Activation of protein kinase C induces rapid internalization and subsequent degradation of muscarinic acetylcholine receptors in neuroblastoma cells

The tumor-promoting phorbol ester 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), which activates protein kinase C, acted synergistically with A23187 to decrease muscarinic acetylcholine receptor (mAChR) number in neuroblastoma cells (clone N1E-115) as determined by a filter binding assay using [3H]quinuclidinyl benzilate in membrane homogenates. After a 6-h incubation, 10(-7) M PMA and 3 X 10(-7) M A23187 reduced mAChR number 30-40%, compared to the 40-50% reduction observed after treatment with 10(-3) M carbachol, a muscarinic agonist. Incubation with 3 X 10(-7) M A23187 and 10(-7) M 4 alpha-phorbol 12,13-didecanoate, an inactive phorbol ester, did not alter mAChR number. The addition of PMA and A23187 to cultures incubated with 10(-3) M carbachol caused only a modest 6% further reduction in mAChR number as compared to incubation with carbachol alone. The kinetics of the decrease in mAChR number produced by PMA/A23187 were similar to those seen after carbachol treatment. Recovery of mAChR number after treatment with either carbachol or PMA/A23187 was blocked by treatment with the protein synthesis inhibitor cycloheximide. Intact cell binding studies employing [3H]N-methylscopolamine showed that treatment with either PMA/A23187 or carbachol caused a rapid (within 15 min) loss of receptors from the cell surface prior to the decrease in total mAChR number. PMA (10(-7) M), but not 4 alpha-phorbol 12,13-didecanoate, promoted the translocation of protein kinase C activity from the cytosol to the membrane. Incubation with carbachol increased membrane-associated protein kinase C activity within 5 min with an EC50 of 3 X 10(-6) M. This increase persisted for at least 60 min in the continued presence of carbachol and was blocked by simultaneous incubation with atropine. These results suggest that activation of protein kinase C may be involved in the regulation of mAChR number in response to agonist.     

Agonist-induced desensitization of cholinergically stimulated phosphoinositide breakdown is independent of endogenously activated protein kinase C in HT-29 human colon carcinoma cells.

Activation of M3 muscarinic receptors in HT-29 cells by carbachol rapidly increases polyphosphoinositide breakdown. Pretreatment of these cells with carbachol (0.1 mM) for 5 h completely inhibits the subsequent ability of carbachol to increase [3H]inositol monophosphate ([3H]InsP) accumulation, paralleled by a total loss of muscarinic binding sites. In contrast, protein kinase C (PK-C)-mediated desensitization by incubation with phorbol esters [PMA (phorbol 12-myristate 13-acetate)], leading to a time- and dose-dependent inhibition of cholinergically stimulated InsP release (95% inhibition after 4 h with 0.1 microM-PMA), is accompanied by only a 40% decrease in muscarinic receptor binding, which suggests an additional mechanism of negative-feedback control. Neither carbachol nor PMA pretreatment had any effect on receptor affinity. Incubation with carbachol for 15 min caused a small increase of membrane-associated PK-C activity (15% increase, P less than 0.05) as compared with the potency of phorbol esters (PMA) (3-4-fold increase, P less than 0.01). Long-term incubation (4-24 h) with PMA resulted in a complete down-regulation of cytosolic and particulate PK-C activity. Stimulation of InsP release by NaF (20 mM) was not affected after a pretreatment with phorbol esters or carbachol, demonstrating an intact function of G-protein and phospholipase-C (PL-C) at the effector side. Determination of PL-C activity in a liposomal system with [3H]PtdInsP2 as substrate, showed no change in PL-C activity after carbachol (13 h) and short-term PMA (2.5 h) pretreatment, whereas long-term preincubation with phorbol esters (13 h) caused a small but significant decrease in PL-C activity (19%, P less than 0.05). Our results indicate that agonist-induced desensitization of phosphoinositide turnover occurs predominantly at the receptor level, with a rapid loss of muscarinic receptors. Exogenous activation of PK-C by phorbol esters seems to dissociate the interaction between receptor and G-protein/PL-C, without major effects on total cellular PL-C activity.     

