[On the chemistry of phorbol. IV. Polybenzoate and acetate of phorbol and phorbol-3-ol and functional derivatives of the allyl grouping of phorbol].
Explore the source record for details and available documents.
Acetates↗
SEARCH · Search PubMed
Results for “Phorbols”
Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Radioimmunoassay for phorbol esters using rabbit antisera against phorbol succinate.
The phorbol nucleus was succinylated and then conjugated to bovine albumin using dicyclohexylcarbodiimide. Rabbits given injections of the conjugate developed antibodies which rose in titer progressively with repeated immunization. By the ninth bleeding, the binding of one antiserum, diluted 1:15,000, was saturated with about 10 nM [3H]phorbol-12,13-dibutyrate [( 3H]-PDBU) and had an average association constant, Ka, of 2.6 X 10(8) M-1. The serological specificity of the antisera was characterized by examining the inhibition of the [3H]PDBU-anti-phorbol succinate immune system by 18 phorbol-related compounds. The specificities of antibodies from two rabbits tested in detail were qualitatively similar. The rank order of inhibitory activity for certain phorbol-related compounds was PDBU [concentration of inhibitor required to give 50% inhibition of PDBU binding (IC50) = 7.6 nM] = phorbol-13-acetate [IC50 = 8.2 nM] greater than phorbol-12,13-dibenzoate greater than 4-beta-phorbol [IC50 = 124 nM] greater than or equal to phorbol-12,13-diacetate greater than or equal to phorbol-12-myristate-13-acetate [IC50 = 184 nM] greater than phorbol-13,20-diacetate greater than phorbol-12-acetate [IC50 = 2300 nM]. The following compounds showed no detectable serological activity: mezerein, 4-0-methylphorbol-12-myristate-13-acetate, ingenol, 4-alpha-phorbol, teleocidin B, and dihydroteleocidin B. These and other results indicated that the 4-beta-phorbol nucleus was required for serological activity, that esterification of the C-13 position with benzoate, acetate, or butyrate enhanced the immunoreactivity of 4-beta-phorbol, and that among the phorbol-related compounds examined there was no direct relationship between serological activity and biological potency as tumor promoters. Using the [3H]PDBU-anti-phorbol succinate immune system, we measured the concentrations of immunoreactive phorbol-related material in crude mixtures such as croton oil and performed pharmacokinetic studies in rats given PDBU s.c.
Animals↗
Role of phorbol ester receptors in the 12-0-tetradecanoyl-phorbol-13-acetate (TPA)-induced down-regulation of colony-stimulating factor (CSF-1) binding to murine peritoneal exudate macrophages.
Treatment of murine peritoneal exudate macrophages (PEM) by tumor-promoting phorbol esters (TPA) results in a rapid loss of binding activity to radioactive-labeled colony-stimulating factor ([125I]-CSF-1) on the cell surface. The inhibitory effect of TPA on PEM is transient; treated cells recover full [125I]-CSF-1 binding activity in less than 6 hr at 37 degrees C either in the presence or after the removal of added TPA. The role of phorbol ester receptors in the induction of [125I]-CSF-1 binding inhibition was studied. The biologically active ligand [3H]-phorbol 12,13-dibutyrate ([3H]-PDBu) bound specifically to cultured murine PEM. At 0 degree C, stable and equilibrium binding occurred after 2-3 hr. Scatchard analysis revealed linear plots with a dissociation constant and receptor number per cell of 20.9 nM and 3.9 X 10(5)/cell, respectively. Treatment of PEM with biologically active phorbol esters at 37 degrees C rapidly inhibited the binding activity of [3H]-PDBu on cell surface (down-regulation) and rendered these cells refractory to the TPA-induced [125I]-CSF-1 binding inhibition by the subsequent TPA treatment. The inhibition of phorbol ester binding activity on TPA-treated PEM is caused by a reduction in the total number of available phorbol ester receptors rather than by a decrease in receptor affinity as judged by Scatchard analysis. The disappearance of [3H]-PDBu binding activity is reversible and transient. However, unlike CSF-1 receptors the restoration of phorbol ester receptors on TPA-treated PEM is a very slow process; a prolonged incubation of up to 72 hr after the removal of TPA was required for PEM to regain fully its [3H]-PDBu binding activity. Furthermore, the degree of TPA-induced CSF-1-receptor down-regulation is closely associated with the number of available phorbol ester receptors present on PEM at the time of treatment. Thus, the refractoriness to TPA diminished as the phorbol ester receptors on PEM recovered. A 72-hr incubation time at 37 degrees C was needed for PEM to lose their refractoriness and again become fully sensitive to TPA-induced CSF-1-receptor down-regulation. This study provides evidence that the loss of CSF-1-receptors induced by TPA treatment requires the presence of phorbol ester receptors and proceeds presumably via a co-internalization of both CSF-1 and phorbol ester receptors; the refractoriness to TPA is thereby induced by a transient loss of available phorbol ester receptors.
