Epidermis-type lipoxygenase 3 (ALOXE3 or eLOX3) is a member of the
lipoxygenase
Lipoxygenases () are a family of (non-heme) iron-containing enzymes most of which catalyze the dioxygenation of polyunsaturated fatty acids in lipids containing a cis,cis-1,4- pentadiene into cell signaling agents that serve diverse roles as aut ...
family of
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s; in humans, it is encoded by the ''ALOXE3''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
.
This gene is located on chromosome 17 at position 13.1 where it forms a cluster with two other lipoxygenases,
ALOX12B
Arachidonate 12-lipoxygenase, 12R type, also known as ALOX12B, 12''R''-LOX, and arachidonate lipoxygenase 3, is a lipoxygenase-type enzyme composed of 701 amino acids and encoded by the ''ALOX12B'' gene. The gene is located on chromosome 17 at po ...
and
ALOX15B
Arachidonate 15-lipoxygenase type II is an enzyme that in humans is encoded by the ''ALOX15B'' gene. ALOX15B, also known as 15-lipoxygenase-2 (15-LO-2 or 15-LOX-2), is distinguished from its related oxygenase, ALOX15 or 15-lipoxygenase-1.
Fun ...
.
Among the human lipoxygenases, ALOXE3 is most closely (54% identity) related in amino acid sequence to ALOX12B.
ALOXE3, ALOX12B, and ALOX15B are often classified as epidermal lipoxygenases, in distinction to the other three human lipoxygenases (
ALOX5
Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme (EC 1.13.11.34) that in humans is encoded by the ''ALOX5'' gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase ...
,
ALOX12
ALOX12 (), also known as arachidonate 12-lipoxygenase, 12-lipoxygenase, 12''S''-Lipoxygenase, 12-LOX, and 12''S''-LOX is a lipoxygenase-type enzyme that in humans is encoded by the ''ALOX12'' gene which is located along with other lipoyxgenases on ...
, and
ALOX15
ALOX15 (also termed arachidonate 15-lipoxygenase, 15-lipoxygenase-1, 15-LO-1, 15-LOX-1) is, like other lipoxygenases, a seminal enzyme in the metabolism of polyunsaturated fatty acids to a wide range of physiologically and pathologically importa ...
), because they were initially defined as being highly or even exclusively expressed and functioning in skin. The epidermis-type lipoxygenases are now regarded as a distinct subclass within the multigene family of mammalian lipoxygenases with mouse Aloxe3 (also termed e-Lox-3) being the
ortholog
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a s ...
to human ALOXE3, mouse Alox12b being the ortholog to human ALOX12B (MIM 603741), and mouse Alox8 being the ortholog to human ALOX15B (MIM 603697)
upplied by OMIM ALOX12B and ALOXE3 in humans, Alox12b and Aloxe3 in mice, and comparable orthologs in other in other species are proposed to act sequentially in a multistep metabolic pathway that forms products that are structurally critical for creating and maintaining the skin's water barrier function.
Tissue distribution
Immunologically detected ALOXE3 and ALOX12B in humans and Aloxe3 and Alox12b in mice have a similar tissue distribution in being highly expressed in the outer, differentiated layers of the epidermis; they co-localize at the surface of
keratinocytes
Keratinocytes are the primary type of cell found in the epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells.
Basal cells in the basal layer (''stratum basale'') of the skin are sometimes referre ...
in the
stratum granulosum
The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis lying above the stratum spinosum and below the stratum corneum ( stratum lucidum on the soles and palms).James, William; Berger, Timothy; Elston, Dirk (2005) '' ...
of mouse skin and during mouse
embryogenesis appear concurrently at the onset of skin development at day 15.5.
ALOXE3 mRNA in humans was also detected at low levels in the pancreas, ovary, brain, testis, placenta, and some secretory epithelia.
Aloxe3 and Alox12b
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
was detected in the tongue,
forestomach
The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. The rumen's microbial favoring environment allow ...
, trachea, brain, testis, and adipose tissue of mice and in the spinal cord of rats.
