The Role of Vitamin D in Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is the most common form of dementia in the elderly individuals and is associated with progressive memory loss and cognitive dysfunction. A significant association between AD and low levels of vitamin D has been demonstrated. Furthermore, vitamin D supplements appear to have a beneficial clinical effect on AD by regulating micro-RNA, enhancing toll-like receptors, modulating vascular endothelial factor expression, modulating angiogenin, and advanced glycation end products. Vitamin D also exerts its effects on AD by regulating calcium-sensing receptor expression, enhancing amyloid-β peptides clearance, interleukin 10, downregulating matrix metalloproteinases, upregulating heme oxygenase 1, and suppressing the reduced form of nicotinamide adenine dinucleotide phosphate expression. In conclusion, vitamin D may play a beneficial role in AD. Calcitriol is the best vitamin D supplement for AD, because it is the active form of the vitamin D₃ metabolite and modulates inflammatory cytokine expression. Therefore, further investigation of the role of calcitriol in AD is needed.

Introduction

Alzheimer’s disease (AD) is the most common form of dementia in elderly individuals and is associated with progressive memory loss and cognitive dysfunction. There is a high prevalence of dementia in patients with low serum 25-hydroxyvitamin D₃ (25OHD) levels. ¹ In a 7-year longitudinal study, serum vitamin D deficiency predicted non-Alzheimer’s dementias in older women. ² In addition, higher vitamin D dietary intake was associated with a lower risk of AD. ³ In a cross section of older adults, vitamin D deficiency was associated with low mood and impaired cognitive performance. ⁴ In older African Americans, vitamin D deficiency was also associated with impaired cognitive performance and low bone density of the hip. ⁵ A significant prevalence of vitamin D insufficiency has been reported in patients with AD. ⁶ Elderly women diagnosed with AD have an increased prevalence of vitamin D deficiency, ⁷ and this deficiency is also associated with low mood and impaired cognitive performance in older adults. ⁸ There is an association between Mini-Mental State Examination (MMSE) test scores and serum 25OHD levels. Vitamin D-sufficient patients had significantly higher MMSE scores compared to vitamin D-insufficient patients. ⁹ In another study, vitamin D deficiency was associated with increased odds of cognitive impairment in the elderly population of the United States. ¹⁰ A vitamin D-free regimen intensified the spatial learning deficit in AD animal models. ¹¹ Long-term treatment with calcitriol resulted in a higher density of CA1 neurons in the middle regions of the hippocampus in aging rats, ¹² which suggests that vitamin D can modulate the biological markers of brain aging. The membrane-associated rapid response steroid-binding receptor (1,25D₃-MARRS) was reported to be expressed on the cell surface and to mediate the rapid response of calcitriol. ¹³ Diosgenin derivative, a plant-derived steroidal sapogenin, significantly reduced amyloid deposits and improved memory dysfunction in amyloid β peptide (Aβ)_1-42-infused AD model rats. ¹⁴ Diosgenin also exhibited significantly improved performance of object recognition memory in AD model mice and significantly reduced amyloid plaques and neurofibrillary tangles in the cerebral cortex and hippocampus. ¹⁵ The 1,25D₃-MARRS was shown to be a target of diosgenin; 1,25D₃-MARRS knockdown completely inhibited diosgenin-induced axonal growth in cortical neurons and treatment with a neutralizing antibody against 1,25D₃-MARRS diminished the axonal regeneration effect of diosgenin in Aβ_1-42-induced axonal atrophy. ¹⁵ These reports suggest that the exogenous stimulator diosgenin activates the 1,25D₃-MARRS pathway, which may be a very critical signaling target for anti-AD therapy. In our previous publication, genetic studies identified a number of proteins that link vitamin D to AD pathology; vitamin D can also act in AD through a number of nongenomic mechanisms. ¹⁶ In the present article, we will further discuss the relationship between vitamin D and AD.

