(Received for publication, November 22, 1996, and in revised form, January 27, 1997)
From the Department of Biology, III University of Rome, 00146 Rome, Italy, the § Institute of Biological Chemistry, University of Verona, 37134 Verona, Italy, and ¶ Department of Biology, University of Rome "Tor Vergata," 00173 Rome, Italy
Previous results indicate that induction of
inducible nitric-oxide synthase (iNOS) expression may be kept
suppressed by the endogenous NO level as produced by a constitutive NOS
(cNOS) enzyme. In cell types possessing both cNOS and iNOS, this may
represent an evident paradox. Here, we report that lipopolysaccharide
and interferon-, which are able to strongly induce iNOS in
astrocytoma cells, can rapidly inhibit the NO production generated by
the constitutive NOS isoform, thus obtaining the best conditions for iNOS induction and resolving the apparent paradox. In fact, a 30-min
treatment of T67 cells with the combination of lipopolysaccharide plus
interferon-
(MIX) strongly inhibits the cNOS activity, as determined
by measuring [3H]citrulline production. In
addition, the effect of MIX is also observed by measuring nitrite, the
stable breakdown product of NO: a 30-min pretreatment of T67 cells with
MIX is able to reduce significantly the
N-methyl-D-aspartate-induced nitrite
production. Finally, using reverse transcriptase-polymerase chain
reaction, we have observed that a 30-min treatment of T67 cells with
MIX does not affect expression of mRNA coding for the neuronal
NOS-I isoform. These results suggest the novel concept of a possible role of a cNOS isoform in astrocytes as a control function on iNOS
induction.
Nitric oxide (NO)1 is an unstable
nitrogen radical which is generated in different cell types by the
concomitant conversion of L-arginine into citrulline
through the enzyme NO synthase (NOS) (1). There are at least three
distinct isoforms of NOS present in human cells. Two enzymes, the
neuronal and the endothelial Ca2+-dependent
isoforms (NOS-I and NOS-III, respectively), are constantly expressed
and termed constitutive NOS (cNOS). The third enzyme is an inducible
Ca2+-independent isoform (iNOS or NOS-II), which is
expressed after stimulation with Escherichia coli
lipopolysaccharide (LPS) and/or cytokines, such as interferon-
(IFN
), interleukin-1
, or tumor necrosis factor-
. The induction
of human iNOS occurs at the transcriptional level (2) and is mediated
by the early activation of some nuclear transcriptional factors such as
NF-
B (3), and interferon regulatory factor-1 (4).
NO generated at low levels by a cNOS plays an important role in physiological processes, whereas uncontrolled and massive iNOS-induced NO production can be potentially detrimental to the tissue integrity (5). In particular, it has been observed recently that in the central nervous system NO released from activated microglia and/or astrocytes may be involved in the pathogenesis of neurodegenerative diseases (6, 7). Thus, the suppression of an undesirable and potentially harmful iNOS-elicited NO production can be fundamental for neuronal survival.
Recently, we have reported that in human ramified microglial cells, a
nearly physiological level of exogenous NO suppresses the LPS/tumor
necrosis factor--elicited NO production, by preventing induction of
iNOS mRNA expression (8). In addition, we have observed that the
early presence of NO donors during induction is essential for
inhibition of iNOS expression, the latter being due to a suppression of
NF-
B activation (8). Taken together, these data imply that induction
of iNOS expression should be prevented by a pre-existing endogenous NO
level, as produced by a cNOS enzyme. Therefore, in the cell types
possessing both a Ca2+-dependent and a
Ca2+-independent NOS isoforms, i.e. astroglial,
epithelial, smooth muscle, or endothelial cells, iNOS gene expression
may be reasonably assumed not to be induced after LPS/cytokines
stimulation, unless the cNOS-generated NO level goes down below a
threshold value in a short time.
