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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Genetica. Author manuscript; available in PMC 2011 July 1.
Published in final edited form as:
PMCID: PMC2896070
NIHMSID: NIHMS199127

Mammalian carboxylesterase 3: comparative genomics and proteomics

Abstract

At least five families of mammalian carboxylesterases (CES) catalyse the hydrolysis or transesterification of a wide range of drugs and xenobiotics and may also participate in fatty acyl and cholesterol ester metabolism. In this study, in silico methods were used to predict the amino acid sequences, secondary and tertiary structures, and gene locations for CES3 genes and encoded proteins using data from several mammalian genome projects. Mammalian CES3 genes were located within a CES gene cluster with CES2 and CES6 genes, usually containing 13 exons transcribed on the positive DNA strand. Evidence is reported for duplicated CES3 genes for the chimp and mouse genomes. Mammalian CES3 protein subunits shared 58–97% sequence identity and exhibited sequence alignments and identities for key CES amino acid residues as well as extensive conservation of predicted secondary and tertiary structures with those previously reported for human CES1. The human genome project has previously reported CES3 mRNA isoform expression in several tissues, particularly in colon, trachea and in brain. Predicted human CES3 isoproteins were apparently derived from exon shuffling and are likely to be secreted extracellularly or retained within the cytoplasm. Mouse CES3-like transcripts were localized in specific regions of the mouse brain, including the cerebellum, and may play a role in the detoxification of drugs and xenobiotics in neural tissues and other tissues of the body. Phylogenetic analyses demonstrated the relationships and potential evolutionary origins of the mammalian CES3 family of genes which were related to but distinct from other mammalian CES gene families.

Keywords: Mammals, Amino acid sequence, Carboxylesterase, Evolution, Gene duplication

Introduction

At least five families of mammalian carboxylesterases (CES; E.C.3.1.1.1) have been reported (Holmes et al. 2008a, b, 2009a, b, c, d) including CES1, the major liver enzyme (Munger et al. 1991; Shibita et al. 1993; Ghosh 2000; Holmes et al. 2009a); CES2, the major intestinal enzyme (Langmann et al. 1997; Schewer et al. 1997; Holmes et al. 2009a); CES3, expressed in liver, colon and brain (Sanghani et al. 2004; Holmes et al. 2009a, b, c); CES5 (also called CES7 or ‘cauxin’), a major urinary protein of the domestic cat (Miyazaki et al. 2003, 2006; Holmes et al. 2008b); and CES6, a predicted CES-like enzyme in brain (Clark et al. 2003; Holmes et al. 2009d). These enzymes catalyze hydrolytic and transesterification reactions with xenobiotics, anticancer pro-drugs and narcotics (Satoh and Hosokawa 1998, 2006; Satoh et al. 2002; Ohtsuka et al. 2003; Redinbo and Potter 2005), and detoxify organophosphates and insecticides (Ahmad and Forgash 1976; Leinweber 1987). CES may also catalyze several cholesterol and fatty acid metabolic reactions (Ghosh 2000; Tsujita and Okuda 1993; Becker et al. 1994; Hosokawa et al. 2007; Diczfalusy et al. 2001), the conversion of lung alveolar surfactant (Ruppert et al. 2006) and assist with the assembly of low density lipoprotein particles in liver (Wang et al. 2007).

Structures for several human and animal CES genes have been determined, including human (Becker et al. 1994; Langmann et al. 1997; Ghosh 2000; Marsh et al. 2004) and rodent CES1 and CES2 ‘like’ genes (Ghosh et al. 1995; Dolinsky et al. 2001; Hosokawa et al. 2007). Predicted gene structures have also been described for other mammalian CES gene families, including CES1, CES2 and CES5 genes, which are localized in two contiguous CES gene clusters on human (chromosome 16), horse (chromosome 3), cow (chromosome 18) and opossum (chromosome 1) genomes (Holmes et al. 2008a, 2009a, b, c). Mammalian CES genes usually contain 12–14 exons of DNA encoding CES enzyme sequences which may undergo exon shuffling generating several CES isoproteins in each case (Thierry-Mieg and Thierry-Mieg 2006). There are similarities for the exon boundaries for each of the mammalian CES genes (Holmes et al. 2008a, b, 2009a, b, c, d) as well as significant sequence identities, especially for key regions previously identified for the major human liver CES1 (Bencharit et al. 2003, 2006; Fleming et al. 2005). Three-dimensional structural analyses of human CES1 have identified three major ligand binding sites, including the promiscuous active site, ‘side door’ and ‘Z-site’, where substrates, fatty acids and cholesterol analogues, respectively, are bound; and the ‘gate’, reportedly functioning in product release (Bencharit et al. 2003, 2006; Fleming et al. 2005).