The Role of Vitamin D in AD

Many studies have suggested a role for vitamin D in brain development and function. The expression of the vitamin D receptor (VDR) occurs early in the developing rodent brain.^17,18 Both VDR and 1α-hydroxylase, the enzyme responsible for the formulating of active vitamin D in the human brain, were found in both neurons and glial cells in a regional and layer-specific pattern; VDR was restricted to the nucleus and 1α-hydroxylase was distributed throughout the cytoplasm. ¹⁹ These findings suggest that vitamin D may have autocrine/paracrine properties in the human brain and plays a role in cellular development within the nervous system. In patients with AD, VDR expression reportedly reduced in different layers of the hippocampus, ²⁰ which is more vulnerable to degeneration in AD. Variations at the VDR locus have been associated with susceptibility and progression to several diseases. The VDR gene variants influence the susceptibility to age-related changes in cognitive functioning. ²¹ In Baltimore Longitudinal Study of Aging, sex-specific VDR gene variation can modify age-related cognitive decline among the US adults. ²² The VDR polymorphisms of Apa1 and Taq1 were associated with the AD risk, particularly in people under the age of 75. ²³ Wang et al ²⁴ also demonstrated an association between VDR polymorphisms and the risk of late-onset AD. The frequency of “TaubF” haplotype (alleles of TaqI, ApaI, Tru9I, BsmI, and FokI, respectively), which was determined by analyzing 5 polymorphisms together, was significantly higher in the patient group with AD compared to the controls. ²⁵

Micro-RNAs (miRNAs) are small, noncoding, single-stranded RNA molecules that exist in cells and body fluids ²⁶ and play important roles in the pathogenesis of various diseases.^27,28 There is evidence that alterations in miRNA expression play a key role in AD pathogenesis. The miRNA-146a binds complementary to the 3’-untranslated region (3’-UTR) of complement factor H (CFH), an important repressor of the inflammatory response of the brain. Upregulation of miRNA-146a coupled to downregulation of CFH was observed in AD brain as a function of disease progression.^29,30 A strong positive correlation between miRNA-125b abundance and the glial cell marker glial fibrillary acidic protein (GFAP) was noted in brains with advanced AD. ³¹ An analysis of the temporal-lobe neocortex-derived extracellular fluid and the cerebrospinal fluid (CSF) of patients with AD showed the abundance of miRNA, including proinflammatory miRNA-146a and miRNA-155 from stressed human primary neural cells. ³² The presence of stable miRNAs in CSF at high enough concentrations to transmit inflammatory signaling in AD and that miRNA bioactivities may extend well outside of the cell cytoplasm to further mediate homeostatic or pathogenic interneural communication. In another study, the levels of urinary miRNA-146a in patients with systemic lupus erythematosus (SLE) were significantly higher than those of the healthy controls. Calcitriol generally reduces the levels of urinary miRNA-146a, which significantly correlates with the estimated glomerular filtration rate in patients with SLE. ³³ Serum miRNA-155 levels have also been reported to be lower in patients with SLE than in healthy controls, ³³ although the urinary levels were significantly higher in the patients with SLE than the controls. Calcitriol also reduces the levels of urinary miRNA-155, which significantly correlate with proteinuria and the SLE disease activity index. ³⁴ Furthermore, many additional studies have documented the relationship between vitamin D and miRNAs. Plasma miRNA-146 levels changed significantly after supplementation with high doses of vitamin D in healthy males compared to placebo. ³⁵ Moreover, VDR 3′-UTR-luciferase activities decreased 40% to 50% in human embryonic kidney 293 cells after transfection with pS-miR-27b or pS-mmu-miR-298. ³⁶ Human vitamin D₃ hydroxylase (CYP24), which catalyzes the inactivation of calcitriol, is posttranscriptionally regulated by miRNA-125b. ³⁷ A potential miRNA-125b recognition element was identified in the 3′-UTR of human VDR messenger RNA (mRNA). ³⁸ Fifteen miRNAs are known to be differentially regulated by calcitriol in prostate cancer cells, ³⁹ and the endogenous level of VDR mRNA is inversely correlated with the expression of miRNA-125b in melanoma cell lines. ⁴⁰ These findings suggest that calcitriol may have an effect on AD by regulating the expression of miRNAs.

