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Original Article
Antihyperglycemic activity of agarwood leaf extracts in STZ-induced diabetic
rats and glucose uptake enhancement activity in rat adipocytes
Ratree Pranakhon1, Patchareewan Pannangpetch2 and Chantana Aromdee1*
1 Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Sciences,
2 Department of Pharmacology, Faculty of Medicine,
Khon Kaen University, Mueang, Khon Kaen, 40002 Thailand.
Received 30 September 2010; Accepted 3 August 2011
Abstract
Agarwood leaf extract was found to possess antipyretic, laxative and antimicrobial activities. A trial of one diabetic
patient taking long term agarwood leaf tea was found to have blood glucose returned to normal. In this work, the effects of
methanol, water and hexane crude extracts of agarwood leaf on hyperglycemia in streptozotocin-induced diabetic rats were
investigated. Only methanol and water extracts at the dose of 1 g/kg body weight lowered the fasting blood glucose levels,
54 and 40%, respectively. The results were comparable to 4 U/kg body weight of insulin (73%). In in vitro experiment, the
effect of th e methanol and water extracts at the concentration of 10 g/mL enh anced glucose uptake activity on rat
adipocytes by 172±10 and 176±21% of the control, respectively. The glucose uptake enhancement activity is comparable to
1.5 nM insulin (166±16%). The thin-layer chromatographic characteristics of all extracts were documented. The findings
suggest that agarwood leaf is a promising potential antidiabetic agent.
Keywords: agarwood, anti-hyperglycemic activity, anti-diabetes mellitus, glucose uptake, rat adipocytes.
Songklanakarin J. Sci. Technol.
33 (4), 405-410, Jul. - Aug. 2011
1. Introduction
At present, herbs are widely used to control diabetes
mellitus (Ranilla et al., 2010; Loarca-Pina et al., 2010; Chee
et al., 2007). Some antidiabetic herbs have been reported to
exert the activity by enhancing insulin secretion (Gray et al.,
1999), or by mimicking insulin activity (Gray et al., 1998).
Aquilaria or agarwood (Aquilaria sinensis Lour., Thyme-
laeaceae) is now widely cultivated for its resin. Various parts
of Aquilaria spp. were reported to have several pharmaco-
logical activities; the aqueous extract of A. agallocha stem
decreases hypersensitivity (Kim et al., 1997), the benzene
extract of agarwood possesses potent central nervous
system depressant activities (Kim et al., 1997). Jinkoh-eremol
and agarospiral from agarwood given by peritoneal and
intracereboventricular administration exert positive effects
on the central nervous system and decrease both metham-
phetamine- and apomorphine-induced spontaneous locomo-
tion in mice (Okugawa et al., 1993; Okugawa et al., 1994).
Ethyl acetate extract of Aquilaria agallocha inhibits nitrite-
induced methaemoglobin formation (Miniyar et al., 2008).
Oral administration of acetone extract of agarwood (A.
sinensis) leaves was found to have laxative activity in mice
(Hideaki et al., 2008). Some other activities of agarwood
extracts are antipyretic, anti-inflammatory (Zhou et al., 2008)
and antimicrobial (Dash et al., 2008). There was a report of
one diabetic patient who drank water infusion of agarwood
leaf instead of water for 6 months. The blood glucose of this
patient decreased from 184 mg/dL to 128 mg/dL. After that,
* Corresponding author.
Email address: chaaro@kku.ac.th
http: //www.sjst.psu.ac. th
R. Pranakhon et al. / Songklanakarin J. Sci. Technol. 33 (4), 405-410, 2011
406
the tea was taken twice a day, once in the morning and once
at bed time, the blood glucose level decreased further to
normal (117 mg/dL) (Akrarapholchote, 2008). However, there
is no other evidence or systematic clinical study to confirm
this antihyperglycemic activity.
Diabetic mellitus is a heterogeneous disease charac-
terized by high blood glucose levels and dyslipidemia. A
major metabolic defect associated with diabetes is the failure
of peripheral tissues in the body to properly utilize glucose,
thereby resulting in chronic hyperglycemia (Rother, 2007).