This paper reports predicted gene structures and amino acid sequences for several mammalian CES3 genes and proteins, the predicted secondary and tertiary structures for mammalian CES3 protein subunits, and the structural and evolutionary relationships for these genes and enzymes with those for other mammalian CES gene families. This study also tests an hypothesis that mammalian CES3 genes are members of a distinct CES gene family to those previously reported, including CES1 (encoding the major liver isozyme) (Munger et al. 1991), CES2 (encoding the major intestinal CES) (Schewer et al. 1997), CES5 (also called ‘cauxin’ or CES7) (Holmes et al. 2008b) and CES6 (transcribed in human brain and also called CES8) (Holmes et al. 2009d).

Methods

In silico mammalian CES gene and protein identification

BLAST (Basic Local Alignment Search Tool) studies were undertaken using web tools from the National Center for Biotechnology Information (NCBI) (http://blast.ncbi.nlm.nih.gov/Blast.cgi) (Altschul et al. 1990). Protein BLAST analyses used mammalian CES amino acid sequences previously described (Table 1). Non-redundant protein sequence databases for several mammalian genomes were examined using the blastp algorithm, including human (Homo sapiens) (International Human Genome Sequencing Consortium 2001); chimp (Pan troglodytes) (Chimpanzee Sequencing and Analysis Consortium 2005); orangutan (Pongo abelii) (http://genome.wustl.edu); the cow (Bos Taurus) (Bovine Genome Project 2008); horse (Equus caballus) (Horse Genome Project 2008); mouse (Mus musculus) (Mouse Genome Sequencing Consortium 2002); and opossum (Monodelphis domestica) (Mikkelsen et al. 2007). This procedure produced multiple BLAST ‘hits’ for each of the protein databases which were individually examined and retained in FASTA format, and a record kept of the sequences for predicted mRNAs and encoded CES-like proteins. These records were derived from annotated genomic sequences using the gene prediction method: GNOMON and predicted sequences with high similarity scores for human CES6. Predicted CES3-like protein sequences were obtained in each case and subjected to in silico analyses of predicted protein and gene structures.

Table 1
Mammalian CES3 and human CES1, CES2, CES5 and CES6 genes and subunits

BLAT analyses were subsequently undertaken for each of the predicted CES3 amino acid sequences using the UC Santa Cruz web browser [http://genome.ucsc.edu/cgi-bin/hgBlat] (Kent et al. 2003) with the default settings to obtain the predicted locations for each of the mammalian CES3 genes, including predicted exon boundary locations and gene sizes. BLAT analyses were similarly undertaken of CES1, CES2, CES5 and CES6-like human genes using previously reported sequences for encoded CES subunits in each case (see Table 1). Structures for human CES3 isoforms (splicing variants) were obtained using the AceView website (http://www.ncbi.nlm.nih.gov/IEB/Research/Acembly/index.html?human) to examine predicted gene and protein structures using the human CES3 cDNA sequence (Gen-Bank NM024922) to interrogate this database of human mRNA sequences (Thierry-Mieg and Thierry-Mieg 2006).

