Cholesterol is important in control of EGF receptor kinase activity but EGF receptors are not concentrated in caveolae

Protein kinase A intersects SRC signaling in membrane microdomains

Regulation of Src kinase activity is tightly coupled to the phosphorylation status of the C-terminal regulatory tyrosine Tyr(527), which, when phosphorylated by Csk, represses Src. Here, we demonstrate that activation of Csk through a prostaglandin E(2)-cAMP-protein kinase A (PKA) pathway inhibits Src. This inhibitory pathway is operative in detergent-resistant membrane fractions where cAMP-elevating agents activate Csk, resulting in a concomitant decrease in Src activity. The inhibitory effect on Src depends on a detergent-resistant membrane-anchored Csk and co-localization of all components of the inhibitory pathway in membrane microdomains. Furthermore, epidermal growth factor-induced activation of Src and phosphorylation of the Src substrates Cbl and focal adhesion kinase are inhibited by activation of the cAMP-PKA-Csk pathway. We propose a novel mechanism whereby G protein-coupled receptors inhibit Src signaling by activation of Csk in a cAMP-PKA-dependent manner.

Lipid rafts: bringing order to chaos

Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids. They exist as distinct liquid-ordered regions of the membrane that are resistant to extraction with nonionic detergents. Rafts appear to be small in size, but may constitute a relatively large fraction of the plasma membrane. While rafts have a distinctive protein and lipid composition, all rafts do not appear to be identical in terms of either the proteins or the lipids that they contain. A variety of proteins, especially those involved in cell signaling, have been shown to partition into lipid rafts. As a result, lipid rafts are thought to be involved in the regulation of signal transduction. Experimental evidence suggests that there are probably several different mechanisms through which rafts control cell signaling. For example, rafts may contain incomplete signaling pathways that are activated when a receptor or other required molecule is recruited into the raft. Rafts may also be important in limiting signaling, either by physical sequestration of signaling components to block nonspecific interactions, or by suppressing the intrinsic activity of signaling proteins present within rafts. This review provides an overview of the physical characteristics of lipid rafts and summarizes studies that have helped to elucidate the role of lipid rafts in signaling via receptor tyrosine kinases and G protein-coupled receptors.

Relationship between cholesterol trafficking and signaling in rafts and caveolae

Caveolae and lipid rafts are two distinct populations of free cholesterol, sphingolipid (FC/SPH)-rich cell surface microdomains. They differ in stability, shape, and the presence or absence of caveolin (present in caveolae) or GPI-anchored proteins (enriched in lipid rafts). In primary cells, caveolae and rafts support the assembly of different signaling complexes, though signal transduction from both is strongly dependent on the presence of FC. It was initially thought that FC promoted the formation of inactive reservoirs of signaling proteins. Recent data supports the concept of a more dynamic role for FC in caveolae and probably, also lipid rafts. It is more likely that the FC content of these domains is actively modulated as protein complexes are formed and, following signal transduction, disassembled. In transformed cell lines with few caveolae, little caveolin and a preponderance of rafts, complexes normally assembled on caveolae may function in rafts, albeit with altered kinetics. However, caveolae and lipid rafts appear not to be interconvertible. The presence of non-caveolar pools of caveolin in recycling endosomes (RE), the trans-Golgi network (TGN) and in mobile chaperone complexes is now recognized. A role in the uptake of microorganisms by cells ascribed to caveolae now seems more likely to be mediated by cell surface rafts.

Caveolae--from ultrastructure to molecular mechanisms

Almost 50 years after the first sighting of small pits that covered the surface of mammalian cells, investigators are now getting to grips with the detailed workings of these enigmatic structures that we now know as caveolae.

Caveolae: anchored, multifunctional platforms in the lipid ocean

The function of caveolae is hotly debated. It now seems clear that caveolae are stable membrane domains that are kept in place by the actin cytoskeleton. However, this stability can be perturbed, leading to caveolar internalization. Caveolae are important in the regulation of various signaling processes, such as nitric oxide activity, and in cholesterol efflux and cholesterol-ester uptake. Caveolin deficiency particularly affects the cardiovascular system and the lungs but, because the knockout mice are viable, none of the proposed functions appears to be essential. Rather than having a specific function, caveolae might be considered to be multifunctional organelles with a physiological role that varies depending on cell type and cellular needs.

