br DGKs inhibition and signaling Local DAG levels
DGKs inhibition and signaling Local DAG levels are strictly controlled by a balance between synthesis and degradation rates. Both receptor controlled PLC-mediated production and DGK-mediated degradation are classically implicated in the control of “signaling” DAG. However, this dogma is starting to be challenged by a series of papers describing a new paradigm: in many circumstances, DGK activity must to be switched off to allow efficient DAG accumulation and downstream signaling (Fig. 2). Relevant reports are discussed below to shed light on whether receptor-induced inhibition of DGK activity represents an exception or a general mechanism for robust DAG accumulation in cells.
The first and best characterized example of DGK modulation is the impairment of DGKθ activity by active RhoA-GTPase. When Houssa and colleagues cloned and characterized DGKθ, they also reported that active RhoA binds specifically to the C-terminal portion of the DGKθ catalytic domain (Houssa et al., 1999, Houssa et al., 1997, Los et al., 2004). This interaction is specific for active RhoA; DGKθ does not bind to inactive RhoA or to other Rho-family GTPases such as Rac or Cdc42 (Houssa et al., 1999). Strikingly, binding to activated RhoA completely inhibits DGKθ catalytic activity presumably by impairing access to the lipid substrate (Houssa et al., 1999). These biochemical observations suggested that, through accumulation of newly produced DAG, RhoA-mediated inhibition of DGKθ may lead to enhanced DAG signaling in response to external stimuli. 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 terbinafine hydrochloride 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).