Periodontics

1. ARACHODONIC ACID
METABOLITES
Dr . P. Sanathana
MDS First Year
Department of Periodontics

2. CONTENTS
• INTRODUCTION
• ARACHODONIC ACID METABOLISM
• PATHWAYS
• ROLE OF ARACHODONIC ACID METABOLITES IN INFLAMMATION
• INHIBITON OF SYNTHESIS
• ARACHODONIC ACID METABOLITES IN PERIODONTAL DESTRUCTION
• ROLE OF ARACHODONIC ACID METABOLITES IN HOST MODULATION THERAPY
• CONCLUSION
• REFERENCES

3. INTRODUCTION
• Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty
acid 20:4(ω-6).
• Its name derives from the New Latin word arachis (peanut), but peanut oil does not
contain any arachidonic acid.

4.  Arachidonic acid (ARA) belongs to unsaturated fatty acids with four double bonds
(tetraenoic acids).
 Its general formula is C20H32O2, systematic name all - cis- 5,8,11,14-eicosatetraenoic acid
, number of carbons and number and position of double bonds 20:4;5,8,11,14 , series ω6 .
 It is an important component of phospholipids in animals. It enters the body either through
food (it is found especially in groundnuts - peanuts), or it is formed from the essential
unsaturated fatty acid linoleic acid (18:2;9,12).

5.  Arachidonic acid is a 20 C polyunsaturated fatty acid derived primarily from dietary
linoleic acid .
 Present as a component of cell membrane phospholipids.
 Products derived from the metabolism of arachidonic acid affect a variety of
metabolic processes including inflammation and haemostasis.

6.  Metabolites of AA are found in virtually all cells and tissues.
 Each cell type appears to have a characteristic balance of metabolites.
 Compounds are synthesised locally, on demand , and are not stored for future
release.
 They act locally in the area in which they are found and in general do not have
distant sites of action, as do many other types of chemical modulators.

7.  On a cellular level, three main phospholipases families can exert their action on phospholipids to
liberate the esterified AA.
 The first enzyme is phospholipase A2 (PLA2), which mediates the hydrolysis of the sn-2
position on phospholipid backbone, yielding a free AA molecule directly in one single step . The
second and the third enzymes are phospholipase C (PLC) and phospholipase D (PLD) that may
also generate free AA.
ARACHODONIC ACID METABOLISM

8.  Moreover, PLD was evidenced to liberate AA by the
following reactions.
 Phosphatidylcholine is catalyzed by PLD generating
phosphatic acid or DAG. The former can be further catalyzed
by phosphatidate phosphohydrolase to form DAG. Then,
DAG-lipase hydrolyzes DAG to generate AA.
 In two consecutive steps, PLC enzyme catalyzes phospholipids
yielding AA through the generation of diacylglycerol (DAG) by
the action of diacylglycerol lipase and lipid products containing
arachidonate by the action of monoacylglycerol lipases .

9. The expression and activation of PLA2 enzyme can be a response to a wide
range of cellular activation signals from receptor dependent events
requiring a G coupled transducing protein as Toll-like receptor 4 (TLR4),
purinergic receptors and inflammation stimulation to calcium ionophores,
melittin (bee venom) and tumor promoting agents.
Furthermore, the activation of PLA2 enzyme can be through the binding of
tumor necrosis factor alpha (TNF-a) to its receptor, P75 and P55, inducing
the release of AA from phosphatidylcholine and phosphatidylethanolamine.

10.  Free AA can have an important role in cell apoptosis as its accumulation that occurs as a result
of arachidonyl CoA transferase inhibition, can promote the activation of sphingomyelinase,
enzymes that trigger the degradation of sphingolipids (known to play an important role in cell
regulation and cell cycle) to phosphocholine and ceramide.
 Free AA can be metabolized via enzymatic reactions. Free AA can undergo four possible
enzymatic pathways: Cyclooxygenase, Lipoxygenase, Cytochrome p450 (CYP 450) and
Anandamide pathways to create bioactive oxygenated PUFA containing 20 C (eicosanoids)
acting as local hormones and other compounds acting as signaling molecules.

12.  Enzymes involved in the cyclooxygenase pathway are COX-
1 and COX-2 (also called prostaglandin H synthase), along
with downstream enzymes that mediate the production of
prostaglandins (PGH2, an unstable intermediate, PGE2,
PGD2 and PGF2alpha, prostacyclins (PGI2), and
thromboxanes (TXA2, TXB2).
 Lipoxygenase pathway consists of LOX-5, LOX-8, LOX-
12, and LOX-15 enzymes and their products, leukotrienes
(LTA4, an unstable intermediate, LTB4, LTC4, LTD4 and
LTE4), lipoxins (LXA4 and LXB4 formed upon LXA4
degradation) and 8– 12- 15- hydroperoxyeicosatetraenoic
acid (HPETE).

