Cistanche Research: Pharmacologic Epigenetic Modulators Of Alkaline Phosphatase in Chronic Kidney Disease Part 1
Mar 12, 2022
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Mathias Haarhausa,b,c, Dean Gilhamd, Ewelina Kulikowskid, Per Magnussonb, and Kamyar Kalantar-Zadehe,f,g
Purpose of review
In chronic kidney disease, disturbance of several metabolic regulatory mechanisms causes premature aging, accelerated cardiovascular disease, and mortality. Single-target interventions have repeatedly failed to improve the prognosis for chronic kidney disease patients. Epigenetic interventions have the potential to modulate several pathogenetic processes simultaneously. Alkaline phosphatase is a robust predictor of cardiovascular disease and all-cause mortality and is implicated in pathogenic processes associated with cardiovascular disease in chronic kidney disease.
Recent findings
In experimental studies, epigenetic modulation of alkaline phosphatase by microRNAs or bromodomain and extra terminal (BET) protein inhibition has shown promising results for the treatment of cardiovascular disease and other chronic metabolic diseases. The BET inhibitor apabetalone is currently being evaluated for cardiovascular risk reduction in phase III clinical study in high-risk cardiovascular disease patients, including patients with chronic kidney disease (ClinicalTrials.gov Identifier: NCT02586155). Phase II studies demonstrate an alkaline phosphatase-lowering potential of apabetalone, which was associated with improved cardiovascular and renal outcomes.
Summary
Alkaline phosphatase is a predictor of cardiovascular disease and mortality in chronic kidney disease. Epigenetic modulation of alkaline phosphatase has the potential to affect several pathogenetic processes in chronic kidney disease and thereby improve cardiovascular outcomes.
Keywords:
kidney function, alkaline phosphatase, apabetalone, chronic kidney disease, epigenetic, microRNA, vascular calcification.
Introduction
Chronic kidney disease (chronic kidney disease) is a state of imbalance of several important physiologic regulatory mechanisms, among them mineral balance, acid-base balance, nutritional balance, and energy balance, resulting in accelerated cardiovascular dis-ease (cardiovascular disease) and mortality. In addition, chronic kidney disease is also associated with chronic inflammation and resembles a model for premature aging [1,2]. In chronic kidney disease, numerous pathways are upregulated that are associated with immunity and inflammation, oxidative stress, endothelial dysfunction, vascular calcification, and coagulation [3]. Pharmacologic epigenetic modulation has the advantage of targeting several disease-related processes simultaneously. Due to its expression in multiple tissues and organs, which is upregulated in response to different pathogenic stimuli, alkaline phosphatase (alkaline phosphatase, EC 3.1.3.1) may be a suitable target for epigenetic modulation (Fig. 1).
FIGURE 1. Alkaline phosphatase is ubiquitously expressed; however, the contribution of alkaline phosphatase from different tissues to the circulating alkaline phosphatase activity may vary. Under healthy conditions, liver and bone isoforms of tissue nonspecific isozyme alkaline phosphatase comprise approximately 50% each of the total circulating alkaline phosphatase activity. Intestine alkaline phosphatase can comprise up to 10% of the circulating alkaline phosphatase activity in individuals with blood group B or 0, but less than 3% in individuals with blood group A. Circulating alkaline phosphatase predicts disease-related outcomes, for example, cardiovascular disease or mortality, but to which extend alkaline phosphatase derived from specific tissues contributes to the total circulating alkaline phosphatase activity in pathologic conditions remains largely undetermined. Designed by Macrovector and Brgfx-Freepik.com.
cistanche can treat kidney disease improve renal function
KEY POINTS
Circulating alkaline phosphatase is a robust risk marker for cardiovascular disease and all-cause mortality in the general population and chronic kidney disease.
Alkaline phosphatase is ubiquitously expressed and is involved in several pathophysiological processes associated with cardiovascular complications in chronic kidney disease, for example, vascular calcification, chronic inflammation, oxidative stress, and fibrosis.
