The first demonstration that RhoA

The first demonstration that RhoA-mediated inhibition of DGKθ had physiological relevance came from genetic studies in Caenorhabditis elegans. The C. elegans ortholog of DGKθ, DGK-1, acts in motor neurons to inhibit 777 symbolism release by removing DAG (Nurrish et al., 1999). Similar to its mammalian counterpart, DGK-1 is also bound and inhibited by the RhoA ortholog RHO-1, which binds to the C-terminal DGK-1 catalytic domain (McMullan et al., 2006). Interestingly, worms expressing constitutively active RHO-1 behave very similarly to both animals exposed to the DAG analog phorbol ester and animals lacking DGK-1, whereas worms overexpressing DGK-1 resemble those that constitutively express C3 transferase, which inhibits Rho (Hajdu-Cronin et al., 1999, Lackner et al., 1999, Miller et al., 1999, Nurrish et al., 1999). Indeed, presynaptic Rho activity increases acetylcholine release by stimulating the accumulation of DAG and the DAG-binding protein UNC-13 at sites of neurotransmitter release, essentially through DGK-1 inhibition (McMullan et al., 2006). Rho-mediated control of DGKθ is an important and evolutionary conserved signaling mechanism, as we reported its relevance for adenosine mediated survival pathways operating during mammalian liver preconditioning. In liver ischemic preconditioning (IP), stimulation of adenosine A2a receptors (A2aR) prevents ischemia–reperfusion injury by promoting DAG-mediated activation of PKC (Carini et al., 2004). We observed that after IP or A2aR activation, a RhoA-mediated decrease in DGKθ activity was associated with the onset of hepatocyte tolerance to hypoxia. This inhibition of DGKθ was associated with the DAG-dependent activation of PKCδ and ɛ and of their downstream target p38 MAPK. Hepatocyte preconditioning is therefore governed by a novel signaling pathway through which adenosine-induced activation of A2aR leads to RhoA-mediated downregulation of DGKθ activity (Baldanzi et al., 2010). Such inhibition is essential for the sustained accumulation of DAG required for triggering PKC-mediated survival signals (Fig. 2A). These findings unveil the general relevance of the Rho-mediated signaling pathway linking G-protein coupled receptors to the negative regulation of DGKθ activity. Our study was also the first to demonstrate the negative regulation of a DGK isoform activity by extracellular ligands, and the significance of such inhibition for signal transduction (Baldanzi et al., 2010). Thus RhoA-mediated inhibition of DGKθ is a well-characterized pathway, conserved from C. elegans to mammalian tissues, which enables G-protein coupled receptors to regulate the extent and kinetics of DAG-mediated signaling by fine tuning DGK enzymatic activity.
Acknowledgments
Introduction Cellular membranes are composed of numerous lipids, with most having structural functions while a few have direct signal-transducing properties (Testerink and Munnik, 2011). In eukaryotes, typical signaling lipids are the phosphatidylinositol lipids (polyphosphoinositides; PPIs), certain lyso-phospholipids, diacylglycerol (DAG), and phosphatidic acid (PA) (Munnik and Testerink, 2009, Munnik and Vermeer, 2010). Several families of enzymes participate in PA production, and phospholipase D (PLD) and phospholipase C (PLC)/diacylglycerol kinase (DGK) have major roles in the stimulus-induced generation of signaling PA (Wang et al., 2014). In the PLC/DGK pathway, diacylglycerol (DAG) is rapidly phosphorylated by DGK to generate PA (Testerink and Munnik, 2005).