Forkhead transcription factors are critical effectors of cell death and cell cycle arrest downstream of PTEN

Nuclear PTEN levels and G2 progression in melanoma cells

The phosphatase and tensin homolog (PTEN) exerts its function, in part, by negatively regulating the well-known phosphatidylinositol-3-kinase/AKT signaling pathway. Previous histological work has suggested that alterations in the nuclear/cytoplasmic compartmentalization of PTEN may play a role in the development and progression of melanoma. In this study, we examined the nuclear/cytoplasmic compartmentalization of PTEN in melanoma cell lines and its correlation with the cell cycle. Studies were performed in melanoma cells lines using classic cell biological techniques. In contrast to breast cancer cell lines, we found that increased levels of nuclear PTEN levels correlate with G2 rather than with G1 arrest. In WM164 and SKmel28 cells, overexpression of PTEN protein did not significantly increase the number of cells in the G2 phase. Differential CDC2 phosphorylation levels in cells that overexpressed PTEN compared with those where PTEN was downregulated suggest some involvement of PTEN in G2 checkpoint regulation. The data suggest that although nuclear PTEN levels correlate with the G2 phase, the role of PTEN in modulating G2/M arrest is not limiting. Further, the specific cell cycle phase regulated by nuclear PTEN is cell-type dependent. Taken together, our observations suggest that in melanoma, nuclear PTEN is involved in G2 progression possibly through the modulation of CDC2, opening up a new arena for investigation.

TSC2 deficiency increases PTEN via HIF1alpha

Substantial evidence suggests roles of TSC2 and PTEN in the development of cancer predisposition syndromes. Loss of TSC2 results in benign tumors, neurological disorders, and angiomyolipomas. We found that PTEN mRNA and protein levels are elevated in Tsc2(-/-) mouse embryo fibroblasts with concomitant reduction in Akt phosphorylation. Reconstitution of TSC2 in Tsc2(-/-) mouse embryo fibroblasts decreases PTEN levels. Interestingly, increased HIF1alpha activity present in Tsc2 null cells is required for PTEN transcription and protein expression. We identified a canonical hypoxia-responsive element in the PTEN promoter, which regulates the transcription of this tumor suppressor protein in a TSC2-dependent manner. Finally, we demonstrate a positive correlation between expression of HIF1alpha and PTEN in renal angiomyolipomas from TSC patients. Our results reveal a unique function of HIF1alpha in up-regulation of PTEN and provide a new mechanism of reduced Akt phosphorylation in Tsc2 null cells. These data suggest that PTEN may safeguard against developing malignant tumors in patients with TSC deficiency.

Astrocyte elevated gene-1 is a proliferation promoter in breast cancer via suppressing transcriptional factor FOXO1

We have previously reported that astrocyte elevated gene-1 (AEG-1) was upregulated in human breast cancer. However, the biological function of AEG-1 in the development and progression of breast cancer remains to be clarified. In this study, we examined the effect of AEG-1 on cell proliferation and found that AEG-1 upregulation was significantly linked to increased Ki67 (P<0.001). Ectopic expression of AEG-1 in MCF-7 and MDA-MB-435 breast cancer cells dramatically enhanced cell proliferation and their ability of anchorage-independent growth, whereas silencing endogenous AEG-1 with shRNAs inhibited cell proliferation and colony-forming ability of the cells on soft agar. Furthermore, these proliferative effects were significantly associated with decreases of p27Kip1 and p21Cip1 two key cell-cycle inhibitors. Moreover, we further demonstrated that AEG-1 could downregulate the transcriptional activity of FOXO1 by inducing its phosphorylation through the PI3K/Akt signaling pathway. These observations were further confirmed in clinical human primary breast cancer specimens, in which high-level expression of AEG-1 was inversely correlated with the expression of FOXO1. Taken together, our results provide the first demonstration of a novel mechanism by which AEG-1 induces proliferation of breast cancer cell, and our findings suggest that AEG-1 might play an important role in tumorigenesis of breast cancer.

EBV and genomic instability--a new look at the role of the virus in the pathogenesis of Burkitt's lymphoma

Epidemiological and molecular evidence links Epstein-Barr virus (EBV) carriage to the pathogenesis of human malignancies of lymphoid and epithelial cell origin but the mechanisms of viral oncogenesis are poorly understood. Burkitt's lymphoma, a tumor occurring in both EBV-positive and -negative forms, provides a convenient model for analysis of the relative contribution of genetic changes and viral products that are expressed in the malignant cells. Here we review recent findings that highlight several mechanisms by which EBV could play an important role in oncogenesis by promoting genomic instability.

Insulin stimulates adipogenesis through the Akt-TSC2-mTORC1 pathway

BACKGROUND

The signaling pathways imposing hormonal control over adipocyte differentiation are poorly understood. While insulin and Akt signaling have been found previously to be essential for adipogenesis, the relative importance of their many downstream branches have not been defined. One direct substrate that is inhibited by Akt-mediated phosphorylation is the tuberous sclerosis complex 2 (TSC2) protein, which associates with TSC1 and acts as a critical negative regulator of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1). Loss of function of the TSC1-TSC2 complex results in constitutive mTORC1 signaling and, through mTORC1-dependent feedback mechanisms and loss of mTORC2 activity, leads to a concomitant block of Akt signaling to its other downstream targets.

METHODOLOGY/PRINCIPAL FINDINGS

We find that, despite severe insulin resistance and the absence of Akt signaling, TSC2-deficient mouse embryo fibroblasts and 3T3-L1 pre-adipocytes display enhanced adipocyte differentiation that is dependent on the elevated mTORC1 activity in these cells. Activation of mTORC1 causes a robust increase in the mRNA and protein expression of peroxisome proliferator-activated receptor gamma (PPARgamma), which is the master transcriptional regulator of adipocyte differentiation. In examining the requirements for different Akt-mediated phosphorylation sites on TSC2, we find that only TSC2 mutants lacking all five previously identified Akt sites fully block insulin-stimulated mTORC1 signaling in reconstituted Tsc2 null cells, and this mutant also inhibits adipogenesis. Finally, renal angiomyolipomas from patients with tuberous sclerosis complex contain both adipose and smooth muscle-like components with activated mTORC1 signaling and elevated PPARgamma expression.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates that activation of mTORC1 signaling is a critical step in adipocyte differentiation and identifies TSC2 as a primary target of Akt driving this process. Therefore, the TSC1-TSC2 complex regulates the differentiation of mesenchymal cell lineages, at least in part, through its control of mTORC1 activity and PPARgamma expression.

AKT-independent signaling downstream of oncogenic PIK3CA mutations in human cancer

Dysregulation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway occurs frequently in human cancer. PTEN tumor suppressor or PIK3CA oncogene mutations both direct PI3K-dependent tumorigenesis largely through activation of the AKT/PKB kinase. However, here we show through phosphoprotein profiling and functional genomic studies that many PIK3CA mutant cancer cell lines and human breast tumors exhibit only minimal AKT activation and a diminished reliance on AKT for anchorage-independent growth. Instead, these cells retain robust PDK1 activation and membrane localization and exhibit dependency on the PDK1 substrate SGK3. SGK3 undergoes PI3K- and PDK1-dependent activation in PIK3CA mutant cancer cells. Thus, PI3K may promote cancer through both AKT-dependent and AKT-independent mechanisms. Knowledge of differential PI3K/PDK1 signaling could inform rational therapeutics in cancers harboring PIK3CA mutations.