13. The CYP 450 pathway involves two enzymes, CYP450 epoxygenase and
CYP450 x-hydroxylase giving rise to epoxyeicosatrienoic acid (EETs)
and 20-hydroxyeicosatetraenoic acid (20-HETE) respectively.
Anandamide pathway comprises the FAAH (fatty acid amide hydrolase)
to produce the endocannabinoid, anandamide

14. PATHWAYS
CYCLOXYGENASE PATHWAY:
 There are two isoforms of cyclooxygenase (COX) or prostaglandin G/H synthases reacting
on free AA and producing ‘prostanoids’ a term used for prostaglandin and thromboxane
products as their basic molecule is prostanoic acid.
 The first isoform is COX-1, an enzyme constitutively expressed in all tissues that induces an
acute inflammation in response to short exposure to lipopolysaccharide (LPS) stimulation
and also directs the cell to promote or suppress the leukotrienes (LTs) biosynthesis .

15.  The term COX refers to enzymes also known as prostaglandin G/H synthases (PGHS),
which metabolize AA to PGH2 and PHG2. These PGs are substrates for a series of
downstream enzymes that generate specific PGs i.e. PGE2, PGI2, PGD2, PGF2, and TXA2.
 COX-1 prefers coupling and co-localization at perinuclear membrane or ER, with
thromboxane synthase, prostaglandin F synthase, and two other prostaglandin D synthases
isozymes, generating thromboxane A2 (TXA2), prostaglandin F2alpha, and prostaglandin
D2, respectively.
 The second, COX-2, is an inducible isoform found in kidney and brain macrophages and is
up regulated by inflammatory stimuli as bacterial endotoxin, cytokines, hormones, and
growth factors after 3 to 24 h stimulation.

16.  COX-2 prefers coupling with prostaglandin I synthase and three prostaglandin E synthases
(cPGES, mPGES-1 and mPGES-2) producing prostacyclin (PGI2) and prostaglandin E2,
respectively. A new isoform, an enzymatically active splice variant of COX-1, COX-3, has
been discovered in the brain and heart though its function is yet to be elucidated.
 The major difference between the 2 COX enzymes is that while COX-1 is more or
less ubiquitously and constitutively expressed, COX-2 is an inducible enzyme, albeit
with some important exceptions.

17.  Both cyclooxygenases are structurally alike but COX-2 active site fits with large substrates
as result of the substitution of isoleucine for valine at position 523 , and both introduce two
O2 molecules to AA to form a cyclic 9,11 endoperoxide, 15 hydroperoxide compound .
 The structure thus formed, PGG2, is an unstable compound that quickly converts to PGH2
through a hydroperoxide glutathione-dependent reaction .
 Further enzymatic sequences contribute in the generation of prostanoids (PGE2) (PGD2)
(PGF2alpha), prostacyclin (PGI2) and thromboxane (TXA2) as aforementioned, through
prostaglandin synthases isomers.

18.  The production of prostanoids is dependent on the expression of these enzymes at site of
inflammation. For example, platelets make primarily thromboxane A2 (TXA2) whereas
endothelial cells generate prostacyclin (PGI2), according to the enzyme content of the tissue
under consideration.
 Thromboxanes (Tx), PGI, PGE, PGF, and PGD metabolites are considered to be ligands for
TP, IP, EP, FP, and DP receptors, respectively.
 A further classification for TP receptor was suggested, including TP alpha and TP beta
subtypes, as well as, EP receptor that was divided into four subtypes EP1, EP2, EP3 and EP4.
The EP subtypes responding to the same agonist PGE2 but differing in their effector function.

19.  The genes encoding the human EP1, EP3, EP4, FP, IP, and TP receptors were mapped to
chromosome bands 19p13.1, 1p31.2, 5p13.1, 1p31.1, 19q13.3, and 19p13.3, respectively.
 Each receptor is encoded by a distinct gene and some variants and isoforms are generated
through alternative splicing of exons in their C-terminal tails region after the seventh
transmembrane domain.