Toll-like receptors (TLRs) are a group of glycoproteins that function as surface transmembrane receptors. These receptors are involved in the innate immune response to exogenous pathogenic microorganisms. Substantial evidence exists supporting the important role of TLRs in the pathogenesis and outcome of AD. The TLR-2 and TLR-4 expression is increased in the peripheral blood mononuclear cells (PBMCs) of patients with AD. ⁴¹ In an AD model, fibrillary Aβ were TLR-4-dependent and stimulated microglial activation and the upregulation of cytokines, such as tumor necrosis factor (TNF)-α, IL-1β, IL-10, and IL-17.^42,43 The TLR-2 is a primary receptor that mediates the neuroinflammatory activation triggered by Aβ.^44,45 In mice, active immunization with Aβ impairs memory performance via the TLR-2/4-dependent activation of the innate immune system. ⁴⁶ The TLR-4 signaling increases the vulnerability of neurons to damage by Aβ and oxidative stress in AD. ⁴⁷ These findings suggest that TLR2/4 signaling increases the vulnerability of neurons to Aβ and oxidative stress in AD. Furthermore, variations at the TLR locus have been associated with susceptibility and progression to AD. The TLR-2 polymorphisms are associated with AD in Han Chinese.^48,49 The TLR-4 polymorphisms are associated with AD in Han Chinese and Italian populations and in a mouse model of AD.^50–53 Soscia et al ⁵⁴ demonstrated that whole AD brain homogenates have significantly higher antimicrobial activity (antimicrobial peptide [AMP]) than age-matched non-AD samples and that AMP action correlated with Aβ-positive tissue. High levels of AMP (LL-37) are also associated with atherosclerotic plaques and induce death in vascular smooth muscle cells. ⁵⁵ In contrast, vitamin D deficiency increases the expression of hepatic mRNA levels of TLR-2, TLR-4, and TLR-9 in obese rats. ⁵⁶ However, calcitriol suppresses the expression of TLR-2 and TLR-4 mRNA and protein in human monocytes and triggers hyporesponsiveness to pathogen-associated molecular patterns. ⁵⁷ Calcitriol has also been shown to downregulate intracellular TLR-2, TLR-4, and TLR-9 expression in human monocytes. ⁵⁸ Interestingly, the TLR activation upregulated the expression of VDR and 1α-vitamin D hydroxylase in human monocytes. ⁵⁹ In addition, calcitriol can increase the vitamin D-induced expression of cathelicidin in bronchial epithelial cells ⁶⁰ and may enhance cathelicidin LL-37 production. ⁶¹ The addition of a VDR antagonist has been reported to inhibit the induction of cathelicidin mRNA by more than 80%, which consequently reduces protein expression of this antimicrobial agent by approximately 70%. ⁶⁰ Taken together, vitamin D may have a role in patients with AD by enhancing TLR pathways.