PTEN suppression of YY1 induces HIF-2 activity in von-Hippel-Lindau-null renal-cell carcinoma

Despite recent advances in cancer therapies, metastatic renal cell carcinoma (RCC) remains difficult to treat. Most RCCs result from inactivation of the von Hippel Lindau (VHL) tumor suppressor, leading to stable expression of Hypoxia-Inducible Factor-alpha (HIF-1alpha, -2alpha, -3alpha) and the induction of downstream target genes, including those responsible for angiogenesis and metastasis. While VHL is inactivated in the majority of RCC cases, expression of the PTEN tumor suppressor is reduced in about 30% of cases. PTEN functions to antagonize PI3K/Akt/mTOR signaling, thereby controlling cell growth and survival. Activation of PI3K/Akt/mTOR leads to increased HIF-1alpha expression in certain cancer cells, supporting the rationale of using mTOR inhibitors as anti-cancer agents. Notably, HIF-2alpha, rather than HIF-1alpha, has been shown to play a critical role in renal tumorigenesis. To investigate whether HIF-2alpha is similarly regulated by the PI3K pathway in VHL(-/-)RCC cells, we manipulated PI3K signaling using PTEN overexpression and siRNA knockdown studies and pharmacologic inhibition of PI3K or Akt. Our data support a novel role for wild-type PTEN in promoting HIF-2alpha activity in VHL null RCC cells. This mechanism is unique to the cellular environment in which HIF-2alpha expression is deregulated, resulting from the loss of VHL function. Our data show that PTEN induces HIF-2alpha transcriptional activity by inhibiting expression of Yin Yang 1 (YY1), which acts as a novel corepressor of HIF-2alpha. Further, PTEN suppression of YY1 is mediated through antagonism of PI3K signaling. We conclude that wild-type PTEN relieves the repressive nature of YY1 at certain HIF-2alpha target promoters and that this mechanism may promote early renal tumorigenesis resulting from VHL inactivation by increasing HIF-2alpha activity.

FOXO3a is broadly neuroprotective in vitro and in vivo against insults implicated in motor neuron diseases

Aging is a risk factor for the development of adult-onset neurodegenerative diseases. Although some of the molecular pathways regulating longevity and stress resistance in lower organisms are defined (i.e., those activating the transcriptional regulators DAF-16 and HSF-1 in Caenorhabditis elegans), their relevance to mammals and disease susceptibility are unknown. We studied the signaling controlled by the mammalian homolog of DAF-16, FOXO3a, in model systems of motor neuron disease. Neuron death elicited in vitro by excitotoxic insult or the expression of mutant SOD1, mutant p150(glued), or polyQ-expanded androgen receptor was abrogated by expression of nuclear-targeted FOXO3a. We identify a compound [Psammaplysene A (PA)] that increases nuclear localization of FOXO3a in vitro and in vivo and show that PA also protects against these insults in vitro. Administration of PA to invertebrate model systems of neurodegeneration similarly blocked neuron death in a DAF-16/FOXO3a-dependent manner. These results indicate that activation of the DAF-16/FOXO3a pathway, genetically or pharmacologically, confers protection against the known causes of motor neuron diseases.

Differential response of glioma cells to FOXO1-directed therapy

Gliomas are the most common adult primary brain tumors, and the most malignant form, glioblastoma multiforme, is invariably fatal. The phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is altered in most glioblastoma multiforme. PTEN, an important negative regulator of the PI3K-Akt pathway, is also commonly mutated in glioma, leading to constitutive activation of Akt. One ultimate consequence is phosphorylation and inactivation of FOXO forkhead transcription factors that regulate genes involved in apoptosis, cell cycle arrest, nutrient availability, DNA repair, stress, and angiogenesis. We tested the ability of a mutant FOXO1 factor that is not subject to Akt phosphorylation to overcome dysregulated PI3K-Akt signaling in two PTEN-null glioma cell lines, U87 and U251. Adenovirus-mediated gene transfer of the mutant FOXO1 successfully restored cell cycle arrest and induced cell death in vitro and prolonged survival in vivo in xenograft models of human glioma (33% survival at 1 year of animals bearing U251 tumors). However, U87 were much more resistant than U251 to mutant FOXO1-induced death, showing evidence of increased nuclear export and Akt-independent phosphorylation of FOXO1 at S249. A cyclin-dependent kinase 2 inhibitor decreased phosphorylation of S249 and rendered U87 cells significantly more susceptible to mutant FOXO1-induced death. Our results indicate that targeting FOXO1, which is at the convergence point of several growth factor receptor tyrosine kinase pathways, can effectively induce glioma cell death and inhibit tumor growth. They also highlight the importance of Akt-independent phosphorylation events in the nuclear export of FOXO1.

Indole-3-carbinol prevents PTEN loss in cervical cancer in vivo

Indole-3-carbinol (I3C) is a phytochemical (derived from broccoli, cabbage, and other cruciferous vegetables) with proven anticancer efficacy including the reduction of cervical intraepithelial neoplasia (CIN) and its progression to cervical cancer. In a breast cancer cell line, I3C inhibited cell adhesion, spreading, and invasion associated with an upregulation of the tumor suppressor gene PTEN, suggesting that PTEN is important in inhibition of late stages in the development of cancer. The goal of this study was to determine the expression of PTEN during the development of cervical cancer and whether I3C affected expression of PTEN in vivo. We show diminished PTEN expression during the progression from low-grade to high-grade cervical dysplasia in humans and in a mouse model for cervical cancer, the K14HPV16 transgenic mice promoted with estrogen. The implication is that loss of PTEN function is required for this transition. Additionally, dietary I3C increased PTEN expression in the cervical epithelium of the transgenic mouse, an observation that suggests PTEN upregulation by I3C is one mechanism by which I3C inhibits development of cervical cancer.

Forkhead box O-class 1 and forkhead box G1 as prognostic markers for bladder cancer

Forkhead box O-class 1 (FOXO1) is a key regulator of glucose homeostasis, cell-cycle progression, and apoptosis. Its functions are modulated by forkhead box G1 (FOXG1), which acts as a transcriptional repressor with oncogenic potential. Real-time PCR and immunohistochemical staining were performed in 174 primary bladder cancer specimens and 21 normal bladder mucosae to evaluate these genes. FOXO1 and FOXG1 mRNA expression in cancer tissues were higher than in normal mucosae (each P<0.001). FOXO1 mRNA levels were significantly higher in samples of non-progressed patients (P<0.001), but FOXG1 were enhanced in those of progressed patients (P=0.019). On univariate analysis, FOXO1 mRNA expression was significantly associated with grade, stage, recurrence, progression and survival (each P<0.05). On multivariate analysis, increased FOXO1 mRNA expression was associated with both reduced disease progression (odds ratio [OR], 0.367; 95% confidence interval [CI], 0.163-0.826, P=0.015) and enhanced disease-free survival (OR, 3.262; 95% CI, 1.361-7.820, P=0.008). At a median follow-up of 33 months (range 2 to 156), the patients with a high FOXO1 mRNA expression had a significantly prolonged survival (P=0.001). Immunohistochemical findings of FOXO1 were generally concordant with mRNA expression levels. In conclusion, FOXO1 may be a promising marker for predicting progression in human bladder cancers.

CXCL12 increases human neural progenitor cell proliferation through Akt-1/FOXO3a signaling pathway

CXCL12, a ligand for the chemokine receptor CXCR4, is well known in mediating neural progenitor cell (NPC) migration during neural development. However, the effects of CXCL12 on human NPC proliferation and its associated signaling pathways remain unclear. The transcription factor, FOXO3a, a downstream target of Akt-1, is critical for cell cycle control and may also play an important role in regulating NPC proliferation. In this study, we found that CXCL12 promotes human NPC proliferation as determined by the proliferation marker Ki67 and BrdU incorporation. This CXCL12-mediated NPC proliferation was associated with an increase in Akt-1 and FOXO3a phosphorylation in a time- and dose-dependent manner. The CXCR4 antagonist (T140) or inhibitors for G proteins (Pertussis toxin) and phosphoinositide 3-kinase (PI3K) (LY294002) abolished CXCL12-mediated NPC proliferation and phosphorylation of Akt-1 and FOXO3a. The roles of Akt-1 and FOXO3a in CXCL12-mediated NPC proliferation were further investigated by using adenoviral over-expression in NPCs. Over-expression of dominant-negative Akt-1 or wild-type FOXO3a in NPC abrogated CXCL12-mediated proliferation. These data suggest that CXCL12-mediated NPC proliferation is reliant upon the phosphorylation of Akt-1 and FOXO3a and gives insight to an essential role of CXCL12 in neurogenesis. Understanding this mechanism may facilitate the development of novel therapeutic targets for NPC proliferation during neurogenesis.

