Enhancement of adenosine A2 and prostaglandin E1 receptor-mediated cAMP generation by prior exposure of Swiss 3T3 fibroblasts to Ca2+-mobilizing receptor agonists or phorbol ester

Accumulation of cyclic AMP in Swiss 3T3 cells adhering to a cellulose biomaterial substratum through interaction with adenylyl cyclase

Controlling cell shape induced by cell-substrata interaction appears of prime importance to influence subsequent biological processes such as cell migration, proliferation, differentiation or apoptosis. Studies on Swiss 3T3 fibroblasts have recently provided evidence that cell spreading is mediated by integrins and RhoA. Our previous studies showed that on Cuprophan, a cellulose membrane (CU) to which vitronectin adhesive protein is poorly adsorbed, Swiss 3T3 cells are rounded and undergo cAMP-dependent aggregation. In contrast, on a polyacrylonitrile membrane (AN69) that favours the adsorption of vitronectin and fibronectin, cells spread out and contain low concentrations of cAM P. We have now examined the parts played by the three components in the cAMP pathway (receptors, G-proteins and adenylyl cyclase itself) in cAM P-dependent cell aggregation on CU. Experiments with intact cells showed no interaction between the CU and receptors, or between the CU and G-proteins. Assays on membrane preparations plus the Mn-ATP substrate, which uncouples G-proteins and adenylyl cyclase, demonstrated that activation of the cAMP pathway by CU depends primarily on the catalytic activity of the adenylyl cyclase. These investigations provide essential data for the development of biomaterials that favour cell functionality.

Protein kinase C plays a key role in the cross-talk between intracellular signalings via prostanoid receptors in a megakaryoblastic cell line, MEG-01s

In a previous study, we characterized prostanoid and thrombin receptors expressed on a megakaryoblastic cell line, MEG-01s (Blood 78, 2328-2336, 1991). In this study, we examines the mechanism of cross-talk between intracellular Ca2+ ([Ca2+]i) and cAMP signalings through prostanoid and thrombin receptors. Addition of a thromboxane (TX)A2 mimetic (U46619 or STA2) or thrombin stimulated the formation of inositol phosphates and dose-dependently augmented a prostaglandin (PG)I2 mimetic (iloprost)- or forskolin-induced cAMP formation. 12-O-tetradecanoylphorbol-13-acetate (TPA) and ionomycin, to lesser extent, also augmented iloprost-induced cAMP formation. The enhancing effect of U46619 or TPA on cAMP formation was inhibited by prolonged pretreatment of the cells with TPA (2.5 microM, 24 h), but not with calmodulin-antagonists; W-7, W-5, or KN-62. The elevation of [Ca2+]i induced by thrombin, STA2 or PGE2 was significantly suppressed by pretreatment of the cells with TPA (100 nM) as well as cAMP mimetics such as dibutyryl cAMP (5 mM), forskolin (5 microM) and iloprost (1 microM). These results suggest the key role of PKC on the cross-talk between [Ca2+]i and cAMP signalings through prostanoid and thrombin receptors; PKC, which is activated with TXA2 or thrombin, concomitantly suppress further [Ca2+]i elevation and enhances the PGI2 receptor-mediated cAMP formation, which, in turn, suppress [Ca2+]i elevation.

Biosynthesis of the endogenous cyclic adenosine monophosphate (AMP) antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), from prostaglandin E and activated inositol polyphosphate in rat liver plasma membranes

The endogenous cyclic adenosine monophosphate (AMP) antagonist, cyclic PIP, has been identified as a prostaglandylinositol cyclic phosphate. It inhibits protein kinase A 100% and activates protein serine phosphatase about sevenfold. It is biosynthesized by an enzyme of the plasma membrane when the assay mixture contains adenosine triphosphate (ATP), Mg2+, prostaglandin E and a novel inositol polyphosphate, which cannot be substituted by commercially available inositol phosphates. This novel inositol polyphosphate is a very labile compound. On anion exchange chromatography it elutes in the range of ATP, which may indicate the presence of three phosphate groups. It adsorbs on charcoal, which suggests the presence of a hydrophobic component, possibly a guanosine. Pyrophosphates obtained from inositol 1,4- and inositol 2,4-bisphosphate are accepted by cyclic PIP synthetase for the synthesis of cyclic PIP. The biosynthesis is characterized by enzyme kinetic parameters like dependence on time, enzyme and substrate concentration. The pH optimum of the enzyme is in the range 7.5-8. The enzyme functions optimally with prostaglandin E and poorly with prostaglandin A as the substrate. The presence of fluoride in the assay causes a three- to fourfold increase in cyclic PIP synthesis, which may be correlated with activation via G proteins. These data support previous reports on the chemical structure and action of cyclic PIP. With respect to the possible isomers of cyclic PIP, these indicate that it is most likely the C4-hydroxyl group of the inositol which binds the C15-hydroxyl group of prostaglandin E. A model of hormone-stimulated synthesis of cyclic PIP is proposed: phospholipase A2 and phospholipase C, activated by G proteins upon alpha-adrenergic stimulation, liberate either unsaturated fatty acids or inositol phosphates, which are transformed to prostaglandins and to novel inositol polyphosphate with an energy-rich bond. The cyclic PIP synthetase combines these two substrates to cyclic PIP.