At present, it is well established that adipose tissue and
skeletal muscle are the primary targets of insulin-stimulated
glucose uptake (Sophie et al., 2009). Glucose is transported
into most tissues by facilitated diffusion or glucose trans-
porters (Sophie et al., 2009). There are several members of
glucose transporters distributed specifically or nonspecifi-
cally in various parts of organs and tissues. GLUT4 is
present in insulin-responsive tissues such as skeletal muscle,
adipose tissue, and the heart. GLUT4 is insulin-sensitive and
has high affinity to glucose (Cushman et al., 1980). GLUT12
found in MCF-7 breast cancer cells is another GLUT sensi-
tive to insulin and present in the heart, prostate, muscle and
white adipose tissue similar to GLUT4 but its affinity to
glucose has not yet been determined (Rogers et al., 2002 and
Stuart et al., 2009). Naowaboot et al. (2009) have reported
that the 1.0 g/kg body weight of ethanolic extract of mulberry
leaf lowers blood glucose in streptozotocin (STZ)-induced
diabetic rats by 23% and 15 g/mL of the extract increases
glucose uptake of adipocytes of diabetic rat by 54±13% over
the control group.
In the present study, hexane, methanol an d water
extracts of A. sinensis were tested on STZ-induced diabetic
rats. The glucose uptake enhancement activity of hexane,
methanol and water extracts were tested in normal rat
adipocytes to check the possible mean of the anti-diabetes
activity. Thin layer chromatographic (TLC) fingerprints of
the extract were detected by UV absorption at 254 and 366
nm wavelengths and color reaction was detected by spray-
ing with anisaldehyde-sulphuric acid reagent (Wagner et al.,
1996). Detection for antioxidant or radical scavenging activity
of the crude extracts by spraying with 0.1 M DPPH was also
performed since oxidative stress is involved in diabetes
mellitus in many ways.
2. Materials and Methods
The chemicals used in this study were commercial
grade for extraction and analytical grade for other tests.
Bovine serum albumin fraction V (BSA), N-[2-hydroxylethyl]
piperazine-N[2-ethanesulfonic acid] (HEPES), insulin powder,
collagenase type 1 and cytochalasin B were purchased from
Sigma Chemical (St. Louis, MO, USA). 2-Deoxy-D-[U-14C]
glucose (14C-2-DG) and aqueous counting scintillant (ACS
II®) were obtained from Amersham Pharmacia Biotech
(Piscataway, NJ, USA).
2.1 Plant extract
Leaves of A. sinensis were obtained from Nakhon
Ratchasima Province, Thailand. This plant voucher (No.
NAT001-002) was deposited at the Faculty of Pharmaceutical
Sciences, Khon Kaen University and validated by comparing
with the voucher specimen, BKF No. 140455 in the Forest
Herbarium, Department of National Parks, Wildlife and Plant
Conservation, Bangkok, Thailand.
Dried leaf powder, 2 kg, was extracted sequentially
with hexane, ethyl acetate and methanol. Each extraction was
carried out three times with 6 L of solvent. Water extraction
was carried out by infusion the dried powdered leaves (150 g)
in boiling water (2.8 L) for 30 min, filtering through cotton
cloth, and then lyophilizing. All extracts were kept at -20°C
until used. TLC-fingerprints were performed to characterize
the extracts.
2.2 TLC-fingerprints
2.2.1 Chromatographic system
Silica gel GF 254 pre-coated plate (Merck, Germany)
was used as a stationary phase. The mobile phase consisted
of toluene : ethyl acetate : methanol : acetic acid (1 : 2 : 1 :
0.04). The chromatograms were detected under the UV light
at 254 nm and 366 nm) and sprayed with anisaldehyde-
sulphuric acid reagent or 0.1M DPPH.
2.2.2 Sample preparation
For freshly prepared solution, 2 g of leaf powder was
sonicated with 20 mL of methanol for 1 h and filtered. For
methanolic, water, and hexane extracts, 0.3 g of each extract
was separately dissolved in 2 mL of the corresponding
solvent used in the preparation of the extracts. The solutions
were centrifuged at 2,500 rpm for 5 min at 4°C. Supernatant
of each solution was separately spotted on a TLC plate.