Regulation of B-cell proliferation and differentiation by pre-B-cell receptor signalling

The pre-B-cell receptor (pre-BCR) is expressed following the productive recombination of the immunoglobulin heavy chain gene. Signals through the pre-BCR are required for initiating diverse processes in pre-B cells, including proliferation and recombination of the light chain gene, which eventually lead to the differentiation of pre-B cells to immature B cells. However, the molecular mechanisms by which the pre-BCR promotes these processes remain largely unresolved. Recent findings suggest that forkhead box O (FOXO) transcription factors connect pre-BCR signalling to the activation of the recombination machinery. In this Review, we discuss how FOXO transcription factors are regulated by the pre-BCR to allow the progression of the cell cycle and the recombination of the light chain gene.

MDM2 acts downstream of p53 as an E3 ligase to promote FOXO ubiquitination and degradation

Members of the FOXO (forkhead O) class of transcription factors are tumor suppressors that also control aging and organismal life span. Mammalian FOXO degradation is proteasome-mediated, although the ubiquitin E3 ligase for FOXO factors remains to be defined. We show that MDM2 binds to FOXO1 and FOXO3A and promotes their ubiquitination and degradation, a process apparently dependent on FOXO phosphorylation at AKT sites and the E3 ligase activity of MDM2. Binding of MDM2 to FOXO occurs through the region of MDM2 that directs its cellular trafficking and the forkhead box of FOXO1. MDM2 promotes the ubiquitination of FOXO1 in a cell-free system, and its knockdown by small interfering RNA causes accumulation of endogenous FOXO3A protein in cells and enhances the expression of FOXO target genes. In cells stably expressing a temperature-sensitive p53 mutant, activation of p53 by shifting to permissive temperatures leads to MDM2 induction and degradation of endogenous FOXO3A. These data suggest that MDM2 acts as an ubiquitin E3 ligase, downstream of p53, to regulate the degradation of mammalian FOXO factors.

Gene expression profile in colon cancer cells with respect to XIAP expression status

BACKGROUND AND AIMS

We observed a marked synergism between peroxisome proliferator-activated receptor gamma (PPARgamma) ligands and X-linked inhibitor of apoptosis (XIAP) down-regulation in colon cancer. In the current study, we detected the gene expression profile in HCT116 cells treated with or without PPARgamma ligand troglitazone.

MATERIALS AND METHODS

HCT116-XIAP(+/+) and HCT116-XIAP(-/-) cells were treated with or without 50 microM troglitazone for 48 h. Gene expressions were detected by microarray, and selected genes were validated by reverse-transcriptase polymerase chain reaction (PCR), real-time PCR, and Western blot.

RESULTS

Relative to HCT116-XIAP(+/+) cells, 58 genes were up-regulated and 33 genes down-regulated in HCT116-XIAP(-/-) cells, all by > or =4-fold. These genes could be classified into a wide variety of functional classes, but we focused on those related to angiogenesis, apoptosis, and proliferation. Thus, two pro-apoptotic genes and one pro-proliferation gene were up-regulated in HCT116-XIAP(-/-) cells. Two pro-proliferation genes, one pro-angiogenesis gene, one anti-angiogenesis gene, and one anti-apoptosis gene were down-regulated in HCT116-XIAP(-/-) cells. Relative to HCT116-XIAP(+/+) cells treated with troglitazone, 137 genes were up-regulated, and 31 genes were down-regulated in troglitazone-treated HCT116-XIAP(-/-) cells, all by > or =4-fold. Among the up-regulated genes were two anti-angiogenesis genes, seven pro-apoptosis genes, and six anti-proliferation genes. Among the down-regulated genes were one anti-angiogenesis gene, one pro-angiogenesis gene, one anti-apoptosis gene, one anti-proliferation gene, and two pro-proliferation genes.

CONCLUSION

Down-regulation of XIAP in HCT116 cells with or without troglitazone treatment was associated with changes of gene expression that favor increased tendency of apoptosis, decreased cell proliferation, and angiogenesis potential.

Dysregulation of WNT/CTNNB1 and PI3K/AKT signaling in testicular stromal cells causes granulosa cell tumor of the testis

Synergistic effects of dysregulation of the WNT/CTNNB1 and phosphatidylinositol 3-kinase (PI3K)/AKT pathways are thought to be important for the development and progression of many forms of cancer, including the granulosa cell tumor of the ovary. Sustained WNT/CTNNB1 signaling in Sertoli cells causes testicular degeneration and the formation of foci of poorly differentiated stromal cells in the seminiferous tubules in mice. To test if concomitant dysregulation of the WNT/CTNNB1 and PI3K/AKT pathways could synergize to cause testicular cancer, Pten(tm1Hwu/tm1Hwu);Ctnnb1(tm1Mmt/+);Amhr2(tm3(cre)Bhr/+) mice that express a dominant, stable CTNNB1 mutant and lack the expression of phosphatase and tensin homolog (PTEN) in their Sertoli cells were generated. These mice developed aggressive testicular cancer with 100% penetrance by 5 weeks of age, and 44% of animals developed pulmonary metastases by 4 months, whereas Pten(tm1Hwu/tm1Hwu);Amhr2(tm3(cre)Bhr/+) controls were phenotypically normal. Surprisingly, the tumors could not be classified as Sertoli cell tumors, but rather bore histologic and ultrastructural characteristics of granulosa cell tumors of the testis (GCTT). Pten(tm1Hwu/tm1Hwu);Ctnnb1(tm1Mmt/+);Amhr2(tm3(cre)Bhr/+) testicular tumors did not express CYP17, CYP19, germ cell nuclear antigen, estrogen receptor 1 or progesterone receptor, but expressed the early granulosa cell markers WNT4 and FOXL2, confirming the diagnosis of GCTT. Immunohistochemical analyses of Pten(tm1Hwu/tm1Hwu);Ctnnb1(tm1Mmt/+);Amhr2(tm3(cre)Bhr/+) GCTT demonstrated a tumor marker profile similar to that reported in human GCTT. Immunoblotting analyses revealed high levels of phosphorylation of AKT and the PI3K/AKT signaling effector FOXO1A in Pten(tm1Hwu/tm1Hwu);Ctnnb1(tm1Mmt/+);Amhr2(tm3(cre)Bhr/+) GCTT, suggesting the involvement of FOXO1A in the mechanism of GCTT development. Together, these data provide the first insights into the molecular etiology of GCTT and the first animal model for the study of GCTT biology.

Chemical genetic analysis of FOXO nuclear-cytoplasmic shuttling by using image-based cell screening

FOXO proteins are direct targets of PI3K/Akt signaling and they integrate the signals of several other transduction pathways at the transcriptional level. FOXO transcription factors are involved in normal cell homeostasis and neoplasia, and they are regulated by multiple post-transcriptional modifications. In cancer research, the regulation of the FOXO factors is receiving increasing attention as their activation has been linked to cell-cycle arrest and apoptosis. Hence, FOXO proteins have been proposed to act as tumor suppressors. Here, we applied a chemical biology approach to study the mechanisms that influence the intracellular localization of the FOXO family member FOXO3a. We established a high-throughput cellular-imaging assay that monitors the nuclear-cytoplasmic translocation of a GFP-FOXO3a fusion protein in tumor cells. Nuclear accumulation of fluorescent signals upon treatment with the known PI3K inhibitors LY294002, wortmannin, PIK-75, and PI-103 was dose dependent and agreed well with the IC(50) values reported for PI3Kalpha inhibition in vitro. Additionally, we identified 17 compounds from a panel of 73 low-molecular-weight compounds capable of inducing the nuclear accumulation of GFP-FOXO. These compounds include chemicals known to interfere with components of the PI3K/Akt signaling pathway, as well as with nuclear export and Ca(2+)/calmodulin (CaM)-dependent signaling events. Interestingly, the therapeutic agent vinblastine induced efficient nuclear translocation of the FOXO reporter protein. Our data illustrate the potential of chemical genetics when combined with robust and sensitive high-content-screening technology.

