The Unfolded Protein Response: Neutron-Induced Therapy Autophagy as a Promising Treatment Option for Osteosarcoma

Abstract

Radiotherapy using high linear energy transfer (LET) radiation results in effectively killing tumor cells while minimizing dose (biological effective) to normal tissues to block toxicity. It is well known that high LET radiation leads to lower cell survival per absorbed dose than low LET radiation. High-linear energy transfer (LET) neutron treatment induces autophagy in tumor cells, but its precise mechanisms in osteosarcoma are unknown. Here, we investigated this mechanism and the underlying signaling pathways. Autophagy induction was examined in gamma-ray-treated KHOS/NP and MG63 osteosarcoma cells along with exposure to high-LET neutrons. The relationship between radiosensitivity and autophagy was assessed by plotting the cell surviving fractions against autophagy levels. Neutron treatment increased autophagy rates in irradiated KHOS/NP and MG63 cells; neutrons with high-LETs showed more effective inhibition than those with lower LET gamma-rays. To determine whether the unfolded protein response and Akt-mTOR pathways triggered autophagy, phosphorylated eIF2α and JNK levels, and phospho-Akt, phosphor-mTOR, and phospho-p70S6 levels were, respectively, investigated. High-LET neutron exposure inhibited Akt phosphorylation and increased Beclin 1 expression during the unfolded protein response, thereby enhancing autophagy. The therapeutic efficacy of high-LET neutron radiation was also assessed in vivo using an orthotopic mouse model. Neutron-irradiated mice showed reduced tumor growth without toxicity relative to gamma-ray-treated mice. The effect of high-LET neutron exposure on the expression of signaling proteins LC3, p-elF2a, and p-JNK was investigated by immunohistochemistry. Tumors in high-LET-neutron radiation-treated mice showed higher apoptosis rates, and neutron exposure significantly elevated LC3 expression, and increased p-elF2a and p-JNK expression levels. Overall, these results demonstrate that autophagy is important in radiosensitivity, cell survival, and cellular resistance against high-LET neutron radiation. This correlation between cellular radiosensitivity and autophagy may be used to predict radiosensitivity in osteosarcoma.

Nuclear protein 1 promotes unfolded protein response during endoplasmic reticulum stress, and alleviates apoptosis induced by cisplatin in non-small cell lung cancer cells

“…In cells, the ER is responsible for the proper folding and processing of polypeptide chains into functional proteins. Some exogenous or endogenous factors lead to UPR, which will have a direct impact on tumor proliferation, migration, and apoptosis, and the associations between UPR and ER stress, autophagy, and apoptosis have already been revealed [8]. Several proteins affect the apoptosis and autophagy of cancer cells via UPR-induced ER stress.…”

“…The unfolded protein response (UPR) is one of the main mechanisms of tumor cell tolerance to chemotherapeutic drugs [8]. Rapid proliferation leads to an environment consisting of ischemia, hypoxia, and malnutrition in tumors.…”

Comparison of the Medical Uses and Cellular Effects of High and Low Linear Energy Transfer Radiation

“…ER stress, the UPR, and autophagy are also activated by both high- and low-LET radiation [ 203 , 204 , 217 , 218 ]. The fate of cells following the activation of these pathways depends on the duration and the degree of the response [ 121 , 202 ].…”

“…The fate of cells following the activation of these pathways depends on the duration and the degree of the response [ 121 , 202 ]. Studies in osteosarcoma and lymphoma cells showed that increased LET induced higher levels of UPR and autophagy [ 139 , 218 ]. ER stress and autophagy were activated by high-LET (I 125 ) radiation in human esophageal squamous cell carcinoma [ 219 ].…”

Recent Insights into Therapy Resistance in Osteosarcoma

“…Oh et al further characterized the link between autophagy and radioresistance by means of high linear energy transfer (LET) neutron radiation. Their study noted that high LET neutron radiation led to increased autophagy via inhibition of Akt and downstream mTOR activity [ 49 ].…”