2.3 Hypoglycemic activity of the extracts
2.3.1 STZ induced diabetes in rats
Male Sprague-Dawley rats (200-280 g) were purchased
from the National Laboratory Animal Center, Mahidol Uni-
versitiy (Nakhon Pathom, Thailand). They were maintained
in an air conditioned room (25±1°C), with a 12 h light - 12 h
dark cycle and fed with standard diet (C.P. mouse feed,
Bangkok, Thailand) and water ad libitum. All procedures
were complied with the National Standards for the care and
use of experimental animals and were approved by the
Animal Ethics Committee of Khon Kaen University, Khon
Kaen, Thailand (Rec. No: AEKKU28/2551). Animals were
housed in the Animal Transit Room, Faculty of Medicine,
Khon Kaen University (Khon Kaen, Thailand) for 7 days
407
R. Pranakhon et al. / Songklanakarin J. Sci. Technol. 33 (4), 405-410, 2011
before starting the experiment. Animals were diabetes-
induced by a single intraperitoneal injection of 45 mg/kg
body weight of STZ dissolved in 0.1 M citrate buffer (pH
4.5). After 7 days of STZ injection, venous blood was
collected from rat tail to determine fastin g blood glucose
level. Only the rats with fasting blood glucose over 200 mg/
dL were considered diabetic and were included in the experi-
ments.
2.3.2 Experimental design
The rats were divided into five groups with six rats in
each group as follows: Group I: diabetic control rats orally
administered with distilled water; Group II: diabetic rats sub-
cutaneously injected with 4 U/kg/day insulin (Mixedtard®)
and Group III-V: diabetic rats orally administered with 1 g/kg
body weight/day of methanol, water and hexane extracts,
respectively. The treatments were continued for 1 week. Fast-
ing blood glucose levels of all rats were determined by a
glucometer (Accu-Chek Advantage II, Roche Diagnostics,
Mannheim, Germany). During fasting, rats were deprived of
food overnight for 12 h but had free access to water.
2.4 Effects of agarwood leaf extracts on glucose uptake by
rat adipocytes
2.4.1 Preparation and isolation of adipocytes
White adipocytes were isolated from the epididymal
fat pad of normal rats by the method described by Rodbell
(1964). The overnight fasted rats were anesthetized with
sodium pentobarbital (50 mg/kg intraperitoneally). The
epididymal fat pad was removed, minced and digested by
incubating with 1 mg/mL of collagenase type 1 in Krebs-
Ringer bicarbonate buffer (KRBB; pH 7.4) with 25 mM
HEPES, 1% w/v BSA and 1.11 mM glucose for 45 min at 37°C
in a shaking water bath set at 120 cycles/min. The fat cell
suspension was filtered through a 500 m nylon mesh to
separate the adipocytes from any undigested tissue frag-
ments and washed 3 times with KRBB without glucose by
floatation (accumulation of a thin cell layer on the top of the
medium after centrifugation at 1,500 rpm for 1 min). Finally,
the cells were suspended in KRBB without glucose to make
40% packed cell volume (approximately 105-106 cells/mL).
Uptake of glucose was measured by using 14C-2-DG as a
tracer.
2.4.2 Experimental design
One milligram of each extract was reconstituted in 1
mL of KRBB without glucose and serially diluted to make
appropriate concentration. Two hundred microliters of cell
suspension was transferred to micro-tube and incubated for
15 min at 37°C in a shaking water bath. Fifty microliters of
KRBB without glucose or 50 L of insulin (1.5 nM, final
concentration) or 50 L of the extracts (1, 3, 10 and 30 g/mL
of final concentrations) was then added to those aliquots of
cell suspensions for basal, insulin and the extract groups,
respectively. Finally, 50 L of 14C-2-DG (0.4 Ci/mL, dissolved
in KRBB with 1.11 mM glucose) was then added to each
treatment group. All the treatments were done in triplicate.
