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From the Faculty of Pharmaceutical Sciences, Teikyo University, Sagami-ko, Kanagawa, Japan.
| Abstract |
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METHODS. Hydrocortisone sodium succinate (HC; 0.25 micromoles) was administered to 15-day-old embryos followed by administration of long-acting recombinant human insulin, 4 and 28 hours later. At the indicated time after HC administration, the incidence of cataractous lenses and any changes in the components of the lenses, liver, and blood were determined.
RESULTS. At 48 hours after HC administration, the following observations were made: opacification of lenses; an elevation of glucose and lipids in the blood and lenses; an increase in lipid peroxide (LPO) in the blood, liver, and lenses; a decrease in glutathione (GSH) in the lens and liver (at 24 hours after HC administration); and a depletion of adenosine triphosphate (ATP) in the liver. These changes in response to HC administration were reversed by a double application of insulin.
CONCLUSIONS. Insulin antagonizes GC-induced gluconeogenesis, stimulates glycolysis, and ultimately leads to recovery of decreased activity in the citric acid cycle. The restoration of ATP by the recovered citric acid cycle may facilitate de novo synthesis of GSH, which in turn may diminish GC-induced elevation of LPO in the liver. Thus, the metabolic changes in response to HC-accelerated gluconeogenesis in the liver, which can be reversed by insulin, are likely to produce oxidative stress that leads to cataract formation. GC-induced metabolic changes in the liver, which are antagonized by insulin, may relate to production of one of the risk factors for cataract formation.
| Introduction |
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Steroid-induced cataracts were first documented by Black et al.1 and were reported in subsequent studies to occur in patients with rheumatoid arthritis, nephrosis, and systemic lupus erythematosus and in organ transplant recipients treated with GC.2 3 4 Lorand et al.5 reported that transglutaminase activity in cataractous lenses from GC-treated patients was higher than that observed in clear lenses. However, no other studies investigating the mechanism of cataract formation in humans have been reported, because it is usually difficult and often impossible to obtain human lenses for analysis. Very few animal model studies have investigated the underlying mechanism of steroid-induced cataracts. Despite this, Manabe et al.6 and Bucala et al.7 demonstrated that the formation of a Schiff base through the amine of lens protein and the C-20 carbonyl of corticoids, followed by a Heyns rearrangement with the C-21 hydroxyl, was involved in cataract formation.
We have demonstrated that steroid-induced cataracts are produced by the biologic activity of GC and not by the chemical formation of the Schiff base.8 We have also shown that no differences exist in transglutaminase activity between clear and cataractous lenses (unpublished data, 1983). In contrast, we observed a loss of lens due to oxidative stress that was indirectly affected by an elevation of blood lipid peroxide (LPO), which was in turn due to an imbalance of redox functions in the liver after hydrocortisone sodium succinate (HC) treatment.9 10 11 Specifically, glutathione (GSH) in the liver decreased until it was approximately 50% of control levels at 24 hours, whereas LPO levels in the liver and blood increased sharply from 20 hours until they were 8 to 10 times that of control levels at 30 to 48 hours after HC administration. In terms of timing, the decrease in GSH and increase in LPO in the lenses were observed 48 hours after HC was administered to a level that elicited maximum opacity. Furthermore, the administration of radical scavengers such as ascorbic acid,12 pyrroloquinoline quinone (PQQ),13 and sulfhydryl compounds14 15 effectively prevents the HC-induced phenomena that we have described. Moreover, among intermediates of the citric acid cycle, isocitrate has shown the most potent and similar protection against HC-induced events, including cataract formation.10 As expected, hyperglycemia was observed in HC-treated chick embryos, but the formation of sorbitol and glycation in the opaque lens was not detectable, suggesting that the osmotic theory of accumulation of polyol16 17 18 and the glycation of protein theory18 are not active in our cataract model.8
Based on the these observations, our cataract model was found to be similar to most other animal cataract models,19 20 in that cataract is probably caused by oxidative stress that is induced by accelerated-gluconeogenesis and a change in metabolic activities in the citric acid cycle of the liver, which is the main target tissue of GC.10
In the present study, we attempted to clarify whether insulin prevents cataract formation through an improvement of hepatic gluconeogenesis, in light of the knowledge that GC and insulin possess antagonistic activities in gluconeogenesis in mammals. This improvement involves a restoration of the imbalance of redox function, LPO production, and a reversal of decrease in adenosine triphosphate (ATP) and GSH levels after GC administration.