Identification of nuclear export inhibitors with potent anticancer activity in vivo

The export protein CRM1 is required for the nuclear export of a wide variety of cancer-related "cargo" proteins including p53, c-Abl, and FOXO-3A. Leptomycin B (LMB) is a highly specific inhibitor of CRM1 with significant in vitro potency but limited in vivo efficacy due to toxicity. We now report a series of semisynthetic LMB derivatives showing substantially improved therapeutic windows. Exposure of cancer cells to these compounds leads to a rapid and prolonged block of nuclear export and apoptosis. In contrast to what is observed in cancer cells, these agents induce cell cycle arrest, but not apoptosis, in normal lung fibroblasts. These new nuclear export inhibitors (NEI) maintain the high potency of LMB, are up to 16-fold better tolerated than LMB in vivo, and show significant efficacy in multiple mouse xenograft models. These NEIs show the potential of CRM1 inhibitors as novel and potent anticancer agents.

A transcription-independent function of FOXO1 in inhibition of androgen-independent activation of the androgen receptor in prostate cancer cells

Increasing evidence suggests that aberrant activation of the androgen receptor (AR) plays a pivotal role in the development and progression of androgen depletion-independent prostate cancer (PCa) after androgen deprivation therapy. Here, we show that loss of the PTEN tumor suppressor gene is associated with hyperactivation of the AR in human PCa cell lines. This effect is mediated primarily by its downstream effector FOXO1. In addition to the inhibition of androgenic activation of the AR, forced expression of FOXO1 in PTEN-negative PCa cells also inhibits androgen-independent activation of the AR in a manner independent of FOXO1 transcriptional function. In contrast, silencing of FOXO1 in PTEN-positive cells not only increases the basal activity of the AR in the absence of androgens, it also markedly sensitizes the AR activation by low levels of androgens or nonandrogenic factors such as interleukin-6. FOXO1-mediated inhibition of the AR is partially attenuated by the histone deacetylase (HDAC) inhibitor trichostatin A. Accordingly, FOXO1 interacts with HDAC3 as shown by coimmunoprecipitation assays, and cotransfection of cells with FOXO1 and HDAC3, but not HDAC1 and HDAC2, results in a greater inhibition of AR activity than in cells transfected with FOXO1 or HDAC3 individually. Together, our findings define a novel corepressor function of FOXO1 in inhibition of androgen-independent activation of the AR.

Forkhead box, class O transcription factors in brain: regulation and behavioral manifestation

BACKGROUND

The mammalian forkhead box, class O (FoxO) transcription factors function to regulate diverse physiological processes. Emerging evidence that both brain-derived neurotrophic factor (BDNF) and lithium suppress FoxO activity suggests a potential role of FoxOs in regulating mood-relevant behavior. Here, we investigated whether brain FoxO1 and FoxO3a can be regulated by serotonin and antidepressant treatment and whether their genetic deletion affects behaviors.

METHODS

C57BL/6 mice were treated with D-fenfluramine to increase brain serotonergic activity or with the antidepressant imipramine. The functional status of brain FoxO1 and FoxO3a was audited by immunoblot analysis for phosphorylation and subcellular localization. The behavioral manifestations in FoxO1- and FoxO3a-deficient mice were assessed via the Elevated Plus Maze Test, Forced Swim Test, Tail Suspension Test, and Open Field Test.

RESULTS

Increasing serotonergic activity by d-fenfluramine strongly increased phosphorylation of FoxO1 and FoxO3a in several brain regions and reduced nuclear FoxO1 and FoxO3a. The effect of D-fenfluramine was mediated by the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway. Chronic, but not acute, treatment with the antidepressant imipramine also increased the phosphorylation of brain FoxO1 and FoxO3a. When FoxO1 was selectively deleted from brain, mice displayed reduced anxiety. In contrast, FoxO3a-deficient mice presented with a significant antidepressant-like behavior.

CONCLUSIONS

FoxOs may be a transcriptional target for anxiety and mood disorder treatment. Despite their physical and functional relatedness, FoxO1 and FoxO3a influence distinct behavioral processes linked to anxiety and depression. Findings in this study reveal important new roles of FoxOs in brain and provide a molecular framework for further investigation of how FoxOs may govern mood and anxiety disorders.

Irs2 inactivation suppresses tumor progression in Pten+/- mice

Mutations in the phosphatase and tensin homologue (PTEN)/phosphatidylinositol-3 kinase-alpha (PI3K) signaling pathway are frequently found in human cancer. In addition, Pten(+/-) mice develop tumors in multiple organs because of the activation of the PI3K signaling cascade. Because activation of PI3K signaling leads to feedback inhibition of insulin receptor substrate-2 (IRS2) expression, an upstream activator of PI3K, we therefore anticipated that IRS2 expression would be low in tumors that lack PTEN. Surprisingly, however, an elevation of IRS2 was often detected in tumor samples in which PTEN levels were compromised. To determine the potential contribution of Irs2 to tumor progression, Pten(+/-) mice were crossed with Irs2(+/-) mice. Deletion of Irs2 did not affect the initiation of neoplasia found in Pten(+/-) mice but suppressed cancer cell growth, proliferation, and invasion through the basement membrane. Deletion of Irs2 also attenuated the expression of Myc in prostatic intraepithelial neoplasia in Pten(+/-) mice. In addition, the expression levels of IRS2 and MYC were highly correlated in human prostate cancer, and IRS2 could stimulate MYC expression in cultured cells. Our findings provide evidence that the PI3K-activating adaptor Irs2 contributes to tumor progression in Pten(+/-) mice by stimulating both Myc and DNA synthesis.

Sulforaphane enhances the therapeutic potential of TRAIL in prostate cancer orthotopic model through regulation of apoptosis, metastasis, and angiogenesis

PURPOSE

The purpose of this study was to examine the molecular mechanisms by which sulforaphane enhances the therapeutic potential of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in prostate cancer.

EXPERIMENTAL DESIGN

Cell viability and apoptosis assays were done by XTT and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay, respectively. Tumor-bearing mice were treated with vehicle, sulforaphane, TRAIL, and sulforaphane plus TRAIL. Markers of apoptosis, angiogenesis, and metastasis were measured by immunohistochemistry.

RESULTS

Sulforaphane enhanced the therapeutic potential of TRAIL in PC-3 cells and sensitized TRAIL-resistant LNCaP cells. Sulforaphane-induced apoptosis in PC-3 cells correlated with the generation of intracellular reactive oxygen species (ROS), collapse of mitochondrial membrane potential, activation of caspase-3 and caspase-9, and up-regulation of DR4 and DR5. Sulforaphane induced the expression of Bax, Bak, Bim, and Noxa and inhibited the expression of Bcl-2, Bcl-X(L), and Mcl-1. The quenching of ROS generation with antioxidant N-acetyl-L-cysteine conferred significant protection against sulforaphane-induced ROS generation, mitochondrial membrane potential disruption, caspase-3 activation, and apoptosis. Sulforaphane inhibited growth of orthotopically implanted PC-3 tumors by inducing apoptosis and inhibiting proliferation and also enhanced the antitumor activity of TRAIL. Sulforaphane up-regulated the expressions of TRAIL-R1/DR4, TRAIL-R2/DR5, Bax and Bak and inhibited the activation of nuclear factor-kappaB P13K/AKT and MEK/ERK pathways in tumor tissues. The combination of sulforaphane and TRAIL was more effective in inhibiting markers of angiogenesis and metastasis and activating FOXO3a transcription factor than single agent alone.

CONCLUSIONS

The ability of sulforaphane to inhibit tumor growth, metastasis, and angiogenesis and to enhance the therapeutic potential of TRAIL suggests that sulforaphane alone or in combination with TRAIL can be used for the management of prostate cancer.