Phosphorylation of endothelin converting enzyme-1 isoforms: relevance to subcellular localization

Endothelin-converting enzyme (ECE)-1 is a metalloenzyme with four subisoforms, which differ only in their amino-terminal domain. ECE-1a and c are the most common isoforms and are found at the plasma membrane and in the Golgi complex, whereas ECE-1b displays lysosomal localization. We have recently shown that ECE-1a but not ECE-1b also colocalizes with nuclear membrane markers, and that maintenance of cells in high glucose (25 mM) promotes relocalization of ECE-1a from the membrane to the intracellular compartment. To investigate the mechanisms involved in this process, we conducted a search for potential phosphorylation sites, which yielded a different number of putative sites for protein kinase (PK)-C and PKA in the amino-terminal region. Stimulation of Chinese hamster ovary (CHO) cells expressing a green fluorescent protein (GFP)-tagged human ECE-1a or ECE-1b with 100 nM phorbol myristate acetate (PMA) resulted in phosphorylation of ECE-1a, as determined by immunoprecipitation with an antibody to GFP followed by immunoblotting with an antibody to phosphoserine. Stimulation of cells with PMA also promoted intracellular relocalization, as seen in cells grown under high-glucose conditions. Incubation of cells grown in 25 mM glucose with the PKC inhibitor, calphostin C (100 nM), partially prevented the relocalization of ECE-1a from the plasma membrane to intracellular compartments. Stimulation of cells with 100 nM forskolin caused phosphorylation of ECE-1b and not ECE-1a, which is consistent with the lack of a putative PKA site in the ECE-1a amino-terminal sequence. Although phosphorylation is not required for ECE-1 enzymatic activity, these results suggest that ECE-1 isoforms are phosphorylated and that phosphorylation might play an important role in the regulation of intracellular trafficking of ECE-1 subisoforms.     

Perturbation of cultured human vascular endothelial cells by phorbol ester or thrombin alters the cellular von Willebrand factor distribution

We have studied the influence of perturbation of cultured human umbilical vein endothelial cells on the distribution of the von Willebrand factor. As shown previously, short-term (less than 1 hr) treatment of endothelial cells with the phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA) or thrombin resulted in the release of cellular stored von Willebrand factor. Long-term treatment with PMA or thrombin evoked a distinct change in the endothelial cell distribution of von Willebrand factor, evident 24 to 48 hrs after exposure. Whereas the contents of the von Willebrand factor storage sites in the cells were gradually restored within 48 hrs, enhanced amounts of von Willebrand factor were secreted into the medium. However, PMA did not increase the endothelial cell contents of mRNA encoding for von Willebrand factor. The number as well as the size of von Willebrand factor storage granules in the endothelial cells increased after exposure to the phorbol ester, as determined by immunofluorescence microscopy. A second treatment with PMA or thrombin, 48 hrs after cells had been stimulated with these agents, resulted again in the instantaneous release of von Willebrand factor. PMA and thrombin caused a decrease in the von Willebrand factor contents of the extracellular matrix. Pulse-chase experiments revealed that PMA blocked the deposition of von Willebrand factor in the subendothelium, whereas PMA did not affect the degradation of matrix von Willebrand factor. Thus, perturbation of endothelial cells changes the cellular distribution of von Willebrand factor.     

