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Significance of increase in glucose 6-phosphatase activity in brown adipose cells of cold-exposed and starved mice.

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THE ANATOMICAL RECORD 219:39-44 (1987)
Significance of Increase in Glucose 6-Phosphatase
Activity in Brown Adipose Cells of Cold-Exposed
and Starved Mice
J U N WATANABE, SHINSUKE KANAMURA, HIROHIKO TOKUNAGA,
MINORU SAKAIDA, AND KAZUO KANA1
Department of Anatomy, Kansai Medical University, Fumizonecho 1, Moriguchi,
Osaka, 570 Japan
ABSTRACT
Cytochemical and biochemical glucose 6-phosphatase (G6Pase) activity was examined in brown adipose tissues of normal, cold-exposed, or starved
mice. In addition, G6Pase activity in white adipose tissue and hexokinase activity
in brown and white adipose tissues were biochemically measured. In normal animals, the reaction product for G6Pase activity was localized in the endoplasmic
reticulum and nuclear envelope of brown adipose cells. The amount of the reaction
product increased in cold-exposed or starved animals. Biochemical G6Pase activity
(259.7 f 48.5 ng Pi/min/mg protein) in brown adipose tissues of normal animals was
higher when the value was compared with values of other organs. Biochemical
G6Pase and hexokinase activities increased rapidly in brown adipose tissues of coldexposed animals, and a close relation was found between activities of the two
enzymes. In brown adipose tissues of animals starved for 3 days, biochemical G6Pase
activity increased, but hexokinase activity did not change. In white adipose tissues
of normal, cold-exposed,or starved animals, G6Pase activity was very low, although
the enzyme activity increased slightly in animals starved for 3 days. The results
show that the high G6Pase activity in brown adipose cells probably relates to
thermogenesis in cold-exposed animals and may be concerned with glucose release
into the blood in starved animals.
Brown adipose cells of rodents produce heat from
stored lipids by oxidation of fatty acids (Rothwell and
Stock, 1979; Nedergaard and Lindberg, 1982). On the
other hand, livers of mammals and flight muscles of
insects make heat by a substrate (futile) cycle; blood
glucose is converted into glucose 6-phosphate (G6P) by
hexokinase activity and G6P is reconverted into glucose
by glucose 6-phosphatase (G6Pase) activity, producing
heat by consumption of ATP (Newsholme and Crabtree,
1973, 1976; Hue and Hers, 1974; Surholt and Newsholme, 1981). Cooney and Newsholme (1982) have observed high hexokinase activity in brown adipose tissue
of rats, which increases in cold-acclimated animals.
Therefore, if brown adipose cells possess G6Pase activity, the substrate cycle, another mechanism for thermogenesis, may be available in the cells. On the other
hand, brown adipose cells are able to produce G6P from
stored lipids via gluconeogenesis (Cannon and Nedergaard, 1979; Frohlich et al., 1977; Seccombe et al., 1977).
If high G6Pase activity is present in brown adipose cells,
the cells may be capable of supplying glucose from stored
lipids into the blood. Thus, information on the presence
of G6Pase activity is of interest in brown adipose cells.
However, there have been no reports on G6Pase activity
in brown adipose cells.
In the present study, therefore, cytochemical and biochemical G6Pase activity was examined in the brown
adipose tissue of normal, cold-exposed, or starved mice.
0 1987 ALAN R. LISS, INC.
In addition, G6Pase activity in the white adipose tissue
and hexakinase activity in brown and white adipose
tissues were biochemically measured. The aim of examining cold-exposed or starved animals was to test the
following working hypotheses: (1)if G6Pase in brown
adipose cells relates to thermogenesis, the enzyme activity will increase with hexokinase activity in the cells
from cold-exposed animals, and (2) if G6Pase in brown
adipose cells is concerned with gluconeogenesis, the enzyme activity will increase in the cells from starved
animals.
MATERIALS AND METHODS
One hundred and eight male ddY mice, 10-12 weeks
old, were used. They had free access to food (NMF, Oriental) and water before the experiments. The animals
were divided into 3 groups: one group was kept at 4°C
with free access to food and water (cold-exposed animals), the second group was kept a t 25°C and given only
water (starved animals), and the third group animals
were kept at 25°C with free access to food and water
(normal animals). These animals were maintained for 3
days (cytochemical experiments) and for 1, 2, or 3 days
Received January 16, 1987; accepted March 24, 1987.