Induction of apoptosis by withaferin A in human leukemia U937 cells through down-regulation of Akt phosphorylation

Withaferin A, a major chemical constituent of Withania somnifera, has been reported for its tumor cell growth inhibitory activity, antitumor effects, and impairing metastasis and angiogenesis. The mechanism by which withaferin A initiates apoptosis remains poorly understood. In the present report, we investigated the effect of withaferin A on the apoptotic pathway in U937 human promonocytic cells. We show that withaferin A induces apoptosis in association with the activation of caspase-3. JNK and Akt signal pathways play crucial roles in withaferin A-induced apoptosis in U937 cells. Furthermore, we have shown that overexpression of Bcl-2 and active Akt (myr-Akt) in U937 cells inhibited the induction of apoptosis, activation of caspase-3, and PLC-gamma1 cleavage by withaferin A. Taken together, our results indicated that the JNK and Akt pathways and inhibition of NF-kappaB activity were key regulators of apoptosis in response to withaferin A in human leukemia U937 cells.

Tat-induced FOXO3a is a key mediator of apoptosis in HIV-1-infected human CD4+ T lymphocytes

The high mutation rate of HIV is linked to the generation of viruses expressing proteins with altered function whose impact on disease progression is unknown. We investigated how HIV-1 viruses lacking Env, Vpr, and Nef affect CD4(+) T cell survival. We found that in the absence of these proteins, HIV-1-infected CD4(+) primary T cells progress to the G(0) phase of the cell cycle and to cell death, indicating that viruses expressing inactive forms of these proteins can contribute to the CD4(+) T cell decline as the wild-type virus, suggesting that other HIV proteins are responsible for inducing apoptosis. Apoptosis in these cells is triggered by the alteration of the Egr1-PTEN-Akt (early growth response-1/phosphate and tensin homolog deleted on chromosome 10/Akt) and p53 pathways, which converge on the FOXO3a (Forkhead box transcription factor O class 3a) transcriptional activator. The FOXO3a target genes Fas ligand and TRAIL, involved in the extrinsic apoptotic pathway, and PUMA, Noxa, and Bim, which are part of the intrinsic apoptotic pathway, were also up-regulated, indicating that HIV infection leads to apoptosis by the engagement of multiple apoptotic pathways. RNAi-mediated knockdown of Egr1 and FOXO3a resulted in reduced apoptosis in HIV-infected HeLa and CD4(+) T cells, providing further evidence for their critical role in HIV-induced apoptosis and G(0) arrest. We tested the possibility that Tat is responsible for the T cell apoptosis observed with these mutant viruses. The induction of Egr1 and FOXO3a and its target genes was observed in Jurkat cells transduced by Tat alone. Tat-dependent activation of the Egr1-PTEN-FOXO3a pathway provides a mechanism for HIV-1-associated CD4(+) T cell death.

Monitoring FoxO1 localization in chemically identified neurons

The PI3K-Akt-FoxO1 pathway contributes to the actions of insulin and leptin in several cell types, including neurons in the CNS. However, identifying these actions in chemically identified neurons has proven difficult. To address this problem, we have developed a reporter mouse for monitoring PI3K-Akt signaling in specific populations of neurons, based on FoxO1 nucleocytoplasmic shuttling. The reporter, FoxO1 fused to green fluorescent protein (FoxO1GFP), is expressed under the control of a ubiquitous promoter that is silenced by a loxP flanked transcriptional blocker. Thus, the expression of the reporter in selected cells is dependent on the action of Cre recombinase. Using this model, we found that insulin treatment resulted in the nuclear exclusion of FoxO1GFP within POMC and AgRP neurons in a dose- and time-dependent manner. FoxO1GFP nuclear exclusion was also observed in POMC neurons following in vivo administration of insulin. In addition, leptin induced transient nuclear export of FoxO1GFP in POMC neurons in a dose dependent manner. Finally, insulin-induced nuclear export was impaired in POMC neurons by pretreatment with free fatty acids, a paradigm known to induce insulin resistance in peripheral insulin target tissues. Thus, our FoxO1GFP mouse provides a tool for monitoring the status of PI3K-Akt signaling in a cell-specific manner under physiological and pathophysiological conditions.

Calpain small-1 modulates Akt/FoxO3A signaling and apoptosis through PP2A

Here, we show that FoxO3A transcription factor is upregulated upon calpain small-1 (CAPNS1) depletion both in mouse embryonic fibroblasts (MEFs) and in the human mammary carcinoma cell line MCF-7. On starvation, CAPNS1 depletion is associated with a higher rate of FoxO3A dephosphorylation and translocation to the nucleus and to a sharper increase in the levels of p27Kip1 and Bim, the products of two FoxO target genes. Notably, FoxO3A depletion in CAPNS1-/- MEFs reduces both the induction of Bim and apoptosis. Both okadaic acid treatment and silencing of the protein phosphatase 2A (PP2A) catalytic subunit can partially reduce starvation-induced FoxO3A activation and apoptosis in CAPNS1-/- fibroblasts. PP2A associates more tightly with Akt in CAPNS1 knockout cells, indicating that PP2A is involved in calpain-mediated FoxO regulation. Finally, we show that PP2A regulatory subunits B56 alpha and gamma are in vitro substrates of calpain, and calpain regulates B56 alpha stability in vivo, suggesting a direct role of calpain in the regulation of PP2A function. In conclusion, for the first time we report that CAPNS1 interferes with PP2A-Akt interaction consequently affecting FoxO3A-dependent cell death. Calpain inhibition might therefore be exploited as a tool to induce apoptosis in tumors sensitive to FoxO activation.

The role of PTEN signaling perturbations in cancer and in targeted therapy

The PTEN tumor suppressor was discovered by its homozygous deletion and other mutations in cancer. Since then, PTEN has been shown to be a non-redundant, evolutionarily conserved phosphatase whose function affects diverse cellular progresses such as cell cycle progression, cell proliferation, chemotaxis, apoptosis, aging, muscle contractility, DNA damage response, angiogenesis and cell polarity. In accordance with its ability to influence multiple crucial cellular processes, PTEN has a major role in the pathogenesis of numerous diseases such as diabetes, autism and almost every cancer examined. This review will discuss the diverse ways in which PTEN signaling is modified in cancer, and how these changes correlate with and might possibly affect the action of targeted chemotherapy.

Significance of nuclear accumulation of Foxo3a in esophageal squamous cell carcinoma

PURPOSE

To investigate the value of Foxo3a in predicting the response to neoadjuvant treatment of, and prognosis for, esophageal squamous cell carcinoma.

METHODS AND MATERIALS

Immunohistochemical staining was performed in a retrospective series of 60 biopsied esophageal squamous cell carcinomas, and the correlation between nuclear accumulation of Foxo3a and clinicopathologic features was analyzed, including patient survival. In addition, in vitro biologic changes, radiosensitivity, and in vivo tumorigenicity of esophageal carcinoma cells after experimental manipulation of Foxo3a expression levels were determined.

RESULTS

Clinical findings point to a significant correlation between the nuclear accumulation of Foxo3a and the survival rate of esophageal cancer patients. In addition, Foxo3a is a significant predictor for the response to neoadjuvant therapy. In cell culture, irradiation and oxidative stress seemed to result in nuclear accumulation of Foxo3a. Down-regulation of Foxo3a significantly decreased radiosensitivity but had no obvious effect on tumor growth, as measured by a clonogenic assay in vitro and growth delay in vivo.

CONCLUSIONS

Nuclear accumulation of Foxo3a in tumor cells was correlated with increased radiosensitivity and with improved patient survival. Thus, it is suggested that Foxo3a may be a potential marker for esophageal cancer.

