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Effectiveness of Chocolate ingredient (Epicatechin) in prevention of stroke

Stroke is one of the most significant health problems in the UK accounting for almost 11% of deaths each year. In England alone, there are more than 100,000 cases of new and recurrent strokes per year costing the economy roughly £7bn (1). In addition to the health and resource burden, nearly one third of patients who survive the event will be dependent on others for basic daily activities making it the leading cause of long-term disability and dependence (2). Symptoms of strokes are often varied, ranging from severe headaches to weakness or paralysis and affect speech and vision and cause severe confusion. The World Health Organisation (WHO) defines stroke as ‘rapidly developing clinical signs of either focal or global disturbance of cerebral function lasting more than 24 hours in duration or leading to death with no apparent cause other than that of vascular origin’ (3).

“The idea of using natural compounds to boost body’s protective mechanisms prior to stroke as well as post-stroke is ever growing”

Current guidelines for treatment of stroke includes thrombolytic therapy, which although effective has a therapeutic window of 4.5 h after onset (4). In addition, further portions of patients are ineligible for the treatment due to co-morbidities, contraindication and drug interactions. Therefore, the idea of using natural compounds to boost body’s protective mechanisms prior to stroke as well as post-stroke is ever growing. In particular, there are growing evidence for flavan-3-ols rich foods, notably epicatechin (EP) rich foods such as apples, blackberries and dark chocolate, in improving vascular functions and reducing the risk of cardiovascular disease and stoke (5-8). There are also several studies indicating the cognitive enhancing abilities of flavonoids from different food sources (9-12). However, the present review will mainly focus on the potential effects of these compounds derived from cocoa and chocolate and in particular their neuroprotection effects on brain activity in stroke.


PATHOGENESIS OF STROKE


Pathogenesis of stroke is complicated and involves a number of risk factors with age being the most important risk factors in all subtypes of strokes. Over 80% of strokes occur in patients over 55 years of age and for each successive decade thereafter, it has been shown that the stroke rate doubles in all patients (13). In addition, due to the aging population, the burden of stroke is expected to greatly increase. Some other risk factors include hypertension, endothelial dysfunction, infections (although still controversial) and production of excessive reactive oxygen specie (ROS).

Hypertension is regarded as the most significant cardiovascular risk factor for stroke as well as a major risk factor for atherosclerosis and atrial fibrillation which, are associated with cardio-embolic ischemic strokes (14,15). In addition, growing numbers of evidence indicate involvement of inflammation in the pathogenesis of stroke (16,17). Endothelial dysfunction as well as infections have also been implicated as contributing to the process of inflammation in stroke (16). Similarly, excessive production of ROS, through induction of endothelial dysfunction is an important mechanism of cerebrovascular damage (18,19), probably at least in part through induction of endothelial dysfunction.

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Figure 1 | The pathogenesis of stroke involves multiple interlinked biological mechanism including aging, hypertension and oxidative stress. (Adopted from Sierra et al. 2011)

Subtypes of stroke

Normal cerebral blood flow (CBF) is approximately 60 ml per 100g of brain tissue each minute and is regionally variants in the brain (20). Ischemic stroke, the more prevalent subtype (80%), is due to a disruption of this blood flow (21). It has been shown that reduction of flow to less than 20 mL/100/minute is associated with ischemia and further reduction to less than 16 mL/100 g leads to tissue death within one hour (22). This reduction in blood flow in ischemic stroke is most commonly due to thrombosis in the internal carotid, middle cerebral or basilar arteries, or following an embolic occlusion of a cerebral blood vessel, the latter usually arising from the atherosclerotic plaques or the heart in patients with atrial fibrillation. (23,24). Haemorrhagic strokes (15-20%), on the other hand, mostly arise from trauma or rupture of either an aneurysm (subarachnoid haemorrhage) or small penetrating arteries (intra-parenchymal haemorrhage) usually due to hypertension. In addition, they are also associated with vascular malformations which together with hypertension leads to rupture in the vasculature (24). These ruptures are the usual source of bleeds and extravasations of blood lead to displacement and compression of adjacent brain tissues (25). Additionally, leakage of blood into the ventricles is associated with increased morbidity and mortality as well as causing irritation and vasospasms of adjacent vessels (22).

