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Identification of Caveolin Sequences involved in Anchorage-dependence of cell growth

Final Activity Report Summary - ICASADEG (Identification of Caveolin Sequences involved in Anchorage-dependence of cell growth)

Development, angiogenesis, wound healing and metastasis all involve the movement of cells in response to changes in the extra-cellular environment. Cell migration and polarization are key biological processes, and both are critically regulated by two families of proteins: Integrins and small GTPases of the Rho/Rac family. Our research focuses on identifying molecular mechanisms by which they regulate migration and polarisation. Integrins regulate membrane targeting of Rac/Rho family GTPases (Del Pozo et al, EMBO J 2000, Nature Cell Biol 2002), and this is mediated by cholesterol-enriched membrane microdomains (CEMM) also named lipid rafts (Del Pozo et al, Science 2004). Internalisation of Rac binding sites in CEMM is mediated by caveolin-1 (Del Pozo et al, Nature Cell Biol 2005). Caveolin-1 inhibits anchorage-independent cell growth (AIG) and is a candidate tumour suppressor, although the mechanism(s) by which this crucial function is exerted remain(s) obscure. By sequestering phosphorylated caveolin in focal adhesions (FA), integrins inhibit CEMM endocytosis, which occurs upon its recruitment to caveolae.

These results may provide a molecular explanation for the role of caveolin-1 as tumour suppressor. The localisation of caveolin phosphorylated on Tyr-14 (pYcav) at Focal Adhesions (FA) is intriguing and suggests that this protein could have an impact in regulating FA dynamics. We have recently demonstrated a role for caveolin-1 in the establishment of cell polarity and directional migration, both intrinsic persistency and the response to external chemotactic cues (Grande-García et al, J Cell Biol , 2007).

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 Src kinase and Rho GTPases signalling.

These ex-vivo findings support the altered phenotype observed in vivo in processes such as angiongenesis and dermal wound repair, which is deficient in Cav-1 deficient mice (Grande-García et al, J Cell Biol , 2007).