Concepción Gómez-Moutón

Segregation of leading-edge and uropod components into specific lipid rafts during T cell polarization

Redistribution of specialized molecules in migrating cells develops asymmetry between two opposite cell poles, the leading edge and the uropod. We show that acquisition of a motile phenotype in T lymphocytes results in the asymmetric redistribution of ganglioside GM3- and GM1-enriched raft domains to the leading edge and to the uropod, respectively. This segregation to each cell pole parallels the specific redistribution of membrane proteins associated to each raft subfraction. Our data suggest that raft partitioning is a major determinant for protein redistribution in polarized T cells, as ectopic expression of raft-associated proteins results in their asymmetric redistribution, whereas non-raft-partitioned mutants of these proteins are distributed homogeneously in the polarized cell membrane. Both acquisition of a migratory phenotype and SDF-1α-induced chemotaxis are cholesterol depletion-sensitive. Finally, GM3 and GM1 raft redistribution requires an intact actin cytoskeleton, but is insensitive to microtubule disruption. We propose that membrane protein segregation not only between raft and nonraft domains but also between distinct raft subdomains may be an organizational principle that mediates redistribution of specialized molecules needed for T cell migration.

Membrane raft microdomains mediate lateral assemblies required for HIV-1 infection

HIV‐1 infection triggers lateral membrane diffusion following interaction of the viral envelope with cell surface receptors. We show that these membrane changes are necessary for infection, as initial gp120–CD4 engagement leads to redistribution and clustering of membrane microdomains, enabling subsequent interaction of this complex with HIV‐1 co‐receptors. Disruption of cell membrane rafts by cholesterol depletion before viral exposure inhibits entry by both X4 and R5 strains of HIV‐1, although viral replication in infected cells is unaffected by this treatment. This inhibitory effect is fully reversed by cholesterol replenishment of the cell membrane. These results indicate a general mechanism for HIV‐1 envelope glycoprotein‐mediated fusion by reorganization of membrane microdomains in the target cell, and offer new strategies for preventing HIV‐1 infection.

Membrane raft microdomains mediate front-rear polarity in migrating cells.

The acquisition of spatial and functional asymmetry between the rear and the front of the cell is a necessary step for cell chemotaxis. Insulin‐like growth factor‐I (IGF‐I) stimulation of the human adenocarcinoma MCF‐7 induces a polarized phenotype characterized by asymmetrical CCR5 chemokine receptor redistribution to the leading cell edge. CCR5 associates with membrane raft microdomains, and its polarization parallels redistribution of raft molecules, including the raft‐associated ganglioside GM1, glycosylphosphatidylinositol‐anchored green fluorescent protein and ephrinB1, to the leading edge. The non‐raft proteins transferrin receptor and a mutant ephrinB1 are distributed homogeneously in migrating MCF‐7 cells, supporting the raft localization requirement for polarization. IGF‐I stimulation of cholesterol‐depleted cells induces projection of multiple pseudopodia over the entire cell periphery, indicating that raft disruption specifically affects the acquisition of cell polarity, but not IGF‐I‐induced protrusion activity. Cholesterol depletion inhibits MCF‐7 chemotaxis, which is restored by replenishing cholesterol. Our results indicate that initial segregation between raft and non‐raft membrane proteins mediates the necessary redistribution of specialized molecules for cell migration.

Concerted Activity of Tyrosine Phosphatase SHP-2 and Focal Adhesion Kinase in Regulation of Cell Motility

ABSTRACT The coordinated interplay of substrate adhesion and deadhesion is necessary for cell motility. Using MCF-7 cells, we found that insulin-like growth factor I (IGF-I) induces the adhesion of MCF-7 to vitronectin and collagen in a dose- and time-dependent manner, suggesting that IGF-I triggers the activation of different integrins. On the other hand, IGF-I promotes the association of insulin receptor substrate 1 with the focal adhesion kinase (FAK), paxillin, and the tyrosine phosphatase SHP-2, resulting in FAK and paxillin dephosphorylation. Abrogation of SHP-2 catalytic activity with a dominant-negative mutant (SHP2-C>S) abolishes IGF-I-induced FAK dephosphorylation, and cells expressing SHP2-C>S show reduced IGF-I-stimulated chemotaxis compared with either mock- or SHP-2 wild-type-transfected cells. This impairment of cell migration is recovered by reintroduction of a catalytically active SHP-2. Interestingly, SHP-2-C>S cells show a larger number of focal adhesion contacts than wild-type cells, suggesting that SHP-2 activity participates in the integrin deactivation process. Although SHP-2 regulates mitogen-activated protein kinase activity, the mitogen-activated protein kinase kinase inhibitor PD-98059 has only a marginal effect on MCF-7 cell migration. The role of SHP-2 as a general regulator of cell chemotaxis induced by other chemotactic agents and integrins is discussed.

