ATM–p53 pathway causes G2/M arrest, but represses apoptosis in pseudolaric acid B-treated HeLa cells

Pseudolaric acid B (PAB) is a diterpene acid, isolated from the root and trunk bark of Pseudolarix kaempferi Gordon (Pinaceae). Previous studies demonstrated that PAB induced G2/M arrest and apoptosis in several cancer cell lines, but the relationship between G2/M arrest and apoptosis is still unclear. We examined the relevant signaling pathways for human cervical carcinoma HeLa cells treated with 1 μM PAB. Intriguingly, we found that activation of ATM–p53 signaling pathway by the treatment with 1 μM PAB played a protective role for the subsequent apoptosis. Although the treatment with 1 μM PAB up-regulated the expression of cyclin B1 and p-Histone 3 (mitotic markers) at 12 h, the expression decreased at 24 and 36 h along with the up-down expression of mitotic markers. The expressions of p-ATM and p-p53 that were involved in G2/M arrest increased at 12 h after treatment with PAB. However, a prolonged treatment with PAB (longer than 24 h) caused cell apoptosis. When the cells were arrested in G1 or S phase by the treatment with serum starvation, cytosine β-d-arabinofuranoside (Ara-C) or hydroxyurea (Hu), the apoptotic ratio induced by PAB decreased.