Isolation and characterization of an ester hydrolase active on phorbol diesters from murine liver

Phorbol esters: structure, biological activity, and toxicity in animals

Phorbol esters are the tetracyclic diterpenoids generally known for their tumor promoting activity. The phorbol esters mimic the action of diacyl glycerol (DAG), activator of protein kinase C, which regulates different signal transduction pathways and other cellular metabolic activities. They occur naturally in many plants of the family Euphorbiacaeae and Thymelaeaceae. The biological activities of the phorbol esters are highly structure specific. The phorbol esters, even at very low concentrations, show toxicological manifestations in animals fed diets containing them. This toxicity limits the use of many nutritive plants and agricultural by-products containing phorbol esters to be used as animal feed. Therefore, various chemical and physical treatments have been evaluated to extract or inactivate phorbol esters so that seed meals rich in proteins could be used as feed resources. However, not much progress has been reported so far. The detoxifying ability has also been reported in some molluscs and in liver homogenate of mice. Besides, possessing antinutritional and toxic effects, few derivatives of the phorbol esters are also known for their antimicrobial and antitumor activities. The molluscicidal and insecticidal properties of phorbol esters indicate its potential to be used as an effective biopesticide and insecticide.

Changes in carboxylesterase isoenzymes of rat liver microsomes during hepatocarcinogenesis

Among the three major carboxylesterase isoenzymes, RH1, RL1 and RL2, present in microsomes from normal rat liver, RL2 shows hydrolyzing activity towards 12-O-tetradecanoylphorbol-13-acetate and 1-oleoy1-2-acetyl-rac-glycerol, both activators of protein kinase C. Since protein kinase C has been suggested to be involved in carcinogenesis and cell proliferation, alterations in hepatic microsomal carboxylesterase isoenzymes including RL2 were studied during hepatocarcinogenesis induced by the Solt-Farber model. Alteration of RL2 was determined by measuring acetanilide-hydrolyzing activity, by quantifying the protein amount using the single radial immunodiffusion method, and by activity staining following electrophoresis of liver microsomes. The isoenzyme composition of hepatic microsomal carboxylesterase was changed after partial hepatectomy, and marked decreases in RL2 activity and protein content were observed at 4 weeks, at the time of preneoplastic foci induction. Partial hepatectomy alone also resulted in decreased RL2 activity. These findings suggest that RL2 may be involved in regulation of protein kinase C activity by metabolizing its activators at an early stage of hepatocarcinogenesis in rats.

Induction of suppressor T cells and inhibition of contact hypersensitivity in mice by 12-O-tetradecanoylphorbol-13-acetate and its analogs

12-O-tetradecanoylphorbol-13-acetate (TPA) and its analogs were surveyed for their abilities to modify contact hypersensitivity (CHS) responses in SENCAR mice. Sensitization of dorsal skin with 2,4-dinitrofluorobenzene (DNFB) and subsequent challenge of the ear 5 d later resulted within 24 h in ear swelling and increased vascular permeability (as measured by the extravasation of Evans Blue dye). Treatment of dorsal or ventral skin with TPA 4 times (application made every 3 or 4 d) prior to sensitization on the dorsum inhibited subsequent induction of CHS by DNFB challenge. Maximum suppression of CHS required sensitization at the site of TPA treatment. Suppression occurred over a narrow dose range of TPA (0.1-1.0 micrograms), and qualitatively correlated with the tumor incidences scored in an initiation-promotion multistage skin carcinogenesis experiment. Multiple applications (4x) of the promoters phorbol-12,13-dibenzoate (10 micrograms) and mezerein (2 micrograms) also suppressed CHS, whereas the non-promoter phorbol (20 micrograms) and the first stage tumor promoter 4-O-methyl TPA (20 micrograms) had no effect. Adoptive transfer of splenocytes isolated from mice pre-treated with TPA prior to DNFB sensitization inhibited the development of CHS in recipient mice that were sensitized and challenged with DNFB, but not oxazolone. Splenocyte preparations depleted of T lymphocytes prior to transfer could not suppress CHS in recipient mice. Conversely, suppressive activity was concentrated in splenocyte preparations depleted of adherent cells/monocytes. Collectively, these studies demonstrate that TPA treatment of murine epidermis prior to sensitization with hapten can inhibit subsequent hapten-dependent elicitation of CHS. This suppression is mediated in part by antigen-specific suppressor T cells. Furthermore, there is a qualitative correlation between the complete and second stage in vivo tumor-promoting activities of TPA and its analogs, and their abilities to inhibit CHS.

Enzymatic deacylations of esterified saccharides--III. Comparison of de-esterifications by serum esterases from different sources

1. 14C-labelled methyl 2,6-di-O-pivaloyl-alpha-D-glucopyranoside (1) was used as a substrate for esterases from rabbit, guinea pig, mouse, donkey, pig, horse, sheep and human sera. 2. Stepwise de-esterification of the diester substrate 1 occurred with rabbit, guinea pig and mouse serum. Data on time-course experiments and kinetic data are reported. 3. The use of donkey, pig, horse, sheep and human serum led to the migration of the 2-O-pivaloyl group in substrate 1 to the position 4- in the sugar molecule, followed by stepwise de-esterifications of both 1 and the newly formed methyl 4,6-di-O-pivaloyl-alpha-D-glucopyranoside (4). A report is given on the time-course experiments.

