Gestational hypoxia up-regulates protein kinase C and inhibits calcium-activated potassium channels in ovine uterine arteries.

OBJECTIVE: The present study tested the hypothesis that gestational hypoxia up-regulates protein kinase C (PKC) and inhibits calcium-activated potassium channels (KCa)-mediated relaxations of uterine arteries in pregnancy. STUDY DESIGN: Uterine arteries were isolated from nonpregnant (NPUA) and pregnant (PUA) (~140 day gestation) sheep maintained at either sea level or high altitude (3,820 m for 110 days, PaO2: 60 mmHg). Contractions of uterine arteries were determined. KEY FINDINGS: In normoxic PUA, selective inhibition of large-conductance KCa (BK) channels significantly enhanced PKC activator phorbol 12, 13-dibutyrate (PDBu)-induced contractions. This effect was abrogated by chronic hypoxia in gestation. Unlike BK channels, inhibition of small-conductance KCa (SK) channels had no significant effect on PDBu-mediated contractions. In normoxic PUA, activation of both BK with NS1619 or SK with NS309 produced concentration-dependent relaxations, which were not altered by the addition of PDBu. However, in uterine arteries treated with chronic hypoxia (10.5% O2 for 48 h), both NS1619- and NS309-induced relaxations were significantly attenuated by PDBu. In NPUAs, inhibition of BK channels significantly enhanced PDBu-induced contractions in both normoxic and hypoxic animals. CONCLUSION: The results suggest that in the normoxic condition BK inhibits PKC activity and uterine vascular contractility, which is selectively attenuated by chronic hypoxia during gestation. In addition, hypoxia induces PKC-mediated inhibition of BK and SK activities and relaxations of uterine arteries in pregnancy.

Phorbol ester-induced PKCepsilon down-modulation sensitizes AML cells to TRAIL-induced apoptosis and cell differentiation.

Despite the relevant therapeutic progresses made in these last 2 decades, the prognosis of acute myeloid leukemia (AML) remains poor. Phorbol esters are used at very low concentrations as differentiating agents in the therapy of myeloid leukemias. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), in turn, is a death ligand that spares normal cells and is therefore currently under clinical trials for cancer therapy. Emerging evidence, however, suggests that TRAIL is also involved in nonapoptotic functions, like cell differentiation. PKCepsilon is differentially modulated along normal hematopoiesis, and its levels modulate the response of hematopoietic precursors to TRAIL. Here, we investigated the effects of the combination of phorbol esters (phorbol ester 4-beta-phorbol-12,13-dibutyrate [PDBu]) and TRAIL in the survival/differentiation of AML cells. We demonstrate here that PDBu sensitizes primary AML cells to both the apoptogenic and the differentiative effects of TRAIL via PKCepsilon down-modulation, without affecting TRAIL receptor surface expression. We believe that the use of TRAIL in combination with phorbol esters (or possibly more specific PKCepsilon down-modulators) might represent a significative improvement of our therapeutic arsenal against AML.

Negative regulation of cyclin-dependent kinase 5 targets by protein kinase C.

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed protein serine/threonine kinase essential for brain development and implicated in synaptic plasticity, dopaminergic neurotransmission, drug addiction, and neurodegenerative disorders. Relatively little is known about the molecular mechanisms that regulate the activity of Cdk5 in vivo. In order to determine whether protein kinase C (PKC) regulates Cdk5 activity in the central nervous system, the phosphorylation levels of two Cdk5 substrates were evaluated under conditions of altered PKC activity in vivo. Treatment of acute striatal slices with a PKC-activating phorbol ester caused a time- and dose-dependent decrease in the levels of phospho-Ser6 inhibitor-1, phospho-Ser67 inhibitor-1, and phospho-Thr75 dopamine- and cAMP-regulated phosphoprotein, Mr 32,000 (DARPP-32). This effect was reversed by the PKC inhibitor, Ro-32-0432. Moreover, phospho-Ser6 inhibitor-1, phospho-Ser67 inhibitor-1, and phospho-Thr75 DARPP-32 levels were elevated in brain tissue from mice lacking the gene for PKC-alpha. PKC did not phosphorylate Cdk5 or its cofactor, p25, in vitro. Striatal levels of the Cdk5 cofactor, p35, did not change in response to phorbol ester treatment. Furthermore, Cdk5 immunoprecipitated from striatal slices treated with phorbol ester had unaltered activity toward a control substrate in vitro. These results suggest that PKC exerts its effects on the phosphorylation state of Cdk5 substrates through an indirect mechanism that may involve the regulatory binding partners of Cdk5 other than its neuronal cofactors.