20. Lipoxygenase pathway:
 The second possibility is for lipoxygenase (LOX) to act on free arachidonate. In this
pathway, oxygenation can take place at many different AA positions, an oxygen atom is
introduced at C-5, C-8, C-9, C-12 or C-15 through an array of lipoxygenases enzymes
numbered according to the oxygen introduced to the carbon atom for example, 5-LOX, 8-
LOX, 9-LOX etc..
 LOX-derived products are of hydroperoxyeicosatetraenoic acid (5, 8, 12, 15 HPETE).
 5-HPETE and 15-HPETE are responsible for leukotrienes and lipoxins production. The
latter have anti-inflammatory effect.

21.  Leukotrienes production require 5-LOX enzyme to produce the 5-
hydroperoxyeicosatetraenoic acid (5-HPETE) which is converted later to LTA4 through
leukotriene synthase (LT synthase).
 Two different enzymes exert their action on LTA4. One enzyme is LTA4 hydrolase that
uses water molecule to give diol, LTB4 that induces inflammation via its chemotactic and
degranulating actions on polymorphonuclear lymphocytes (PMN).
 The other one is glutathione S-transferase enzyme that adds a glutathione molecule to
generate LTC4. Further conversion occurs to LTC4 with the addition of amino acids to
produce peptidoleukotrienes LTD4 and LTE4.

22.  BLT1 and BLT2 receptors can recognize LTB4 and it is
mapped on 14q11.2 – q12 and on 14q12, respectively.
 CysLT1 and CysLT2 receptors can bind to
peptidoleukotrienes and those receptors are located on
Xq13.2-q21.1 and 13q14.2.
 On the other hand, biosynthesis of lipoxins can occur
through the following pathways.
 1- lipoxins can be carried out by 5-LOX in leukocytes
following the action of 12-LOX in platelets .
 2- In epithelial cells, lipoxins can be synthesized by the
15-LOX and followed by 5-LOX in leukocytes.

23.  3- Norris et al. Was able to demonstrate lipoxins production upon inflammation signals
mediated by TLR4 and P2X7 receptor in macrophages.
 Subsequent to toll-like receptor (TLR4) activation, 1 to 3 percent of oxygenated AA
converts to 15-HETE, mediated by the peroxide active site.
 The generated compound is either secreted and diffused from cells or incorporated in
membrane phospholipids via fatty acyl COA ligase.
 The incorporated 15-HETE can be hydrolyzed by the action of 5-LOX enzyme coupled
with cPLA2 and with the assistance of arachidonate 5-lipoxygenase-activating protein
(FLAP) necessary for LXA4 and epi-LXA4 (lipoxins) production.

24.  The last pathway usually is enhanced by the action of aspirin to inhibit COX-2 .
 Lipoxins act as ligands for G-protein coupled receptor (GPCR) known as ALXR. It was
identified as one of formylpeptide receptors (FPR) family that recognizes the N-formyl
peptides derived from bacterial degradation . Its gene is mapped on chromosome 19.

26. ROLE OF ARACHODONIC ACID
METABOLITES IN INFLAMMATION
 In one study , Cavanaugh et al. first performed the immunohistochemical analysis of
cyclooxygenase-2 protein expression in inflamed gingival tissues.
 They found that cyclooxygenase-1 and cyclooxygenase-2 proteins were expressed in
fibroblasts, gingival epithelial cells, endothelial cells, and inflammatory mononuclear
cells. They examined the expression of cyclooxygenase-1 and cyclooxygenase-2 proteins
in clinically healthy and inflamed human gingiva.
 In both types of gingiva, cyclooxygenase-1-immunoreactive cells were detected in
subepithelial connective tissue including fibroblasts and endothelial cells and some
gingival epithelial cells were slightly immunopositive for cyclooxygenase-1.

27.  In inflamed gingiva, inflammatory cells were also immunopositive for cyclooxygenase-1.
However, cyclooxygenase-2 protein was detected in fibroblasts, gingival epithelial cells,
endothelial cells, and inflammatory cells in inflamed gingiva, whereas in clinically healthy
gingiva, it was detected at low levels in only gingival epithelial cells and fibroblasts.
 In vitro studies have shown that cyclooxygenase-2 plays an important role in producing
prostaglandins in various cells stimulated with proinflammatory stimuli.
 Cultured monocytes/macrophages produce prostaglandin E2 in response to
lipopolysaccharides derived from periodontopathic bacteria including Actinobacillus
actinomycetemcomitans and P. gingivalis.