Angiogenesis is a complex process that involves the coordinated steps of endothelial cell activation, proliferation, migration, tube formation, and capillary sprouting. In addition, angiogenesis requires the participation of intracellular signaling pathways. Vascular endothelial growth factor (VEGF) is a key mediator of angiogenesis. Pathological angiogenesis could be a key event in the pathogenesis of AD. The abnormal regulation of VEGF expression has been reported in AD pathogenesis. Homogenates in the brains of APP23 mice, a transgenic model of AD, induced the formation of new vessels during in vivo angiogenesis, and could be blocked by a VEGF antagonist. ⁶² Angiopoietin 2 and VEGF are highly expressed in the microcirculatory system of patients with AD compared to controls. ⁶³ Enhanced VEGF immunoreactivity in clusters of reactive astrocytes was found in the neocortex of patients with AD but not in elderly controls. ⁶⁴ Increased VEGF levels in CSF were observed in patients with AD and vascular dementia compared to healthy controls. ⁶⁵ These findings suggest that angiogenic changes occur in the microcirculation of the AD brain and may contribute to disease pathogenesis. The VEGF interacts with Aβ and coaccumulates with Aβ in the brains of patients with AD. ⁶⁶ The Aβ also inhibits the VEGF-induced migration of endothelial cells, as well as VEGF-induced permeability in an in vitro model of the blood brain barrier. ⁶⁷ The VEGF gene variability was suggested as genetic factors influencing the lifespan in the cohort of Italian. ⁶⁸ The VEGF polymorphisms have been associated with AD in Italian, Han Chinese, and Tunisian populations.^69–72 Interestingly, calcitriol has been reported to inhibit angiogenesis both in vitro and in vivo. ⁷³ Calcitriol significantly inhibits VEGF-induced endothelial cell spouting and elongation in a dose-dependent manner and decreases the production of VEGF. ⁷⁴ Calcitriol has also been shown to be a potent inhibitor of retinal neovascularization in a mouse model of oxygen-induced ischemic retinopathy. ⁷⁵ In addition, vitamin D and its analogs have been shown to reduce VEGF expression in various cancer cell lines.^76,77 These findings suggested that vitamin D may modulate VEGF expression in AD.

Angiogenin is endocytosed by subconfluent endothelial cells, translocated to the nucleus, accumulates in the nucleolus, and is a potent inducer of neovascularization. ⁷⁸ Angiogenin protects cultured motor neurons against excito-toxic injury, whereas the knockdown of angiogenin levels potentiated excitotoxic motorneuron death. ⁷⁹ The patients with AD exhibited lower serum angiogenin and higher VEGF levels than control participants. Cognitive function was positively correlated with serum levels of angiogenin. ⁸⁰ Mouse and human angiogenin are induced during inflammatory process. ⁸¹ These findings implicate angiogenin may be involved in the pathogenesis of AD. However, vitamin D-deficient mice showed significantly decreased angiotensin and impaired colonic antibacterial activity, and were predisposed to colitis. ⁸² Lagishetty et al ⁸² also revealed that putative vitamin D response elements are located within the promoter for the angiogenin gene. Taken together, vitamin D may modulate angiogenin in AD.

Glyoxalase 1 (Glyo-1) catalyzes the first and rate-limiting step of methylglyoxal (MG) removal, which is the major precursor to advanced glycation end product (AGE) formation. The AGEs are a heterogeneous group of macromolecules that are formed by the nonenzymatic glycation of proteins, lipids, and nucleic acids. The receptors for AGE (RAGEs) are multiligand receptors, and their ligands are also likely to recognize several receptors while mediating their biological effects. ⁸³ Glyoxalase I-specific anti-serum revealed many intensely stained flame-shaped neurons in AD brains compared to brains from controls without dementia. ⁸⁴ Moreover, Glyo-I protein levels increase 1.5 times in patients with early AD and continuously decrease in the intermediate and late stages of AD. ⁸⁵ The accumulation of AGEs occurs in the brain during the aging process and might be involved in the pathogenesis of AD. The level of CSF AGEs was 1.7 times higher in patients with AD than in a normal age-matched group. ⁸⁶ The plasma AGE levels were almost twice as high in patients with AD as in controls. ⁸⁷ In AD, most astrocytes contained both AGE- and RAGE-positive granules and their distribution was almost the same. ⁸⁸ Micro-vascular RAGE levels increase in conjunction with the onset of AD and continue to increase linearly as a function of the pathologic severity of AD. ⁸⁹ The Aβ has been demonstrated to serve as a binding ligand for RAGE, which triggers a cascade of events leading to the generation of reactive oxygen species (ROS) in microglial cells.^90,91 Variations at the RAGE locus have been associated with susceptibility and progression to AD. There is an association between the RAGE G82S polymorphism and AD in Chinese and European populations.^92,93 Anti-RAGE and anti-Aβ IGs correlate strongly with global scores of dementia. ⁹⁴ These findings suggest potential for anti-Aβ and anti-RAGE immunoglobulins G as blood biomarkers for AD. Moreover, high levels of AGEs are thought to play a role in the development of chronic vasculopathies in advanced chronic kidney disease (CKD). The addition of calcitriol normalized the expression, secretion, and activity of RAGEs and IL-6 in cultured endothelial cells that were incubated in a CKD-like environment. ⁹⁵ Calcitriol may act as a vascular protective agent counteracting the deleterious effects of AGEs on endothelial cell activities. ⁹⁶ Calcitriol also decreased the expression of adhesion molecules, as well as the lipopolysaccharide (LPS)-induced mRNA expression of RAGEs and IL-6. In addition, calcitriol blunted the proatherosclerotic parameters via nuclear factor κB (NF-κB) and p38 in vitro. ⁹⁷ These findings suggest that vitamin D may protect against the delirious effects of AGEs in AD.

