Prostaglandin endoperoxidase H synthase (EC
126.96.36.199, PGHS) is a bifunctional enzyme. Firstly it catalyzes the conversion
of arachidonic acid to PGG2 via
activity, and then PGG2
is converted to prostaglandin
the peroxidase activity. Two PGHS isoenzymes, also named COX-1 and
COX-2, were identified and cloned (1-6). COX-derived PGH2
occupies a central position in the biosynthesis of prostanoids. Since 1976 it
is known that in arterial wall PGH2
synthase (PGI-S) to prostacyclin (7)
and in platelets by TXA2
synthase (TX-S) to
(8). Other prostanoids such as PGD2
were thought to be generated non- enzematically from PGH2
(9-12). In early eighties, Gerritsen and Printz proposed an unknown enzyme to
be responsible for isomerisation of PGH2
, but their attempts to purify microsomal
PGE isomerase failed (13). Not long ago Jakobsson et al.
cloned human prostaglandin E synthase (EC 188.8.131.52, PGE-S) as a membrane-bound,
microsomal, glutathione-dependent, inducible enzyme (14). The second PGE-S isoform
in cytosol was suggested (15-17). Cytosolic PGE-S seems to be identical to p23,
a chaperon for the hsp90/glucocorticoid receptor complex (17). In many types
of cells microsomal PGE-S is induced by cytokines (i.e. interleukin 1ß
(IL-1ß) or tumour necrosis factor a (TNFalpha
and it is glucocorticoid repressible (16). Abundant expression of PGE-S was
reported in cancer cell line as A549 and HeLa. The PGE-S is also expressed at
intermediate levels in prostate, testis, placenta, mammary gland and bladder
(14). The presence of PGE-S is moreover reported in several colorectal adenomas
(18). Some time ago PGE-S was found in heart and brain (19). In vasculature
the reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed
that PGE-S mRNA was present in smooth muscle cells but not in human umbilical
vein endothelial cells (HUVEC) and not in endothelial cells from human saphenous
vein, even following stimulation with IL-1ß, TNFalpha
phorbol myristate acetate or lipopolysaccharide (11). However, it is known that
endothelial cells, along with PGI2
, do produce
, in particular after cytokine stimulation
Here we report on the mechanism of induction of functional PGE-S in HUVEC using
interleukin 1ß as an inducer.
MATERIAL AND METHODS
Endothelial cell culture and drug used
Human umbilical vein endothelial cells (HUVEC) were cultured as previously described
(23). All experiments were performed on HUVEC at the second passage on reaching
confluence. Prior to experiments, the cells were maintained 24 hours in the
culture medium (Optimem, Gibco, Invitrogen, UK) without serum. The culture medium
was replaced before adding of human recombinant interleukin 1ß (IL-1ß,
0.02-200 pM, R&D Systems, Germany), actinomycin D (5 µg ml-1
, Sigma,.USA), dexamethasone (0.3-3 µM, Sigma, USA), indomethacin (1-10 µM,
Sigma, USA) and acetaminophen (30-300 µM, Sigma, USA) during various periods
of time. Then culture supernants were collected for PGE2
assay while the cells were subjected to RNA extraction or protein analysis.
Reverse-transcription polymerase chain reaction for endothelial enzymes
Total RNA was extracted from HUVEC using TRIZOL®
Reagent (Invitrogen, Life Technologies, UK). Reverse transcription (RT) of total
RNA (1 µg) was performed with oligo- (dT)12-18
primer and M-MLV reverse transcriptase (Gibco Invitrogen, UK) during 2 hours
at 42°C. PCR experiments were performed with 1 µl of the first strand DNA (cDNA)
in 25 µl mixture containing 200 µM dNTPs, 1.5 mM MgCl2
40 nM of specific primer pairs and 1U of recombinant HotStart Taq polymerase
(Qiagen, USA) using T3 Thermocycler (Biometra, Germany) (24). The sequence of
the sense and antisense primers for human PGE-S were: 5'-ctctgcagcacgctgctgg-3'
and 5'-gtaggtcacggagcggatgg-3' (11). After an initial denaturation of PCR sample
for 15 min at 95°C, 34 cycles of amplification (94°C for 45 sec, 65°C for 45
sec and 72°C for 45 sec) were performed followed by 10 min extention at 72°C.