Shear stress attenuates endothelin and endothelin-converting enzyme expression through oxidative stress

Shear stress is known to dilate blood vessels and exert an antiproliferative effect on vascular walls. These effects have partly been ascribed to shear stress-induced regulation of the secretion of endothelium-derived vasoactive substances. In this study, to elucidate the role of shear stress in endothelin production by endothelial cells, we examined the effect of physiological shear stress on the mRNA expression of endothelin-converting enzyme-1 (ECE-1) as well as endothelin-1 (ET-1) in cultured bovine carotid artery endothelial cells (BAECs) and human umbilical vein endothelial cells (HUVECs), using a parallel plate-type flow chamber. ECE-1 mRNA expression was significantly down-regulated by shear stress in an intensity- and time-dependent manner within the physiological range (1.5 to 15 dyn/cm(2)). ET-1 mRNA expression decreased together with ECE-1 mRNA expression. Shear stress at 15 dyn/cm(2) for 30 min induced a significant increase in the intracellular peroxide concentration, and the down-regulation of ECE-1 and ET-1 mRNA expression by shear stress was attenuated almost completely on treatment with N-acetyl cysteine (NAC), an antioxidant (20 mM). Furthermore, when H(2)O(2) (0.5 to 2 mM) was added to BAECs in static culture, the ECE-1 as well as ET-1 mRNA expression was attenuated in proportion to the concentration of H(2)O(2). It is suggested that endothelial cells sense shear stress as oxidative stress and transduce signal for the regulation of the gene expression of ECE as well as ET to attenuate vascular tone and inhibit the proliferation of vascular smooth muscle cells.     

Phorbol Ester- and Retinoic Acid-Induced Regulation of the Protein Kinase C Substrate MARCKS in Immortalized Hippocampal Cells

Abstract: The expression of MARCKS, a major protein kinase C (PKC) substrate, was examined in the immortalized hippocampal cell line HN33, following differentiation using phorbol esters or retinoic acid. In cells exposed to phorbol esters, MARCKS protein levels were reduced through an apparent PKC-dependent mechanism. Exposure to 1 µ M phorbol 12-myristate 13-acetate (PMA) for 10 min resulted in a rapid loss of PKC activity in the soluble fraction with a concurrent increase in membrane-associated PKC activity. PKC activity was reduced to <20% of control values in both soluble and membrane fractions following 1 h of PMA exposure. Significant reductions in MARCKS protein levels were initially observed in membrane and soluble fractions following PMA exposure for 4 and 8 h, respectively. The reduction in MARCKS protein levels was maximal following 24 h of PMA exposure. MARCKS protein expression was also down-regulated in a dose-dependent manner on exposure of HN33 cells to retinoic acid. In cells exposed to 10 µ M retinoic acid, the MARCKS protein level was reduced in the membrane fraction within 4 h. Reduction of MARCKS protein levels was maximal (>90%) by 12 h with no evidence for any alteration in PKC activity. Reduced levels of MARCKS protein were also observed in the soluble fraction of retinoic acid-exposed cells, but to a significantly lesser extent. Addition of the PKC inhibitor GF109203X blocked the down-regulation of MARCKS protein in PMA-treated cultures but not in retinoic acid-treated cells. These findings suggest that the down-regulation of MARCKS may play an important role in both phorbol ester- and retinoic acid-induced differentiation in cells of neuronal origin.     

Dynamic regulation of Na(+)-K(+)-2Cl(-) cotransporter surface expression by PKC-{epsilon} in Cl(-)--secretory epithelia

In secretory epithelia, activation of PKC by phorbol ester and carbachol negatively regulates Cl^– secretion, the transport event of secretory diarrhea. Previous studies have implicated the basolateral Na⁺-K⁺-2Cl^– cotransporter (NKCC1) as a target of PKC-dependent inhibition of Cl^– secretion. In the present study, we examined the regulation of surface expression of NKCC1 in response to the activation of PKC. Treatment of confluent T84 intestinal epithelial cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (PMA) reduced the amount of NKCC1 accessible to basolateral surface biotinylation. Loss of cell surface NKCC1 was due to internalization as shown by 1) the resistance of biotinylated NKCC1 to surface biotin stripping after incubation with PMA and 2) indirect immunofluorescent labeling. PMA-induced internalization of NKCC1 is dependent on the -isoform of PKC as determined on the basis of sensitivity to a panel of PKC inhibitors. The effect of PMA on surface expression of NKCC1 was specific because PMA did not significantly alter the amount of Na⁺-K⁺-ATPase or E-cadherin available for surface biotinylation. After extended PMA exposure (>2 h), NKCC1 became degraded in a proteasome-dependent fashion. Like PMA, carbachol reduced the amount of NKCC1 accessible to basolateral surface biotinylation in a PKC--dependent manner. However, long-term exposure to carbachol did not result in degradation of NKCC1; rather, NKCC1 that was internalized after exposure to carbachol was recycled back to the cell membrane. PKC--dependent alteration of NKCC1 surface expression represents a novel mechanism for regulating Cl^– secretion. endocytosis; recycling; ion transporters     

