Caveolae are submicroscopic, plasma membrane pits that are abundant in many mammalian cell types. The past few years have seen a quantum leap in our understanding of the formation, dynamics and functions of these enigmatic structures. Caveolae have now emerged as vital plasma membrane sensors that can respond to plasma membrane stresses and remodel the extracellular environment. Caveolae at the plasma membrane can be removed by endocytosis to regulate their surface density or can be disassembled and their structural components degraded. Coat proteins, called cavins, work together with caveolins to regulate the formation of caveolae but also have the potential to dynamically transmit signals that originate in caveolae to various cellular destinations. The importance of caveolae as protective elements in the plasma membrane, and as membrane organizers and sensors, is highlighted by links between caveolae dysfunction and human diseases, including muscular dystrophies and cancer.

Summary Mechanotransduction, a key determinant of tissue homeostasis and tumor progression, is driven by intercellular adhesions, cell contractility and forces generated with the microenvironment, dependent on extracellular matrix composition, organization and compliance. Caveolin-1 (Cav1) favors cell elongation in 3D cultures and promotes Rho-and force-dependent contraction, matrix alignment and microenvironment stiffening through regulation of p190RhoGAP. In turn, microenvironment remodeling by Cav1-fibroblasts forces cell elongation. Cav1-deficient mice have disorganized stromal tissue architecture. Stroma associated with human carcinomas and melanoma metastases is enriched in Cav1-expressing carcinoma-associated fibroblasts (CAFs). Cav1 expression in breast CAFs correlates with low survival, and Cav1 depletion in CAFs decreases CAF contractility. Consistently, fibroblast expression of Cav1, through p190RhoGAP regulation, favors directional migration and invasiveness of carcinoma cells in vitro. In vivo, stromal Cav1 remodels peri- and intratumoral microenvironments to facilitate tumor invasion, correlating with increased metastatic potency. Thus, Cav1 modulates tissue responses through force-dependent architectural regulation of the microenvironment.

Fluid shear stress is a critical determinant of vascular remodeling and atherogenesis. Both integrins and the small GTPase Rho are implicated in endothelial cell responses to shear but the mechanisms are poorly understood. We now show that shear stress rapidly stimulates conformational activation of integrin avb3 in bovine aortic endothelial cells, followed by an increase in its binding to extracellular cell matrix (ECM) proteins. The shear-induced new integrin binding to ECM induces a transient inactivation of Rho similar to that seen when suspended cells are plated on ECM proteins. This transient inhibition is necessary for cytoskeletal alignment in the direction of¯ow. The results therefore de®ne the role of integrins and Rho in a pathway leading to endothelial cell adaptation to¯ow.

Translocation of the small GTP-binding protein Rac1 to the cell plasma membrane is essential for activating downstream effectors and requires integrin-mediated adhesion of cells to extracellular matrix. We report that active Rac1 binds preferentially to low-density, cholesterol-rich membranes, and specificity is determined at least in part by membrane lipids. Cell detachment triggered internalization of plasma membrane cholesterol and lipid raft markers. Preventing internalization maintained Rac1 membrane targeting and effector activation in nonadherent cells. Regulation of lipid rafts by integrin signals may regulate the location of membrane domains such as lipid rafts and thereby control domain-specific signaling events in anchorage-dependent cells.