After incubation for 15 min at 37°C, the reaction was termi-
nated by adding 10 L of cytochalasin B (20 M) and kept at
4°C for 10 min. The fat cells were then washed 3 times in 1.5
mL cold KRBB with glucose, filtered through Whatman®
microfiber filters and placed in scintillation vials filled with
3 mL of aqueous scintillation cocktail. The radioactivity was
counted by a Beckman LS6500 Liquid Scintillation Counter.
Glucose uptake value was corrected for non-specific uptake
by subtracting the activity of cells pretreated with 20 M
cytochalasin B from the measured activity.
2.5 Statistical Analysis
All results are expressed as mean ± SEM. Compari-
sons of blood glucose levels between base line and after
treatments were performed using Student’s paired t-test. For
glucose uptake, the values among groups were compared
and tested by analysis of variance (ANOVA) followed by
Student Newman-Keuls test to show specific group differ-
ences. A P value less than 0.05 was considered statistically
significant.
3. Results
3.1 Extraction and characterization of extracts
The yields of hexane, ethyl acetate, methanol extracts
were 0.9, 0.9 and 7.95% w/w, respectively. The yield of the
water extract was 13.33% w/w. The TLCs of dr ied crude
extracts and of freshly prepared powdered leaves in methanol
are shown respectively in lanes 1, 2, 5 and 6 of Figure 1.
Trolox and –sitosterol were used as markers (lanes 3 and
4). Detections were carried out by examining under UV light
at the wavelengths of 254 and 365 nm. Anisaldehyde-sulfuric
acid was used to detect compounds which can be coupled
by the reagent. DPPH was sprayed to detect the antioxidant
activity of the extracts. Methanol and water extracts
contained high radical scavenging antioxidant (Figure 1 C,
lanes 1 and 5) but low antioxidant activities in hexane extract
(Figure 1 C, lane 2).
3.2 Hypoglycemic activity
Effects of methanol and water extracts of agarwood
on fasting blood glucose levels are shown in Table 1. Interest-
ingly, 1.0 g/kg of methanol and water extracts, but not hexane
extract significantly (p<0.05) reduced blood glucose levels
by 54.29% and 40.54%, respectively. The oral administration
of 1.0 g/kg of methanol extract reduced blood glucose levels
in diabetic rats to almost the same degree as 4 U/kg of
insulin. After the treatments had been discontinued for 1
R. Pranakhon et al. / Songklanakarin J. Sci. Technol. 33 (4), 405-410, 2011
408
week, the blood glucose of all of the diabetic rats returned to
their pretreatment levels (Table 1), indicating the diabetic
condition persisted throughout the period of the experiment.
3.3 Effects of agarwood leaf crude extracts on glucose
uptake by rat adipocytes
Effects of methanol and water extracts to increase
glucose uptake were similar to that of 1.5 nM insulin. How-
ever, the increases in glucose uptake of the adipocytes by
both extracts were not concentration dependent (Table 2).
The 10 mg/mL concentration of both extracts seemed to
produce maximum increase in glucose uptake, whereas
hexane extract exerted no activity (data not shown).
4. Discussion
The results from this study showed that the methanol
and the water extracts had anti-hyperglycemic activity in
STZ-induced diabetic rats, increased glucose uptake by
adipocytes from normal rats and contained antioxidant acti-
vities. However, hexane extract had no anti-hyperglycemic
activity.
The decrease in fasting blood glucose levels of the
methanol and water extracts paralleled to their enhancement
of glucose uptake by adipocytes. This implies that the anti-
hyperglycemic activities of methanol and water extracts may
be mediated via increasing glucose uptake by adipocytes
similar to the effects exerted by insulin. It is very interesting
that only 1 g/mL of the extract exhibited uptake close to the
positive control. Thus, doses lower than 1 g/mL should be
tried. At the concentrations of 3 and 10 g/mL, both extracts
provided slightly higher glucose uptake effects than 1.5 nM
of insulin used as a positive control. However, the increase
in glucose uptake was not concentration dependent at the
concentrations tested. The concentration of 10 g/mL of
both extracts seemed to produce maximum effect. At the
highest dose of both extracts tested, 30 g/mL, the glucose
uptake tended to decrease. The true concentration depend-
ent effects of both extracts on glucose uptake may be
Figure 1. TLC fingerprints of agarwood crude extracts; a) under the uv light 254 nm; b) uv light 365 nm; c) sprayed with DPPH; d)
sprayed with anisaldehyde-sulphuric acid reagent; lane 1 = water extract; lane 2 = hexane extract; lane 3 = trolox; lane 4 = -
sitosterol; lane 5 = methanol extract and lane 6 = freshly prepared solution.