The calcium-sensing receptor (CaSR) is a G-protein-coupled, transmembrane receptor that responds to changes in Ca²⁺ levels. The CaSR plays a central role in calcium homeostasis, primarily by regulating parathyroid hormone (PTH) secretion and renal tubular calcium reabsorption,^98,99 and CaSR is also expressed in the brain, particularly in parts of the hippocampus, hypothalamus, and cerebellum. ¹⁰⁰ Calcium dysregulation increases the susceptibility to neuronal cell damage, and CaSR is altered in patients with AD. In AD brains, the intracellular immunostaining for the neuronal calcium sensor proteins, vicilin-like protein 1 and -3 (VILIP-1 and -3), was reduced compared to the controls. ¹⁰¹ Also, significantly fewer VILIP-1-immunoreactive neurons were found in the temporal cortex of patients with AD compared to the controls. ¹⁰¹ The VILIP-1 expression enhanced the hyper-phosphorylation of tau protein compared to the nontransfected or calbindin-D28K-transfected cells and was associated with amyloid plaques and fibrillary tangles in AD brains. ¹⁰² The Aβ deposited on hippocampal neurons in AD overactivated CaSR, resulting in neuronal dysfunction.^103,104 These morphological and neurochemical findings suggest an involvement of these neuronal calcium sensor proteins in pathology and possibly pathophysiology of changed calcium homeostasis in AD. In CaSR-null mice, CaSR activation was reported to be critical for the expansion and differentiation of oligodendrocyte progenitor cells. ¹⁰⁵ The CaSR polymorphism was significantly associated with AD susceptibility, particularly in individuals without the Apolipoprotein E4 allele. ¹⁰⁶ These findings demonstrate that the CASR has a role in AD susceptibility. However, CaSR expression is upregulated by calcitriol in the kidney, parathyroid gland, adipocyte, and in genetic hypercalciuric stone forming rats.^107–110 In addition, calcitriol regulates CaSR in human colorectal cancer. ¹¹¹ Taken together, vitamin D may have a role in AD by regulating CaSR expression.