Animals↗
The 65-kDa phorbol-diester hydrolase in mouse plasma is esterase 1 and is immunologically distinct from the 56-kDa phorbol-diester hydrolase in mouse liver.
Esterase 1, a well-characterized mouse plasma protein of unknown function, has activity against a wide range of ester substrates including beta-alanine nitrophenyl esters and 17 beta-esters of estradiol. In this article, we report that esterase 1 is also responsible for a majority of the phorbol-12-ester hydrolase activity in mouse plasma. Incubation of homogeneous esterase 1 with 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) at either 4 or 37 degrees C for up to 18 h yielded phorbol 13 alpha-acetate as the only hydrolysis product. Specific polyclonal antibodies to esterase 1 inhibited 95% of PMA hydrolysis by a purified esterase 1 preparation and 65% of PMA hydrolysis by mouse plasma. Perfused mouse liver homogenates contain two distinct phorbol diester hydrolases with apparent molecular masses of 65 kDa and 56 kDa, respectively. The 65-kDa protein appears to be immunologically identical to the plasma enzyme, while the 56-kDa protein, found in liver but not in plasma, is immunologically distinct. Phorbol 12-myristate, phorbol 12,13-dibutyrate, and PMA were found to be competitive inhibitors of the beta-alanine-nitrophenyl esterase activity of esterase 1 with Ki values of approximately 7 microM. Phorbol 13-acetate and phorbol itself were less effective with Ki values of 37 and 140 microM, respectively. Sodium salts of valeric and myristic acids did not inhibit at 10 microM. The above results indicate that efficient substrate binding requires a phorbol 12-ester. Similar results were obtained with estradiol 17 beta-valerate which is a better substrate for esterase 1 than is PMA. Our results strongly suggest that esterase 1 and a recently described phorbol ester hydrolase isolated from mouse serum (Saito, M., and Egawa, K. (1984) J. Biol. Chem. 259, 5821-5826) are the same and are immunologically and kinetically distinct from the 56-kDa phorbol 12-ester hydrolase in mouse liver.
Animals↗
Characterization of specific binding of [3H]phorbol 12,13-dibutyrate and [3H]phorbol 12-myristate 13-acetate to mouse brain.
[20-3H]Phorbol 12,13-dibutyrate ([3H]PDBU) and [20-3H]-phorbol 12-myristate 13-acetate ([3H]PMA) bound specifically and with high affinity to one class of saturable binding sites in particulate preparations from mouse brain. The dissociation constants for binding of [3H]PDBU and [3H]PMA were 7 nM and 66 pm, respectively. At half-maximal specific binding, binding of [3H]PDBU was 96% specific and binding of [3H]PMA was 91% specific. At saturation, 28 pmol of [3H]PDBU were bound per mg protein. Nonradioactive phorbol 12-myristate 13-acetate blocked > 95% of the specific [3H]PDBU binding. Conversely, nonradioactive phorbol 12,13-dibutyrate blocked > 95% of the specific [3H]PMA binding. The binding sites in mouse brain for [3H]PDBU and [3H]PMA thus appear to be equivalent. In addition to phorbol 12-myritate 13-acetate, seven other biologically active phorbol-related diterpenes inhibited [3H]PDBU binding with potencies which correlated with their biological activities. In the case of phorbol 12,13-diacetate, inhibition of [3H]PDBU binding was shown to be competitive. Phorbol (20 microgram/ml) and 4 alpha-phorbol 12,13-didecanoate (20 microgram/ml), two compounds devoid of biological activity, had no effect on binding. Binding was sensitive to heat, papain, and phospholipase A2 but was insensitive to neuraminidase. Based on the high level of specific phorbol ester binding in brain, we postulate that the phorbol ester target plays a functional rather than information-transducing role in the cell.
