The effect of curcumin (turmeric) on Alzheimer’s disease: An overview

This paper discusses the effects of curcumin on patients with Alzheimer’s disease (AD). Curcumin (Turmeric), an ancient Indian herb used in curry powder, has been extensively studied in modern medicine and Indian systems of medicine for the treatment of various medical conditions, including cystic fibrosis, haemorrhoids, gastric ulcer, colon cancer, breast cancer, atherosclerosis, liver diseases and arthritis. It has been used in various types of treatments for dementia and traumatic brain injury. Curcumin also has a potential role in the prevention and treatment of AD. Curcumin as an antioxidant, anti-inflammatory and lipophilic action improves the cognitive functions in patients with AD. A growing body of evidence indicates that oxidative stress, free radicals, beta amyloid, cerebral deregulation caused by bio-metal toxicity and abnormal inflammatory reactions contribute to the key event in Alzheimer’s disease pathology. Due to various effects of curcumin, such as decreased Beta-amyloid plaques, delayed degradation of neurons, metal-chelation, anti-inflammatory, antioxidant and decreased microglia formation, the overall memory in patients with AD has improved. This paper reviews the various mechanisms of actions of curcumin in AD and pathology.

Keywords: Alternative approach to Alzheimer’s, beta amyloid plaques, curcumin, curcumin and dementia, epidemiology, turmeric
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Introduction
Alzheimer’s disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease. It is characterized by progressive cognitive deterioration together with declining activities of daily living and behavioral changes. It is the most common type of pre-senile and senile dementia. According to the World Health Organization (WHO), 5% of men and 6% of woman of above the age of 60 years are affected with Alzheimer’s type dementia worldwide.[1] In India, the total prevalence of dementia per 1000 people is 33.6%, of which AD constitutes approximately 54% and vascular dementia constitutes approximately 39%. AD affects approximately 4.5 million people in the United States or approximately 10% of the population over the age of 65, and this number is projected to reach four times by 2050. The frequency increases to 50% by the age of 80 years. Every year more than $100 billion is spent for health care in the U.S. to treat AD in primary care settings alone.

Neuropathology of AD:
The neuropathological process consists of neuronal loss and atrophy, principally in the temporoparietal and frontal cortex, with an inflammatory response to the deposition of amyloid plaques and an abnormal cluster of protein fragments and tangled bundles of fibres (neurofibillary tangles). Neurotic plaques are relatively insoluble dense cores of 5-10 nm thick amyloid fibrils with a pallor staining “halo” surrounded by dystrophic neuritis, reactive astrocytes and activated microglia. There is an increased presence of monocytes/macrophages in the cerebral vessel wall and reactive or activated microglial cells in the adjacent parenchyma.[2,3] The main protein component of amyloid in AD is the 39-42 amino acid (beta) amyloid peptide (A-beta)

Curcumin

Curcumin (Curcuma longa – Haldi) is the source of the spice Turmeric [Figure 2] and is used in curries and other spicy dishes from India, Asia and the Middle East. Similar to many other herbal remedies, people first used curcumin as a food and later discovered that it also had impressive medicinal qualities. It has been used extensively in Ayurveda (Indian system of Medicine) for centuries as a pain relieving, anti-inflammatory agent to relieve pain and inflammation in the skin and muscles. It has also proven to have anti-cancer properties.[4,5] Curcumin holds a high place in Ayurvedic medicine as a “cleanser of the body,” and today, science is finding a growing list of diseased conditions that can be healed by the active ingredients of turmeric

Botanical name: Curcuma longa; Family: Zingiberaceae, the ginger family. Turmeric is a sterile plant and does not produce any seeds [Figure 2]. The plant grows up to 3-5 ft tall and has dull yellow flowers. The underground rhizomes or roots of the plant are used for medicinal and food preparation. The rhizome is an underground stem that is thick and fleshy ringed with the bases of old leaves. Rhizomes are boiled and then dried and ground to make the distinctive bright yellow spice, turmeric.