Effect of phorbol 12,13-dibutyrate on smooth muscle tone in rat stomach fundus.

We investigated the effects of phorbol 12,13-dibutyrate (PDBu), a typical protein kinase C (PKC) activator, on smooth muscle tone in the rat stomach fundus. In 5-hydroxytriptamine (5-HT)-precontracted stomach fundus strips, PDBu induced dose-dependent relaxation, but 4alpha-phorbol 12,13-didecanoate, a phorbol ester that does not activate PKC, did not induce relaxation. A PDBu-induced dose-dependent relaxation was also observed in strips precontracted with platelet-activating factor (PAF), carbachol, or 60 mM K+. In stomach fundus strips pretreated with PDBu, the contractile responses to 5-HT and PAF were completely blocked, but those induced by carbachol and endothelin-1 (ET-1) were only partially inhibited. In stomach fundus strips preincubated with carbachol in Ca2+-free medium, the Ca2+-induced contraction was decreased by preincubation with PDBu. In strips preincubated with 5-HT, PAF, or ET-1 in Ca2+-free medium, Ca2+-induced contractions were greatly inhibited by pretreatment with PDBu. These results suggest that in rat stomach fundus strips, PDBu-induced relaxation is mediated by activation of PKC. We speculate that a major factor mediating the relaxant action of PDBu in rat stomach fundus smooth muscle is represented by a reduction in Ca2+ influx via an inhibition of Ca2+ channels.

Bidirectional regulation of renal cortical Na+,K+-ATPase by protein kinase C.

We examined the role of protein kinase C (PKC) in the regulation of Na+,K+- ATPase activity in the renal cortex. Male Wistar rats were anaesthetized and the investigated reagents were infused into the abdominal aorta proximally to the renal arteries. A PKC-activating phorbol ester, phorbol 12,13-dibutyrate (PDBu), had a dose-dependent effect on cortical Na+,K+-ATPase activity. Low dose of PDBu (10(-11) mol/kg per min) increased cortical Na+,K+-ATPase activity by 34.2%, whereas high doses (10(-9) and 10(-8) mol/kg per min) reduced this activity by 22.7% and 35.0%, respectively. PDBu administration caused changes in Na+,K+-ATPase Vmax without affecting K(0.5) for Na+, K+ and ATP as well as Ki for ouabain. The effects of PDBu were abolished by PKC inhibitors, staurosporine, GF109203X, and Gö 6976. The inhibitory effect of PDBu was reversed by pretreatment with inhibitors of cytochrome P450-dependent arachidonate metabolism, ethoxyresorufin and 17-octadecynoic acid, inhibitors of phosphatidylinositol 3-kinase (PI3K), wortmannin and LY294002, and by actin depolymerizing agents, cytochalasin D and latrunculin B. These results suggest that PKC may either stimulate or inhibit renal cortical Na+,K+-ATPase. The inhibitory effect is mediated by cytochrome P450-dependent arachidonate metabolites and PI3K, and is caused by redistribution of the sodium pump from the plasma membrane to the inactive intracellular pool.

Parathyroid hormone (PTH)-(1-34), [Nle(8,18),Tyr34]PTH-(3-34) amide, PTH-(1-31) amide, and PTH-related peptide-(1-34) stimulate phosphatidylcholine hydrolysis in UMR-106 osteoblastic cells: comparison with effects of phorbol 12,13-dibutyrate.

Studies were performed to determine the effects of PTH and related compounds on phosphatidylcholine (PC) hydrolysis in UMR-106 cells and the pathway by which the PTH effects occurred. The responses were compared with those of phorbol 12,13-dibutyrate (PDBu). Both bovine PTH-(1-34) [bPTH-(1-34)] and PDBu stimulated PC hydrolysis within 10 min. Significant effects were elicited by concentrations of 0.3-1 nM bPTH-(1-34) and 5 nM PDBu. Dose-dependent increases were seen at higher concentrations of both compounds, however, the response to bPTH-(1-34) was reduced at 30 nM. Bovine or human PTH-(1-34) and human PTH-related peptide-(1-34) [hPTHrP-(1-34)] were equipotent in their effects, whereas bovine [Nle(8,18)Tyr34]PTH-(3-34) amide [bPTH-(3-34)] and hPTH-(1-31) amide [hPTH-(1-31)] were less potent than bPTH-(1-34). bPTH-(3-34) did not antagonize the effects of bPTH-(1-34). Down-regulation of protein kinase C isozymes by 24-h treatment with PDBu completely prevented the stimulatory effect of PDBu on PC hydrolysis, but did not significantly affect the stimulatory effect of bPTH-(1-34). Both bPTH-(1-34) and PDBu stimulated transphosphatidylation of PC, indicating a phospholipase D-stimulated mechanism. The results suggest that in the UMR-106 cell line PTH can stimulate activation of PLD by a mechanism other than through protein kinase C.