28.  This prostaglandin E2 production is completely inhibited by NS-398, a specific
cyclooxygenase- 2 inhibitor, and cyclooxygenase-2 messenger ribonucleic acid and protein
expression is induced, whereas cyclooxygenase-1 messenger ribonucleic acid and protein
expression is not changed, which suggests that human monocytes produce prostaglandin E2
via cyclooxygenase-2 in response to the lipopolysaccharides of periodontopathic bacteria.
 Prostaglandin E2 production was enhanced in lipopolysaccharide-stimulated peripheral blood
monocytes from patients with localized aggressive periodontitis, compared with healthy
subjects.

29.  Interleukin-1b is a potent stimulator of prostaglandin production via cyclooxygenase-2 in
human gingival fibroblasts .
 Interleukin-1b challenged human gingival fibroblasts induce cyclooxygenase- 2
expression via tyrosine kinase pathways.
 Tumor necrosis factor-a is a less potent stimulator of prostaglandin E2 production
compared with interleukin-1, but tumor necrosis factor-a and interleukin-1 synergistically
enhance prostaglandin E2 production .
 Human gingival fibroblasts challenged with periodontopathic bacteria
lipopolysaccharides produce prostaglandin E2 via cyclooxygenase- 2 induction, which is
regulated by tyrosine kinases.

30.  In human periodontal ligament cells, it has been reported that interleukin-1a and
interleukin-1b potently induce prostaglandin E2 via cyclooxygenase- 2 induction .
 Mechanical stress also induces cyclooxygenase-2 .
 Tumor necrosis factor-a alone is a weak stimulator of prostaglandin E2 production in
periodontal ligament cells.
 However, tumor necrosis factor-a synergistically produces prostaglandin E2 with
interleukin-1, like human gingival fibroblasts.

31.  Leukotriene B4 is probably the most potent neutrophil chemotactic agent produced by the
arachidonic acid cascade.
 Compared with other lipoxygenase by products that are chemotactic for neutrophils (for
example, 5-, 12-, and 15-hydroxyeicosatetraenoic acid and 5-, 12-, and 15
hydroperoxyeicosatetraenoic acid), LTB4 exerts substantially stronger chemokinetic effects
on human cells .
 Intratracheal instillation of LTB4 induces the selective recruitment of functionally active
neutrophils into bronchoalveolar lavage fluid in humans . Subcutaneous injection of LTB4
into humans causes neutrophils to accumulate rapidly in the affected tissue.

32.  Leukotriene B4 may play a pivotal role in the induction of neutrophil-endothelial cell
adherence .
 In vivo, the topical application of LTB4 to the vascular network within the hamster cheek
pouch results in immediate and reversible adherence of neutrophils to venular endothelial cells.
 Leukotriene B4-induced endothelial cell hyper adhesiveness for neutrophils depends on
increased CD11/CD18 expression on the neutrophil surface and possibly a specific domain of
the adhesion molecule CD54 found on endothelial cells.

33.  At nanomolar concentrations, LTB4 causes the release of substantial quantities of
glucuronidase and lysozyme from neutrophils, although less effectively than the chemotactic
fragment of the complement component C5 .
 Leukotriene B4-induced enzyme secretion is mediated by LTB4 recognition of a surface
receptor of substantially lower affinity than that which mediates neutrophil aggregation,
adherence to endothelium, and chemotaxis.
 In vitro, LTB4 stimulates myelopoiesis, a phenomenon that may be linked to the secretion of
significant quantities of LTB4 by human bone marrow cells .

34.  Leukotriene B4 augments interleukin-6 production in human monocytes by increasing
both interleukin-6 gene transcription and messenger RNA (mRNA) stabilization ;
activation of nuclear factor (NF)-K B and NF-interleukin-6 transcriptional factors may
be important in this enhancement of interleukin-6 release .
 Leukotriene B4 may modulate the production of other cytokines by stimulating gene
transcription of the proto-oncogenes c-jun and c-fos in mononuclear cells.

35.  T-lymphocyte proliferation in response to mitogen also appears to depend on LTB4.
 Leukotriene B4 may exert its proliferative effect on T lymphocytes through the
stimulation of interleukin-2 secretion .
 Leukotriene B4 can replace interleukin-2 in the induction of interferon- y by T cells.
 Moreover, T cells treated with LTB4 appear to be able to suppress lymphocyte
proliferation through interferon-y-modulated augmentation of monocyte interleukin-1
secretion.

36.  Lipoxin (LX) and aspirin-triggered LX (ATL) are arachidonic acid-derived bioactive lipids that
are formed by interactions between individual lipoxygenases (LO) and appear to play an
important role in downregulating neutrophil responses in inflammation .
 In the nanomolar range, LXA4 and its 15R epimer (15-epi-LXA4) triggered by aspirin each
inhibit fMLP- and LTB4-stimulated PMN adhesion and transmigration and hence represent
potential counterregulatory signals operative in the resolution of inflammatory site.
 Like most autacoids and lipid mediators, LX are rapidly generated, act within a local
microenvironment, and are rapidly enzymatically inactivated.

