Differences in the induction of carboxylesterase isozymes in rat liver microsomes by perfluorinated fatty acids

Early-life exposure to PCBs and PFAS exerts negative effects on the developing central nervous system

Persistent organic pollutants (POPs) are ubiquitous in the environment and display the capacity to bioaccumulate in living organisms, constituting a hazard to both wildlife and humans. Although restrictions have been applied to prohibit the production of several POPs since the 1960s, high levels of these compounds can still be detected in many environmental and biological matrices, due to their chemical properties and significantly long half-lives. Some POPs can be passed from mother to the fetus and can gain entry to the central nervous system (CNS), by crossing the blood-brain barrier (BBB), resulting in significant deleterious effects, including neurocognitive and psychiatric abnormalities, which may lead to long-term socio-economic burdens. A growing body of evidence obtained from clinical and experimental studies has increasingly indicated that these POPs may influence neurodevelopment through several cellular and molecular mechanisms. However, studies assessing their mechanisms of action are still incipient, requiring further research. Polychlorinated biphenyls (PCBs) and per- and polyfluoroalkyl substances (PFAS) are two of the main classes of POPs associated with disturbances in different human systems, mainly the nervous and endocrine systems. This narrative review discusses the main PCB and PFAS effects on the CNS, focusing on neuroinflammation and oxidative stress and their consequences for neural development and BBB integrity. Moreover, we propose which mechanisms could be involved in POP-induced neurodevelopmental defects. In this sense, we highlight potential cellular and molecular pathways by which these POPs can affect neurodevelopment and could be further explored to propose preventive therapies and formulate public health policies.

Hepatic carboxylesterases are differentially regulated in PPARα-null mice treated with perfluorooctanoic acid

Hepatic carboxylesterases (Ces) catalyze the metabolism of drugs, environmental toxicants, and endogenous lipids and are known to be regulated by multiple nuclear receptors. Perfluorooctanoic acid (PFOA) is a synthetic fluorochemical that has been associated with dyslipidemia in exposed populations. In liver, PFOA can activate nuclear receptors such as PPARα, and alter the metabolism and excretion of chemicals. Here, we sought to test the ability of PFOA to modulate Ces expression and activity in the presence and absence of the PPARα receptor. For this purpose, male C57BL/6 NCrl mice were administered PFOA (1 or 3 mg/kg, po, 7 days) and livers collected for assessment of Ces expression and activity. PFOA increased Ces1 and 2 protein and activity. Notably, PFOA increased Ces1d, 1e, 1f, 1 g, 2c, and 2e mRNAs between 1.5- and 2.5-fold, while it decreased Ces1c and 2b. Activation of PPARα by PFOA was confirmed by up-regulation of Cyp4a14 mRNA. In a separate study of PFOA-treated wild-type (WT) and PPARα-null mice, induction of Ces 1e and 1f mRNA and in turn, Ces1 protein, was PPARα-dependent. Interestingly, in PPARα-null mice, Ces1c, 1d, 1 g, 2a, 2b, and 2e mRNAs and Ces2 protein were up-regulated by PFOA which contributed to sustained up-regulation of Ces activity, although to a lower extent than observed in WT mice. Activation of the CAR and PXR receptors likely accounted for up-regulation of select Ces1 and 2 subtypes in PPARα-null mice. In conclusion, the environmental contaminant PFOA modulates the expression and function of hepatic Ces enzymes, in part through PPARα.

Carboxylesterases are uniquely expressed among tissues and regulated by nuclear hormone receptors in the mouse

Carboxylesterases (CES) are a well recognized, yet incompletely characterized family of proteins that catalyze neutral lipid hydrolysis. Some CES have well-defined roles in xenobiotic clearance, pharmacologic prodrug activation, and narcotic detoxification. In addition, emerging evidence suggests other CES may have roles in lipid metabolism. Humans have six CES genes, whereas mice have 20 Ces genes grouped into five isoenzyme classes. Perhaps due to the high sequence similarity shared by the mouse Ces genes, the tissue-specific distribution of expression for these enzymes has not been fully addressed. Therefore, we performed studies to provide a comprehensive tissue distribution analysis of mouse Ces mRNAs. These data demonstrated that while the mouse Ces family 1 is highly expressed in liver and family 2 in intestine, many Ces genes have a wide and unique tissue distribution defined by relative mRNA levels. Furthermore, evaluating Ces gene expression in response to pharmacologic activation of lipid- and xenobiotic-sensing nuclear hormone receptors showed differential regulation. Finally, specific shifts in Ces gene expression were seen in peritoneal macrophages following lipopolysaccharide treatment and in a steatotic liver model induced by high-fat feeding, two model systems relevant to disease. Overall these data show that each mouse Ces gene has its own distinctive tissue expression pattern and suggest that some CES may have tissue-specific roles in lipid metabolism and xenobiotic clearance.