Enzymatic deacylations of esterified saccharides. II. De-esterifications of radiolabelled O-acylglucopyranosides by mice serum and liver esterases

1. 14C-Labelled methyl 2,6-di-O-pivaloyl-alpha-D-glucopyranoside (1) was used as a novel substrate for esterases from mouse serum and liver. 2. Stepwise de-esterification of the diester substrate 1 was achieved, and data on time-course experiments are reported. 3. Kinetic studies were undertaken to compare deacylation rates for the enzymatic de-esterification of the diester substrate 1 using both, mice sera and liver microsomal fractions. 4. Serum and liver esterase activities were studied in mice treated with an immunostimulating agent, peptidoglycan monomer (PGM), and a comparison made with esterases from untreated mice.

Isolation and characterization of a specific receptor for biologically active phorbol and ingenol esters

A high-affinity, specific receptor for biologically active phorbol and ingenol esters has been purified to electrophoretic homogeneity from murine brains using ammonium sulfate fractionation, and DEAE-cellulose, Sephadex G-200, Affi-Gel Blue, and phenyl-Sepharose chromatographies. The receptor is a single-chain hydrophobic protein with a molecular weight (Mr) of 81,500, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The receptor has a sedimentation coefficient of 5.2 S and Stokes radius of 30.3 A. It has an isoelectric point (pI) of 5.5. The receptor is heat and acid labile. The receptor absolutely depends upon phosphatidylserine or phosphatidylinositol (optimum concentration approximately 4-8 micrograms/ml) for its activity. A variety of divalent cations stimulates the binding activity of the receptor. A molecule of receptor binds 1-2 molecules of phorbol-12, 13-dibutyrate (PDBu) with a Kd value of 4.2 nM. Those phorbol and ingenol esters which stimulate cell growth in culture and have tumor-promoting activity in vivo inhibit the binding of labeled PDBu to its homogeneous receptor, while the biologically inactive derivatives fail to do so. The homogeneous receptor protein contains a protein kinase activity.

Tumor promoting phorbol diesters: substrates for diacylglycerol lipase

Enzyme activity in rat serum was examined utilizing the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and various glycerolipids as substrates. The serum activity was specific for hydrolysis of the long chain tetradecanoate moiety of TPA, hydrolyzed mono- and diacylglycerols, but was not effective against triacylglycerols, cholesterylesters, or phospholipids. Heating the enzyme preparation at 56 degrees C for 1 min was dually effective in reducing the hydrolysis of both TPA and dioleoylglycerol by 83-86% of control levels. The potent diacylglycerol lipase inhibitor, RHC 80267, inhibited the hydrolysis of TPA in the 0.2-1.0 microM range and was also a potent blocker of monoacyl- and diacylglycerol hydrolysis. In substrate competition studies, exogenous unlabeled TPA was added to the [14C]dioleoylglycerol-containing reaction mixture, however, this produced an approximate 3-fold stimulation of [14]dioleoylglycerol hydrolysis. Although we have not established whether the hydrolysis of TPA and diacylglycerol is the work of one enzyme, the effectiveness of the specific lipase inhibitor, RHC 80267, demonstrates that diacylglycerol lipase can utilize TPA as substrate, a finding never before documented. This point is of interest in light of the theory that phorbol esters act by mimicry of the natural lipid mediator, diacylglycerols.

Acylhydrolases from parsley (Petroselinum hortense). Relative distribution and properties of four esterases hydrolyzing malonic acid hemiesters of flavonoid glucosides

In parsley, malonylated flavonoid glycosides are formed in response to ultraviolet irradiation and accumulate in the vacuoles. Involvement of malonyltransferases, which catalyze the transfer of malonic acid from malonyl-coenzyme A to either flavone/flavonol 7-O-glucosides or flavonol 3-O-glucosides, has been described previously. These enzymes are present in very young leaf buds, and their activities decrease rapidly when leaves begin to unfold, while at the same time esterase activity is developing. The latter enzyme activity continues to increase with tissue age. Four esterases, distinguished by pI's of 3.8, 3.9, 4.0, and 4.05, were purified to apparent homogeneity from parsley leaves and shown to hydrolyze malonic acid hemiesters of flavonoid glucosides. These esterases are unspecific and are best described as one acetyl- and three arylesterases on the basis of inhibition studies by 4-chloromercuribenzoic acid and diisopropyl fluorophosphate. Esterases and malonic acid hemiesters appear to be separated from each other within the parsley leaf cell, and only on disruption of the cells do the respective substrates become available to the enzymes. Involvement of esterases in formation of wound periderm in parsley plants is suggested.

Enhancement by fluphenazine of dimethylbenz[a]-anthracene-induced mammary tumorigenesis in rats

Mammary tumor formation in female Sprague-Dawley rats was studied as a 2-stage protocol of initiation with 7,12 dimethylbenz[a]anthracene (DMBA) followed by repeated treatment with fluphenazine decanoate. No mammary tumors were found in the untreated control group or in the fluphenazine-treated groups. The repeated fluphenazine treatment was found to increase the number of mammary tumors in rats who had previously received DMBA and also to shorten the tumor latency period. The significance of these findings is discussed.