The nuclear lamina is implicated in the organization of the eukaryotic nucleus. Association of nuclear lamins with the genome occurs through large chromatin domains including mostly, but not exclusively, repressed genes. How lamin interactions with regulatory elements modulate gene expression in different cellular contexts is unknown. We show here that in human adipose tissue stem cells, lamin A/C interacts with distinct spatially restricted subpromoter regions, both within and outside peripheral and intra-nuclear lamin-rich domains. These localized interactions are associated with distinct transcriptional outcomes in a manner dependent on local chromatin modifications. Down-regulation of lamin A/C leads to dissociation of lamin A/C from promoters and remodels repressive and permissive histone modifications by enhancing transcriptional permissiveness, but is not sufficient to elicit gene activation. Adipogenic differentiation resets a large number of lamin-genome associations globally and at subpromoter levels and redefines associated transcription outputs. We propose that lamin A/C acts as a modulator of local gene expression outcome through interaction with adjustable sites on promoters, and that these position-dependent transcriptional readouts may be reset upon differentiation.

Analyses of molecular events associated with reprogramming somatic nuclei to pluripotency are scarce. We previously reported the reprogramming of epithelial cells by extract of undifferentiated embryonal carcinoma (EC) cells. We now demonstrate reprogramming of DNA methylation and histone modifications on regulatory regions of the developmentally regulated OCT4 and NANOG genes by exposure of 293T cells to EC cell extract. OCT4 and NANOG are transcriptionally up-regulated and undergo mosaic cytosine-phosphate-guanosine demethylation. OCT4 demethylation occurs as early as week 1, is enhanced by week 2, and is most prominent in the proximal promoter and distal enhancer. Targeted OCT4 and NANOG demethylation does not occur in 293T extract-treated cells. Retinoic acid-mediated differentiation of reprogrammed cells elicits OCT4 promoter remethylation and transcriptional repression. Chromatin immunoprecipitation analyses of lysines K4, K9, and K27 of histone H3 on OCT4 and NANOG indicate that primary chromatin remodeling determinants are acetylation of H3K9 and demethylation of dimethylated H3K9. H3K4 remains di-and trimethylated. Demethylation of trimethylated H3K9 and H3K27 also occurs; however, trimethylation seems more stable than dimethylation. We conclude that a central epigenetic reprogramming event is relaxation of chromatin at loci associated with pluripotency to create a conformation compatible with transcriptional activation. INTRODUCTIONReprogramming of a differentiated somatic cell into a pluripotent cell may have applications in regenerative medicine, and as such, several approaches are being examined to produce embryonic stem (ES)-like cells. Nuclear transplantation into oocytes has demonstrated that functional nuclear reprogramming is possible, through the production of nuclear transfer ES cells (Cibelli et al., 1998;Munsie et al., 2000;Wakayama et al., 2001) and cloned animals (Wilmut et al., 2002;. Fusion of somatic cells with ES or embryonal carcinoma (EC) cells also elicits a reprogramming of the somatic genome within the hybrids, demonstrated by X chromosome reactivation (Tada et al., 2001), changes in gene expression profile, and acquisition of ES cell properties, including contribution to all germ layers in teratomas and in aggregation chimeras (Tada et al., 1997(Tada et al., , 2001Pells et al., 2002;Terada et al., 2002;Ying et al., 2002;Cowan et al., 2005). Recently, retroviral transduction and constitutive expression of four factors (Oct4, Sox2, Klf4, and c-Myc) was also shown to induce an ES cell-like behavior in mouse fibroblasts, similar to that reported by fusion with ES cells (Takahashi and Yamanaka, 2006). A fourth approach to reprogramming entails treatment of reversibly permeabilized somatic cells with an extract of another differentiated cell type or of undifferentiated, pluripotent ES or EC cells (Taranger et al., 2005). Notably, epithelial 293T cells treated with extract of undifferentiated human EC (NCCIT) cells induces expression of genes associated with pluripotency, such as OCT4...