37.  In some studies, LXA4, PGE2, andLTB4 were identified in human CF, and an oral
microbe clinically associated with periodontal disease, Porphyromonas gingivalis, was
examined in an animal model of leukocyte trafficking and activation.
 This microbe potently attracted leukocytes in vivo and also upregulated the expression of
COX-2 from infiltrating leukocytes. Moreover, topical administration of metabolically
stable analogues of LX-ATL within the pouch cavity potently blocked P. gingivalis
mediated neutrophil infiltration.

38. INHIBITION OF SYNTHESIS
 There are several endogenous inhibitors to suppress prostaglandin production in vivo.
 Glucocorticoid can down-regulate cyclooxygenase-2 expression to reduce prostaglandin
production . In addition to glucocorticoid, interleukin-4, interleukin-10, and interleukin-
13 are well known as anti-inflammatory cytokines.
 The cytokines can inhibit the production of proinflammatory cytokines such as
interleukin-1, interleukin-6, interleukin-8, and tumor necrosis factor-a by monocytes and
suppress bone resorption . Interleukin-4, interleukin-10, and interleukin-13 can inhibit
prostaglandin production via down-regulation of cyclooxygenase-2 expression in human
monocytes and neutrophils.

39.  In human gingival fibroblasts and periodontal ligament cells, interleukin-4 decreased
interleukin-1-induced prostaglandin production via inhibition of cyclooxygenase- 2
expression with no effect on cyclooxygenase-1 expression.
 Interleukin-13 also inhibits interleukin-1-induced prostaglandin production via inhibition of
cyclooxygenase-2 expression although it is less potent compared to interleukin-4 .
 Recently, it has been shown that interleukin-4 receptor and interleukin-13 receptor
(interleukin-13 receptor a1 chain) are expressed in human gingival fibroblasts .
 Interleukin-4, interleukin- 10, and interleukin-13 are detected in inflamed periodontal tissues.

40.  Furthermore, interferon- c, a Th1 cytokine, also decreases interleukin-1-elicited
prostaglandin E2 production in human gingival fibroblasts and human periodontal ligament
cells .
 Hayashi et al. reported that interferon-c inhibited interleukin 1-induced cyclooxygenase- 2
expression, but could not find the inhibitory effect of interferon-c on cyclooxygenase-2
expression .
 It is plausible that in periodontal lesions there are inhibitory systems to regulate
prostaglandin production.

41. Hypothetical regulatory mechanism of cyclooxygenase-2
(COX-2) expression and prostaglandin E2
(PGE2)production through cell–cell interaction in
periodontal lesions. As a stimulatory mechanism for PGE2
production in periodontal lesions, periodontopathic
pathogens may directly activate monocytes/macrophages
(M/ and fibroblasts to induce COX-2 expression, resulting
in PGE2 production, or may stimulate
monocytes/macrophages and gingival epithelial cells to
produce proinflammatory cytokines including interleukin-
1 (IL-1) and tumor necrosis factor-a (TNF-a), which
induce COX-2 protein in monocyte/macrophages and
fibroblasts to produce PGE2.
In some settings, anti-inflammatory cytokines including
interleukin-4 (IL-4), IL-10, and IL-13 may be involved in
inhibiting PGE2 overproduction by down-regulating COX-
2 expression.

42. Traditional non-steroidal anti-inflammatory drugs effectively inhibit the progression of periodontal
diseases.

43.  Thromboxane inhibitors are broadly classified as either those that inhibit the synthesis of
thromboxane , or those that inhibit the target effect of it.
 Thromboxane synthesis inhibitors , in turn , can be classified regarding which step in the
synthesis they inhibit.
 The widely used drug aspirin acts by inhibiting the ability of the COX enzyme to synthesise
the precursors of thromboxane within platelets.
 Low dose , long-term aspirin use irreversibly blocks the formation of thromboxane A2 in
platelets , producing an inhibitory effect on platelet aggregation.

44.  Thromboxane synthase inhibitors inhibit the final enzyme (thromboxane synthase) in the
synthesis of thromboxane .
 Ifetroban is a potent and selective thromboxane receptor antagonist.
 Dipyridamole antagonizes this receptor too, but has various other mechanisms of antiplatelet
activity as well.
 High dose Naproxen can induce near complete suppression of platelet thromboxane throughout
the dosing interval and appears not to increase cardiovascular risk , where as other high dose
NSAID regimens have only transient effects on platelet COX -1 and have been found to be
associated with a small but definite vascular hazard.