Tumor suppressor Foxo3a is involved in the regulation of lipopolysaccharide-induced interleukin-8 in intestinal HT-29 cells

Enteric bacteria and their products play an important role in intestinal inflammation; however, the complete mechanisms are not elucidated yet. Tumor suppressor Foxo3a regulates gene expression in the nucleus, and its translocation to the cytosol leads to inactivation. Proximally, Foxo3a is regulated by different pathways including the phosphoinositide 3-kinase (PI3K) pathway. The aim of this study was to determine the effect of bacterial infection on Foxo3a in intestinal epithelial cells and to examine the contribution of Foxo3a in intestinal inflammation. Bacterial lipopolysaccharide (LPS) and infection with mouse pathogen Citrobacter rodentium induce translocation of the nuclear Foxo3a into the cytosol, where it degrades in human HT-29 and mouse CMT-93 cells. In colonic epithelia of healthy mice, Foxo3a is localized in the epithelia at the bottom of the crypts in both the nucleus and the cytosol, while in C. rodentium-infected colon Foxo3a is expressed along the crypts and located mainly in the cytosol, suggesting its inactivation. LPS utilized the PI3K pathway to inhibit Foxo3a. Additionally, inhibition of PI3K attenuated LPS-induced proinflammatory interleukin-8 (IL-8). LPS-induced IL-8 is increased in HT-29 cells with silenced Foxo3a. Moreover, in HT-29 cells with silenced Foxo3a, the amount of IkappaBalpha, an NF-kappaB inhibitor, is decreased. In conclusion, LPS and bacterial infection inactivate Foxo3a in intestinal epithelia via the PI3K pathway and inactivated Foxo3a leads to the upregulation of IL-8 by suppressing inhibitory IkappaBalpha.

Antineoplastic effect of rapamycin is potentiated by inhibition of IRS-1 signaling in prostate cancer cells xenografts

Proper activation of phosphoinositide 3-kinase-Akt pathway is critical for the prevention of tumorigenesis. Recent data have characterized a negative feedback loop, wherein mammalian target of rapamycin (mTOR) blocks additional activation of the Akt/mTOR pathway through inhibition insulin receptor substrate 1 (IRS-1) function. However, the potential of IRS-1 inhibition during rapamycin treatment has not been examined. Herein, we show that IRS-1 antisense oligonucleotide and rapamycin synergistically antagonize the activation of mTOR in vivo and induced tumor suppression, through inhibition of proliferation and induction of apoptosis, in prostate cancer cell xenografts. These data demonstrate that the addition of agents that blocks IRS-1 potentiate the effect of mTOR inhibition in the growth of prostate cancer cell xenografts.

Hormone-dependent nuclear export of estradiol receptor and DNA synthesis in breast cancer cells

In breast cancer cells, cytoplasmic localization of the estradiol receptor alpha (ERalpha) regulates estradiol-dependent S phase entry. We identified a nuclear export sequence (NES) in ERalpha and show that its export is dependent on both estradiol-mediated phosphatidylinositol-3-kinase (PI3K)/AKT activation and chromosome region maintenance 1 (CRM1). A Tat peptide containing the ERalpha NES disrupts ERalpha-CRM1 interaction and prevents nuclear export of ERalpha- and estradiol-induced DNA synthesis. NES-ERalpha mutants do not exit the nucleus and inhibit estradiol-induced S phase entry; ERalpha-dependent transcription is normal. ERalpha is associated with Forkhead proteins in the nucleus, and estradiol stimulates nuclear exit of both proteins. ERalpha knockdown or ERalpha NES mutations prevent ERalpha and Forkhead nuclear export. A mutant of forkhead in rhabdomyosarcoma (FKHR), which cannot be phosphorylated by estradiol-activated AKT, does not associate with ERalpha and is trapped in the nucleus, blocking S phase entry. In conclusion, estradiol-induced AKT-dependent phosphorylation of FKHR drives its association with ERalpha, thereby triggering complex export from the nucleus necessary for initiation of DNA synthesis and S phase entry.

Splice variants of the forkhead box protein AFX exhibit dominant negative activity and inhibit AFXalpha-mediated tumor cell apoptosis

BACKGROUND

Loss-of-function in the apoptosis-inducing genes is known to facilitate tumorigenesis. AFX (FOXO4), a member of forkhead transcription factors functions as a tumor suppressor and has 2 isoforms, AFXalpha (505 a.a.) and AFXzeta (450 a.a.). In human cancer cells, we identified an N-terminally deleted form of AFXalpha (alpha198-505), translated from a downstream start and 2 short N-terminal AFX proteins (90, and 101 a.a.) produced by aberrant splicing.

METHODS AND FINDINGS

We investigated the expression and role of these AFX variants. Cell transduction study revealed that short N-terminal AFX proteins were not stable. Though alpha(198-505) protein expression was detected in the cytoplasm and nucleus, alpha(198-505) expressing cells did not show a nucleocytoplasmic shuttling mediated by PI3 kinase signaling. Whereas, we observed this shuttling in cells expressing either AFXalpha or AFXzeta protein. AFXzeta and alpha(198-505) lost the ability to transactivate BCL6 or suppress cyclin D2 gene expression. These variants did not induce cancer cell death whereas AFXalpha resulted in apoptosis. We found that AFXzeta and alpha(198-505) suppress the AFXalpha stimulation of BCL6 promoter in a dose dependent manner, indicating dominant negative activity. These variants also inhibited AFXalpha induction of apoptosis.

CONCLUSIONS

Loss of function by aberrant splicing and the dominant negative activity of AFX variants may provide a mechanism for enhanced survival of neoplastic cells.

Dysregulation of apoptotic signaling in cancer: molecular mechanisms and therapeutic opportunities

Apoptosis is a tightly regulated cell suicide program that plays an essential role in the maintenance of tissue homeostasis by eliminating unnecessary or harmful cells. Defects in this native defense mechanism promote malignant transformation and frequently confer chemoresistance to transformed cells. Indeed, the evasion of apoptosis has been recognized as a hallmark of cancer. Given that multiple mechanisms function at many levels to orchestrate the regulation of apoptosis, a multitude of opportunities for apoptotic dysregulation are present within the intricate signaling network of cell. Several of the molecular mechanisms by which cancer cells are protected from apoptosis have been elucidated. These advances have facilitated the development of novel apoptosis-inducing agents that have demonstrated single-agent activity against various types of cancers cells and/or sensitized resistant cancer cells to conventional cytotoxic therapies. Herein, we will highlight several of the central modes of apoptotic dysregulation found in cancer. We will also discuss several therapeutic strategies that aim to reestablish the apoptotic response, and thereby eradicate cancer cells, including those that demonstrate resistance to traditional therapies.

Targeting prostate cancer based on signal transduction and cell cycle pathways

Prostate cancer remains a leading cause of death in men despite increased capacity to diagnose at earlier stages. After prostate cancer has become hormone independent, which often occurs after hormonal ablation therapies, it is difficult to effectively treat. Prostate cancer may arise from mutations and dysregulation of various genes involved in regulation signal transduction (e.g., PTEN, Akt, etc.,) and the cell cycle (e.g., p53, p21(Cip1), p27(Kip1), Rb, etc.,). This review focuses on the aberrant interactions of signal transduction and cell cycle genes products and how they can contribute to prostate cancer and alter therapeutic effectiveness.

Constitutive ERK MAPK activity regulates macrophage ATP production and mitochondrial integrity

A unique feature of human alveolar macrophages is their prolonged survival in the face of a stressful environment. We have shown previously that the ERK MAPK is constitutively active in these cells and is important in prolonging cell survival. This study examines the role of the ERK pathway in maintaining mitochondrial energy production. The data demonstrate that ATP levels in alveolar macrophages depend on intact mitochondria and optimal functioning of the electron transport chain. Significant levels of MEK and ERK localize to the mitochondria and inhibition of ERK activity induces an early and profound depletion in cellular ATP coincident with a loss of mitochondrial transmembrane potential. The effect of ERK suppression on ATP levels was specific, since it did not occur with PI3K/Akt, p38, or JNK suppression. ERK inhibition led to cytosolic release of mitochondrial proteins and caspase activation. Both ERK inhibition and mitochondrial blockers induced loss of plasma membrane permeability and cell death. The cell death induced by ERK inhibition had hallmarks of both apoptotic (caspase activation) and necrotic (ATP loss) cell death. By blocking ERK inhibition-induced reactive oxygen species, caspase activation was prevented, although necrotic pathways continued to induce cell death. This suggests that mitochondrial dysfunction caused by ERK inhibition generates both apoptotic and necrotic cell death-inducing pathways. As a composite, these data demonstrate a novel mitochondrial role for ERK in maintaining mitochondrial membrane potential and ATP production in human alveolar macrophages.

