Bcl-2 blocks 2-methoxyestradiol induced leukemia cell apoptosis by a p27(Kip1)-dependent G1/S cell cycle arrest in conjunction with NF-kappaB activation.

2-Methoxyestradiol (2-ME2) induces leukemia cells to undergo apoptosis in association with Bcl-2 inactivation but the mechanisms whereby Bcl-2 contributes to protection against programmed cell death in this context remain unclear. Here we showed that 2-ME2 inhibited the proliferation of Jurkat leukemia cells by markedly suppressing the levels of cyclins D3 and E, E2F1 and p21(Cip1/Waf1) and up-regulating p16(INK4A). Further, 2-ME2 induced apoptosis of Jurkat cells in association with down-regulation and phosphorylation of Bcl-2 (as mediated by JNK), up-regulation of Bak, activation of caspases-9 and -3 and PARP-1 cleavage. To determine the importance and mechanistic role of Bcl-2 in this process, we enforced its expression in Jurkat cells by retroviral transduction. Enforcing Bcl-2 expression in Jurkat cells abolished 2-ME2-induced apoptosis and instead produced a G1/S phase cell cycle arrest in association with markedly increased levels of p27(Kip1). Bcl-2 and p27(Kip1) were localized mainly in the nucleus in these apoptotic resistant cells. Interestingly, NF-kappaB activity and p50 levels were increased by 2-ME2 and suppression of NF-kappaB signaling reduced p27(Kip1) expression and sensitized cells to 2-ME2-induced apoptosis. Importantly, knocking-down p27(Kip1) in Jurkat Bcl-2 cells sensitized them to spontaneous and 2-ME2-induced apoptosis. Thus, Bcl-2 prevented the 2-ME2-induced apoptotic response by orchestrating a p27(Kip1)-dependent G1/S phase arrest in conjunction with activating NF-kappaB. Thus, we achieved a much better understanding of the penetrance and mechanistic complexity of Bcl-2 dependent anti-apoptotic pathways in cancer cells and why Bcl-2 inactivation is so critical for the efficacy of apoptosis and anti-proliferative inducing drugs like 2-ME2.