Rapid desensitization and resensitization of bombesin-stimulated phospholipase D activity in Swiss 3T3 cells

Intrathecal bombesin is sympathoexcitatory and pressor in rat

Bombesin, a 14 amino-acid peptide, is pressor when administered intravenously in rat and pressor and sympathoexcitatory when applied intracerebroventricularly. To determine the spinal effects of bombesin, the peptide was administered acutely in the intrathecal space at around thoracic spinal cord level six of urethane-anesthetized, paralyzed, and bilaterally vagotomized rats. Blood pressure, heart rate, splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, and end-tidal CO(2) were monitored to evaluate changes in the cardiorespiratory systems. Bombesin elicited a long-lasting excitation of sSNA associated with an increase in blood pressure and tachycardia. There was a mean increase in arterial blood pressure of 52 ± 5 mmHg (300 μM; P < 0.01). Heart rate and sSNA also increased by 40 ± 4 beats/min (P < 0.01) and 162 ± 33% (P < 0.01), respectively. Phrenic nerve amplitude (PNamp, 73 ± 8%, P < 0.01) and phrenic expiratory period (+0.16 ± 0.02 s, P < 0.05) increased following 300 μM bombesin. The gain of the sympathetic baroreflex increased from -2.8 ± 0.7 to -5.4 ± 0.9% (P < 0.01), whereas the sSNA range was increased by 99 ± 26% (P < 0.01). During hyperoxic hypercapnia (10% CO(2) in O(2), 90 s), bombesin potentiated the responses in heart rate (-25 ± 5 beats/min, P < 0.01) and sSNA (+136 ± 29%, P < 0.001) but reduced PNamp (from 58 ± 6 to 39 ± 7%, P < 0.05). Finally, ICI-216,140 (1 mM), an in vivo antagonist for the bombesin receptor 2, attenuated the effects of 300 μM bombesin on blood pressure (21 ± 7 mmHg, P < 0.01). We conclude that bombesin is sympathoexcitatory at thoracic spinal segments. The effect on phrenic nerve activity may the result of spinobulbar pathways and activation of local motoneuronal pools.

Endogenously generated 2-arachidonoylglycerol plays an inhibitory role in bombesin-induced activation of central adrenomedullary outflow in rats

We previously reported the involvement of brain diacylglycerol lipase and cyclooxygenase in intracerebroventricularly (i.c.v.) administered bombesin-induced secretion of noradrenaline and adrenaline from the adrenal medulla in rats. Diacylglycerol can be hydrolyzed by diacylglycerol lipase into 2-arachidonoylglycerol, which may be further hydrolyzed by monoacylglycerol lipase into arachidonic acid, a substrate of cyclooxygenase. 2-Arachidonoylglycerol is a major endocannabinoid, which can inhibit synaptic transmission by presynaptic cannabinoid CB(1) receptors. Released 2-arachidonoylglycerol is rapidly inactivated by uptake into cells and enzymatic hydrolysis. In the present study, we examined the involvement of brain 2-arachidonoylglycerol and its regulatory role in the bombesin-induced central activation of adrenomedullary outflow using anesthetized rats. The elevation of plasma noradrenaline and adrenaline induced by a sub-maximal dose of bombesin (1 nmol/animal, i.c.v.) was reduced by MAFP (monoacylglycerol lipase inhibitor) (0.28 and 0.7 μmol/animal, i.c.v.), JZL184 (selective monoacylglycerol lipase inhibitor) (0.7 and 1.4 μmol/animal, i.c.v.), ACEA (CB(1) receptor agonist) (0.7 and 1.4 μmol/animal, i.c.v.) and AM 404 (endocannabinoid uptake-inhibitor) (80 and 250 nmol/animal, i.c.v.), while AM 251 (CB(1) receptor antagonist) (90 and 180 nmol/animal, i.c.v.) potentiated the response induced by a small dose of bombesin (0.1 nmol/animal, i.c.v.). These results suggest a possibility that 2-arachidonoylglycerol is endogenously generated in the brain during bombesin-induced activation of central adrenomedullary outflow, thereby inhibiting the peptide-induced response by activation of brain CB(1) receptors in rats.

