Schistosoma mansoni: evidence for protein kinase-C-like modulation of muscle activity

The phorbol esters, phorbol-12,13-dibutyrate, phorbol-12-myristate-13-acetate, phorbol-12,13-didecanoate, and phorbol-12,13-diacetate, as well as mezerin at concentrations as low as 10 nM produce a spastic paralysis of the schistosome musculature. The action of these protein kinase-C activators is dependent on the sites of esterification and is stereo-specific since phorbol-13,20-diacetate, phorbol-12,13,20-triacetate, 20-oxo, 20-deoxy-beta-phorbol-12,13-dibutyrate, alpha-phorbol-12,13-didecanoate, and alpha-phorbol are inactive. A phospholipid and phorbol ester-dependent protein kinase is identified. This kinase is stimulated by all of the phorbol esters that increase muscle tone but is not stimulated by phorbol esters that do not affect muscle tone. A high affinity, stereo-specific phorbol ester receptor is identified. Dose-response curves of phorbol-12,13-dibutyrate-induced muscle tension and -stimulated kinase activity and receptor binding indicate that these responses are mediated by the same system. These results indicate that protein kinase-C-like enzyme may play an important role in modulating activity of the schistosome musculature.     

Transient modulation and internalization of T4 antigen induced by phorbol esters

Phorbol esters are known to alter the expression of surface antigens and receptors on a variety of mammalian cell types. On T lymphoblastoid cell lines and peripheral blood T cells, phorbol esters have been shown to selectively reduce the expression of the T4 antigen. To more fully characterize this process, we have examined the metabolic requirements for this phorbol ester effect, and have evaluated the relationship between phorbol ester-induced T4 loss and the expression of receptors for phorbol-12,13-dibutyrate (PDB) on purified peripheral blood T4 cells. We observed that the loss of T4 on peripheral blood lymphocytes (PBL) occurred at PDB concentrations at which 10 to 15% of phorbol ester binding sites were occupied. The loss of T4 was inhibited at 4 degrees C, and by azide, methylamine, and sodium fluoride, but not by inhibitors of DNA synthesis. When cells were exposed to phorbol esters for greater than 2 days, the T4 antigen was again expressed on the cell surface despite the continued presence of phorbol esters. Cells which had recovered T4 were resistant to the effects of freshly added PDB on this antigen, and this resistance correlated with a 55% reduction in phorbol ester binding sites. Studies on fixed PBL T4 cells and MOLT-4 cells by immunofluorescence microscopy demonstrated that the decreased expression of T4 from the cell surface correlated with a bright clustering of T4 within the cytoplasm, indicating that PDB had induced an internalization of this antigen. These observations demonstrate that the binding of phorbol esters to specific receptors on lymphocytes initiates metabolically dependent events which result in the internalization of the T4 antigen. These findings may be relevant to mechanisms by which T4 functions as a signal-transducing molecule in vivo.     

Tumor promoters induce membrane changes detected by fluorescence polarization.

The probe, 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to determine if tumor promoting agents alter cell membranes. The active tumor promoters TPA (12-0-tetra-decanoyl-phorbol-13-acetate), PDD (phorbol-12,13-didecanoate) and PDB (phorbol-12,13-dibenzoate) were found to decrease fluorescence polarization of DPH in rat embryo cells, whereas the inactive tumor promoting compounds phorbol and 4α-PDD failed to induce this change.     

Purification to homogeneity of protein kinase C from bovine brain--identity with the phorbol ester receptor

The calcium- and phospholipid-dependent kinase activity (protein kinase C) was isolated from bovine brains by a combination of DEAE-cellulose chromatography, gel filtration and hydrophobic chromatography on octyl-Sepharose and phenyl-Sepharose. The phorbol ester receptor co-purifies with the protein kinase C throughout the procedure yielding a homogeneous protein of 79 500 daltons on SDS-polyacrylamide gels. The purified kinase incorporated approximately 5000 nmol phosphate into substrate/min/mg protein at saturating concentrations of Ca2+ and phosphatidyl serine. Reciprocal plots of protein kinase activity at varying phosphatidyl serine concentrations were biphasic and yielded two apparent Ka values for phosphatidyl serine of 0.6-2 and 35-80 micrograms/ml). These apparent Ka values were reduced 2- to 3-fold by either diolein (20 micrograms/ml) or phorbol-12,13-dibutyrate (10 micrograms/ml). The protein binds [3H]phorbol-12,13-dibutyrate ( [3H]PDB) with high affinity (Ka = 15 nM) in a phosphatidyl serine-dependent manner. At saturating phosphatidyl serine concentrations 0.89 mol [3H]PDB are bound per mol protein. The identification of protein kinase C as the phorbol ester receptor is discussed with respect to the function and regulation of this protein.     