Activity
ALOXE3 is an atypical lipoxygenase in that under most but not all experimental conditions, it lacks the dioxygenase activity that converts
polyunsaturated fatty acids
Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic proper ...
(PUFAs) to hydroperoxide metabolites; rather, it possess
hepoxilin
Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol (i.e. hydroxyl) residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid ...
synthase (i.e. hydroperoxy isomerase) activity — that is, it converts hydroperoxy-containing PUFAs to hepoxilin-like epoxyalcohol products. These products, unlike those formed by non-enzymatic transformations, are specific isomers with just one form of the chiral hydroxy and epoxy residues. ALOX3E metabolizes 12''R''-HpETE to 8''R''-hydroxy-11''R'',12''R''-epoxy-eicosatrienoic acid
and metabolizes 9''R''-HpODE to products that contain either an epoxyalcohol or a
ketone residue.
It exhibits relatively weak activity in conducting this conversion on free 9''R''-HODE but stronger activity when 9''R''-HpODE is presented as its
methyl ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
. ALOXE3's primary function in epidermal tissue appears to be to metabolize the 9''R''-HpODE moiety that is not free but rather esterified to certain
ceramide
Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetyl sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make ...
lipids.
Linoleic acid
Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt or ester of this acid.
...
is the most abundant fatty acid in the skin
epidermis, being present mainly
esterified
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
to the omega-
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
residue of
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
-linked omega-hydroxylated
very long chain fatty acid A very-long-chain fatty acid (VLCFA) is a fatty acid with 22 or more carbons. Their biosynthesis occurs in the endoplasmic reticulum. VLCFA's can represent up to a few percent of the total fatty acid content of a cell.
Unlike most fatty acids, VL ...
s (VLCFAs) in a unique class of
ceramide
Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetyl sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make ...
s termed esterified omega-hydroxyacyl-
sphingosine
Sphingosine (2-amino-4-trans-octadecene-1,3-diol) is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phos ...
(EOS). EOS is an intermediate component in a proposed multi-step metabolic pathway which delivers very long-chain fatty acids (VLCFAs) to the cornified lipid envelope in the skin's
stratum corneum; the presence of these
wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...
-like, hydrophobic VLCFAs is needed to maintain the skin's integrity and functionality as a water barrier (see
Lung microbiome#Role of the epithelial barrier).
ALOX12B metabolizes the LA in EOS to its 9''R''-hydroperoxy derivative which ALOXE3 then converts to three ceramide-esterified products: a) 9''R'',10''R''-trans-
epoxide
In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale ...
,13''R''-hydroxy-10''E''-octadecenoic acid, b) 9-keto-10''E'',12''Z''-octadecadienoic acid, and c) 9''R'',10''R''-trans-epoxy-13-keto-11''E''-octadecenoic acid.
The ALOX12B/ALOE3-oxidized products, it is proposed, signal for their
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
(i.e. removal) from EOS; this allows the multi-step metabolic pathway to proceed in delivering the VLCFAs to the cornified lipid envelop in the skin's Stratum corneum.
AloxE3 appears responsible for forming hepoxilins A and/or B from 12''R''-HpETE in the spinal fluids of rats
and ALOXE3 is proposed to be responsible for the formation of these hepoxilins in various human tissues
although the presence and activity of ALOXE3 in many of these hepoxilin-forming tissues has not yet been demonstrated.
Spinal Aloxe3, apparently through its ability to make hepoxilins, appears responsible for the
hyperalgesia
Hyperalgesia ( or ; 'hyper' from Greek ὑπέρ (huper, “over”), '-algesia' from Greek algos, ἄλγος (pain)) is an abnormally increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves and can ...
which accompanies inflammation in rats.
Aloxe3 appears necessary and sufficient for the differentiation of mouse 3T3-L1
fibroblast cells into
adipocytes
Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...
