Abstract LB-270: Regulation of the cortical actin cytoskeleton and amoeboid motility through the mDia2 formin;DIP complex
Authors: Eisenmann Kathryn 
Published: February 2011

Abstract
Tumor cells display a remarkable morphological plasticity that supports malignant progression. From abnormal survival/proliferation in host tissue to tumor egress during metastasis, dynamic remodeling of the actin and microtubule cytoskeletons drives membrane protrusions, trafficking, and adhesion events essential to cell migration. Central to this process are Rho family small GTP binding proteins and their downstream effectors, such as the Wasp/Scar activators of Arp2/3 branched actin assembly. While GTPase-activated Arp2/3 has key roles in regulated changes in cell morphology, only recently has it become clear that formins, which assembly linear actin filaments, have fundamental roles in generating protrusive cell structures, including filopodia and lamellipodia. The canonical mammalian Diaphanous (mDia)-related Formin-2 regulates the cortical actin cytoskeleton through association with Diaphanous-interacting protein (DIP). Upon interacting with mDia2, DIP inhibits mDia2's ability to nucleate, processively elongate and bundle non-branched F-actin. Direct interaction between the Formin homology-2 (FH2) domain and the leucine-rich DIP C-terminus (LRR)) disrupts the cortical actin cytoskeleton, triggering non-apoptotic plasma membrane blebbing on two-dimensional (2D) matrices. Membrane blebbing is a physiological process engaging elements of the contractile machinery and functions in amoeboid cell motility (ACM). ACM contrasts from mesenchymal-type cell motility, as it is a focal adhesion (FA)/integrin- and MMP-independent process utilizing Rho-, as opposed to Rac-based signaling mechanisms. ACM was described as a specialized mode of breast cancer cell migration and may play an essential role in metastasis. Here we demonstrate a tightly regulated complex between mDia2 and DIP that deregulates mDia2-dependent F-actin dynamics in vitro and in vivo and induces membrane blebbing. Both DIP and mDia2 are dynamically localized to blebs in constitutively blebbing cancer cells in both 2D and 3D, suggesting a critical role in cortical contractility fundamental to the amoeboid phenotype. Conversely, depleting DIP inhibits ACM and promotes elongated, filopodia-rich morphology in breast cancer cells predisposed to this type of 3D migration. We propose that DIP and mDia2 comprise a precise regulatory mechanism controlling cortical actin assembly and are required for driving cortical contractility and the amoeboid-type of cancer cell invasion. As few therapeutic treatments effectively treat metastatic disease, understanding the molecular basis of amoeboid cell movement will lend novel insight into mechanisms controlling and contributing to cancer cell migration, invasion, and metastasis and may provide a critically needed alternate therapeutic for metastatic disease.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-270.