Evaluation of cyclic AMP accumulation in EBV-transformed human B-lymphocytes: effects of dopamine agonists, isoproterenol, prostaglandin E1, cholera toxin, forskolin, and phorbol 12-myristate-13 acetate

1. Phorbol 12-myristate-13-acetate (PMA), a protein kinase C activator, elevated cyclic AMP accumulation in EBV-transformed human B-lymphocytes, and potentiated isoproterenol-, prostaglandin- (PGE1), cholera toxin-, and forskolin-stimulated cyclic AMP accumulation. 2. The dopamine D1 receptor agonist, SKF38393 (10(-7) to 10(-5) MH, had no effect on cyclic AMP accumulation in transformed human B-lymphocytes. 3. The dopamine D2 receptor agonist, quinpirole (10(-7) to 10(-5) MH did not inhibit cyclic AMP accumulation even when cyclic AMP accumulation was maximized by the addition of PMA and forskolin. 4. These data suggest that dopamine D1- and D2-receptor coupling to a cyclic AMP generating system is not present at detectable levels in transformed human B-lymphocytes.

Indomethacin treatment causes loss of insulin action in rats: involvement of prostaglandins in the mechanism of insulin action

Glucose tolerance tests in rats showed that after indomethacin treatment plasma insulin levels rose five-fold higher than in untreated controls. Accordingly, the pancreatic islets of indomethacin-treated rats secreted insulin at a threefold higher rate. Glucose tolerance tests additionally showed that indomethacin treatment led to a retarded disposal of the elevated blood glucose. Both effects appear to be caused by an attenuation of the hormone responsiveness for insulin and noradrenaline (alpha-adrenoceptor action) by indomethacin. The following observations support this view: insulin and adrenaline (alpha-adrenoceptor action) lost their ability to lower cyclic adenosine monophosphate (AMP) levels in hepatocytes; the glycogen content of liver and skeletal muscle was reduced by 95% and 65%, respectively; in adipocytes the stimulation of glucose transport by insulin was reduced by 60%. These effects of indomethacin can be reversed by the addition of exogenous prostaglandin E (PGE), as elevated cyclic AMP synthesis was again sensitive to alpha-adrenergic inhibition in the liver. These results indicate a relationship between prostaglandins and insulin action. These effects of indomethacin could result from reduced synthesis of cyclic PIP (prostaglandylinositol cyclic phosphate), a proposed second messenger for insulin and alpha-adrenoceptor action, whose synthesis was decreased by indomethacin treatment and increased by the addition of exogenous PGE. Stimulation of glucose transport by cyclic PIP was unaffected by indomethacin treatment, in contrast to the stimulation by insulin. Inhibition of PGE and cyclic PIP synthesis resulted in a metabolic state comparable to insulin resistance in non-insulin-dependent diabetes mellitus.

Inhibition of protein kinase A fails to alter mast cell adenosine responsiveness

Adenosine activates adenylate cyclase and phospholipase C in mast cells and potentiates stimulated mediator release. To determine whether activation of adenylate cyclase is necessary for the effects of adenosine on the mast cell secretory process, a specific inhibitor of cAMP-dependent protein kinase, KT5720, was used. Antigen and adenosine each induced a rapid increase in mast cell cAMP-dependent protein kinase activity within 30 s. Preincubation with KT5720 (100 nM-10 microM) suppressed cAMP-dependent protein kinase activity and inhibited antigen-stimulated beta-hexosaminidase and leukotriene C4 releases. Adenosine retained its ability to potentiate beta-hexosaminidase release in antigen- and A23187-stimulated cells even in the presence of complete cAMP-dependent protein kinase inhibition. Mast cells rendered unresponsive to adenosine-related signals by preincubation with adenosine analogs maintained this hyporesponsiveness after incubation with KT5720. It appears that the abilities of adenosine to augment mast cell degranulation and induce receptor hyporesponsiveness are independent of changes in cAMP.

