its C-terminal SET domain (44). "
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    ABSTRACT: Conventional paradigm ascribes the cell proliferative function of the human oncoprotein mouse double minute2 (MDM2) primarily to its ability to degrade p53. Here we report that in the absence of p53, MDM2 induces replication stress eliciting an early S-phase checkpoint response to inhibit further firing of DNA replication origins. Partially synchronized lung cells cultured from p53-/-:MDM2 transgenic mice enter S phase and induce S-phase checkpoint response earlier than lung cells from p53-/- mice and inhibit firing of DNA replication origins. MDM2 activates chk1 phosphorylation, elevates mixed lineage lymphoma histone methyl transferase levels and promotes checkpoint-dependent tri-methylation of histone H3 at lysine 4, known to prevent firing of late replication origins at the early S phase. In the absence of p53, a condition that disables inhibition of cyclin A expression by MDM2, MDM2 increases expression of cyclin D2 and A and hastens S-phase entry of 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 entry of cells in the absence of p53.
    Nucleic Acids Research 10/2013; 42(2). DOI:10.1093/nar/gkt944 · 9.11 Impact Factor
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    • "Recent studies on E2F1 co-regulators involved in its downstream target activation suggested the recruitment of MLL1 to E2F-responsive genes either through direct mechanism or by host cell factor-1 (HCF-1) (Tyagi et al., 2007). Interestingly, just as E2F1, MLL1 protein is also a direct substrate for Skp2- mediated ubiquitylation and degradation (Liu et al., 2007) and, thus, got accumulated in the tissue lysates of transgenic mice (Fig. 5A). Moreover, in contrast to E2F1 occupancy, there was a shift in the promoter occupancy of MLL1 and the associated histone H3K4Me3 marker (Fig. 6). "
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    ABSTRACT: The E2F family of transcription factors are considered versatile modulators poised at biological crossroads to execute diverse cellular functions. Despite extensive studies on E2F, the molecular mechanisms that control specific biological functions of E2F1 transcription factor is still not fully understood. Here we have addressed the molecular underpinnings of paradoxical functions of E2F1 in a tumour microenvironment using the 'X15-myc' oncomouse model of hepatocellular carcinoma. We observed that the HBx oncoprotein of hepatitis B virus regulates E2F1 functions by interfering with its binding to Skp2 E3 ubiquitin ligase. The HBx-Skp2 interaction led to the accumulation of transcriptionally active E2F1 and histone methyltransferase mixed lineage leukemia 1 (MLL1) protein. During early stages of hepatocarcinogenesis, the increased E2F1 activity promoted cellular proliferation by stimulating the genes involved in cell cycle control and replication. However, during the late stages, E2F1 triggered replicational stress-induced DNA damage and sensitized cells to apoptotic death in a p53-independent manner. Interestingly, the differential promoter occupancy of MLL1 during the early and late stages of tumour development seemed to specify the proliferative and apoptotic functions of E2F1 through its dynamic interaction with co-activator CBP or co-repressor Brg1. Thus, the temporally-regulated promoter occupancy of histone methyltransferase could be a novel regulatory mechanism associated with diverse cellular functions of E2F family of transcription factors.
    Journal of Cell Science 07/2013; 126(20). DOI:10.1242/jcs.126235 · 5.33 Impact Factor
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