Here, we have used human astrocytoma cells (T67), as a model of
astroglia for a better understanding of the molecular mechanisms of the
fine regulation of NOS expression. We hypothesize that LPS and IFN,
which are able to strongly induce iNOS in T67 cells as reported
elsewhere (9, 10), may directly and rapidly inhibit the NO production
generated by the cNOS isoform, thus obtaining the best conditions for
iNOS induction.
Materials
N-Nitro-L-arginine
methyl ester (L-NAME), E. coli LPS (Serotype
0127:B8), N-methyl-D-aspartate (NMDA),
L-arginine (L-Arg) were obtained from Sigma
(Milan, Italy). Human recombinant interferon-
(IFN
) was supplied
by Biogen SA (Geneva, Switzerland; specific activity: 2 × 107 IU/mg protein) and
L-[2,3,4,5-3H]arginine by Amersham Corp.
Methods
Preparation of Astrocytoma CellsHuman astrocytoma T67 cells were obtained from explant of III WHO gemistocytic astrocytoma and were characterized in our laboratory, as described previously (11).
Assay of Astrocytoma NOS ActivityNOS activity was estimated by measuring the conversion of L-[2,3,4,5-3H]arginine to L-[2,3-3H]citrulline according to the modification of the method described by Bredt and Snyder (12). 2 × 106 human astrocytoma cells (T67) were homogenized with Ultra-Turrax homogenizer (5-mm blade) for 20 s in 200 µl of a buffer containing 50 mM HEPES, pH 7.4, 1 mM DL-dithiothreitol, 1 mM EDTA, 10 µg/ml leupeptin, 10 µg/ml soybean trypsin inhibitor, 10 µg/ml antipain, and 1 mM phenylmethylsulfonyl fluoride. For Ca2+-dependent activity, after centrifugation (10,000 × g, 30 min at 4 °C), an aliquot of the supernatant was added to a reaction mixture of a final volume of 100 µl containing 50 mM HEPES, pH 7.4, 20 nM [3H]arginine, 1 µM arginine, 1 mM NADPH, 1 mM EDTA, 1.2 mM CaCl2, 1 µg/ml calmodulin, 10 µM FAD, 0.1 mM (6R)-5,6,7,8-tetrahydro-1-biopterin, and 1 mM dithiothreitol. For Ca2+-independent activity, instead, 1.2 mM CaCl2 and 1 µg/ml calmodulin were omitted from, and 1 mM EGTA was added to the reaction mixture. The reactions were stopped by adding 0.4 ml of 1:1 slurry of Dowex AG50WX-8 (Bio-Rad, Na+ form) in 50 mM HEPES, pH 5.5, and after 15-min shaking, radioactivity in the supernatant was measured. Enzyme activity was expressed as picomoles of citrulline formed in 1 min by 1 mg of protein. Protein concentration in the samples was determined by the method of Bradford (13).
Nitrite Analysis5 × 105 human
astrocytoma cells (T67) were resuspended in Krebs' buffer and stirred
for 10 min at 1000 rpm in a platelet aggregometer. The intact cells
were then centrifuged at 13,000 × g for 5 min and
nitrite (NO2) levels were determined
in supernatants by the Griess reaction, as described elsewhere (9, 10).
Briefly, aliquots of the cell supernatants were mixed with an equal
volume of Griess reagent (1% sulfanilamide, 0.1%
naphthylethylenediamine dihydrochloride, 2.5%
H3PO4). The absorbance was measured at 546 nm
and NO2
concentration was determined
using sodium nitrite as a standard. Results were expressed as nmol
ml
1.