BLAT (BLAST-Like Alignment Tool) in silico studies were undertaken using the UC Santa Cruz web browser [http://genome.ucsc.edu/cgi-bin/hgBlat] (Kent et al. 2003) with the default settings. UniProtKB/Swiss-Prot Database [http://au.expasy.org] and GenBank [http://www.ncbi.nlm.nih.gov/Genbank/] sequences for human, orangutan and mouse CES sequences (Table 1) were used to interrogate human, chimp, orangutan, horse, cow, mouse and opossum genome sequences. Gene locations, predicted gene structures and CES protein subunit sequences were observed for each CES examined for those regions showing identity with the respective mammalian CES gene products.

Predicted structures and properties of mammalian CES subunits

Predicted secondary and tertiary structures for human and other mammalian CES-like subunits were obtained using the PSIPRED v2.5 web site tools [http://bioinf.cs.ucl.ac.uk/psipred/psiform.html] (McGuffin et al. 2000) and the SWISS MODEL web tools [http://swissmodel.expasy.org/], respectively (Guex and Peitsch 1997; Kopp and Schwede 2004). The reported tertiary structure for the rabbit CES1 4-piperidino–piperidine complex (Bencharit et al. 2003) served as the reference for the predicted human CES3 tertiary structure, with a modeling range of residues 34–553. Theoretical isoelectric points and molecular weights for mammalian CES3 subunits were obtained using Expasy web tools (http://au.expasy.org/tools/pi_tool.html). SignalP 3.0 web tools were used to predict the presence and location of signal peptide cleavage sites (http://www.cbs.dtu.dk/services/SignalP/) for each of the predicted mammalian CES sequences (Emmanuelsson et al. 2007). The NetNGlyc 1.0 Server was used to predict potential N-glycosylation sites for human CES and mammalian CES3 subunits (http://www.cbs.dtu.dk/services/NetNGlyc/). Predictions of subcellular locations for mammalian CES3 ‘like’ enzymes and for human CES1, CES2, CES5 and CES6 were conducted using PSORT 11 (http://psort.ims.u-tokyo.ac.jp/form2.html) (Horton and Nakai 1997).

Mouse brain CES3 expression

The Allen Mouse Brain Atlas was interrogated for cellular transcription of the mouse CES3 gene using the GenBank BC061004 sequence (see Table 1) (Lein et al. 2007). Data for sagittal sections of mouse brain were examined and recorded for mouse CES3 transcripts using web tools available on the Allen Brain Atlas web site (http://www.brain-map.org).

Phylogenetic analyses and sequence divergence

ClustalW-derived alignments of CES protein sequences were assembled using BioEdit v.5.0.1 and the default settings (Hall 1999). Alignment ambiguous regions, including the amino and carboxyl termini, were excluded prior to phylogenetic analysis yielding alignments of 448 residues for comparisons of mammalian CES1, CES2, CES3, CES5 and CES6 and Drosophila melanogaster CES6 (or Est6) sequences (Table 1). Evolutionary distances were calculated using the Kimura option (Kimura 1983) in TREECON (Van de Peer and De Wachter 1994). Phylogenetic trees were constructed from evolutionary distances using the neighbor-joining method (Saitou and Nei 1987) and were rooted using the Drosophila melanogaster CES6 (Est6) sequence. Tree topology was re-examined by the boot-strap method (1,000) of resampling (Felsenstein 1985).

Results and discussion

Alignments of human CES subunits and mammalian CES3 amino acid sequences

The deduced amino acid sequences for horse, cow and mouse CES3-like subunits are shown in Fig. 1 together with the previously reported sequences for human CES1 (Munger et al. 1991; Shibita et al. 1993); human CES2 (Langmann et al. 1997; Schewer et al. 1997); human CES3 (Sanghani et al. 2004); human CES5 (Holmes et al. 2008b); and human CES6 (Holmes et al. 2009d) (Table 1). Alignments of the human and other mammalian CES3 subunits examined in this figure showed between 68 and 77% sequence identities, suggesting that these CES protein subunits are products of the same family of genes, whereas sequence alignments of mammalian CES3 subunits with human CES1, CES2, CES5 and CES6 subunits exhibited lower levels of sequence identities with human CES3 (47, 50, 48 and 45%, respectively), indicating that these are members of distinct CES families (Table 2).

