Forskolin is a diterpene that activates adenylate cyclase in a variety of mammalian cells. In addition of forskolin to blowfly salivary glands increased cyclic AMP accumulation and salivary secretion. There was a small increase in transepithelial movement of labelled Ca2+. Forskolin did not induce breakdown of labelled phosphatidylinositol or inhibit the stimulation of phosphatidylinositol breakdown caused by 5-hydroxytryptamine. These data indicate that forskolin can mimic all the effects of 5-hydroxytryptamine on salivary-gland secretion that have been attributed to cyclic AMP.
Protein kinase C (PKC) isoforms phosphorylated phospholipase C-beta1 (PLC-beta1) in vitro as follows: PKCalpha >> PKCepsilon; not PKCzeta. PLC-beta3 was not phosphorylated by PKCalpha. G-protein betagamma subunits inhibited the PKCalpha phosphorylation of PLC-beta1 in a concentration-dependent manner. Half-maximal inhibition occurred with 500 nM betagamma. G-protein betagamma subunits also antagonized the PKCalpha-mediated inhibition of PLC-beta1 enzymic activity. PKCalpha, in turn, inhibited the stimulation of PLC-beta1 activity by betagamma. There was little effect of PKCalpha on the stimulation of PLC-beta1 by alphaq/11-guanosine 5'[gamma-thio]triphosphate (GTP[S]). These findings demonstrate that G protein betagamma subunits antagonize PKCalpha regulation of PLC-beta1. Thus betagamma subunits might have a role in modulating the negative feedback regulation of this signalling system by PKC.
Guanyl-5'-yl imidodiphosphate (p[NH]ppG) stimulated a rapid phospholipase C-mediated breakdown of exogenously added phosphatidylinositol 4,5-bisphosphate (PIP2) in rat cerebral-cortical membranes, with half-maximal activation at approx. 33 microM. NaF stimulated phospholipase C activity, with half-maximal activation at 0.5 mM. Stimulation of phospholipase C activity by NaF exhibited pH optima at approx. 5.5 and 7.0, with the stimulatory activity at pH 7.0 greater than that at pH 5.5. With p[NH]ppG, only stimulation at pH 7.0 was observed. Neither p[NH]ppG nor NaF stimulated hydrolysis of added phosphatidylinositol (PI) or phosphatidylinositol 4-phosphate (PIP). Mg2+ (0.5 mM) potentiated p[NH]ppG-stimulated breakdown of PIP2. Ca2+ increased basal and p[NH]ppG-stimulated breakdown of PIP2. PI breakdown was stimulated only by high Ca2+ concentrations and was unaffected by p[NH]ppG at any Ca2+ concentration examined. These results indicate that, in cerebral-cortical membranes, activation of phospholipase C by guanine nucleotides or fluoride directly increases a phospholipase C activity which specifically hydrolyses PIP2.
The role of phosphatidic acid (PA) in regulating phospholipase C-beta(1) (PLC-beta(1)) activity was determined. PA promoted the binding of PLC-beta(1) to sucrose-loaded unilamellar vesicles (SLUV) containing phosphatidylcholine. PA increased enzymatic activity over a range of Ca(2+) concentrations and reduced the Ca(2+) concentration required for half-maximal stimulation of activity. PA did not affect the apparent K(m) for phosphatidylinositol 4, 5-bisphosphate. Lysophosphatidic acid also enhanced the binding of PLC-beta(1) to SLUV but was less effective in stimulating enzymatic activity. Diacylglycerol, phosphatidylserine, and oleic acid had little effect on activity. Anionic and neutral detergents did not stimulate activity. PA stimulation was relatively independent of acyl chain length. Dipalmitoyl-PA (16:0) was comparable to PA from egg lecithin and dimyristoyl-PA (C14:0) in stimulating activity, while dilauroyl-PA (C12:0) was slightly less effective. A 100 kDa catalytic fragment of PLC-beta(1) lacking amino acid residues C-terminal to His(880) did not bind to PA and was insensitive to stimulation by 7-15 mol % PA. Stimulation of 100 kDa enzymatic activity required 30 mol % PA. PA increased receptor-G protein stimulation of PLC-beta(1) activity in membranes. These results demonstrate that PA stimulates basal and receptor-G protein-regulated PLC-beta(1) activity. PA stimulation occurs through both a C-terminal-dependent and an independent mechanism. The C-terminal-mediated mechanism for stimulation may constitute an important pathway for conferring specific regulation of PLC-beta(1) in response to increases in cellular PA levels.
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