Cholesterol depletion from the plasma membrane triggers ligand-independent activation of the epidermal growth factor receptor

We recently demonstrated that depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) caused activation of MAPK (Chen, X., and Resh, M. D. (2001) J. Biol. Chem. 276, 34617-34623). MAPK activation was phosphatidylinositol 3-kinase (PI3K)-dependent and involved increased tyrosine phosphorylation of the p85 subunit of PI3K. We next determined whether MbetaCD treatment induced tyrosine phosphorylation of other cellular proteins. Here we report that cholesterol depletion of serum-starved COS-1 cells with MbetaCD or filipin caused an increase in Tyr(P) levels of a 180-kDa protein that was identified as the epidermal growth factor receptor (EGFR). Cross-linking experiments showed that MbetaCD induced dimerization of EGFR, indicative of receptor activation. Reagents that block release of membrane-bound EGFR ligands did not affect MbetaCD-induced tyrosine phosphorylation of EGFR, indicating that MbetaCD activation of EGFR is ligand-independent. Moreover, MbetaCD treatment resulted in increased tyrosine phosphorylation of EGFR downstream targets and Ras activation. Incubation of cells with the specific EGFR inhibitor AG4178 blocked MbetaCD-induced phosphorylation of EGFR, SHC, phospholipase C-gamma, and Gab-1 as well as MAPK activation. We conclude that cholesterol depletion from the plasma membrane by MbetaCD causes ligand-independent activation of EGFR, resulting in MAPK activation by PI3K and Ras-dependent mechanisms. Moreover, these studies reveal a novel mode of action of MbetaCD, in addition to its ability to disrupt membrane rafts.

Ganglioside induces caveolin-1 redistribution and interaction with the epidermal growth factor receptor

Although caveolin-1 is thought to facilitate the interaction of receptors and signaling components, its role in epidermal growth factor receptor (EGFR) signaling remains poorly understood. Ganglioside GM3 inhibits EGFR autophosphorylation and may thus affect the interaction of caveolin-1 and the EGFR. We report here that endogenous overexpression of GM3 leads to the clustering of GM3 on the cell membrane of the keratinocyte-derived SCC12 cell line and promotes co-immunoprecipitation of caveolin-1 and GM3 with the EGFR. Overexpression of GM3 does not affect EGFR distribution but shifts caveolin-1 to the detergent-soluble, EGFR-containing region; consistently, caveolin-1 is retained in the detergent-insoluble membrane when ganglioside is depleted. GM3 overexpression inhibits EGFR tyrosine phosphorylation and receptor dimerization and concurrently increases both the content and tyrosine phosphorylation of EGFR-associated caveolin-1, providing evidence that tyrosine phosphorylation of caveolin-1 inhibits EGFR signaling. Consistently, depletion of ganglioside both increases EGFR phosphorylation and prevents the EGF-induced tyrosine phosphorylation of caveolin-1. GM3 also induces delayed serine phosphorylation of EGFR-unassociated caveolin-1, suggesting a role for serine phosphorylation of caveolin-1 in regulating EGFR signaling. These studies suggest that GM3 modulates the caveolin-1/EGFR association and is critical for the EGF-induced tyrosine phosphorylation of caveolin-1 that is associated with its inhibition of EGFR activation.

Localization of phospholipase D1 to caveolin-enriched membrane via palmitoylation: implications for epidermal growth factor signaling

Phospholipase D (PLD) has been suggested to mediate epidermal growth factor (EGF) signaling. However, the molecular mechanism of EGF-induced PLD activation has not yet been elucidated. We investigated the importance of the phosphorylation and compartmentalization of PLD1 in EGF signaling. EGF treatment of COS-7 cells transiently expressing PLD1 stimulated PLD1 activity and induced PLD1 phosphorylation. The EGF-induced phosphorylation of threonine147 was completely blocked and the activity of PLD1 attenuated by point mutations (S2A/T147A/S561A) of PLD1 phosphorylation sites. The expression of a dominant negative PKCalpha mutant by adenovirus-mediated gene transfer greatly inhibited the phosphorylation and activation of PLD1 induced by EGF in PLD1-transfected COS-7 cells. EGF-induced PLD1 phosphorylation occurred primarily in the caveolin-enriched membrane (CEM) fraction, and the kinetics of PLD1 phosphorylation in the CEM were strongly correlated with PLD1 phosphorylation in the total membrane. Interestingly, EGF-induced PLD1 phosphorylation and activation and the coimmunoprecipitation of PLD1 with caveolin-1 and the EGF receptor in the CEM were significantly attenuated in the palmitoylation-deficient C240S/C241S mutant, which did not localize to the CEM. Immunocytochemical analysis revealed that wild-type PLD1 colocalized with caveolin-1 and the EGF receptor and that phosphorylated PLD1 was localized exclusively in the plasma membrane, although some PLD1 was also detected in vesicular structures. Transfection of wild-type PLD1 but not of C240S/C241S mutant increased EGF-induced raf-1 translocation to the CEM and ERK phosphorylation. This study shows, for the first time, that EGF-induced PLD1 phosphorylation and activation occur in the CEM and that the correct localization of PLD1 to the CEM via palmitoylation is critical for EGF signaling.

