Autophagy and apoptosis have a
Autophagy and apoptosis have a very complex relationship, and the precise mechanism remains to be determined. At present, a large amount of research has confirmed that autophagy can protect the cell from apoptosis in special conditions such as nutrient deficiency or growth factors deprivation; the enhancement of autophagy can thus improve cell survival via inhibiting apoptosis (Gump and Thorburn, 2011, Mariño et al., 2014). However, the interaction between autophagy and apoptosis induced by high levels of iodine is unknown.
Materials and methods
Discussion Due to geological structures resulting in variations of iodine concentrations, more and more attention has been paid to the epidemic state of high concentrations of iodide in drinking water. It has been estimated that the number of iodine-excessive WHO member states has increased from 5 in 2003 to 10 up in 2016 (Gizak, 2016). And in China, high levels of iodine in drinking water were found in eleven provinces where there were approximately 31 million people living (Shen et al., 2011), the report shows that the water iodine concentrations may reach up to 2804 μg/L in China (Zhao et al., 2000), so it is important to study the health effects of high iodine. The concentrations used in our study were higher than those in the environment because taking into account that iodine could be taken up and accumulated in tissues, meanwhile, the rapid renal clearance of iodide makes it difficult to predict the extracellular concentration to trigger apoptosis (Vitale et al., 2000), it is possible that cells are actually exposed to a much higher level of iodine in vitro than is environmentally relevant, which is more consistent with reality. Our experimental iodide concentrations were based on the cytotoxic effects derived from the CCK-8 trial. The concentrations of iodide were largely consistent with other reports investigating the toxic effects of high concentrations of iodine on human thyroid cells (Vitale et al., 2000, Kostic et al., 2009). Iodine is required for the production of thyroid hormones that control metabolic processes, growth and development, especially of the manufacturer and central nervous system (Horn and Heuer, 2010, Brough, 2017). However, the relationship between high iodine and brain development remains unknown. There is one study demonstrating that high iodine had no obvious damaging effects on brain development in mice (Liu et al., 1988). However, our previous epidemiological study showed that exposure to high iodine concentrations in drinking water significantly reduced the intelligence of children (Liu et al., 2009). The present study showed that high iodide treatment results in adverse changes of multiple cellular phenotypes, including decreased number and viability of cells, changed cellular morphology that demonstrated high concentrations of iodide could damage human neuroblastoma SH-SY5Y cells, which sets a stage for us to further investigate the possible deeper mechanism underlying the relationship between high levels of iodide exposure and risk of intelligence impairment. Researches had proved that apoptosis plays a key role in the functional perfection of the nervous system. Recognized as a distinctive and important mode of “programmed” cell death, apoptosis is essential for the normal functioning and survival of most multicellular organisms. Previous studies have confirmed the effect of excessive iodine exposure on apoptosis to thyroid cells in vitro (Vitale et al., 2000) and in vivo (Chen et al., 2011). In our present study, high iodide exposure caused morphological changes of apoptosis and elevation of four major apoptotic factors including Cleaved-Caspase3, PARP, cytochrome c and P53 in cultured SH-SY5Y cells. The variations of these factors suggest that high iodide treatment can induce apoptosis in SH-SY5Y cells. Autophagy is a fundamental biological process by removing damaged organelles, but disordered autophagy is involved in a variety of diseases such as neurodegeneration and microbial infection (Mizushima et al., 2008). Singh et al. (2011) showed that molecular iodine (I2) treatment enhanced the features of autophagy in the human breast cancer cell line. Therefore, the other effects of high concentrations of iodide on SH-SY5Y cells were investigated. In this study, we focused on macroautophagy, the most prevalent form of autophagy, which is morphologically characterized by the formation of double-membrane autophagosomes (Shen and Codogno, 2012). Beclin1 and LC3 are essential proteins that regulate the autophagosomal membrane. The LC3-II/LC3-I ratio is positively correlated with the autophagosome number. These data indicate that high iodide may induce autophagy by upregulating the expression of LC3-II and Beclin1. In addition, the structure of p62 makes itself specially required both for the formation and the degradation of autophagosome by autophagy (Pankiv et al., 2007, Komatsu and Ichimura, 2010). The increased expression of p62 revealed that the formation speed of autophagosome was faster than the degradation velocity of autolysosome, indicating autophagosome accumulation (Dodson and Zhang, 2017). From the above, the expression levels of autophagy-related proteins demonstrate that autophagy may be an event accompanying with high iodide-induced apoptosis in SH-SY5Y cells.