International Union of Pharmacology. LXVIII. Mammalian bombesin receptors: nomenclature, distribution, pharmacology, signaling, and functions in normal and disease states

The mammalian bombesin receptor family comprises three G protein-coupled heptahelical receptors: the neuromedin B (NMB) receptor (BB(1)), the gastrin-releasing peptide (GRP) receptor (BB(2)), and the orphan receptor bombesin receptor subtype 3 (BRS-3) (BB(3)). Each receptor is widely distributed, especially in the gastrointestinal (GI) tract and central nervous system (CNS), and the receptors have a large range of effects in both normal physiology and pathophysiological conditions. The mammalian bombesin peptides, GRP and NMB, demonstrate a broad spectrum of pharmacological/biological responses. GRP stimulates smooth muscle contraction and GI motility, release of numerous GI hormones/neurotransmitters, and secretion and/or hormone release from the pancreas, stomach, colon, and numerous endocrine organs and has potent effects on immune cells, potent growth effects on both normal tissues and tumors, potent CNS effects, including regulation of circadian rhythm, thermoregulation; anxiety/fear responses, food intake, and numerous CNS effects on the GI tract as well as the spinal transmission of chronic pruritus. NMB causes contraction of smooth muscle, has growth effects in various tissues, has CNS effects, including effects on feeding and thermoregulation, regulates thyroid-stimulating hormone release, stimulates various CNS neurons, has behavioral effects, and has effects on spinal sensory transmission. GRP, and to a lesser extent NMB, affects growth and/or differentiation of various human tumors, including colon, prostate, lung, and some gynecologic cancers. Knockout studies show that BB(3) has important effects in energy balance, glucose homeostasis, control of body weight, lung development and response to injury, tumor growth, and perhaps GI motility. This review summarizes advances in our understanding of the biology/pharmacology of these receptors, including their classification, structure, pharmacology, physiology, and role in pathophysiological conditions.

Brain phospholipase C/diacylglycerol lipase are involved in bombesin BB2 receptor-mediated activation of sympatho-adrenomedullary outflow in rats

Bombesin receptors are mainly divided into two subtypes: BB1 receptor (neuromedin B-preferring receptor) and BB2 receptor [gastrin-releasing peptide (GRP)-preferring receptor]. Previously, we reported that intracerebroventricularly (i.c.v.) administered bombesin elevates plasma noradrenaline and adrenaline by production of brain arachidonic acid in rats. Arachidonic acid is released mainly by phospholipase A2 (PLA2)-dependent pathway or phospholipase C (PLC)/diacylglycerol lipase-dependent pathway. In the present study, bombesin and GRP elevated plasma catecholamines in a dose-dependent manner (1 and 5 nmol/animal, i.c.v.), while neuromedin B (1, 5 and 10 nmol/animal, i.c.v.) had no effect in urethane-anesthetized rats (bombesin=GRP>>neuromedin B). The bombesin (1 nmol/animal, i.c.v.)-induced response was dose-dependently attenuated by [D-Phe6, des-Met14]-bombesin (6-14) ethylamide (bombesin BB2 receptor antagonist) (15.3 and 30.6 nmol/animal, i.c.v.) and also by U-73122 (PLC inhibitor) (10 and 100 nmol/animal, i.c.v.) and RHC-80267 (diacylglycerol lipase inhibitor) (1.3 and 2.6 micromol/animal, i.c.v.). However, D-Nal-cyclo[Cys-Tyr-d-Trp-Orn-Val-Cys]-Nal-NH2 (bombesin BB1 receptor antagonist) (30 and 100 nmol/animal, i.c.v.), mepacrine (PLA2 inhibitor) (1.1 and 2.2 micromol/animal, i.c.v.) and U-73343 (inactive analog of U-73122) (100 nmol/animal, i.c.v.) had no effect. These results suggest the involvement of brain PLC/diacylglycerol lipase in the brain bombesin BB2 receptor-mediated activation of sympatho-adrenomedullary outflow in rats.