Specific primer pairs for COX-1 and COX-2 were purchased from Ambion (USA) with
the thermocycler profile: an initial denaturation for 15 min at 95°C, 23 - 34
cycles of amplification (94°C for 30 sec, 59°C for 30 sec and 72°C for 30 sec)
followed by 10 min extention at 72°C. The quality of RNA samples was evaluated
using ß-actin specific primers: 5'-agcgggaaatcgtgcgtg-3' (sense) and 5'-agcgggaaatcgtgcgtg-3'
(antisense). PCR products were detected by electophoresis on 2% agarose gels
stained with ethidium bromide. Images were captured electronically with DC40
digital camera (Kodak, USA) and the bands were quantified using image analysis
software (NIH Image, Scion Co., USA). Results were normalized and expressed
as ratio of pixel density units for specific mRNA to ß-actin mRNA. For
position and size of observed specific bands DNA marker M1 (pUC19/Mspl, Gdañsk,
Poland) was run in parallel.
Expression of PGE-S, COX-1 and COX-2 proteins
HUVEC were lysed in phosphate-buffered saline containing 0,1% sodium dodecyl sulphate
(SDS) and samples (30 mg of total protein per lane) were applied to SDS-polyacrylamide (12% for
COXs and 15% for PGE-S) gels and subdued to electrophoresis using Mini Protean II (Bio-Rad,
USA) and Laemmli buffer system (20;25). Then proteins were electroblotted onto nitrocelulose
membranes with a semidry blotter (Bio-Rad, USA). The membranes were blocked overnight in 4°C
with 5% non-fat dried milk. Then the membranes were incubated 2 hrs in room temperature with
specific murine antibody to PGE-S(COX-1 or COX-2) protein (Cayman Chemical, USA). Bands
were visualized with nitro blue tetrazolium and 5-bromo-4chloro-3-indolyl phosphate as substrates
for alkaline phosphatase-conjugated secondary antibody (Sigma, USA) Protein bands were scanned
with CD40 digital camera (Kodak, USA).
Assay of PGE2
Culture supernatants (500 µl) were collected in Eppendorff tubes and stored
at -70°C not longer than for a week. PGE2 was assayed using the monoclonal enzyme
immunoassay (EIA) kit (Cayman Chemical, USA) according to the manufacturer's
protocol. Results were expressed in pg ml-1
|Fig. 1. Induction
of COX-2 and PGE-S transcripts by IL-1ß at a range of concentrations
0.02 pM - 200 pM. Panel A - actual gels. M1 - the marker lane, 1 - 6 lanes:
RT-PCR products after incubation with IL-1ß at concentrations indicated
in the graph. Panel B shows levels of specific mRNA for PGE-S (filled
bars) and COX-2 (patterned bars) expression normalized with respect to
b-actin in HUVEC and treated with increasing concentrations of
Three hours after the exposition of HUVEC to IL-1ß at concentrations from
0.02 pM to 200 pM, total RNA was extracted and specific mRNA for PGE-S and 645.COX-2
were analysed by RT-PCR. Fig.1A
shows data obtained from gels. Fig.1B
shows the expression levels of PGE-S mRNA and COX-2 mRNA standardized to ß-actin.