CDK1 promotes cell proliferation and survival via phosphorylation and inhibition of FOXO1 transcription factor

The forkhead box O (FOXO) transcription factor FOXO1 functions as a tumor suppressor by regulating expression of genes involved in apoptosis, cell cycle arrest and oxidative detoxification. Here, we demonstrate that cyclin-dependent kinase 1 (CDK1) specifically phosphorylates FOXO1 at serine 249 (S249) in vitro and in vivo. Coimmunoprecipitation assays demonstrate that both endogenous CDK1 and ectopically expressed CDK1 form a protein complex with FOXO1 in prostate cancer (PCa) cells. In vitro protein binding assays reveal that CDK1 interacts directly with FOXO1. Accordingly, overexpression of CDK1 inhibits the transcriptional activity of FOXO1 in PCa cells through S249 phosphorylation on FOXO1. Consistent with the roles of FOXO3a and FOXO4 (two other members of the FOXO family) in cell cycle regulation, forced expression of FOXO1 causes a delay in the transition from G2 to M phase. This effect is blocked completely by overexpression of CDK1 and cyclin B1. Ectopic expression of constitutively active CDK1 also inhibits FOXO1-induced apoptosis in PCa cells. Moreover, we demonstrate that the inhibitory effect of FOXO1 on Ras oncogene-induced colony formation in fibroblasts is diminished by overexpression of CDK1. Given that CDK1 and cyclin B1 are often overexpressed in human cancers including PCa, our findings suggest that aberrant activation of CDK1 may contribute to tumorigenesis by promoting cell proliferation and survival via phosphorylation and inhibition of FOXO1.

Many forks in the path: cycling with FoxO

FoxO transcription factors are an evolutionary conserved subfamily of the forkhead transcription factors, characterized by the forkhead DNA-binding domain. FoxO factors regulate a number of cellular processes involved in cell-fate decisions in a cell-type- and environment-specific manner, including metabolism, differentiation, apoptosis and proliferation. A key mechanism by which FoxO determines cell fate is through regulation of the cell cycle machinery, and as such the cellular consequence of FoxO deregulation is often manifested through perturbation of the cell cycle. Consequently, the deregulation of FoxO factors is implicated in the development of numerous proliferative diseases, in particular cancer.

FOXOs, cancer and regulation of apoptosis

Forkhead box O (FOXO) transcription factors are involved in multiple signaling pathways and play critical roles in a number of physiological and pathological processes including cancer. The importance of FOXO factors ascribes them under multiple levels of regulation including phosphorylation, acetylation/deacetylation, ubiquitination and protein-protein interactions. As FOXO factors play a pivotal role in cell fate decision, mounting evidence suggests that FOXO factors function as tumor suppressors in a variety of cancers. FOXOs are actively involved in promoting apoptosis in a mitochondria-independent and -dependent manner by inducing the expression of death receptor ligands, including Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand, and Bcl-2 family members, such as Bim, bNIP3 and Bcl-X(L), respectively. An understanding of FOXO proteins and their biology will provide new opportunities for developing more effective therapeutic approaches to treat cancer.

Endothelin-1 couples betaPix to p66Shc: role of betaPix in cell proliferation through FOXO3a phosphorylation and p27kip1 down-regulation independently of Akt

The phosphorylation of forkhead transcription factor FOXO3a by Akt is critical regulator of cell proliferation induced by serum. We show that endothelin-1 (ET-1) stimulation of primary human mesangial cells (HMCs) induces betaPix and p66Shc up-regulation, resulting in the formation of the betaPix/p66Shc complex. In transformed HMCs, ET-1 induces a biphasic phosphorylation of p66Shc and FOXO3a. The second phase leads to p27(kip1) down-regulation independently of Akt. Depletion of betaPix blocks the second phase of p66Shc and FOXO3a phosphorylation and prevents p27(kip1) down-regulation induced by ET-1. Depletion of either betaPix or p66Shc inhibits ET-1-induced cell proliferation. The expression of beta(1)Pix induces FOXO3a phosphorylation through activation of Rac1, ERK1/2, and p66Shc. Using either p66Shc- or Akt-depleted cells; we show that beta(1)Pix-induced FOXO3a phosphorylation requires p66Shc but not Akt. beta(1)Pix-induced p27(kip1) down-regulation was blocked by U0126 but not by wortmannin. Endogenous betaPix and FOXO3a are constitutively associated with endogenous p66Shc. FOXO3a and p66Shc binding requires beta(1)Pix homodimerization. Expression of beta(1)Pix homodimerization deficient mutant abrogates beta(1)Pix-induced p27(kip1) down-regulation and cell proliferation. Our results identify p66Shc and FOXO3a as novel partners of beta(1)Pix and represent the first direct evidence of beta(1)Pix in cell proliferation via Erk/p66Shc-dependent and Akt-independent mechanisms.

The regulation and function of the forkhead transcription factor, Forkhead box O1, is dependent on the progesterone receptor in endometrial carcinoma

In many type I endometrial cancers, the PTEN gene is inactivated, which ultimately leads to constitutively active Akt and the inhibition of Forkhead box O1 (FOXO1), a member of the FOXO subfamily of Forkhead/winged helix family of transcription factors. The expression, regulation, and function of FOXO1 in endometrial cancer were investigated in this study. Immunohistochemical analysis of 49 endometrial tumor tissues revealed a decrease of FOXO1 expression in 95.9% of the cases compared with the expression in normal endometrium. In four different endometrial cancer cell lines (ECC1, Hec1B, Ishikawa, and RL95), FOXO1 mRNA was expressed at similar levels; however, protein levels were low or undetectable in Ecc1, Ishikawa, and RL95 cells. Using small interfering RNA technology, we demonstrated that the low levels of FOXO1 protein were due to the involvement of Skp2, an oncogenic subunit of the Skp1/Cul1/F-box protein ubiquitin complex, given that silencing Skp2 increased FOXO1 protein expression in Ishikawa cells. Inhibition of Akt in Ishikawa cells also increased nuclear FOXO1 protein levels. Additionally, progestins increased FOXO1 protein levels, specifically through progesterone receptor B (PRB) as determined by using stably transfected PRA-specific and PRB-specific Ishikawa cell lines. Finally, overexpression of triple mutant (Tm) FOXO1 in the PR-specific Ishikawa cell lines caused cell cycle arrest and significantly decreased proliferation in the presence and absence of the progestin, R5020. Furthermore, TmFOXO1 overexpression induced apoptosis in PRB-specific cells in the presence and absence of ligand. Taken together, these data provide insight into the phosphoinositide-3-kinase/Akt/FOXO pathway for the determination of progestin responsiveness and the development of alternate therapies for endometrial cancer.

Inhibition of PI3K/AKT and MEK/ERK pathways act synergistically to enhance antiangiogenic effects of EGCG through activation of FOXO transcription factor

BACKGROUND

We have recently shown that epigallocatechin-3-gallate (EGCG), a polyphenolic compound from green tea, inhibits angiogenesis. However, the molecular mechanisms by which EGCG inhibits angiogenesis have never been investigated. In this study, we examined the interaction of PI3K/AKT and MEK/ERK pathways on the regulation of FOXO transcription factors, which ultimately control the antiangiogenic effects of EGCG.

