FOXO1 promotes cancer cell growth through MDM2-mediated p53 degradation.

FOXO1 is a transcription factor and potential tumor suppressor that is negatively regulated downstream of PI3K-PKB/AKT signaling. Paradoxically, FOXO also promotes tumor growth, but the detailed mechanisms behind this role of FOXO are not fully understood. In this study, we revealed a molecular cascade by which the Thr24 residue of FOXO1 is phosphorylated by AKT and is dephosphorylated by calcineurin, which is a Ca2+-dependent protein phosphatase. Curiously, single nucleotide somatic mutations of FOXO1 in cancer occur frequently at and near Thr24. Using a calcineurin inhibitor and shRNA directed against calcineurin, we revealed that calcineurin-mediated dephosphorylation of Thr24 regulates FOXO1 protein stability. We also found that FOXO1 binds to the promoter region of MDM2 and activates transcription, which in turn promotes MDM2-mediated ubiquitination and degradation of p53. FOXO3a and FOXO4 are shown to control p53 activity; however, the significance of FOXO1 in p53 regulation remains largely unknown. Supporting this notion, FOXO1 depletion increased p53 and p21 protein levels in association with the inhibition of cell proliferation. Taken together, these results indicate that FOXO1 is stabilized by calcineurin-mediated dephosphorylation and that FOXO1 supports cancer cell proliferation by promoting MDM2 transcription and subsequent p53 degradation.

Dephosphorylation of NFAT by Calcineurin inhibits Skp2-mediated degradation.

The transcription factor NFAT plays key roles in multiple biological activities, such as immune responses, tissue development and malignant transformation. NFAT is dephosphorylated by calcineurin, which is activated by intracellular calcium levels, and translocated into the nucleus, resulting in transcriptional activation. Calcineurin dephosphorylates various target proteins and regulates their functions. However, the regulation of NFAT degradation is largely unknown, and it is unclear whether calcineurin contributes to the stability of NFAT. We investigated the effect of calcineurin inhibition on NFAT protein stability and found that the dephosphorylation of NFAT by calcineurin promotes the NFAT stabilization, whereas calcineurin mutant that is defective in phosphatase activity was unable to stabilize NFAT. Increased intracellular calcium ion concentration, which is essential for calcineurin activation, also induced NFAT stability. In addition, we identified S-phase kinase associated protein 2 (Skp2), an F-box protein of the SCF ubiquitin ligase complex, as a factor mediating degradation of NFAT when calcineurin was depleted. In summary, these findings revealed that the dephosphorylation of NFAT by calcineurin protects NFAT from degradation by Skp2 and promotes its protein stability.

Calcineurin-mediated dephosphorylation enhances the stability and transactivation of c-Myc.

c-Myc, a transcription factor, induces cell proliferation and is often aberrantly or highly expressed in cancers. However, molecular mechanisms underlying this aberrantly high expression remain unclear. Here, we found that intracellular Ca2+ concentration regulates c-Myc oncoprotein stability. We identified that calcineurin, a Ca2+-dependent protein phosphatase, is a positive regulator of c-Myc expression. Calcineurin depletion suppresses c-Myc targeted gene expression and c-Myc degradation. Calcineurin directly dephosphorylates Thr58 and Ser62 in c-Myc, which inhibit binding to the ubiquitin ligase Fbxw7. Mutations within the autoinhibitory domain of calcineurin, most frequently observed in cancer, may increase phosphatase activity, increasing c-Myc transcriptional activity in turn. Notably, calcineurin inhibition with FK506 decreased c-Myc expression with enhanced Thr58 and Ser62 phosphorylation in a mouse xenograft model. Thus, calcineurin can stabilize c-Myc, promoting tumor progression. Therefore, we propose that Ca2+ signaling dysfunction affects cancer-cell proliferation via increased c-Myc stability and that calcineurin inhibition could be a new therapeutic target of c-Myc-overexpressing cancers.

Impact of FKBP52 on cell proliferation and hormone-dependent cancers.

FK506 binding protein 52 (FKBP52) (gene name FKBP4) is a 52 kDa protein that belongs to the FKBP family; it binds to the immunosuppressant FK506 and has proline isomerase activity. In addition to its FK domain-containing peptidylprolyl isomerase activity, FKBP52 also acts as a cochaperone through the tetratricopeptide repeat domain that mediates binding to heat shock protein 90. Previous studies have reported that FKBP52 is associated with hormone-dependent, stress-related, and neurodegenerative diseases, revealing its diverse functions. In particular, the effects of FKBP52 on cancer have attracted considerable attention. FKBP52 promotes the growth of hormone-dependent cancers by activating steroid hormone receptors. Recent studies have shown that the expression of FKBP52 is increased not only in steroid hormone-dependent cancer cells but also in colorectal, lung, and liver cancers, revealing its diverse functions that contribute to cancer growth. This review summarizes reports related to hormone-dependent cancer and cell proliferation in terms of the structure of FKBP52 and its function on interacting molecules.