A concentration-dependent sharp increase in the expression level of PGE-S (filled
bars) and COX-2 (patterned bars) was evoked by IL-1ß at concentrations
from 0.02 pM to 2.0 pM. At a higher concentration of IL-1ß no further
significant increase in response occurred. To confirm the specificity of RT-PCR
products, they were sequenced and then blasted against known sequences in GenBank
database. Our specific PGE-S amplified in RT-PCR product had >99% homology to
that stored in GenBank (data nor shown). After 3 hours of stimulation of endothelial
cells with increasing concentration of IL-1ß, HUVEC were lysed and analysed
with Western blot technique. Fig.2
shows the appearance of the band of
PGE-S protein in lane four, five and to lesser extent in lane 6. We observed
band for COX-2 protein in control, unstimulated endothelial cells. However,
the treatment with IL-1ß increased the COX-2 protein expression in a concentration
dependent manner. No effect of stimulation with IL-1ß was observed in
the case of COX-1 protein expression. Three hours after stimulation of HUVEC
with increasing concentration of IL-1ß, we collected supernatants and
was assayed with EIA. Fig.3A
an increase in release of PGE2
while the concentration
of IL-1ß was rising from 0.02 pM to 2 pM. Then PGE2
release into the culture medium reached its plateau and IL-1ß at higher
concentrations no longer augmented the release of PGE2
We also measured the kinetics of PGE2
into HUVEC supernatants. Cells were stimulated with IL-1ß (2 pM) and samples
were collected at time intervals shown on Fig.3B
release increased linearly at all time intervals and maximum, almost 20 fold
increase in PGE2
release, occurred at 180 min
of stimulation with the cytokine. As shown on Fig.4A
when pre-treated with actinomycin D (Act-D, 5 µg ml-1
), did not respond with formation of PGE-S mRNA to stimulation with IL-1ß
(2 pM). Filled bars (Fig.4B
) show normalized expression levels of specific
PGE-S mRNA plotted against ß-actin. Similarly, pre-treatment with act-D
inhibits expression of COX-2 mRNA (Fig.4A
). Patterned bars show normalized
against ß-actin level of specific COX-2 mRNA expression (Fig.4B
In contrast, expression level of COX-1 remained unchanged both after act-D pre-treatment
and after IL-1ß stimulation (Fig.4
). Finally, we performed pharmacological
analysis of PGE2
release from HUVEC stimulated
3 hours with IL-1ß (2 pM). Actinomycin D (5 µg ml-1
) prevented IL-1ß-induced PGE2
into supernatant (see Fig.5
). Also dexamethasone (0.3 -3 µM), acetaminophen
(30- 300 µM) and indomethacin (1-10 µM) blocked PGE2
release from HUVEC after IL-1ß stimulation (Fig.5
||Fig. 2. PGE-S and COX-2 but
not COX-1 protein expression in HUVEC is dependent on IL-1ß concentration
as evidenced by Western blot.
Our results indicate clearly that endothelial cells (HUVEC) did express the
PGE-S transcript and the PGE-S protein next to the stimulation with IL-1ß.
The presence of functional PGE-S in HUVEC was confirmed by semi-quantitative
RT-PCR analysis (Fig.1
), immunostaining of PGE-S protein (Fig.2
and the rapid release of PGE2
into culture supernatants
). Actinomycin D inhibited at the transcriptional level the IL-1ß
induced appearance of PGE-S mRNA (Fig.4
), and in consequence it inhibited
the release of PGE2
into the culture medium
). Our above finding differs from the data presented by Soler et
(11) even though we used the same specific primer pair and RT-PCR profile
for PGE-S mRNA analysis. The reason for this discrepancy remains unclear. A
number of subcultivations of cultured endothelial cells does influence their
production (26, 27). Might it influence
generating system in a similar way? We
used HUVEC of the 2nd
passage while Soler et
used endothelial cells of the 1st
|Fig. 3. Effect
of IL-1ß on PGE2 release from HUVEC. Panel A - Concentration
dependence (IL-1ß, 0.02 - 200 pM). Panel B - Time-dependence
(15-180 min) for IL1ß at a concentration 2 pM.
In line with the report of Jakobsson et al.
(14) we used IL-1ß
as an inducer for PGE2
generating system. This
proinflammatory cytokine might be of relevance in the endothelial cell dysfunction
accompanying atherosclerosis (28). Our results (Fig.1-3
) combined with
known effect of IL-1ß on COX-2 induction also suggest that PGE-S and COX-2
are coregulated and that the biosynthesis of PGE2
may depend on the presence of both of these enzymes. Indeed, an inducible PGE
synthase activity that coincides with COX-2 expression was described in lipopolysaccharide-stimulated
rat macrophages (29-31). The functional coupling between microsomal COX-2 and
PGE-S that leads to the efficient PGE2
from arachidonate was reported by Murakami et al.
(16). In the same paper
colocalization of these two enzymes was visualized by confocal microscopic analyses
and demonstrated in cell lines cotransfected with COX-2 and PGE-S.
|Fig. 4. Panel
A - HUVEC when pre-treated with actinomycin D (Act-D, 5µg ml-1
) did not respond with formation of PGE-S mRNA and COX-2 mRNA to stimulation
with IL-1ß (2 pM) as evidenced by RT-PCR. The expression level of
COX-1 remained unchanged neither to IL-1ß stimulation nor Act-D
pre-treatment. Panel B - Bars shows the normalized expression levels
of specific PGE-S mRNA, COX-2 mRNA and COX-1 mRNA plotted against ß-actin.
Cytokine-induced COX-2 and PGE2
strongly repressed by dexamethasone in a number of experimental systems (32-34).