Animals↗
Phorbol esters and related analogs regulate the subcellular localization of beta 2-chimaerin, a non-protein kinase C phorbol ester receptor.
The novel phorbol ester receptor beta2-chimaerin is a Rac-GAP protein possessing a single copy of the C1 domain, a 50-amino acid motif initially identified in protein kinase C (PKC) isozymes that is involved in phorbol ester and diacylglycerol binding. We have previously shown that, like PKCs, beta2-chimaerin binds phorbol esters with high affinity in a phospholipid-dependent manner (Caloca, M. J., Fernandez, M. N., Lewin, N. E., Ching, D., Modali, R., Blumberg, P. M., and Kazanietz, M. G. (1997) J. Biol. Chem. 272, 26488-26496). In this paper we report that like PKC isozymes, beta2-chimaerin is translocated by phorbol esters from the cytosolic to particulate fraction. Phorbol esters also induce translocation of alpha1 (n)- and beta1-chimaerins, suggesting common regulatory mechanisms for all chimaerin isoforms. The subcellular redistribution of beta2-chimaerin by phorbol esters is entirely dependent on the C1 domain, as revealed by deletional analysis and site-directed mutagenesis. Interestingly, beta2-chimaerin translocates to the Golgi apparatus after phorbol ester treatment, as revealed by co-staining with the Golgi marker BODIPY-TR-ceramide. Structure relationship analysis of translocation using a series of PKC ligands revealed substantial differences between translocation of beta2-chimaerin and PKCalpha. Strikingly, the mezerein analog thymeleatoxin is not able to translocate beta2-chimaerin, although it very efficiently translocates PKCalpha. Phorbol esters also promote the association of beta2-chimaerin with Rac in cells. These data suggest that chimaerins can be positionally regulated by phorbol esters and that each phorbol ester receptor class has distinct pharmacological properties and targeting mechanisms. The identification of selective ligands for each phorbol ester receptor class represents an important step in dissecting their specific cellular functions.
Animals↗
Highly lipophilic phorbol esters as inhibitors of specific [3H]phorbol 12,13-dibutyrate binding.
We examined the ability of a series of highly lipophilic phorbol esters to inhibit [20-3H]phorbol 12,13-dibutyrate binding to the cytosolic aporeceptor from mouse brain. If added in the usual fashion directly into the aqueous phase of the assay mixture, phorbol 12,13-distearate, phorbol 12,13-dioleate, and phorbol 12,13-dimyristate showed very weak inhibitory activities, with apparent inhibitor equilibrium dissociation constant values above 4 microM. In contrast, if incorporated directly into the liposomes used to reconstitute the aporeceptor, all three derivatives inhibited binding with high apparent affinities, 7.4 to 34 nM. The less lipophilic derivative phorbol 12,13-didecanoate showed a similar high affinity, 2.4 to 3.2 nM, by either route of addition. Consistent with the activity of the lipophilic derivatives being masked by an inability to transfer from the aqueous to the lipid phase, phorbol 12,13-distearate added to the aqueous phase inhibited efficiently (apparent inhibitor equilibrium dissociation constant, 14 nM) in the presence of 0.03% Triton X-100. The results suggest that the phorbol ester receptor recognizes phorbol esters which are inserted into the lipid bilayer. They indicate, moreover, that the apparent low activity of the more lipophilic phorbol esters is strongly influenced by factors other than equilibrium binding affinities.
Animals↗
Phorbol ester receptors-insights into the initial events in the mechanism of action of the phorbol esters.
