Endocytosis of the Type III Transforming Growth Factor-β (TGF-β) Receptor through the Clathrin-independent/Lipid Raft Pathway Regulates TGF-β Signaling and Receptor Down-regulation

Gerard C. Blobe,Elizabeth C. Finger,Nam Y. Lee,Hye-jin You

doi:10.1074/jbc.m804741200

Abstract

Transforming growth factor-beta (TGF-beta) signals through three highly conserved cell surface receptors, the type III TGF-beta receptor (T beta RIII), the type II TGF-beta receptor (T beta RII), and the type I TGF-beta receptor (T beta RI) to regulate diverse cellular processes including cell proliferation, differentiation, migration, and apoptosis. Although T beta RI and T beta RII undergo ligand-independent endocytosis by both clathrin-mediated endocytosis, resulting in enhanced signaling, and clathrin-independent endocytosis, resulting in receptor degradation, the mechanism and function of T beta RIII endocytosis is poorly understood. T beta RIII is a heparan sulfate proteoglycan with a short cytoplasmic tail that functions as a TGF-beta superfamily co-receptor, contributing to TGF-beta signaling through mechanisms yet to be fully defined. We have reported previously that T beta RIII endocytosis, mediated by a novel interaction with beta arrestin-2, results in decreased TGF-beta signaling. Here we demonstrate that T beta RIII undergoes endocytosis in a ligand and glycosaminoglycan modification-independent and cytoplasmic domain-dependent manner, with the interaction of Thr-841 in the cytoplasmic domain of T beta RIII with beta-arrestin2 enhancing T beta RIII endocytosis. T beta RIII undergoes both clathrin-mediated and clathrin-independent endocytosis. Importantly, inhibition of the clathrin-independent, lipid raft pathway, but not of the clathrin-dependent pathway, results in decreased TGF-beta1 induced Smad2 and p38 phosphorylation, supporting a specific role for clathrin-independent endocytosis of T beta RIII in regulating both Smad-dependent and Smad-independent TGF-beta signaling.

Highlights

Entiation, adhesion, angiogenesis, and embryonic development [1,2,3,4]
This phosphorylation, on residue Thr841, promotes association of the scaffolding protein ␤-arrestin2 and endocytosis of T␤RIII and T␤RII with subsequent down-regulation of TGF-␤ signaling [25]. ␤-Arrestin2 is a member of a family of versatile adapter proteins known to regulate the signaling and internalization of certain activated G-protein-coupled receptors [25]. ␤-Arrestin2 can form specific interactions with members of the endocytic machinery, including clathrin and adaptor protein 2 (AP2), resulting in internalization of cell surface receptors, and triggering additional signaling events [26]. ␤-Arrestin2 contains a consensus mitogen-activated protein (MAP) kinase recognition sequence within its C-terminal domain, which enables it to function as a scaffold for two MAP kinase cascades, ERK1/2 and c-Jun NH2terminal kinase, potentially increasing signaling efficiency of activated receptors [26, 27]
Because we have previously demonstrated that T␤RII can phosphorylate the T␤RIII cytoplasmic domain in vivo, allowing association with ␤-arrestin2, co-internalization, and co-localization [25], we focused on this interaction by performing a cell-surface ELISA in COS7 cells overexpressing T␤RIII⌬gag Ϯ ␤-arrestin2

Summary

Introduction

Entiation, adhesion, angiogenesis, and embryonic development [1,2,3,4]. Three highly conserved and tissue-specific TGF-␤ isoforms signal through heteromeric complexes of three cell surface receptors, the type III TGF-␤ receptor (T␤RIII, or betaglycan), the type II TGF-␤ receptor (T␤RII), and the type I TGF-␤ receptor (T␤RI). We have demonstrated that T␤RII can phosphorylate T␤RIII on its cytoplasmic domain in vivo [25] This phosphorylation, on residue Thr841, promotes association of the scaffolding protein ␤-arrestin and endocytosis of T␤RIII and T␤RII with subsequent down-regulation of TGF-␤ signaling [25]. A specialized form of the structures that form on the plasma membrane in lipid raft endocytosis, are abundant in many mammalian cells and closely associate with caveolin-1 [31] This method of endocytosis, whereas not as well studied, is known to be sensitive to cholesterol depletion and has been shown to both cross-talk with and be segregated from common endosomal compartments [32]. Previous studies have demonstrated the importance of investigating this method of endocytosis, with a growing number of cell surface receptors undergoing lipid raft-mediated endocytosis: interleukin-2 receptor ␤ [33], autocrine motility factor receptor [34], SV40 [35], T␤RI, and T␤RII [8, 32]

Objectives

Results

Conclusion