Statins Inhibit HIV-1 infection by down-regulating rho activity

Human immunodeficiency virus (HIV)-1 infectivity requires actin-dependent clustering of host lipid raft–associated receptors, a process that might be linked to Rho guanosine triphosphatase (GTPase) activation. Rho GTPase activity can be negatively regulated by statins, a family of drugs used to treat hypercholesterolemia in man. Statins mediate inhibition of Rho GTPases by impeding prenylation of small G proteins through blockade of 3-hydroxy-3-methylglutaryl coenzyme A reductase. We show that statins decreased viral load and increased CD4+ cell counts in acute infection models and in chronically HIV-1–infected patients. Viral entry and exit was reduced in statin-treated cells, and inhibition was blocked by the addition of l-mevalonate or of geranylgeranylpyrophosphate, but not by cholesterol. Cell treatment with a geranylgeranyl transferase inhibitor, but not a farnesyl transferase inhibitor, specifically inhibited entry of HIV-1–pseudotyped viruses. Statins blocked Rho-A activation induced by HIV-1 binding to target cells, and expression of the dominant negative mutant RhoN19 inhibited HIV-1 envelope fusion with target cell membranes, reducing cell infection rates. We suggest that statins have direct anti–HIV-1 effects by targeting Rho.

Dynamic redistribution of raft domains as an organizing platform for signaling during cell chemotaxis

Spatially restricted activation of signaling molecules governs critical aspects of cell migration; the mechanism by which this is achieved nonetheless remains unknown. Using time-lapse confocal microscopy, we analyzed dynamic redistribution of lipid rafts in chemoattractant-stimulated leukocytes expressing glycosyl phosphatidylinositol–anchored green fluorescent protein (GFP-GPI). Chemoattractants induced persistent GFP-GPI redistribution to the leading edge raft (L raft) and uropod rafts of Jurkat, HL60, and dimethyl sulfoxide–differentiated HL60 cells in a pertussis toxin–sensitive, actin-dependent manner. A transmembrane, nonraft GFP protein was distributed homogeneously in moving cells. A GFP-CCR5 chimera, which partitions in L rafts, accumulated at the leading edge, and CCR5 redistribution coincided with recruitment and activation of phosphatidylinositol-3 kinase γ in L rafts in polarized, moving cells. Membrane cholesterol depletion impeded raft redistribution and asymmetric recruitment of PI3K to the cell side facing the chemoattractant source. This is the first direct evidence that lipid rafts order spatial signaling in moving mammalian cells, by concentrating the gradient sensing machinery at the leading edge.

The Matrix Metalloproteinase-9 Regulates the Insulin-like Growth Factor-triggered Autocrine Response in DU-145 Carcinoma Cells

The androgen-independent human prostate adenocarcinoma cell line DU-145 proliferates in serum-free medium and produces insulin-like growth factors (IGF)-I, IGF-II, and the IGF type-1 receptor (IGF-1R). They also secrete three IGF-binding proteins (IGFBP), IGFBP-2, -3, and -4. Of these, immunoblot analysis revealed selective proteolysis of IGFBP-3, yielding fragments of 31 and 19 kDa. By using an anti-IGF-I-specific monoclonal antibody (mAb), we detect surface receptor-bound IGF-I on serum-starved DU-145 cells, which activates IGF-1R and triggers a mitogenic signal. Incubation of DU-145 cells with blocking anti-IGF-I, anti-IGF-II, or anti-IGF-I plus anti-IGF-II mAb does not, however, inhibit serum-free growth of DU-145. Conversely, anti-IGF-1R mAb and IGFBP-3 inhibit DNA synthesis. IGFBP-3 also modifies the DU-145 cell cycle, decreases p34 cdc2 levels, and IGF-1R autophosphorylation. The antiproliferative IGFBP-3 activity is not IGF-independent, since des-(1–3)IGF-I, which does not bind to IGFBP-3, reverses its inhibitory effect. DU-145 also secretes the matrix metalloproteinase (MMP)-9, which can be detected in both a soluble and a membrane-bound form. Matrix metalloproteinase inhibitors, but not serpins, abrogate DNA synthesis in DU-145 associated with the blocking of IGFBP-3 proteolysis. Overexpression of an antisense cDNA for MMP-9 inhibits 80% of DU-145 cell proliferation that can be reversed by IGF-I in a dose-dependent manner. Inhibition of MMP-9 expression is also associated with a decrease in IGFBP-3 proteolysis and with reduced signaling through the IGF-1R. Our data indicate an IGF autocrine loop operating in DU-145 cells, specifically modulated by IGFBP-3, whose activity may in turn be regulated by IGFBP-3 proteases such as MMP-9.