Specific phorbol ester receptors are found in the particulate fraction of cells. In addition, cytosol contains a phorbol ester apo-receptor, which requires phospholipids for reconstitution. The apo-receptor corresponds to protein kinase C, and the quantitatively major membrane receptor appears to be a protein kinase C-phospholipid complex. The ability to reconstitute the phorbol ester apo-receptor into different lipid domains permits analysis of the role of the lipid domain in phorbol ester receptor function. Studies reviewed here indicate that diacylglycerols competitively inhibit phorbol ester binding, consistent with their being the postulated endogenous phorbol ester analogs. Highly lipophilic phorbol esters only inhibit effectively if incorporated into the lipid phase, indicating that the membrane dissolved form of the ligand can be recognized. The binding affinity of [3H]phorbol 12,13-dibutyrate for holo-receptor depends markedly (greater than 20-fold range) on the phospholipid environment, and heterogeneous phorbol ester binding (i.e., curved Scatchard plots) can be generated by use of heterogeneous lipid environments in the reconstitution. The possible existence of other phorbol ester receptors in addition to protein kinase C-phospholipid complexes remains to be resolved.
Animals↗
Increased diacylglycerol content with phospholipase C or hormone treatment: inhibition of phorbol ester binding and induction of phorbol ester-like biological responses.
The purpose of these studies was to determine whether increased cellular diacylglycerol could modulate phorbol ester receptor properties, in order to demonstrate that diacylglycerol can interact with and modulate the phorbol ester receptor in intact cells. Treatment of GH4C1 cells with bacterial phospholipase C caused an increase in cellular diacylglycerol. This was accompanied by increased PRL secretion and decreased epidermal growth factor (EGF) binding, two responses that also occur with phorbol ester treatment of GH4C1 cells. Phospholipase C treatment led to decreased apparent affinity for phorbol esters with no change in receptor number when measured in intact cells. This is consistent with increased concentrations of a competitive inhibitor of phorbol ester binding in treated cultures. Phospholipase C treatment caused a change in subcellular distribution of phorbol ester receptors, another response characteristic of phorbol ester treatment. TRH is known to activate endogenous phospholipase C activity in these cells, leading to a transient increase in diacylglycerol levels. TRH treatment also led to a transient change in subcellular distribution of phorbol ester receptors. In addition, a coordinate change in subcellular distribution of protein kinase C was observed. These data suggest that diacylglycerol is an endogenous ligand for the target for phorbol ester action in GH4C1 cells.
Animals↗
Low- and high-affinity phorbol ester and diglyceride interactions with protein kinase C: 1-O-alkyl-2-acyl-sn-glycerol enhances phorbol ester- and diacylglycerol-induced activity but alone does not induce activity.
Phorbol ester-induced conventional protein kinase C (PKCalpha, -betaIota/IotaIota, and -gamma) isozyme activities are potentiated by 1,2-diacyl-sn-glycerol. This has been attributed to a "cooperative" interaction of the two activators with two discrete sites termed the low- and high-affinity phorbol ester binding sites, respectively [Slater, S. J., Milano, S. K., Stagliano, B. A., Gergich, K. J., Ho, C., Mazurek, A., Taddeo, F. J., Kelly, M. B., Yeager, M. D., and Stubbs, C. D. (1999) Biochemistry 38, 3804-3815]. Here, we report that the 1-O-alkyl ether diglyceride, 1-O-hexadecyl-2-acetyl-sn-glycerol (HAG), like its 1,2-diacyl counterpart, 1-oleoyl-2-acetyl-sn-glycerol (OAG), also potentiated PKCalpha, -betaI/II, and -gamma activities induced by the phorbol ester 4beta-12-O-tetradecanoylphorbol-13-acetate (TPA). Similar to OAG, HAG was found to bind to the low-affinity phorbol ester binding site and to enhance high-affinity phorbol ester binding, and to decrease the level of Ca(2+) required for phorbol ester-induced activity, while being without effect on the Ca(2+) dependence of membrane association. Thus, similar to OAG, HAG may also potentiate phorbol ester-induced activity by interacting with the low-affinity phorbol ester binding site, leading to a reduced level of Ca(2+) required for the activating conformational change. However, HAG was found not to behave like a 1,2-diacyl-sn-glycerol in that alone it did not induce PKC activity, and also in that it enhanced OAG-induced activity. The results reveal HAG to be a member of a new class of "nonactivating" compounds that modulate PKC activity by interacting with the low-affinity phorbol ester binding site.