Keloid fibroblasts are refractory to inhibition of DNA synthesis by phorbol esters. Altered response is accompanied by reduced sensitivity to prostaglandin E2 and altered down-regulation of phorbol ester binding sites.

To investigate abnormal growth regulation in keloid fibroblasts, responses to phorbol esters were examined. Treatment of quiescent cultures with phorbol 12-myristate 13-acetate (PMA) blocked a normally occurring (20-24 h) peak of serum-stimulated thymidine incorporation in normal and keloid cells. In keloid fibroblasts PMA induced a delayed peak of DNA synthesis. When indomethacin was added with PMA the delayed peak appeared in normal fibroblasts. The ED50 for inhibition of the 20-24-h peak was 1 nM, whereas the delayed peak required a 50-fold-higher PMA concentration. In both cell types PMA induced prostaglandin E2 (PGE2) synthesis, and exogenous PGE2 caused 50% inhibition of the 20-24-h peak. When PMA and indomethacin were added with PGE2 the delayed peak was inhibited 90% in normal fibroblasts, whereas inhibition of keloid cells was the same as with PGE2 alone. Normal and keloid fibroblasts had the same number of phorbol ester binding sites. However, in normal cells, phorbol 12,13-dibutyrate bound with greater affinity, and down-regulation of phorbol ester binding occurred to a greater extent. These findings suggest that altered expression of protein kinase C isozymes or another molecule that binds phorbol esters may play a role in abnormal growth regulation of keloid cells.     

Chemiluminescent response to PMA in isolated rat liver after in situ ischemia-reperfusion

The median and left lateral lobes of rat liver in situ were rendered ischemic for 30 min, then blood flow reinstituted. After 1, 3, 6, 24, or 48 h, livers were removed and set up for isolated perfused organ study. Luminol enhanced chemiluminescence (LEC) was recorded from the surface of the median and left lateral lobes before and for 90 min following phorbol myristate acetate (PMA, 1.6 × 10⁻⁸ M) perfusion. An increase in PMA induced LEC was evident at 1 h and continued to increase up to 6 h. By 24 h the magnitude of the PMA response had returned to within control values. This indicates that a large influx of inflammatory cells had occurred in the liver following the in vivo ischemia-reperfusion insult and that these cells were well fixed in the tissue and capable of mounting a very large and sustained burst of radical production on stimulation with PMA. This combined in vivo/in vitro technique is ideally suited for the assessment of interventions designed to ameliorate damage following oxidative stress.     

Molecular mechanism of the desensitization of beta-adrenergic receptors and adenilate cyclase by hypoxia in human endothelial cells

Long-term oxygen deficiency in vivo leads to the progressive blunting of responsiveness to sympathetic stimulation and blood catecholamines in many human and animal tissues. In order to better understand the molecular processes that underlie this phenomenon we examined the effect of hypobaric hypoxia (290 mm Hg, pO2 = 40 mM Hg) on the--beta-adrenoreceptor (beta-AR) density and the activity of adenylate cyclase (AC) and phosphoinositide turnover (PI-turnover) in cultures of human pulmonary artery and umbilical vein cells. We discovered that 30 min of hypobaric hypoxia increased basal levels of inositol mono-, bis- and tris-phosphate, products of PI-turnover in endothelial cells (EC). After 60 min of hypoxia their content amounted to 250-300% of the basal level. Desensitization of PI-turnover to histamine stimulation in EC was observed after 60 min of hypoxia. Basal and isoproterenol (beta-AR-agonist)-stimulated AC activities therewith were markedly reduced. beta-AR-density was decreased in EC membranes after 2-3 hrs of hypoxia. Similar desensitization of beta-AR and AC occurred after 1-2 hrs treatment of EC with histamine and platelet activating factor (stimulators of PI-turnover) and with phorbol myristate acetate (PK C activator). Neither hyproxia nor phorbol myristate acetate influenced beta-AR density or AC activity in protein kinase C-deficient EC (72 hrs treatment with phorbol myristate acetate). The data suggest that hypoxia-induced desensitization of beta-AR and AC in endothelial cells is mediated via hypozia-stimulated turnover and subsequent protein kinase C activation.     

