Introduction
Diabetes mellitus (DM) is the third most common chronic disease worldwide and a
serious threat to human health and life [1].
DM-related macrovascular and microvascular complications, including coronary heart
disease, cerebrovascular disease, heart failure, peripheral vascular disease,
diabetic retinopathy (DR), neuropathy, and nephropathy, impair the quality of life
and cause disability and premature death [2].
Fasting plasma glucose (FPG), postprandial glucose excursions, and hemoglobin A1c
(HbA1c), described as the “glucose triad,” are the main parameters
used in monitoring patients with type 2 diabetes (T2D) [3]. Among elderly patients with T2D, all-cause
mortality [4], including cardiovascular
disease mortality [5], is mainly related to
the variability or instability of fasting glycemia rather than its absolute values.
In the Veteran Affairs Diabetes Trial, longitudinal variations in FPG were
associated with all-cause mortality, even when accounting for standard measures of
glucose control, as well as comorbidity and lifestyle factors [6]. Notably, many studies have found that HbA1c
does not fully explain the risk of chronic complications of T2D. Therefore, other
reliable and accurate monitoring parameters of diabetic complications need to be
explored.
Good glucose control is one of the most effective means to prevent the complications
of advanced DM [7]. Impaired glucose
homeostasis is the main risk factor for cardiovascular disease [8]; thus, glycemic variability (GV) might be as
important as the glucose triad [9].
Additionally, several studies have confirmed that patients with DM and fluctuant
hyperglycemia have higher risks of chronic vascular complications than those with
persistent hyperglycemia [10]
[11]. Furthermore, greater degrees of glycemic
fluctuation are associated with a higher incidence of complications and worse
prognosis [12]. Thus, GV has become a research
hotspot in the field of DM prevention and treatment.
Hyperglycemia fluctuation is an important factor that causes aggravation of
DM-associated vascular complications. Vascular endothelial dysfunction is the
initiating factor for the development of atherosclerosis and is an important
pathophysiological basis for diabetic vascular disease. Platelet hyperactivation
induced by thrombosis is also essential in the development of vascular events.
Furthermore, both increased platelet reactivity and endothelial dysfunction are
considered a “prothrombotic state” in DM [13]. This review aimed to provide a deeper
understanding of the role and mechanism of GV-induced diabetic complications. Toward
this goal, we focused on the relationship between endothelial dysfunction, platelet
hyperactivation, and GV.
Indicators of GV
Glucose levels can be measured repeatedly in one day (within-day glucose
variability) or during more days (between-day glucose variability) [14]. Another method is continual
measurement of glucose levels using a continual glucose monitoring (CGM) system
[15].
Dysglycemia in diabetes can be classified into two mechanisms: sustained chronic
hyperglycemia and acute fluctuant fluctuations over a daily period [16]
[17]. The former is integrated by HbA1c, which depends on both
interprandial and postprandial hyperglycemia, and the percentage of each
contributor is modulated by the degree of diabetic control [18]. There are many parameters for the
clinical evaluation of GV [19], with the
most common measures being the following ([Table 1]): (1) assessment of within-day blood glucose variability,
including the mean amplitude of glycemic excursions (MAGE), largest amplitude
glycemic excursion (LAGE), standard deviation (SD) of all blood glucose
measurements, and high/low blood glucose index (HBGI/LBGI); (2)
assessment of day-to-day blood glucose variability, including the FPG
coefficient of variation (CV), mean of daily difference (MODD), and average
daily risk range (ADRR); (3) assessment of postprandial blood glucose
fluctuation, such as the mean indices of meal excursions (MIME); and (4) special
assessment for islet transplantation of type 1 DM (T1D), such as the lability
index (LI).
Table 1 Definition of the various indices used to assess
glycemic variability (GV).