The Aβ is produced in excess in AD and contribute to neuronal dysfunction and degeneration. The Aβ triggers neurodegeneration not only by inducing L-type voltage-sensitive calcium channels expression and nerve growth factor (NGF) levels but also by suppressing VDR expression. ¹¹² Administration of vitamin D to animal AD model protected neurons by preventing cytotoxicity and apoptosis, and also by downregulating LVSCC A1C and upregulating VDR. ¹¹³ In patients with AD, PBMCs are defective in the phagocytosis and degradation of Aβ. ¹¹⁴ The transcription of MGAT3 stimulated Aβ and distinguished macrophages into types 0-2. In vivo, the immune cell function was improved by calcitriol in both type I and type II macrophages.^114–117 In both type I and type II macrophages, calcitriol strongly stimulated Aβ phagocytosis and clearance while protecting against apoptosis. ¹¹⁷ In aged mice, vitamin D administration significantly reduced the levels of Aβ accumulation in the retinal inflammation and in the numbers of retinal macrophage and marked a shift in macrophage morphology. ¹¹⁸ The overexpression of VDR or vitamin D treatment suppressed the amyloid precursor protein (APP) transcription in neuroblastoma cells. ²⁴ A vitamin D-enriched diet is correlated with a decrease in the number of amyloid plaques, a decrease in Aβ, a decrease in inflammation, and an increase in NGF in the brains of Aβ precursor protein transgenic mice. ¹¹⁹ These observations suggest that a vitamin D₃-enriched diet may benefit patients with AD. Calcitriol also enhances the cerebral clearance of human Aβ across the blood–brain barrier of mouse brains and appears to enhance brain-to-blood Aβ_1-40 efflux transport by both genomic and nongenomic actions. ¹²⁰ Taken together, vitamin D may benefit to AD by enhancing Aβ clearance.

It is known that inflammation plays an important role in the pathogenesis of AD. Certain proinflammatory cytokines and chemokines are closely related to cognitive dysfunction and neurodegeneration. Interleukin-10 (IL-10) is an anti-inflammatory cytokine that may modulate disease progression. Compared with the controls, IL-10 production was reduced in patients with AD after stimulating the PMBC with Aβ. ¹²¹ In blood cells stimulated with LPS, IL-10 release was reduced in patients with multiinfarct dementia compared to normal participants. ¹²² The IL-10 enhances neurogenesis and cognitive function in APP + PS1 transgenic mice. ¹²³ The findings suggest that decreased IL-10 production point to a reduction of suppressor cell function in patients with AD. Positive associations between AD and the IL-10 polymorphism are reported in Chinese and Italian populations.^124,125 The homozygous A allele of the -1082 polymorphism of IL-10 increased the risk of AD and reduced the IL-10 generation in peripheral cells after amyloid cleavage. ¹²⁶ A systemic meta-analysis suggested an association between IL-19 polymorphisms and AD.^127,128 These reports suggest that polymorphisms of cytokine genes can affect neurodegeneration. Notably, IL-10 signaling is essential for the calcitriol-mediated inhibition of experimental autoimmune encephalomyelitis. ¹²⁹ In addition, the genes encoding the cytokines IL-2, IL-19, and IL-12B are primary calcitriol target genes. ¹³⁰ Note that low vitamin D status is associated with low cord blood levels of the immune-suppressive cytokine IL-10. ¹³¹ Calcitriol and calcipotriol have been shown to induce IL-10 receptor gene expression in human epidermal cells, ¹³² and calcitriol directly affects Treg cell growth and promotes IL-10 production. ¹³³ Vitamin D treatment downregulated TLR-4, TLR-5, and triggering receptor expressed on myeloid cells (TREM)-2; upregulated Toll interacting protein, CD-80, TREM-1, and the anti-inflammatory cytokine IL-10 in human myometrial cells. ¹³⁴ Taken together, vitamin D may have a role in AD by modulating IL-10.

