1. Dr Alok Tripathi

2. The eicosanoids
The eicosanoids are a family of lipophilic hormones derived from the twenty carbon
fatty acid, arachidonic acid.
Although they are diverse in structure, many eicosanoids have roles in inflammation,
including regulation of vasodilation, vascular permeability, pain, and recruitment of
leukocytes.
Most members of this family are rapidly metabolized near their site of synthesis, so they
act locally on neighbouring cells, not distant parts of the body.
They are also not stored in cells, but synthesized rapidly in response to stimuli, making
regulation of their synthesis a key to their activity.
Many drugs act through modulating the production of eicosanoids or modulating
their signalling pathways.
Hormones in the eicosanoid family include the prostaglandins, thromboxanes,
leukotrienes, and prostacyclins.

3. the production of eicosanoids is
extracellular stimuli.
the release of arachidonic acid
from either diacylglycerol or
phospholipids by membrane
bound phospholipases
Arachidonic acid has several
possible fates,
•including oxygenation by
lipoxygenases to make HPETEs
(hydroperoxy-eicosatetraenoic acids),
•or production of prostaglandin H2 by
PGH2 synthase.
5-lipoxygenase acts with the
membrane bound protein
FLAP (five lipoxygenase
activating protein) to produce
the epoxide leukotriene LTA4
which is hydrolyzed to
•produce LTB4 or
•has glutathione added by a
glutathione S-transferase to produce
LTC4 and LTD4.

4.  A G-protein coupled receptor for LTD4, CysLT1, mediates an important component of the
inflammatory response of leukotrienes on airway constriction and recruitment of
leukocytes, and several marketed asthma drugs act as antagonists of the CysLT1 receptor.
PGH2 synthase actually consists of two enzyme components, a cyclooxygenase and a
peroxidase, and there are more than one type of cycloxygenase, including Cox-1 and Cox-
2. Recent NSAIDS acting selectively as Cox-2 inhibitors like Vioxx are widely used for the
treatment of arthritis and other inflammatory conditions, inhibiting the production of
downstream thromboxanes and prostaglandins. PGH2 also has several possible fates,
including conversion by thromboxane synthase to Tpx2, an eicosinoid with potent
coagulation and vasoconstriction activity. PGI2, or prostacyclin, synthesized by
prostacyclin synthase, has properties opposite those of thromboxane, causing
vasodilation and a reduction in clotting through the IP receptor, causing thromboxanes
and PGI2 to act in opposition to each other. Thromboxane antagonists and prostacyclin
agonists both provide tools and drugs to reduce vasoconstriction. The prostaglandins
include PGD2, PGE2 and PGF2, with varying degrees of selectivity among their receptors,
DP, EP and FP, respectively. PGE2 exerts biological effects including induction of pain,
fever and vasodilation through at least four receptors, EP1, EP2, EP3 and EP4, and EP3 is
found in multiple splice variants. The diversity of the eicosanoids and their receptors and
their involvement in many disease states makes it likely that this pathway will continue as
a major research focus.

6. Prostaglandins, thromboxanes & leukotrienes Synthesis
PG: prostaglandins - PGG2, PGH2 (constriction), PGD2 (constriction or vasodilation), PGE2 (vasodilation), PGF2a (constriction), PGI2
(prostacyclin, dilation)
LT: leukotrienes - LTB4, LTC4, LTD4, LTE4 (multiple roles, microvascular vasoconstriction)
TBX: thromboxanes - TXA2 (constriction), TXB2 (constriction)
NSAIDs: nonsteroidal anti-inflammatory drugs, aspirin, ibuprofen, acetaminophen (not really an NSAID); anti-inflammatory steroids work
by boosting levels of lipocortin (an annexin, Ca2+-dependent inhibitor protein/enzyme that acts on phospholipase A2)
Most of the enzymes are located in the smooth endoplasmic reticulum
Brain/nerves - PGD2, PGE2, and PGF2
Kidneys - PGE2 and PGI2
Lungs - PGD2
Synovial cells - PGE2 and PGI2 when stimulated by interleukin-1
Vascular beds - PGE2 and PGI2 & PGH2 and TXA2

7. PG:
prostaglandin
AC: adenylyl
cyclase
cAMP: cyclic
adenosine
monophophate
VSM: vascular
smooth muscle
TXA2:
thromboxane
A2

9. Pathway overview
IL: interleukin-1
IL-1R: interleukin-1 receptor
NSAIDS: nonsteroidal anti-inflammatory drugs, aspirin
(irreversible inhibitor of COX-1), ibuprofen (lesser ratio of COX-
1/COX-2), acetaminophen (Tylenol, does not affect COX-1 or COX-
2 but may indicate presence of a COX-3 or PCOX-1a or PCOX-1b
isoforms that are not involved in PG synthesis but address fever
and pain); anti-inflammatory steroids boost levels of Ca2+-
dependent inhibitor protein lipocortin

10. PG: prostaglandin
GSH: glutathione (reduced form)
GSSG: glutathione disulfide (oxidized form)
PC: PGI2 or prostacyclin
PGE2 synthase is also denoted PG endoperoxidase E
isomerase, microsomal form is key enzyme
Recall that most of the enzymes are located in the
smooth endoplasmic reticulum

12. COX: cyclooxygenase, COX1 constitutive (endoplasmic
reticulum), COX2 inducible (perinuclear envelope),
COX3 brain
NSAIDs: nonsteroidal anti-inflammatory drugs, aspirin,
ibuprofen, acetaminophen
PG: prostaglandins (PGI2 = prostacyclin, endothelial cells)
TX: thromboxane (TXA2 = thromboxane, platelets)

13. Arachidonic
Acid
COX-1 and COX-2 serve
identical functions prostanoids.
IL-1
Following exposure to
interleukin-1, synovial cells
→considerably more PGE2
and prostacyclin, but they
still do not synthesize
PGD, TXB2 or PGF2a.
The IL1-induced increase
in PGE2 and prostacyclin is
mediated through COX-2.
The specific prostanoid(s) generated in any given cell is determined by which distal
enzymes in the prostanoid synthetic pathways are expressed. For example, stimulated
human synovial cells synthesize small amounts of PGE2 and prostacyclin but not
thromboxane or PGD or PGF2a.

14. Thus,
the species of prostanoid
synthesized in a cell ἃ specific
distal synthetic enzyme(s)
expressed,
the amount synthesized is
determined by the amount of
COX —1 and —2 activities
expressed.
COX-1 is expressed in nearly all
cells in their basal
(unstimulated) state.
COX-1 mediated production of
thromboxane in platelets
promotes normal clotting.
And COX-1 mediated synthesis
of prostaglandins in the kidney
appears to be responsible for
maintaining renal plasma flow
in the face of vasoconstriction

16. COX-1 & COX-2 Comparison
Aspirin will inactivate
COX-1. Its effects take
longer to wear off because
it takes 24 hours for new
enzyme synthesis.
Aspirin will acylate COX-2,
but the larger active site
can still bind arachidonic
acid and may produce
other mediators.
Glucocorticosteroids have
effects at both the gene and
protein level.
Recall that COX-1 is found
primarily in the
endoplasmic reticulum and
COX-2 is located in the
perinuclear envelope.
There is recent work
suggesting that inducible
NO synthase activates
COX-2 (no effect on COX-
1).
Oxidized low-density
lipoprotein (LDL) appear
to increase gene expression
of COX-2.

18. Ar: arachidonic
acid
PG:
prostaglandin
PC:
prostacyclin
TX:
thromboxane