Abstract
In general, transforming growth factor-beta-activated kinase 1 (TAK1) plays a role to inhibit apoptosis in response to multiple stresses. To assess the role of TAK1 in X-irradiated cells, cell death and cell cycle distributions were investigated in TAK1 knockdown and its parental HeLa cells. Furthermore, changes of gene expressions were also examined to elucidate the molecular mechanisms.
After X-irradiation, radiosensitivity estimated by colony formation assay increased in the TAK1 knockdown cells. Apoptosis induction indicated by caspase-3 cleavage also supported the enhanced radiosensitivity in the TAK1 knockdown cells. Simultaneously, irradiated cells in G2/M phases decreased and those in S and SubG1 phases increased by TAK1 depletion, indicating loss of cell cycle checkpoint regulation promotes radiosensitivity in the TAK1 knockdown cells. However, we could not identified significant difference of increased NF-kappa B, p38 MAPK and ERK phosphorylation, major downstream molecules of TAK1, between TAK1 knockdown cells and its parental control cells after irradiation. Instead, global gene expression analysis revealed the differentially expressed genes after irradiations and bioinformatics analysis identified a genetic network associated with cell cycle regulation. Several genes associated with cell cycle in the genetic network showed different expression patterns between TAK1 knockdown and its parental cells after X-irradiation. Especially, CDKN1A (coding p21WAF1), which was central in the identified network, was up-regulated in control cells but not in TAK1 knockdown cells after X-irradiations. RNA interference of p21 decreased cells in G2/M phases and increased cells in S and SubG1 phases after X-irradiations. In conclusion, these findings showed the role of TAK1 in cell cycle regulation and apoptosis in a manner independent on NF-kappa B, p38 MAPK, and ERK phosphorylations but dependent on p21 induction, at least in part.