The Toxicology of Perfluorooctanoate

PFOA is a peroxisome proliferator (PPAR agonist) and exerts morphological and biochemical effects characteristic of PPAR agonists. These effects include increased beta-oxidation of fatty acids, increases in several cytochrome P-450 (CYP450)-mediated reactions, and inhibition of the secretion of very low-density lipoproteins and cholesterol from the liver. These effects on lipid metabolism and transport result in a reduction of cholesterol and triglycerides in serum and an accumulation of lipids in the liver. The triad of tumors observed (liver, Leydig cell, and pancreatic acinar-cell) is typical of many PPAR agonists and is believed to involve nongenotoxic mechanisms. The hepatocellular tumors observed in rats are likely to have been the result of the activation of the peroxisome proliferator activated receptor alpha (PPARalpha). The tumors observed in the testis (Leydig-cell) have been hypothesized to be associated with an increased level of serum estradiol in concert with testicular growth factors. The mechanism responsible for the acinar-cell tumors of the pancreas in rats remains the subject of active investigation. The mechanism resulting in the hepatocellular tumors in rats (PPARalpha activation) is not likely to be relevant to humans. Similarly, the proposed mechanism for Leydig-cell tumor formation is of questionable relevance to humans. Acinar tumors of the pancreas are rare in humans, and the relevance of the these tumors, as found in rats, to humans is uncertain. Epidemiological investigations and medical surveillance of occupationally exposed workers have not found consistent associations between PFOA exposure and adverse health effects.

Studies on the toxicological effects of PFOA and PFOS on rats using histological observation and chemical analysis

As an emerging class of environmentally persistent and bioaccumulative contaminants, perfluorinated compounds (PFCs), especially perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), have been ubiquitously found in the environment. Increasing evidence shows that the accumulated levels of PFCs in animals and the human body might cause potential impairment to their health. In the present study, toxicological effects of PFOA and PFOS on male Sprague-Dawley rats were examined after 28 days of subchronic exposure. Abnormal behavior and sharp weight loss were observed in the high-dose PFOS group. Marked hepatomegaly, renal hypertrophy, and orchioncus in treated groups were in accordance with the viscera-somatic indexes of the liver, kidney, and gonad. Histopathological observation showed that relatively serious damage occurred in the liver and lung, mainly including hepatocytic hypertrophy and cytoplasmic vacuolation in the livers and congestion and thickened epithelial walls in the lungs. PFOA concentrations in main target organs were in the order of kidney > liver > lung > (heart, whole blood) > testicle > (spleen, brain), whereas the bioaccumulation order for PFOS was liver > heart > kidney > (whole blood) > lung > (testicle, spleen, brain). The highest concentration of PFOA detected in the kidney exposed to 5 mg/kg/day was 228+/-37 microg/g and PFOS in the liver exposed to 20 mg/kg/day reached the highest level of 648+/-17 microg/g, indicating that the liver, lung, and kidney might serve as the main target organs for PFCs. Furthermore, a dose-dependent accumulation of PFOS in various tissues was found. The accumulation levels of PFOS were universally higher than PFOA, which might explain the relative high toxicity of PFOS. The definite toxicity and high accumulation of the tested PFCs might pose a great threat to biota and human beings due to their widespread application in various fields.

