Possible role of phospholipase D in cellular differentiation and apoptosis

Abstract

Phospholipase D (PLD) is widely distributed in mammalian cells and is implicated in a variety of physiological processes that reveal it to be a member of the signal transducing phospholipases. Recently, two related PLD isozymes, PLD1 and PLD2, were cloned. The former activity is regulated in vitro by protein kinase C and small molecular weight GTP-binding proteins (Arf and Rho family). By contrast, the basal activity of the latter is high and it is unresponsive in vitro to these activators. The cellular PLD activity and mRNA levels of these PLD isozymes drastically changed during differentiation and apoptosis in several types of cells. The general trend was that the mRNA level of PLD1 increased during differentiation, as did the observed GTPγS-dependent PLD activity which presumably derived from PLD1-specific catalysis. In contrast, the PLD activity and mRNA level of PLD1 were down-regulated during apoptosis. In addition to these PLD isozymes, there exists another PLD isozyme which is activated by unsaturated fatty acids such as oleic acid, although its molecular nature and physiological roles are not well defined. We have observed that this type of PLD activity is drastically increased during apoptosis of Jurkat T cells, which mainly possess this kind of PLD activity. These results suggest the possibility that PLD activity is controlled at the transcriptional level in certain circumstances, and that PLD plays roles in differentiation, survival and apoptosis in mammalian cells.

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.