Animals↗
Effects of phorbol ester on mitogen-activated protein kinase kinase activity in wild-type and phorbol ester-resistant EL4 thymoma cells.
Phorbol ester-sensitive and -resistant EL4 thymoma cell lines differ in their ability to activate mitogen-activated protein kinase (MAPK) in response to phorbol ester. Treatment of wild-type EL4 cells with phorbol ester results in the rapid activations of MAPK and pp90rsk kinase, a substrate for MAPK, while neither kinase is activated in response to phorbol ester in variant EL4 cells. This study examines the activation of MAPK kinase (MAPKK), an activator of MAPK, in wild-type and variant EL4 cells. Phosphorylation of a 40-kDa substrate, identified as MAPK, was observed following in vitro phosphorylation reactions using cytosolic extracts or Mono Q column fractions prepared from phorbol ester-treated wild-type EL4 cells. MAPKK activity coeluted with a portion of the inactive MAPK upon Mono Q anion-exchange chromatography, permitting detection of the MAPKK activity in fractions containing both kinases. This MAPKK activity was present in phorbol ester-treated wild-type cells, but not in phorbol ester-treated variant cells or in untreated wild-type or variant cells. The MAPKK from wild-type cells was able to activate MAPK prepared from either wild-type or variant cells. MAPKK activity could be stimulated in both wildtype and variant EL4 cells in response to treatment of cells with okadaic acid. These results indicate that the failure of variant EL4 cells to activate MAP kinase in response to phorbol ester is due to a failure to activate MAPKK. Therefore, the step that confers phorbol ester resistance to variant EL4 cells lies between the activation of protein kinase C and the activation of MAPKK.
Amino Acid Sequence↗
Phorbol regulation of topoisomerases I and II in human leukemia cells. Studies in an additional cell pair sensitive or resistant to phorbol-induced differentiation.
We previously reported (Zwelling et al., Cancer Res 50: 7116-7122, 1990) that etoposide-induced DNA cleavage and mRNA coding for topoisomerase II are reduced in HL-60 cells induced to differentiate by phorbol ester. Reduction of etoposide-induced cleavage and topoisomerase II message did not occur in the derived cell line 1E3 (which is resistant to phorbol-induced differentiation), implying that topoisomerase II activity may be related to the state of cell differentiation. We have extended these studies using a new phorbol sensitive/resistant cell pair, S (sensitive) and PET (phorbol ester tolerant). Phorbol ester exposure not only reduced etoposide-induced DNA cleavage and topoisomerase II mRNA in S cells but also decreased the amount of immunoreactive topoisomerase II enzyme in whole S cells. However, immunoreactive topoisomerase II extracted from the nuclei of phorbol-treated S cells was not reduced compared with that from the nuclei of untreated S cells. This suggests that topoisomerase II contained in nuclear extracts is not always representative of the total cellular enzyme. Dramatic decreases in the amount, activity, or gene expression of topoisomerase II were not observed after phorbol treatment of the resistant PET cells; this is consistent with the potential involvement of topoisomerase II in monocytoid differentiation. Levels of topoisomerase I enzyme and mRNA fell in both S and PET cells after phorbol treatment; therefore, the genes for topoisomerases I and II did not appear to be regulated coordinately.
Cell Differentiation↗
Specific binding of [3H]phorbol dibutyrate to phorbol diester-responsive and -resistant clones of a human myeloid leukemia (KG-1) line 1.