Modulation of the formalin-induced nociceptive response by diabetes: possible involvement of protein kinase C.

Injection of formalin into the hind paw of mice produced a biphasic nociceptive response consisting of immediate (first-phase) and tonic (second-phase) components. Although the duration of the first-phase response was significantly longer in diabetic mice than in nondiabetic mice, the second phase was significantly shorter in diabetic mice. The first-phase response was dose-dependently and significantly reduced by pretreatment with calphostin C (0.3 to 3 pmol, i.t.), a specific protein kinase C inhibitor, in diabetic mice. The second-phase response was markedly increased when diabetic mice were pretreated with calphostin C. However, calphostin C (3 nmol, i. t.) had no significant effect on either the first-phase or second-phase response in nondiabetic mice. On the other hand, pretreatment with phorbol-12,13-dibutyrate (50 pmol, i.t.), a protein kinase C activator, significantly enhanced the first-phase response in nondiabetic mice. These results suggest that the change in the formalin-induced nociceptive response in diabetic mice may be due, at least in part, to the modification of nociceptive transmission in the spinal cord by the activation of protein kinase C.

Activation of protein kinase C alpha enhances human growth hormone-binding protein release.

The effect of phorbol ester on human growth hormone-binding protein (hGH-BP) release was investigated. The hGH-BP release from human IM-9 cells measured by immunoblotting was dose-dependently enhanced by a phorbol ester, phorbol 12, 13-dibutyrate (PDBu), and reached plateau at 100 nM. The increased hGH-BP release was shown after 10 min incubation with PDBu and reached a plateau at 60 min after stimulation. Similarly, a diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol, enhanced hGH-BP release. The enhancement was not inhibited by cycloheximide pretreatment, suggesting that the enhanced hGH-BP release does not require de novo protein synthesis. The PDBu-enhanced hGH-BP release was strongly inhibited by extracellular EDTA, and was dose-dependently inhibited by protein kinase C (PKC)-specific inhibitor, Ro 31-8220. These results suggest that activation of PKC mediates the PDBu-enhanced hGH-BP release. Of the 11 known PKC isoforms in human cells, PKCalpha, delta, mu and iota were detected in IM-9 cells by immunoblotting. Of these isoforms, PKCalpha, delta and mu were present in the membrane fraction, which is a known activation marker of PKC. Furthermore, when several PKC-specific inhibitors (Gö 6976, GF 109203X or bisindolylmaleimide III) with different specificities for each isoform were used, there was a good correlation between inhibition of the enhancement of hGH-BP release and inhibition of the phosphorylation of PKC isoforms, another activation marker of PKC, in PKCalpha but not in PKCdelta and mu. These results suggest that activation of PKCalpha is involved in PDBu-enhanced hGH-BP release.

Pretreatment with the protein kinase C activator phorbol 12,13-dibutyrate attenuates the ethanol-induced loss of the righting reflex in mice: modification by diabetes.

The effects of the protein kinase C (PKC) activator phorbol 12,13-dibutyrate (PDBu) on the ethanol-induced loss of the righting reflex were studied in diabetic and non-diabetic mice. The ethanol-induced loss of the righting reflex was significantly less in diabetic mice than in non-diabetic mice. Intracerebroventricular (i.c.v.) pretreatment with PDBu dose- and time-dependently reduced the ethanol-induced loss of the righting reflex in non-diabetic mice. The reduction of the ethanol-induced loss of the righting reflex caused by PDBu was reversed by concomitant i.c.v. pretreatment with calphostin C, a selective PKC inhibitor. On the other hand, PDBu had no effect on the ethanol-induced loss of the righting reflex in diabetic mice. I.c.v. pretreatment with calphostin C (10 pmol) increased the ethanol-induced loss of the righting reflex in diabetic mice but not in non-diabetic mice. These results suggest that the activation of PKC reduces the ethanol-induced loss of the righting reflex in mice. Furthermore, it is possible that this attenuation of the ethanol-induced loss of the righting reflex in diabetic mice may be due in part to increased PKC activity.

Protein kinase C induced changes in human airway smooth muscle tone: the effects of Ca2+ and Na+ transport.