Table 1. Effects of oral administration of agarwood leaf extracts on fasting blood glucose in STZ-induced diabetic
rats
Mean fasting blood glucose
Groups
Pre-treatment (mg/mL) After 1-week treatments 1 week after withdrawal (mg/mL)
mg/mL % change
Control diabetic 314 ± 16 310 ± 20 +1.27 356 ± 25
Diabetic + Ins 4 U/kg 365 ± 28 97 ± 43a-73.42b335 ± 25
Diabetic + ME 1 g/kg 305 ± 22 141 ± 16a-53.77 278 ± 26
Diabetic + WE 1 g/kg 257 ± 18 153 ± 18a-40.46b247 ± 23
Diabetic + HE 1 g/kg 280 ± 14 316 ± 26 +12.86 383 ± 23
a p < 0.001 by paired t-test comparing to the pre-treatment
b p < 0.05 by ANOVA test and followed by Student Newman-Keuls test to compare with the diabetic control group.
Ins: insulin, ME: methanol extract, WE: water extract, HE: hexane extract
409
R. Pranakhon et al. / Songklanakarin J. Sci. Technol. 33 (4), 405-410, 2011
evidenced if lower concentrations are used and should be
investigated in future study. Several plant extracts have been
reported to have glucose uptake enhancing activity. Mul-
berry extract at 15 g/mL concentration enhances glucose
uptake of STZ-induced diabetic rat adipocytes by 54±13%
(Naowaboot et al., 2009) whereas 15 g/mL of roselle
(Hibiscus sabdariffa) extract increases glucose uptake to
111% in high fructose and fat fed diet diabetic rat (Yosaph,
2009). Gray et al. (1998 and 1999) also found that some herbal
extracts, e.g. Agaricus campestris and Coriandum sativum,
imitate insulin effects on in vitro glucose metabolism.
Although methanolic and water extracts of agarwood
leaves exhibited antioxidant activities in the present study,
their contribution to anti-hyperglycemic and enhancement of
glucose uptake activities are not known. Antioxidants have
been reported to be beneficial in the prevention or treatment
of various diseases including diabetes (Maritim et al., 2003).
However, lowering blood glucose levels in STZ-induced
diabetes by antioxidants in agarwood extract is unlikely,
since the damage induced by STZ is a permanent one.
GLUT4 is the most active transporter for glucose
uptake by insulin-responsive tissues present in adipocytes,
skeletal muscle, and the heart (Holman et al., 1994). The anti-
hyperglycemic and glucose uptake enhancement activities of
agarwood methanol and water extracts are similar to those of
insulin, however, whether these effects are mediated via the
mechanism involving GLUT4 or not is not known with
certainty. Therefore, further investigation is warr anted to
uncover the exact mechanism of agarwood extracts as well as
to isolate compound(s) responsible for the activity.
5. Conclusion
The methanol and water crude extracts have anti-
hyperglycemic activity in diabetic rats, most likely to be
associated with glucose uptake increasing mechanism. Lower
doses of the extract should be tried in future study to esta-
blish the most appropriate dose for clinical trial. Agarwood
leaf has shown to be a potential anti-diabetic and thus agar-
wood can be a promising source for anti-diabetic agent.
Acknowledgement
We thank the Rajamangala University of Technology
Isan for Mrs Pranakhon’ study leave and for the project
grant. Mr Nuthawongse Siriyodaroon is thanked for provid-
ing dried leaves of Aquilaria sinensis.
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