Induction of tumor cell resistance to macrophage-mediated lysis by phorbol esters: a postbinding event

Activated macrophages can recognize, bind to, and lyse tumor cells in an antibody-independent manner. We have found that tumor cells pretreated with phorbol esters are markedly less susceptible to macrophage-mediated cytolysis, although the initial binding step is unaffected. Tumor cells preincubated with tumor-promoting phorbol esters (10(-8)-10(-6) M) were rendered resistant to macrophage kill whereas non-tumor-promoting derivatives were inactive in protecting tumor cells against cytolysis. Inhibition of [3H]phorbol-12,13-dibutyrate binding by other phorbol esters correlated with their potency as tumor promoters and their ability to render tumor cells resistant to macrophage killing. The role of protein kinase C as the receptor to phorbol esters was implicated by inhibition of PDBu binding by phenothiazine derivatives. This suggests a possible mechanism for the resistance of phorbol ester-treated tumor cells to macrophage-mediated cytolysis.     

Prolactin sensitivity of mammary epithelial cells from mice exposed neonatally to diethylstilbestrol

We examined the responsiveness to prolactin and growth hormone of mammary epithelial cells from mice exposed neonatally to diethylstilbestrol (DES) and from control mice. The mammary epithelial cells were cultured inside collagen gels with serum-free medium containing insulin, epidermal growth factor, and linoleic acid. This produces prolactin-sensitive cells with low levels of casein production, as measured in cellular homogenates with a specific enzyme-linked immunosorbent assay for alpha-casein. The collagen gels containing these cells were then released and the medium supplements changed to insulin, linoleic acid, and prolactin at concentrations from 10 to 1000 ng/ml and growth hormone at 0, 10, or 100 ng/ml. This second phase of the culture, the differentiation phase, allows the cells to accumulate casein if they have this capacity. When cultured with prolactin only (no growth hormone), the cells from DES-exposed mice consistently accumulated 50-100% of the casein content of normal cells, but never more. Growth hormone, when added to prolactin-containing medium, increased casein accumulation above the levels seen with prolactin alone. Combinations of prolactin and growth hormone enhanced the difference between casein accumulation in DES-exposed and control cells, and DES-exposed cells were much less responsive to growth hormone. In our studies, the isolated mammary epithelial cells of estrogen-exposed mice are not more sensitive to prolactin than cells from normal animals as previous reports reports had suggested, but rather are generally less sensitive to hormonal stimulants.     

Effects of phorbol-12,13-diesters on prostaglandin production and phospholipase activity in canine kidney (MDCK) cells

Two tumor promoting phorbol-12,13-diesters, 12-0-tetradecanoyl-phorbol-13-acetate and phorbol-12,13-didecanoate, at concentrations of 10^?9 to 10^?10 M, stimulated prostaglandin production by dog kidney (MDCK) cells cultured in serum-supplemented medium. The non-tumor producing phorbol diester, 4α-phorbol-12,13-didecanoate, at a concentration of 10^?7 M, had no effect. The two biologically active phorbol diesters, but not the non-tumor promoting analog, stimulated deacylation of the cellular phospholipids of MDCK cells radioactively labelled with [³H]arachidonic acid. Most of the arachidonic acid was converted into prostaglandins.     

Effects of amiloride on the induction of DNA synthesis and casein gene expression in rabbit mammary explants

Amiloride, an inhibitor of Na+/H+ exchange, was added at various concentrations to the culture medium of rabbit mammary explants. In the concentration range 100-250 microM, amiloride progessively inhibited 14C-thymidine incorporation induced by insulin, EGF or prolactin. Up to 250 microM, amiloride, which did not inhibit basal protein synthesis, was not cytotoxic, but it reduced basal DNA synthesis at the highest concentration. Addition of amiloride to the culture medium of mammary explants also strongly inhibited the induction of casein synthesis and casein mRNA accumulation by prolactin. The inhibition by amiloride is therefore not specific of the mitogenic action of prolactin since this drug also prevented its lactogenic action. The data reported here describe a new inhibitory action of amiloride on the transmission of the lactogenic signals.     