Structure and catalytic properties of carboxylesterase isozymes involved in metabolic activation of prodrugs

Mammalian carboxylesterases (CESs) comprise a multigene family whose gene products play important roles in biotransformation of ester- or amide-type prodrugs. They are members of an α,β-hydrolase-fold family and are found in various mammals. It has been suggested that CESs can be classified into five major groups denominated CES1-CES5, according to the homology of the amino acid sequence, and the majority of CESs that have been identified belong to the CES1 or CES2 family. The substrate specificities of CES1 and CES2 are significantly different. The CES1 isozyme mainly hydrolyzes a substrate with a small alcohol group and large acyl group, but its wide active pocket sometimes allows it to act on structurally distinct compounds of either a large or small alcohol moiety. In contrast, the CES2 isozyme recognizes a substrate with a large alcohol group and small acyl group, and its substrate specificity may be restricted by the capability of acyl-enzyme conjugate formation due to the presence of conformational interference in the active pocket. Since pharmacokinetic and pharmacological data for prodrugs obtained from preclinical experiments using various animals are generally used as references for human studies, it is important to clarify the biochemical properties of CES isozymes. Further experimentation for an understanding of detailed substrate specificity of prodrugs for CES isozymes and its hydrolysates will help us to design the ideal prodrugs.

Structure, function and regulation of carboxylesterases

This review covers current developments in molecular-based studies of the structure and function of carboxylesterases. To allay the confusion of the classic classification of carboxylesterase isozymes, we have proposed a novel nomenclature and classification of mammalian carboxylesterases on the basis of molecular properties. In addition, mechanisms of regulation of gene expression of carboxylesterases by xenobiotics and involvement of carboxylesterase in drug metabolism and enzyme induction are also described.

Dexamethasone-induced methylprednisolone hemisuccinate hydrolase: Its identification as a member of the rat carboxylesterase 2 family and its unique existence in plasma

Carboxylesterases (CESs) play important roles in the metabolism of many ester-drugs. In the present study, we identified and characterized dexamethasone-induced methylprednisolone hemisuccinate (MPHS) hydrolase in rat liver microsomes. Intraperitoneal injection of dexamethasone resulted in a significant increase in the level of MPHS hydrolase activity accompanied by induction of a specific CES isozyme. Since the biochemical characteristics of the induced CES isozyme were very similar to those of rat CES RL4, we hypothesized that these were the same enzymes. The results of nano-electrospray ionization tandem mass spectrometry analysis revealed that both dexamethasone-induced CES isozyme and CES RL4 possessed identical peptide fragments to those of , a rat CES2 isozyme, supporting our hypothesis. Furthermore, the results of reverse transcription-polymerase chain reaction showed that the amount of mRNA in dexamethasone-treated liver was greater than that in control liver. To confirm that encodes dexamethasone-induced CES isozyme, cDNA cloning was performed and the obtained cDNA was expressed in Sf9 cells by using a baculovirus-mediated expression system. The recombinant CES protein could hydrolyze MPHS and exhibited biochemical characteristics similar to those of CES RL4. Collectively, the results indicated that dexamethasone-induced MPHS hydrolase in liver microsomes is a rat CES2 isozyme. Interestingly, the results also showed that this rat CES2 isozyme exists in plasma and that the amount of this protein is increased by dexamethasone. These findings, together with the findings described above, provide important information for the study of phramacokinetics and pharmacodynamics of ester-drugs as well as for the study of CESs.

Synthesis of environmentally relevant fluorinated surfactants--a review

In the past years there has been a growing interest in fluorinated persistent organic pollutants such as perfluorooctanesulfonic acid, perfluorooctanesulfonamides, perfluorinated carboxylic acids and fluorotelomer alcohols. Although these compounds have probably been present in the environment for many decades, we are only now beginning to realize that these environmental contaminants may have serious environmental and health effects. This article gives a state-of-the-art review of synthetic approaches that have been employed for the synthesis of these environmentally relevant fluorinated compounds. Perfluorooctanesulfonic acid derivatives, in particular, pose a problem because only a few perfluorooctanesulfonic acid derivatives are available from commercial sources--a fact that limits the ability of researchers worldwide to further study these compounds. Because of the limited literature available, this article also describes synthetic approaches for shorter chain homologues or perfluoroether analogues that can potentially be applied for the synthesis of perfluorooctanesulfonic acid derivatives. The preparation of typical starting materials for the synthesis of perfluorooctanesulfonic acid derivatives such as the perfluoroalkanesulfonyl fluorides and chlorides will be discussed. Subsequently, their conversion into relevant perfluoroalkane sulfonate salts (R(F)SO3M), sulfonamides (R(F)SO2NH2), N-alkyl sulfonamides (R(F)SO2NHR, R = alkyl), N,N-dialkyl sulfonamides (R(F)SO2NR2, R = alkyl), sulfonamidoethanol (R(F)SO2NRCH2CH2OH, R = -H, -CH3 or -C2H5) and sulfonamidoacetates (R(F)SO2NRCH2CO2H, R = -H, -CH3 or -C2H5) will be described. Many perfluorinated carboxylic acids and fluorotelomer alcohols are available from commercial sources. The review of the synthesis of these two classes of fluorinated compounds includes a review of their industrial synthesis and the synthesis of relevant degradation products.