Fig. 1
Amino acid sequence alignments for mammalian CES3 and human CES1, CES2, CES5 and CES6 subunits. See Table 1 for sources of CES sequences; * shows identical residues for CES subunits;: similar alternate residues;. dissimilar alternate residues; residues ...
Table 2
Percentage identities for mammalian CES3 and human CES1, CES2, CES5 and CES6 subunit amino acid sequences

The amino acid sequence for human CES3 contained 571 residues whereas other predicted mammalian CES3 subunits contained fewer amino acids: 570 residues for cow and horse CES3; 554 residues for mouse CES3; and 568 residues for mouse CES3L (Fig. 1). Previous studies on human CES1 have identified key residues which contribute to the catalytic, subcellular localization, oligomeric and regulatory functions for this enzyme (sequence numbers refer to human CES1). These include the catalytic triad for the active site (Ser228; Glu345; His458) (Cygler et al. 1993); microsomal targeting sequences, including the hydrophobic N-terminus signal peptide (von Heijne 1983; Zhen et al. 1995; Potter et al. 1998) and the C-terminal endoplasmic reticulum (ER) retention sequence (His-Ile-Glu-Leu) (Robbi and Beaufay 1991); disulfide bond forming residues (Cys95/Cys123 and Cys280/Cys291) (Lockridge et al. 1987); and ligand binding sites, including the ‘Z-site’ (Gly358), the ‘side door’ (Val424-Met425-Phe426) and the ‘gate’ (Phe551) residues. Identical residues were observed for each of the mammalian CES3 subunits for the active site triad and disulfide bond forming residues for human CES1 although several changes were observed for the corresponding residues for CES3 subunits of other key human CES1 residues: the ‘side-door’ identified for human CES1 (Val424-Met425-Phe426) (Bencharit et al. 2003, 2006) has a Val416-Phe417-Ile418 sequence for human CES3; the ‘Z-site’ (Gly358 for human CES1) has been changed to Ser355 for human CES3, although other mammalian CES3 sequences examined have retained a glycine residue at this position; the hydrophobic N-terminal sequence for mammalian CES3 sequences has undergone major changes, however, this region retains in each case a predicted signal peptide property; the mammalian CES3 C-terminal sequences have undergone significant changes in sequence (QEDL, QEEL or PEEL) in comparison with human CES1 (HIEL). This may be responsible for a change in the predicted microlocalization for the CES3 subunits (secreted or extracellular enzymes), as compared with human CES1, which is retained by the endoplasmic reticulum (Table 1). The ‘gate’ residue has also undergone changes for the mammalian CES3 sequences examined from Phe551 (CES1) to Tyr (human and horse CES3) or Leu (cow and mouse CES3).

Other key human CES1 sequences included two charge clamps which are responsible for subunit–subunit interaction, namely residues Lys78/Glu183 and Glu72/Arg193 (Bencharit et al. 2003, 2006; Fleming et al. 2005). Predicted mammalian CES3 subunit sequences for these sites indicated that only one of the charge clamps would be retained for human CES3 subunits: Arg88/Glu191 (Fig. 1), whereas other mammalian CES3 subunits examined showed no potential for forming charge clamps at the corresponding sequence positions. Previous reports have shown that human and baboon CES2 subunits, which lack these charge clamp residues, behave as monomers (Pindel et al. 1997; Holmes et al. 2009a). With the exception of human CES3, the mammalian CES3 sequences have undergone amino acid substitutions for those residues which contribute to the oligomeric subunit structure for human CES1. The identified human CES1 charge clamp residues Lys78/Glu183 and Glu72/Arg193 have other residues at the corresponding sites (Arg88/191Lys and Gln82/Ala194 for the horse CES3 sequence) (Fig. 1), and it is likely that these mammalian CES3 subunits are also monomeric in subunit structure. This may have a major influence on the kinetics and biochemical roles for mammalian CES3 since three dimensional studies have suggested that ligand binding to the human CES1 ‘Z-site’ shifts the trimer–hexamer equilibrium towards the trimer facilitating substrate binding and enzyme catalysis (Redinbo and Potter 2005). This property is unlikely to be shared by mammalian CES3 which is expected to display distinct kinetic properties to the oligomeric CES1 enzyme.