Measure [Ref]
|
Description
|
Advantages
|
Limits
|
Mean amplitude of glycemic excursions (MAGE) [20]
[21]
[22]
|
Mean of glycemic excursions from nadir to peak blood glucose
level and vice versa that are>1 SD of blood
glucose mean
|
It is a diabetes-specific metric of the amplitude of glucose
excursions.
|
It considers glycemic peaks and nadirs occurring daily but
does not account for the total number of fluctuations; it
depends on sampling frequency; it is ambiguous as to where
peaks and nadirs begin and end.
|
Largest amplitude glycemic excursion (LAGE) [23]
|
Maximal sensor glucose levels minus the minimal daily sensor
glucose levels
|
It can reflect variations in the characteristics of
within-day and day-to-day blood glucose.
|
It cannot reflect the frequency of fluctuations or full level
of GV for a single day or several days.
|
Standard deviation (SD) [24]
[25]
|
Variation around the mean blood glucose (intra-day or
inter-day) [26]
|
It is a simple, classical statistical method.
|
It combines information on variability from different
sources; it does not address non-Gaussian skewed data.
|
Coefficient of variation (CV)=SD/mean
|
Magnitude of variability relative to mean blood glucose [27]
[28]
|
It can be used to assign more importance to hypoglycemia than
to hyperglycemia.
|
It is subject to the same limitations as SD. It fails to
provide enough weight to hypoglycemic values.
|
Low blood glucose index (LBGI) [29]
|
For glucose values<112.5 mg/dl,
average of 27.695×{[log(glucose)] 1.084 –
5.381}
|
Heavier weights are assigned to severe hypoglycemic
values.
|
The mathematical form is obscure.
|
High blood glucose index (HBGI) [30]
|
For glucose values>112.5 mg/dl,
average of 27.695×{[log(glucose)] 1.084 –
5.381}
|
Heavier weights are assigned to severe hyperglycemic
values.
|
The mathematical form is obscure.
|
Mean of daily difference (MODD) [31]
|
It is calculated as the average of the absolute difference
between values on different days but at the same time for
two consecutive days.
|
It can be used to assess the continuous variability of blood
glucose at the same time between different days.
|
It may be affected by insulin injections.
|
Average daily risk range (ADRR) [32]
|
Blood glucose is continuously monitored for 14–28
days at least four times a day. The results are converted to
obtain ADRR, which is used to evaluate long-term GV.
|
It is the best predictor for variations of hypoglycemia and
hyperglycemia, independent of the type of diabetes.
|
Patients are required to master self-monitoring of blood
glucose. Because of the high monitoring frequency, long
duration, and low patient compliance, this is less
frequently applied in clinical practice.
|
Mean indices of meal excursions (MIME) [33]
|
These include postprandial spike (PPGE), peak-reaching time,
and percentage decrease in blood glucose 1 h after
peaking (BR). PPGE is the difference between a postprandial
spike and the corresponding preprandial glucose.
|
Dynamic changes in PPGE can be visually shown in detail.
|
It is related to mealtime, type of food, and eating style.
Changes in postprandial levels during different days cannot
be observed.
|
Lability index (LI) [34]
|
It is calculated based on changes in glucose levels over time
using 4-week glucose records and compared with a clinical
assessment of glycemic lability.
|
It can be used as an indicator of patient prognosis.
|
It is only applicable to patients with type 1 diabetes
mellitus having solitary islet transplantation with
recurrent severe hypoglycemia and labile glucose
control.
|
Continuous overall net glycemic action (CONGA-n) [35]
|
It measures the intraday glycemic swings occurring over
predetermined intervals.
|
It provides an accurate measure of intra-day glycemic
variability.
|
It is difficult to calculate.
|
The development of CGM technology greatly expands the ability to assess glycemic
control throughout the day and has enabled research on the influence of acute
blood glucose fluctuations in real life [36]. The 2017 Advanced Technologies & Treatments for Diabetes
consensus conference [37] identified
“time in ranges” as a metric of glycemic control that provides
more actionable information than HbA1c alone. The metric includes three key CGM
measurements: percentage of readings and time per day within target glucose
range [TIR, 70–180 mg/dl
(3.9–10.0 mmol/l)], time below target glucose range
[TBR, 70 mg/dl (3.9 mmol/l)], and time above
target glucose range [TAR, 180 mg/dl
(10.0 mmol/)] [38].
These variability parameters are important in selecting the optimal treatment
strategies and estimating the risk of chronic DM complications. GV is closely
related to diabetic complications and monitoring it could help in controlling or
reducing the risk of complications. Therefore, the parameters of both short- and
long-term GV should be further explored. Recently, numerous studies have
supported the hypothesis that GV acts as an important determinant in both the
genesis and development of diabetic vascular complications [39]
[40]
[41]
[42]
[43]
[44].