45.  The inhibitors of the target effects of thromboxane are the thromboxane receptor
antagonist including terutroban.
 Picotamide has activity both as a thromboxane synthase inhibitor and as a
thromboxane receptor antagonist,
 Ridogrel is another example.

46.  One of the most promising is aimed at inhibiting the 5-
lipoxygenase enzyme, which plays a crucial role in the
biosynthesis of LTA4 .
 Inhibiting this enzyme would lead to suppression of the
formation of both LTB4 and sulfidopeptide
leukotrienes.
 The 5-lipoxygenase inhibitor most studied in clinical
trials is zileuton. A hydroxyurea compound with
chelating activity, zileuton inhibits the active-site iron
of 5-lipoxygenase at concentrations that do not inhibit
cyclooxygenase, 12-lipoxygenase, or 15-lipoxygenase .

47.  MK-0591 is a representative 5-lipoxygenase-activating protein inhibitor that effectively
blocks leukotriene generation and is under clinical evaluation.
 Whereas 5-lipoxygenase and 5-lipoxygenase-activating protein-inhibitor drugs proximally
block the arachidonic acid cascade, thus preventing leukotriene formation, an alternative
pharmacologic approach is distal selective blockade of the actions of LTB4 and
sulfidopeptide leukotrienes by specific LTB4 and LTD4 receptor antagonists, respectively .
 The availability of selective leukotriene inhibitors and antagonists has helped elucidate the
role of leukotrienes in the pathogenesis of many inflammatory diseases, most notably
asthma, ulcerative colitis, rheumatoid arthritis, and psoriasis.
 Leukotriene formation can also be inhibited by compounds that bind tightly to 5-
lipoxygenase-activating protein , thus preventing translocation of 5-lipoxygenase to the
cell membrane.

49. ARACHODONIC ACID METABOLITES IN
PERIODONTAL DESTRUCTION
• Periodontitis is a chronic inflammatory condition of the periodontium involving interactions
between bacterial products, numerous cell populations and inflammatory mediators.
• It is generally accepted that periodontitis is initiated by complex and diverse microbial biofilms
which form on the teeth, i.e. dental plaque.
• Substances released from this biofilm such as lipopolysaccharides, antigens and other virulence
factors, gain access to the gingival tissue and initiate an inflammatory and immune response,
leading to the activation of host defence cells.
• As a result of cellular activation, inflammatory mediators, including cytokines, chemokines,
arachidonic acid metabolites and proteolytic enzymes collectively contribute to tissue destruction
and bone resorption.

50.  A range of arachidonic acid metabolites are produced in the gingival tissues.
 These eicosanoids include prostanoids and leukotrienes, which are produced from arachidonic
acid through distinct enzymatic systems.
 Leukotrienes, known to play an important role in asthma and allergy, are also involved in bone
remodeling.
 Leukotriene B4 (LTB4) in particular has been implicated in RA, which is highly similar to
periodontitis in that it is a chronic inflammatory condition which affects bone remodeling.
 A possible role for LTB4 has been suggested in the progression of periodontal disease because
of the findings that the substantial increase in GCF LTB4 concentrations, which are associated
with the severity of periodontal disease, decreased following periodontal treatment.

51.  Some leukotrienes also have anti-inflammatory effects, and one such leucotriene investigated
in relation to periodontal disease is Resolvin E1 (RvE1).
 This anti-inflammatory eicosanoid has been reported to down-regulate inflammation-induced
bone loss in experimental periodontitis and inhibit osteoclast growth and bone resorption by
interfering with osteoclast differentiation.
 It was also recently reported that RvE1 restored impaired phagocytic activity in macrophages
from the blood of patients with aggressive periodontitis and inhibited LTB4-induced
production of the antimicrobial peptide LL-37 from PMNs, thus terminating the LL-37/LTB4
proinflammatory circuit.

52.  Prostaglandins are a group of potent arachidonic acid-derived inflammatory mediators with
the capacity to induce a wide variety of biological responses .
 They influence many biological processes, including vasodilatation, vascular permeability,
oedema, pain and fever, and the mediator also play an immunoregulatory role in neutrophil
and monocyte chemotaxis .
 Prostaglandins, synthesised by virtually all mammalian cells, are local hormones, acting at or
near the site of their synthesis.
 They function in both an autocrine and a paracrine fashion and modulate the responses of
other hormones, which have profound effects on many cellular processes.
 Among prostaglandins, PGE2 is the most prominent in the pathogenesis of periodontitis.