RESULTS

Inhibition of PI3K/AKT and MEK/ERK pathways interact synergistically to inhibit migration and capillary tube formation of HUVEC cells and further enhanced the antiangiogenic effects of EGCG. Inhibition of AKT and MEK kinases synergistically induced FOXO transcriptional activity, which was further enhanced in the presence of EGCG. Phosphorylation deficient mutants of FOXO induced FOXO transcriptional activity, inhibited HUVEC cell migration and capillary tube formation. Inhibition of FOXO phosphorylation also enhanced antiangiogenic effects of EGCG through transcriptional activation of FOXO.

CONCLUSION

Inhibition of PI3K/AKT and MEK/ERK pathways act synergistically to regulate antiangiogenic effects of EGCG through activation of FOXO transcription factors. The activation of FOXO transcription factors through inhibition of these two pathways may have physiological significance in management of diabetic retinopathy, rheumatoid arthritis, psoriasis, cardiovascular diseases, and cancer.

Expression of Foxo3a in non-Hodgkin's lymphomas is correlated with cell cycle inhibitor p27

OBJECTIVE

Cell cycle arrest by FOXO transcription factors involves in transcriptional activation of p27(kip1), although the exact mechanism remains unclear. And it has been evidenced that reduced level of p27(kip1) which is frequently occurred in human cancers has been associated with poor prognosis. In this study, our purpose is to investigate the clinical relevance of altered patterns of Foxo3a and p27(kip1) expression in Chinese patients with localized non-Hodgkin's lymphomas (NHL).

METHODS

We analyzed the Foxo3a and p27(kip1) expression of Chinese NHL patients by immunohistochemistry and protein levels using Western Blot.

RESULTS

There was a direct relationship between the low level of Foxo3a and the rapid proliferation in immunohistochemical analyses. We also found a positive correlation between Foxo3a and p27(kip1) in immunohistochemical analyses and cell culture. Additionally we revealed that activation of Foxo3a could induce the accumulation of p27(kip1) at protein levels when cell cycle was arrested.

CONCLUSIONS

The expression of Foxo3a may be correlated with patients' survival.

PTEN regulates p300-dependent hypoxia-inducible factor 1 transcriptional activity through Forkhead transcription factor 3a (FOXO3a)

The tumor suppressor PTEN is mutated or deleted in many tumors, causing the activation of the PI3K pathway. Here, we show that the loss of PTEN increases the transcriptional activity of hypoxia-inducible factor 1 (HIF-1) through the inactivation of Forkhead transcription factors (FOXO) in PTEN-null cells. Reintroduction of PTEN into the nucleus, overexpression of a nonphosphorylatable FOXO3a, which accumulates in the nucleus, or inhibition of nuclear export of FOXO3a by leptomycin B represses HIF-1 transcriptional activity in PTEN-null cells. HIF-1 transcriptional activity increases in PTEN-positive cells depleted of FOXO3a with siRNA. PTEN and FOXO3a regulate the transactivation domain of HIF-1alpha. Chromatin immunoprecipitation indicates that FOXO3a complexes with HIF-1alpha and p300 on the Glut-1 promoter, a HIF-1 target gene. Overexpression of p300 reverses FOXO3a-mediated repression of HIF-1 transcriptional activity. Coimmunoprecipitation and GAL4-HIF-1alpha transactivation assays reveal that FOXO3a interferes with p300-dependent HIF-1 transcriptional activity. Thus, FOXO3a negatively regulates HIF-1 transcriptional activity.

Normal development is an integral part of tumorigenesis in T cell-specific PTEN-deficient mice

PTEN is a tumor suppressor gene but whether cancer can develop in all PTEN-deficient cells is not known. In T cell-specific PTEN-deficient (tPTEN-/-) mice, which suffer from mature T cell lymphomas, we found that premalignancy, as defined by elevated AKT and senescence pathways, starts in immature T cell precursors and surprisingly not in mature T cells. Premalignancy only starts in 6-week-old mice and becomes much stronger in 9-week-old mice although PTEN is lost since birth. tPTEN-/- immature T cells do not become tumors, and senescence has no role in this model because these cells exist in a novel cell cycle state, expressing proliferating proteins but not proliferating to any significant degree. Instead, the levels of p27(kip1), which is lower in tPTEN-/- immature T cells and almost nonexistent in tPTEN-/- mature T cells, correlate with the proliferation capability of these cells. Interestingly, transient reduction of these cancer precursor cells in adult tPTEN-/- mice within a crucial time window significantly delayed lymphomas and mouse lethality. Thus, loss of PTEN alone is not sufficient for cells to become cancerous, therefore other developmental events are necessary for tumor formation.

EBV latent membrane protein 1 activates Akt, NFkappaB, and Stat3 in B cell lymphomas

Latent membrane protein 1 (LMP1) is the major oncoprotein of Epstein-Barr virus (EBV). In transgenic mice, LMP1 promotes increased lymphoma development by 12 mo of age. This study reveals that lymphoma develops in B-1a lymphocytes, a population that is associated with transformation in older mice. The lymphoma cells have deregulated cell cycle markers, and inhibitors of Akt, NFκB, and Stat3 block the enhanced viability of LMP1 transgenic lymphocytes and lymphoma cells in vitro. Lymphoma cells are independent of IL4/Stat6 signaling for survival and proliferation, but have constitutively activated Stat3 signaling. These same targets are also deregulated in wild-type B-1a lymphomas that arise spontaneously through age predisposition. These results suggest that Akt, NFκB, and Stat3 pathways may serve as effective targets in the treatment of EBV-associated B cell lymphomas.

Epstein-Barr virus (EBV) is linked to the development of multiple cancers, including post-transplant lymphoma, Hodgkin disease, and nasopharyngeal carcinoma. Latent membrane protein 1 (LMP1) is expressed in many EBV-associated cancers and is responsible for most of the altered cellular growth properties that are induced by EBV infection. This study reveals that LMP1 induces lymphomas in B-1a lymphocytes, a cell type that is susceptible to transformation in aged mice. The lymphomas require Akt, NFκB, and Stat3 signaling for enhanced growth and survival. The activation of the Stat3, Akt, and NFκB signaling pathways likely underlies the ability of LMP1 to promote malignant transformation.

Regulation of cardiomyocyte proliferation and myocardial growth during development by FOXO transcription factors

Cardiomyocytes actively proliferate during embryogenesis and withdraw from the cell cycle during neonatal stages. FOXO (Forkhead O) transcription factors are a direct target of phosphatidylinositol-3 kinase/AKT signaling in skeletal and smooth muscle and regulate expression of the Cip/Kip family of cyclin kinase inhibitors in other cell types; however, the interaction of phosphatidylinositol-3 kinase/AKT signaling, FOXO transcription factors, and cyclin kinase inhibitor expression has not been reported for the developing heart. Here, we show that FOXO1 and FOXO3 are expressed in the developing myocardium concomitant with increased cyclin kinase inhibitor expression from embryonic to neonatal stages. Cell culture studies show that embryonic cardiomyocytes are responsive to insulin-like growth factor 1 stimulation, which results in the induction of the phosphatidylinositol-3 kinase/AKT pathway, cytoplasmic localization of FOXO proteins, and increased myocyte proliferation. Likewise, adenoviral-mediated expression of AKT promotes cardiomyocyte proliferation and cytoplasmic localization of FOXO. In contrast, increased expression of FOXO1 negatively affects myocyte proliferation. In vivo myocyte-specific transgenic expression of FOXO1 during heart development causes embryonic lethality at embryonic day 10.5 because of severe myocardial defects that coincide with premature activation of p21(cip1), p27(kip1), and p57(kip2) and decreased myocyte proliferation. Transgenic expression of dominant negative FOXO1 in cardiomyocytes does not obviously affect heart development at embryonic day 10.5, but results in abnormal morphology of the myocardium by embryonic day 18.5 along with decreased cyclin kinase inhibitor expression and increased myocyte proliferation. These data support FOXO transcription factors as negative regulators of cardiomyocyte proliferation and promoters of neonatal cell cycle withdrawal during heart development.