Cellular and Physiological Roles for Phospholipase D1 in Cancer

Abstract

Phospholipase D enzymes have long been proposed to play multiple cell biological roles in cancer. With the generation of phospholipase D1 (PLD1)-deficient mice and the development of small molecule PLD-specific inhibitors, in vivo roles for PLD1 in cancer are now being defined, both in the tumor cells and in the tumor environment. We review here tools now used to explore in vivo roles for PLD1 in cancer and summarize recent findings regarding functions in angiogenesis and metastasis. The phospholipase D (PLD)2 enzyme superfamily is defined by a conserved catalytic site and a functional commonality of transphosphatidylation activity at phosphodiester bonds found in a wide range of substrates. PLDs are present in bacteria and viruses as well as in yeast, plants, and animals. Classical PLD enzymes hydrolyze the abundant membrane lipid phosphatidylcholine to yield the second messenger phosphatidic acid (PA) and choline (1). Most vertebrates have two classic isoforms, PLD1 (2) and PLD2 (3), which are 50% identical in protein sequence in mammals and arose from a gene duplication event early in the vertebrate lineage. PLD2 is particularly intriguing in that it has a compact gene structure (4, 5) and resides within a 50-kb intron of another gene. Other animals, such as Drosophila melanogaster and Caenorhabditis elegans, have single PLD genes that are intermediate between mammalian PLD1 and PLD2 (1, 6, 7). PLD1 and PLD2 encode an identical series of protein regions, including Pleckstrin homology (PH) and Phox (PX) domains and a phosphatidylinositol 4,5-bisphospate-interacting motif that regulate association with specific subcellular membranes during signaling events, in addition to the pair of split catalytic domains (1). PLD function has been studied using biochemical, cell biological, and now physiological approaches. Potential roles for PLD in general or for PLD1 specifically have been reported in numerous physiological settings including ones relevant to cancer such as survival signaling (8 -11), control of cell polarity (12, 13), Ras activation (14 -19), and cell migration (13, 20 -26). Moreover, a PLD1 single nucleotide polymorphism (SNP) associates with the risk of non-small cell lung cancer and increased PLD1 expression and/or PLD activity have been reported in multiple types of cancer (27)(28)(29)(30)(31)(32)(33), although the mechanisms underlying this increase and the specific advantage this confers to the tumor cells are not known. As will be discussed as well, roles for PLD1 in the tumor microenvironment have also been uncovered, specifically in relationship to platelet activation (34 -36) and angiogenesis (22,26,37).In this review, we discuss physiological roles, in particular in the context of cancer, that have been identified for PLD1 using PLD lipase activity inhibitors and genetically modified animal models. Tools Used for Study of PLD FunctionCell biological roles for PLD enzymes have long been explored using a variety of types of inhibitors, the most popular of which has involved primary alcohols. Alt...

Molecular Pathways: Targeting Diacylglycerol Kinase Alpha in Cancer

“…In addition to the DGKs, the lysophosphatidic acid acyltransferases (LPAATs) and phospholipase D (PLD) enzymes also generate PA. LPAAT and PLD enzymes have also been linked to cancer, further supporting roles for PA in malignant cells (21–24). Though all DGKs convert DAG to PA, they may still play unique roles in cancer given their different cellular localizations.…”

Targeting Phospholipid Metabolism in Cancer

“…The combination of PLD1 and autophagy inhibition was shown to synergize in inducing tumor cell apoptosis and tumor regression, providing a potential rational target for anticancer therapy (91). In earlier studies, short-chain primary alcohols were used as PLD inhibitor, but concerns were raised about insufficient suppression of PA production and off-target effects (92). Several PLD-specific inhibitors have been screened and evaluated in preclinical studies (81, 92, 93).…”

“…In earlier studies, short-chain primary alcohols were used as PLD inhibitor, but concerns were raised about insufficient suppression of PA production and off-target effects (92). Several PLD-specific inhibitors have been screened and evaluated in preclinical studies (81, 92, 93). 5-Fluoro-2-indolyl des-chlorohalopemide (FIPI), a dual inhibitor of PLD1 and PLD2 (93), has shown anticancer potential.…”

The phospholipase D superfamily as therapeutic targets

“…The development of a pharmacological approach was sparked by the identification of halopemide, a neuropsychiatric drug, as a robust PLD inhibitor [29, 30]. An analog, denoted FIPI [23], is a potent inhibitor of both PLD1 and PLD2, has a half-life and bioavailability parameters that have permitted it to be used widely in cell culture and animal studies, and thus far has phenocopied outcomes observed with knockout cells and animals and with RNAi [24, 31–34].…”

“…As discussed above, the vast majority of studies more than five years old relied on alcohol-mediated inhibition or other non-specific means of altering PLD activity and thus need to be interpreted with caution. These are discussed in extensive detail in recent reviews [29, 30]. However, studies using modern small molecule inhibitors, animals lacking PLD isoforms, and genetic models are starting to provide a clearer picture of potential utility for PLD inhibitors.…”