Phorbol diesters induce macrophage-like differentiation in KG-1 and HL-60 human acute myelogenous leukemia cell lines. We developed a cloned subline of KG-1, known as KG-1a, that does not differentiate when exposed to phorbol diesters. Both KG-1 and KG-1a cells have a single class of specific high-affinity receptors for labeled phorbol-12,13-dibutyrate with a mean Kd of 1.47 +/- 0.10 (S.E.) X 10(-8) M and 0.85 +/- 0.20 X 10(-8) M for the sensitive parental KG-1 line and the resistant KG-1a subline, respectively (p less than 0.025). The number of [3H]phorbol-12,13-dibutyrate binding sites (mean +/- S.E.) per cell was 3.85 +/- 0.98 X 10(5) and 3.94 +/- 0.31 X 10(5) on KG-1 and resistant KG-1a cells, respectively. We observed no significant decrease of specific binding with time (down regulation) in either KG-1, KG-1a, or HL-60 cells, suggesting that down regulation of specific phorbol-12,13-dibutyrate binding is not critical to induction of differentiation. Our data also confirm that the presence of specific high-affinity phorbol receptors on leukemic cells does not assure that phorbol diesters can trigger their differentiation.
Cell Differentiation↗
Relative contribution of phosphoinositides and phosphatidylcholine hydrolysis to the actions of carbamylcholine, thyrotropin, and phorbol esters on dog thyroid slices: regulation of cytidine monophosphate-phosphatidic acid accumulation and phospholipase-D activity. II. Actions of phorbol esters.
The effects of phorbol dibutyrate (PDBu) on phosphatidylbutanol (PtdBut) generation in [3H]palmitate- or [3H]myristate-prelabeled dog thyroid slices were measured to assess the activity of phospholipase-D (PLD) in the presence or absence of the two inhibitors of protein kinase-C (PKC), staurosporine (STSP) and calphostin-C. The actions of the same agents on [3H]cytidine monophosphate-phosphatidic acid accumulation were also determined to evaluate phosphatidic (PA) generation and inositol recycling to phosphatidylinositol. The effluxes of [3H]choline and [3H]ethanolamine induced by the phorbol ester from prelabeled slices were also evaluated. PDBu (5 x 10(-9) to 5 x 10(-6) M) potently stimulated PLD activity, with a concomitant increase in fatty acids incorporation in phosphatidylcholine (PtdCho). However, under no condition did the phorbol ester result in cytidine monophosphate-phosphatidic acid accumulation. It stimulated the efflux of choline and ethanolamine while decreasing choline and ethanolamine phosphates in the slices and incubation medium. Calphostin-C, inhibiting PKC, decreased PtdBut and PtdCho formation induced by the phorbol ester, as opposed to STSP (5 x 10(-6) M), which did not affect these actions of PDBu and, moreover, reproduced by itself the effects of the phorbol ester on choline efflux and PtdBut generation despite efficient inhibition of other effects of PKC. These data demonstrate the existence in thyroid tissue of a PLD-hydrolyzing PtdCho, which was stimulated by phorbol esters and STSP. They also suggest that the PA formed after PKC stimulation and subsequent PLD activation is channeled toward PtdCho resynthesis when intracellular Ca2+ is not increased, whereas the PA accumulated with a concomitant increase in intracellular Ca2+ is diverted toward phosphatidylinositol synthesis. The physiological relevance of this Ca-independent stimulation of a PKC-coupled PLD in thyroid metabolism could be related to the growth-inducing and dedifferentiating effects of the phorbol esters.
Animals↗
Regulation of [3H]phorbol-12,13-dibutyrate binding sites in mouse neuroblastoma cells: simultaneous down-regulation by phorbol esters and desensitization of their inhibition of muscarinic receptor function.
