monensin and betadex

Clathrin-dependent endocytosis is believed to be involved in TGFbeta-stimulated cellular responses, but the subcellular locus at which TGFbeta induces signaling remains unclear. Here, we demonstrate that inhibitors of clathrin-dependent endocytosis, which are known to arrest the progression of endocytosis at coated-pit stages, inhibit internalization of cell-surface-bound TGFbeta and promote colocalization and accumulation of TbetaR-I and SARA at the plasma membrane. These inhibitors enhance TGFbeta-induced signaling and cellular responses (Smad2 phosphorylation/nuclear localization and expression of PAI-1). Dynasore, a newly identified inhibitor of dynamin GTPase activity, is one of the most potent inhibitors among those tested and, furthermore, is a potent enhancer of TGFbeta. Dynasore ameliorates atherosclerosis in the aortic endothelium of hypercholesterolemic ApoE-null mice by counteracting the suppressed TGFbeta responsiveness caused by the hypercholesterolemia, presumably acting through its effect on TGFbeta endocytosis and signaling in vascular cells.

Topics: Animals; Apolipoproteins E; beta-Cyclodextrins; Cadaverine; Carrier Proteins; Cell Line; Clathrin; Endocytosis; Enzyme Inhibitors; Female; GTP-Binding Proteins; Hydrazones; Mice; Mice, Inbred C57BL; Mice, Knockout; Monensin; Plasminogen Activator Inhibitor 1; Protein Serine-Threonine Kinases; Receptor, Transforming Growth Factor-beta Type I; Receptors, Transforming Growth Factor beta; Signal Transduction; Smad2 Protein; Transforming Growth Factor beta; Triflupromazine

There are an increasing number of studies reporting the presence of Hsps in human serum. We have investigated the release of Hsp70 into blood and culture medium from peripheral blood mononuclear cells (PBMCs), and whether this release is due to cell damage or active secretion from the cells. Intact Hsp70 was released from cells within whole blood and from purified PBMCs under normal culture conditions. Hsp70 release was rapid (0.1 ng/10(6) cells/h) over the first 2 h of culture and continued at a reduced rate up to 24 h (<0.025 ng/10(6) cells/h). Using viable cell counts and lactate dehydrogenase release we were able to confirm that the release of Hsp70 was not due to cellular damage. Hsp70 release was inhibited by monensin, methyl-beta-cyclodextrin, and methylamine, but not by brefeldin A. These data suggest that Hsp70 is released from cells via a non-classical pathway, possibly involving lysosomal lipid rafts.

Topics: B-Lymphocytes; beta-Cyclodextrins; Blotting, Western; Brefeldin A; Cell Survival; Culture Media; HSP70 Heat-Shock Proteins; Humans; L-Lactate Dehydrogenase; Leukocytes, Mononuclear; Lysosomes; Membrane Microdomains; Methylamines; Monensin; Protein Synthesis Inhibitors; RNA, Messenger; T-Lymphocytes; Temperature; Time Factors

In addition to their central role in triglyceride storage, fat cells are a primary depot of unesterified cholesterol (FC) in the body. In comparison, peripheral cells contain very little FC. This difference in adipocytes versus peripheral tissues is inconsistent with the current theory of cholesterol homeostasis. Attempting to resolve this discrepancy, we examined intracellular storage sites of FC in murine 3T3-F442A adipocytes. Using the cholesterol-binding antibiotic, filipin, in combination with high resolution fluorescence microscopy, intense fluorescent staining characteristically decorated the periphery of triglyceride droplets (TGD) as well as the plasma membrane (PM) of fat cells. Filipin-staining was not visible inside the lipid droplets. Purification of TGD by subcellular fractionation demonstrated that the rise in total FC content of adipocytes upon differentiation was attributable to an increase in TGD-FC, which contributed up to one third of the total cellular FC. The protein component of purified TGD from cultured adipocytes as well as from murine adipocytes obtained from fresh tissues contained the lumenal endoplasmic reticulum (ER) immunoglobulin binding protein (BiP) and the integral ER membrane protein calnexin. Efflux experiments using the extracellular FC acceptors (&bgr;)-cyclodextrin or apolipoprotein A-I demonstrated that TGD-associated FC was releasable from TGD. Whereas FC efflux from adipocytes was unaffected in the presence of brefeldin A or monensin, the secretion of a control protein, lipoprotein lipase, was effectively reduced. In summary, our findings identify the TGD surface layer as primary intracellular storage site for FC within adipocytes. We suggest that the structural role of ER-resident proteins in this adipocyte TGD envelope has been previously neglected. Our findings support the suggestion that an ER-like structure, albeit of modified lipid composition, constitutes the lipid droplets' surface layer. Finally, the efflux process of FC from adipocytes upon extracellular stimulation with (beta)-cyclodextrin provides evidence for an energy-dependent intracellular trafficking route between the TGD-FC pool and the PM-FC sites which is distinct from the secretory pathway of proteins.

Topics: Adipocytes; Animals; Apolipoprotein A-I; beta-Cyclodextrins; Biological Transport; Brefeldin A; Calcium-Binding Proteins; Calnexin; Carrier Proteins; Cell Differentiation; Cell Line; Cell Membrane; Cells, Cultured; Cholesterol; Cyclodextrins; Endoplasmic Reticulum; Endoplasmic Reticulum Chaperone BiP; Filipin; Heat-Shock Proteins; Ionophores; Lipoprotein Lipase; Mice; Microscopy, Fluorescence; Molecular Chaperones; Monensin; Triglycerides