No abstract

The transcription factor E2F activates genes required for S phase, such as cyclin E and cyclin A. We show that, contrary to long term eects of E2F-1 overexpression, short ectopic overexpression of this transcription factor in logarithmically growing cells does neither aect the cell cycle distribution nor the cell size, but heavily induces cyclin E and A expression as well as cyclin Eand A-dependent kinase activities. We further separated logarithmically growing E2F-1-overexpressing cells according to their dierent cell cycle phases by centrifugal elutriation. These experiments revealed that deregulated E2F-1 expression triggers high levels of cyclin E and A expression and kinase activities in small early G 1 cells, normally not exhibiting these activities. These eects on the regulation of cyclin E-and A-associated kinases are not accompanied by any detectable alteration in the rate of progression through the cell cycle, suggesting that these changes are independent of any mitogenic properties of E2F-1.

No abstract

LR7/8B is a member of the low density lipoprotein receptor gene family that is specifically synthesized in the brain. Here we have functionally expressed in 293 cells the splice variant harboring eight ligand binding repeats (LR8B). As assessed by confocal microscopy, the expressed receptor is localized to the plasma membrane. Importantly, in cell binding experiments, we demonstrate that this protein is a receptor for activated ␣ 2 -macroglobulin. Because to date low density lipoprotein receptor-related protein (LRP) has been shown to be the only ␣ 2 -macroglobulin receptor in brain, we became interested in the expression pattern of both proteins at the cellular level in the brain. LR7/8B is expressed in large neurons and Purkinje cells of the cerebellum and in cells constituting brain barrier systems such as the epithelial cells of the choroid plexus, the arachnoidea, and the endothelium of penetrating blood vessels. Anti-LR7/8B antibody stains the plasma membrane, dendrites, and vesicular structures close to the cell membrane of neurons, especially of Purkinje cells. In contrast, LRP is present in patchy regions around large neurons and most prominently in the glomeruli of the stratum granulare of the cerebellum. This suggests that, contrary to LR7/8B, LRP is expressed in synaptic regions of the neurons; furthermore, there is a striking difference in the expression patterns of LR7/8B and LRP in the choroid plexus. Whereas LRP shows baso-lateral and apical localization in the epithelial cells, LR7/8B is restricted to the apical cell aspect facing the cerebrospinal fluid. Finally, these studies were extended to cultured primary rat neurons, where double immunofluorescence labeling with anti-LR7/8B and anti-microtubuli-associated protein 2 (MAP2) confirmed the somatodendritic expression of the receptor. Based upon these data, we propose that LR7/8B is involved in the clearance of ␣ 2 -macroglobulin⅐proteinase complexes and/or of other substrates bound to ␣ 2 -macroglobulin from the cerebrospinal fluid and from the surface of neurons.The LDL 1 receptor gene family specifies a group of highly related composite membrane proteins engaged in receptor-mediated endocytosis of a variety of independent ligands. The members of the family are, listed in the order of their discovery, LDL receptor (LDLR), LDL receptor-related protein (LRP), gp330/megalin, VLDL receptor (or LR8), LR11, apolipoprotein E receptor 2 (apoER2), and LR8B (1, 2). Common features of these proteins are structurally and functionally distinct modules that are defined by distinct exons in the corresponding genes. These modules are (i) the type A binding repeats of ϳ40 residues, each harboring 6 paired cysteines; (ii) type B repeats, also containing 6 cysteines each; (iii) modules of ϳ50 residues with a consensus tetrapeptide, Tyr-Trp-Thr-Asp (YWTD) (together with the type B repeats they constitute the epidermal growth factor precursor homology domain); (iv) a short stretch containing many serines and threonines carrying O-linked sugars (in the case of LR8...