In the present work, the long-term administration of aspartame induced major alterations in the light and electron microscopic structures of both the liver and the kidney. Several possible mechanisms were considered to be involved in the hepatotoxic and nephrotoxic effects of aspartame.
It has been reported that after ingestion of aspartame, it is metabolized in the gastrointestinal tract into three constituents: aspartic acid, phenylalanine, and methanol and further breakdown products including formaldehyde and formic acid. Methanol has a slow rate of oxidation and its major effect is found mainly in the liver and the kidneys .
In the present study, cytoplasmic vacuoles in hepatocytes might be responsible for the physical changes in the structure of plasma membranes of protein and lipids of different organelles. This affected the Na+/K+ pump function and caused the accumulation of sodium and migration of water to the cells. This might occur as a result of the release of the free radicals secondary to the production of methanol and aspartic acid after aspartame ingestion . In agreement with our explanation, it was suggested that methanol could increase the lipid peroxidation products as well as the surface charge density . This might result in membrane liver cell damage. Some investigators have reported that these vacuoles most probably represented a cellular defense mechanism against toxic substances, in which these substances were aggregated in the vacuoles, thus preventing their interference with cellular metabolism . However, aspartame did not affect DNA-damaging activity in the rat hepatocyte .
It was suggested that the degenerative changes observed in the liver treated with aspartame might be inflammatory similar to a hepatitis-like condition. This was confirmed by other researchers as they found that the disturbance in the secretion and formation of coagulation factor VII and fibrinogen induced by aspartame caused prolonged hepatitis . However, aspartame might disrupt the delicate balance between a positively and a negatively charged amino acid residue in humans . This might lead to the formation of a salt bridge between these amino acid residues and facilitate autoantigen presentation and CD4 T-helper cell activation as well as a decrease in the serum concentrations of the growth hormone. This causes a decrease in the activity of several hepatic cytochromes P-450 and other drug-metabolizing enzymes. Finally, the patients developed autoimmune hepatitis.
The prominent appearance of von kupffer cells in some hepatic lobules of aspartame-treated rats was attributed to accumulated formaldehyde, which led to damage of the protein molecules. These molecules were recognized by specific detection sites on kupffer cells [24,25]. Some authors added that macrophages could destroy these proteins at a rate 100 times faster than proteins not treated with formaldehyde .
In the present work, damage of liver cells might be secondary to the activation of Kupffer cells that secrete tumor necrosis factor alpha, interleukins, reactive oxygen, nitrogen species, proteases, and prostaglandins. These mediators could act directly on hepatocytes to cause cell death. This was in accordance with others, where they proved that Kupffer cells might cause hepatocellular damage in acetaminophen hepatotoxicity [26,27]. In contrast, some authors found that Kupffer cells played a protective role in hepatic injury .
In the present study, it was observed that aspartame led to structural changes in both glomeruli and tubules. The appearance of acidophoilic material inside the tubules might be simulating those described as hyaline material in male rats . The presence of this material could indicate the accumulation of protein secondary to renal dysfunction. Similar findings were obtained in rat kidney after feeding with waste fat released from grilled chicken .
The renal cortex was more affected than other parts in the kidney as the cortex received most of the blood nutrient flow to the organ. Thus, when a blood-borne toxicant is delivered to the kidney, a high percentage of the toxin will reach the cortex .
In the present study, there was a highly significant increase in the mean diameter of the proximal and distal tubules’ lumen in aspartame-treated rats. It was suggested that aspartame induced apoptotic changes in most of the renal tubule lining cells that had hetrochromatic irregular nuclei, ballooned mitochondria with destroyed cristae, loss of brush borders, and secondary lysosomes. These changes could be secondary to DNA damage that occurred as a result of breakage and then cross linking occurred within the genetic material by formaldehyde exposure. The chronic use of aspartame led to damage of nucleic acids, mainly DNA .
The mitochondrial changes observed might be considered as early manifestations of apoptosis and an adaptive process to unfavorable environments such as excess exposure of the cell to free radicals . Some authors agreed with this explanation as they proved that methanol significantly increased the malondialdehyde level and caspase-3 activity . This caused an increase in the level of lipid peroxidation and activation of the intrinsic pathway of apoptosis, where mitochondria permeability increased due to the formation of a high conductance channel in the inner mitochondrial membrane. This led to the release of proapoptotic molecules into the cytoplasm that activated caspases, followed by the release of death-inducing molecules such as cytochrome C, apoptosis-inducing factor, and endonuclease G .
It was suggested that the disruption of the intercellular junctions and detachment of the cells from the basement membrane in the proximal tubules might be a precancerous sign . Multiple studies have shown a correlation between the reduction of the integrity of these junctions and tumor initiation and progression [38,39]. In agreement with our finding, it was proved that the changes in the genetic material caused by aspartame may lead to cancer such as brain tumor in humans [40,41]. In contrast to this, some authors reported that aspartame was safe and it was not mutagenic or clastogenic in animals [3,42].
In the present work, there was a highly significant increase in the mean distance between the visceral and the parietal layer of the renal corpuscle. This could be secondary to shrinkage of the glomerulus. It was suggested that the drug concentration in the blood was affected by capillary constriction, leading to a decrease in glomerular filtration of that drug, which minimized its effect and protected the tubular cells . This might cause shrinkage and atrophy of the glomeruli. At the same time, the mesangial cell processes may be retracted due to the contraction of their filaments, which might be stimulated by angiotensin II.
The thickening of the glomerular and tubular basement membrane might have resulted from an increase in the amount of collagenous fibers secondary to overproduction of collagen fibers after repeated damage to the epithelial cells with regeneration and secretions of the new fibers. However, this thickening might be due to an increase in the deposition of glycoproteins .
In this study, it was observed that the proximal tubules were affected more than the distal ones. This was explained by some authors by the fact that PCTs are the first to come into contact with the toxic agent after its filtration by the glomeruli .
The beading of glomerular capillary endothelium might result in a vascular leakage with migration of neutrophils and monocular cells into the interstitial tissue, which led to inflammatory cellular infiltration. In addition, formaldehyde could conjugate with human serum albumin and yield a new antigenic determinant with stimulation of the body to produce anti formaldehyde human serum antibodies and to raise the antigen memory cells. This led to sustained stimulation of the immune system .
In conclusion, the results of our study showed that aspartame causes hepatorenal changes and consequently affects their functions. P. anisum oil provided direct protection from these changes; thus, the intake of aspartame should be restricted unless necessary and, if required, should be used for a short time. The optimum dose of anisum to exert good prophylactic effect against aspartame will need further investigations. To determine which chemical component in the P. anisum oil is responsible for its effects, chromatographic analysis of the oil should be performed and its ingredients should be studied in an aspartame-treated model.
There is no conflict of interest to declare.
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