Visualization by BiFC of different C/EBPβ dimers and their interaction with HP1α reveals a differential subnuclear distribution of complexes in living cells

How the co-ordinated events of gene activation and silencing during cellular differentiation are influenced by spatial organization of the cell nucleus is still poorly understood. Little is known about the molecular mechanisms controlling subnuclear distribution of transcription factors, and their interplay with nuclear proteins that shape chromatin structure. Here we show that C/EBP beta not only associates with pericentromeric heterochromatin but also interacts with the nucleoskeleton upon induction of adipocyte differentiation of 3T3-L1 cells. Different C/EBP beta dimers localize in different nuclear domains. Using BiFC in living cells, we show that LAP (Liver Activating Protein) homodimers localize in euchromatin and heterochromatin. In contrast, LIP (Liver Inhibitory Protein) homodimers localize exclusively in heterochromatin. Importantly, their differential subnuclear distribution mirrors the site for interaction with HP1 alpha. HP1 alpha inhibits LAP transcriptional capacity and occupies the promoter of the C/EBP beta-dependent gene c/ebp alpha in 3T3-L1 preadipocytes. When adipogenesis is induced, HP1 alpha binding decreases from c/ebp alpha promoter, allowing transcription. Thus, the equilibrium among different pools of C/EBP beta associated with chromatin or nucleoskeleton, and dynamic changes in their interaction with HP1 alpha, play key roles in the regulation of C/EBP target genes during adipogenesis. (C) 2010 Elsevier Inc. All rights reserved.