title: Down-regulation of transforming growth factor-beta receptors: cooperativity between the types I, II, and III receptors and modulation at the cell surface.
abstract: The types I, II, and III receptors (RI, RII, RIII) for transforming growth factor-beta (TGF-beta) become down-regulated in response to ligand, presumably via their internalization from the cell surface. This report examines the down-regulation of full-length RI, RII, and RIII in cells endogenously or transiently expressing these receptors. Down-regulation occurred rapidly (within 2 h after TGF-beta1 treatment at 37 degrees C) and showed a dose response, between 10 and 200 pM TGF-beta1, in cells expressing RI, RII, and RIII (Mv1lu and A549 cells). A comparison between Mv1Lu and mutant cell derivatives R-1B (lacking RI) or DR-26 (lacking RII) indicated that all three receptors were necessary for efficient down-regulation. Down-regulation experiments, utilizing TGF-beta-treated 293 cells transiently expressing different combinations of these receptors indicated that neither RII or RIII were down-regulated when expressed alone and that RI was required for maximal down-regulation of RII. RII and RIII were partially down-regulated when these receptors were coexpressed in the absence of RI (in R-1B and 293 cells). Surprisingly, TGF-beta receptors were partially down-regulated in Mv1Lu, A549, and 293 cells treated with TGF-beta1 at 4 degrees C. Microscopic examination of 293 cells coexpressing RI fused to green fluorescent protein (RI-GFP) and RII indicated that, after treatment with TGF-beta1 at 4 degrees C, RI-GFP formed aggregates at the cell surface at this temperature. RI-GFP was not detected at the surface of these cells after TGF-beta1 treatment at 37 degrees C. Our results suggest a two phase mechanism for TGF-beta1 receptor down-regulation involving receptor modulation (aggregation) at the cell surface and internalization.
authors.affiliation: Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, H4P 2R2, Canada. john.zwaagstra@nrc.ca
authors.forename: J C
authors.initials: JC
authors.lastname: Zwaagstra
authors.forename: Z
authors.initials: Z
authors.lastname: Kassam
authors.forename: M D
authors.initials: MD
authors.lastname: O'Connor-Mccourt
mesh_terms: D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology*; D015536:Down-Regulation; D011940:Receptor Aggregation / Q000502:physiology*; D018125:Receptors, Transforming Growth Factor beta / Q000502:physiology*
chemical_list: D018125:Receptors, Transforming Growth Factor beta