Phospholipase D activation regulates endothelin-1 stimulation of phosphoinositide-specific phospholipase C in SK-N-MC cells

Endothelin-1 stimulates hydrolysis of phosphatidylcholine by phospholipases C and D in intact rat mesenteric arteries

A characteristic of endothelin-1 (ET-1)-induced contraction is the prolonged duration of the response. Phosphatidylcholine (PC) hydrolysis has been implicated in sustained agonist-induced effects through the formation of the second messengers phosphatidic acid (PA) and diacylglycerol (DAG). Therefore, we have investigated the activation of PC-phospholipase D (PLD) and PC-phospholipase C (PLC) in intact rat mesenteric small arteries stimulated with ET-1 and determined whether PC-derived DAG is necessary for ET-1-induced contraction. PLD activity, measured as phosphatidylethanol (PEt) production in vessels labelled with [3H] or [14C]myristate, was increased in a concentration- and time-dependent manner following ET-1 stimulation, as were [3H]PA levels, peaking at 10 min (ET-1 100 nM) and remaining above control levels for up to 20 min. Inclusion of 0.5% ethanol during ET-1 stimulation reduced [3H]PA levels, but did not alter the time course of formation. In addition, [14C]choline release was increased, confirming PLD-mediated PC hydrolysis. In contrast, DAG levels, measured by [3H]myristate labelling and mass assay, increased transiently at 5 min of ET-1 stimulation only. Analysis of the subclasses of DAG demonstrated an increase in all types of DAG without any enrichment with arachidonate-containing species, indicating PC, not inositol lipid hydrolysis was the source of DAG. The PC-PLC inhibitor D609, 2.5 microg/ml, completely abolished the ET-1-induced increase in [14C]DAG without affecting the increase in [14C]PA, PLD activity or the contractile response. In [14C]choline-labelled vessels, [14C]phosphocholine levels were increased by ET-1 with a similar time course to DAG production. Removal of extracellular Ca2+ and addition of 0.1 or 2 mM EGTA completely inhibited ET-1-stimulated PLD activity. The tyrosine kinase inhibitor tyrphostin A23 (100 microM) abolished ET-1-induced [3H]PA, [3H]PEt and [14C]DAG increases, whilst the negative analogue A1 was without effect. These data suggest that ET-1 couples via a calcium-dependent tyrosine kinase mechanism to PLD and PC-PLC in vascular smooth muscle. PC-derived DAG did not appear to be necessary for the contractile response, whereas sustained formation of PA, generated by PLD activity, implies that this lipid second messenger could be involved in the prolonged contractile response to this peptide.

Overexpression of myristoylated alanine-rich C-kinase substrate enhances activation of phospholipase D by protein kinase C in SK-N-MC human neuroblastoma cells

Signal transduction can involve the activation of protein kinase C (PKC) and the subsequent phosphorylation of protein substrates, including myristoylated alanine-rich C kinase substrate (MARCKS). Previously we showed that stimulation of phosphatidylcholine (PtdCho) synthesis by PMA in SK-N-MC human neuroblastoma cells required overexpression of MARCKS, whereas PKCalpha alone was insufficient. We have now investigated the role of MARCKS in PMA-stimulated PtdCho hydrolysis by phospholipase D (PLD). Overexpression of MARCKS enhanced PLD activity 1.3-2.5-fold compared with vector controls in unstimulated cells, and 3-4-fold in cells stimulated with 100 nM PMA. PMA-stimulated PLD activity was blocked by the PKC inhibitor bisindolylmaleimide. Activation of PLD by PMA was linear with time to 60 min, whereas stimulation of PtdCho synthesis by PMA in clones overexpressing MARCKS was observed after a 15 min time lag, suggesting that the hydrolysis of PtdCho by PLD preceded synthesis. The formation of phosphatidylbutanol by PLD was greatest when PtdCho was the predominantly labelled phospholipid, indicating that PtdCho was the preferred, but not the only, phospholipid substrate for PLD. Cells overexpressing MARCKS had 2-fold higher levels of PKCalpha than in vector control cells analysed by Western blot analysis; levels of PKCbeta and PLD were similar in all clones. The loss of both MARCKS and PKCalpha expression at higher subcultures of the clones was paralleled by the loss of stimulation of PLD activity and PtdCho synthesis by PMA. Our results show that MARCKS is an essential link in the PKC-mediated activation of PtdCho-specific PLD in these cells and that the stimulation of PtdCho synthesis by PMA is a secondary response.