Total cellular RNA was purified from 1 × 106 human astrocytoma T67 cells by the method Chomczynski and Sacchi (14). Briefly, a single extraction with an acid guanidinium thiocyanate-phenol-chloroform mixture was performed. Whole RNA was reverse-transcribed into cDNA by Moloney murine leukemia virus-reverse transcriptase using oligo(dT)12-18 as primers. PCR was carried out with Taq DNA polymerase in an automatic DNA thermal cycler (GeneAmp 2400, Perkin-Elmer). For NOS-I cDNA amplification, the gene-specific primers (BN-1, BN-2, and BN-3) were chosen according to Asano et al. (15). The cycle program was set as follow: 94 °C for 30 s, 68 °C for 30 s, and 72 °C for 30 s using BN-1 and BN-2 primers for 35 cycles followed by an extension step at 72 °C for 7 min. Then, PCR products from BN-1 and BN-2 were reamplified using BN-2 and BN-3 primers for another 30 cycles under the same conditions. Ten µl of each final PCR product (NOS-I, 204 bp) were electrophoresed on 1.5% agarose gel and then visualized after ethidium bromide staining. The bands so obtained were quantified with Sigma Scan/Image software (Jandel Corp.) after scanner recording from the gel photography. The mRNA for the constitutive glycerol-3-phosphate Dehydrogenase (GAPDH) was examined as the reference cellular transcript. GAPDH mRNA amplification products (195 bp) were present at equivalent levels in all cell lysates. The reaction was performed using specific primers as described elsewhere (16).
As described previously, human cultured astrocytoma cells (T67)
possess an inducible Ca2+-independent NO synthase (iNOS)
which is stimulated time-dependently by treatments with LPS
or cytokines, such as IFN (9, 10). Here, we have demonstrated that
the homogenates of unstimulated T67 cells also exhibit a basal NOS
activity (0.68 ± 0.06 nmol mg protein
1
min
1), as determined by measuring
[3H]citrulline production (Fig. 1). The
elimination of free Ca2+ by EGTA (1 mM) from
the mixture reaction almost totally abolished this activity (0.68 ± 0.06 to 0.012 ± 0.02 nmol mg protein
1
min
1), demonstrating that the isoform of the enzyme was a
constitutive Ca2+-dependent NOS (cNOS) (Fig.
1).
Interestingly, we found that a quick treatment of T67 cells with the
combination of LPS (1 µg/ml) and IFN (1000 units/ml) (MIX) for 30 min strongly inhibited the basal NOS activity (0.68 ± 0.06 to
0.015 ± 0.01 nmol mg protein
1 min
1),
as determined in the absence of EGTA (Fig. 1). In MIX-treated T67
cells, NOS activity resulted very low also in the presence of EGTA,
thus demonstrating that a 30-min MIX treatment of cells was too short
to induce the Ca2+-independent NOS expression (Fig. 1). So,
the inhibitory effect on cNOS activity elicited by MIX treatment might
be triggered by the direct NOS modifications.
The effect of LPS/IFN on cNOS-dependent NO production
was also confirmed by measuring nitrite, the stable breakdown product of NO (17), in stirred T67 cells. In the supernatants of stirred unstimulated T67 cells, we have found a basal nitrite
(NO2
) level (about 1.1 nmol
ml
1). The latter was not abolished by NOS inhibitor
L-NAME (500 µM) (data not shown), indicating
that the basal level of NO2
was not
due to L-arginine metabolism. However, this
NO2
basal level can somewhat hide the
small amount of NO produced by the cNOS enzyme in untreated T67 cells.
To enhance cNOS-dependent NO production, we have stimulated
T67 cells with NMDA, a typical cNOS activator in the nervous system
(18). When T67 cells were incubated with NMDA (500 µM)
for 0.5, 1, and 2 h, a time-dependent increase of
nitrite synthesis (above the background of ~1.1 nmol ml
1) was observed (Fig. 2). The maximal
peak was reached after 2 h of treatment (5.0 ± 0.26 nmol
ml
1), and this effect was inhibited by L-NAME
(500 µM) (5.0 ± 0.26 to 0.2 ± 0.1 nmol
ml
1), thus demonstrating that the NMDA-increased nitrite
generation was associated with the L-arginine-NO pathway
(Fig. 2). As already observed, T67 cells possess a NMDA-activated NO
pathway. In fact, the treatment of T67 cells with NMDA was able to
increase both nerve growth factor mRNA and prostaglandin E2 levels,
these effects being mediated by NO synthesis (19, 20).
