Aluminium-induced impairment of transcellular calcium absorption in the small intestine: calcium uptake and glutathione influence

The putative role of environmental aluminium in the development of chronic neuropathology in adults and children. How strong is the evidence and what could be the mechanisms involved?

The conceptualisation of autistic spectrum disorder and Alzheimer's disease has undergone something of a paradigm shift in recent years and rather than being viewed as single illnesses with a unitary pathogenesis and pathophysiology they are increasingly considered to be heterogeneous syndromes with a complex multifactorial aetiopathogenesis, involving a highly complex and diverse combination of genetic, epigenetic and environmental factors. One such environmental factor implicated as a potential cause in both syndromes is aluminium, as an element or as part of a salt, received, for example, in oral form or as an adjuvant. Such administration has the potential to induce pathology via several routes such as provoking dysfunction and/or activation of glial cells which play an indispensable role in the regulation of central nervous system homeostasis and neurodevelopment. Other routes include the generation of oxidative stress, depletion of reduced glutathione, direct and indirect reductions in mitochondrial performance and integrity, and increasing the production of proinflammatory cytokines in both the brain and peripherally. The mechanisms whereby environmental aluminium could contribute to the development of the highly specific pattern of neuropathology seen in Alzheimer's disease are described. Also detailed are several mechanisms whereby significant quantities of aluminium introduced via immunisation could produce chronic neuropathology in genetically susceptible children. Accordingly, it is recommended that the use of aluminium salts in immunisations should be discontinued and that adults should take steps to minimise their exposure to environmental aluminium.

Oxidative stress, antioxidants and intestinal calcium absorption

The disequilibrium between the production of reactive oxygen (ROS) and nitrogen (RNS) species and their elimination by protective mechanisms leads to oxidative stress. Mitochondria are the main source of ROS as by-products of electron transport chain. Most of the time the intestine responds adequately against the oxidative stress, but with aging or under conditions that exacerbate the ROS and/or RNS production, the defenses are not enough and contribute to developing intestinal pathologies. The endogenous antioxidant defense system in gut includes glutathione (GSH) and GSH-dependent enzymes as major components. When the ROS and/or RNS production is exacerbated, oxidative stress occurs and the intestinal Ca2+ absorption is inhibited. GSH depleting drugs such as DL-buthionine-S,R-sulfoximine, menadione and sodium deoxycholate inhibit the Ca2+ transport from lumen to blood by alteration in the protein expression and/or activity of molecules involved in the Ca2+ transcellular and paracellular pathways through mechanisms of oxidative stress, apoptosis and/or autophagy. Quercetin, melatonin, lithocholic and ursodeoxycholic acids block the effect of those drugs in experimental animals by their antioxidant, anti-apoptotic and/or anti-autophagic properties. Therefore, they may become drugs of choice for treatment of deteriorated intestinal Ca2+ absorption under oxidant conditions such as aging, diabetes, gut inflammation and other intestinal disorders.

Dietary and pharmacological compounds altering intestinal calcium absorption in humans and animals

The intestine is the only gate for the entry of Ca to the body in humans and mammals. The entrance of Ca occurs via paracellular and intracellular pathways. All steps of the latter pathway are regulated by calcitriol and by other hormones. Dietary and pharmacological compounds also modulate the intestinal Ca absorption process. Among them, dietary Ca and P are known to alter the lipid and protein composition of the brush-border and basolateral membranes and, consequently, Ca transport. Ca intakes are below the requirements recommended by health professionals in most countries, triggering important health problems. Chronic low Ca intake has been related to illness conditions such as osteoporosis, hypertension, renal lithiasis and incidences of human cancer. Carbohydrates, mainly lactose, and prebiotics have been described as positive modulators of intestinal Ca absorption. Apparently, high meat proteins increase intestinal Ca absorption while the effect of dietary lipids remains unclear. Pharmacological compounds such as menadione, dl-butionine-S,R-sulfoximine and ursodeoxycholic acid also modify intestinal Ca absorption as a consequence of altering the redox state of the epithelial cells. The paracellular pathway of intestinal Ca absorption is poorly known and is under present study in some laboratories. Another field that needs to be explored more intensively is the influence of the gene × diet interaction on intestinal Ca absorption. Health professionals should be aware of this knowledge in order to develop nutritional or medical strategies to stimulate the efficiency of intestinal Ca absorption and to prevent diseases.