 

Aging and changes in the brain

Age-related changes in cellular processes increase the vulnerability of the vasculature to the vascular diseases contributing to stroke (26). Neuronal atrophic changes (0.1% decrease in brain volume per year from 30 years of age to 0.5% per year after 70 years of age (27)) are associated with glial cell changes leading to hyperactivity of astrocytes and microglial cells and white matter degeneration (28). The presence of cerebral white matter lesions (WML) has been seen in up to 44% of patients with stroke or transient ischemic attacks and is associated with increased risks of recurrent events (29, 30). In addition, the permeability of blood-brain-barrier (BBB) increases with normal aging and may play a significant role to the disruption CBF (31). This disruption in CBF is associated with decreased cerebral protein synthesis and changes in cellular pH and build up of neurotransmitters such as glutamate as well as lactate in brain interstitial fluid that may lead to imbalances in electrolyte concentrations and ischemic neuronal death and damage to cerebral endothelium (32,33).

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Hypertension and risk of stroke

Cerebral arterioles are capable of compensating for an increase or decrease in cerebral perfusion pressure by vasoconstriction or vasodilatation respectively. This process of auto-regulation protects the brain against changes in the systemic blood pressure by keeping CBF constant (19). However, long-term increase in intraluminal pressure as seen in hypertension causes adaptive vascular changes such as release of several vasoactive substances (e.g. nitric oxide (NO), cytokines, and ROS) resulting in the formation of vasogenic oedema (34,35) and increase resistance in the cerebral circulation due to narrowing by lipohyalinosis and microatherosclerosis. Hypertension may also plays a role in development of silent cerebrovascular damage such as WML by shifting the limits of auto-regulatory capacity towards higher blood pressure, rendering hypertensive patients especially vulnerable to episodes of hypotension (19).

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Figure 2 | Randomised controlled trials comparing anti-hypertensive drugs with a placebo by subgroup (Adopted from Lawes et al. 2004). This meta analysis confirmed a 40% reduction in stroke risk by lowering blood pressure. The diamonds are centred on the pooled estimated of effect and represent 95% CI. The solid diamond represents the pooled relative risk and 95% CI for all contributing trials. 

There are further mechanisms of hypertensive strokes such as formation of atherosclerotic plaques in both intracranial and systemic vessels that usually lead to intraluminal narrowing and ischemic stroke by thrombotic occlusion of this narrowed lumen or, more commonly resulting in their rupture, which causes atheroembolism with possible occlusion of smaller, more distal branches of intracranial arteries. It has been shown that with every 10mmHg rise in ambulatory systolic blood pressure the odds of complex atherosclerosis are increased by further 43% (36). Hypertension, similar to the process of aging, may also exert changes in the BBB leading to lesions in the white matter. This changes have been suggested to associated with cerebral edema, activation of astrocytes, and the effects of enzymes or other agents that will pass through the damaged vessel walls (34,35). The significance of hypertension as a risk factor for stroke is well established. There are growing numbers of randomised clinical trials (RCT) that indicate the benefits of anti-hypertensive treatments in active management of patients suffered from stroke as well as a preventive measure for patients at increased risk of stroke. Meta-analyses of these RCTs (Fig. 2) confirm a reduction of stroke risks by approximately 40% with BP lowering (37).

Endothelial dysfunction, oxidative stress and reactive oxygen species

Endothelial dysfunction (ED), manifested as diminished nitric oxide (NO) bioavailability and production of peroxynitrite (38) is one of the common features of vascular-related diseases such as stroke (39,40), and it has been shown to increase risk of acute ischemic stroke (41). The endothelial cells of BBB with their unique characteristics serve the important role of controlling the penetration of chemicals and immune cells from the systemic circulation into the central nervous system (39). Endothelium-derived NO plays a key role in regulation of BBB homeostasis. It stimulates soluble guanylate cyclase, which upon activation increases cyclic GMP levels resulting in alteration in vascular smooth muscle tone and prevents platelet aggregation and leukocyte adhesion (42). Disruption in this regulatory mechanism as seen in ED (Table 2) is a result of increased oxidative stress in the vascular wall that is mostly attributable to increased activation of vascular nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and uncoupling of endothelial nitric oxide synthase (eNOS) (43) leading to an imbalance between ROS production and metabolism. ROS, at increased concentrations, lead to vascular disease specifically in the brain due to high oxygen consumption and high concentrations of peroxidisable lipids and low levels of protective anti-oxidants. (38,44). There are also other alterations such as over-expression of endothelin-1, imbalance between the production of vasodilator and vasoconstrictor prostanoids, and induction of adhesion molecules and other pro-inflammatory mediators (43). Additionally, increased levels of superoxide in the superoxide dismutase deficient animal models have shown prominent effect on impairing NO mediated regulation of vascular muscle tone as well as hypertrophy of cerebral arterioles in absence of any other risk factors (45-47). Table 2 summarises the damaging effects of increased oxidative stress through reduced NO bioavailability and production of peroxynitrite. In addition, there are several studies that indicate elevated systemic concentrations of inflammatory biomarkers (e.g. high-sensitivity C-reactive protein (hs-CRP)) may increase risk of first stroke (16,48), however, the mechanisms of hs-CRP association with stroke remains uncertain.