Dephosphorylation of the EGFR protein by calcineurin at serine 1046/1047 enhances its stability.

The epidermal growth factor receptor (EGFR) is highly expressed or abnormally activated in several types of cancers, such as lung and colorectal cancers. Inhibitors that suppress the tyrosine kinase activity of EGFR have been used in the treatment of lung cancer. However, resistance to these inhibitors has become an issue in cancer treatment, and the development of new therapies that inhibit EGFR is desired. We found that calcineurin, a Ca2+/calmodulin-activated serine/threonine phosphatase, is a novel regulator of EGFR. Inhibition of calcineurin by FK506 treatment or calcineurin depletion promoted EGFR degradation in cancer cells. In addition, we found that calcineurin dephosphorylates EGFR at serine (S)1046/1047, which in turn stabilizes EGFR. Furthermore, in human colon cancer cells transplanted into mice, the inhibition of calcineurin by FK506 decreased EGFR expression. These results indicate that calcineurin stabilizes EGFR by dephosphorylating S1046/1047 and promotes tumor growth. These findings suggest that calcineurin may be a new therapeutic target for cancers with high EGFR expression or activation.

FKBP52 and FKBP51 differentially regulate the stability of estrogen receptor in breast cancer.

Estrogen receptor α (ERα) is a transcription factor that induces cell proliferation and exhibits increased expression in a large subset of breast cancers. The molecular mechanisms underlying the up-regulation of ERα activity, however, remain poorly understood. We identified FK506-binding protein 52 (FKBP52) as a factor associated with poor prognosis of individuals with ERα-positive breast cancer. We found that FKBP52 interacts with breast cancer susceptibility gene 1 and stabilizes ERα, and is essential for breast cancer cell proliferation. FKBP52 depletion resulted in decreased ERα expression and proliferation in breast cancer cell lines, including MCF7-derived fulvestrant resistance (MFR) cells, suggesting that inhibiting FKBP52 may provide a therapeutic effect for endocrine therapy­resistant breast cancer. In contrast, FKBP51, a closely related molecule to FKBP52, reduced the stability of ERα. Consistent with these findings, FKBP51 was more abundantly expressed in normal tissues than in cancer cells, suggesting that these FKBPs may function in the opposite direction. Collectively, our study shows that FKBP52 and FKBP51 regulate ERα stability in a reciprocal manner and reveals a regulatory mechanism by which the expression of ERα is controlled.

FKBP51 and FKBP52 regulate androgen receptor dimerization and proliferation in prostate cancer cells.

The growth of prostate cancer is dependent on the androgen receptor (AR), which serves as a ligand-specific transcription factor. Although two immunophilins, FKBP51 and FKBP52, are known to regulate AR activity, the precise mechanism remains unclear. We found that depletion of either FKBP51 or FKBP52 reduced AR dimer formation, chromatin binding, and phosphorylation, suggesting defective AR signaling. Furthermore, the peptidyl-prolyl cis/trans isomerase activity of FKBP51 was found to be required for AR dimer formation and cancer cell growth. Treatment of prostate cancer cells with FK506, which binds to the FK1 domain of FKBPs, or with MJC13, an inhibitor of FKBP52-AR signaling, also inhibited AR dimer formation. Finally, elevated expression of FKBP52 was associated with a higher rate of prostate-specific antigen recurrence in patients with prostate cancer. Collectively, these results suggest that FKBP51 and FKBP52 might be promising targets for prostate cancer treatment through the inhibition of AR dimer formation.

Calcineurin regulates the stability and activity of estrogen receptor α.

Estrogen receptor α (ER-α) mediates estrogen-dependent cancer progression and is expressed in most breast cancer cells. However, the molecular mechanisms underlying the regulation of the cellular abundance and activity of ER-α remain unclear. We here show that the protein phosphatase calcineurin regulates both ER-α stability and activity in human breast cancer cells. Calcineurin depletion or inhibition down-regulated the abundance of ER-α by promoting its polyubiquitination and degradation. Calcineurin inhibition also promoted the binding of ER-α to the E3 ubiquitin ligase E6AP, and calcineurin mediated the dephosphorylation of ER-α at Ser294 in vitro. Moreover, the ER-α (S294A) mutant was more stable and activated the expression of ER-α target genes to a greater extent compared with the wild-type protein, whereas the extents of its interaction with E6AP and polyubiquitination were attenuated. These results suggest that the phosphorylation of ER-α at Ser294 promotes its binding to E6AP and consequent degradation. Calcineurin was also found to be required for the phosphorylation of ER-α at Ser118 by mechanistic target of rapamycin complex 1 and the consequent activation of ER-α in response to ß-estradiol treatment. Our study thus indicates that calcineurin controls both the stability and activity of ER-α by regulating its phosphorylation at Ser294 and Ser118 Finally, the expression of the calcineurin A-α gene (PPP3CA) was associated with poor prognosis in ER-α-positive breast cancer patients treated with tamoxifen or other endocrine therapeutic agents. Calcineurin is thus a promising target for the development of therapies for ER-α-positive breast cancer.