Address reprint requests to Dr. Shinsuke Kanamura, Department
of Anatomy, Kansai Medical University, Fumizonocho 1, Moriguchi,
Osaka, 570 Japan.
J. WATANABE ET AL.
40
(biochemical experiments) in individual cages, and sacAll biochemical data were subjected to statistical analrificed by cervical dislocation except for those used for ysis (analysis of variance and Student's t-test). Spearthe cytochemical experiments.
man's rank order correlation coefficient was used for
ascertaining the relationship between G6Pase and hexCytochernical Method
okinase activities.
Interscapular brown adipose tissues of the animals
RESULTS
under sodium pentobarbital anesthesia were perfused
via the left ventricle, first with ice-cold saline for 15
The average body weight of normal mice was 35.6
seconds and then with 2% glutaraldehyde in 0.1 M so- 2.2 gm (means f S.D., n = 6). The weight of cold-exposed
dium cacodylate (pH 7.4,4"C)for 5 min (6 ml/min). The animals for 1 day (35.3 f 2.3 gm), for 2 days (36.3 f 2.5
fixed tissues were sectioned a t 40 pm with a freezing gm), and for 3 days (35.5 k 2.3 gm), and of animals
microtome. The sections were washed with 5% sucrose starved for 1day (33.7 f 2.8 gm) did not differ from the
in 0.1 M sodium cacodylate (pH 7.4) at 4°C for 1 hour value of normal animals. The weight decreased signifi(Kanamura, 19731, and incubated at room temperature cantly at 2 days (29.1 k 1.9 gm) and 3 days (27.3 f 2.6
for 30 minutes in a reaction medium (3.7 mM G6P, 3.6 gm) of starvation (P < 0.05).
mM lead nitrate, 80 mM sodium cacodylate, 230 mM
Cytochernical Results
sucrose and 10 mM levamisole, pH 6.7; Watanabe et al.,
1983,1986;Kanai et al., 1983,1986; Shugyo et al., 1986).
The reaction product for G6Pase activity was observed
Levamisole was used to inhibit alkaline phosphatase in the endoplasmic reticulum and nuclear envelope of
activity (Van Belle, 1972). The sections were washed brown adipose cells from normal, cold-exposed,or starved
with 5% sucrose in 0.1 M sodium cacodylate (pH 7.4) at animals (Figs. 1, 2). However, the reaction product be4°C for 30 minutes, postfixed in 1% buffered osmium came abundant in cold-exposed or starved animals (see
tetroxide (pH 7.4) a t 4°C for 1 hour, dehydrated in a Figs. 4-6, 8). No deposition of the reaction product was
graded series of ethanol, and embedded in Spurr's resin. seen in mitochondria, Golgi apparatus, and cell matrix
Thin sections were stained with uranyl acetate and lead of brown adipose cells. However, small amounts of final
citrate and examined in a JEM-100s electron microscope. product were occasionally present in lysosomes, lipid
Control experiments for ascertaining the specificity of droplets, and the plasma membrane of the cells. Small
the reaction product (Kanamura, 1971a,b71975a,b)con- amounts of reaction product for G6Pase activity was also
sisted of: (1) incubation of the sections in the reaction observed in the endoplasmic reticulum and nuclear enmedium lacking G6P, (2) incubation in the reaction medium containing 3.7 mM 0-glycerophosphate in place of
G6P, (3) preincubation in 0.25 M sucrose containing 10
mM NaF at 37°C for 15 minutes, and then incubation
in the reaction medium containing 10 mM Nal?.
Biochemical Methods
Interscapular brown adipose tissues or epididymal
white adipose tissues were homogenized at 4°C with 50
volumes of 0.25 M sucrose in a Potter-Elvehejem homogenizer for 2 minutes at 2,000 rpm. The homogenate was
centrifuged at 4°C for 10 minutes a t 3,000 g.
G6Pase activity in a n aliquot (0.1 ml) of the supernatant was assayed by the method of Leskes et al. (1971).