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Ischemic Penumbra (IP)

At the onset of the cerebral ischemic event, there is a core of infarction surrounded by a still potentially salvageable zone know as the ischemic penumbra (IP) where auto-regulation is ineffective and therefore the tissue exhibits zones of reversible ischemia and injury (49,50). The pathophysiology of this zone is closely linked to generation of spontaneous waves of a cortical spreading depolarisation originating from either the ischemic core or within the penumbra due to increases in synaptic neurotransmitter, specifically glutamate as well as extracellular potassium ions. Eventually, hypoxic rapid depolarisations appear before irreversible neuronal death (51-53). The critical time period, referred to as the window of opportunity, is clinically significant period during which the neurological deficits and ischemic injury can be reversed, sometimes even fully, by appropriate treatment (54,55).


CURRENT GUIDELINES FOR TREATMENT OF STROKE


Immediately after suffering from stroke, treatment goals include stabilisation of the patient by maintain a patent airway and control of blood pressure and cardiac rhythm with beta adrenergic antagonist if needed (22). It is then crucial to establish the underlying mechanism of stroke i.e. ischemic or haemorrhagic.

In ischemic events, treatment will include the use of thrombolytic therapy as well as anticoagulants, and antiplatelet agents. However, these treatments are associated with increased risk of haemorrhage. This has led to recombinant tissue plasminogen activator (r-tPA), e.g. Alteplase, being the preferred treatment for acute ischemic stroke (22,56,57) Because of the existence of the therapeutic window, it is recommended that the thrombolytic agent be administered as soon as possible and within 3 hours of the onset of symptoms (58). Studies suggest that prompt administration of thrombolytic agents reduces the risk of disability following stroke by as much as 30% (59). Antiplatelet therapy (Aspirin 300mg) and the use of anticoagulants have been shown to reduce mortality as well as preventing further clot formation (60). Management of cerebral haemorrhages are slightly more challenging as they develop rapidly over 30 to 90 minutes, often with rapid loss of consciousness. After the initial treatment that includes intubation and hyperventilation, the reversal of pre-existing anticoagulation is crucial (57).

Blood pressure is one of many modifiable risk factors that can be targeted in long-term prevention of recurrent stroke. In addition, smoking cessation and reduction of alcohol intake must be part of lifestyle modification(61,62). Because of the aging population and increasing prevalence of stroke, the added financial burden of management of patients, and the growing evidence for use of naturally occurring substances, there is potential for significant changes in management of this condition.


EPICATECHIN METABOLISMS AND CROSSING BBB


The flavan-3-ol compounds are mostly present in the cocoa bean in the form of epicatechin (EC) and catechin (63). Cocoa bean also contains low amounts of caffeine (0.06-0.4%) and theobromine both of which have been linked to enhancement of cognition and mental performance (64,65) as well as improvements in some neurodegenerative diseases (66). In addition, cocoa contains some biogenic amines such as serotonin and tryptophan however, these compounds are metabolised in the kidney and liver and therefore do not cross the BBB (67). Although there are many biologically active compounds in cocoa, it has been shown that the main neuroprotective mechanisms concerned in protection from stroke is due to flavanol compounds, in particular EC. There are growing numbers of studies indicating that EC protects against both haemorrhagic (68) and ischemic (69,70) strokes through proposed mechanism involving Nrf2/HO1 pathways against neuronal cell death induced oxidative stress (71) and thus they may serve as a potential intervention. However, it is crucial to first consider the bioavailability and absorption of EC and furthermore its penetration through BBB.