Identification of di-(2-ethylhexyl) phthalate-induced carboxylesterase 1 in C57BL/6 mouse liver microsomes: purification, cDNA cloning, and baculovirus-mediated expression

Several mouse carboxylesterase (CES) isozymes have been identified, but information about their roles in drug metabolism is limited. In this study, we purified and characterized a mouse CES1 isozyme that was induced by di-(2-ethylhexyl) phthalate. Purified mouse CES1 shared some biological characteristics with other CES isozymes, such as molecular weight of a subunit and isoelectronic point. In addition, purified mouse CES1 behaved as a trimer, a specific characteristic of CES1A subfamily isozymes. The purified enzyme possessed temocapril hydrolase activity, and it was found to contribute significantly to temocapril hydrolase activity in mouse liver microsomes. To identify the nucleotide sequences coding mouse CES1, antibody screening of a cDNA library was performed. The deduced amino acid sequence of the obtained cDNA, mCES1, exhibited striking similarity to those of CES1A isozymes. When expressed in Sf9 cells, recombinant mCES1 showed hydrolytic activity toward temocapril, as did purified mouse CES1. Based on these results, together with the findings that recombinant mouse CES1 had the same molecular weight of a subunit, the same isoelectronic point, and the same native protein mass as those of purified mouse CES1, it was concluded that mCES1 encoded mouse CES1. Furthermore, tissue expression profiles of mCES1 were found to be very similar to those of the human CES1 isozyme. This finding, together with our other results, suggests that mCES1 shares many biological properties with the human CES1 isozyme. The present study has provided useful information for study of metabolism and disposition of ester-prodrugs as well as ester-drugs.

Regulation of triacylglycerol hydrolase expression by dietary fatty acids and peroxisomal proliferator-activated receptors

Triacylglycerol hydrolase (TGH) is an enzyme that catalyzes the lipolysis of intracellular stored triacylglycerol (TG). Peroxisomal proliferator-activated receptors (PPAR) regulate a multitude of genes involved in lipid homeostasis. Polyunsaturated fatty acids (PUFA) are PPAR ligands and fatty acids are produced via TGH activity, so we studied whether dietary fats and PPAR agonists could regulate TGH expression. In 3T3-L1 adipocytes, TGH expression was increased 10-fold upon differentiation, compared to pre-adipocytes. 3T3-L1 cells incubated with a PPARgamma agonist during the differentiation process resulted in a 5-fold increase in TGH expression compared to control cells. Evidence for direct regulation of TGH expression by PPARgamma could not be demonstrated as TGH expression was not affected by a 24-h incubation of mature 3T3-L1 adipocytes with the PPARgamma agonist. Feeding mice diets enriched in fatty acids for 3 weeks did not affect hepatic TGH expression, though a 3-week diet enriched in fatty acids and cholesterol increased hepatic TGH expression 2-fold. Two weeks of clofibrate feeding did not significantly affect hepatic TGH expression or microsomal lipolytic activities in wild-type or PPARalpha-null mice, indicating that PPARalpha does not regulate hepatic TGH expression. Therefore, TGH expression does not appear to be directly regulated by PPARs or fatty acids in the liver or adipocytes.

Current progress on esterases: from molecular structure to function

This article reports on a symposium sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the April 2001 Experimental Biology meeting. Current developments in molecular-based studies into the structure and function of cholinesterases, carboxylesterases, and paraoxonases are described. This article covers mechanisms of regulation of gene expression of the various esterases by developmental factors and xenobiotics, as well as the interplay between physiological and chemical regulation of enzyme activity.

