Effects of Ionizing Radiation and RF-EMF on Amyloid Precursor Protein Processing and its Association with HSP27
Alzheimer’s disease is a disease of the brain that causes problems with memory, thinking and behavior. Alzheimer's disease has a steady increase in the incidence of elderly people aged 65 years and older. There are hypothesis that amyloid β, which is toxic to cells, aggregates and causes alzheimer’s disease. Currently, the spread of LTE (Long term evolution) mobile communication network has increased. Therefore, it is necessary to study the effect of LTE radio frequency electromagnetic fields (RF-EMF) on the nearest exposed brain, especially focus on alzheimer disease development. Ionizing radiation (IR) is currently one of the most common therapies for brain tumors, but it has been reported that it causes cognitive dysfunction, one of the pathological elements of alzheimer's disease. However, there are few reports to identify the molecules that cause alzheimer's disease. Moreover, in this study, HSP27 was evaluated for the candidate protein for inhibition of alzheimer disease development. The RF-EMF was exposed to the cells for 24 hours at an intensity of 8W/kg and the amyloid precursor protein (APP) processing and cell death effects were examined. IR was irradiated to cells at 5 Gy or 10 Gy for 48 hours and the protein related to APP processing and cell death were also examined. Immunoprecipitation and cycloheximide assays were performed to determine the association of HSP27 with APP. As a result, the expression of BACE1 (β-secretase 1) was increased in a dose dependent manner after 48 hours of IR. However, the tendency of amyloidogenic APP processing product, CTFβ (C-terminal fragment β) and nonamyloidogenic APP processing product, CTFα were not increased. IR dose dependent degradation of APP was observed accompanied with decreased expression of CTFβ and CTFα. In this time points, IR induced cell death when it was determined with flow cytometry or apoptosis markers expression such as cleaved PARP and cleaved caspase 3. In the case of RF-EMF, no significant changes of ADAM10 and BACE1 were observed, but the degradation of APP was observed similar to IR. However, unlike IR, a significant decrease in CTFs was not observed. RF-EMF has also proven to be a weak damage factor in experiments for measuring cell death. Immunoprecipitation (IP) and cycloheximide (CHX) assays were performed to determine the pathway to increase the stability of APP. As a result, HSP27 (HSP25) increased the stability of APP by binding directly to APP. In addition, the half life of APP was measured by the CHX assay and the half life of APP was found to be longer when HSP27 (HSP25) was overexpressed. Taken together, HSP27 can directly bind to APP to improve APP stability and increase protein half life. Identification of more detailed mechanisms will be needed in the further studies.