The Roles of MLL and Taspase1-Mediated MLL Proteolysis in Embryogenesis and the HGF-MET Signaling Pathway

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