The assay medium contained 30 mM G6P, 30 mM sodium cacodylate, and 10 mM levamisole (pH 6.7). Incubation was done a t 37°C for 30 minutes. The inorganic
phosphorus released was determined by Phosphor C-test
(Wako Pure Chemicals, Osaka, Japan). Protein was estimated by the method of Lowry et al. (1951). The enzyme activity was expressed as nanograms of phosphorus
liberated per minute per milligram of protein.
Hexokinase activity was measured according to the
method of Joshi and Jagannathan (1966).An aliquot (0.1
ml) of the supernatant was mixed with 2.8 ml of the
medium containing 15 mM glucose, 10 mM MgC12, 40
mM Tris-HC1 buffer, 1.4 IU glucose 6-phosphate dehydrogenase, 0.01 mM ethylenediaminetetraacetate, and
0.3 mM 0-nicotinamide adenine dinucleotide phosphate
(NADP), pH 7.4. Then, 0.1 ml of 3.3 mM adenosine
triphosphate (ATP) was added to the mixture and the
formation of NADP-reduced form (NADPH) was estimated at 25°C from the second to the tenth minute after
adding ATP. The enzyme activity was expressed as nanomoles of NADPH formed per min per milligram of
protein.
Figs. 1-8. Cytochemical demonstration of G6Pase activity in brown
adipose tissues from mice. Sections (40 pm) cut from perfusion-fixed
tissues were incubated in the reaction medium at room temperature
for 30 minutes.
Figs. 1, 2. Portions of brown adipose cells from normal animals. The
reaction product for G6Pase activity is seen in the endoplasmic reticulum (arrows) and nuclear envelope (arrowheads)of brown adipose cells.
Deposition of final products is also seen in lysosomes (Ly) and lipid
droplets (L).Fig. 1:(a) X8,OOO; 6)~28,000;Fig. 2, X20,OOO.
Fig. 3. Control experiment; portion of a brown adipose cell from
normal animal. Fixed section was preincubated in 0.25 M sucrose
containing 10 mM NaF for 15 minutes and incubated in the reaction
medium containing 10 mM NaF for 30 minutes. No reaction product is
seen in the endoplasmic reticulum and nuclear envelope. L, lipid drop
let. ~8,000.
Figs. 4,5. Portions of brown adipose cells and endothelial cells from
animals kept at 4°C for 3 days. The reaction product is abundant in
the endoplasmic reticulum and nuclear envelope of brown adipose
cells. Deposition of final products is present in lysosomes Cy), lipid
droplets (L), and pits of plasma membrane (P).The reaction product is
also observed in the endoplasmic reticulum of an endothelial cell (El.
Fig. 4, x 8,000; Fig. 5, X 12,000.
Fig. 6. High-power magnification of a portion (lower right) in Figure
5. Abundant reaction product is localized in the cisternae of smooth
endoplasmic reticulum surrounding lipid droplet (L). ~80,000.
Fig. 7. Small amount of the reaction product (see Figs. 4,5) is seen in
nuclear envelope of a n endothelial cell (arrowheads). The reaction
product in the nuclear envelope of a brown adipose cell @) is abundant.
Deposition of final products is also seen in the plasma membrane of
both cells. x 10,000.
Fig. 8. Portion of a brown adipose cell from an animal starved for 3
days. The reaction product is abundant in the endoplasmic reticulum
and nuclear envelope. ~8,000.
GLUCOSE 6-PHOSPHATASE ACTIVITY IN MICE
41
42
-
J. WATANABE ET AL.
1
BROWN
E 600
0)
4-
9
n
0
E 400\
F
E
\
< 200m
C
200
z
g
t
2
0-
L
WHITE
2
DAYS
OF
3
STARVATION
o
\
3
(0
s
0,
'D
1
4
NORMAL
I
L
3
m
3
Fig. 10. Changes of G6Pase and hexokinase activities in brown
(upper) and white (lower) adipose tissues of normal (fed) and starved
mice. Normal animals and animals given only water for 1,2, or 3 days
Fig. 9. Changes of G6Pase and hexokinase activities in brown (upper are sacrificed at 10 A.M. See legend of Figure 9 for explanation.
figure) and white (lower figure) adipose tissues of normal and coldexposed mice. Normal animals (kept at 25°C) and animals kept at 4°C
for 1, 2, or 3 days are sacrificed at 10 A.M. Dotted column: G6Pase
activity. Open column: hexokinase activity. Values are means k S.D. However, the two enzyme activities did not change in
for 6 animals. *P < 0.05, **P < 0.01: significantly different from the
white adipose tissues if animals were exposed to cold.
value of normal animals (Student's t-test).