Phorbol ester enhances activation of adenylate cyclase in bovine aortic endothelial cells

Endothelial cells possess beta-adrenoceptors linked to adenylate cyclase which may regulate several aspects of endothelial cell function. The potential for this second messenger system to be modulated by protein kinase C activity was investigated. Bovine aortic endothelial cells (BAECs) were cultured in the absence or presence of phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C. Basal and forskolin-, sodium fluoride (NaF)-, and isoproterenol-stimulated adenylate cyclase activity was measured in homogenates from BAECs. beta-adrenoceptor density on membranes from BAECs was measured by 125I-iodocyanopindolol binding. Sodium dodecylsulfate-polyacrylamide gel electrophoresis of immunoprecipitated proteins was used to identify phosphorylated proteins. Pretreatment of BAECs with 100 nM PMA for 30 min increased basal adenylate cyclase activity above control levels, and also increased enzyme activity stimulated by forskolin, NaF, or isoproterenol. Pretreatment of BAECs for 60 min with 100 nM staurosporine, an inhibitor of protein kinase C, prevented the enhancement of adenylate cyclase activity caused by PMA. Treatment of BAECs with PMA did not trigger phosphorylation of the inhibitory guanine nucleotide-binding protein, and there was no change in BAEC beta-adrenoceptor density following PMA pretreatment. Exposure of BAECs to ATP or bradykinin did not mimic the effects of phorbol ester. In conclusion, activation of protein kinase C by PMA enhanced adenylate cyclase activity in BAECs. However, ATP and bradykinin which activate endothelial cell surface receptors linked to phospholipase C did not mimic this effect.

Signal-transduction mechanisms of ATP-stimulated phosphatidylcholine secretion in rat type II pneumocytes: interactions between ATP and other surfactant secretagogues

ATP stimulates phosphatidylcholine secretion in type II cells, an effect that is mediated by both adenosine A2 receptors coupled to adenylate cyclase and P2 receptors coupled to phosphoinositide-specific phospholipase C. Activation of these effector enzymes leads to formation of cAMP, diacylglycerols and inositol trisphosphate (IP3). cAMP in turn activates cAMP-dependent protein kinase, diacylglycerols activate protein kinase C and IP3 promotes Ca2+ mobilization. To further investigate the signal-transduction mechanisms mediating the ATP effect, we examined its action in combination with that of other surfactant secretagogues: 5'(N-ethylcarboxyamido)adenosine (NECA), a A2 agonist that activates adenylate cyclase; TPA (12-O-tetradecanoylphorbol-13-acetate), a direct activator of protein kinase C; and ionomycin, an ionophore that increases intracellular Ca2+. The effects of NECA, TPA and ionomycin were additive and thus consistent with independent signaling mechanisms. However, the effects of all combinations of three or four secretagogues that contained ATP were 10-20% less than additive. This suggested that ATP and other secretagogues act via common mechanisms. Calmodulin antagonists decreased the effects of ionomycin and ATP by approx. 60% and 30%, respectively, but did not decrease the effects of NECA, terbutaline or TPA. Complete inhibition of the effect of ATP was achieved with a combination of a calmodulin antagonist, an A2 antagonist and a protein kinase C inhibitor. These and previous data suggest that the stimulatory effect of ATP on phosphatidylcholine secretion in type II cells is mediated by three signal-transduction mechanisms: activation of cAMP-dependent protein kinase; activation of protein kinase C; and a calmodulin-dependent mechanism.