Altered expression of the carboxylesterases ES-4 and ES-10 by peroxisome proliferator chemicals

The nonspecific carboxylesterases (EC.3.1.1.1) are a large group of enzymes that play important roles in the metabolism of foreign xenobiotics and endogenous lipids, including activators of the peroxisome proliferator-activated receptor alpha, a nuclear receptor that is the central mediator of peroxisome proliferator (PP) effects in the rodent liver. A number of reports have demonstrated that PP exposure leads to alterations in levels of carboxylesterases in the liver. In this study, we determined by Western blot analysis whether exposure to diverse PP results in alteration of expression of two highly expressed microsomal carboxylesterases. Chronic exposure to the PP WY-14,643 (WY) and gemfibrozil (GEM), but not di-n-butyl phthalate (DBP), led to decreases in ES-4 in male rat livers. ES-4 was increased in female rat livers treated with GEM. WY exposure led to decreases in ES-10 in male and female rat livers. ES-10 was increased in female rats treated with DBP. Compared with other end points that are altered within days after PP exposure, the downregulation of ES-4 and ES-10 by WY was considerably slower, occurring between 1 and 5 weeks of exposure. Decreased expression of ES-4 was observed at doses of WY or GEM as low as 10 or 8000 ppm, respectively, whereas decreased expression of ES-10 was more resistant to changes by any PP occurring only with WY at doses as low as 50 ppm. After chronic exposure to WY or diethylhexyl phthalate in wild-type mice, kidney, but not liver, expression of ES-4 and ES-10 was downregulated. These decreases in kidney ES expression were not observed in PPARalpha-null mice lacking a functional PPARalpha gene, demonstrating the importance of this transcription factor in these changes. These studies demonstrate that ES protein expression is under complex control by PP that is sex- and compound-dependent. These results lend support to the hypothesis that PP exposure leads to a reprogramming of expression of enzymes important in the metabolism of PPARalpha activators.

Purification, molecular cloning, and functional expression of dog liver microsomal acyl-CoA hydrolase: a member of the carboxylesterase multigene family

To clarify the reason for the high acyl-CoA hydrolase (ACH) activity found in dog liver microsomes, the ACH was purified to homogeneity using column chromatography. The purified enzyme, named ACH D1, exhibited a subunit molecular weight of 60 KDa. The amino terminal amino acid sequence showed a striking homology with rat liver carboxylesterase (CES) isozymes. ACH D1 possessed hydrolytic activities toward esters containing xenobiotics in addition to acyl-CoA thioesters, and these activities were inhibited by a specific inhibitor of CES or by CES RH1 antibodies. These findings suggest that this protein is a member of the CES multigene family. Since ACH D1 appears to be a protein belonging to the CES family, we cloned the cDNA from a dog liver lambdagt10 library with a CES-specific probe. The clone obtained, designated CES D1, possessed several motifs characterizing CES isozymes, and the deduced amino acid sequences were 100% identical with those of ACH D1 in the first 18 amino acid residues. When it was expressed in V79 cells, it showed high catalytic activities toward acyl-CoA thioesters. In addition, the characteristics of the expressed protein were identical with those of ACH D1 in many cases, suggesting that CES D1 encodes liver microsomal ACH D1.

The mammalian carboxylesterases: from molecules to functions

Multiple carboxylesterases (EC 3.1.1.1) play an important role in the hydrolytic biotransformation of a vast number of structurally diverse drugs. These enzymes are major determinants of the pharmacokinetic behavior of most therapeutic agents containing ester or amide bonds. Carboxylesterase activity can be influenced by interactions of a variety of compounds either directly or at the level of enzyme regulation. Since a significant number of drugs are metabolized by carboxylesterase, altering the activity of this enzyme class has important clinical implications. Drug elimination decreases and the incidence of drug-drug interactions increases when two or more drugs compete for hydrolysis by the same carboxylesterase isozyme. Exposure to environmental pollutants or to lipophilic drugs can result in induction of carboxylesterase activity. Therefore, the use of drugs known to increase the microsomal expression of a particular carboxylesterase, and thus to increase associated drug hydrolysis capacity in humans, requires caution. Mammalian carboxylesterases represent a multigene family, the products of which are localized in the endoplasmic reticulum of many tissues. A comparison of the nucleotide and amino acid sequence of the mammalian carboxylesterases shows that all forms expressed in the rat can be assigned to one of three gene subfamilies with structural identities of more than 70% within each subfamily. Considerable confusion exists in the scientific community in regards to a systematic nomenclature and classification of mammalian carboxylesterase. Until recently, adequate sequence information has not been available such that valid links among the mammalian carboxylesterase gene family or evolutionary relationships could be established. However, sufficient basic data are now available to support such a novel classification system.