Effects of Al(III) and nano-Al13 species on malate dehydrogenase activity

The effects of different aluminum species on malate dehydrogenase (MDH) activity were investigated by monitoring amperometric i-t curves for the oxidation of NADH at low overpotential using a functionalized multi-wall nanotube (MWNT) modified glass carbon electrode (GCE). The results showed that Al(III) and Al₁₃ can activate the enzymatic activity of MDH, and the activation reaches maximum levels as the Al(III) and Al₁₃ concentration increase. Our study also found that the effects of Al(III) and Al₁₃ on the activity of MDH depended on the pH value and aluminum speciation. Electrochemical and circular dichroism spectra methods were applied to study the effects of nano-sized aluminum compounds on biomolecules.

Electrochemical and spectral study on the effects of Al(III) and nano-Al13 species on glutamate dehydrogenase activity

A functionalized multi-wall nanotube (MWNT) modified glass carbon electrode (GCE) was used to study the effects of aluminum species on glutamate dehydrogenase (GLDH) activity by monitoring amperometric i-t curve for the oxidation of the enzymatically generated NADH. The conformational changes of the coenzyme nicotinamide adenine dinucleotide (NAD(+)) induced by Al(III) and nanometer-sized tridecameric aluminum polycation (nano-Al(13)) were investigated by the fluorescence technique. The results showed that the electrochemical method may be a potential tool to investigate the activity of enzymes in the biological system. It may also be useful in studying the effects of nano-sized aluminum compounds on biomolecules in order to discuss their safety to the environment and human.

Effects of duodenal redox status on calcium absorption and related genes expression in high-fat diet-fed mice

OBJECTIVE

This study investigated whether duodenal redox imbalance induced by high-fat diet (HFD) influenced expression of genes involved in transcellular calcium absorption, thus leading to reduced intestinal calcium absorption.

METHODS

Male C57BL/6 mice were randomly assigned to one of four groups with eight mice in each group. The control group consumed an ordinary diet (4.9% fat, w/w). The other three groups were fed a HFD (21.2% fat), the HFD plus 0.1% lipoic acid, or the HFD plus an additional 0.9% calcium supplement. After 9 wk, plasma and duodenal oxidative stress biomarkers including malondialdehyde, superoxide dismutase, catalase, total antioxidant capacity, reduced glutathione/oxidized glutathione ratio, and reactive oxygen species were examined. The intestinal calcium absorption state was evaluated through examining the calcium balance, bone mineral density, and calcium metabolism biomarkers. Furthermore, quantitative reverse transcription-polymerase chain reaction was carried out to analyze the changes in expression of transcellular calcium absorption-related genes.

RESULTS

The HFD induced marked decreases in intestinal calcium absorption and bone mineral density of the whole body, accompanied by redox imbalance and increased oxidative damage in duodenum; duodenal expression of calbindin-D(9K), plasma membrane calcium ATPase (PMCA(1b)), and sodium-calcium exchanger was significantly down-regulated by 1.9-, 2.7-, and 1.5-fold, respectively. Furthermore, duodenal glutathione and oxidized glutathione (GSH/GSSG) ratios were strongly positively correlated with the apparent calcium absorption rate and the expression of PMCA(1b) and Calbindin-D(9K), whereas reactive oxygen species levels were negatively correlated with them.

CONCLUSION

Our results demonstrated that a HFD-induced duodenal oxidation state could significantly down-regulate expression of calbindin-D(9K), PMCA(1b), and sodium-calcium exchanger, thus causing an inhibitory effect on intestinal calcium absorption.