Differential recruitment of accessory molecules by FcgammaRI during monocyte differentiation

Aggregation of the human high-affinity receptor for immunoglobulin G, FcgammaRI, results in initiation of intracellular signaling cascades. However, as the receptor contains no known signaling motif, it is required to recruit an accessory molecule. The gamma chain has been proposed to fulfil this role. Here, we show that in U937 cells differentiated to a more macrophage-like phenotype with dibutyryl cAMP, FcgammaRI no longer signals through the gamma chain but rather uses FcgammaRIIa to initiate tyrosine phosphorylation. Expression of the gamma chain is, however, increased in the dbcAMP-induced cells, but here the gamma chain specifically associates with the IgA receptor, FcalphaRI. Recruitment of the gamma chain either by FcgammaRI in cytokine-primed cells or by FcalphaRI in dbcAMP-induced cells couples ligand binding to the activation of phosphatidyl choline-specific phospholipase D.

Differentiation-dependent switch in protein kinase C isoenzyme activation by FcgammaRI, the human high-affinity receptor for immunoglobulin G

Aggregation of receptors for the constant region (Fc) of immunoglobulin G on myeloid cells results in endocytosis or phagocytosis and cellular activation. Previous work has shown, using the cell line U937, that the high-affinity immunoglobulin G receptor, FcgammaRI, activates alternate intracellular signalling pathways depending on the cell differentiation state, which results in a marked change in the nature of calcium transients within the cell. Here, we show that protein kinase C (PKC) is activated in both interferon-gamma (IFN-gamma) -primed and dibutyryl cyclic AMP (dbcAMP) -differentiated cells but that the nature of the particular isoenzymes recruited differs. Thus, in IFN-gamma-primed U937 cells, FcgammaRI aggregation results in an increase of PKC activity which is essentially calcium independent resulting from the translocation to the membrane of the novel PKCs, delta and epsilon, together with the atypical PKC zeta. However, in cells differentiated to a more macrophage phenotype, all PKC enzyme activity after receptor aggregation is calcium dependent. Consistent with this finding, the isoenzymes translocated to the nuclear-free membrane fraction are the conventional PKCs alpha, beta and gamma; results consistent with our previous finding that FcgammaRI couples to phospholipase C in such dbcAMP-differentiated cells. Thus, the nature of PKC isoenzyme activated following FcgammaRI aggregation is defined by differentiation. The calcium dependence of the PKC isoenzyme is consistent with the duration of calcium transients previously reported in the two differentiation states.

Neuromedin B activates phospholipase D through both PKC-dependent and PKC-independent mechanisms

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca2+]i at 0.2 nM, in [3H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 microM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca2+-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca2+]i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca2+]i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

FcgammaRI coupling to phospholipase D initiates sphingosine kinase-mediated calcium mobilization and vesicular trafficking

Aggregation of receptors specific for the constant region of immunoglobulin G activates a repertoire of monocyte responses that can lead ultimately to targeted cell killing via antibody-directed cellular cytotoxicity. The high affinity receptor, FcgammaRI, contains no recognized signaling motif in its cytoplasmic tail but rather utilizes the gamma-chain of FcepsilonRI as an accessory molecule to recruit tyrosine kinases for signal transduction. We show here that, in a human monocytic cell line primed with interferon-gamma, FcgammaRI mobilizes intracellular calcium stores using a novel pathway that involves tyrosine kinase coupling to phospholipase D and resultant downstream activation of sphingosine kinase. Moreover, FcgammaRI is not coupled to phospholipase C; hence, calcium release from intracellular stores occurred in the absence of any measurable rise in inositol triphosphate. Finally, as this novel activation pathway is also shown to be responsible for mediating the vesicular trafficking of internalized immune complexes for degradation, it is likely to play a key role in controlling intracellular events triggered by FcgammaRI.

A molecular switch changes the signalling pathway used by the Fc gamma RI antibody receptor to mobilise calcium

BACKGROUND

Leukocytes express Fc gamma receptors, which are specific for the constant region of immunoglobulin G. Aggregation of these receptors activates a repertoire of responses that can lead to targeted cell killing by antibody-directed cellular cytotoxicity. The nature of the myeloid response to Fc gamma receptor aggregation is highly variable and depends on the maturation state of the cell, but little is known about the signalling mechanisms underlying this variability.