BET inhibitors and microRNAs are epigenetic modulators with the potential to simultaneously target several different pathogenic mechanisms upregulated in chronic diseases.
The novel epigenetic modulator apabetalone targets pathogenetic processes associated with the induction of alkaline phosphatase and improves cardiovascular prognosis in high-risk patients, including patients with chronic kidney disease while lowering circulating alkaline phosphatase activity.
Cistanche can improve kidney functions and avoid chronic kidney disease.
ALKALINE PHOSPHATASE IN HEALTH AND DISEASE
Alkaline phosphatase is a ubiquitously expressed enzyme that catalyzes the hydrolytic removal of phosphate groups from biochemical compounds [4]. Four different isozymes are known in humans. The tissue-nonspecific isozyme (TN-alkaline phosphatase) is expressed in different organs, for example, bone, liver, kidneys, brain, cardiovascular system, and leukocytes, whereas tissue-specific isozymes are expressed in the intestine (I-alkaline phosphatase), the placenta, and the testis (germ cell alkaline phosphatase) [5]. In most healthy individuals, circulating total alkaline phosphatase activity is comprised of approximately 50% of bone-specific isoforms of TN-alkaline phosphatase (B-alkaline phosphatase) and an equal percentage of liver-specific TN-alkaline phosphatase isoforms. However, in patients with blood groups B and 0, I-alkaline phosphatase can contribute up to 10% of the circulating alkaline phosphatase activity. In individuals with blood group A, I-alkaline phosphatase contributes less than 3% of total alkaline phosphatase activity, as blood group A red cells bind I-alkaline phosphatase in the circulation. alkaline phosphatase is an ectoenzyme attached to the outer layer of cell membranes. It is released into circulation as a soluble homodimer and cleared from the circulation via hepatic asialoglycoprotein receptors after desialylation by circulating neuraminidase [6–8].
FIGURE 2. Summary of mechanisms linking dephosphorylation by alkaline phosphatase to normal and pathophysiological processes. LPS, lipopolysaccharides; MMP, metalloproteinase; OPN, osteopontin; Pi, phosphate; PL, pyridoxal; PLP, pyridoxal phosphate; PPi, pyrophosphate.
TN-alkaline phosphatase is involved in the regulation of biomineralization, inflammation, oxidative stress, endothelial dysfunction, fibrosis, and cellular hypertrophy [9&,10–12]. TN-alkaline phosphatase dephosphorylates compounds of the extracellular matrix quite unspecifically. Known biological functions of alkaline phosphatase include the inactivation of calcification inhibitors, the dephosphorylation of nucleotides in purinergic signaling, the activation of matrix metalloproteinases (MMPs), and the local regulation of vitamin B6 metabolism (Fig. 2). I-Alkaline phosphatase contributes to the regulation of the gut microbiome, nutrient uptake, and the systemic immune response [5].
FIGURE 3. Chromatin is comprised of DNA and proteins that generate a compact structure critical for the packaging and stability of eukaryotic chromosomes. The primary protein components are histones, around which the DNA is wound to form a nucleosome. Epigenetics involves covalent modifications to chromatin that does not affect the underlying DNA sequence. Covalent modifications to chromatin impact both chromatin structure and recruitment of transcription complexes that, in effect, switch genes on or off. These dynamic epigenetic modifications are carried out by adding (writing) and removing (erasing) posttranslational modifications, followed by ‘reading’, which dictates gene expression and eventual phenotypic response.
Alkaline phosphatase is present in many species including humans, and is routinely applied as a marker for liver disease or bone turnover; however, until recently, its biologic relevance was poorly understood. Similar to the evolutionary science behind the emergence of the C-reactive protein (CRP) from an inflammatory modulator to now a novel cardiovascular disease marker, over the past 2 decades, alkaline phosphatase, too, has been emerging with newly discovered roles in biological homeostasis [9& ]. Emerging evidence suggests that circulating alkaline phosphatase is a strong predictor of adverse cardiovascular outcomes and all-cause mortality [9& ]. Despite being a novel cardiovascular risk marker and potential therapeutic target for cardiovascular risk, no clinical-stage therapeutics aimed at lowering serum alkaline phosphatase are available to date.