Role of Protein Kinase C in Endothelin Converting Enzyme-1 trafficking and shedding from endothelial cells

This study aimed to determine the consequences of Protein Kinase C (PKC) mediated Endothelin Converting Enzyme-1 (ECE-1) phosphorylation and its relationship to ECE-1 expression and shedding. The proteins on the surface of EA.hy926 cells were labelled with EZ-Link NHS-SS-Biotin both prior to (control) and following stimulation by 2 μM phorbol 12-myristate 13-acetate (PMA) which activates PKC. The biotinylated proteins were isolated using neutravidin beads, resolved by gel electrophoresis and analysed by western blotting using anti-ECE-1 antibodies. Significant increase in ECE-1 expression at the cell surface was observed following stimulation by PMA, compared to unstimulated control cells (170 ± 32.3% of control, n = 5). The ECE-1 activity (expressed as μM substrate cleaved/min) was determined by monitoring the cleavage of a quenched fluorescent substrate. The specificity of cleavage was confirmed using the ECE-1 inhibitor CGS35066. The stimulation of cells by PMA (1 μM, 6 h) significantly increased the ECE-1 activity (0.28 ± 0.02; n = 3) compared to the control (0.07 ± 0.02; n = 3). This increase was prevented by prior incubation with the PKC inhibitor bisindolymaleimide (BIM; 2 μM for 1 h; 0.10 ± 0.01; n = 3). Treatment with PMA also increased the activity of ECE-1 in the media (0.18 ± 0.01; n = 3) compared to control (0.08 ± 0.01; n = 3). In addition, this study confirmed by western immunoblotting that only the extracellular region of ECE-1 is released from the cell surface. These data indicate for the first time that PKC activation induces the trafficking and shedding of ECE to and from the cell surface, respectively.     

Distribution of active protein kinase C in smooth muscle.

To localize activated protein kinase C (PKC) in smooth muscle cells, an antibody directed to the catalytic site of the enzyme was used to assess PKC distribution by immunofluorescence techniques in gastric smooth muscle cells isolated from Bufo marinus. An antibody to vinculin was used to delineate the cell membrane. High-resolution three-dimensional images of immunofluorescence were obtained from a series of images collected through focus with a digital imaging microscope. Cells were untreated or treated with agents that increase PKC activity (10 microM carbachol for 1 min, 1 microM phorbol 12-myristate 13-acetate (PMA) for 10 min), or have no effect on PKC activity (1 micrometer 4-alpha phorbol, 12,13-didecanoate (4-alpha PMA)). In unstimulated cells, activated PKC and vinculin were located and organized at the cell surface. Cell cytosol labeling for activated PKC was sparse and diffuse and was absent for vinculin. After treatment with carbachol, which stimulates contraction and PKC activity, in addition to the membrane localization, the activated PKC exhibited a pronounced cytosolic fibrillar distribution and an increased total fluorescence intensity relative to vinculin. The distributions of activated PKC observed after PMA but not 4-alpha PMA were similar to those observed with carbachol. Our results indicate that in resting cells there is a pool of activated PKC near the cell membrane, and that after stimulation activated PKC is no longer membrane-confined, but is present throughout the cytosol. Active PKC appears to associate with contractile filaments, supporting a possible role in modulation of contraction.     