RESULTS

We show here that differentiation of a monocytic cell line, U937, to a more macrophage phenotype resulted in an absolute and fundamental switch in the nature of the phospholipid signalling pathway recruited following Fc gamma receptor aggregation. In cytokine-primed monocytes, aggregation of the high-affinity receptor Fc gamma RI resulted in the activation of phospholipase D and sphingosine kinase, which in turn led to the transient release of stored calcium; these effects were mediated by the gamma chain, an Fc gamma RI accessory protein. In contrast, in cells differentiated to a more macrophage type, aggregation of Fc gamma RI resulted in the Fc gamma RIIa-mediated activation of phospholipase C, and the resulting calcium response was prolonged as calcium entry was stimulated.

CONCLUSIONS

The switch in Fc gamma RI signalling pathways upon monocyte differentiation is mediated by a switch in the accessory molecule recruited by Fc gamma RI, which lacks its own intrinsic signal transduction motif. As many immune receptors have separate polypeptide chains for ligand binding and signal transduction (allowing a similar switch in signalling pathways), the mechanism described here is likely to be widely used.

Prolonged activation of phospholipase D in Chinese hamster ovary cells expressing platelet-activating-factor receptor lacking cytoplasmic C-terminal tail

The mechanism and role of phospholipase D (PLD) activation by platelet-activating factor (PAF) were examined with Chinese hamster ovary cells stably expressing wild-type PAF receptor (WT-H cells) and truncated PAF receptor lacking the C-terminal cytoplasmic tail (D-H cells). Treatment of D-H cells with PAF resulted in the rapid formation of Ins(1,4,5)P3, which was followed by a sustained phase for more than 10 min. In these cells, PAF-induced PLD activation lasted for more than 20 min. In contrast, PLD activation in WT-H cells was transient. PAF stimulation caused the biphasic formation of 1,2-diacylglycerol (DG) in both types of cell. The first phase was rapid and transient, coinciding with the Ins(1,4,5)P3 peak. The second sustained phase of DG formation was attenuated by butanol, which produces phosphatidylbutanol at the expense of phosphatidic acid (PA) by transphosphatidylation activity of PLD, and by propranolol, a selective inhibitor for PA phosphohydrolase catalysing the conversion of PA into DG. The DG level returned nearly to basal at 20 min after PAF stimulation in WT-H cells, whereas in D-H cells the elevated DG level was sustained for more than 20 min. The profile of translocation of protein kinase Calpha (PKCalpha) to membrane was similar to that of DG formation. In WT-H cells, PKCalpha was transiently associated with membranes and then returned to the cytosol. However, in D-H cells PKCalpha was rapidly translocated to and remained in membranes for more than 20 min. Butanol suppressed this sustained translocation of PKCalpha. Furthermore the mRNA levels of c-fos and c-jun by PAF in WT-H cells were much lower than those in D-H cells. Propranolol and butanol at concentrations that inhibited the formation of DG suppressed the PAF-induced mRNA expression of c-fos and c-jun. Taken together, the prolonged PLD activation in D-H cells confirmed a primary role for phospholipase C/PKC in PLD activation by PAF. Furthermore the results obtained here suggest that sustained PLD activation in turn leads to chronic activation and membrane translocation of PKCalpha, which might play an important role in the expression of c-fos and c-jun.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

Purinergic agonist and G protein stimulation of phospholipase D in rat liver plasma membranes. Independence from phospholipase C activation