Overproduction of urokinase-type plasminogen activator is regulated by phospholipase D- and protein kinase C-dependent pathways in murine mammary adenocarcinoma cells

Urokinase-type plasminogen activator (uPA) initiates a proteolytic cascade with which invasive cells eliminate barriers to movement. The signaling pathways regulating uPA production in tumor cells remain unclear. We first studied the effects of n-butanol, a phospholipase D (PLD) and protein kinase C (PKC) inhibitor, on the production of uPA in murine mammary adenocarcinoma cells. Tumor cell monolayers treated during 24 h with 0.3% v/v n-butanol, secreted 45-50% less uPA to the culture medium than control monolayers (P < 0.001) as determined by radial caseinolysis, zymography and western blot. This inhibition occurred also with 5-h treatments and remained up to 5 h after the removal of the alcohol. Treatment with the phorbol ester PMA or with EGF, strongly increased uPA production (P < 0.001). Interestingly, a mild inhibition of uPA production was observed when PMA stimulation was assayed in cotreatments with n-butanol. In contrast EGF was unable to reverse the inhibition induced by n-butanol. H7 significantly inhibited uPA activity (P < 0.001) secreted to the culture media. Furthermore, phosphatidic acid significantly stimulated uPA production meanwhile propranolol, which blocks phosphatidic acid availability, reduced it, suggesting a main regulatory role for this intermediary metabolite. These results suggest for the first time that uPA production is regulated by PLD and PKC signal transduction pathways in murine mammary adenocarcinoma cells.

Contrasting effects of phorbol ester and agonist-mediated activation of protein kinase C on phosphoinositide and Ca2+ signalling in a human neuroblastoma

The effects of protein kinase C (PKC) activation on muscarinic receptor-mediated phosphoinositide and Ca2+ signalling were examined in the human neuroblastoma, SH-SY5Y. Carbachol evoked rapid transient elevations of Ins(1,4,5)P3 and intracellular [Ca2+] followed by lower sustained elevations. Phorbol 12,13-dibutyrate (PDBu) preferentially attenuated transient phases. Removal of the transplasmalemmal Ca2+ gradient coupled with depletion of intracellular Ca2+ stores with thapsigargin also reduced carbachol-mediated Ins(1,4,5)P3 accumulation. Under these conditions, PDBu virtually abolished Ins(1,4,5)P3 responses to carbachol thereby implicating both Ca(2+)- and PKC-sensitive components. PDBu also reduced agonist-mediated accumulation of inositol phosphates and depletion of lipids, thereby eliminating an effect of PKC on Ins(1,4,5)P3 metabolism or phosphoinositide synthesis. In electroporated cells, PDBu inhibited Ins(1,4,5)P3 accumulation mediated by carbachol or guanosine 5'-[gamma-thio]-triphosphate, the latter indicating that some PDBu-sensitive elements were downstream of the receptor. The PKC inhibitor, Ro-318220, protected against PDBu but did not enhance responses to maximal concentrations of carbachol, indicating no feedback inhibition by agonist-activated PKC. Muscarinic antagonist activity of Ro-318220 complicated such assessment at low agonist concentrations. Carbachol or PDBu induced cytosol to membrane translocation of PKC alpha. This was faster and possibly greater with PDBu, which may explain the lack of feedback by agonist-activated PKC. These results indicate that, in SH-SY5Y cells, PDBu activation of PKC preferentially inhibits rapid muscarinic receptor-mediated phosphoinositide and Ca2+ responses via suppression of PtdIns(4,5)P2 hydrolysis. This is at least partially through inhibition of Gq-protein/phosphoinositidase C coupling. However, at least at high agonist concentrations, a major agonist-mediated PKC feedback is not present in these cells.