Nesprins form a novel class of nuclear envelope-anchored spectrin-repeat proteins. We show that a direct association of their highly conserved C-terminal luminal domain with the inner nuclear membrane protein Sun1 mediates their nuclear envelope localisation. In Nesprin-1 and Nesprin-2 the conserved C-terminal amino acids PPPX are essential for the interaction with a C-terminal region in Sun1. In fact, Sun1 is required for the proper nuclear envelope localisation of Nesprin-2 as shown using dominant-negative mutants and by knockdown of Sun1 expression. Sun1 itself does not require functional A-type lamins for its localisation at the inner nuclear membrane in mammalian cells. Our findings propose a conserved nuclear anchorage mechanism between Caenorhabditis elegans and mammals and suggest a model in which Sun1 serves as a `structural bridge' connecting the nuclear interior with the actin cytoskeleton.

Polarized hepatocytes expressing hyperactive Ha-Ras adopt an invasive and metastatic phenotype in cooperation with transforming growth factor (TGF)-β. This dramatic increase in malignancy is displayed by an epithelial to mesenchymal transition (EMT), which mimics the TGF-β-mediated progression of human hepatocellular carcinomas. In culture, hepatocellular EMT occurs highly synchronously, facilitating the analysis of molecular events underlying the various stages of this process. Here, we show that in response to TGF-β, phosphorylated Smads rapidly translocated into the nucleus and activated transcription of target genes such as E-cadherin repressors of the Snail superfamily, causing loss of cell adhesion. Within the TGF-β superfamily of cytokines, TGF-β1, -β2 and -β3 were specific for the induction of hepatocellular EMT. Expression profiling of EMT kinetics revealed 78 up- and 235 downregulated genes, which preferentially modulate metabolic activities, extracellular matrix composition, transcriptional activities and cell survival. Independent of the genetic background, platelet-derived growth factor (PDGF)-A ligand and both PDGF receptor subunits were highly elevated, together with autocrine secretion of bioactive PDGF. Interference with PDGF signalling by employing hepatocytes expressing the dominant-negative PDGF-α receptor revealed decreased TGF-β-induced migration in vitro and efficient suppression of tumour growth in vivo. In conclusion, these results provide evidence for a crucial role of PDGF in TGF-β-mediated tumour progression of hepatocytes and suggest PDGF as a target for therapeutic intervention in liver cancer.

Lamina-associated polypeptide (LAP) 2α is a nonmembrane-bound LAP2 isoform that forms complexes with nucleoplasmic A-type lamins. In this study, we show that the overexpression of LAP2α in fibroblasts reduced proliferation and delayed entry into the cell cycle from a G0 arrest. In contrast, stable down-regulation of LAP2α by RNA interference accelerated proliferation and interfered with cell cycle exit upon serum starvation. The LAP2α-linked cell cycle phenotype is mediated by the retinoblastoma (Rb) protein because the LAP2α COOH terminus directly bound Rb, and overexpressed LAP2α inhibited E2F/Rb-dependent reporter gene activity in G1 phase in an Rb-dependent manner. Furthermore, LAP2α associated with promoter sequences in endogenous E2F/Rb-dependent target genes in vivo and negatively affected their expression. In addition, the expression of LAP2α in proliferating preadipocytes caused the accumulation of hypophosphorylated Rb, which is reminiscent of noncycling cells, and initiated partial differentiation into adipocytes. The effects of LAP2α on cell cycle progression and differentiation may be highly relevant for the cell- and tissue-specific phenotypes observed in laminopathic diseases.