| Materials and Methods |
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Treatment of Animals
Chick embryos were treated in accordance with the ARVO Statement
for the Use of Animals in Ophthalmic and Vision Research. The procedure
used to produce cataracts in developing chick embryos by HC has been
described elsewhere.10
21
Preparation and Administration of Insulin
Insulin was dissolved in vehicle (17 mM sodium acetate buffer [pH
7.0] containing 0.12 M NaCl and 15 mM ZnCl2),
and 0.4 ml of the solution containing 2.5 to 10 units of insulin was
administered at 4 and 28 hours after HC administration in the same
manner as HC.
Classification of Lenses
Lenses were removed from chick embryos at 48 hours after HC
administration, and visually classified as described previously, except
that stages IV and V were not separated.12
Determination of Glucose in Blood and Lens
Blood was centrifuged at 2000g for 15 minutes at 4°C,
and serum was collected. A 10µl aliquot of the serum was used for
the determination. Ten lenses were homogenized in 0.25 ml of
redistilled water by a sonicator (Handy Sonic model UR-20P; Tomy Seiko,
Tokyo, Japan) and centrifuged at 15,000g for 10 minutes at
4°C. After centrifugation, 10 µl of the supernatant was used for
determination. Glucose concentrations were determined by a test kit
(Glucose-C; Wako, Osaka, Japan) based on the method of Miwa et
al.22
Analysis of Ketone Bodies in Blood and Allantoic Fluid
Blood and allantoic fluid were both centrifuged at
2000g for 15 minutes at 4°C. A 10-µl aliquot of the
supernatant was blotted onto test paper for determination of ketone
bodies and classified into six grades (from - to 4+ ) according
to the manufacturers protocol: -, not detected; ±, 5 mg/dl; +, 15
mg/dl; 2+, 40 mg/dl; 3+, 80 mg/dl; 4+, 160 mg/dl. Acetoacetic acid was
used as the positive control.23
Determination of GSH in Lens and Liver
The method of Ellmanns reagent was used for determining GSH in
the lens and liver.24
Nonprotein sulfhydryl residue levels
in chick embryo lenses, as well as in other animal lenses, can be
derived mostly from the amount of glutathione present, whereas 80% of
nonprotein sulfhydryl residues in the liver are glutathione, determined
by the glyoxal I-methylglyoxalase method.14
In the present
study, GSH was used instead of nonprotein sulfhydryl residues.
Determination of ATP in the Liver
Livers were homogenized in two volumes of PBS by a sonicator
(Handy Sonic model UR-20P). Three volumes of 12% trichloroacetic acid
(TCA) were added to the liver homogenate. After a centrifugation at
15,000g for 10 minutes at 4°C, 0.25 ml of the supernatant
was used for determination of ATP by an enzymatic method using an ATP
kit (Sigma), based on the reactions described by Bucher25
as modified by Adams.26
Determination of Lipids in Blood and Lens
Eight lenses from four embryos were isolated and homogenized in a
test tube containing 0.08 ml phosphate-buffered saline (PBS; pH 7.4) by
a microhomogenizer (Micro Multi Mixer C; Ieda, Tokyo, Japan), and
centrifuged at 15,000g for 10 minutes at 4°C. After
centrifugation, the supernatant was collected and used for assay. Serum
and lenticular lipids were determined by enzymatic tests (Triglyceride
[TG] E-Test, Nonesterified Fatty Acid [NEFA] C-Test, and
Cholesterol [T-Chol]) E-Test, all kits from Wako).