The N-glycosylation site reported for human CES1 at Asn79-Ala80-Thr81 (Kroetz et al. 1993; Bencharit et al. 2003, 2006; Fleming et al. 2005) has not been retained for any of the mammalian CES3 sequences, although additional potential N-glycosylation sites were observed at one or several positions (Table 3). Given the reported role of the N-glycosylated carbohydrate group in contributing to mammalian CES stability and maintaining catalytic efficiency (Kroetz et al. 1993), this property may be shared by the mammalian CES3 subunits as well, especially for those containing multiple potential sites for N-glycosylation, such as the mouse CES3L sequence, which contains four such sites.

Table 3
Predicted N-glycosylation sites for mammalian CES3 and human CES1, CES2, CES5 and CES6 subunits

Predicted secondary and tertiary structures for mammalian CES3 subunits

Analyses of predicted secondary structures for mammalian CES3 sequences were compared with the previously reported secondary structure for human CES1 (Bencharit et al. 2003, 2006) and with those predicted for human CES2, CES5 and CES6 (Holmes et al. 2008b, 2009d) (Fig. 1). Similar α-helix β-sheet structures were observed for all of the mammalian CES3 and human CES gene products examined. Consistent structures were apparent near key residues or functional domains including the α-helix within the N-terminal signal peptide; the β-sheet and α-helix-helix structures near the active site Ser228 (human CES1) and ‘Z-site’ (Glu354/Gly356, respectively); the α-helices bordering the ‘side door’ site; and the α-helix containing the ‘gate’ residue (Phe551 for human CES1). In addition, two random coil regions (residues 51–115 and 169–188 for human CES1) were predominantly retained for all forms of mammalian CES3 examined which have been shown for human CES1 to contain two charge clamps sites: (Lys79/Glu183 and Glu73/Arg186); an N-glycosylation site at Asn79-Ala80-Thr81; a second potential N-glycosylation site for human CES2, and one of the disulfide bridges (87Cys/117Cys) reported for human CES1. The human CES5 secondary structure, however, predicted an additional helix at the hydrophobic C-termini, in comparison with mammalian CES3 C-terminal sequences. In addition, the predicted 3-D structure for human CES3 is similar to the reported for human CES1 (Bencharit et al. 2003, 2006) (Fig. 2). These were based on structures reported for residues 21–553 of human CES1 and a predicted structure for residues 34–553 of human CES3. The single helix and following two β-sheet structures at the C-termini (shown in red at the top) for the human CES1 and CES3 subunits are readily apparent, as are four β-sheet structures at the N-termini of the CES subunits (shown in blue at the base), in each case. It is apparent that human CES3 is likely to be highly similar in its secondary and tertiary structure to that reported for human CES1.

Fig. 2
Tertiary structure for human CES1 subunit and predicted tertiary structure for human CES 3 subunit. The structure for human CES1 is taken from Bencharit et al. (2003). Predicted human CES3 3-D structure was obtained using the SWISS MODEL web site http://swissmodel.expasy.org/workspace/index.php ...

Predicted gene locations and exonic structures for mammalian CES3 genes

Table 1 summarizes the predicted locations for mammalian CES3 genes based upon BLAT interrogations of several mammalian genomes using the reported sequences for human CES3, mouse CES3 and CES3L, and the predicted sequences for horse and cow CES3 and the UC Santa Cruz Web Browser (Kent et al. 2003). The predicted mammalian CES3 genes were located in a CES2/CES3/CES6 gene cluster in each case, although the gene order underwent changes for different species. In addition, mouse showed evidence of duplicate CES3 genes, with predicted CES3 and CES3L genes being located within a four CES gene cluster on chromosome 8: CES2-CES3-CES3L-CES6 (Table 1). Chimp CES3 also provided evidence for duplicate forms of this gene on chromosome 16 but with a changed order of genes: CES6-CES3L-CES2-CES3 (Table 1). BLAT interrogations of mammalian genomes with the corresponding CES sequences suggested that the CES2-CES3-CES6 gene cluster was syntenic for chromosomes 16 (human, chimp and orangutan), 3 (horse), 18 (cow), 8 (mouse) and 1 (opossum), respectively.