(AT25"C)
DAYSOF
I
EXPERIMENT(AT4Y)
velope in endothelial cells of capillaries in brown adipose tissues (see Figs. 5,7). Deposition of a little final
product was occasionally seen in lysosomes, cytoplasmic
vesicles, and the plasma membrane of the endothelial
cells.
Omission of G6P from the incubation medium resulted
in a n absence of the reaction product except for that in
lipid droplets. Use of 0-glycerophosphate in place of G6P
in the reaction medium caused a loss of the reaction
product except for that in lysosomes, lipid droplets, and
the plasma membrane. Preincubation of the sections in
0.25 M sucrose containing 10 mM NaF, followed by
incubation in the reaction medium containing 10 mM
NaF, abolished the total reaction (Fig. 31, but the final
product was occasionally visible in lipid droplets and the
plasma membrane. These results indicate that the reaction product in the endoplasmic reticulum and nuclear
envelope in brown adipose cells and endothelial cells is
due to G6Pase activity. The deposition of final product
in lysosomes and the plasma membrane is probably
related to nonspecific phosphatase or acid phosphatase
activity, and the final product in lipid droplets is nonspecific deposition of lead.
Biochemical Results
In normal, cold-exposed, and starved animals, G6Pase
and hexokinase activities in brown adipose tissues were
always higher than those in white adipose tissues (Figs.
9,101.
G6Pase and hexokinase activities in brown adipose
tissues of cold-exposed animals became higher than those
of normal animals at all time points examined (Fig. 9).
G6Pase activity in brown and white adipose tissues of
animals starved for 1 or 2 days was not different from
that of normal animals (Fig. lo), but the activity increased in animals starved for 3 days. Hexokinase activity in brown and white adipose tissues was unchanged
in starved animals.
A strong positive correlation between hexokinase and
G6Pase activities was observed in brown adipose tissues
of cold-exposed animals (correlation coefficient, 0.937;
Fig. 11). Correlation coefficients between the two enzyme activities in white adipose tissues of cold-exposed
animals, brown adipose tissues of starved animals, and
white adipose tissues of starved animals were 0.735,
- 0.473, and -0.322, respectively.
DISCUSSION
As revealed in the present study, G6Pase activity was
present in brown adipose cells from mice. The reaction
product for the activity was localized in the endoplasmic
reticulum and nuclear envelope. The biochemical activity in the brown adipose tissue of normal animals (259.7
f 48.5 ng Pi/min/mg protein) was higher than that of
the white adipose tissue (14.3 f 3.81, submandibular
gland and pancreas (76.3 f 3.2 and 94.4 f 4.4, Watanabe et al., 1983), and leg muscles (139 & 20 to 217 28,
Watanabe et al., 1986; Hirose et al., 1986), and was
similar to that of the seminal vesicle, which is considered to be high (270.0 & 28.6, Kanai et al., 1986). Further, the amount of the reaction product in brown
adipose cells and biochemical activity in the brown adipose tissue increased in cold-exposedor starved animals.
These results probably show the presence of thermogenesis by a substrate (futile) cycle and suggest the ability
to supply glucose into the blood in brown adipose cells
of mice, supporting our working hypothesis.
GLUCOSE 6-PHOSPHATASEACTIVITY IN MICE
c
>
I
600
400
E
a
0
0%
m
-> 200
C
r = 0.937
00
43
brown adipose cells from starved rats. The increased
G6Pase activity possibly hydrolizes G6P produced from
gluconeogenesis. Glucose thus produced may be released
into the blood. In starved animals, however, the greater
part of blood glucose is produced in the liver from lactate
and alanine derived from the skeletal muscle (Felig et
al., 1970; Kusaka and Ui, 1977), and from fatty acids
derived from white and brown adipose tissues (ZaragozaHermans, 1974; Nedergaard and Lindberg, 19821, and is
from liver glycogen (Kanamura et al., 1980). The role of
glucose released directly from brown adipose cells, even
if present in small amounts, is possibly to elevate the
glucose level, which inclined to be low in the blood of
mice starved for 3 days.