(i.e. fat cells); the function of Aloxe3 in this differentiation appears to be to its metabolism 12''R''-HpETE into
hepoxilin
Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol (i.e. hydroxyl) residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid ...
s A3 or B3 which directly activate(s)
Peroxisome proliferator-activated receptor gamma
Peroxisome proliferator- activated receptor gamma (PPAR-γ or PPARG), also known as the glitazone reverse insulin resistance
receptor, or NR1C3 (nuclear receptor subfamily 1, group C, member 3) is a type II nuclear receptor functioning as a tran ...
which in turn initiates the expression of adipocyte-differentiation genes.
Clinical significance
Congenital ichthyosiform erythrodema
Deletions of ''Alox12b'' or ''Aloxe3'' genes by
gene knockout in mice cause a congenital scaly skin disease which is characterized by a greatly reduced skin water barrier function and other features found in the
autosomal recessive nonbullous
Congenital ichthyosiform erythroderma Congenital ichthyosiform erythroderma (CIE), also known as nonbullous congenital ichthyosiform erythroderma, is a rare type of the ichthyosis family of skin diseases which occurs in 1 in 200,000 to 300,000 births. CIE comes under the umbrella term a ...
(ARCI) disease of humans.;
homozygous recessive deleterious mutations in ALOXE3 or ALOX12B are likewise causes, albeit rare, of this congenital disease in humans.
ARCI refers to nonsyndromic (i.e. not associated with other signs or symptoms)
congenital Ichthyosis
Ichthyosis is a family of genetic skin disorders characterized by dry, thickened, scaly skin. The more than 20 types of ichthyosis range in severity of symptoms, outward appearance, underlying genetic cause and mode of inheritance (e.g., dominant ...
including
Harlequin-type ichthyosis
Harlequin-type ichthyosis is a genetic disorder that results in thickened skin over nearly the entire body at birth. The skin forms large, diamond/trapezoid/rectangle-shaped plates that are separated by deep cracks. These affect the shape of th ...
,
Lamellar ichthyosis
Lamellar ichthyosis, also known as ichthyosis lamellaris and nonbullous congenital ichthyosis, is a rare inherited skin disorder, affecting around 1 in 600,000 people.
Presentation
Affected babies are born in a collodion membrane, a shiny, waxy- ...
, and
Congenital ichthyosiform erythroderma Congenital ichthyosiform erythroderma (CIE), also known as nonbullous congenital ichthyosiform erythroderma, is a rare type of the ichthyosis family of skin diseases which occurs in 1 in 200,000 to 300,000 births. CIE comes under the umbrella term a ...
.
ARCI has an incidence of about 1/200,000 in European and North American populations; 40 different mutations in ''ALOX12B'' and 13 different mutations in ''ALOXE3'' genes account for a total of about 10% of ARCI cases; these mutations are homozygous recessive (see
Dominance (genetics)), cause a total loss of ALOX12B or ALOXE3 function (see
mutations), and can be associated with any of the three cited forms of the disease.
Hepoxilin synthase
In mice lacking Aloxe3 activity due to
gene knockout of the ''Alox3'' gene, levels in skin of
hepoxilin
Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol (i.e. hydroxyl) residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid ...
s A3 and B3, as well as their metabolites, trioxilins A3 and B3, are greatly reduced.
Furthermore, rat Aloxe3 has been implicated in the production of hepoxilin B3 in studies that transfected its gene into cultured
HEK 293 cells
Human embryonic kidney 293 cells, also often referred to as HEK 293, HEK-293, 293 cells, or less precisely as HEK cells, are a specific immortalised cell line derived from a spontaneously miscarried or aborted fetus or human embryonic kidney cel ...
and similarly implicated in the inflammation-induced production of hepoxilin B3 in the spine of rats as well as the perception of pain (i.e.
allodynia
Allodynia is a condition in which pain is caused by a stimulus that does not normally elicit pain. For example, bad sunburn can cause temporary allodynia, and touching sunburned skin, or running cold or warm water over it, can be very painful. It i ...
) by these animals using pharmacological inhibitor and
siRNA
Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA at first non-coding RNA molecules, typically 20-24 (normally 21) base pairs in length, similar to miRNA, and operating ...