Alkaline phosphatase and biomineralization
Biomineralization is regulated by a complex interplay of calcification promotors and inhibitors. In chronic kidney disease, disturbance of this interplay is common and can cause extensive soft-tissue calcification such as medial artery calcification or calcification of atherosclerotic plaques. alkaline phosphatase is essential for bone mineralization, as demonstrated by hypophosphatasia, a hereditary disease with loss-of-function mutations of the alkaline phosphatase gene that encodes TN-alkaline phosphatase [13]. In addition, alkaline phosphatase plays a central role in pathological soft-tissue calcification [14,15]. Alkaline phosphatase is actively enhanced in matrix vesicles derived from mineralization-competent cells. These vesicles function as nidi for matrix mineralization. The process is similar in physiologically mineralizing tissues, such as bone and dentin, and pathological soft-tissue calcification. Alkaline phosphatase promotes the propagation of matrix mineralization by dephosphorylation of mineralization inhibitors such as pyrophosphate and the phosphoprotein osteopontin, and by the generation of inorganic phosphate, rendering a more procalcific extracellular milieu [16–18]. A role in the regulation of additional phosphoproteins in the extracellular matrix can be speculated. Matrix Gla protein (MGP) is one of the most important physiological mineralization inhibitors [19]. Its activity is determined by post-translational phosphorylation in addition to vitamin K-dependent carboxylation [20,21]. The effect of MGP inhibition by pharmacological vitamin K antagonists on the propagation of medial artery calcification and calcific uremic arteriolopathy in chronic kidney disease is well known [22,23]. Lower circulating levels of the nonphosphorylated form of MGP are associated with vascular calcification and mortality in dialysis patients, independent of its carboxylation status [24]. However, the mechanisms of MGP dephosphorylation are yet unknown and a role for alkaline phosphatase in this process can only be hypothesized.
Alkaline phosphatase and fibrosis
A novel mechanism has been suggested for alkaline phosphatase in fibrosis and cardiovascular fibro calcification, which is a feature of congestive heart failure [25]. The upregulation of alkaline phosphatase in cardiac myocytes leads to increased fibrosis via dephosphorylation of metalloproteinases 2 and 9 [26]. Indeed, increased circulating alkaline phosphatase activities have been observed in chronic kidney disease patients with myocardial hypertrophy and congestive heart failure [27–29]. Further, alkaline phosphatase in bronchoalveolar lavage has been identified as a marker of pulmonary fibrosis, connecting alkaline phosphatase to fibrotic processes in the lung [30].
Alkaline phosphatase and inflammation
Several mechanisms link Alkaline phosphatase to inflammation. Circulating alkaline phosphatase correlates well with circulating CRP, and alkaline phosphatase has been suggested as a component of the hepatic acute-phase reaction [31]. Also, circulating I-alkaline phosphatase is enhanced in inflammatory conditions [32]. However, CRP and inflammatory cytokines have an inhibitory effect on alkaline phosphatase activity in osteoblasts [33,34] as circulating CRP was only associated with total alkaline phosphatase, not B-alkaline phosphatase, in a large cohort of dialysis patients [35], suggesting an extra-skeletal source for the increased circulating alkaline phosphatase activity during inflammation. In contrast to the effect of inflammation on alkaline phosphatase in bone, inflammatory mediators can increase alkaline phosphatase activity in vascular smooth muscle cells (VSMCs) and mesenchymal stem cells [36,37], which is concordant with the clinical finding of opposing effects of inflammation on bone versus vascular mineralization in chronic kidney disease [38]. Alkaline phosphatase modulates the cellular inflammatory response via purinergic signaling by contributing to the enzymatic conversion of proinflammatory extracellular adenosine triphosphate to anti-inflammatory adenosine [39]. Alkaline phosphatase is also expressed by inflammatory cells in the vascular wall and may mediate a link between inflammation and vascular calcification, commonly seen in the atherosclerotic plaque and diseases of the metabolic syndrome, such as type 2 diabetes mellitus and chronic kidney disease [40–43].