With the exception of the opossum CES3 gene, mammalian CES3 ‘like’ genes were apparently transcribed on the positive strand, as for the human CES2 and CES6 genes. In contrast, human CES1 and CES5 genes were transcribed on the negative strand (Table 1). Figure 1 summarizes the predicted exonic start sites for cow, horse and mouse CES3 genes with each having 13 exons, in identical or similar positions to those predicted for the human CES3 gene (Holmes et al. 2009a). The predicted chimp CES3L gene, however, exhibited only 11 predicted exons. Human CES1 (Munger et al. 1991; Shibita et al. 1993), CES2 (Tang et al. 2008), CES5 (Ota et al. 2004) and CES6 (Holmes et al. 2009d) genes contained 14, 12, 13 and 14 exons, respectively (Fig. 1), in similar positions to those predicted for the mammalian CES3 genes (Fig. 1).

Figure 3 shows the predicted nucleotide sequences for exons 2–4 and introns 2 and 3 of mouse CES3 and CES3L genes. The sequences for the exons were 98% identical with nearly identical amino acid sequences involving five amino acid substitutions for these genes, whereas mouse CES3 and CES3L introns 2 and 3 showed distinct nucleotide sequences which were 85 and 94% identical, respectively. In addition, BLAT analyses of the mouse genome revealed distinct but closely localized predicted positions for these genes, being separated by ~26 kilobases of DNA on chromosome 8 (Table 1). These data support an hypothesis for two closely related and localized CES3-like genes in the mouse, for which the subunits have been previously designated as esterase-31 (also called CES3) (Aida et al. 1993) and esterase-31L (also called CES3L) (Mouse Genome Sequencing Consortium 2002).

Fig. 3
Nucleotide sequence alignments for mouse CES3 ‘Like’ genes: CES3 (Es31) and CES3L (Es31L): predicted exons 2–4-; and predicted introns 2 and 3. Exon sequences are in CAPITALS and intron sequence in lower case. Note the different ...

Mouse brain CES3 and CES3L expression

Figure 4 shows a sagittal section of mouse brain examining the distribution of CES3 and CES3L transcripts (GenBank mRNA BC061004 and BC019047) within different regions of the brain (provided by the Allen Institute for Brain Science) (Lein et al. 2007). The cerebellum folds exhibited highest staining levels for both transcripts although staining was observed throughout the brain, including the hippocampus, amygdalar nuclei, the olfactory bulb, the cortex, the pons and the medulla regions.

Fig. 4
Sagittal section of mouse brain showing the distribution of mouse CES3 and CES3L transcripts. The two distinct regions of the mouse brain (C57BL6/6J strain) included the cerebellum (right) and the cerebrum (left). Note major staining of CES3 and CES3L ...

The differential tissue and subcellular distributions of CES family members may assist in determining the differential roles of CES family members within mammalian organisms. Mammalian liver is predominantly responsible for drug and xenobiotic clearance from the body with CES1 and CES2 (with CES1 > CES2) playing major roles, following absorption of drugs and xenobiotics into the circulation (Pindel et al. 1997; Imai 2006). Mammalian intestine (with CES2 > CES1) is predominantly responsible for first pass clearance of several drugs and xenobiotics, with the activity occurring mostly in the ileum and jejunum and processed via CES2 (Imai et al. 2003). CES1 and CES2 also serve different roles in prodrug activation, as shown for the anti-cancer drug irinotecan (CPT-11) which is converted to its active form SN-38 predominantly by CES2 (Humerickhouse et al. 2000; Xu et al. 2002). In contrast with CES1 and CES2, mammalian CES5 is predominantly expressed in peripheral tissues, including brain, kidney, lung and testis (Thierry-Mieg and Thierry-Mieg 2006), and in cat, is a secreted form of CES enzyme appearing in cat urine (Miyazaki et al. 2003, 2006). Mammalian CES5 may serve in two major roles within mammalian fluids and peripheral tissues, including regulating the production of a pheromone precursor in urine (Miyazaki et al. 2003, 2006) and contributing to lipid and cholesterol transfer processes within male reproductive fluids (Ecroyd et al. 2006).