Activation of protein kinase C by phorbol 12,13-dibutyrate (PDB) (1 nM-3 microM) caused a concentration-dependent contractile response in human isolated bronchus. The mean maximal contraction was 26 +/- 4.4% (n = 11) of that induced by acetylcholine (1 mM). The contraction was increased by the presence of the Ca2+ ionophore (A23187) to 47 +/- 6% (n = 7, P < 0.05) by the Ca2+ channel agonist, Bay K 8644 to 59.5 +/- 4.5% (n = 4, P < 0.05) and by KCl to 47.4 +/- 6%, while it was unaffected by carbachol (28.7 +/- 6.8%, n = 4, P > 0.05). The Ca2+ channel antagonist, verapamil (1 microM) significantly reduced the contraction from 32.3 +/- 4.9 to 12.5 +/- 1% (n = 4, P < 0.05) and in the presence of nifedipine (1 microM), the contractile response was abolished. A single concentration of 10 microM PDB produced a biphasic response-relaxation (6 +/- 1%) followed by contraction (76 +/- 4%, n = 4) which was greater than that produced when responses were obtained cumulatively. The relaxation response was inhibited by the addition of a Na-/K+ exchange antagonist, ouabain (10 microM) which also markedly potentiated the contractile response to 110 +/- 10% (n = 4, P < 0.05). These results suggest that the protein kinase C-mediated contraction in human airway smooth muscle is dependent on extracellular Ca2+ influx. Protein kinase C may also phosphorylate Na+/K(+)-ATPase resulting in a relaxation response.

The protein kinase C activators, phorbol 12-myristate,13-acetate and phorbol 12,13-dibutyrate, are convulsant in the pico-nanomolar range in mice.

Administration of phorbol 12-myristate,13-acetate (PMA, 10 fmol-10 nmol) or phorbol 12,13-dibutyrate (PDB, 0.2-495 nmol) (i.c.v.) to mice induced: hindlimb scratching, tremor, myoclonic jerks, hyperlocomotion, clonic seizure, followed by death or recovery. CD50 values for clonic seizures for PMA and PDB were 1.0 pmol and 1.2 nmol. 4-alpha-Phorbol (68-686 nmol) was inactive. The effects of PDB (24-247 nmol) were reduced by pretreatment with staurosporine (30 nmol, i.c.v.). Protein kinase C activators are potent convulsants in vivo.

Acute phorbol ester treatment improves spatial learning performance in rats.

Recent findings have lead researchers to speculate that hippocampal protein kinase C (PKC) in rodents is involved in spatial learning and memory. The purpose of this study was to determine if treating rats with a compound known to increase PKC activity would improve performance in a task that requires spatial learning processes. Rats were treated with a single intracerebroventricular injection of a phorbol ester, phorbol 12,13-dibutyrate (PDBu) that is known to increase PKC activity and then tested on the hidden-platform version of the Morris water taks. Results showed that PDBu-treated subjects' ability to learn to locate the escape platform was better than controls. In addition, PDBu-treated subjects showed signs of having remembered the location of the platform better than controls when tested 24 h later. These results support a role of brain PKC in processes required to learn the Morris water task.

Phorbol esters stimulate the transport of anionic amino acids in cultured human fibroblasts.

The effect of phorbol esters on the transport of amino acids has been evaluated in cultured human fibroblasts. The activity of the Na(+)-dependent system XAG- for anionic amino acids is selectively and markedly stimulated by phorbol esters. The effect is maximal within 15 min; it is attributable to an increase in transport maximum (Vmax) and not prevented by inhibitors of protein synthesis. The half-maximal stimulation is observed at concentrations of phorbol 12,13-dibutyrate lower than 100 nM. Prolonged incubations in the presence of 1 microM phorbol 12,13-dibutyrate lower the binding of the ligand to its receptor with a loss of the stimulatory effect on transport. The results presented indicate that the stimulation of amino acid transport through system XAG- by phorbol esters requires the activation of protein kinase C.

Phorbol 12,13-dibutyrate potentiates alpha 2-adrenoceptor-mediated contraction in the rat saphenous vein.

The responses of the rat saphenous vein to potassium depolarization, noradrenaline and B-HT 920 were potentiated significantly by phorbol 12,13-dibutyrate (PDBu) (10 nM). In contrast, only the contraction induced by potassium depolarization was potentiated by PDBu in the rat tail artery. The responses to noradrenaline and methoxamine were slightly inhibited. These results suggest that PDBu may have a differential effect on the responses of the two subtypes of alpha-adrenoceptors-potentiation for the alpha 2-adrenoceptors in the rat saphenous vein and inhibition for the alpha 1-adrenoceptors in the rat tail artery.