Absence of MGST1 mRNA and protein expression in human neuroblastoma cell lines and primary tissue

A recent study identified a haplotype on a small region of chromosome 12, between markers D12S1725 and D12S1596, shared by all patients with familial neuroblastoma (NB). We previously localized the human MGST1 gene, whose gene product protects against oxidative stress, to this very same chromosomal region (12p112.1–p13.33). Owing to the chromosomal location of MGST1; its roles in tumorigenesis, drug resistance, and oxidative stress; and the known sensitivity of NB cell lines to oxidative stress, we considered a role for MGST1 in NB development. Surprisingly there was no detectable MGST1 mRNA or protein in either NB cell lines or NB primary tumor tissue, although all other human tissues, cell lines, and primary tumor tissue examined to date express MGST1 at high levels. The mechanism behind the failure of NB cells and tissue to express MGST1 mRNA is unknown and involves the failure of MGST1 pre-mRNA expression, but does not involve chromosomal rearrangement or nucleotide variation in the promoter, exons, or 3' untranslated region of MGST1. MGST1 provides significant protection against oxidative stress and constitutes 4 to 6% of all protein in the outer membrane of the mitochondria. As NB cells are extremely sensitive to oxidative stress, and often used as a model system to investigate mitochondrial response to endogenous and exogenous stress, these findings may be due to the lack of expression MGST1 protein in NB. The significance of this finding to the development of neuroblastoma (familial or otherwise), however, is unknown and may even be incidental. Although our studies provide a molecular basis for previous work on the sensitivity of NB cells to oxidative stress, and possibly marked variations in NB mitochondrial homeostasis, they also imply that the results of these earlier studies using NB cells are not transferable to other tumor and cell types that express MGST1 at high concentrations.     

Protein kinase C-dependent and -independent pathways of proto-oncogene induction in human astrocytoma cells

We compared the abilities of the muscarinic agonist carbachol, epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) to induce proto-oncogene mRNA accumulation and other cellular responses in normal and protein kinase C-deficient 1321-N1 human astrocytoma cells. PMA, carbachol, and EGF all stimulated rapid accumulation of mRNA for the proto-oncogenes c-fos and c-myc in the normal cells; in the protein kinase C-deficient cells, carbachol and EGF, but not PMA, retained this effect, which was not mimicked by the calcium ionophore A23187. Both carbachol and PMA activated protein kinase C in these cells, as evidenced by the stimulated phosphorylation of an acidic Mr 80,000 protein kinase C substrate protein with phosphoamino acid and peptide map identity. This response was mimicked by several other neurotransmitters in these cells, including epinephrine, histamine, oxotremorine, and serotonin, and was abolished in cells made protein kinase C-deficient by preincubation with high concentrations of PMA. Both PMA and carbachol promoted the phosphorylation of the ribosomal protein S6 and activated an S6 protein kinase in the normal but not in the protein kinase C-deficient cells. EGF, in contrast, did not appear to activate protein kinase C, but promoted the phosphorylation of S6 and activation of the S6 kinase in both normal and protein kinase C-deficient cells. We conclude that, in 1321-N1 cells, induction of c-fos and c-myc mRNA can occur through a protein kinase C-dependent pathway and one or more independent pathways, exemplified by the responses to carbachol and EGF in the protein kinase C-deficient cells.     

A transfected m1 muscarinic acetylcholine receptor stimulates adenylate cyclase via phosphatidylinositol hydrolysis

