Potocytosis is an endocytic process that is specialized for the internalization of small molecules. Recent studies on the uptake of 5-methyltetrahydrofolate by the folate receptor have suggested that the glycosyl-phosphatidylinositol anchor on this protein causes it to cluster and be internalized by caveolae instead of coated pits. To test this hypothesis directly, we have constructed a chimeric folate receptor that has the glycosyl-phosphatidylinositol anchor replaced with the transmembrane domain and cytoplasmic tail of the low density lipoprotein receptor. The cells with wild-type receptors delivered 5-methyltetrahydrofolate to the cytoplasm more rapidly than did cells expressing the chimeric receptor. This suggests that efficient delivery to the cytoplasm depends on caveolae. In sharp contrast to cells with wild-type folate receptors, cells internalizing folate by clathrin-coated pits were unable to decrease vitamin uptake when they were either folate replete or confluent.Cells use specific membrane receptors to concentrate various types of molecules before they are sequestered and delivered to the interior of the cell. There are at least two different sets of receptors: those that govern the uptake of macromolecules such as low density lipoprotein (LDL) and those that handle small molecules such as 5-methyltetrahydrofolate (5-MeTHF). Cells take up macromolecules by receptor-mediated endocytosis (1), using receptors that are internalized by clathrincoated pits. Many of these receptors use a tight ,B-turn motif (2, 3) in their cytoplasmic domain to cluster in coated pits (4, 5) prior to internalization. Small molecules, by contrast, appear to enter cells through caveolae by a process called potocytosis (6, 7). The receptors in this latter group are linked to the membrane by glycosyl-phosphatidylinositol (GPI) and it is the lipid anchor that mediates receptor clustering in association with caveolae (8-13).The molecules that are internalized by these two pathways have quite different fates. Clathrin-coated pits bud from the membrane and form vesicles that merge by a series of regulated fusion reactions with endosomes, lysosomes, and portions of the trans-Golgi network (14,15). The principal function of this pathway is to deliver macromolecules by lysosomes for hydrolytic processing or to transport them across polarized cells by transcytosis (16). Caveolae, on the other hand, seal off from the plasma membrane but appear to remain separate from other endocytic compartments. The small molecules that are concentrated within each closed caveola reach the cytoplasm by diffusing across the membrane through water-filled channels.The existence of two separate endocytic pathways, operating side-by-side in the same cell, suggests that caveolae are better able to deliver small molecules to the cytoplasm than areThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fac...
Fibrosis is characterized by loss of normal structure and function of a tissue or organ resulting from excessive fibroblast proliferation and extracellular matrix production. Currently, there is no efficient treatment for fibrosis. Herein, we test effects of the drug mithramycin, which targets the Sp1 family of transcription factors, on mRNA expression by human gingival fibroblasts. Mithramycin reduced expression of connective tissue growth factor and type I collagen mRNAs. Microarray profiling revealed that mithramycin selectively blocked expression of cell proliferation and transforming growth factor-beta (TGF-β) signalling clusters. These microarray data were validated using real-time polymerase chain reaction and western blot analyses. Mithramycin suppressed expression of key profibrotic TGF-β signalling mediators, Smad3 and p300, as well as cell proliferation. Taken together, these data suggest that the Sp1 family of transcription factors may contribute to expression of fibrogenic genes in human gingival fibroblasts; drugs targeting the Sp1 family may be beneficial in treatment of fibro-proliferative diseases.
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