Sepsis-induced inflammation can cause acute kidney injury and loss of renal function that leads to morbidity and mortality [44]. Serum alkaline phosphatase predicts infection-related mortality [45] and has been proposed as a component of a clinical prediction model for bacteremia in chronic kidney disease stage 5D patients [46]. Circulating alkaline phosphatase has the potential to inactivate endotoxins and other highly phosphorylated proinflammatory compounds [31,32]. Intestinal alkaline phosphatase detoxifies lipopolysaccharide (LPS) to reduce its inflammatory properties and interaction with Toll-like receptors and prevents inflammation in zebrafish in response to the gut microbiota [47]. Indeed Resolvin E1-induced intestinal alkaline phosphatase promotes resolution of inflammation through LPS detoxification [48]. This concept is being challenged in clinical trials. For example, in patients with acute kidney injury and sepsis, injection of recombinant alkaline phosphatase promoted a decrease in all-cause mortality, supporting a physiological role for alkaline phosphatase in mitigating the deleterious and morbid actions arising from sepsis [49]. Hence, similar to CRP, there is a biologically plausible role for increased levels of alkaline phosphatase under such pathologic circumstances, which may elicit maladaptive consequences. I-alkaline phosphatase may also exert a protective effect against inflammation-induced complications of diabetes mellitus type 1, such as cardiovascular disease or diabetic nephropathy [50].
Alkaline phosphatase and oxidative stress Increased oxidative stress is associated with adverse cardiovascular outcomes [51]. Oxidative stress induces alkaline phosphatase and calcification in calcifying vascular cells [52]. Increased oxidative stress is also associated with osteoporosis [53] because mineralization is inhibited in osteoblasts [52]. The reduction of cardiovascular oxidative stress in chronic kidney disease patients by exercise treatment is associated with a reduction of circulating alkaline phosphatase [54]. However, the origin of the increased serum alkaline phosphatase activity in patients with oxidative stress has yet to be determined.
Alkaline phosphatase and hypertension alkaline phosphatase contributes to the regulation of hypertension and vascular tone. Inhibition of alkaline phosphatase in isolated perfused kidneys and experimental animals in vivo decreased the hypertensive blood pressure (BP) response to norepinephrine [55]. The effect is partially explained by the role of alkaline phosphatase in purinergic signaling and increased adenosine production. Circulating alkaline phosphatase activity is inversely correlated to maximal vasodilatory response to acetylcholine, indicative of endothelial dysfunction [56]. An additional mechanism linking alkaline phosphatase to BP control is the association with arterial stiffness [57], possibly explained by vascular calcification [58]. A contribution of Alkaline phosphatase to the increased fibrotic transformation of capacity arteries can also be speculated [59].
Alkaline phosphatase and cognitive impairment
Circulating alkaline phosphatase is associated with impaired cognition [60–62]. Cognitive impairment is a serious complication in aging and chronic kidney disease. Underlying abnormalities include neurodegenerative processes and impaired microcirculation. In Alzheimer’s disease, alkaline phosphatase in the brain and circulation is inversely correlated with cognitive function, and dephosphorylation of tau has been suggested as a putative pathomechanism [63]. Increased circulating alkaline phosphatase is also associated with cerebral small vessel disease, a hallmark of vascular cognitive impairment [64]. alkaline phosphatase contributes to the regulation of gamma-amino butyrate and other neurotransmitters [65]. The association of reduced circulating alkaline phosphatase after parathyroidectomy in chronic kidney disease patients with improved cognition suggests a possible therapeutic implication for alkaline phosphatase lowering in cognitive impairment [66].