53.  PGE2 is produced by immune cells, fibroblasts and other resident gingival cells and has
a wide range of biological effects on the cells of the diseased gingiva .
 The actions of PGE2 include the stimulation of inflammatory mediators and MMPs, as
well as osteoclast formation via receptor activator of nuclear factor-κB ligand
(RANKL) .
 The effect of PGE2 on a specific cell type depends on the prostaglandin receptors, EP1
through EP4. The receptors most relevant to the pathogenesis of periodontitis are EP2
and EP4, which are reported to activate adenylate cyclase and protein kinase A
signalling .
 In rodent models, these two receptors have been shown to be involved in bone
resorption in response to PGE2.

54.  Several clinical alterations observed in periodontal disease can be associated with PGE2,
especially when IL-1 and TNFα are present in the gingival tissue.
 PGE2 is detected at significantly higher levels in human inflamed gingival tissue and
especially from periodontal sites exhibiting recent attachment loss .
 Higher levels of PGE2 are also found in the GCF of patients with periodontitis compared
with levels found in GCF of healthy individual.
 Accordingly, increasing levels of PGE2 in crevicular fluid have been suggested to serve as
a predictor of periodontal attachment loss .

55.  Furthermore, polymorphisms within the cyclooxygenase-2 (COX-2) gene as well as the methylation
levels within the COX-2 promoter, which affect COX-2 mRNA expression, have been repeatedly
implicated in periodontitis.
 Altogether, over-production of PGE2 is suggested to have a significant role in the pathobiology of
periodontitis.
 Maintenance of the extracellular matrix is important for normal development and function of gingival
tissue.
 Proteolytic MMP enzymes and their endogenous inhibitors, tissue inhibitors of metalloproteinases
(TIMPs), are involved in the homoeostasis of the extracellular matrix in healthy tissue, but they are also
key players in the process of tissue destruction in inflammatory diseases.
 Besides modifying the extracellular matrix, MMPs are also involved in regulating the activities of
cytokines and cytokine receptors.

56.  In periodontitis, both host and bacteria-derived proteolytic enzymes contribute to the
degradation of the extracellular matrix of the connective tissue.
 Numerous host proteolytic enzymes such as MMPs, elastase, mast cell tryptase,
dipeptidyl peptidase, plasminogen activators and the lysosomal cysteine proteinases,
cathepsins and protease 3 have been detected in the GCF of patients with periodontitis .
 Increased expressions of MMPs (gelatinase and collagenase) are associated with
pathological conditions including RA and periodontitis.

57.  MMP expression and activity are in general low in noninflamed periodontium but increase
to pathologically high levels in inflamed gingiva, where increased levels of inflammatory
mediators upregulate MMP expression.
 In in vitro studies, the inflammatory mediators IL-1β, TNFα and bacterial LPS upregulate
MMP-1, -3, -8 and -9 expression in gingival fibroblasts .
 Moreover, the periodontal pathogen Porphyromonas gingivalis, in the presence of cigarette
smoke condensate, increases collagen degradation and protein levels of MMP-1, -2, -3 and -
14 in gingival fibroblasts.
 The cytokine IL-1β stimulates MMP-2 expression via a PGE2-dependent mechanism in
human chondrocytes.

58.  The close interactions between PGE2 and the MMPs are further emphasised by the key role
of PGE2 in the regulation of MMP-9 expression in macrophages and in the induction of
MMP-3 and MMP-13 in chondrocytes via the PGE2-regulatory enzyme microsomal
prostaglandin E synthase-1 (mPGES-1) .
 Moreover, PGE2 stimulates MMP-1 production in human gingival fibroblasts via activation
of mitogen-activated protein kinases (MAPKs)/activator protein-1 (AP- 1) and nuclear factor-
κB (NF-κB) and in mouse osteoblasts via the cAMP-PKA signalling pathway.

59.  The major pathway by which the inflammatory mediator PGE2 stimulates bone resorption
is generally considered to be via the up-regulation of RANKL expression and the
inhibition of OPG expression in osteoblastic cells .
 In osteoclastogenesis, the stimulatory effect of oral pathogen sonicates has been
demonstrated to be mainly mediated through the PGE2/RANKL pathway in primary
mouse osteoblasts cocultured with bone marrow cells .
 It has also been reported that RANKL-stimulated osteoclastogenesis can be enhanced by
PGE2 and LPS through direct effects on the haematopoietic cell lineage .
 PGE2 has been shown both to inhibit and stimulate OPG expression , a contradiction
which may be the result of differing incubation times, as has been suggested for the effect
of PGE2 on osteoclast formation.