Turmeric History:
Probably originating from India, turmeric has been used in India for at least 2500 years. It is most common in southern Asia and particularly in India. Turmeric was probably cultivated at first as a dye and later on it was used as cosmetic and as an auspicious and aromatic food substance. It possesses antiseptic, anti-inflammatory detoxifying properties as well as carminative properties. Turmeric has a long history of medicinal use in South Asia and was widely used in Ayurvedic, Siddha and Unani systems. It is thought to be a hybrid selection and vegetative propagation of wild turmeric (Curcuma aromatica), which is native to India, Sri Lanka and the eastern Himalayas and some other closely related species.

Worldwide, there are over 1000 published animal and human studies, both in vivo and in vitro in which the effects of curcumin on various diseases have been examined. Studies include epidemiological, basic and clinical research on AD.

Various studies and research[9,10] results indicate a lower incidence and prevalence of AD in India. The prevalence of AD among adults aged 70-79 years in India is 4.4 times less than that of adults aged 70-79 years in the United States.[9] Researchers investigated the association between the curry consumption and cognitive level in 1010 Asians between 60 and 93 years of age. The study found that those who occasionally ate curry (less than once a month) and often (more than once a month) performed better on a standard test (MMSE) of cognitive function than those who ate curry never or rarely.[10]

Mechanism of action of curcumin on Alzheimer’s disease:
The process through which AD degrades the nerve cells is believed to involve certain properties: inflammation, oxidative damage and most notably, the formation of beta-amyloid plaques, metal toxicity [Figure 3]. There have been several studies on effects of curcumin on AD. Outlined below are some of the studies and their conclusions.

A study conducted at UCLA found that curcumin may help the macrophages to clear the amyloid plaques found in Alzheimer’s disease. Macrophages play an important role in the immune system. They help the body to fight against foreign proteins and then effectively clear them. Curcumin was treated with macrophages in blood taken from nine volunteers: six AD patients and three healthy controls. Beta amyloid was then introduced. The AD patients, whose macrophages were treated with curcumin, when compared with patients whose macrophages were not treated with curcumin, showed an improved uptake and ingestion of the plaques. Thus, curcumin may support the immune system to clear the amyloid protein.[11]

Curcumin on glial cells:
Recent histological studies reveal the presence of activated microglia and reactive astrocytes around A-beta plaques in brains from patients with AD. The chronic activation of microglia secretes cytokines and some reactive substances that exacerbate A-beta pathology. So neuroglia is an important part in the pathogenesis of AD. Curcumin has a lipophilic property and can pass through all cell membranes and thus exerts its intracellular effects. Curcumin has anti-proliferative actions on microglia. A minimal dose of curcumin affects neuroglial proliferation and differentiation. Its inhibition of microglial proliferation and differentiation were studied and researched by the University of Southern California Los Angeles (UCLA). Researchers[12] using doses of 4, 5, 10, 15, 20 microM concentration of curcumin in C-6 rat glioma 2B-clone cells, a mixed colony of both neuroglial cells in a six- day trial, showed that curcumin dose dependently stops the proliferation of neuroglial cells, by differentiate into a mature cell or undergo apoptosis. It inhibits neuroglial cells proliferation dose dependently (i.e.) higher the concentration, the greater the inhibition. It has shown to decrease the glutamine synthetase (GS) assay, a marker enzyme for astrocytes. In the same study, curcumin was shown to increase CNP (2′3′- cyclic Nucleotide 3′-phosphohydrolase), a marker enzyme for oligodendrocytes. The overall effect of curcumin on neuroglial cells involves decreased astrocytes proliferation, improved myelogenesis and increased activity and differentiation of oligodendrocytes.