Survival of Chick Embryonic Sensory Neurons in Culture Is Supported by Phorbol Esters

Sensory neurons of the chick embryo are supported in culture by several neurotrophic factors, including the phorbol esters. Because phorbol esters are known to activate one of the second messengers, namely, protein kinase C, it was of interest to see if the neurotrophic action of phorbol 12,13-dibutyrate (PDB) was related to the activation of protein kinase C in sensory neurons. Sensory neurons were obtained from dorsal root ganglia of 10-day-old chick embryos and maintained in a serum-free medium for several days to quantify survival and analyze protein kinase C activity. PDB (30 nM) supported the survival of approximately 50% of the total number of neurons plated. This value was comparable to that supported by nerve growth factor (NGF; 40 ng/ml). If PDB and NGF were added together, there was no additive effect on the survival. The protein kinase C activity of the particulate and cytosolic fractions of sensory neurons supported by NGF for 3 days was 1.26 +/- 0.1 and 2.9 +/- 0.32 pmol/min/mg of protein, respectively. In contrast, neurons supported by PDB showed an approximately 500% increase in enzyme activity in their particulate fraction. The enzyme activity of the cytosolic fraction was decreased by approximately 40%. If NGF-supported neurons were treated with PDB (30 nM) for 15 min, protein kinase C activity increased greater than 400% in the particulate fraction, whereas an approximately 50% decrease was observed in the cytosolic fraction. The protein kinase C value, expressed as a ratio of the activities in the particulate to cytosol fractions, showed large increases after phorbol treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparative studies on the mode of action of proctolin and phorbol-12,13-dibutyrate in their ability to contract the locust mandibular closer muscle.

The role of proctolin has been further investigated in the locust (Locusta migratoria) mandibular closer muscles. Radioactive calcium uptake measurements were made using protease-dissociated muscle cells. Both the phorbol ester, phorbol-12,13-dibutyrate, and proctolin produce tonic contractions which are associated with the influx of extracellular calcium. The thresholds for proctolin and the phorbol ester to contract the muscle were 1-10 nM and 10-100nM, respectively, while their respective thresholds for evoking measurable calcium influx into the muscle cells were 0.1-1 nM for proctolin, and 0.1-1 pM for phorbol-12,13-dibutyrate. The effect of phorbol-12,13-dibutyrate is blocked by a number of protein kinase inhibitors (at a concentration of 0.1 mM), suggesting that an activation of a protein kinase can lead to calcium influx. These inhibitors, however, do not block the effect of proctolin, indicating that these two compounds work through different pathways, possibly converging on the same final target. In light of this finding, a number of other compounds have been tested to try to ascertain how proctolin mediates an increased calcium influx.     

Inhibition of denuded mouse oocyte meiotic maturation by tumor-promoting phorbol esters and its reversal by retinoids

Two tumor-promoting phorbol esters, phorbol 12,13-dibutyrate (PDBu) and phorbol 12-myristate 13-acetate (PMA), when added to the culture medium of denuded mouse oocytes prevent their spontaneous meiotic maturation, whereas phorbol 13-acetate, which is inactive as a tumor promoter, does not inhibit this process. Retinoids appear to antagonize this inhibitory action of tumor promoters. However, the inhibitory effect of forskolin on meiotic maturation is not prevented, but is potentiated by retinal. These data indirectly suggest a role for calcium and/or phospholipids in the regulation of meiotic maturation. They also suggest that forskolin and phorbol esters mediate their effects through different pathways.     

Response of hepatic ornithine decarboxylase and polyamine concentration to surgical stress in the rat: evidence for a permissive effect of catecholamines on glucocorticoid action

The effects of dibutyryl cAMP (Bt2cAMP) and phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and phorbol-12,13-didecanoate (PDD) on VIP/PHM-27 gene expression in human neuroblastoma cells in culture were investigated. Bt2cAMP and phorbol esters increased the VIP/PHM-27 mRNA level by about 9- and 4-fold, respectively. In the presence of both Bt2cAMP and phorbol esters, the VIP/PHM-27 mRNA level increased by about 36-fold. The intracellular cAMP level was essentially unaffected by phorbol esters. The VIP/PHM-27 gene dosage was unchanged by Bt2cAMP and phorbol esters. The results suggest that cAMP and phorbol esters synergistically induce the VIP/PHM-27 gene expression through independent pathways.     

An inhibitory role for the protein kinase C pathway in ovarian steroidogenesis. Studies with cultured swine granulosa cells.