GV and vascular complications of DM
Diabetic vascular complications are conventionally classified as microvascular
and macrovascular according to the size and location of the blood vessels
involved [45]. Several studies have
demonstrated that the presence of macro- and microvascular complications in
patients with DM is related not only to chronic hyperglycemia represented by
HbA1c but also to acute glycemic fluctuations [46]
[47]. There is substantial
evidence supporting that GV has drawn a great attention for its role in macro-
and microvascular complications in patients with T1D or T2D [48].
GV and microvascular disease of DM
Microvascular disease, mainly including retinopathy, nephropathy, and neuropathy,
is strongly associated with hyperglycemia. Recently, studies have shown a
positive association between GV and microvascular complications of diabetes
[49]. The present review suggests that
increased levels of short-term glucose variability, particularly in FPG levels,
may contribute to the development of microvascular complications in T2D, whereas
the role of increased short-term glucose variability in the development of
microvascular complications in T1D is less evident [50].
Retinopathy
The risk of development and progression of retinopathy has been linked to
glycemic exposure in many studies from the Diabetes Control and Complications
Trial in 1995 [51]. DR is currently the
leading cause of blindness among working-aged persons in the developed world
[52]. Many studies in the present
literature indicate that short-term GV may contribute to the development or
progression of DR in T2D, while long-term GV, represented by HbA1c, appears to
play a more important role in retinopathy in patients with T1D or T2D [53]. One study of 415 DR patients with T1D
in a tertiary referral centre suggested prevention and early detection of
retinopathy in T1D patients, to take HbA1c variability into account when
optimizing glycemic control [54].
Nephropathy
In a cross-sectional study of patients with T1D, higher glucose variability, as
estimated by SD, CV, and MAGE, was found significantly more often in those with
elevated albuminuria than in those with normal albuminuria, although their mean
HbA1c was comparable [50]. Moreover, in
vivo studies have revealed that in patients with T2D, GV results in
chronic kidney disease characterized by progressive proteinuria, which
ultimately leads to end-stage renal failure [55].
Neuropathy
Many forms of neuropathy can occur, including sensory, motor, and autonomic
neuropathies, in the setting of diabetes after the exclusion of other causes.
Several retrospective longitudinal studies on patients with T1D have
demonstrated that glycemic fluctuations may contribute to diabetic peripheral
neuropathy [56] and cardiovascular
autonomic neuropathy [57]. Moreover, high
levels of GV, assessed via CGM, appear to have even more deleterious effects
than sustained hyperglycemia in the pathogenesis of cardiovascular autonomic
neuropathy and other cardiovascular complications in patients with T2D [58]
[59].
Both GV, as derived from visit-to-visit FPG measurements using CV, and HbA1c
≥53 mmol/mol were potent predictors of diabetic
peripheral polyneuropathy (DPN) in patients with T2D. The associations among
HbA1c, GV, and DPN suggest a linked pathophysiologic mechanism, which may play a
crucial role in clinical risk assessments [60].
GV and macrovascular disease of DM
There are three major manifestations of macrovascular disease: coronary artery
disease, cerebrovascular disease, and peripheral artery disease. In patients
with T1D and T2D, the role of increased short-term GV in the development of both
micro- and macrovascular complications is less evident [50].
Coronary artery disease
DM increases the risk of myocardial infarction (MI) more than any other risk
factor aside from cigarette smoking. Coronary artery disease is the most common
macrovascular complication recorded in patients with DM [61]. Fluctuant hyperglycemia has adverse
effects on blood vessels that lead to cardiovascular and peripheral vascular
diseases. Patients with T2D complicated by coronary heart disease have higher GV
than those without coronary heart disease [62]. In addition, increased visit-to-visit GV has been shown to
predict mortality in patients with T2D and acute MI [63]. Previous studies have already
correlated an increased GV to the occurrence of adverse events in patients with
acute coronary syndromes undergoing percutaneous coronary revascularization
(PCI) [64]
[65].
Cerebrovascular disease
Stroke incidence is elevated in the diabetic population, claiming a high cost in
terms of morbidity and mortality [66].
More severe glycemic fluctuations in patients with T2D are associated with a
greater risk of cerebral infarction and worse prognoses [67].