One of the important pathogenesis in Alzheimer’s disease is the chronic inflammation of nerve cells. Several studies have demonstrated the associated inflammatory changes such as microgliosis, astrocytosis and the presence of pro-inflammatory substances that accompany the deposition of amyloid-β (Aβ) peptide. Patients with the prolonged use of certain nonsteroidal anti-inflammatory (NSAID) drugs such as ibuprofen have been shown to have a reduced risk of developing the symptoms of AD; however, the chronic use of NSAID can cause a toxic effect on the kidneys, liver and GI track. Curcumin has a potent anti-inflammatory effect. Through its various anti-inflammatory effects, it may have a role in the cure of AD. Curcumin inhibits Aβ-induced expression of Egr-1 protein and Egr-1 DNA-binding activity in THP-1 monocytic cells. Studies have shown the role of Egr-1 in amyloid peptide-induced cytochemokine gene expression in monocytes. By inhibition of Egr-1 DNA-binding activity by curcumin, it reduces the inflammation. The chemotaxis of monocytes, which can occur in response to chemokines from activated microglia and astrocytes in the brain, can be decreased by curcumin.[13,14]

Curcumin is found to inhibit cyclooxygenase (COX-2), phospholipases, transcription factor and enzymes involved in metabolizing the membrane phospholipids into prostaglandins. The reduction of the release of ROS by stimulated neutrophils, inhibition of AP-1 and NF-Kappa B inhibit the activation of the pro-inflammatory cytokines TNF (tumor necrosis factor)-alpha and IL (interleukin)-1 beta.[15,16] Overall, curcumin decreases the main chemical for inflammation and the transcription of inflammatory cytokines. Curcumin inhibits intracellular IL-12 p40/p70 and IL-12 p70 expression. The exposure to curcumin also impaired the production of pro-inflammatory cytokines (IL-1, IL-6 and TNF-). These studies indicate a potent inhibitor of pro-inflammatory cytokine production by curcumin and it may differ according to the nature of the target cells.

Curcumin inhibits the activity of AP-1, a transcription factor involved in expression of amyloid, which is linked to AD. Curcuminoids are proven to have strong antioxidant action demonstrated by the inhibition of the formation and propagation of free radicals. It decreases the low-density lipoprotein oxidation and the free radicals that cause the deterioration of neurons, not only in AD but also in other neuron degenerative disorders such as Huntington’s and Parkinson’s disease.[16] In one study, curcuma oil (500 mg Kg(-1) i.p.) was given 15 min before 2 h middle cerebral artery occlusion, followed by 24 h reflow in rats. This significantly diminished the infarct volume, improved neurological deficit and counteracted oxidative stress.[17]

A study conducted at Nanjing Medical University (China) showed that a single injection of curcumin (1 and 2 mg/kg, i.v.) after focal cerebral ischemia/reperfusion in rats significantly diminished the infarct volume, improved neurological deficit, decreased mortality and reduced the water content in the brain.[18]

Curcumin has powerful antioxidant and anti-inflammatory properties; according to the scientists, these properties believe help ease Alzheimer’s symptoms caused by oxidation and inflammation.[19] A study conducted at Jawaharlal Nehru University (India) demonstrated that the administration of curcumin significantly reduced lipid peroxidation and lipofuscin accumulation that is normally increased with aging.[20] It also increased the activity of superoxide dismutase, sodium-potassium ATPase that normally decreased with aging. In another study, curcumin has been shown to protect the cells from betaA (1-42) insult through antioxidant pathway.[21] Curcumin protects brain mitochondria against various oxidative stress. Pre-treatment with curcumin protects brain mitochondria against peroxynitrite (a product of the reaction of nitric oxide with superoxide) a potent and versatile oxidant that can attack a wide range of cells in vitro by direct detoxification and in vivo by the elevation of total cellular glutathione levels

Natural antioxidant curcumin has been identified as a potent inducer of hemoxygenase, a protein that provides efficient cytoprotection against various forms of oxidative stress. By promoting the inactivation of Nrf2-keap1 complex and increased binding to no-1ARE, curcumin induces hemoxygenase activity. The incubation of astrocytes with curcumin at a concentration that promoted hemoxygenase activity resulted in an early increase in reduced glutathione, followed by a significant elevation in oxidized glutathione content.[23–25] Glutathione is an important water-phase antioxidant and essential cofactor for antioxidant enzymes protecting the mitochondria against endogenous oxygen radicals. Its level reflects the free radical scavenging capacity of the body. GSH depletion leads to tissue damage due to lipid peroxidation and oxidative damage.

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