Abstract
Backgrounds: There is still little progress in the effective treatment of radiation-induced lung injury (RILI), a key dose-limiting factor for thoracic radiotherapy. Ras-related C3 botulinum toxin substrate1, Rac1, is a small guanosine triphosphatase involved in various mechanisms of radiation-induced damage and is over-expressed/mutated in various tumors. The gain-of-function mutation of Rac1 mediates tumor cells’ resistance to radiotherapy. Therefore, inhibiting Rac1 has the potential of protecting normal tissues from radiation-induced injury, and at the same time, sensitizing tumor to radiation therapy, which makes it a promising ideal target for radiation protection. To investigate the protective effects and mechanisms of Rac1 inhibition on RILI, and explore the possible mechanisms that mediate the differential effects of Rac1 inhibition on normal lung tissue and tumor cells.Methods: 60Co radioactive source was used for ionizing radiation (IR). RILI mouse model was constructed. Influence of Rac1 inhibition which was achieved by Rac1-specific inhibitor, NSC23766, on RILI were studied by H & E and Masson staining, and immunohistochemical staining of vimentin, TGF-βand γ-H2AX. Normal mouse lung epithelial cell line, MLE-12, and mouse lung cancer cell line, LLC, were used to study the effects of Rac1 inhibition on the cellular level. RNA-seq analysis was used for screening differential gene expression caused by Rac1 knockdown. The molecular mechanisms of Rac1 inhibition were studied at the cellular level. Subcutaneous tumor-bearing nude mouse model and orthotopic lung tumor-bearing mouse model were constructed to verify the bidirectional effects of Rac1 inhibition in vivo. Results: RILI of mouse was alleviated by intraperitoneal injection of NSC23766. Rac1 inhibition/knockdown reduced the radiation-induced damage of MLE-12 while aggravated that of LLC. Rac1 translocated from cytoplasm to nucleus after radiation. Tumor protein p53-inducible nuclear protein 1, Trp53inp1, was down-regulated by Rac1 knockdown. In vivo study further proved the differential effects of Rac1 inhibition. Rac1 was over-expressed and mutated in LLC cells, and the expression level of Trp53inp1 significantly lower, compared with that of MLE-12.Conclusion: Rac1 inhibition reduced the radiation-induced damage of normal lung epithelial cells, thereby alleviating RILI of mouse. These effects were partially mediated by down-regulating the expression of Trp53inp1. However, Rac1 inhibition significantly increased the sensitivity of LLC to radiation damage and inhibited its growth. The over-expression and mutation of Rac1, and the significant low expression of Trp53inp1 in LLC, may be the fundamental reasons mediating the differential effects of Rac1 inhibition.
Inhibition of Rac1 Attenuates Radiation-Induced Lung Injury while Sensitizes Lung Tumor to Radiotherapy