Peripheral artery disease
The risk of developing peripheral artery disease (PAD) is increased two- to
four-fold in patients with DM compared to those without [68]. One-fourth of patients with PAD
demonstrate progression of symptoms over 5 years and a rate of amputation of
around 4%. Diabetes not only affects large-calibre peripheral vessels
but distal arterioles as well [66]. A
recent large, retrospective cohort study reported that HbA1c and GV, as
estimated using FPG-CV, were risk factors for PAD aside from other conventional
risk factors in persons with T2D [69].
Endothelial dysfunction in GV induces DM-associated complications
Endothelial dysfunction is defined as a disorder in the capacity of the
endothelium to maintain vascular homeostasis [70]. It represents one of the most important determinants of coronary
vascular disease [71]. Endothelial
dysfunction could contribute to insulin resistance, potentially unifying the
etiology of DM and coronary vascular disease [72]. Many studies have confirmed that fluctuant hyperglycemia is more
likely to cause vascular endothelial dysfunction than persistent hyperglycemia
[73]. Moreover, glucose fluctuation
negatively influences endothelial function [74].
Endothelial dysfunction arising from a chronic hyperglycemic state is the result
of increased oxidative stress and overproduction of reactive oxygen species
(ROS), reduced nitric oxide (NO), and increased expression of adhesion molecules
and inflammatory reactions ([Fig.
1]).
Fig. 1 Schematic representation of the main processes involved in
the pathogenesis of endothelial dysfunction in patients with diabetes
with glycemia variability (GV).
Oxidative stress plays a critical role in the pathogenesis of diabetic
complications and several vascular diseases [75]. As stated in the unifying theory for DM complications proposed
by Brownlee, oxidative stress response accompanied by sharp fluctuations in
blood sugar is an important mechanism of vascular endothelial dysfunction [76]. The association between GV and
oxidative stress has also been investigated using CGM. Monnier et al. [77] found that the average 24-hour urinary
excretion rate of free 8-iso-prostaglandin (PG) F2α, a marker of
oxidative stress, in 21 patients with T2D significantly correlated with MAGE, a
marker of GV assessed via CGM, and with the area under the curve of the mean
postprandial increment of glucose level above preprandial values. This suggests
that glycemic fluctuations during postprandial periods are more likely to induce
oxidative stress. In vitro studies and animal models have also
substantiated these results.
Our study found that intermittent high glucose levels
(5.56–25 mmol/every 24 h) induced oxidative
stress injury with an increase in advanced oxidative protein products (AOPPs)
and a decrease in total antioxidant capacity (T-AOC), which led to increased
cellular apoptosis in human umbilical vein endothelial cells (HUVECs), compared
with a constant high-glucose setting (25 mmol/l). Furthermore,
the trend of these indices was verified in streptozotocin (STZ)-induced diabetic
rats with fluctuating hyperglycemia treatment [78]. These effects were amplified during glucose fluctuations,
consistent with previous observations [79]
[80].
NO produced by nitric oxide synthases (NOS) is the smallest gaseous intercellular
messenger involved in the modulation of several processes (e. g., blood
flow and platelet aggregation control) and is essential for maintaining vascular
homeostasis [81]. Aside from being a
vasodilator, NO reduces vascular permeability and the synthesis of monocyte and
lymphocyte adhesion molecules, which contribute to the reduction of tissue
oxidation and inflammation, platelet aggregation, and thrombogenic factor
activation. These processes, in turn, lead to the reduction of typical
inflammatory processes induced by hyperglycemia [82]. Impaired endothelial NOS activity and enhanced ROS production in
DM result in diminished NO bioavailability and vascular damage [83]. Therefore, NO is considered an
important anti-atherogenic molecule that is necessary to contain diabetic
endothelial alterations [84]
[85].