When T67 cells were preincubated with MIX for 30 min and then washed,
the NMDA-induced nitrite production was significantly reduced (5.0 ± 0.26 to 1.0 ± 0.09 nmol ml1; Fig. 2). As
described previously, LPS or cytokines such as IFN
are able to
induce a high-output NO in T67 cells (9, 10). In fact, a 24-h
stimulation of T67 cells with MIX enhanced iNOS-elicited nitrite
production. On the contrary, a 30-min treatment of T67 cells with MIX
alone did not affect nitrite synthesis (Fig. 2), thus demonstrating
that a short MIX treatment of cells did not induce the iNOS enzyme, in
agreement with the NOS activity results shown in Fig. 1.
The swiftness of MIX-elicited inhibition of cNOS pathway would seem to exclude a possible involvement of cNOS mRNA transcript during the first 30 min. To verify this hypothesis, we have analyzed the effect of MIX on cNOS mRNA expression in T67 cells using the reverse transcriptase-PCR.
Here, we have shown that human astrocytoma T67 cell cultures expressed
constitutively mRNA coding for the neuronal NOS-I isoform (Fig.
3). As expected, the reverse transcriptase-PCR NOS-I
product consisted of a 204-bp fragment amplified using brain
NOS-specific primers (BN-1, BN-2, and BN-3), as described elsewhere
(15). In agreement with those authors, the first PCR product from BN-1 and BN-2 primers was reamplified by using BN-2 and BN-3 primers. A
higher specificity of NOS-I sequence amplification was derived from the
use of the BN-3 primer, which was chosen inside the first fragment from
BN-1 and BN-2 primers. Our finding is supported by the recent study of
Kugler and Drenckhahn (21), which provides immunocytochemical evidence
of NOS-I in rat astrocytes, as well as in Bergmann glia.
When 1 × 106 T67 cells were incubated with MIX for 30 min, no inhibition of NOS-I mRNA expression was observed (Fig. 3), thereby demonstrating that MIX-elicited inhibition of the Ca2+-dependent NO production was not due to modifications of cNOS mRNA transcription. These findings were confirmed by further experiments carried out using different decreasing template concentrations. In all experiments, untreated and MIX-treated T67 cells showed no significant differences (data not shown).
Some mechanisms explaining the fast inhibitory effect on cNOS enzyme have already been described. As an example, phosphorylation of NOS-I and NOS-III by protein kinase C, which is activated by immunologically active substances including endotoxin, is associated with a reduction of the enzymatic activity (22-24). Alternatively, endotoxin directly inhibits agonist (bradikinin and ADP)-mediated NO production from endothelial cells by affecting cytosolic free Ca2+ mobilization mechanisms leading, in turn, to subsequent decreases in the activity of NOS-III (25).
Taken together, our results provide evidence that LPS and IFN were
able to inhibit NO production in a very short time, by affecting cNOS
activity but not cNOS mRNA expression. On the other hand,
LPS/cytokines also induce iNOS gene, by quick activation of the
transcriptional factor NF-
B (8). However, the early presence of
physiological concentrations of NO can keep the iNOS expression
suppressed by preventing NF-
B activation (8). Thus, a very fast
reduction of the cNOS-generated NO levels below a threshold value could
explain how LPS/cytokines are able to provide the best conditions for
NF-
B activation and subsequent iNOS induction. Note that
inflammatory stimuli such as LPS/IFN
have already been reported to
trigger endothelial cells to switch from constitutive to inducible NO
synthase activity (26, 27).
In conclusion, a novel concept may be introduced that the presence of cNOS isoform in astrocytes, as well as in other cell types possessing both cNOS and iNOS, may have a functional implication as a controller of the iNOS induction, this being consistent with a protective role of physiological NO.
We thank Carolina Muscoli for technical support and Luisella Mattace for editorial assistance.