Matrix metalloproteinases (MMPs) are proteolytic enzymes that are responsible for remodeling the extracellular matrix and regulating the leukocyte migration through the extracellular matrix. This migration is an important step in inflammatory and infectious pathophysiology. The MMPs are produced by many cell types, including lymphocytes, granulocytes, astrocytes, and activated macrophages. There is growing evidence that MMPs play an important role in the pathogenesis of AD. Leake et al ¹³⁵ identified an approximately 50% increase in the cortex levels of MMP-1 in AD. This finding is consistent with the presence of an inflammatory state within the brain in AD and contributes to the blood–brain barrier dysfunction seen in AD. Plasma MMP-3 was significantly elevated in patients with AD. ¹³⁶ The MMP-3 was expressed predominantly in white matter of brain and was also expressed in senile plaques in the cortices of patients with AD. ¹³⁷ The MMP-9 levels were significantly elevated in the plasma of patients with AD compared to the controls. ¹³⁸ In the brain tissue of patients with AD, MMP-9 expression was found in the neurons’ cytoplasm, neurofibrillary tangles, senile plaques, and vascular walls. ¹³⁹ There were inverse correlations between the Global Cognitive and MMSE scores and MMP-9 activity. ¹⁴⁰ The Aβ is a potent stimulator of MMP-9 and MMP-2 activity in mixed hippocampal astrocyte cultures. ¹⁴¹ The interaction of Aβ and RAGEs induces MMP-2 expression in brain endothelial cells. ¹⁴² The MMP genotypes may influence the risk of dementia. The MMP gene polymorphisms reportedly associated with vascular dementia and AD. ¹⁴³ The MMP-3 variants are associated with changes in the Aβ levels in humans and an increased risk of dementia.^144,145 An MMP-9 inhibitor improves the Aβ-mediated cognitive impairment and neurotoxicity in mice. ¹⁴⁶ These finding further suggest that MMP-9 plays a causal role in Aβ-induced cognitive impairment and neurotoxicity. Moreover, VDR-knock-out mice were shown to have an influx of inflammatory cells, phospho-acetylation of NF-κB, and upregulated expression of MMP-2, MMP-9 and MMP-12 in the lungs. ¹⁴⁷ The VDR TaqI polymorphism is associated with the decreased production of TIMP-1, a natural MMP-9 inhibitor. ¹⁴⁸ In addition, calcitriol modulates tissue MMP expression under experimental conditions, ¹⁴⁹ downregulates MMP-9 levels in keratinocytes, and may attenuate the deleterious effects of excessive TNF-α-induced proteolytic activity, which is associated with cutaneous inflammation. ¹⁵⁰ Calcitriol also inhibits both the basal levels and the staphylococcus-stimulated production of MMP-9 in human blood monocytes and alveolar macrophages. ¹⁵¹ Moreover, a vitamin D analog has also been reported to reduce the expression of MMP-2, MMP-9, VEGF, and PTH-related peptide in Lewis lung carcinoma cells. ¹⁵² Furthermore, calcitriol significantly attenuates the Mycobacterium tuberculosis-induced expression of MMP-7 and MMP-10, and suppresses the secretion of MMP-7 by M tuberculosis-infected PBMCs. The MMP-9 gene expression, secretion, and activity were also shown to be significantly inhibited by calcitriol, irrespective of the infection status. ¹⁵³ In another study, calcitriol also suppressed the production of MMPs (MMP-7 and MMP-9) and increased the level of TMP-1 in patients with tuberculosis. ¹⁵⁴ In human articular chondrocytes, calcitriol was shown to significantly suppress the increased production of MMP-9-induced by phorbol myristate acetate. ¹⁵⁵ Together, these studies suggest that calcitriol may play an important role in the pathological processes of AD by downregulating the level of MMPs and regulating the level of TIMPs.