61. ARACHODONIC ACID METABOLITES
IN HOST MODULATION THERAPY
 Periodontitis is a complex disease in which the host immune inflammatory response caused by the
bacterial challenge results in connective tissue destruction and bone resorption.
 During disease activity, numerous inflammatory cells and resident cells in the periodontium express
and/or stimulate inflammatory mediators including PGE2, cytokines, chemokines, MMPs and
proteins of signal transduction pathways collectively contributing to the destruction of soft and hard
tissue.
 Traditional periodontal therapy has focused on decreasing the microbial challenge by mechanically
disrupting and removing bacterial biofilms that form on tooth surfaces and adjacent soft tissue.
 A growing number of studies, however, have indicated strong potential for adjunctive host-
modulating drugs as new therapeutic strategies in the management of periodontal disease

62.  As Nobel Laureate John R. Vane first suggested in 1971 , the COX enzymes are the primary
targets for nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin.
 NSAIDs inhibit the first step of the reaction, the formation of PGG2. Specific COX-2 inhibitors
have been developed to achieve inhibition of inflammation-induced PGE2 production without the
detrimental inhibition of baseline, COX-1- derived prostaglandin production was thought to
account for the gastrointestinal side-effects of traditional NSAIDs .
 Treatment strategies with nonselective NSAIDs and selective COX-2 inhibitors have suggested a
potential adjuvant role for COX-inhibitors in periodontal therapy .
 Evidence from animal experiments and clinical trials demonstrates that both NSAIDs and
selective COX-2 inhibitors are generally responsible for stabilisation of periodontal conditions by
reducing the rate of alveolar bone resorption.

63.  Recently, it was also reported, in a small sample size, that “low-dose” aspirin may reduce the
risk of periodontal attachment loss .
 In contrast, adjunctive treatment with oral administration of meloxicam does not seem to
improve clinical parameters or GCF levels of PGE2 and IL-1β .
 In experimental periodontitis of rats, the selective COX-2 inhibitor celecoxib and
prophylactic omega-3 fatty acid, alone and in combination, inhibit gingival tissue MMP-8
expression.
 One of the most promising groups of inhibitors are the disubstituted phenanthrene
imidazoles, which were found to be orally active in a guinea pig model.
 The indole 5- lipoxygenase-activating protein inhibitor MK- 886 and its derivatives have
been shown to inhibit mPGES-1 in enzyme assays.

64.  Several mPGES-1 inhibitors are being studied in animal models, but none are as yet
available for use in humans.
 In experimental periodontitis in rats, the mPGES-1 inhibitor curcumin effectively inhibited
cytokine gene expression at the mRNA and the protein level and inhibited activation of NF-
κB in the gingival tissues although the inhibitor did not prevent alveolar bone resorption .
 It was also recently reported that aminothiazoles targeting mPGES-1 decrease PGE2
synthesis in vitro and ameliorate experimental periodontitis in vivo.
 PGE2 inhibitors, targeting the enzyme COX using NSAIDs or specific COX-2 inhibitors,
have been shown to block periodontal PGE2 synthesis and prevent disease progression in
numerous animal models and a few clinical studies .

65. CONCLUSION
 Arachidonic acid metabolites contribute to inflammation by
1. Increasing capillary permeability
2. Inducing local vasodilation and thus redness
3. Promoting infiltration of inflammatory cells
4. Production of tissue injuring oxygen free radicals during the synthesis of prostaglandins and
leukotrienes.
5. Producing inflammation associated hyperalgesia (increased pain).

66. REFERENCES
• Carranza – 13 th edition
• Hanna VS, Hafez EAA. Synopsis of arachidonic acid metabolism: A review. J Adv Res.
2018 Mar 13;11:23-32.
• Wang, B., Wu, L., Chen, J. et al. Metabolism pathways of arachidonic acids: mechanisms
and potential therapeutic targets. Sig Transduct Target Ther 6, 94 (2021).
• Noguchi K, Ishikawa I. The roles of cyclooxygenase-2 and prostaglandin E2 in
periodontal disease. Periodontol 2000. 2007;43:85-101.
• Page RC. The role of inflammatory mediators in the pathogenesis of periodontal disease.
J Periodontal Res. 1991 May;26(3 Pt 2):230-42.

67. THANK YOU