Association of Cancer Stem Cell Radio-Resistance Under Ultra-High Dose Rate FLASH Irradiation With Lysosome-Mediated Autophagy

Abstract

Cancer stem cell (CSC) is thought to be the major cause of radio-resistance and relapse post radiotherapy (RT). Recently ultra-high dose rate “FLASH-RT” evokes great interest for its decreasing normal tissue damages while maintaining tumor responses compared with conventional dose rate RT. However, the killing effect and mechanism of FLASH irradiation (FLASH-IR) on CSC and normal cancer cell are still unclear. Presently the radiation induced death profile of CSC and normal cancer cell were studied. Cells were irradiated with FLASH-IR (∼109 Gy/s) at the dose of 6–9 Gy via laser-accelerated nanosecond particles. Then the ratio of apoptosis, pyroptosis and necrosis were determined. The results showed that FLASH-IR can induce apoptosis, pyroptosis and necrosis in both CSC and normal cancer cell with different ratios. And CSC was more resistant to radiation than normal cancer cell under FLASH-IR. Further experiments tracing lysosome and autophagy showed that CSCs had higher levels of lysosome and autophagy. Taken together, our results suggested that the radio-resistance of CSC may associate with the increase of lysosome-mediated autophagy, and the decrease of apoptosis, necrosis and pyroptosis. To our limited knowledge, this is the first report shedding light on the killing effects and death pathways of CSC and normal cancer cell under FLASH-IR. By clarifying the death pathways of CSC and normal cancer cell under FLASH-IR, it may help us improve the understanding of the radio-resistance of CSC and thus help to optimize the future clinical FLASH treatment plan.

Autophagy Regulation on Cancer Stem Cell Maintenance, Metastasis, and Therapy Resistance

“…Radiotherapy (RT) targets tumor cells with photons (e.g., X-ray) and particles (e.g., electrons, protons, and heavy ions) to directly induce irreparable DNA damage by DNA ionization or indirectly through the generation of ROS by interacting with water molecules [ 72 , 73 ]. CSCs have been shown to elevate levels of the antioxidant glutathione (GSH) to protect against ROS-induced oxidative stress [ 79 ].…”

“…Autophagy inhibition via ATG5 KD or BafA1 increased radio sensitization and induction of apoptosis [ 72 ]. Another study on breast CSCs demonstrated increased autophagic vesicles paired with a delayed response to irradiation and reduced initial apoptosis induction compared to normal cancer cells [ 73 ]. Both studies suggest that autophagy may play a protective role against radiotherapy for CSCs.…”

Mechanisms of FLASH effect

“…More importantly, Fouillade et al compared the lung injury between TERC-/-mice (mice with extremely short telomeres, simulating the state of stem cells senescence) and wild mice, the phenomenon of FLASH-RT on lung protection disappeared in TERC-/-mice. Yang et al (30) compared the damage ability of FLASH-RT to tumor stem cells and normal tumor cells in vitro. They concluded that under the condition of FLASH-RT (10 9 Gy/ s, 6~9Gy), tumor stem cells and normal tumor cells both will undergo apoptosis, scorch, and necrosis after irradiation.…”

“…The radiation resistance of cancer stem cells may be related to the increase of lysosome mediated autophagy. Due to Yang et al (30) did not compare the damage of FLASH-RT and COVN-RT on tumor stem cells, whether tumor stem cells affect the retention anti-tumor effect of FLASH-RT needs further study.…”

Deciphering the Biological Effects of Radiotherapy in Cancer Cells

“…In addition, it has been found that S100 calcium-binding protein A9 (S100A9) mediates the properties of CSCs in the brain microenvironment by activating the RAGE/NF-κB pathway [ 34 ]. It has been reported that with ultra-high doses of irradiation (~10 9 Gy/s), the CSCs may enhance lysosome-mediated autophagy and decrease pyroptosis and apoptosis, as well as necrosis [ 35 ].…”