The 26S Proteasome and Histone Modifying Enzymes Regulate

Major Histocompatibility Complex Class-II (MHC-II) molecules are critical regulators of adaptive immunity that present extracellular antigens required to activate CD4 + T cells. MHC-II are regulated at the level of transcription by master regulator, the Class II Transactivator (CIITA), whose association with the MHC-II promoter is necessary to initiate transcription. Recently, much research focused on novel mechanisms of transcriptional regulation of critical genes like MHC-II and CIITA; findings that the macromolecular complex of the 26S-proteasome is involved in transcription have been perhaps the most exciting as they impart novel functions to a well studied system. Proteasome is a multi-subunit complex composed of a 20S-core particle capped by a 19S-regulatory particle. The 19S contains six ATPases which are required for transcription initiation and elongation. We demonstrate that 19S ATPase-S6a inducibly associates with CIITA promoters. Decreased expression of S6a negatively impacts recruitment of the transcription factors STAT-1 and IRF-1 to the CIITA due to significant loss in histone H3 and H4 acetylation. S6a is robustly recruited to CIITA coding regions, where S6a binding coordinates with that of RNA polymerase II. RNAi mediated S6a knockdown significantly diminishes recruitment of Pol II and P-TEF-b components to CIITA coding regions, indicating S6a plays important roles in transcriptional elongation. Our research is focused on the ways in which accessibility to and transcription of DNA is regulated. While cancers are frequently linked to dysregulated gene expression, contribution of epigenetics to cancers remains unknown. To achieve metastatic ability, tumors alter gene expression to escape host immunosurveilance. MHC-II and CIITA expression are significantly downregulated in highly metastatic MDA-MB-435 breast cancer cells. This suppression correlates with elevated levels of the silencing modification H3K27me3 at CIITA and a significant reduction in Pol II recruitment. We observe elevated binding of the histone methyltransferase to CIITApIV and demonstrate this enzyme is a master regulator of CIITA gene expression. EZH2 knockdown results in significant increases in CIITA and MHC-II transcript levels in metastatic cells. In sum, transcriptional regulation by the 19S-proteasome and histone modifying enzymes represents novel mechanisms of control of mammalian gene expression and present novel therapeutic targets for manipulating MHC expression in disease. INDEX WORDS: Major histocompatibility class II, Class II transactivator, Transcription regulation, 26S proteasome, 19S ATPase, S6a, epigenetic, histone modifications, histone methyltransferase EZH2 THE 26S PROTEASOME AND HISTONE MODIFYING ENZYMES REGULATE TRANSCRIPTION OF THE CLASS II TRANSACTIVATOR, CIITA