Potentiation of A2 purinoceptor-stimulated surfactant phospholipid secretion in primary cultures of rat type II pneumocytes

Surfactant secretion is mediated by a number of different signal-transduction mechanisms. Positive and negative interactions between different signaling pathways can have an important influence on the overall regulation of secretion. To examine interactions between the adenosine A2 receptor-mediated pathway and those involving activation of protein kinase C and a Ca++/calmodulin-dependent system, we examined the effect of the A2 agonist 5'-(N-ethylcarboxyamido) adenosine (NECA) in combination with 2 activators of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate (TPA) and dioctanoylglycerol, and the Ca++ ionophore ionomycin on phosphatidylcholine secretion in primary cultures of rat type II cells. The individual agonists increased secretion 3-5-fold over the rate in control cells. The stimulatory effects of NECA+TPA, NECA+dioctanoylglycoerol, and NECA+ionomycin were 44%, 20%, and 44% greater, respectively, than expected by addition of the effects of the individual agonists. NECA increased cAMP formation while the other agonists did not. However, the effect of NECA on cAMP formation was significantly enhanced by TPA and dioctanoylglycerol, while the duration of the increase in cAMP level was prolonged by dioctanoylglycerol and ionomycin. Although the possible involvement of other second messenger systems cannot be excluded, we speculate that the synergistic interaction between the agonists in stimulating phosphatidylcholine secretion is mediated by increased cAMP levels.

Glutamate Inhibits Adenylate Cyclase Activity in Dispersed Rat Hippocampal Cells Directly via an N-Methyl-d-Aspartate-Like Metabotropic Receptor

Three major subtypes of glutamate receptors that are coupled to cation channels--N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors--are known as ionotropic receptors in the mammalian CNS. Recently, an additional subtype that is coupled to GTP binding proteins and stimulates (or inhibits) metabolism of phosphoinositides has been proposed as a metabotropic receptor. Incubation of dispersed hippocampal cells from adult rats with glutamate or NMDA decreased forskolin-stimulated cyclic AMP (cAMP) accumulation; half-maximal effects were obtained with 5.6 +/- 2.2 and 6.4 +/- 2.3 microM, respectively. Kainate and quisqualate were less potent. The effect of glutamate was antagonized by 2,3-diaminopropionate and 2-amino-5-phosphonovalerate, NMDA/glutamate receptor antagonists, but not by 0.5 microM Joro spider toxin, a specific blocker of the AMPA receptor. The inhibitory effect of glutamate on cAMP formation was not blocked by 2 microM tetrodotoxin or by the absence of Ca2+. In hippocampal membranes, glutamate, similar to carbachol, inhibited adenylate cyclase activity in a GTP-dependent manner. These findings suggest that the glutamate inhibition of adenylate cyclase is direct and is not due to a result of the release of other neurotransmitters. The effect of glutamate on cAMP accumulation was observed in an assay medium containing 0.7 mM MgCl2, which is known to inhibit both ionotropic NMDA receptor/channels in the hippocampus and metabotropic NMDA receptors in the cerebellum. The inhibitory effect of glutamate was abolished by pertussis toxin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C differentially modulates PTH- and PGE2-sensitive adenylate cyclase in osteoblast-like cells

The effects of phorbol 12-myristate 13-acetate (PMA), a known activator of protein kinase C, on receptor-mediated stimulation of adenylate cyclase were evaluated in a rat osteosarcoma cell line (UMR-106) with the osteoblast phenotype. Pretreatment of UMR-106 cells with PMA increased parathyroid hormone (PTH)-stimulated adenylate cyclase activity and inhibited prostaglandin E2 (PGE2)-responsive enzyme activity. In addition, PMA enhanced enzyme activation by forskolin, which is thought to exert a direct stimulatory action on the catalytic subunit of adenylate cyclase. The regulatory effects of PMA were concentration dependent and of rapid onset (less than or equal to 1 min). Treatment with PMA also resulted in translocation of protein kinase C activity from the cytosol to the particulate cell fraction. Pertussis toxin, which attenuates inhibition of adenylate cyclase mediated by the inhibitory guanine nucleotide-binding regulatory protein (Gi), augmented PTH-sensitive adenylate cyclase activity and reduced the incremental increase in PTH response produced by PMA. The results suggest that activation of protein kinase C increases PTH-stimulated adenylate cyclase activity by actions on Gi and/or the catalytic subunit and decreases PGE2 responsiveness by a mechanism involving the PGE2 receptor.

