Possible role of phospholipase D in cellular differentiation and apoptosis
Introduction
Phospholipase D (PLD) hydrolyzes phospholipids, especially phosphatidylcholine (PC) to phosphatidic acid (PA) and choline. PA acts as a second messenger and can be converted to other messenger molecules, such as 1,2-diacylglycerol (DG) and lysophosphatidic acid (LPA). In addition to hydrolysis, PLD also catalyzes specific transphosphatidylation reactions in which a primary alcohol acts as acceptor in place of H2O (Billah and Anthes, 1990, Exton, 1990). The production of phosphatidylalcohol is often used as an indicator for the activity of PLD. Recent cDNA cloning studies have revealed the existence of at least two isozymes (PLD1, PLD2) in mammalian cells (Hammond et al., 1995, Colley et al., 1997, Kodaki and Yamashita, 1997, Nakashima et al., 1997, Hammond et al., 1997, Lopez et al., 1998). For PLD1, two alternatively spliced forms, PLD1a and PLD1b (PLD1b is a shorter form lacking 38 amino acids) have been identified (Hammond et al., 1997). The splicing does not affect the catalytic activity of PLD1. Both PLD1a and PLD1b have low basal activity and are activated in vitro by ADP ribosylation factor (Arf), Rho family GTP-binding proteins (G proteins), and protein kinase C (PKC) in the presence of phosphatidylinositol 4,5-bisphosphate (PIP2). By contrast, PLD2 has a very high basal activity in the presence of PIP2 but is unresponsive in vitro to small G proteins and PKC (Colley et al., 1997). In addition to these, another type of PLD which is activated by unsaturated fatty acids such as oleic acid has been suggested (Massenburg et al., 1994, Okamura and Yamashita, 1994), although the molecular nature of this so-called ‘oleic acid-dependent PLD’ is not known yet.
In a budding yeast Saccharomyces cerevisiae, SPO14 which was identified as a gene essential for meiosis has been revealed to be a PLD gene (Rose et al., 1995). In mammalian cells, PLD is activated by a variety of extracellular signals in a wide range of cells and is recognized to play an important role in signal transduction. The receptor-mediated PLD activation is thought to be involved in a variety of cellular responses including rapid responses such as secretion and superoxide generation, and as well long-term responses such as proliferation, differentiation and apoptosis (Border, 1994, Klei et al., 1995, Frohman and Morris, 1996, Exton, 1997, Gomez-Cambronero and Keire, 1998). Moreover, PLD is also believed to be involved in vesicular trafficking (Chen et al., 1997, West et al., 1997) because ARF appears to be one of the regulatory factors (Brown et al., 1993, Cockcroft et al., 1994). However, the exact role and regulation of PLD in each cell response are not fully disclosed. We have recently found the evidence that the mRNA levels of PLD1 and PLD2 as well as PLD activity are markedly changed in response to stimuli which induce cell differentiation or apoptosis (Yoshimura et al., 1996, Ohguchi et al., 1997, Yoshimura et al., 1997a, Hayakawa et al., 1998, Nakashima et al., 1998). In this review the transcriptional changes of PLD isozymes during differentiation and apoptosis of several types of cells are summarized and their possible implication in these cellular processes are discussed.
Section snippets
Differentiation of promyelocytic leukemia HL60 cells
Neutrophils are attracted to inflammatory sites and undergo respiratory burst, degranulation and phagocytosis. The human promyelocytic leukemia cell line (HL60 cell) can be differentiated to neutrophil-like cells upon addition of dibutyryl cyclic AMP (dbcAMP), all-trans retinoic acid (ATRA), and dimethylsulfoxide (DMSO) (Collins, 1987). Thus, this cell-line is a useful model for studying regulatory mechanisms of neutrophil-like differentiation and functions of neutrophils. In neutrophils and
Apoptosis of C6 glioma cells
In addition to glycerophospholipids, sphingomyelin hydrolysis is induced by a variety of extracellular signals including cytokines, tumor necrosis factor-α (TNFα), interferons and chemotheraputic agents. Hydrolysis of sphingomyelin by sphingomyelinase results in the production of ceramide, which is regarded as a second messenger of differentiation and apoptosis (Obeid et al., 1993, Hannun, 1994, Nakamura et al., 1996). Cell-permeable ceramides are commonly used in studying the mechanism of
Possible implication of PLD in cell proliferation
Accumulating evidence suggests that during apoptosis, signals regulating growth and/or survival are turned off in addition to the activation of death signals (Cardone et al., 1997, Franke et al., 1997, Kothakota et al., 1997, Widmann et al., 1998). Therefore, the decrease of GTPγS-dependent PLD activity and mRNA of PLD1 suggests that this type of PLD is implicated in cell survival and/or growth. PLD has been suggested to be involved in the regulation of cell growth, since it is activated by a
Perspective
There is no doubt that PLD is involved in a variety of physiological processes. The results obtained suggest that even in resting cells PLD produces survival signals. In response to growth or differentiation signals, it is activated and increases the strength of this signal. On the other hand, apoptotic agents trigger death signal transduction as well as shutting off survival signals including GTPγS-dependent PLD activity. By contrast, unregulated activation of PLD, as observed in Jurkat T
Acknowledgements
The authors thank Dr M.A. Frohman for reading of the manuscript. This work was supported in part by Grant-in Aid for Scientific Research on Priority Areas (09273104, 09259214, 10212204), Grant-in Aid for Creative Basic Research (09NP0601), Grant-in-Aids for Scientific Research (B) (09480162) and (C) (08670143, 10670136) from The Ministry of Education, Science, Spots and Culture of Japan, Japan-U.S. Cooperative Science Program, Ichiro Kanehara Foundation, Terumo Life Science Foundation, and
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