The binding characteristics of [3H]phorbol-12,13-dibutyrate ([3H]PDBu) in mouse neuroblastoma N1E-115 cells were studied. The specific binding of [3H]PDBu to intact cells was saturable and to a homogeneous class of binding sites, with a Kd of 21 nM. Phorbol 12-myristate-13-acetate and PDBu competed for [3H]PDBu binding whereas 4 alpha-phorbol did not. The binding of [3H]PDBu to the cells was selective, as it was not affected by several agents that interact with various neurotransmitter receptors in N1E-115 cells. The density of the phorbol ester binding site decreased as the cell passage increased, although the Kd of [3H]PDBu binding remained relatively constant. Upon exposure of the cells to 100 nM PDBu for 1 hr at 37 degrees C, a translocation of the binding sites from the cytosol to the particulate fraction was observed. A similar pretreatment of the cells with 1 mM carbamylcholine, however, was ineffective. The specific binding of [3H]PDBu was down-regulated in both a time- and a concentration-dependent fashion by exposure of the cells to PDBu. When the cells were treated with 100 nM PDBu for 24 hr, the maximum binding site density of [3H]PDBu was decreased to 47% of control, with no change in the Kd. Recovery of [3H]PDBu binding after exposure to the phorbol ester for 24 hr was slow and incomplete, and was dependent on protein synthesis. The down-regulation of [3H]PDBu binding after pretreatment of the cells with PDBu for 24 hr was accompanied by an attenuation of the ability of phorbol 12-myristate-13-acetate to inhibit carbamylcholine-induced cyclic GMP formation as well as inositol phosphates accumulation in these cells, indicating desensitization of protein kinase C function.
Animals↗
Phorbol ester binding and phorbol ester-induced arachidonic acid metabolism in a highly responsive murine fibrosarcoma cell line and in a less-responsive variant.
Phorbol ester binding was examined in two lines of murine fibrosarcoma cells. The two cell lines were isolated from the same parent tumor but respond differentially to stimulation with phorbol esters. In one of the lines, these agents stimulate a rapid attachment and spreading response and induce directional migration. The other cell line does not migrate in response to stimulation with phorbol esters and the attachment and spreading response is slow. The cell line which responds actively to phorbol ester stimulation is highly malignant when injected into syngeneic animals while the other line is of low tumorigenicity and is virtually non-metastatic. In spite of these differences, both lines were found in the present study to bind [3H]4 beta-phorbol-12 beta, 13 alpha-dibutyrate in a receptor-mediated fashion. The characteristics of binding were virtually identical between the two cell lines. In additional studies, arachidonic acid metabolism was examined in the same two lines. In the highly responsive line, PMA stimulated a rapid release of [3H]arachidonic acid and its conversion into cyclooxygenase and lipoxygenase products. In the less-responsive line, PMA stimulated a slower release of [3H]arachidonic acid from prelabeled cells. The quantity of arachidonic acid metabolites produced was also much less. These studies suggest that the disparity between the two cell lines in their response to phorbol ester stimulation is not the result of differences in the initial interaction between the cells and ligand but may result from alterations in their signal transductance mechanism. This may be the result of inherent differences in capacity for arachidonic acid metabolism.
Animals↗
Protein kinase C isoenzymes display differential affinity for phorbol esters. Analysis of phorbol ester receptors in B cell differentiation.
Protein kinase C (PKC) comprises a family of distinct isoenzymes that are involved in signal transduction pathways linking the cell to triggers perceived via membrane receptors. These isoenzymes differ in their tissue distribution, activation requirements, and substrate specificity. One common denominator among different PKC subspecies is their activation by phorbol esters. We have developed a sensitive method permitting the measurement of phorbol ester binding sites, their quantitation, as well as their dissociation kinetics, by performing cytofluorometric analyses on intact cells or on isolated PKC associated to phosphatidylserine vesicles incubated in the presence of fluorochrome-labeled phorbol ester. Both PKC isozymes beta I/beta II and alpha from brain and spleen after incorporation into phosphatidylserine vesicles, display affinities with apparent Kd of 120 and 50 nM, respectively; although PKC gamma from brain exhibits a Kd of 210 nM. In addition to these receptors, on PKC isozymes from spleen, an intermediate affinity phorbol ester receptor (Kd of 3 nM) and an additional high affinity phorbol ester binding site with a Kd of 0.1 to 0.5 nM were also detected. This latter receptor comigrates with high m.w. PKC isoforms. In different cell lines, the phorbol ester binding patterns, as well as the expression of individual PKC isoenzymes, could be positively correlated.
Animals↗
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.
Binding Sites↗