The type III TGFβ receptor regulates filopodia formation via a Cdc42-mediated IRSp53-N-WASP interaction in epithelial cells.

Adhesion and migration of both tumor cells and host cells are essential components of the metastatic process. Both cell adhesion and migration are accompanied by changes in the actin cytoskeleton during epithelial to mesenchymal transition, intravasation and heterotypic cell interaction with blood cells and platelets during circulation, extravasation and colonization [1]. Filopodia are thin, actin-rich plasma-membrane protrusions that function as antennae for cells to probe their environment and have an important role in cell migration [2]. The Rho (Ras homology) family small guanosine triphosphatase (GTPase) Cdc42 is a well-known regulator in the formation of filopodia [3]. Deregulation of Cdc42 has been demonstrated in several pathogenic processes including cancer, cardiovascular disease, and neuronal degenerative disease [4]. Several Cdc42 effectors N-WASP, IRSp53, PAK and MRCK have been implicated in filopodia formation. [5]. The Wiskott-Aldrich syndrome protein and neuronal-Wiskott-Aldrich syndrome protein (WASP/N-WASP) are scaffolds that link upstream signals to the activation of the ARP2/3 complex, leading to actin polymerization [6]. IRSp53 was identified as an effector for Rac1 and Cdc42, participating in filopodia and lamellipodia production. [7]. Recent studies have demonstrated that N-WASP is an essential mediator of IRSp53-induced filopodia formation. The SH3 domain of IRSp53 can bind N-WASP directly, with IRSP53 failing to induce filopodia in N-WASP KO fibroblast, while still maintaining the ability to induce lamellipodia formation and membrane ruffling [8]. IRSP53 can also generate filopodia by coupling membrane protrusion through its I-BAR domain with actin dynamics through SH4 domain binding partners including N-WASP and Mena [5]. Upstream mechanisms for regulating Cdc42:IRSP53: NWASP-mediated filopodial formations remain to be defined. The ubiquitously expressed type III TGF-β receptor (TβRIII/betaglycan) is the most abundantly expressed TGF-β superfamily receptor [9]. TβRIII is classically thought to function as a coreceptor, presenting TGF-β superfamily ligands to their respective signaling receptors [10]. TβRIII mediates TGF-β superfamily ligand dependent as well as ligand independent signaling to both Smad and non-Smad signaling pathways [9]. TβRIII has essential roles in murine and chick development [11, 12], and has been defined as a suppressor of cancer progression/metastasis suppressor, with loss of TβRIII expression correlating with disease progression, advanced stage, and/or a poorer prognosis for patients [13–15]. TβRIII/betaglycan suppresses cancer progression primarily by regulating cancer cell motility and invasion [16–19]. Mechanistically, TβRIII suppresses motility of epithelial derived cancer cells via activation of Cdc42 in a β-arrestin2 dependent manner [18] and by regulating cell adhesion via integrin α5β1 trafficking [20], which can act upstream of Cdc42 as well [21]. Here we examine the mechanism by which TβRIII regulates Cdc42 function in MCF10A and HMEC mammary epithelial cells.