We have used primary cultures of swine granulosa cells to investigate the regulatory role of the protein kinase C pathway in the ovary. In this system, we observed the following. Swine granulosa cells bound [3H]phorbol 12,13-dibutyrate [( 3H]PDB) specifically with high affinity [apparent Ki for 12-O-tetradecanoylphorbol 13-acetate (TPA) = 3.1 (2.1-4.7) nM] and low capacity [0.68 (0.34-0.99) pmol/10(7) cells]. The cytosol of granulosa cells contained functionally active protein kinase C capable of phosphorylating distinct proteins in response to stimulation with active phorbol ester. TPA and PDB induced dose-dependent inhibition (greater than 85%) of follicle-stimulating-hormone (FSH)-stimulated progesterone production. Half-maximally inhibitory concentrations were 0.10 and 0.75 nM for TPA and PDB respectively, whereas phorbol analogues that do not activate protein kinase C were not inhibitory. TPA did not impede cyclic AMP generation in response to FSH, cholera toxin or forskolin acutely (within 48 h), but did inhibit the stimulatory effects of 8-bromo cyclic AMP, insulin and oestradiol on progesterone biosynthesis. In the presence of maximally effective concentrations of 25-hydroxy-, 20 alpha-hydroxy- or 22R-hydroxy-cholesterol as exogenous sterol substrates for cholesterol side-chain cleavage, treatment with TPA suppressed pregnenolone, progesterone and 20 alpha-hydroxypregn-4-en-3-one biosynthesis by more than 80%. The inhibitory effects of phorbol esters were not attributable to non-specific cytotoxicity, since prostaglandin F2 alpha production increased in the same cultures and aromatization of exogenously supplied testosterone to oestradiol was not suppressed. In intact granulosa cells, the effects of phorbol esters were mimicked by a synthetic non-diterpene diacylglycerol, 1-octanoyl-2-acetylglycerol, and the tumour promoter, mezerein, which specifically activates protein kinase C. We conclude that swine granulosa cells contain specific high-affinity receptors for phorbol esters that are functionally coupled to protein phosphorylation. Moreover, treatment with phorbol esters or non-phorbol activators of protein kinase C results in selective inhibition of cholesterol side-chain cleavage activity without impairing cyclic AMP generation or oestrogen biosynthesis.     

Phorbol-ester-induced alterations of free calcium ion transients in single rat hepatocytes.

The effect of the phorbol esters phorbol 12-myristate 13-acetate (TPA) and phorbol 12,13-dibutyrate (PDB) on changes in free Ca2+ concentration ([Ca2+]i) in single rat hepatocytes, microinjected with the photoprotein aequorin, were investigated. [Arg8]vasopressin and phenylephrine induced a series of repetitive [Ca2+]i transients. Phorbol esters inhibited the alpha 1-adrenoceptor-induced response; sub-nanomolar concentrations decreased the transient frequency, and higher concentrations abolished the transients. The inhibitory effect of PDB was readily reversible. Phorbol esters were less effective in decreasing the frequency of [Arg8]-vasopressin-induced transients, and the inhibition could be overcome by high [Arg8]vasopressin concentrations.     

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.     

Homologous and heterologous mitogenic desensitization of Swiss 3T3 cells to phorbol esters and vasopressin: role of receptor and postreceptor steps

Prolonged treatment of Swiss 3T3 cells with phorbol 12,13 dibutyrate (PDB) rendered the cells refractory to subsequent mitogenic stimulation by both PDB and vasopressin. In contrast, the cells retained full responsiveness to a wide variety of other mitogens. An early response to vasopressin and phorbol esters, inhibition of (125I)-labeled epidermal growth factor [(125I)-EGF] binding, was also substantially decreased in PDB pretreated cells. The cross desensitization was not produced by vasopressin; this ligand induced homologous but not heterologous desensitization. Exposure of Swiss 3T3 cells to PDB caused a down regulation of (3H)-PDB receptors but did not reduce the binding of vasopressin to refractory cells. The time-course (t1/2 = 7 h) and dependence on PDB concentration (half maximal at 20 nM) for this phorbol ester receptor loss paralleled the induction of the mitogenic desensitizations to both PDB and vasopressin. However, the time-course of recovery revealed an important dissociation between receptor presence and mitogenic response. When Swiss 3T3 cultures, which had been pretreated with PDB, were washed to remove this ligand and incubated in its absence for 24 h, both (3H)-PDB receptors and PDB or vasopressin inhibition of (125I)-EGF binding were almost completely restored to control levels. However the homologous and heterologous mitogenic desensitizations showed a very different reversal time. After a 24-h recovery period PDB-treated refractory cells were still unable to synthesize DNA in response to PDB or vasopressin. The mitogenic desensitizations were however completely reversible; after a 48-h incubation in the absence of PDB the cells responded fully to the mitogenic actions of PDB or vasopressin. This finding suggests that a further postreceptor step was also desensitized by prolonged PDB treatment. The presence of a low level of cycloheximide during the PDB pretreatment blocked induction of this postreceptor refractoriness. We propose that this refractory postreceptor step selectively blocks both PDB and vasopressin stimulation of DNA synthesis and may represent the point at which the mitogenic pathways of phorbol esters and vasopressin converge.