Objective To investigate the protective effect and mechanism of CDK4/6 inhibitor palbociclib against radiation-induced injury in rats'small intestinal crypt epithelial cell line(IEC-6).Methods(1)Using CCK-8 colorimetric assay,colony formation assay,the PI staining method,Annexin V-FITC/PI dual staining and Western blot,the impact of palbociclib on proliferation,cell cycle dynamics,apoptosis,DNA damage,as well as phosphorylation events at Rb-780 and 795 sites in irradiated IEC-6 cells was investigated.(2)An IEC-6 cell line overexpressing Rb was established,and the effects of Rb overexpression on proliferation,cell cycle,and apoptosis in the IEC-6 cell line were assessed using the CCK-8 colorimetric assay,PI staining method,Annexin V-PE/7-AAD dual staining and Western blot.Additionally,the impact of Rb overexpression on the protective effect of palbociclib against radiation damage was verified in the Rb-overexpressing IEC-6 cell line.Results(1)Administration of palbociclib was demonstrated to enhance the proliferative capacity of IEC-6 cells following exposure to 10 Gy X-ray irradiation,increase colony formation ability post 4-8 Gy X-ray exposure while concomitantly reducing the proportion of cells in the G2/M phase after 10 Gy irradiation and diminishing apoptosis in irradiated IEC-6 cells.Protein expression analysis revealed that palbociclib inhibited phosphorylation at Rb-780 and 795 sites and attenuated the production of γ-H2AX in IEC-6 cells following irradiation.(2)After transfection of IEC-6 cells with Rb overexpression lentivirus,there was a significant increase in the total protein expression of Rb.The phosphorylation levels at Rb-780 and 795 sites were markedly elevated.Consequently,cell proliferation was accelerated,the proportion of cells in the S phase of the cell cycle increased,and the apoptotic rate significantly rose after exposure to 10 Gy X-ray irradiation.(3)Treatment with a lower dose of palbociclib(0.05 μmol/L)was found to suppress the phosphorylation of Rb at 780 and 795 sites in IEC-6 cells overexpressing Rb.Additionally,it reduced apoptosis in IEC-6 cells overexpressing Rb following exposure to 10 Gy X-ray irradiation.However,this treatment did not inhibit the phosphorylation of Rb at 780 and 795 sites in IEC-6 cells transfected with the empty vector,nor did it exert a protective effect against radiation-induced damage in these cells.Conclusion The CDK4/6 inhibitor palbociclib demonstrates significant protective effects against radiation-induced damage in rats'intestinal crypt epithelial cell line(IEC-6),with the Rb protein potentially playing a crucial role in mediating this radioprotective response.