Investigation of the Relationship between Aspartame and Swi6 in Schizosaccharomyces pombe

Aspartame (APM) is a non-nutritive first generation artificial sweetener used widely in many products including pharmaceuticals since 1981. Although it is a substance that is approved by FDA, previous medical studies have shown that it induces lymphomas and leukaemias. Molecular studies have found that it alters the expression of tumor suppressor gene TP53 and oncogenes c-Myc and Ha-ras; the formaldehyde it metabolizes to, forms DNA-DNA and DNA-protein crosslinks and sister chromatid exchanges. With these results, it has been confirmed that aspartame is a carcinogenic substance. In this research, our aim is to reveal molecular mechanisms of such effects in a simpler model organism Schizosaccharomyces pombe which has similar cellular processes with mammals. For the evaluation, human HP1 (heterochromatin protein 1) family ortholog of Swi6 is selected. Swi6 is a telomere, centromere, and mating-type locus binding protein for the regulation of heterochromatin structure. On the other hand, it has been found that HP1 gene expression decreases in several cancer types. Since Swi6 interacts with cell cycle through Cdc18, mitosis through cohesin, and chromatin remodeling through histones, it is possible that Swi6 also plays a key role in cancer or apoptosis. To verify if the carcinogenic effects of APM is related to Swi6; S. pombe parental and swi6Δ strains were analyzed with cell viability, intracellular oxidation, glucose consumption, nucleus DAPI staining and real time polimerase chain reaction (qRT-PCR) methods. According to the results, 1 mg/mL APM did not affect viability or glucose consumption and did not cause reactive oxygen species (ROS) production for both strains. But microscopic examinations showed that while APM did not induce nuclear fragmentation, cell size of the parental strain was increased. Additionally, when swi6Δ was compared with parental strain, slower rate of growth and glucose consumption, presence of intracellular ROS and cell size elongation were detected. Cell size difference was also found significant in the statistical interaction analysis of Swi6 and APM. Furthermore, qRT-PCR data showed a decrease in the expression of swi6 in parental and expression differences between parental and swi6Δ in fbp1, hxk2, tup11, sod1 and ctt1 genes were found significant, depending on the effect of APM. Taken together, it has been seen that S. pombe parental strain has adapted to APM effects by activating the stress response pathway and swi6Δ did not present a meaningful response. Thus, we proposed that there is a relationship between APM and Swi6, and that APM may be carrying out its effects through Swi6. Nevertheless, it is understood that aspartame is not an efficient agent for carcinogenesis, since its effects are weak when compared with swi6Δ phenotype.