Pioneer factor Pax7 initiates two-step cell-cycle-dependent chromatin opening.

Pioneer transcription factors direct cell differentiation by deploying new enhancer repertoires through their unique ability to target and initiate remodelling of closed chromatin. The initial steps of their action remain undefined, although pioneers have been shown to interact with nucleosomal target DNA and with some chromatin-remodeling complexes. We now define the sequence of events that enables the pioneer Pax7 with its unique abilities. Chromatin condensation exerted by linker histone H1 is the first constraint on Pax7 recruitment, and this establishes the initial speed of chromatin remodeling. The first step of pioneer action involves recruitment of the KDM1A (LSD1) H3K9me2 demethylase for removal of this repressive mark, as well as recruitment of the MLL complex for deposition of the activating H3K4me1 mark. Further progression of pioneer action requires passage through cell division, and this involves dissociation of pioneer targets from perinuclear lamin B. Only then are the SWI-SNF remodeling complex and the coactivator p300 recruited, leading to nucleosome displacement and enhancer activation. Thus, the unique features of pioneer actions are those occurring in the lamin-associated compartment of the nucleus. This model is consistent with previous work that showed a dependence on cell division for establishment of new cell fates.

O-GlcNAcylation of FOXK1 orchestrates the E2F pathway and promotes oncogenesis.

Gene transcription is a highly regulated process, and deregulation of transcription factors activity underlies numerous pathologies including cancer. Albeit near four decades of studies have established that the E2F pathway is a core transcriptional network that govern cell division in multi-cellular organisms1,2, the molecular mechanisms that underlie the functions of E2F transcription factors remain incompletely understood. FOXK1 and FOXK2 transcription factors have recently emerged as important regulators of cell metabolism, autophagy and cell differentiation3-6. While both FOXK1 and FOXK2 interact with the histone H2AK119ub deubiquitinase BAP1 and possess many overlapping functions in normal biology, their specific functions as well as deregulation of their transcriptional activity in cancer is less clear and sometimes contradictory7-13. Here, we show that elevated expression of FOXK1, but not FOXK2, in primary normal cells promotes transcription of E2F target genes associated with increased proliferation and delayed entry into cellular senescence. FOXK1 expressing cells are highly prone to cellular transformation revealing important oncogenic properties of FOXK1 in tumor initiation. High expression of FOXK1 in patient tumors is also highly correlated with E2F gene expression. Mechanistically, we demonstrate that FOXK1, but not FOXK2, is specifically modified by O-GlcNAcylation. FOXK1 O-GlcNAcylation is modulated during the cell cycle with the highest levels occurring during the time of E2F pathway activation at G1/S. Moreover, loss of FOXK1 O-GlcNAcylation impairs FOXK1 ability to promote cell proliferation, cellular transformation and tumor growth. Mechanistically, expression of FOXK1 O-GlcNAcylation-defective mutants results in reduced recruitment of BAP1 to gene regulatory regions. This event is associated with a concomitant increase in the levels of histone H2AK119ub and a decrease in the levels of H3K4me1, resulting in a transcriptional repressive chromatin environment. Our results define an essential role of O-GlcNAcylation in modulating the functions of FOXK1 in controlling the cell cycle of normal and cancer cells through orchestration of the E2F pathway.