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Date of Award

Summer 8-15-2010

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Developmental, Regenerative, & Stem Cell Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Mammalian embryogenesis is achieved through precisely organized temporal and spatial control of gene activities, including epigenetic regulation. MLL is a conserved epigenetic regulator that belongs to the Trithorax protein family. The SET domain of MLL exerts a histone methyltransferase (HMT) activity that specifically catalyzes methylation of histone H3 lysine 4 (H3K4), an epigenetic mark for active transcription. MLL is site-specifically proteolyzed by a threonine endopeptidase, Taspase1, to generate a MLLN320/MLLC180 heterodimer. To investigate the in vivo functions of Taspase1, we generated Taspase1-/- animals. Disruption of Taspase1 resulted in homeotic transformation and impaired cell proliferation. Importantly, uncleaved MLL displayed diminished HMT activity. Taspase1 deficiency in MEFs showed down-regulation of Cyclin Es, As and Bs, accompanied by reduced H3K4 tri-methylation at the respective promoters. We discovered that MLL interacts with E2Fs transcription factors to target the cyclin loci for H3K4 tri-methylation.

The biological roles of Taspase1-mediated cleavage of MLL were specifically investigated by the generation and analyses of transgenic MLLnc/nc mice that express a non-cleavable form of MLL. MLLnc/nc mice exhibited multiple homeotic transformation phenotypes, indicating the prerequisite of MLL proteolysis for proper Hox expression. Unexpectedly, MLLnc/nc animals also showed defects in the outgrowth of the hypoglossal nerve, a phenotype associated with the deficiency of HGF-MET signaling. Importantly, this defect was also observed in Taspase1-/- and MLL-/- embryos, indicating a novel involvement of MLL in the HGF-MET signaling pathway.

HGF-MET signaling modulates diverse physiological and pathological processes, including proliferation, migration and invasion. The molecular mechanisms by which MLL engages in the HGF-MET signaling pathway were characterized using HepG2 hepatocellular carcinoma cells. Cell migration and invasion assays demonstrated the requirement of MLL for HGF-induced migration/invasion. Consistently, silencing of MLL abolished up-regulation of critical HGF-downstream effecter genes, matrix metalloprotease (MMP) 1 and 3. The induction of MMP1/3 is dependent on Ets2, a transcription factor activated by HGF-MET pathway. Importantly, we revealed a novel complex of MLL and Ets2 by co-immunoprecipitation. Lastly, the chromatin immunoprecipitation assay demonstrated that HGF directs the recruitment of the MLL-Ets2 complex to the promoter of MMP1, where MLL confers H3K4 tri-methylation to activate MMP1.

In summary, my thesis reveals a critical post-translational regulation of MLL through Taspase1-mediated proteolysis and discovers a novel participation of MLL in the HGF-MET signaling pathway that is commonly dysregulated in human cancers.

Language

English (en)

Chair and Committee

James B Skeath

Committee Members

Aaron Diantonio, Joseph Corbo

Comments

Permanent URL: https://doi.org/10.7936/K7Z899MW

Available for download on Friday, August 15, 2110

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