The metabolic role(s) for mammalian CES3 has not been extensively investigated although the enzyme is capable of activating prodrugs such as irinotecan (Sanghani et al. 2004) and the gene is expressed as distinct isoforms in tissues of the body, including the colon, placenta and neural tissues, such as the cerebellum and hippocampus (Fig. 5; Table 4) (Thierry-Mieg and Thierry-Mieg 2006). Human CES3 mRNAs encode isoforms which are predicted to be either secreted or extracellular (CES3a–c), cytosolic (CES3d) or extracellular (CES3e–u), and may play distinct roles in carboxyl-ester metabolism in the body. A comparison of the predicted cellular locations for mammalian CES3 reveals that it is likely to be extracellular (including the cell wall) in primate (human, chimp and orangutan) and mouse tissues, whereas horse and bovine CES3 are predicted to be localized in the endoplasmic reticulum, which is comparable with human CES1 and CES2. The extracellular locations predicted for human and mouse CES3 may reflect distinct roles for this enzyme in drug metabolism within peripheral tissues of the body.

Fig. 5
Gene structures and splicing variants for the human CES3 (NM024922) and the mouse CES3 (BC061004) genes. Derived from AceView website (Thierry-Mieg and Thierry-Mieg 2006) http://www.ncbi.nlm.nih.gov/IEB/Research/Acembly/. Mature isoform variants (a, b, ...
Table 4
Tissue distribution of human mRNA transcripts and predicted encoded CES3 isoforms

Phylogeny and divergence of CES3 and other mammalian CES sequences

Phylogenetic trees (Fig. 6) were constructed from ClustalW-derived alignments of human, chimp, orangutan, horse, cow, mouse and opossum CES3-like amino acid sequences with human CES1, CES2, CES5 and CES6 and Drosophila melanogaster CES6 sequences. The dendrogram was rooted using the latter Drosophila CES6 sequence and showed clustering of all of the CES3-like sequences which were distinct from the other human CES gene families. In addition, the CES3 and CES3L mouse sequences showed clustering with the mouse (and rat) CES3-like genes, which is consistent with these genes being products of recent duplication events during mammalian evolution. Holmes et al. (2008a) have previously demonstrated that the gene duplication events which generated the ancestral mammalian CES1, CES2, CES3, CES5 and CES6 genes predated the common ancestor for marsupial and eutherian mammals [estimated at ~ 173–193 million years ago by Woodburne et al. (2003)], and appeared to coincide with the early diversification of tetrapods approximately 350–360 million years ago (Donoghue and Benton 2007). The mammalian CES3 family of genes may therefore be considered to be of an ancient origin, prior to the appearance of mammals in evolutionary time.

Fig. 6
Phylogenetic tree of mammalian CES3, human CES1, CES2, CES5 and CES6 and fruit fly (Drosophila melanogaster) CES6 amino acid sequences. The tree is labeled with the gene name and the name of the animal. Note the major cluster for mammalian CES3 ‘like’ ...

The mammalian CES gene complex: differential roles and comparison with other gene family complexes

The mammalian CES3 gene family is localized within a CES2-CES3-CES6 gene complex on eutherian and marsupial mammalian genomes, with a consistent gene order for most genomes examined, with the exception of the chimpanzee (Pan troglodytes) and orangutan (Pongo abelii) genomes (Table 1). The other two CES gene families (CES1 and CES5) are also usually located on the same mammalian chromosome, but within a separate distantly located gene complex (see Holmes et al. 2008b, 2009d). The existence of these two mammalian CES gene complexes may reflect the evolutionary origins of these genes by gene duplication processes, as well as the timing and sequence of gene duplication events generating the CES1-CES5 and CES2-CES5-CES6 gene complexes prior to mammalian radiation and evolution.