Growth of normal cells is anchorage-dependent because signalling through multiple pathways including Erk, PI 3-kinase and Rac requires integrin-mediated cell adhesion 1 . Components of these pathways localize to low density, cholesterol-rich domains in the plasma membrane named "lipid rafts" 2 , 3 or "cholesterol enriched membrane microdomains" (CEMM) 4 . We previously reported that integrin-mediated adhesion regulates CEMM trafficking such that cell detachment from the extracellular matrix (ECM) triggers CEMM internalisation and clearance from the plasma membrane 5 . We now report that this internalisation is mediated by dynamin-2 and caveolin-1. Internalisation requires phosphorylation of caveolin-1 on tyrosine 14. A shift in localisation of phospho-caveolin-1 from focal adhesions to caveolae induces CEMM internalisation upon cell detachment, which mediates inhibition of Erk, PI 3-kinase and Rac. These data define a novel molecular mechanism for growth and tumour suppression by caveolin-1. Keywordsanchorage-dependent cell growth; cancer; integrin signalling; caveolin; cholesterol-enriched membrane microdomains (CEMM); Rho GTPases Loss of anchorage dependence of growth in vitro is closely associated with tumour growth and metastasis in vivo 1 . The effects of integrins on multiple growth regulatory pathways mediate anchorage-dependence. Conversely, anchorage-independence in cancer cells is due to constitutive activation of these pathways such that integrin-mediated adhesion is no longer Supplementary Information accompanies the paper on the Nature Cell Biology's website. Competing Interests statementThe authors declare that they have no competing financial interests. The best defined subtype of CEMM is caveolae, which contain caveolin-1, or in muscle, caveolin-3 7 . Caveolae are ∼100 nm invaginations of the plasma membrane involved in clathrin-independent membrane traffic 8 and intracellular cholesterol transport 9 . Caveolin is a 21 kD protein first identified as a substrate for the v-src tyrosine kinase, which, among several other kinases, phosphorylates caveolin on tyr14 10 , 11 . Caveolin is also implicated in modulation of signal transduction. Caveolin inhibits a number of enzymes 11 , 12 and has been identified as a candidate tumour suppressor 13 -16 . Many tumour cells show loss of caveolin expression, and its re-expression reverses anchorage-independent growth 12 . 16% of human breast cancers contain a caveolin mutation 17 and caveolin knockout mice show dramatic acceleration of tumorigenesis in response to carcinogenic stimuli 15 , 16 . NIH Public AccessCaveolin is involved in internalisation of GM1 8 . We therefore investigated the role of caveolin in CEMM internalisation in anchorage-dependent cells after integrin signalling was inactivated by detaching cells from the ECM 1 . When cells were detached, caveolin-1 showed timedependent movement from the plasma membrane to an intracellular compartment ( Fig. 1a; results quantified in 1b) on the same time scale as the raft marker GM1 5 . This...

Development, angiogenesis, wound healing, and metastasis all involve the movement of cells in response to changes in the extracellular environment. To determine whether caveolin-1 plays a role in cell migration, we have used fibroblasts from knockout mice. Caveolin-1–deficient cells lose normal cell polarity, exhibit impaired wound healing, and have decreased Rho and increased Rac and Cdc42 GTPase activities. Directional persistency of migration is lost, and the cells show an impaired response to external directional stimuli. Both Src inactivation and p190RhoGAP knockdown restore the wild-type phenotype to caveolin-1–deficient cells, suggesting that caveolin-1 stimulates normal Rho GTP loading through inactivation of the Src–p190RhoGAP pathway. These findings highlight the importance of caveolin-1 in the establishment of cell polarity during directional migration through coordination of the signaling of Src kinase and Rho GTPases.

M.A.del Pozo and L.S.Price contributed equally to this workThe small GTPase Rac regulates cytoskeletal organization, cell cycle progression, gene expression and oncogenic transformation, processes that depend upon both soluble growth factors and adhesion to the extracellular matrix (ECM). We now show that growth factors and adhesion to the ECM both contribute independently and approximately equally to Rac activation. However, activated Rac in non-adherent cells failed to stimulate the Rac effector PAK. V12 Rac or Rac activated by serum translocated to the membrane fraction of adherent cells but remained mainly cytoplasmic in suspended cells. An activated Rac mutant lacking a membrane-targeting sequence did not activate PAK in adherent cells, while mutations that forced membrane targeting restored PAK activation in suspended cells. In vitro, V12 Rac showed greater binding to membranes from adherent relative to suspended cells, indicating that cell adhesion regulated membrane binding sites for Rac. These results show that ECM regulates the ability of Rac to couple with PAK via an effect on membrane binding sites that facilitate their interaction.

Hemodynamic shear stress is a fundamental determinant of vascular remodeling and atherogenesis. Changes in focal adhesions, cytoskeletal organization and gene expression are major responses of endothelial cells to shear stress. Here, we show that activation of the small GTPase Rac is essential for gene expression and for providing spatial information for shear stress-induced cell alignment. Fluorescence resonance energy transfer (FRET) localizes activated Rac1 in the direction of¯ow. This directional Rac1 activation is downstream of shear-induced new integrin binding to extracellular matrix. Additionally, Rac1 mediates ow-induced stimulation of nuclear factor kB (NF-kB) and the subsequent expression of intercellular cell adhesion molecule 1 (ICAM-1), an adhesion receptor involved in the recruitment of leukocytes to atherosclerotic plaque. These studies provide a unifying model linking three of the main responses to shear stress that mediate both normal adaptation to hemodynamic forces and in¯ammatory dysfunction of endothelial cells in atherosclerosis.