Extraperoxisomal targets of peroxisome proliferators: mitochondrial, microsomal, and cytosolic effects. Implications for health and disease

Peroxisome proliferators are a structurally diverse group of compounds that include the fibrate hypolipidemic drugs, the phthalate ester industrial plasticizers, the phenoxy acid herbicides, and the anti-wetting corrosion inhibitors perfluorinated straight-chain monocarboxylic fatty acids. Administration of these chemicals to rodents results in a number of effects, the most prominent being hepatomegaly and induction of peroxisomal enzyme activities. Several of these compounds have also been associated with the production of liver tumors in rodents and are classified as nongenotoxic hepatocarcinogens. Experimental evidence suggests that humans are not susceptible to these effects following exposure to peroxisome-proliferating compounds. This has led to the proposal that an "actual threat to humans" from exposure to one of these compounds seems "rather unlikely". Indeed, recent reports suggest that peroxisome proliferators may prove valuable as antitumor agents in humans. However, this assessment is preliminary given that peroxisome proliferators also produce a myriad of extraperoxisomal effects in livers and other tissues of experimental animals. Such effects include both stimulation and inhibition of mitochondrial and microsomal metabolism and alteration of the activities of various cytosolic enzymes. These responses may be directly or indirectly related to the effects on peroxisomes or may be totally independent of these events. Whether the extraperoxisomal effects of these compounds occur in humans is not known and their potential impact on human health remains to be investigated.

Activity of esterases from different tissues of freshwater fish and responses of their isoenzymes to inhibitors

Activity of nonspecific esterase from different tissues (i.e., liver, gallbladder, heart, intestine, and muscle) of five species of freshwater fish, namely, topmouth gudgeon (Pseudorasbora parva), goldfish (Carassius auratus), nile tilapia (Tilapia nilotica), mosquitofish (Gambusia affinis), and rainbow trout (Salmo gairdneri) was tested using alpha-naphthyl acetate as substrate. The results indicated that activity of the enzyme was mainly concentrated in the digestive system (i.e., intestine, liver, bile). The overall activity was highest in nile tilapia, followed by mosquitofish, topmouth gudgeon, goldfish, and lowest in rainbow trout. Electrophoresis and the following in vitro treatment of the isoenzymes with triphenol phosphate (TPP, an inhibitor of carboxylesterase) indicated the TPP-sensitive esterase was mainly distributed in liver of the five species. The enzyme was not found in the other five tissues (including gill) except in gallbladder of topmouth gudgeon and goldfish. The correlation was obviously improved between susceptibility and detoxification capacity if activity of the TPP-sensitive esterase was employed instead of that of the nonspecific esterase to make the comparison. In vitro treatment of nonspecific esterase in liver with malaoxon proved that the active metabolite of malathion inhibited a different isoenzyme from the TPP-sensitive one.

Molecular aspects of carboxylesterase isoforms in comparison with other esterases

The involvement of carboxylesterase, acetylcholinesterase, butyrylcholinesterase and cholesterol esterase in pharmacology and toxicology are well recognized. However, there are few papers concerning the comparative studies of these serine hydrolases in terms of molecular level. Recently, we have studied various aspects of carboxylesterases using cDNAs of carboxylesterase isozymes purified from 9 animal species and human liver microsomes, and found that there is high homology of the N-terminal amino acid sequences of the isozymes tested. On the other hand, we compared the amino acid sequences at the active site of the individual esterases and found that the sequences of all esterases tested are strictly conserved. These results strongly suggest that the esterases involved are classified into the serine hydrolase super family.