The m1 muscarinic acetylcholine receptor gene was transfected into and stably expressed in A9 L cells. The muscarinic receptor agonist, carbachol, stimulated inositol phosphate generation, arachidonic acid release, and cAMP accumulation in these cells. Carbachol stimulated arachidonic acid and inositol phosphate release with similar potencies, while cAMP generation required a higher concentration. Studies were performed to determine if the carbachol-stimulated cAMP accumulation was due to direct coupling of the m1 muscarinic receptor to adenylate cyclase via a GTP binding protein or mediated by other second messengers. Carbachol failed to stimulate adenylate cyclase activity in A9 L cell membranes, whereas prostaglandin E2 did, suggesting indirect stimulation. The phorbol ester, phorbol 12-myristate 13-acetate (PMA), stimulated arachidonic acid release yet inhibited cAMP accumulation in response to carbachol. PMA also inhibited inositol phosphate release in response to carbachol, suggesting that activation of phospholipase C might be involved in cAMP accumulation. PMA did not inhibit prostaglandin E2-, cholera toxin-, or forskolin-stimulated cAMP accumulation. The phospholipase A2 inhibitor eicosatetraenoic acid and the cyclooxygenase inhibitors indomethacin and naproxen had no effect on carbachol-stimulated cAMP accumulation. Carbachol-stimulated cAMP accumulation was inhibited with TMB-8, an inhibitor of intracellular calcium release, and W7, a calmodulin antagonist. These observations suggest that carbachol-stimulated cAMP accumulation does not occur through direct m1 muscarinic receptor coupling or through the release of arachidonic acid and its metabolites, but is mediated through the activation of phospholipase C. The generation of cytosolic calcium via inositol 1,4,5-trisphosphate and subsequent activation of calmodulin by m1 muscarinic receptor stimulation of phospholipase C appears to generate the accumulation of cAMP.     

Phorbol esters inhibit agonist-induced [3H] inositol-1-phosphate accumulation in rat hippocampal slices

In rat hippocampal slices, carbachol and norepinephrine induce an accumulation of [3H]-inositol-1-phosphate which is markedly amplified in the presence of lithium. The tumor-promoting agents phorbol 12,13-dibutyrate (PDB) and 4 beta phorbol, 12 beta-myristate, 13 alpha-acetate (PMA) have no effect on [3H] inositol-1-phosphate accumulation alone, but when preincubated with hippocampal slices significantly inhibit the accumulation of [3H]-inositol-1-phosphate induced by carbachol and norepinephrine. The IC50 values for PDB and PMA are 0.2 microM and 25 microM respectively. In contrast, the weak tumor promoting agents 4-O-methylphorbol 12 myristate 13 acetate (MPMA) and phorbol 13,20-diacetate (P 13,20 DA) only slightly attenuate the agonist-induced response at concentrations less than or equal to 100 microM, whereas 4 alpha-phorbol (4 alpha-PHR), a biologically inactive phorbol, has no effect. These data suggest that phorbol ester receptor-mediated events may be negatively coupled to agonist-induced phosphatidylinositol hydrolysis.     

Combined incubation of colon carcinoma cells with phorbol ester and mitochondrial uncoupling agents results in synergic elevated reactive oxygen species levels and increased γ-glutamyltransferase expression

The NADPH oxidase (NOX) is a significant determinant for the expression and activity of γ-glutamyltransferase (GGT), which is frequently upregulated after increased levels of reactive oxygen species (ROS) and oxidative stress. Earlier studies on human colon carcinoma HT-29 cells have shown that treatment with phorbol 12-myristate 13-acetate (PMA) activates NOX thus increasing the intracellular level of ROS and upregulating GGT. Another important source of cellular ROS is the mitochondria, and treatment with the mitochondria uncoupler carbonylcyanide-4-(trifluoromethoxy)-phenylhydrazone (FCCP) results in increased ROS levels. The present study shows that when HT-29 cells were simultaneously treated with both agents, a significant and synergic increase in intracellular ROS was detected. NOX activity contributed at least 50 % of this increase as inhibiting NOX activity with apocynin or downregulating the NOX activity using siRNA against p22 phox reduced the synergic ROS production. The combined FCCP and PMA treatment also provoked highly increased GGT mRNA levels after 24 h whereas only minor and delayed increases in GGT protein and enzyme activity levels were detected. The results strongly indicate that ROS production by both mitochondria and NOX is involved in the regulation of GGT expression in colon carcinoma cells.