It takes approximately 30 min for traces of EC to be detectable in plasma after consumption of flavanol-rich chocolate in humans and although, there are variations in metabolism rates, the concentrations peak at 3 hours after ingestion and return to baseline value by 8 hours (72). It has been shown that EC is modified in vivo by small intestine and then the liver (73) where it is modified to a number of metabolites but mainly glucuronides conjugates. These compounds have been shown to then be able to cross the BBB cell lines in rats and also in humans (74,75). In animals in vivo, epicatechin and catechin and their metabolites were detected in the brain following oral ingestions (75,76). These studies further indicated that the passage of these compounds across BBB cells is a stero-selective and time-dependent process and these cells are capable of further metabolising EC metabolites resulting in possibility of free epicatechin penetration into BBB (74). However, these studies have not yet been able to assure whether the glucuronides conjugates are the biologically active form of EC or whether they bind to B-glucuronidase and release free EC that may exert cellular effects (78). Interesting to note that epicatechin metabolites are detected as early as 60 min in animal brain after oral administration (74) and it has been observed in animal models for up to 10 days following ingestion (77). In addition, there are a number of studies indicating that the BBB might be regionally selective for the uptake of EC as higher concentrations of EC were found in structures such as striatum and hypothalamus following oral ingestion than other regions in the brain (79).

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Figure 3 | Transport efficiency of 30 μM catechin and epicatechin through (A) rat cerebral endothelial cells and (B) human cerebral endothelial cells. (Adopted from Faria A et al. 2011). Transport efficiency percentages were calculated based on (compound concentrations at the baso-lateral side overtime)/(compound concentrations at the apical side at the zero hours) x100. White bars represent transport efficiency of catechin and dark bars represent epicatechin transport.

 

There is increasing evidence on the relation between flavonoid intake, particularly EC and improved neuronal function following stroke. A 29% reduction of the risk of stroke has been reported in meta-analysis of 3 studies with approximately 114,000 participants in individual with higher consumptions of chocolate (80). Likewise, there are reports of improved neurological scores as well as reduction in lesion volume in the brain following middle cerebral artery occlusion (MCAO) in mice that were pre-treated orally with EC 90 minutes before the occlusion was performed (70). Interestingly, the mice that were post-treated with EC also had significantly smaller lesion volume (68,70). These studies indicate bioavailability of EC in the brain as soon as 90 minutes. The effect of flavonoids has been linked to their interaction with intracellular signalling pathways that control survival and differentiation of neurons as well expression of several genes involved in cellular response to oxidative stress (81,82). In addition, these compounds are able to inhibit micro-glial mediated inflammation via mechanisms including inducible nitric oxide synthase (iNOS) and cyclo-oxygenase-2 (COX-2) expression and NADPH oxidase activation, which would compromise neuronal survival if not inhibited (83-85). Although, so far many of these proposed mechanisms remain hypothetical, in the last few years, it has been shown that neuroprotective benefits of EC is eliminated in animal models where neuroprotective enzyme heme oxygenase 1 (HO1) or the transcriptional factor nuclear factor (erythroid-derived 2)-like 2, (Nrf2) have been knocked out, suggesting EC may exerts part of its beneficial effect through activation of Nrf2/HO1 pathway (70). In addition, while not fully investigated, EC has been shown to rescue cells from apoptosis by inhibiting β-amyloid induced apoptosis in PC12 cells (86,87).

 

Nrf2/HO1 antioxidant-response element (ARE) pathway

Transcriptional factor Nrf2, in response to cellular stress, induces the expression of various cytoprotective genes encoding many phase II antioxidant enzymes such as HO1 and NADPH (70,88,89). These enzymes may play a significant role in providing neuroprotection against ischemic stroke and oxidative stress by regulating cellular redox states (90). In particular, HO1 is up-regulated through the Nrf2 mediated mechanism in stroke offering protection by mainly degrading its pro-oxidant heme substrate (91,92). This reaction generates the antioxidants biliverdin and bilirubin as well as releasing one molecule of iron that may increase cellular ferritin levels and hence provide protection from generation of additional free radicals (93). Other by-products of this reaction that may provide beneficial effects in stroke includes carbon monoxide (CO), which has been reported to prevent programmed cell deaths as well as eliciting vaso-dilatory actions and anti-inflammatory properties at low concentrations (94).

Interestingly however, it appears that in intracerebral haemorrhage (ICH), HO1 acts as a pro-oxidant (95) and a recent study (68) suggested that in haemorrhagic brain EC diminishes HO1 induction via down-regulation of activator protein (AP)-1 that has been linked to expression of many pro-inflammatory mediators including, matrix metalloproteinases (MMPs) that tribute to BBB breakdown. This process is know as Nrf2-independent pathway (96).

