Cell Biology and Pathophysiology of the Diacylglycerol Kinase Family: Morphological Aspects in Tissues and Organs

Kaoru Goto*; Yasukazu Hozumi*; Tomoyuki Nakano*; Sachiko S. Saino*; Hisatake Kondo †      * Department of Anatomy and Cell Biology, Yamagata University School of Medicine, Yamagata 990-9585, Japan
† Division of Histology, Department of Cell Biology, Tohoku University Graduate School of Medical Science, Sendai 980-8575, Japan

1 Introduction

Lipid metabolism plays a pivotal role in a variety of cellular functions. It is well known that phosphoinositide (PI), a minor component of the lipid, is metabolized sequentially to produce several metabolites, including diacylglycerol (DG), phosphatidic acid (PA), and phosphatidylinositol 4,5-bisphosphate (PIP2) (Rhee and Bae, 1997). Among them, DG and PA serve not only as major intermediate products in the synthesis of several kinds of lipids but also as bioactive molecules (English et al., 1996; Wakelam, 1998).

In this regard, it should be noted that DG is not a single entity because it is also derived from phosphatidylcholine (PC) in addition to PI, and different DGs from different sources contain distinct acyl chains at the sn-1 and sn-2 position (Hodgkin et al., 1998; Wakelam, 1998). It has been reported that DG constitutes at least 50 structurally distinct molecular species, whose fatty-acyl groups can be polyunsaturated, diunsaturated, monounsaturated, or saturated. DG derived from PI is largely composed of polyunsaturated acyl chains (i.e., 1-stearoyl-2-arachidonoyl species), whereas DG originating from PC contains monounsaturated and saturated chains (Holub and Kuksis, 1978). The molecular diversity of DG is directly reflected in the diversity of PA because it is mostly produced by phosphorylation of DG. Therefore it is conceivable that different DG and PA species exert unique effects on cellular functions.

In addition, previous studies have also shown that DG (and presumably PA also) can be produced preferentially in various subcellular compartments including the plasma membrane, internal membranes, cytoskeleton, and nucleus (Divecha et al., 1991; Mazzotti et al., 1995; Nishizuka, 1992; Payrastre et al., 1991). These studies clearly show that the lipid second messenger DG may be generated locally in response to external stimuli or internal conditions in order to support different signal transduction pathways. From these data it is conceivable that the PI cycle, including DG metabolism, occurs throughout a cell and is regulated differently in each location, suggesting that PI-related molecules should also be posted at specific sites.

It was long believed that DG acts solely through the protein kinase C (PKC) family of isoforms (Becker and Hannun, 2005), but in addition to PKC DG also targets the guanine nucleotide exchange factor vav and Ras or Rap guanyl nucleotide-releasing proteins (Ron and Kazanietz, 1999). Furthermore, DG recruits a number of proteins to membrane compartments, including chimerins, protein kinase D, and the Munc 13 proteins (Topham, 2006; van Blitterswijk and Houssa, 2000). Among those a direct link is provided between DG generation and Ras activation via RasGRP (Ebinu et al., 1998; Lorenzo et al., 2001; Rong et al., 2002). RasGRP is composed of at least four members, RasGRP1–4 (Quilliam et al., 2002). All these GRP members have a pair of atypical EF hands (a calcium-binding motif) and the C1 domain, which represents a motif that is involved in the recognition of phorbol ester and DG.

PA is also believed to elicit many biological responses by itself. In fact, PA plays a role in cytoskeletal organization by inducing actin polymerization and stress fiber formation (Cross et al., 1996). It is also involved in the regulation of enzymes including inositide kinases, PAK1, PKC-ζ, Ras-GAP, and protein phosphatase 1 (Topham, 2006; van Blitterswijk and Houssa, 2000).

Under most circumstances phosphorylation of DG to PA is the major route for signaling DG metabolism. This reaction is catalyzed by an enzyme referred to as diacylglycerol kinase (DGK) (Kanoh et al., 1990). Therefore DGK is thought to play one of the central roles in lipid signal transduction because it is involved in the metabolism of two messenger molecules. Since molecular cloning of DGKα was first reported from the porcine cDNA library (Sakane et al., 1990), 10 mammalian DGK isozymes have been identified to date, including the recently cloned new member, DGKκ (Imai et al., 2005). Initially, DGK was considered to regulate PKC via attenuation of the DG signal and was often referred to as a regulator of PKC, because PKC was the only one known to be activated by DG as noted. However, recent progress in molecular biology has identified a number of “isoforms/isozymes,” which is also true for DGK and PKC and other molecules. Then, questions arose as to why so many isozymes exist, how each isozyme is involved in the metabolism of DG, and how the specificity of the DG signal is secured in a crowd of isozymes. One answer may be in the diversity of DG species containing different acyl chain compositions as described above, suggesting that different DG species might convey distinct signals (Marignani et al., 1996; Schachter et al., 1996). Another may come from the recent findings on binding partners. For example, it was reported that DGK isozymes may regulate RasGRPs in specific manners (Regier et al., 2005): DGKα regulates RasGRP1, whereas DGK ι binds and regulates RasGRP3. On the other hand, DGKζ inhibits the activities of RasGRP1, -3, and -4. In addition, it was also shown that DGK γ interacts with and activates β 2-chimerin, a Rac-specific GAP, in response to epidermal growth factor (Yasuda et al., 2007). These studies clearly show that the DGK family is intimately involved in the regulation of the initiation or promotion of cancer in an isozyme-specific manner (Regier et al., 2005). Beyond the specific interaction in each case, these findings have also suggested that a signaling specificity is guaranteed by direct molecular interaction and an on–off switch might be mediated by the enzymatic conversion of DG to PA in the molecular complex.

Diversities of molecular structures and the biological actions of DG and PA, together with their various subcellular signaling sites, as revealed by previous studies in mammalian cells, may be the reason why so many isozymes have been diversified for the DGK family in the course of evolution, though only one or a few isozymes have been reported in Escherichia coli (Preiss et al., 1986), Dictyostelium discoideum (De La Roche et al., 2002), Drosophila melanogaster (Harden et al., 1993; Masai et al., 1992, 1993), and other lower organisms (Luo et al., 2004; Topham, 2006; van Blitterswijk and Houssa, 2000). Therefore it is conceivable that each member of the DGK family plays a unique role in the regulation of the signal transduction mediated by DG and PA at distinct subcellular sites in a specific signaling complex.

A growing number of papers have been published on the functional implications of DGKs, and various findings have been reported on cultured cells using gene transfection. Despite scores of new findings, however, findings on cultured cells sometimes provoke more questions than answers. This may be partly due to the “diversity” in and around the DGK family, as is also true for other molecules. Furthermore, recent advances in molecular biology provide more experimental choices than nature can provide. In addition, combined information from different conditions and distinct cells may lead to a misunderstanding of the whole system. Therefore it is tempting to see whether the phenomenon observed in the cell culture system simulates pathophysiological conditions at an organism level.

This review focuses on pathophysiological...