The corneal epithelium expresses all three pathways of AA
metabolism, each of which can each generate bioactive eicosanoids.
However, the relative activity of the individual pathways can be
differentially modulated by pathophysiological conditions such as
hypoxia. COX-derived PGE
2 has long been
implicated as a mediator of ocular inflammation because of its
established role in inflammation in other tissues and its increased
production seen in association with ocular inflammation. The partial
effectiveness of nonsteroidal anti-inflammatory drugs (NSAIDs) in
alleviating ocular inflammation has furthered this notion. The current
data did not exclude a role for PGE
2 in ocular
inflammation under normoxic conditions; however, the marked decrease
(>90%) in PGE
2 synthesis in hypoxia argues
strongly against a role for PGE
2 in mediating
disease associated with oxygen deprivation. Consistent with this point,
Duffin et al.
20 found a significant decrease in
neovascularization with gas-permeable contact lenses when compared with
gas-impermeable lenses (thus, under oxygen conditions in which
PGE
2 levels are predicted to be higher, there is
less neovascularization). It is noteworthy that
PGE
2 formation was severely diminished under
hypoxic culture conditions compared with normoxic culture, despite
nearly equivalent levels of total COX expression in each condition.
Equally interesting was the observation that in hypoxia a more than
threefold increase in COX-2 was associated with a more than 90%
decrease in PGE
2 synthesis. Moreover, we found
that corneas that had been cultured for 24 hours in hypoxia displayed
COX activity that was nearly equivalent to that in normoxia-cultured
corneas once oxygen was reintroduced during a 2-hours incubation with[
14C]AA. Juranek et al.
21 have
reported that most AA oxygenases (COX, LOX) are saturated with oxygen
above 30 μM (normoxia, ≈250 μM) but that under hypoxic conditions
(5–20 μM O
2) the activity of these enzymes
becomes limited. The current findings suggest that oxygen availability
is the major limitation for COX catalysis under hypoxic conditions.
This hypothesis is supported by the observations that the accumulation
of endogenous PGE
2 in the medium of corneas
cultured in hypoxia is only 10% of that produced by corneas cultured
under atmospheric oxygen tension; the acute ability of COX to
metabolize exogenously added [
14C]AA is rapidly
inhibited by hypoxia; there are equivalent amounts of COX protein under
normoxic and hypoxic conditions, and these proteins exhibit nearly
equivalent catalytic activity when normal oxygen conditions are
restored; and phospholipase activity, and thus free AA, is elevated
under hypoxic conditions,
22 23 which should serve to
increase COX activity.