Hormonal regulation of phospholipase D (PLD) was studied in isolated rat liver plasma membranes. Purinergic agents and a submaximal concentration of guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of GTP, synergistically stimulate phosphatidylethanol formation, a measure of PLD activity. The rank order of efficacy for stimulation of PLD activity in the presence of 0.2 microM GTP gamma S was beta, gamma-methylene-ATP > adenosine 5'-0-(3-thiotriphosphate) = ATP = ADP = 2-methylthio-ATP > alpha, beta-methylene-ATP = UTP. This pattern of activation does not conform to the series at known P2 receptors. GTP gamma S stimulated PLD activity in a dose-dependent manner, and the GTP gamma S dose-response curve for phosphatidylethanol formation was shifted to the left by an analog of ATP. Activation of PLD by purinergic agents in the presence of GTP gamma S supports the involvement of a purinergic receptor of the P2 class and a GTP-binding protein. Purinergic agents competitively inhibited [35S]adenosine 5'-0-(3-thiotriphosphate) binding to plasma membranes in the rank order adenosine 5'-0'(3-thiotriphosphate) > ATP > alpha,beta-methylene-ATP = UTP >> beta, gamma-methylene-ATP = ADP. Stimulation of phosphoinositide phospholipase C (PI-PLC) by purinergic agents, as measured by release of radioactivity from endogenously myo[3H]inositol-labeled plasma membranes, occurred in the order alpha, beta-methylene-ATP >> 2-methylthio-ATP. Beta, gamma-methylene-ATP had little effect on PI-PLC activity. Different dose-response relationships for agonist-stimulation of PI-PLC and PLD indicate that activation of PI-PLC is not involved in stimulation of PLD in rat liver plasma membranes, and suggest that purinergic activation of PLD occurs via a pathway involving a G protein and a heretofore uncharacterized P2 receptor.

Rapid and persistent desensitization of m3 muscarinic acetylcholine receptor-stimulated phospholipase D. Concomitant sensitization of phospholipase C

Activation of muscarinic acetylcholine receptors (mAChR) in human embryonic kidney (HEK) cells stably expressing the human m3 subtype leads to stimulation of both phospholipase C (PLC) and D (PLD). mAChR-stimulated PLD was turned off after 2 min of receptor activation with either the full (carbachol) or partial agonist (pilocarpine) and remained completely suppressed for at least 4 h. Partial recovery was observed 24 h after agonist removal. This rapid arrest of PLD response was not due to a loss of cell surface receptors and was also not caused by negative feedback due to concomitant activation of protein kinase C, tyrosine phosphorylation, increase in cytosolic calcium, or activation of Gi proteins. Furthermore, PLD stimulation by directly activated protein kinase C and GTP-binding proteins was unaltered in carbachol-pretreated cells. Finally, neither prevention of PLD stimulation during carbachol pretreatment by genistein nor inhibition of protein synthesis by cycloheximide, added before or after carbachol challenge, resulted in recovery of mAChR-stimulated PLD. The short term carbachol pretreatment nearly completely abolished agonist-induced binding of guanosine 5'-O-(3-thiotriphosphate) to membranes or permeabilized adherent cells. Full recovery of this response was achieved after 4 h. Similar to transfected m3 mAChR, PLD stimulation by endogenously expressed purinergic receptors was also fully blunted after 2 min of agonist (ATP) treatment. Preexposure of HEK cells to either receptor agonist partially, but not completely, reduced PLD stimulation by the other agonist. In contrast to desensitization of PLD stimulation, 2 min of carbachol treatment led to a sensitization, by up to 2-fold, of mAChR-stimulated inositol phosphate formation. This supersensitivity was also observed with pilocarpine, which acted as a full agonist on PLC. On the basis of these results, we conclude that the m3 mAChR stimulates PLD and PLC in HEK cells with distinct efficiencies and with very distinct durations of each response. The rapid and long lasting desensitization of the PLD response is apparently not due to a loss of cell surface receptors or PLD activation by GTP-binding proteins, but it may involve, at least initially, an uncoupling of receptors from GTP-binding proteins and most likely a loss of an as yet undefined essential transducing component.