Muscarinic receptor-linked elevation of cAMP in SH-SY5Y neuroblastoma cells is mediated by Ca2+ and protein kinase C

The mechanisms of muscarinic receptor-linked increase in cAMP accumulation in SH-SY5Y human neuroblastoma cells has been investigated. The dose-response relations of carbachol-induced cAMP synthesis and carbachol-induced rise in intracellular free Ca2+ were similar. The stimulated cAMP synthesis was inhibited by about 50% when cells were entrapped with the Ca2+ chelator BAPTA or in the presence of the protein kinase C (PKC) inhibitor staurosporine. Production of cAMP could be induced also by the Ca2+ ionophore, ionomycin and by TPA, an activator of PKC. When added together TPA and ionomycin had a synergistic effect. When cAMP synthesis was activated with cholera toxin, PGE1 or PGE1 + pertussis toxin carbachol stimulated cAMP production to the same extent as in control cells. Ca2+ and protein kinase C thus seem to be the mediators of muscarinic-receptor linked cAMP synthesis by a direct action on adenylate cyclase.

Phorbol ester plus calcium ionophore induces release of arachidonic acid from membrane phospholipids of a human B cell line

Binding of LA350, a lymphoblastoid human B cell line, by phorbol myristate acetate (PMA) plus a calcium ionophore, either ionomycin or A23187, produced unique alterations in the release of arachidonic acid (AA) from cellular phospholipids. After equilibrium labeling of cells with radioactive fatty acids, [14C]AA demonstrated a selective enhanced release from the cells in response to the binding of PMA plus calcium ionophore as compared to the release of [14C]stearic acid (STE), [3H]oleic acid (OLE) and [3H]palmitic acid (PAL). The major phospholipid sources of the released [14C]AA were shown to be phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. The participation of protein kinase C (PKC) in the enhanced synergistic release of [14C]AA was demonstrated by the inhibition of the release by the PKC inhibitor, staurosporine. Approximately 2-6% of the labeled AA liberated was converted to 5-hydroxyeicosatetraenoic acid by an endogenous 5-lipoxygenase. Therefore during cell activation the B cell is capable of liberating AA via a PKC-dependent mechanism, implicating AA and/or its metabolites in signal transduction.

Desensitization of the epidermal adenylate cyclase system: agonists and phorbol esters desensitize by independent mechanisms

Exposure of pig epidermis to adenylate cyclase stimulators results in receptor-specific desensitization. We investigated the nature of the agonist-induced desensitization, which was compared with the phorbol ester-induced, receptor-nonspecific desensitization. Both phorbol ester-induced desensitization and the agonist-induced desensitization were accompanied by an increase in forskolin- and cholera toxin-induced cyclic AMP accumulations. The magnitude of the increase in the agonist-induced desensitization was parallel to the degree of the initial cyclic AMP accumulation; histamine and adenosine, which increase more cyclic AMP than epinephrine, resulted in a more marked increase in forskolin- and cholera toxin-induced cyclic AMP accumulations. Similarly, epidermis desensitized to multiple receptors revealed more marked forskolin- and cholera toxin-induced cyclic AMP accumulations than epidermis desensitized to a single receptor. In contrast to the phorbol ester-induced desensitization, agonist-induced desensitization was not affected by the protein kinase C inhibitors H-7 and staurosporin. Further, agonist-induced desensitization was still inducible in phorbol ester-desensitized epidermis and vice versa. In contrast to the agonist-induced desensitization, which is accompanied by the preceding adenylate cyclase stimulation, no evidence for the stimulation of the adenylate cyclase during phorbol ester treatment was obtained. Neither agonist-induced desensitization nor phorbol ester-induced desensitization affected the content of inhibitory guanine nucleotide binding protein of the epidermis, which was monitored by the pertussis toxin (IAP)-catalyzed ADP ribosylation reaction. Our results indicate that agonist-induced desensitization and the phorbol ester-induced desensitization are independent of each other. Although both processes are characterized by increased forskolin- and toxin-induced cyclic AMP accumulations, the former is accompanied by initial cyclic AMP accumulation; the latter is not.

