ABSTRACT
PURPOSE: To evaluate changes in expression of cell cycle related genes during apoptosis induced in HL60 cells by X-irradiation to understand molecular biologic aspects in mechanism of radiation therapy. MATERIAL AND METHODS: HL-60 cell line (promyelocytic leukemia cell line) was grown in culture media and irradiated with 8 Gy by linear accelerator (6 MV X-ray). At various times after irradiation, ranging from 3 to 48 hours were analyzed apoptotic DNA fragmentation assay for apoptosis and by western blot analysis and semi-quantitative RT-PCR for expression of cell cycle related genes (cyclin A, cyclin B, cyclin C, cyclin D1, cyclin E, cdc2, CDK2, CDK4, p16INK4a, p21WAF1, p27KIP1, E2F, PCNA and Rb). RESULTS: X-irradiation (8 Gy) induced apoptosis in HL-60 cell line. Cycline A protein increased after reaching its peak 48 h after radiation delivery and cyclin E, E2F, CDK2 and RB protein increased then decreased after radiation. Radiation induced up-regulation of the expression of E2F is due to mostly increase of phosphorylated retinoblastoma proteins (ppRb). Cyclin D1, PCNA, CDC2, CDK4 and p16INK4a protein underwent no significant change at any times after irradiation. There was not detected p21WAF1 and p27KIP1 protein. Cyclin A, B, C mRNA decreased immediately after radiation and then increased at 12 h after radiation. Cyclin D1 mRNA increased immediately and then decreased at 48 h after radiation. After radiation, cyclin E mRNA decreased with the lapse of time. CDK2 mRNA decreased at 3 h and increased at 6h after radiation. CDK4 mRNA rapidly increased at 6 to 12 h after radiation. There was no change of expression of p16INK4a and not detected in expressin of p21WAF1 and p27KIP1 mRNA. CONCLUSION: We suggest that entry into S phase may contribute to apoptosis of HL60 cells induced by irradiation. Increase of ppRb and decrease of pRb protein are related with radiation induced apoptosis of HL60 cells and tosis of HL60 cells induced by irradiation. Increase of ppRb and decrease of pRb protein are related with radiation induced apoptosis of HL60 cells and this may be associated with induction of E2F and cyclinE/CDK2. These results support that p21WAF1 and p27KIP1 are not related with radiation induced-apoptosis.