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Figure 4 | Effect of EC on brain injury volume. (Adapted from Chang et al. 2014) Stained brain sections at 72 h post intracerebral haemorrhage (ICH). Areas lacking stain are injured and are circled with dotted lines. Similar images exist for EC treated mice with ischemic stroke.


DISCUSSION


There is approximately a 6-hour therapeutic window between the onset of ischemia and irreversible neuronal damage during which administration of appropriate interventions could lead to reversal of the neurological deficits and ischemic injury specially in the penumbra regions (97,98). Animal studies have demonstrated that administration of EC 15 mg/kg, equivalent to a human dose of 1.22 mg/kg per day is the optimal dose in reducing all negative outcomes in all types of strokes (99). Following ingestion of flavonoid-rich food, EC is absorbed into circulation (100) and after further metabolism by the intestine and the liver, available evidence indicates that EC metabolites (mainly epicatechin glucuronides) crosses the BBB (75,101). Moreover, after an ischemic event, the BBB is prone to be more penetrable, especially in the surrounding regions and hence there might be increased concentrations of EC intracranially.

Taken together, all the evidence suggests that EC is neuroprotective against brain injury in both haemorrhagic and ischemic stroke. EC has shown to protect the brain from ischemia-induced injury when administered 90 minutes before or 3.5 hours following occlusion of middle cerebral artery (69,70). In a study of haemorrhagic stroke, EC also reduced early brain injury in mice and improved neurologic function for at least 4 weeks after intracranial haemorrhage (ICH) (68). Nevertheless, administration of EC in Nrf2/HO1 knocked out mice failed to provide any protection against the damage induced by brain ischemia, providing additional evidence that EC exert its neuroprotective effect through Nrf2 pathway by reducing oxidative protein damage and dampening microglia activation (90,102).

Numbers of studies suggest that Nrf2 pathway is partially mediated through up-regulation of HO1 expression in ischemic stroke (90,103). HO1 catalyses the cleavage of heme (a pro-oxidant) to form iron, carbon monoxide and biliverdin (Dore, 2002). HO1, and therefore EC, elicit their beneficial neuroprotective through production of these antioxidants by-product specially biliverdin and bilirubin. It is evident that HO1 plays a role in protecting the brain from permanent focal ischemia (94,104). Interestingly however, studies of haemorrhagic stroke have shown that EC elicits similar neuroprotective benefits through Nrf2 pathway although it decreases HO1 protein expression and iron decomposition through decreases in AP-1 activity. This is known as the Nrf2-independet pathway (68). The discrepancy in findings between these findings could be due to the differences between studies, most importantly the differences in pathological events in haemorrhagic and ischemic strokes.

Pathology of haemorrhagic stroke leads to HO1 over-expression and hence leading to iron overload in the brain (105,106). Iron damages neurons by catalysing reactions with hydrogen peroxide to generate hydroxyl radicals. A recent study (68), reported for the first time that EC has iron eliminating properties after haemorrhagic stroke through decrease in HO1 protein expression (107). Interestingly also, post stroke administration of EC in Nrf2 knocked out mice still elicited some protection suggesting that an Nrf2-independent pathway might act synergistically under ICH conditions (68). All the evidence so far for EC protection mediated through Nrf2 activation suggests its implications for the future benefits of EC and EC-enriched extracts in reducing stroke-induced damage. In addition, there are many studies suggesting that EC is also protective in many more conditions as well as positively influencing cognitive performance (9).


CONCLUSIONS


In conclusion, the flavanol EC protects against both ischemic and haemorrhagic strokes when given both before and after the event. The neuroprotective mechanism has been shown to involve activation of Nrf2/HO1 pathway. In addition, EC diminishes HO1 induction by down regulating AP-1 in haemorrhagic strokes. The evidence so far provide a good understanding of the mechanisms involved and can be beneficial in future developments of dietary agents for use in both prevention and treatment of stroke.

 

Cite this article as: Ali Ansaripour, "Effectiveness of Chocolate ingredient (Epicatechin) in prevention of stroke," in Projmed, September 22, 2015, https://www.projmed.com/2015/09/effectiveness-of-chocolate-ingredient-epicatechin-in-prevention-of-ischemic-and-haemorrhagic-stroke/.

  


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