Modulation of responsiveness to cAMP stimulating agonists by phorbol ester in fetal rat osteoblasts

We studied the effect of activation of protein kinase C (PKC) by a phorbol ester on cAMP accumulation in fetal rat osteoblasts. Activation of PKC by phorbol 12-myristate 13-acetate (PMA) caused a potentiation of cAMP accumulation induced by parathyroid hormone (PTH), forskolin, and cholera toxin. The results suggest that the potentiating effect of PMA on PTH-induced cAMP accumulation was not due to an effect on the PTH-receptor nor to an effect on cAMP degradation, as the effect of PMA persisted in the presence of a phosphodiesterase inhibitor. Pretreatment of the cells with pertussis toxin did not prevent the action of PMA, indicating that PMA does not act via the inhibitory G-protein. PMA had a biphasic effect on prostaglandin E2 (PGE2)-induced cAMP accumulation; i.e., at concentrations greater than or equal to 10(-6) M, PMA potentiated the PGE2-induced cAMP response but PMA attenuated cAMP accumulation induced by concentrations of PGE2 less than or equal to 5.10(77) M. From our data we conclude that PKC can interact with a stimulated cAMP pathway in a stimulatory and inhibitory manner. Potentiation of cAMP accumulation is probably due to modification of the adenylate cyclase complex, whereas attenuation of stimulated cAMP accumulation appears to be due to an effect on a different site of the cAMP generating pathway, which may be specific to PGE2-induced cAMP accumulation.

Enhancement of cyclic AMP metabolism in a B cell line by protein kinase C

In a human B cell line in which we previously demonstrated an inverse relationship between cyclic adenosine monophosphate (cAMP) content and immunoglobulin secretion, the phorbol ester, phorbol myristate acetate, (PMA), was shown to augment the cAMP elevating ability of cholera toxin (CT), suggesting a regulatory linkage between the two transmembrane signaling pathways, cAMP and phospholipid (J. Immunol. 141, 1678-1686, 1988). We now extend these studies and provide additional evidence that activated protein kinase C, a principal product of the activation of the hydrolytic phospholipid pathway, plays a direct role in the augmentation of cAMP levels in cells stimulated by diverse cAMP-elevating ligands. Prostaglandin E1 (PGE1), forskolin (FSK) and CT, all of which demonstrated a concentration and time-dependent elevation of intracellular cAMP, produced even greater (up to twofold) elevations of cAMP in the presence of PMA or the diacylglycerol analogs, 1,2-dioctanoylglycerol (DiC8), and 1-oleoyl-2-acetylglycerol (OAG). In the absence of CT, PGE1, or FSK, these protein kinase C activators produced only small increases in cAMP content of the cells. Several tests of protein kinase C specificity in these PMA-, DiC8-, and OAG-induced augmentations were made: (i) only phorbol esters known to activate protein kinase C worked, (ii) PMA augmentation was abolished by down-regulation of protein kinase C, (iii) Staurosporine (a known inhibitor of protein kinase C) selectively inhibited the effects of PMA on cAMP generation and on immunoglobulin secretion in the LA350 cell line.

Regulation of mannose 6-phosphate/insulin-like growth factor II receptor distribution by activators and inhibitors of protein kinase C

The tumor-promotor phorbol dibutyrate (PDBt) increases the binding of a neoglycoprotein containing mannose 6-phosphate (Man6P) and of insulin-like growth factor II (IGF-II) to the Man6P/IGF-II receptor at the cell surface. This effect is dependent on time and concentration and is also seen with synthetic 1-oleoyl-2-acetyl-sn-glycerol, but not with 4 alpha-phorbol, an inactive tumor-promoter. The increase is due to a 3-4-fold increase in the number of cell-surface, receptors, accompanied by a 1.6-fold increase in ligand-binding affinity. The internalization rate of the Man6P/IGF-II receptor is not affected by PDBt, suggesting that the redistribution of these receptors to the cell surface is due to an accelerated externalization rate. The redistribution of Man6P/IGF-II receptors did not impair the sorting of newly synthesized Man6P-containing ligands while uptake of these ligands is 2-4-fold increased. Inactivation or down regulation of protein kinase C decreased the binding of the Man6P-containing neoglycoprotein to 65% of controls. Incubation of cells with Man6P, IGF-I, IGF-II or epidermal growth factor induces a rapid redistribution of Man6P/IGF-II receptors to the plasma membrane [Braulke, T., Tippmer, S., Neher, E. & von Figura, K. (1989) EMBO J. 8, 681-686]. Incubation with PDBt prevented the effect of growth factors but not that of Man6P on receptor redistribution. Inactivation of protein kinase C did not affect the Man6P/IGF-II receptor redistribution induced by Man6P and growth factors. These data suggest that Man6P, growth factors and activation of protein kinase C by phorbol esters and diacylglycerols modulate Man6P/IGF-II receptor cell-surface binding by at least two independent mechanisms, receptor redistribution as well as an increase of binding affinity, which might be involved in regulation of endocytosis of ligands.