-based gene knockdown studies.
Finally, cultured human skin cells, which are rich in ALOXE3 readily convert arachidonic acid as well as 12''S''-hydroperoxy-eicosatetraenoic acid to Hepoxilin B3; this production, in keeping with the higher content of ALOXE3, is far greater in the skin cells isolated from subjects with
psoriasis
Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by raised areas of abnormal skin. These areas are red, pink, or purple, dry, itchy, and scaly. Psoriasis varies in severity from small, localized patches to complete ...
.
These results suggest that ALOXE3 and its
orthologs
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
contribute greatly to or are the hepoxylin synthase activity responsible for producing bioactive hepoxilins (see
hepoxilin
Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol (i.e. hydroxyl) residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid ...
) in the skin and other ALOXE3/ortholog-rich tissues of mammals, possibly including humans.
ALOXE3 may be a key effector of the therapeutic response to fasting. Expressing ALOXE3 specifically in
hepatocyte
A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass.
These cells are involved in:
* Protein synthesis
* Protein storage
* Transformation of carbohydrates
* Synthesis of cholesterol, ...
s (liver parenchymal cells) retards weight gain and
hepatic steatosis (fatty liver) in mice rendered obese by feeding them a high-fat/high-sugar diet and in the
db/db mouse, which overeats due to a mutation in the
leptin receptor
Leptin receptor, also known as LEP-R or OB-R, is a type I cytokine receptor, a protein that in humans is encoded by the ''LEPR'' gene. LEP-R functions as a receptor for the fat cell-specific hormone leptin. LEP-R has also been designated as CD2 ...
.
In these mice, ALOXE3 overexpression stimulates higher basal
thermogenesis
Thermogenesis is the process of heat production in organisms. It occurs in all warm-blooded animals, and also in a few species of thermogenic plants such as the Eastern skunk cabbage, the Voodoo lily ('' Sauromatum venosum''), and the giant w ...
and cuts the link between obesity and
insulin resistance.
Some of these effects are recapitulated when ALOXE3 is activated by the
sugar alcohol
Sugar alcohols (also called polyhydric alcohols, polyalcohols, alditols or glycitols) are organic compounds, typically derived from sugars, containing one hydroxyl group (–OH) attached to each carbon atom. They are white, water-soluble solids ...
trehalose
Trehalose (from Turkish '' tıgala'' – a sugar derived from insect cocoons + -ose) is a sugar consisting of two molecules of glucose. It is also known as mycose or tremalose. Some bacteria, fungi, plants and invertebrate animals synthesize it ...
and its degradation-resistant analog lactotrehalose.
The mechanism appears to be through ALOXE3's synthesis of the
eicosanoid
Eicosanoids are signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosanoids are a s ...
12-KETE in hepatocytes, which act as a
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
for the insulin-sensitizing
nuclear receptor peroxisome proliferator-activated receptor gamma
Peroxisome proliferator- activated receptor gamma (PPAR-γ or PPARG), also known as the glitazone reverse insulin resistance
receptor, or NR1C3 (nuclear receptor subfamily 1, group C, member 3) is a type II nuclear receptor functioning as a tran ...
(PPAR-γ), the target of the
thiazolidinedione
The thiazolidinediones , abbreviated as TZD, also known as glitazones after the prototypical drug ciglitazone, are a class of heterocyclic compounds consisting of a five-membered C3NS ring. The term usually refers to a family of drugs used i ...
class of
diabetes drugs.
A caution about the human relevance of these findings is that humans with elevated of trehalose in their serum were found to be at elevated risk of incident diabetes.
Other possible clinical significances
The distribution of ALOXE3 suggests that this lipoxygenase may serve functions not only in the skin but also in other tissues. The pain perception and adipocyte differentiation activities of Aloxe3 in rodents might also occur in humans.
Toxicity
Interuterine delivery of eLox3 to mice at gestational day 14.5 resulted in fetal growth restriction and intrauterine death, apparently due to a strongly negative effect on placental development.
References
External links
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Further reading
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{{refend
Enzymes