PP1 regulatory subunit NIPP1 regulates transcription of E2F1 target genes following DNA damage.

DNA damage induces transcriptional repression of E2F1 target genes and a reduction in histone H3-Thr11 phosphorylation (H3-pThr11 ) at E2F1 target gene promoters. Dephosphorylation of H3-pThr11 is partly mediated by Chk1 kinase and protein phosphatase 1γ (PP1γ) phosphatase. Here, we isolated NIPP1 as a regulator of PP1γ-mediated H3-pThr11 by surveying nearly 200 PP1 interactor proteins. We found that NIPP1 inhibits PP1γ-mediated dephosphorylation of H3-pThr11 both in vivo and in vitro. By generating NIPP1-depleted cells, we showed that NIPP1 is required for cell proliferation and the expression of E2F1 target genes. Upon DNA damage, activated protein kinase A (PKA) phosphorylated the NIPP1-Ser199 residue, adjacent to the PP1 binding motif (RVxF), and triggered the dissociation of NIPP1 from PP1γ, leading to the activation of PP1γ. Furthermore, the inhibition of PKA activity led to the activation of E2F target genes. Statistical analysis confirmed that the expression of NIPP1 was positively correlated with E2F target genes. Taken together, these findings demonstrate that the PP1 regulatory subunit NIPP1 modulates E2F1 target genes by linking PKA and PP1γ during DNA damage.

UV-induced activation of ATR is mediated by UHRF2.

UHRF1 (Ubiquitin-like with PHD and ring finger domains 1) regulates DNA methylation and histone modifications and plays a key role in cell proliferation and the DNA damage response. However, the function of UHRF2, a paralog of UHRF1, in the DNA damage response remains largely unknown. Here, we show that UHRF2 is essential for maintaining cell viability after UV irradiation, as well as for the proliferation of cancer cells. UHRF2 was found to physically interact with ATR in a DNA damage-dependent manner through UHRF2's TTD domain. In addition, phosphorylation of threonine at position 1989, which is required for UV-induced activation of ATR, was impaired in cells depleted of UHRF2, suggesting that UHRF2 is essential in ATR activation. In conclusion, these results suggest a new regulatory mechanism of ATR activation mediated by UHRF2.

Targeting EZH2 as cancer therapy.

Enhancer of zeste homolog 2 (EZH2) is the catalytic subunit of polycomb repressive complex 2 (PRC2) that mediate repression of target genes by trimethylation of Lys27 in histone 3 (H3K27me3). Given the reported roles of EZH2 in cancer, it is perhaps not surprising that targeting EZH2 in cancer therapy has become a hot research topic. Indeed, different types of EZH2 inhibitors are currently under development and are being evaluated by clinical trials. Recently, Murashima et al. identified NPD13668, a novel EZH2 inhibitor, by using a cell-based high-throughput screening assay. NPD13668 inhibited EZH2 methyltransferase activity, and repressed cell growth in multiple cancer cell lines, indicating a potential role for this compound in cancer treatment. In this review, we will focus on the current knowledge regarding the biological significance of PRC2 and H3K27me, and the recent advances in developing and testing drugs that target PRC2.

Calcineurin regulates cyclin D1 stability through dephosphorylation at T286.

The Calcineurin/NFAT (nuclear factor of activated T cells) pathway plays an essential role in the tumorigenic and metastatic properties in breast cancer. The molecular mechanism of the antiproliferative effect of calcineurin inhibition, however, is poorly understood. We found that calcineurin inhibition delayed cell cycle progression at G1/S, and promoted cyclin D1 degradation by inhibiting dephosphorylation at T286. Importantly, overexpression of cyclin D1 partially rescued delayed G1/S progression, thereby revealing cyclin D1 as a key factor downstream of calcineurin inhibition. Cyclin D1 upregulation is observed in human invasive breast cancers, and our findings indicate that dysregulation of T286 phosphorylation could play a role in this phenomenon. We therefore propose that targeting site specific phosphorylation of cyclin D1 could be a potential strategy for clinical intervention of invasive breast cancer.