Determination of LPO, a Thiobarbituric AcidReacting Substance, in
Lens, Blood, and Liver
After four lenses from four embryos had been homogenized in
redistilled water by a microhomogenizer, the amount of LPO in the
homogenate was determined fluorometrically with thiobarbituric acid by
a modification of Yagis method, as described
previously.9
A solution of 1,1,3,3-tetraethoxypropane,
which quantitatively produces malondialdehyde, was used as a standard.
Determination of Serum Insulin
Serum insulin was determined as an immunoreactive insulin (IRI)
using a radioimmunoassay kit (Phadeseph; Pharmacia & Upjohn, Uppsala,
Sweden).
Statistical Analysis
For determination of the lens components, a lens from one animal
was used unless otherwise stated. For all experiments, an unpaired
Students t-test was used. Levels of statistical
significance were set at P < 0.05.
| Results |
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Ketone Bodies in Blood and Allantoic Fluid
As shown in Table 1
, when 15-day-old chick embryos were treated with HC for 48 hours,
ketone bodies were detected in their serum and allantoic fluid but were
not detected in either fluid from embryos treated with a double
application of insulin after HC administration.
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Effect of Insulin on Lipids in the Serum and the Lens
During HC-induced cataract formation, we also observed not only
hyperglycemia but also hyperlipidemia as an adverse effect of HC in
this model. Thus, we investigated the effect of insulin administration
on hyperlipidemia caused by HC. As shown in Table 3
, when HC was administered to 15-day-old chick embryo, the levels of
serum TG, NEFA, and T-Chol were found to have increased at 48 hours
after HC treatment. However, a double application of insulin after HC
administration effectively suppressed HC-induced hyperlipidemia.
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Preventive Effect of Insulin on HC-Induced Elevation of LPO
The preventive effect of insulin on the elevation of LPO caused by
HC is shown in Table 4
. As previously reported,9
28
29
the levels of LPO in the
lens, blood, and liver of chick embryos at 48 hours after HC treatment
were approximately 1.3, 5.0, and 6.2 times that of the matched
controls, respectively. However, a double application of insulin after
HC administration effectively suppressed HC-induced LPO production in
those tissues.
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| Discussion |
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Glucose in the Lens
Several hypotheses have been made concerning the mechanism of
sugar cataract formation, including an accumulation of
polyol16
17
18
and a formation of glycation between glucose
and protein.18
None of these hypotheses was supported by
our findings, given that no sorbitol, fructose, or glycation of protein
was detected in either control or HC-induced cataractous lenses
obtained 48 hours after HC administration.8
Because sugar
cataract was caused by HC-induced hyperglycemia, it appears likely that
the decrease in glucose in the lens by insulin acted as a direct
trigger to prevent HC-induced cataract formation. However, the
transparent stage I lenses obtained from embryos treated with HC and
agents possessing anticataract activity such as
isocitrate,10
propylene glycol,30
and
ascorbic acid12
generally but not always further
decreased the amount of glucose compared with stage IVV lenses
treated with HC alone. Thus, our results and those of previous
reports10
12
30
failed to demonstrate direct correlation
between high glucose levels and loss of lens transparency.
GSH and LPO in the Lens
A decline in GSH levels has been observed during formation of most
cataracts. However, in developing chick embryos treated with buthionine
sulfoximine (an inhibitor of GSH synthesis), although the levels of GSH
in lenses were much lower than those in HC-treated embryos with severe
cataracts, the lenses were clear, and the amount of LPO did not
increase.31
One of the mechanisms proposed for cataract
formation is that oxidative changes in the lens leading to GSH
consumption may be involved in the loss of lens
transparency.19
As described in the Results section,
insulin prevented the elevation of LPO and formation of HC-induced
cataracts. The same results were obtained when agents possessing
anticataract activity were used.9
10
12
Because the marked
elevation of LPO in the blood and liver after HC administration is
suppressed by VC9
and isocitrate,10
we
speculated that LPO produced in the liver flows through the blood
stream and aqueous humor to reach the lens, and ultimately leads to a
loss of lens transparency and consumption of GSH. Thus, we concluded
that the regulation of LPO production in the liver is very important
for clarifying the mechanism of GC-induced adverse effects, including
cataract formation.