Inflammation is widely considered a key etiological factor that plays a vital
role in the development of diabetic complications [86]. Otsuka et al. injected glucose into
the intraperitoneal space of Sprague-Dawley rats to cause a temporary increase
in blood glucose. The results indicated that a transient increase in blood
glucose could induce increased adhesion of monocytes and endothelial cells in
the thoracic aorta [87]. Studies by Watada
et al. [88] and Mita et al. [89] in rats have also shown that repetitive
postprandial hyperglycemia could promote the adhesion of monocytes, macrophages,
and aortic endothelial cells more than sustained hyperglycemia, thus increasing
the surface area of arteriosclerotic injury. Additionally, studies in HUVECs
have also yielded similar results, further confirming that fluctuant
hyperglycemia can significantly increase the levels of inflammatory indicators
(interleukin-6, tumor necrosis factor-α) and expression of adhesion
molecules (ICAM-1, VCAM-1, and E-selectin) [90]
[91]
[92]. In human circulation, fluctuant
hyperglycemia could induce tumour necrosis factor-α production in
vivo
[93]. A previous study showed
that high glucose altered the expression profile of cytokines and chemokines via
specific signalling pathways and can increase monocyte-endothelial adhesion in
monocytes [94]. Acute glucose fluctuation
may cause significant oxidative stress and inflammation in rat aortic
endothelial cells, increase the adhesion of monocytes to rat aortic endothelial
cells, and elevate endothelial cell apoptosis, resulting in severe
cardiovascular injury [95].
Platelet hyperactivation in GV-induced DM-associated complications
Increased coagulation, impaired fibrinolysis, and endothelial dysfunction result
in a prothrombotic state for which platelet hyperreactivity is said to be an
established contributing factor. Complications arise owing to these hyperactive
platelets that play a vital role in the pathophysiology of thrombotic events
[96].
Platelets could both trigger thrombus formation and release oxidative, mitogenic,
and vasoconstrictive substances that induce the development of local vascular
lesions [97]. Studies have shown that
platelet aggregation is significantly enhanced and is in a hyperactive state in
patients with DM, with consequent increase in microcapillary embolization and
accelerated local vascular lesions [98]
[99]. Increased platelet
activities, such as altered morphology and function, may play a vital role in
the development of diabetic vascular complications [100]. A study found that increased levels
of the mean platelet volume, plateletcrit (PCT), and platelet-large cell ratio
(P-LCR) are positively associated with the occurrence of increased HbA1c,
retinopathy, nephropathy, and neuropathy individually in patients with DM [101]. Chronic hyperglycemia has been
established as the cause of platelet activation and platelet hyperactivity in
patients with DM [102]. For example, the
profound increase in urinary excretion of 11-dehydro-TXB2 in patients with T2D
suggests that acute hyperglycemia induces increased platelet activation from
high shear-stress conditions [103]. T2D is
associated with a greater production of 8-iso-PGF2α, a stable compound
of non-cyclooxygenase peroxidation of arachidonic acid inducing vasoconstriction
and platelet activation [104]. Basili et
al. [105] found that acute glucose
fluctuations strongly correlated with urinary excretion of 8-iso-PGF2α,
but no relationship was observed when urinary 8-iso-PGF2α excretion
rates were plotted against main markers of sustained hyperglycemia (HbA1c and
mean daily glucose concentrations). In our previous study, we explored platelet
aggregation in HUVECs exposed to different GV media and healthy platelets and
found that endothelial cells intermittently incubated with high glucose showed a
more relevant increase in maximum platelet aggregation rate than the increase
observed in the stable high-glucose condition [106].
The significant factors causing increased platelet reactivity in patients with DM
are hyperglycemia and insulin resistance. Hyperglycemia can cause dysfunctional
platelet adhesion, aggregation, and release through the following mechanisms
([Fig. 2]) [107]
[108]. First, hyperglycemia is responsible for non-enzymatic glycation
of proteins on the surface of the platelet, which decreases membrane fluidity
and increases the propensity of platelets to become activated [109]. Next, the platelet activation
signalling pathway is ultimately mediated by glycoprotein IIb/IIIa
receptor (GPIIb/IIIa) platelet-fibrin interaction. Hyperglycemia leads
to the release of larger platelets with more GPIb and GPIIb/IIIa
receptors, thus increasing the aggregation baseline activation and
thromboxane-forming capacity [110]. Third,
both acute and chronic hyperglycemia induce increased protein kinase C (PKC)
in vivo, a transduction pathway that triggers platelet activation
[111]. Fourth, hyperglycemia promotes
increased non-enzymatic glycation of circulating low-density lipoprotein, which
may cause platelet dysfunction by increasing intracellular calcium concentration
and platelet NO production [112]
[113]. As an important second messenger,
calcium participates in the regulation of a series of platelet functions,
including morphological changes, secretion, aggregation, and thromboxane
synthesis. Lastly, hyperglycemia induces the coagulation mechanism by increasing
the release of prothrombotic molecules (e. g., tissue factor and von
Willebrand factor [vWF]) while inhibiting fibrinolysis by increasing plasminogen
activator inhibitor-1 (PAL-1) concentration [114]
[115]. A high level of vWF
in the circulation correlates with increased platelet activation, suggesting
that acute, short-term hyperglycemia in T2D may precipitate vascular occlusions
by facilitating platelet activation [116].