In brown adipose cells, lipolysis and thermogenesis
are stimulated rapidly by P-adrenergic effect of norepinephrine (Nedergaard and Lindberg, 1982) and high
concentration of glucagon (Kuroshima and Yahata,
1979). Glucocorticoids stimulate slowly the synthesis of
gluconeogenic enzymes in brown adipose cells (Nedergaard and Lindberg, 1982). Therefore, rapid increase in
G6Pase activity in the cells of cold-exposedanimals may
relate to actions of norepinephrine and glucagon, and
slow increase in G6Pase activity of starved animals
might be concerned with glucocorticoids.
In white adipose cells, G6Pase may not be concerned
with thermogenesis, because the activity did not increase in the cells of cold-exposed animals. Further, the
activity was low if it increased in animals starved for 3
days. Therefore, the significance of G6Pase in white
adipose cells appears unclear from these results. On the
other hand, the white adipose tissue maintained relatively high hexokinase activity during starvation. Pilkis
(1970) showed t h a t the major source of G6P in white
adipose cells is neither glycogen nor gluconeogenic precursors but blood glucose. Further, the rate of lipid synthesis is low in the white adipose tissue of starved rats
(Zaragoza-Hermans, 1974). Therefore, the intracellular
concentration of G6P is probably relatively high in white
adipose cells from starved animals. A possible role of
G6Pase in white adipose cells is to regulate the intracellular concentration of G6P, hydrolyzing any excess.
:L
3
3
a
0
0
(
200
400
HEXOKINASEACTIVITY
(D
n mot NADPH/min/mg protein)
Fig. 11. Relationship between GGPase and hexokinase activities in
brown adipose tissues of normal and cold-exposed mice. G6Pase activity in brown adipose tissue of each animal is plotted as a function of
hexokinase activity in the tissue of the same animal. Open circle:
normal animal. Closed symbols: animals kept at 4°C for 1 (circle), 2
(triangle) and 3 (square) days. r: coefficient of correlation.
Generally, sources of G6P are gluconeogenic precursors, blood glucose, and glycogen. In brown adipose cells
from normal nonfetal animals, glycogen is not detected
(Nedergaard and Lindberg, 1982). Cooney and Newsholme (1982) showed that brown adipose cells of rats have
a potent ability to produce G6P from blood glucose by
high hexokinase activity. Further, Cannon and Nedergaard (1979), Frohlich et al. (1977), and Seccombe e t al.
(1977) suggest that brown adipose cells can convert gluconeogenic precursors produced from stored lipids into
G6P. Thus, the sources of G6P in brown adipose cells
are probably the blood glucose and gluconeogenic
precursors.
In the brown adipose tissue of cold-exposed animals,
G6Pase and hexokinase activities increased rapidly with
a strong positive correlation. G6Pase is a multifunctional enzyme (Nordlie, 1971). However, in vivo, the
hydrolysis of G6P is probably its sole function (Arion et
al., 1972). Therefore, the results show that G6P is produced steadily from the blood glucose by the increased
hexokinase activity and then G6P is converted into glucose by the increased G6Pase activity in brown adipose
cells of cold-exposed animals. Therefore, a substrate cycle
between glucose and G6P, which produces large amounts
of heat (Newsholme and Crabtree, 1973, 1976; Hue and
Hers, 1974; Surholt and Newsholme, 19811, probably
exists in addition to the established heat production
mechanism, P-oxidation of fatty acids (Rothwell and
Stock, 1979; Nedergaard and Lindberg, 19821, in brown
adipose cells from cold-exposed mice.
In the brown adipose tissue of animals starved for 3
days, G6Pase activity increased, but hexokinase activity
did not change. Gluconeogenic precursors produced from
stored lipid can be changed into G6P by brown adipose
cells (Frohlich et al., 1977; Seccombe et al., 1977; Cannon and Nedergaard, 1979). Feldman and Hirst (1978)
found an increase in the rate of gluconeogenesis in the
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