Mammalian CES gene families have, however, diverged significantly in terms of their differential gene regulation and in the properties of encoded CES isozymes, which may reflect their differential and specialized roles in the body. In addition to being enzymes of broad substrate specificities in carboxyl-ester metabolism, CES family members perform several specialized roles in the body, including: CES1: liver drug clearance (see Satoh and Hosokawa 2006; Hosokawa et al. 2007), lung surfactant conversion (Ruppert et al. 2006); and macrophage cholesterol ester metabolism (Ghosh 2000); CES2: intestine drug clearance (see Marsh et al. 2004; Imai 2006); CES3: colon and neural drug metabolism (Sanghani et al. 2004); CES5: pheromone and lipid transfer metabolism (Miyazaki et al. 2003, 2006; Ecroyd et al. 2006); and CES6: predicted roles in drug and xenobiotic metabolism in the brain (Holmes et al. 2009d). The mammalian CES gene complexes stand in contrast to the CES gene families reported in a model plant, Arabidopsis thaliana, for which 20 genes were identified (Marshall et al. 2003). With the exception of three clusters of tandemly duplicated genes, these were present in four of five plant chromosomes.

The mammalian CES gene complexes are comparable to those previously reported for other mammalian genes and enzymes, including alcohol dehydrogenase (ADH), which occurs as a gene complex on the same chromosome in human (Homo sapiens) (chromosome 4) (see Jornvall et al. 2000) and mice (Mus musculus) (chromosome 3) (Holmes et al. 1983) genomes, and comprises at least five families of ADH genes, for which encoded isozymes perform differential roles on a wide range of alcohol substrates (see Hoog et al. 2003). The number and diversity of genes found within the CES gene complex is in contrast, however, with the large numbers of genes reported for the major histocompatibility complex (MHC), for which a 3.6 Mb MHC region on human chromosome 6 contains 140 genes, which are divided into three classes, with differential roles in the immune system of the body (see MHC Sequencing Consortium 1999).

Conclusion

The results of the present study indicate that mammalian CES3 genes and encoded CES3 enzymes represent a distinct CES gene and enzyme family which share key conserved sequences and structures that have been reported for human CES1 and have family specific sequences consistent with the oligomeric and monomeric subunit structures for CES1 and CES3, respectively. This study also reports that chimp and mouse genomes contain at least two CES3 ‘like’ genes, which are located in tandem with the CES2 and CES6 genes, and with the more distantly located CES1 and CES5 genes on chromosomes 16 and 8, respectively. Predicted secondary structures and tertiary structures for mammalian CES3 subunits showed a high degree of conservation with human CES1. Phylogeny studies using several mammalian CES3 subunits (human, chimp, orangutan, mouse, horse, cow and opossum) indicated that CES3’ like’ genes have apparently appeared during mammalian evolution well prior to the eutherian and marsupial common ancestor more than 170 million years ago, which is consistent with previous studies (Holmes et al. 2008a).

Acknowledgments

This project was supported by NIH Grants P01 HL028972 and P51 RR013986. In addition, this investigation was conducted in facilities constructed with support from Research Facilities Improvement Program Grant Numbers 1 C06 RR13556, 1 C06 RR15456, 1 C06 RR017515. We gratefully acknowledge the assistance of Dr B. Patel in studying the phylogeny of CES3 and related CES gene families.

Contributor Information

Roger S. Holmes, Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, TX 78227, USA ; rholmes/at/sfbrgenetics.org. Southwest National Primate Research Center, Southwest Foundation for Biomedical Research, San Antonio, TX, USA. School of Biomolecular and Physical Sciences, Griffith University, Nathan, QLD, Australia.

Laura A. Cox, Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, TX 78227, USA. Southwest National Primate Research Center, Southwest Foundation for Biomedical Research, San Antonio, TX, USA.

John L. VandeBerg, Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, TX 78227, USA. Southwest National Primate Research Center, Southwest Foundation for Biomedical Research, San Antonio, TX, USA.

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