Heme oxygenase-1 (HO-1) is a stress protein that may confer cytoprotection by enhancing the catabolism of prooxidant heme to the radical scavenging bile pigments biliverdin and bilirubin. In various models of oxidative tissue injuries, the induction of HO-1 protects tissues from further damage by removing the heme. ¹⁵⁶ Several studies have suggested that HO-1 may be involved in the pathogenesis of neurodegenerative diseases, such as AD. The HO-1 mRNA and its transcripted proteins were increased in the neocortical and cerebral vessels of patients with AD compared to the controls. ¹⁵⁷ The percentage of GFAP-positive astrocytes that coexpress HO-1 in the hippocampus in AD is significantly increased relative to age-matched controls. ¹⁵⁸ In rat astrocytes, human HO-1 overexpression resulted in significant oxidative damage to mitochondrial lipids, proteins, and nucleic acids and increased cell death. ¹⁵⁹ The long-term overexpression of HO-1 promotes tau aggregation in mouse brains. ¹⁶⁰ Astroglial HO-1 expression in the temporal cortex was associated with decreased scores of global cognition, semantic, and working memory, and was correlated with the burden of neurofibrillary pathology. ¹⁶¹ Another study found that exposing a group of obese rats to a Westernized diet (high fat/high fructose corn syrup) deficient in vitamin D exacerbated nonalcoholic fatty liver disease and resulted in an increased expression of the oxidative stress marker HO-1. ¹⁶² Additionally, pretreatment with vitamin D₃ ameliorates systemic IL-6 levels following ischemia and reperfusion of bilaterally occluded vessels in rats improves lung and muscle injury, and results in a significant increase in leukocytes HO-1 expression. ¹⁶³ Moreover, following the focal cortical ischemia that is elicited by photothrombosis, calcitriol treatment results in a transient but significant upregulation of glial HO-1 immunoreactivity. This upregulation was concomitant with a reduction in GFAP in remote cortical regions that are affected by the secondary spread of injury in glial cells. ¹⁶⁴ These results support the protective role of calcitriol in postcellular injury.

The reduced form of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) enzyme complex mediates critical physiological and pathological processes including cell signaling, inflammation, and mitogenesis, by generating ROS from molecular oxygen. The NOX is widely expressed in various immune cells, including microglia, macrophages, and neutrophils. In AD, NOX is activated in microglia, resulting in the formation of ROS that are toxic to neighboring neurons. ¹⁶⁵ The Aβ induces mitochondrial dysfunction and oxidative stress in astrocytes and the death of neurons via NOX activation.^166,167 The NOX expression and activity are upregulated specifically in vulnerable brain regions of mildly cognitively impaired patients. ¹⁶⁸ The NOX is also upregulated in the frontal and temporal cortex and contributes to AD progression. ¹⁶⁹ The inhibition of NOX or the gene deletion of its functional p47^phox subunit promotes alternative and anti-inflammatory microglial activation during neuroinflammation. ¹⁷⁰ However, vitamin D deprivation in rats decreased the activity of cytosolic NADPH-dependent 3,5,3′-triodo-l-thyronine binding in the liver. This decrease was restored by administering calcitriol. ¹⁷¹ In heart mitochondria, NAD⁺-dependent isocitrate dehydrogenase decreased notably in vitamin D-deficient rats, but calcitriol subsequently restored normal values. ¹⁷² In rat centrilobular hepatocytes, a vitamin D-deficient diet induced a significant increase in NADPH. ¹⁷³ Husain et al ¹⁷⁴ reported that cardiac NOX activity increased by 300% in uremic rats compared to the normal controls. Treatment with paricalcitol protected the uremic rats from cardiac oxidative stress by inhibiting NOX activity (by 50%), thus lowering superoxide production in the heart. Taken together, vitamin D may have a role in AD by suppressing NADPH expression.

Conclusions

Vitamin D may play a role in AD. Genetic studies have provided the opportunity to determine which proteins link vitamin D to the pathology of AD, including miRNA, TLRs, VEGF, angiogenin, and AGEs. Vitamin D also exerts its effects on AD via CaSR, Aβ, IL-10, MMPs, HO-1, and the reduced form of NADP. Calcitriol is the best potential vitamin D supplement for patients with AD because it is an active form of vitamin D₃ metabolite, suppresses PTH (a cellular growth factor), and binds to the VDR receptors on human nerve cells. The monitor of serum 25OHD₃ levels after calcitriol administration is unnecessary because calcitriol inhibits the liver production of serum 25OHD₃.^175,176 The main limitations to the widespread use of calcitriol are its hypercalcemic side effects.