Interaction of extracellular-signal molecules with cell-surface receptors often activates a phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine and other phospholipids, generating phosphatidic acid. The activation of PLD is believed to play an important role in the regulation of cell function and cell fate. Multiple PLD activities were characterized in eukaryotic cells, and, more recently, several PLD genes have been cloned. A PLD gene superfamily, defined by a number of structural domains and sequence motifs, also includes phosphatidyltransferases and certain phosphodiesterases. Among the eukaryotic PLD genes are those from mammals, nematodes, fungi and plants. The present review focuses on the structure, localization, regulation and possible functions of cloned mammalian and yeast PLDs. In addition, an overview of plant PLD genes, and of several distinct PLD activities that have not yet been cloned, is provided. Emerging evidence from recent work employing new molecular tools indicates that different PLD isoforms are localized in distinct cellular organelles, where they are likely to serve diverse functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

Cancer chemotherapy often results in the development of multidrug resistance (MDR), which is commonly associated with overexpression of P-glycoprotein (P-gp), a plasma membrane drug efflux ATPase. It was shown recently that glycosphingolipids are elevated in MDR cells. Sphingolipids are major constituents of caveolae and of detergent-insoluble, glycosphingolipid-rich membrane domains. Here we report that multidrug-resistant HT-29 human colon adenocarcinoma cells exhibit a 12-fold overexpression of caveolin-1, a 21-kDa coat/adaptor protein of caveolae. Similar observations were made in adriamycin-resistant MCF-7 human breast adenocarcinoma cells. Caveolin-2 expression is also up-regulated in MCF-7-AdrR cells, but neither caveolin-1 nor caveolin-2 were detected in MCF-7 cells stably transfected with P-gp. The up-regulation of caveolins is associated with a 5-fold increase in the number of caveolae-like structures observed in plasma membrane profiles of HT-29-MDR cells and with the appearance of a comparable number of caveolae in MCF-7-AdrR cells. A significant fraction (ϳ40%) of cellular P-gp is localized in low density detergent-insoluble membrane fractions derived from either HT-29-MDR or MCF-7-AdrR cells. The distribution of recombinant P-gp in stably transfected MCF-7 cells was similar, even though these cells do not express caveolins and are devoid of caveolae. The possibility that caveolae contribute to the multidrug resistance and thus are coselected with P-gp during the acquisition of this phenotype is discussed.Although chemotherapy improves long term survival in cancer patients, the treatment often results in the development of tumors that are resistant to most cytotoxic drugs commonly used in chemotherapy, leading to an untreatable and incurable disease (1). Known as multidrug resistance (MDR), 1 this phenomenon may be defined as the ability of cancer cells exposed to a given drug to resist the cytotoxic actions of a broad range of structurally and functionally unrelated drugs. MDR is often caused by overexpression of a plasma membrane ATPase called P-glycoprotein (P-gp) (2). P-gp acts as an energy-dependent drug efflux pump, increasing outward transport of active drugs and thereby decreasing their intracellular concentration and reducing their cytotoxic efficacy. However, additional mechanisms that contribute to MDR have been described (see Ref. 3 for review). Recent studies have indicated that glucosylceramide accumulates to a major extent in various types of MDR cells (4). Glucosylceramide and other glycosphingolipids are important constituents of detergent-insoluble membrane domains termed DIGs (5) that are enriched also in sphingomyelin and cholesterol (6). DIGs are related in their lipid composition and their insolubility in cold non-ionic detergents to nonclathrin-coated, plasma membrane vesicular invaginations termed caveolae (reviewed in Ref. 7). Caveolin-1, a 21-kDa integral membrane protein, is a major caveolar coat protein (8) that has the ability to engage in complex interactions with other...

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