Additionally, mechanisms of platelet dysfunction in DM include upregulation of
platelet P2Y purinoceptor 12 signalling, increased generation of thrombin,
increased production of thromboxane A2 from arachidonic acid metabolism, and
accelerated platelet turnover [117].
Fig. 2 Schematic representation of the main processes involved in
the pathogenesis of platelet hyperactivation in patients with diabetes
with glycemia variability (GV). PKC: Protein kinase C; GP:
Glycoprotein.
Nowadays, several studies have demonstrated that some antidiabetic agents also
have antithrombotic effects. The potential benefit to platelets may be related
to the normalization of glycemic control, but other additional direct
antithrombotic and anti-inflammatory mechanisms may be involved. The modulation
of platelet activation by antidiabetic drugs may mitigate the risk of thrombotic
events and contribute to cardiovascular protection in patients with DM [118]. Metformin, recommended as first-line
therapy for newly diagnosed T2D by the American Diabetes Association, is
associated with decreased cardiovascular risk, such as significant reduction in
cardiovascular endpoints (MI and stroke) or all-cause mortality [119] and decreased macrovascular
complications (MI, stroke, peripheral vascular disease) in patients with DM
already on insulin therapy [120].
Interaction between endothelial dysfunction and platelet hyperactivation in
vascular complications of DM
Several studies have demonstrated that hyperglycemia is the main mediator of
endothelial dysfunction and platelet hyperaggregation in DM, which contributes
to the development and progression of vascular complications. Interestingly,
both endothelial dysfunction and platelet hyperactivation also regulate
hyperglycemia-induced diabetic complications ([Fig. 3]). However, their influence on this process remains unclear
and needs further investigation.
Fig. 3 Schematic representation of the interaction between
endothelial dysfunction and platelet hyperactivation in diabetic
vascular complications with glycemia variability (GV). vWF: von
Willebrand factor; AGE: Advanced glycation end-product.
Oxidative stress is the first link in the interaction between endothelial
dysfunction and platelet hyperactivation in vascular complications of DM.
Superoxide may increase platelet reactivity by enhancing intraplatelet activity
to release calcium after activation [121].
In addition, oxidative stress impairs endothelial function and reduces the
production of NO, thus increasing platelet reactivity [122].
The second link of the interaction is vWF, which is a glycoprotein released into
the circulation by secretion from endothelial cells. Studies have reported that
an elevated level of vWF is associated with a higher risk of thrombotic
cardiovascular events in patients with DM [123]
[124]. Hu et al. [125] showed that hyperglycemia-induced
repression of microRNA-24 increases vWF expression and secretion in both
patients with DM and diabetic mouse models. When the endothelial layer is
disrupted, vWF binds to exposed collagen and then anchors platelets to the
subendothelium. In addition, vWF is also able to bind to GPIb-IX and IIb-IIIa
platelet receptors, promoting platelet aggregation and causing platelet plug
formation [126].
Advanced glycation end-products (AGEs) mediate the linkage between endothelial
dysfunction and platelet hyperactivation. Previous data have indicated that
increased AGE production under hyperglycemic conditions can induce decreased
endothelial NOS expression and increased endothelin-1 expression and ROS through
AGE-specific receptors, leading to endothelial dysfunction [127]
[128]. This further results in impaired vasodilatory response to NO
[129] and enhancement of platelet
aggregation in vivo and in vitro
[130]. Another mechanism linking AGEs and platelet hyperreactivity is
the increased expression of CD36, CD62, and CD63 on the platelet surface
membrane [131]
[132], which is associated with enhanced
platelet reactivity in vitro as well as enhanced arterial thrombosis
in vivo.