Three-dimensional matrix induces sustained activation of ERK1/2 via Src/Ras/Raf signaling pathway

Research in cell signaling often depends on tissue culture, but the artificial substrates used to grow cells in vitro are likely to distort the conclusions, particularly when adhesion-mediated signaling events are investigated. Studies of signal transduction pathways operating in cells grown in three-dimensional (3D) matrices provide a better system, giving a closer insight of the cell signaling in vivo. We compared the steady-state levels of ERK1/2 activity in primary human fibroblasts, induced by cell-derived 3D fibronectin matrix or fibronectin, coated on flat surfaces. 3D environment caused ERK1/2 stimulation concomitant with a 2.5-fold increase in Ras GTP loading and Src activation. Under these conditions FAK autophosphorylation was suppressed. Treatment with Src inhibitor PP2 abolished these effects indicating that 3D fibronectin matrix activated ERK1/2 through Src/Ras/Raf pathway, bypassing FAK. These observations suggest that within in vivo-like conditions Src may have a leading role in the induction of sustained ERK1/2 activation.     

Intake of Xylooligosaccharides Alters the Structural Organization of Liver Plasma Membrane Bilayer

Xylooligosaccharides (XOS) are non-digestible carbohydrate prebiotics that beneficially affect the host by selective stimulation of specific bacteria in the gastro-intestinal tract. The impact of XOS on gastrointestinal microflora and blood lipids is well known but the exact mechanism of action on liver membranes is still unclear. The organization of membrane lipids in domains is known to be important for the proper functioning of various receptors and mechanisms triggering cell signaling. In this study the influence of XOS-enriched diet on the lipid bilayer structure of rat liver plasma membrane was investigated. XOS intake caused a slight decrease of the fluidity of lipid extracts from liver plasma membranes compared to the controls. This observation was based on the increased generalized polarization (GP) and blue shifted emission spectra of Laurdan. The elevated amount of membrane sphingomyelin may be one possible reason for the reported effects. The micron-scale phase separation of the lipid extracts was also investigated by fluorescence microscopy. A different temperature of phase separation and domain pattern was observed in plasma membrane lipid extracts from XOS-fed animals. We presume that it could be assigned to the altered lipid composition of the membrane bilayer, in particular to the changes in the sphingomyelin/cholesterol ratio. All observed alterations are discussed in the light of the impact of XOS on human health and physiology.