Cardiovascular Journal of Africa: Vol 32 No 2 (MARCH/APRIL 2021)

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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 32, No 2, March/April 2021 AFRICA 95
The elevated blood pressure observed in the HFD animals
could potentially be as a result of physical compression of
the kidneys due to the accumulation of fat in and around the
kidneys, and stimulation of the sympathetic nervous system as
a result of increased leptin levels,37,38 since leptin has been shown
to be one of the factors that mediates increased blood pressure
in obesity.39,40 The HFD animals also presented with an increase
in oxidative stress, reflected by an increase in lipid peroxidation
and downregulation of CAT, GPx and SOD activity in the liver
(Table 4). HFD has previously been documented to induce
oxidative stress in obese Wistar rats.28
Treatment with GRT extract resulted in less weight
gain, a decrease in IP fat mass, leptin levels, liver mass and
blood pressure, as observed in the treated HFD animals.
It also decreased vasocontraction (Fig. 3A) and improved
vasorelaxation (Fig. 3B). Additionally, the GRT extract did not
affect the body weight of the control animals. This corresponds
with previous studies done in mice that used unfermented
rooibos and green rooibos extract (GRE).14,41
It is believed that these herbal substances bring about the
anti-obesity effects by inhibiting adipocyte differentiation
and downregulate mRNA expression of the transcription
factors responsible for the adipocyte differentiation, such
as peroxisome proliferator-activated receptor-gamma (γ)
(PPAR-γ).25 This further results in reduction in leptin levels,
AMPK activation, an increase in glucose uptake and a
decrease in lipolysis and lipogenesis. Rooibos extracts have
been shown to decrease the size and number of adipocytes.25
These changes may also have resulted in decreased liver weight,
as fat accumulation inside the liver primarily contributes to an
increase in liver weight.
The decrease in leptin levels may also have resulted in
lowering of blood pressure, subsequently improving vascular
function. However, further mechanisms need to be explored,
such as measuring endothelial-derived vasorelaxation and
vasocontraction factors, as well as the renin–angiotensin system
intermediates. The anti-hypertensive effects of unfermented
rooibos have previously been documented.8,42,43 Effects on the
expression and phosphorylation state of the main proteins
involved in the activation of eNOS were affected by the
ingestion of GRT observed in this study. This could be one of
the mechanisms that potentially contributed to the reduction
in blood pressure, as observed in the HFD animals treated
with the GRT extract. Previously, treatment with rooibos and
aspalathin have been shown to activate AMPK.25,44 Captopril
possibly also resulted in upregulation of the production of
NO by improved phosphorylation of AMPK, PKB and eNOS,
decreasing blood pressure. Captopril is a well-known inhibitor
of the angiotensin converting enzyme, thereby lowering the
activity of the renin–angiotensin–aldosterone system.45
Treatment with GRT extract improved glucose metabolism
(Fig. 2) in the treated HFD animals. Previously, it was reported
that GRE extract and aspalathin improved glucose uptake
in vivo via phosphorylation of AMPK and PKB signalling
proteins and lipid metabolism.12 Furthermore, aspalathin
decreased fasting blood glucose levels and improved glucose
intolerance in a diabetic rat model, confirming the effects
of aspalathin on glucose homeostasis.11 Interestingly, in the
current study, the GRT extract showed no significant effect on
phosphorylation of AMPK and PKB (Figs 4, 5), respectively,
in the aorta. However, according to the P:T AMPK ratio,
GRT extract increased the AMPK phosphorylation state in the
treated HFD group (Fig. 4). This could mean that the GRT
extract may have induced upregulation of glucose uptake in the
tissues via the AMPK pathway.
In half of the control animals and in all the HFD animals,
GRT interestingly increased urinary glucose levels (Table 3).
The excretion of glucose in the urine is closely associated with
inhibition of the sodium/glucose co-transporter 2 (SGLT2),
thereby speculating that GRT might act as an SGLT2 inhibitor.
Confirming this suggestion, a previous study reported that
aspalathin can inhibit SGLT2.46 SGLT2 is predominantly
expressed in the renal proximal tubules of the kidneys, and to a
lesser extent in the liver, muscle, heart47 and pancreatic α-cells.48
Since glucose is mainly reabsorbed by the kidney via SGLT2,
inhibition of SGLT2 offers an insulin-independent novel
mechanism for the treatment of type 2 diabetes. We speculate
that the improved glucose clearance in the HFD animals
could be ascribed to the inhibition of SGLT2 by the GRT
extract. Further studies are needed to explore the postulated
mechanism at the level of this transporter. Lastly, the GRT
extract restored the oxidant–antioxidant imbalance in the
treated HFD animals, thus restoring the antioxidant defence
system (Table 4).
Conclusion
There is a strong correlation between visceral obesity and
the pathogenesis of CVD risk factors. The use of natural,
safe and affordable therapeutics may be useful in alleviating
these pathologies. As demonstrated in the HFD rats in this
study, obesity was strongly associated with impaired glucose
homeostasis, increased blood pressure, ED, dyslipidaemia and
oxidative stress, synergistically increasing cardiovascular risk.
These were, however, attenuated by treatment with GRT extract,
but the mechanisms need to be explored to further elucidate
some of the findings. Therefore, GRT extract may be a potential
therapeutic agent against obesity-related vascular dysfunction,
impaired glucose homeostasis, elevated blood pressure, oxidative
stress, leptin resistance and weight gain.
We thank Afriplex (Pty) Ltd and the South African Medical Research
Council (SAMRC) for provision of the Afriplex GRTTM extract and HPLC
analysis, Harry Crossley Foundation for financial support and Mr LDM
Bennie for performing the glutathione peroxidase and lipid peroxidation
assays. We declare that the rats received humane care in compliance with the
revised South African National Standard for the Animal Care and Use for
Scientific Purposes (South African Bureau of Standards, SANS 10386, 2008)
and the protocol was approved by Stellenbosch University Animal Ethics
Committee, Tygerberg, South Africa (ethics number: SU-ACUM15-00102).
References
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3. World Health Organization. Obesity and overweight. World Health
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factsheets/fs311/en/