The E2F1 transcription factor is a critical downstream target of the tumor suppressor pRB. The retinoblastoma (RB) pathway is often inactivated in human tumors, resulting in deregulated E2F activity that can induce both proliferation and apoptosis. Bcl-2 homology 3 (BH3)-only proteins are pro-apoptotic members of the Bcl-2 protein family that trigger apoptosis in response to diverse stimuli. We show here that E2F1 up-regulates the expression of the pro-apoptotic BH3-only proteins PUMA, Noxa, Bim, and Hrk/DP5 through a direct transcriptional mechanism. Expression of the E7 protein of HPV16, which disrupts RB/E2F complexes, also up-regulates the expression of these four BH3-only proteins, implicating endogenous E2F in this phenomenon. Indeed, endogenous E2F1 binds the promoters of these four genes. Furthermore, inhibition of E2F1-induced expression of either Noxa or PUMA results in a significant reduction in E2F1-induced apoptosis, indicating that increased Noxa and PUMA levels mediate this E2F1-induced apoptosis. Importantly, inhibition of E2F activity abolishes DNA damage-induced elevation of PUMA levels, implicating E2F in the physiological regulation of PUMA expression. These data provide a novel direct link between E2F and the apoptotic machinery and may explain the increased sensitivity of cells with a defective RB/E2F pathway to chemotherapy.

The E2F1 transcription factor is a critical downstream target of the tumor suppressor RB. When activated, E2F1 induces cell proliferation. In addition, E2F1 can induce apoptosis via both p53-dependent and p53-independent pathways. A number of E2F-regulated genes, including ARF, ATM and Chk2, contribute to E2F-induced p53 stabilization. However, it is not known how E2F directs p53 activity towards apoptosis rather than growth arrest. We show that E2F1 upregulates the expression of four proapoptotic cofactors of p53 -ASPP1, ASPP2, JMY and TP53INP1 -through a direct transcriptional mechanism. Adenovirus E1A protein also induces upregulation of these genes, implicating endogenous E2F in this effect. TP53INP1 was shown to mediate phosphorylation of p53 on serine 46. We demonstrate that activation of E2F1 leads to phosphorylation of p53 on serine 46 and this modification is important for E2F1-p53 cooperation in apoptosis. Overall, these data provide novel functional links between RB/E2F pathway and p53-induced apoptosis.

The E2F family of transcription factors regulates a diverse array of cellular functions, including cell proliferation, cell differentiation, and apoptosis. Recent studies indicate that E2F can also regulate transcription of upstream components of signal transduction pathways. We show here that E2F1 modulates the activity of the p38 MAPK pathway via E2F1-induced transient up-regulation of p38 MAPK phosphorylation. The mechanism by which E2F1 modulates p38 MAPK phosphorylation involves transcriptional induction of the kinase ASK1, a member of the MAPKKK family that phosphorylates p38 MKKs. Subsequent E2F-dependent down-regulation of the p38 signaling pathway is achieved through E2F-induced up-regulation of Wip1, a phosphatase that dephosphorylates and inactivates p38. Both ASK1 and Wip1 are essential mediators of the E2F-p38 connection: knock down of ASK1 inhibits E2F1-induced phosphorylation of p38, whereas knock down of Wip1 prolongs E2F1-induced p38 phosphorylation. Furthermore, Wip1 knock down enhances E2F1-induced apoptosis. Therefore, our data reveal a novel link between a central signaling pathway and the transcription factor E2F and identify Wip1 as a modulator of E2F1-induced apoptosis.

No abstract

No abstract