Alkaline phosphatase in chronic kidney disease
In chronic kidney disease, circulating alkaline phosphatase is commonly used in conjunction with parathyroid hormone for the approximation of bone turnover due to its association with bone formation [10,67]. In the absence of liver disease, variations in total alkaline phosphatase typically arise from B-alkaline phosphatase and can identify extremes of high and low bone turnover [68]. Furthermore, circulating alkaline phosphatase is a better predictor of incident fractures in dialysis patients than bone mineral density [69]. Circulating alkaline phosphatase is also a strong and independent predictor of mortality and cardiovascular complications in chronic kidney disease [9& ]. In non-chronic kidney disease populations, the association between alkaline phosphatase and inflammation is predictive of mortality [35]. In contrast, circulating B-alkaline phosphatase levels in patients with advanced chronic kidney disease is an even stronger predictor of mortality than total alkaline phosphatase [70]. This could be due to its association with the extensive vascular calcification arising in patients with chronic kidney disease on dialysis [71]. As all of the pathomechanisms discussed above are upregulated in chronic kidney disease [3], the contribution of alkaline phosphatase to the increased chronic kidney disease-related mortality, cardiovascular complications, and impaired cognition is presumably multifactorial.
REGULATION OF Alkaline phosphatase L GENE EXPRESSION
Human TN-alkaline phosphatase is encoded by the alkaline phosphatase L gene (accession number, NM_000478), which is located on the short arm of chromosome 1, 1p36.12 [72–74]. The alkaline phosphatase L gene exceeds 50 kb and comprises 12 exons. The first exon is part of the 50-untranslated region of the TN-alkaline phosphatase mRNA, which consists of either exon 1A or 1B that respond to different promoters and results in two mRNAs, each encoding an identical polypeptide, but with different 50 -untranslated regions [75]. The expression of TN-alkaline phosphatase is ubiquitous; however, transcription of the two variants of exon 1 results in cell-specific and tissue-specific expression. One of these transcripts is termed ‘bone alkaline phosphatase L transcript' inactive osteoblasts comprising exon 1A, whereas exon 1B is driven by a separate promoter active in liver and kidney tissues [75,76].
The regulation of alkaline phosphatase L expression is best studied in osteoblast-like cells. Bone formation by cells from the osteoblast lineage and functional actions, for example, biomineralization, involve multiple developmental signals such as hormones, growth factors, cytokines, Wingless-related integration site (WNT) ligands, and bone morphogenetic proteins. In addition, several transcription factors regulate the expression of a variety of osteoblast-specific genes expressing proteins pivotal for biomineralization, for example, collagen type I, bone-specific alkaline phosphatase, and osteocalcin [77]. The bone essential transcription factor runt-related transcription factor 2 (Runx2) has been identified as the master regulator for osteoblast differentiation [78]. Osterix (Osx; Sp7 gene), a zinc-finger containing transcription factor with a Runx2-binding sequence, is also essential for osteoblast differentiation and bone mineralization. Osx is not expressed in Runx2-deficient mice, whereas the expression of Runx2 is not affected in Osx-deficient mice [79], which implies that Osx regulates osteoblast differentiation downstream of Runx2 [80].
Other key transcription factors involved in osteoblast differentiation are the homeobox gene Msx2 and members of the distal-less homeobox (Dlx) family. Msx2 represses the expression of alkaline phosphatase L by directly binding to its promoter, whereas Dlx5 activates alkaline phosphatase L expression by interfering with the action of Msx2 [81]. Dlx3 is another potent regulator of Runx2 activation during osteogenic differentiation [82]. It has also been demonstrated that overexpression of Dlx2 has no effect on RUNX2, DLX5, and MSX2 expression upon osteogenic induction, but stimulates alkaline phosphatase L and osteocalcin expression [83]. Thus, Dlx2 may directly upregulate alkaline phosphatase L to promote osteoblastogenesis.
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