We observed that repression of IL-1ß-induced COX-2 and IL-1ß-induced
PGE-S by dexamethasone involved some degree (<30%) of transcriptional repression
(data not shown). Newton et al.
also observed 25-40% suppression of COX-2
transcription (34). Mechanisms of dexamethasone action on transcriptional repression
includes transrepression of glucocorticoid response elements (35), repression
of AP-1-dependent transactivation (36), upregulation of NF-
(37-39). However, this is not sufficient to account for the observed profound
inhibition at the product level (Fig.5
). Additional postranscriptional
mechanisms involving loss of poly(A) mRNA are proposed for the total dexamethasone-dependent
blockage of PGE2
release (34). The complex mechanisms
of the suppression by glucocorticoides of endothelial prostanoids generating
systems certainly deserve further studies.
||Fig. 5. The effect of acti-
nomycin D, acetaminophen, indomethacin and dexame- thasone on IL-1ß
(2 pM) - dependent release of PGE2 determined
after incubation with IL-1ß alone or in the presence of indicated
The anti-inflammatory benefits of nonsteroidal anti-inflammatory drugs (non-
selective COX inhibitors, NSAIDs) are usually ascribed to inhibition of COX-2,
while their undesirable side effects rely on COX-1 inhibition (40). Recent data
with selective COX-2 inhibitors (e.g. coxibs) stay in line with the above reasoning.
As shown in Fig.5
indomethacin (a NSAID) at concentrations of 1 µM and
10 µM strongly inhibited PGE2
inhibition of PGE2
release from IL-1ß-induced
HUVEC by acetaminophen (Fig.5
) was rather an unexpected finding. Although
acetaminophen is a potent antipyretic and analgesic drug, in vitro
is a weak inhibitor of COX-1 and COX-2. In mouse macrophage cell line, the diclofenac-induced
COX-2 was more sensitive to inhibition with acetaminophen than the lipopolysaccharide-induced
COX-2 (41;42). This may point out to this particular COX-2 variant with specific
subcellular localization, protein modifications or redox state (41;42) as to
a possible candidate to COX-3 (43;44). Other possibilities of inhibitory action
of acetaminophen might derive from its side effects such as producing mitochondrial
dysfunction, peroxynitrite formation or glutathione depletion (45, 46). Since
PGE-S is glutathione- dependent inducible enzyme (14), inhibitory activity of
acetaminophen may relay not only on the COX-2 inhibition but also on the glutathione
depletion. Indeed, in bovine or ram seminal vesicle microsomes we had proved
that acetaminophen in presence of glutathione inhibited COX activity, while
in absence of glutathione it stimulated COX activity (47).
COX-2 is hardly detected in most mammalian tissues, but its expression can be
rapidly induced (2-6 hours) in endothelial cells (48). In this paper we confirmed
this fast (3 hours) induction of COX-2 mRNA (Fig.1
and COX-2 protein (Fig.2
). In addition, we observed a rapid induction
of PGE-S transcripts (Fig.1
), PGE-S protein (Fig.2
and PGE-S product (Fig.3B
). In our model PGE-S behaved as a product of
an immediate early gene.
So far, in many cellular systems, induction of COX-2 was associated with shifting
generation (29-31;49). Here is an explanation that we can offer. In endothelial
cells along with the induction of COX-2 occurs induction of PGE-S. It may well
be that there exists a special pool of COX-2 ("COX-2-bis") selectively coupled
with PGE-S. This "COX-2-bis" may show distinct distribution in tissues and cells
or may be governed by divergent transcriptional and postranscriptional regulation
(41-43). Our data show that unstimulated HUVEC express both COX-2 mRNA (see
) and COX-2 protein (Fig.2
) but they do
not release PGE2 into the culture supernatant (Fig.3
). This happens after
the treatment with IL-1ß. The above speaks for "COX- 2 bis" hypothesis.
One may speculate that in endothelial cells shear stress (50, 51) and bradykinin
(52) induce "classical" COX-2 coupled with PGI-S while proinflammatory cytokines
induce "COX-2 bis". This assumption certainly needs more studies to be verified.
Acknowledgements: The authors thank Ms. Renata Budzyñska
for her excellent technical help. These studies were supported by grant 4 P05A
050 19 from the State Committee for Scientific Research and grants 501/P/150/L
and W³/149/P/L from Medical College of Jagiellonian University.
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