p53- and p21-independent apoptosis of squamous cell carcinoma cells induced by 5-fluorouracil and radiation.

Apoptosis-inducing therapy is becoming a new strategy in cancer therapy. We investigated the influence of 5-fluorouracil (5-FU) and radiation (gamma-ray) on the cell cycle of tumor cells, and their apoptosis-inducing activity using four oral squamous cell carcinoma lines (OSC-1 and OSC-4 with wild type p53; OSC-2 and OSC-3 with mutant type p53). The expression of p53 and cyclin-dependent kinase 2 (Cdk2) proteins was not increased even after cell treatment with 5-FU and gamma-rays in any cell lines. Although the promoter of p21 gene was not activated, p21-mRNA expression was increased by 5-FU and gamma-rays. p21 protein was expressed by irradiation in parallel with the increase in the messages but not by 5-FU in any OSC lines. Despite the increased p21 protein expression, cyclin E/Cdk2 kinase activity was not suppressed in irradiated cells. With the increased expression of cyclin E protein, 5-FU augmented the kinase activity in OSC-1, OSC-2 and OSC-3 cells. However, with a constant cyclin E level the kinase activity in OSC-4 was not increased by 5-FU. Without correlation to the kinase activity, 5-FU strongly induced apoptosis in OSC-2, OSC-3 and OSC-4 accumulating cells in the S phase, but 5-FU only very weakly induced apoptosis in OSC-1. While irradiated cells were in the G2/M phase, they exhibited apoptosis, to the same degree, in all OSC lines. Furthermore, the expression of Bax protein was not increased by 5-FU or gamma-rays, although apoptosis was induced by both treatments. These findings indicate that 5-FU and gamma-rays induce apoptosis of squamous cell carcinoma cells in p53- and p21-independent manners, in the S and G2/M phases, respectively.