Gluconeogenesis, GSH, and LPO in the Liver
Insulin treatment after HC administration effectively prevented
the induced decline of GSH and elevation of LPO in the liver and the
elevation of LPO and glucose in the blood. We have suggested that LPO
synthesis occurred at an accelerated rate in the liver of HC-treated
chick embryos, given the decrease in the superoxide dismutase,
catalase, and glutathione peroxidase activities and the increase in
hydroxidase activity in the liver.11
The present results
assumed that the stimulation of gluconeogenesis and its associated
metabolic changes in the liver of chick embryos by HC involve a
mechanism similar to that seen in mammals, and may have produced an
imbalance of redox functions that induced a decrease in GSH and an
overproduction of LPO as a result of oxidative stress in the liver.
Gluconeogenesis by GC is demonstrated by inducing glucose-6-phosphatase, fructose-1,6-bisphosphatase, pyruvate calboxylase, and phosphoenolpyruvate carboxykinase.32 These metabolic changes suggest that the amount of oxaloacetate in the liver decreased as phosphoenolpyruvate carboxykinase converted oxaloacetate to phosphoenol pyruvate. In fact, Agius et al.33 demonstrated that dexamethasone (one of the potent derivatives of glucocorticoids) decreases the reduced nicotine adenine dinucleotide (NADH)/ nicotine adenine dinucleotide (NAD) ratio that acts as an indicator of a mitochondria redox state in hepatoma cells. Accordingly, the lower levels of oxaloacetate may decrease the metabolic activities of the citric acid cycle, leading to a decline in the NADH/NAD ratio, which in turn reduces ATP production through a respiratory chain reaction in the liver. These metabolic changes seem to take place in chick embryos, evidenced by our observation of the decline of the hepatic ATP pool after HC administration in this study.
In contrast, HC administration elevated ketone bodies in addition to hyperlipidemia and elevated NEFA, TG, and T-Chol, a finding that has been observed in chick embryos as well as mammals. Accordingly, the production of ATP was probably due to ß-oxidation of fatty acids. However, in a recent study Letteron et al.34 reported that GC inhibited ß-oxidation of fatty acids in the livers of mice. In the present study, we also observed a decline in the hepatic ATP pool after HC administration (Fig. 3B) . This depletion appears to have been the result of HC-induced systemic changes in glucose and lipid metabolism, as mentioned above.
De novo synthesis of 1 mole glutathione requires 2 mole ATP and 1 mole each glutamic acid, cysteine, and glycine. Although glutathione biosynthesis is influenced by various other factors, ATP is one of the important factors in the liver. In our experiment, a double application of insulin after HC administration prevented cataract formation and promoted the recovery of ATP depletion in the liver. We postulate that the recovery of ATP depletion may facilitate de novo synthesis of glutathione consumed by the scavenging of reactive oxide substances such as LPO in the liver.
However, the biological activity of GC is complex and not well understood. Therefore, it is impossible to draw conclusions based on our limited results, given the absence of any direct evidence. Based on the present findings, we speculate that the acceleration of gluconeogenesis and related metabolic changes by HC suppressed de novo synthesis of GSH, resulting in an imbalance in redox activity and a tendency to produce oxidative stress.
In conclusion, we demonstrated that GC produced a short-term diabetic condition in the developing chick embryo with cataracts. The processes underlying cataract formation by GC were closely related to acceleration of GC-induced gluconeogenesis, which can be recovered by insulin. Further research is currently under way.
| Footnotes |
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Submitted for publication July 26, 1999; revised December 29, 1999; accepted January 18, 2000.
Commercial relationships policy: N.
Corresponding author: Hideo Nishigori, Faculty of Pharmaceutical Sciences, Teikyo University, 1091-1, Suarashi, Sagamiko-machi, Tsukui-gun, Kanagawa 199-0195, Japan. h-nishig{at}pharm.teikyo-u.ac.jp
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