Desensitization and resensitization of human platelets to 5-hydroxytryptamine at the level of signal transduction

Desensitization of platelets to 5-hydroxytryptamine (5HT) (1 microM), during active removal of the agonist by the platelet 5HT-uptake system, was studied at the level of signal transduction. Desensitization to 5HT was dose-dependent and homologous. Without occupation of the 5HT2 receptor, neither an increase in cytosolic [Ca2+] (30 nM ionomycin), nor a separate or simultaneous activation of protein kinase C (by 10 microM 1-oleoyl-2-acetylglycerol), could induce desensitization to 5HT (1 microM). During the early phase of desensitization, the 5HT2 receptor was coupled to phospholipase C, whereas during the late phase of desensitization this coupling was disconnected. However, after disappearance of the agonist, the coupling in the resting platelet recovered quickly, and was nearly complete (82%) after 30 min. During this resensitization, the 5HT-inducibility of activation of phospholipase C, of the increase in cytosolic [Ca2+] and of stimulation of protein kinase C were restored in parallel. The time course for resensitization of the 5HT-induced increase in cytosolic [Ca2+] was independent of the presence of extracellular Ca2+. It is concluded that, after dissociation of 5HT from the platelet 5HT2-receptor, 5HT-induced responses rapidly resensitize. Because of its short duration and the parallelism in recovery between the different 'down-stream phospholipase C' intracellular transduction signals, it is considered that desensitization arises from a reversible change in the transduction mechanism at a step up to or including the activation of phospholipase C. Neither desensitization nor resensitization to 5HT is dependent on the presence of extracellular Ca2+.

Heterologous desensitization of bombesin- and vasopressin-stimulated phospholipase D activity in Swiss 3T3 fibroblasts

Bombesin- and vasopressin-stimulated phospholipase D (PLD) activities are rapidly desensitized in 3T3 cells, in addition both agonists are subject to heterologous desensitization. Binding studies showed that homologous desensitization was partly a result of loss of cell surface receptors, whilst heterologous desensitization was independent of receptor changes. Pretreatment with either agonist reduced subsequent GTP gamma S-stimulated PLD activity by 50% whereas a pretreatment with GTP gamma S did not attenuate the response, suggesting that the G-protein or downstream effector systems were affected by receptor activation resulting in desensitization. The desensitization of receptor-stimulated PLD activation provides support for the phospholipase functioning in a key signalling pathway.

Stimulation of phospholipase D by epidermal growth factor requires protein kinase C activation in Swiss 3T3 cells

The proposal that epidermal growth factor (EGF) activates phospholipase D (PLD) by a mechanism(s) not involving phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis was examined in Swiss 3T3 fibroblasts. EGF, basic fibroblast growth factor (bFGF), bombesin, and platelet-derived growth factor (PDGF) activated PLD as measured by transphosphatidylation of butanol to phosphatidylbutanol. The increase in inositol phosphates induced by bFGF, EGF, or bombesin was significantly enhanced by Ro-31-8220, an inhibitor of protein kinase C (PKC), suggesting that PtdIns(4,5)P2-hydrolyzing phospholipase is coupled to the receptors for these agonists but that the response is down-regulated by PKC. Activation of PLD by EGF was inhibited dose dependently by the PKC inhibitors bis-indolylmaleimide and Ro-31-8220, which also inhibited the effects of bFGF, bombesin, and PDGF. Down-regulation of PKC by prolonged treatment with 4 beta-phorbol 12-myristate 13-acetate also abolished EGF- and PDGF-stimulated phosphatidylbutanol formation. EGF and bombesin induced biphasic translocations of PKC delta and epsilon to the membrane that were detectable at 15 s. In the presence of Ro-31-8220, translocation of PKC alpha became evident, and membrane association of the delta- and epsilon-isozymes was enhanced and/or sustained in response to the two agonists. The inhibitor also enhanced EGF-stimulated [3H]diacylglycerol formation in cells preincubated with [3H]arachidonic acid, which labeled predominantly phosphatidylinositol, but inhibited [3H]diacylglycerol production in cells preincubated with [3H]myristic acid, which labeled mainly phosphatidylcholine. These data support the conclusion that EGF can stimulate diacylglycerol formation from PtdIns(4,5)P2 and that PKC performs the dual role of down-regulating this response as well as mediating phosphatidylcholine hydrolysis. In summary, all of the results of the study indicate that PLD activation by EGF is downstream of PtdIns(4,5)P2-hydrolyzing phospholipase and is dependent upon subsequent PKC activation.