Filamin-A regulates actin-dependent clustering of HIV receptors

Human immunodeficiency virus (HIV)-1 infection requires envelope (Env) glycoprotein gp120-induced clustering of CD4 and coreceptors (CCR5 or CXCR4) on the cell surface; this enables Env gp41 activation and formation of a complex that mediates fusion between Env-containing and target-cell membranes. Kinetic studies show that viral receptors are actively transported to the Env-receptor interface in a process that depends on plasma membrane composition and the actin cytoskeleton. The mechanisms by which HIV-1 induces F-actin rearrangement in the target cell remain largely unknown. Here, we show that CD4 and the coreceptors interact with the actin-binding protein filamin-A, whose binding to HIV-1 receptors regulates their clustering on the cell surface. We found that gp120 binding to cell receptors induces transient cofilin-phosphorylation inactivation through a RhoA–ROCK-dependent mechanism. Blockade of filamin-A interaction with CD4 and/or coreceptors inhibits gp120-induced RhoA activation and cofilin inactivation. Our results thus identify filamin-A as an adaptor protein that links HIV-1 receptors to the actin cytoskeleton remodelling machinery, which may facilitate virus infection.

Secreted MMP9 promotes angiogenesis more efficiently than constitutive active MMP9 bound to the tumor cell surface

Association of matrix metalloprotease 9 (MMP9) to the cell membrane is considered important in tumor growth and angiogenesis. To dissect this regulatory mechanism, we generated raft and non-raft MMP9 chimeras to force membrane expression in the MCF-7 human breast carcinoma cell line. MMP9 targeting to non-raft cell surface domains rendered a constitutive active membrane MMP9 form, suggesting a contribution by the lipid environment in MMP activation. We generated human breast cancer xenograft models using MCF-7 cells overexpressing secreted and membrane-anchored MMP9. The non-raft MMP9 chimera was constitutively active at the cell membrane in xenografts, but this activation did not correlate with an increase in MMP9-induced angiogenesis. Capillary number and vessel perimeter were specifically increased only in tumors overexpressing wild-type MMP9 (the secreted form); this increase was inhibited when tumors were induced in doxycycline-treated mice. Xenografts from tumor cells overexpressing wild-type MMP9 showed increased vascular endothelial growth factor (VEGF)/VEGFR2 receptor association, which was also dependent on MMP9 activity. These observations indicate that membrane location can influence MMP9 activity in vitro and in vivo, and confirm the relevance of stromal-associated, but not tumor-bound MMP9 in mediating tumor-induced angiogenesis.

CCR5 Expression Influences the Progression of Human Breast Cancer in a p53-dependent Manner

Chemokines are implicated in tumor pathogenesis, although it is unclear whether they affect human cancer progression positively or negatively. We found that activation of the chemokine receptor CCR5 regulates p53 transcriptional activity in breast cancer cells through pertussis toxin–, JAK2-, and p38 mitogen–activated protein kinase–dependent mechanisms. CCR5 blockade significantly enhanced proliferation of xenografts from tumor cells bearing wild-type p53, but did not affect proliferation of tumor xenografts bearing a p53 mutation. In parallel, data obtained in a primary breast cancer clinical series showed that disease-free survival was shorter in individuals bearing the CCR5Δ32 allele than in CCR5 wild-type patients, but only for those whose tumors expressed wild-type p53. These findings suggest that CCR5 activity influences human breast cancer progression in a p53-dependent manner.