Inhibitory effect of aluminium on calcium absorption in small intestine of rats with different thyroid hormone status

To analyse the influence of thyroid status on the effect of aluminium (Al) upon intestinal calcium (Ca) absorption, adult male Wistar rats with experimentally altered thyroid hormones circulating levels, were orally treated (o.g.) with 0 (control), or 50 mg elemental Al (as chloride)/kg body weight (b.w.) per day, for a 14 d period. Hyper- and hypo-thyroid conditions were respectively achieved by means of administration of either sodium levothyroxine (50 microg/kg b.w. per day, o.g.) or methimazole, a thyroxine synthesis inhibitor (1mg/kg b.w. per day, o.g.). In duodenum-jejunum segments, in vitro mucosa-to-serosa (45)Ca flux (JCa(ms)) and kinetics of (45)Ca uptake in isolated enterocytes, were determined. In serum, concentrations of thyroxine (T4) and triiodothyronine (T3) were measured by chemiluminescent enzyme immunoassay. Unlike non-Al-treated rats, JCa(ms) of Al-exposed rats decreased as serum levels of T4 and T3 increased, showing a significant inverse correlation in both cases (T4: r(2)=0.414, P=0.024; T3: r(2)=0.443, P=0.018). Enterocytes isolated from rats treated with Al plus thyroxine showed a reduction of both maximum Ca uptake (4.86+/-0.44 vs. 6.85+/-1.04 nmol Ca/mg protein, P<0.05) and K(m) (0.84+/-0.18 vs. 1.05+/-0.36 mM, P<0.05) when compared to control. The observed variability in the Al effect on Ca transport with thyroid status of rats could be reflecting a negative interaction of Al with thyroid hormone action mechanisms on intestinal Ca absorption, which would take place mainly at Ca entry into enterocyte from lumen.

Effect of aluminium on duodenal calcium transport in pregnant and lactating rats treated with bromocriptine

The aim of present work was to study the effect of oral aluminium (Al) overload on intestinal calcium (Ca) absorption in the critical stages of pregnancy and lactation of rats and to find out possible relationships with prolactin (PRL) and 17beta-estradiol (E2) circulating levels. Adult female Wistar rats were orally treated from day 1 of pregnancy with 0 (control), or 50 mg elemental Al (as chloride)/kg body weight per day. Ca transport was determined by everted duodenal sacs technique using 2 microCi of (45)CaCl(2) as flux marker (JCa(ms)). Al treatment reduced JCa(ms) either in late pregnancy (day 19) or in middle lactation (day 9 postpartum). Oral administration of bromocriptine (BrC), an inhibitor of PRL secretion, at dose of 10 mg/kg body weight given 18 h before JCa(ms) measurements were done, produced a significant decrease in the inhibitory effect of Al on JCa(ms), expressed as percent of control, at day 9 of nursing (vehicle: 51+/-7%, BrC: 28+/-4%, P <0.05). A positive correlation between Al effects on JCa(ms) and the physiological variations of E2 serum levels along pregnancy and lactation in BrC-treated rats was also found (r(2)=0.277, P =0.001). We conclude Al could reduce transcellular Ca absorption in the duodenum by interfering with physiological mechanisms of Ca transport partially mediated by serum level increments of E2 and PRL, observed in late pregnancy and mainly during middle lactation of rats.

Effect of Aluminium on the Levels of Some Essential Elements in Occupationally Exposed Workers

The mechanism of aluminium-induced cytotoxicity has not yet been defined. This study investigated possible changes in essential elements in workers occupationally exposed to Al fumes. It included 60 exposed workers and a matching control group of 60 employees not occupationally exposed to Al. Mean serum copper, calcium, zinc and iron were significantly lower in the exposed group than in controls. In addition, mean plasma and urine levels of Al were significantly higher in the exposed employees than in the controls. A statistically significant negative correlation was found between plasma and urinary Al and the studied essential elements. These findings corroborate the hypothesis that Al exposure has an adverse effect on essential elements in humans, with subsequent impact on the cellular enzymatic and metabolic processes.