Figure 6.

E2F1 is not responsible for prodigiosin-induced transcriptional repression of SKP2. (A) Dose-dependent reduction in E2F1 by prodigiosin. A549 cells were treated with increasing doses of prodigiosin (0–100 nM) for 24 h, and the protein levels of E2F1 were determined thereafter using immunoblotting analysis. Human β-tubulin was used as the loading control. (B) Ectopic expression of E2F1 fails to restore SKP2 expression in prodigiosin-treated cells. A549 cells were infected with pBabe.puro vector alone (pBabe) or with pBabe vector expressing HA-tagged E2F1 (HA-E2F1). The effect of E2F1 overexpression on SKP2 levels in prodigiosin-treated cells was examined thereafter. E2F1 activity was preserved in E2F1-overexpressing cells treated with prodigiosin, as shown by the high levels of cyclin E expression, a bona fide transcriptional target of E2F1. (C) A proposed model for the mechanistic understanding of prodigiosin's antiproliferative action. In this study, we established that prodigiosin induces PKB inhibition for transcriptional repression of SKP2, leading to cellular accumulation of p27^KIP1 to suppress cell proliferation. Neither GSK-3β nor E2F1 is involved in this process. The effectors downstream of PKB responsible for prodigiosin-induced repression of SKP2 transcription remain to be identified.