Lipid rafts and the control of neurotrophic factor signaling in the nervous system: variations on a theme

Lipid rafts are specialized, liquid-ordered subdomains of the plasma membrane. Through their ability to promote specific compartmentalization of lipids and membrane proteins, lipid rafts have emerged as membrane platforms specialized for signal transduction. In recent years, signaling by neurotrophic factors and their receptors has been shown to depend upon the integrity and function of lipid rafts and associated components. It has also been shown that these microdomains play critical roles in selective axon-dendritic sorting and the proteolytic processing of several neurotrophic ligands and receptors in neuronal cells. The available evidence supports an important role for lipid rafts in the initiation, propagation and maintenance of signal transduction events triggered by different neurotrophic factors and their receptors in the nervous system.

Cholesterol levels modulate EGF receptor-mediated signaling by altering receptor function and trafficking

A variety of signal transduction pathways including PI turnover, MAP kinase activation, and PI 3-kinase activation have been shown to be affected by changes in cellular cholesterol content. However, no information is available regarding the locus (or loci) in the pathways that are susceptible to modulation by cholesterol. We report here that depletion of cholesterol with methyl-beta-cyclodextrin increases cell surface (125)I-EGF binding by approximately 40% via a mechanism that does not involve externalization of receptors from an internal pool. Cholesterol depletion also enhances in vivo EGF receptor autophosphorylation 2-5-fold without altering the rate of receptor dephosphorylation. In vitro kinase assays, which are done under conditions where phosphotyrosine phosphatases are inhibited and receptor trafficking cannot occur, demonstrate that treatment with methyl-beta-cyclodextrin leads to an increase in intrinsic EGF receptor tyrosine kinase activity. EGF receptors are localized in cholesterol-enriched lipid rafts but are released from this compartment upon treatment with methyl-beta-cyclodextrin. These data are consistent with the interpretation that localization to lipid rafts partially suppresses the binding and kinase functions of the EGF receptor and that depletion of cholesterol releases the receptor from lipid rafts, relieving the functional inhibition of the receptor. Cholesterol depletion also inhibits EGF internalization and down-regulation of the EGF receptor, and this likely contributes to the enhanced ability of EGF to stimulate downstream signaling pathways such as the activation of MAP kinase.

Plasma membrane phospholipid scramblase 1 is enriched in lipid rafts and interacts with the epidermal growth factor receptor

We have identified physical and functional interactions between the epidermal growth factor (EGF) receptor and phospholipid scramblase 1 (PLSCR1), an endofacial plasma membrane protein proposed to affect phospholipid organization. PLSCR1, a palmitoylated protein, was found to partition with the EGF receptor in membrane lipid rafts. Cell stimulation with EGF transiently elevated Tyr-phosphorylation of PLSCR1, peaking at 5 min. Although PLSCR1 is a known substrate of c-Abl [Sun, J., et al. (2001) J.Biol. Chem. 276, 28984-28990], the Abl inhibitor STI571 did not substantially affect its EGF-dependent phosphorylation, suggesting PLSCR1 is a substrate of the EGF receptor kinase, or another EGF-activated kinase. Coinciding with phosphorylation, there was a transient increase in physical association of PLSCR1 with both the EGF receptor and the adapter protein Shc, as determined by immunoprecipitation and Western blotting. Confocal immunofluorescence analysis revealed that EGF initiates rapid internalization of both the EGF receptor and PLSCR1, with trafficking into both distinct and common endosomal pools. These data also suggested that whereas the EGF receptor is ultimately degraded, much of the endocytosed PLSCR1 is recycled to the cell surface within 3 h after EGF treatment. Consistent with this interpretation, Western blotting revealed neither ubiquitination nor proteolysis of PLSCR1 under these conditions, whereas the ubiquitination and degradation of the EGF receptor were readily confirmed. Finally, stimulation with EGF was also found to markedly increase the total cellular expression of PLSCR1, suggesting that in addition to its initial interactions with activated EGF receptor, PLSCR1 may also contribute to posttranscriptional effector pathway(s) mediating the cellular response to EGF.