The p38 MAPK-MK2 axis regulates E2F1 and FOXM1 expression after epirubicin treatment

E2F1 is responsible for the regulation of FOXM1 expression, which plays a key role in epirubicin resistance. Here, we examined the role and regulation of E2F1 in response to epirubicin in cancer cells. We first showed that E2F1 plays a key role in promoting FOXM1 expression, cell survival, and epirubicin resistance as its depletion by siRNA attenuated FOXM1 induction and cell viability in response to epirubicin. We also found that the p38-MAPK activity mirrors the expression patterns of E2F1 and FOXM1 in both epirubicin-sensitive and -resistant MCF-7 breast cancer cells, suggesting that p38 has a role in regulating E2F1 expression and epirubicin resistance. Consistently, studies using pharmacologic inhibitors, siRNA knockdown, and knockout mouse embryonic fibroblasts (MEF) revealed that p38 mediates the E2F1 induction by epirubicin and that the induction of E2F1 by p38 is, in turn, mediated through its downstream kinase MK2 [mitogen-activated protein kinase (MAPK)-activated protein kinase 2; MAPKAPK2]. In agreement, in vitro phosphorylation assays showed that MK2 can directly phosphorylate E2F1 at Ser-364. Transfection assays also showed that E2F1 phosphorylation at Ser-364 participates in its induction by epirubicin but also suggests that other phosphorylation events are also involved. In addition, the p38-MK2 axis can also limit c-jun-NH(2)-kinase (JNK) induction by epirubicin and, notably, JNK represses FOXM1 expression. Collectively, these findings underscore the importance of p38-MK2 signaling in the control of E2F1 and FOXM1 expression as well as epirubicin sensitivity.

Phosphorylation of FOXO3a on Ser-7 by p38 promotes its nuclear localization in response to doxorubicin

FOXO3a is a forkhead transcription factor that regulates a multitude of important cellular processes, including proliferation, apoptosis, differentiation, and metabolism. Doxorubicin treatment of MCF-7 breast carcinoma cells results in FOXO3a nuclear relocation and the induction of the stress-activated kinase p38 MAPK. Here, we studied the potential regulation of FOXO3a by p38 in response to doxorubicin. Co-immunoprecipitation studies in MCF-7 cells demonstrated a direct interaction between p38 and FOXO3a. We also showed that p38 can bind and phosphorylate a recombinant FOXO3a directly in vitro. HPLC-coupled phosphopeptide mapping and mass spectrometric analyses identified serine 7 as a major site for p38 phosphorylation. Using a phosphorylated Ser-7 FOXO3a antibody, we demonstrated that FOXO3a is phosphorylated on Ser-7 in response to doxorubicin. Immunofluorescence staining studies showed that upon doxorubicin treatment, the wild-type FOXO3a relocalized to the nucleus, whereas the phosphorylation-defective FOXO3a (Ala-7) mutant remained largely in the cytoplasm. Treatment with SB202190 also inhibits the doxorubicin-induced FOXO3a Ser-7 phosphorylation and nuclear accumulation in MCF-7 cells. In addition, doxorubicin caused the nuclear translocation of FOXO3a in wild-type but not p38-depleted mouse fibroblasts. Together, our results suggest that p38 phosphorylation of FOXO3a on Ser-7 is essential for its nuclear relocalization in response to doxorubicin.

ATM and p53 regulate FOXM1 expression via E2F in breast cancer epirubicin treatment and resistance

In this report, we investigated the role and regulation of forkhead box M1 (FOXM1) in breast cancer and epirubicin resistance. We generated epirubicin-resistant MCF-7 breast carcinoma (MCF-7-EPI(R)) cells and found FOXM1 protein levels to be higher in MCF-7-EPI(R) than in MCF-7 cells and that FOXM1 expression is downregulated by epirubicin in MCF-7 but not in MCF-7-EPI(R) cells. We also established that there is a loss of p53 function in MCF-7-EPI(R) cells and that epirubicin represses FOXM1 expression at transcription and gene promoter levels through activation of p53 and repression of E2F activity in MCF-7 cells. Using p53(-/-) mouse embryo fibroblasts, we showed that p53 is important for epirubicin sensitivity. Moreover, transient promoter transfection assays showed that epirubicin and its cellular effectors p53 and E2F1 modulate FOXM1 transcription through an E2F-binding site located within the proximal promoter region. Chromatin immunoprecipitation analysis also revealed that epirubicin treatment increases pRB (retinoblastoma protein) and decreases E2F1 recruitment to the FOXM1 promoter region containing the E2F site. We also found ataxia-telangiectasia mutated (ATM) protein and mRNA to be overexpressed in the resistant MCF-7-EPI(R) cells compared with MCF-7 cells and that epirubicin could activate ATM to promote E2F activity and FOXM1 expression. Furthermore, inhibition of ATM in U2OS cells with caffeine or depletion of ATM in MCF-7-EPI(R) with short interfering RNAs can resensitize these resistant cells to epirubicin, resulting in downregulation of E2F1 and FOXM1 expression and cell death. In summary, our data show that ATM and p53 coordinately regulate FOXM1 via E2F to modulate epirubicin response and resistance in breast cancer.

Characterization of Drosophila melanogaster JmjC+N histone demethylases

In this article, we characterize histone demethylase activity of the entire family of JmjC+N proteins of Drosophila melanogaster. Our results show that Lid (little imaginal discs), which is structurally homologous to JARID1, demethylates H3K4me3. However, contrary to what would be inferred from its demethylase activity, lid contributes to the establishment of transcriptionally competent chromatin states as: (i) is required for histone H3 acetylation; (ii) contributes to expression of the homoeotic gene Ultrabithorax (Ubx); and (iii) antagonizes heterochromatin-mediated gene silencing (PEV). These results, which are consistent with the identification of lid as a trithorax group (trxG) gene, are discussed in the context of current models for the contribution of H3K4me3 to the regulation of gene expression. Here, we also show that the two Drosophila JMJD2 homologues, dJMJD2(1)/CG15835 and dJMJD2(2)/CG33182, are capable of demethylating both H3K9me3 and H3K36me3. dJMJD2(1)/CG15835 regulates heterochromatin organization, as its over-expression induces spreading of HP1, out of heterochromatin, into euchromatin, without affecting the actual pattern of histone modifications of heterochromatin. dJMJD2(1)/CG15835 is excluded from heterochromatin and localizes to multiple euchromatic sites, where it regulates H3K36 methylation. These results indicate that dJMJD2(1)/CG15835 contributes to delimit hetero- and euchromatic territories through the regulation of H3K36 methylation in euchromatin. On the other hand, dJARID2/CG3654 shows no demethylase activity on H3K4me3, H3K9me3, H3K27me3, H3K36me3 and H4K20me3.