Localization of platelet-derived growth factor-stimulated phosphorylation cascade to caveolae

A role for caveolin in transport of cholesterol from endoplasmic reticulum to plasma membrane

Caveolin is a 22-kDa membrane protein found associated with a coat material decorating the inner membrane surface of caveolae. A remarkable feature of this protein is its ability to migrate from caveolae directly to the endoplasmic reticulum (ER) when membrane cholesterol is oxidized. We now present evidence caveolin is involved in transporting newly synthesized cholesterol from the ER directly to caveolae. MA104 cells and normal human fibroblasts transported new cholesterol to caveolae with a half-time of approximately 10 min. The cholesterol then rapidly flowed from caveolae to non-caveolae membrane. Cholesterol moved out of caveolae even when the supply of fresh cholesterol from the ER was interrupted. Treatment of cells with 10 microg/ml progesterone blocked cholesterol movement from ER to caveolae. Simultaneously, caveolin accumulated in the lumen of the ER, suggesting cholesterol transport is linked to caveolin movement. Caveolae fractions from cells expressing caveolin were enriched in cholesterol 3-4-fold, while the same fractions from cells lacking caveolin were not enriched. Cholesterol transport to the cell surface was nearly 4 times more rapid in cells expressing caveolin than in matched cells lacking caveolin.

Endothelial nitric oxide synthase is regulated by tyrosine phosphorylation and interacts with caveolin-1

The regulation of endothelial nitric oxide synthase (eNOS) by phosphorylation is poorly understood. Here, we demonstrate that eNOS is tyrosine-phosphorylated in bovine aortic endothelial cells (BAEC) using 32P metabolic labeling followed by phosphoamino acid analysis and by phosphotyrosine specific Western blotting. Treatment of BAEC with hydrogen peroxide and the protein tyrosine phosphatase inhibitor, sodium orthovanadate, increases eNOS tyrosine phosphorylation. Utilizing a novel immunoNOS assay, the increase in tyrosine phosphorylation is associated with a 50% decrease in the specific activity of the enzyme. Because eNOS is localized in plasmalemma caveolae, we examined if tyrosine phosphorylated eNOS interacts with caveolin-1, the coat protein of caveolae. Immunoprecipitation of eNOS from bovine lung microvascular endothelial cells resulted in the co-precipitation of caveolin-1. Conversely, immunoprecipitation of caveolin-1 resulted in the co-precipitation of tyrosine-phosphorylated eNOS. Thus, tyrosine phosphorylation is a novel regulatory mechanism for eNOS and caveolin-1 is the first eNOS-associated protein. Collectively, these observations provide a novel regulatory mechanism for eNOS and suggest that tyrosine phosphorylation may influence its activity, subcellular trafficking, and interaction with other caveolin-interacting proteins in caveolae.

Localization and turnover of phosphatidylinositol 4,5-bisphosphate in caveolin-enriched membrane domains

Caveolae are small, plasma membrane invaginations that have been implicated in cell signaling. In A431 cells, approximately half of the total cellular phosphatidylinositol 4,5-bisphosphate (PtdIns 4, 5-P2) was found to be localized in low density, Triton-insoluble membrane domains enriched in caveolin. Treatment of cells with either epidermal growth factor or bradykinin for 5 min at 37 degrees C resulted in approximately a 50% decrease in this caveolar PtdIns 4,5-P2 with no change in the levels of plasma membrane PtdIns 4,5-P2. These data suggest that the PtdIns 4,5-P2 present in cells is largely compartmentalized and that the caveolar PtdIns 4,5-P2 is subject to hydrolysis by hormone-stimulated phospholipase C. As growth factor receptors, seven transmembrane domain receptors, heterotrimeric G proteins, and the inositol trisphosphate receptor have all been shown to be enriched in caveolae, these findings suggest that both the generation and response to inositol trisphosphate is highly compartmentalized within the cell.

Clustered folate receptors deliver 5-methyltetrahydrofolate to cytoplasm of MA104 cells

Previously, a high affinity, glycosylphosphatidylinositol-anchored receptor for folate and a caveolae internalization cycle have been found necessary for potocytosis of 5-methyltetrahydrofolate in MA104. We now show by cell fractionation that folate receptors also must be clustered in caveolae for potocytosis. An enriched fraction of caveolae from control cells retained 65-70% of the [3H]folic acid bound to cells in culture. Exposure of cells to the cholesterol-binding drug, filipin, which is known to uncluster receptors, shifted approximately 50% of the bound [3H]folic acid from the caveolae fraction to the noncaveolae membrane fraction and markedly inhibited internalization of [3H]folic acid. An mAb directed against the folate receptor also shifted approximately 50% of the caveolae-associated [3H]folic acid to noncaveolae membrane, indicating the antibody perturbs the normal receptor distribution. Concordantly, the mAb inhibited the delivery of 5-methyl[3H]tetrahydrofolate to the cytoplasm. Receptor bound 5-methyl[3H]tetrahydrofolate moved directly from caveolae to the cytoplasm and was not blocked by phenylarsine oxide, an inhibitor of receptor-mediated endocytosis. These results suggest cell fractionation can be used to study the uptake of molecules by caveolae.