Earlier studies have shown that macroautophagy is not a constitutively activated process, however, the mechanism of activation is not fully understood. Here, we report that autophagy is a dynamic process in cancer cells in response to glucose starvation. In addition, we determined that FOXO1 turnover is involved in the regulation of this dynamic process. X-box binding protein 1u (XBP1u) plays a critical role in FOXO1 degradation by recruiting FOXO1 to the 20S proteasome. Moreover, the phosphorylation of XBP1u by mitogen-activated protein kinases 1 and 3 (MAPK1/3, also known as ERK2/1) on serine residues 61 and 176 was found to be essential for the enhancement of the interaction between XBP1u and FOXO1. Thus, our findings support the hypothesis that the turnover of FOXO1 induced by MAPK1/3 and XBP1u is a critical factor regulating the autophagic process.
FOXO1; XBP1u; ERK; autophagy; cancer
Genetic abnormalities have been conventionally considered as hallmarks of cancer. However, studies over the past decades have demonstrated that epigenetic regulation also participates in the development of cancer. The fundamental patterns of epigenetic components, such as DNA methylation and histone modifications, are frequently altered in tumor cells. Acetylation is one of the best characterized modifications of histones, which is controlled by histone acetyltransferases (HATs) and histone deacetylases (HDACs). HDACs are a group of enzymes which catalyze the removal of the acetyl groups of both histones and non-histone proteins. HDACs are involved in modulating most key cellular processes, including transcriptional regulation, apoptosis, DNA damage repair, cell cycle control, autophagy, metabolism, senescence and chaperone function. Because HDACs have been found to function incorrectly in cancer, various HDAC inhibitors are being investigated to act as cancer chemotherapeutics. The primary purpose of this paper is to summarize recent studies of the links between HDACs and cancer, and further discuss the underlying mechanisms of anti-tumor activities of HDAC inhibitors and clinical implications.
HDAC; HDAC inhibitor; epigenetic therapy; cancer.
Forkhead box O (FOXO) transcriptional protein family members, including FOXO1 and FOXO3, are involved in the modulation of autophagy. However, whether there is redundancy between FOXO1 and FOXO3 in the ability to induce autophagy remains unclear. In this study, we showed that FOXO3 induced a transcription-dependent autophagy, and FOXO1 was required for this process. Overexpression of wild-type FOXO3 (WT) or FOXO3 (3A), which harbors alanine mutations at residues Thr32, Ser253 and Ser315, but not transcription-inactive FOXO3 (∆DB3A), significantly induced autophagy in the human embryonic kidney cell line HEK293T and mouse embryonic fibroblast (MEF) cell lines. Interestingly, depletion of FOXO1 by siRNA attenuated FOXO3-induced autophagy. Our data also showed that FOXO3 overexpression did not increase the expression of FOXO1 at the protein level, although FOXO3 was capable of binding the promoter region of FOXO1 and inducing an increase in the transcription of FOXO1 mRNA. Furthermore, our results showed that FOXO3 promoted the translocation of FOXO1 from the nucleus to the cytoplasm, resulting in an increase in FOXO1-induced autophagy. Moreover, our results supported a mechanism whereby FOXO3 dramatically increased the expression of the class I PtdIns3K catalytic subunit PIK3CA, leading to an increase in AKT1 activity, which resulted in the phosphorylation and nuclear export of FOXO1. To the best of our knowledge, our data are the first to suggest that FOXO1 plays a central role in FOXO3-induced autophagy.
FOXO1; FOXO3; autophagy; PIK3CA; AKT1
The prognostic significance of chemokine receptors in stage I/II colon cancer is unclear. We assessed the prognostic value of chemokine receptor CXCR3 and CXCR4 in stage I/II colon cancer.
145 patients with stage I/II colon cancer who underwent curative surgery alone from 2000 to 2007 were investigated. Chemokine receptor expression was assessed by immunohistochemistry. The associations between CXCR3, CXCR4 and clinicopathological variables were analysed using the χ2 test, and the relationships between chemokine receptors and a 5-year disease-free survival were analysed by univariate and multivariate analyses.
The high-expression rates of CXCR3 and CXCR4 were 17.9% (26/145) and 38.6% (56/145), respectively. There were no significant associations between the expressions of CXCR3, CXCR4 and clinicopathological factors including gender, age, tumour location, histological differentiation, pathological stage, lymphovascular invasion and pretreatment serum carcinoembryonic antigen (CEA). The 5-year disease-free survival was not significantly different between low-expression groups and high-expression groups of CXCR3 and CXCR4. Multivariate analysis revealed that serum CEA and a number of retrieved lymph nodes, rather than chemokine receptors, were independent prognosticators.
CXCR3 and CXCR4 are not independent prognosticators for stage I/II colon cancer after curative surgery.
Histone deacetylase (HDAC) inhibitors have been shown to induce cell cycle arrest and apoptosis in cancer cells. However, the mechanisms of HDAC inhibitor induced apoptosis are incompletely understood. In this study, depsipeptide, a novel HDAC inhibitor, was shown to be able to induce significant apoptotic cell death in human lung cancer cells. Further study showed that Bim, a BH3-only proapoptotic protein, was significantly upregulated by depsipeptide in cancer cells, and Bim's function in depsipeptide-induced apoptosis was confirmed by knockdown of Bim with RNAi. In addition, we found that depsipeptide-induced expression of Bim was directly dependent on acetylation of forkhead box class O1 (FoxO1) that is catalyzed by cyclic adenosine monophosphate-responsive element-binding protein-binding protein, and indirectly induced by a decreased four-and-a-half LIM-domain protein 2. Moreover, our results demonstrated that FoxO1 acetylation is required for the depsipeptide-induced activation of Bim and apoptosis, using transfection with a plasmid containing FoxO1 mutated at lysine sites and a luciferase reporter assay. These data show for the first time that an HDAC inhibitor induces apoptosis through the FoxO1 acetylation-Bim pathway.
The tumour suppressor ARF is specifically required for p53 activation under oncogenic stress1–6. Recent studies showed that p53 activation mediated by ARF, but not that induced by DNA damage, acts as a major protection against tumorigenesis in vivo under certain biological settings7,8, suggesting that the ARF–p53 axis has more fundamental functions in tumour suppression than originally thought. Because ARF is a very stable protein in most human cell lines, it has been widely assumed that ARF induction is mediated mainly at the transcriptional level and that activation of the ARF–p53 pathway by oncogenes is a much slower and largely irreversible process by comparison with p53 activation after DNA damage. Here we report that ARF is very unstable in normal human cells but that its degradation is inhibited in cancerous cells. Through biochemical purification, we identified a specific ubiquitin ligase for ARF and named it ULF. ULF interacts with ARF both in vitro and in vivo and promotes the lysine-independent ubiquitylation and degradation of ARF. ULF knockdown stabilizes ARF in normal human cells, triggering ARF-dependent, p53-mediated growth arrest. Moreover, nucleophosmin (NPM) and c-Myc, both of which are commonly overexpressed in cancer cells, are capable of abrogating ULF-mediated ARF ubiquitylation through distinct mechanisms, and thereby promote ARF stabilization in cancer cells. These findings reveal the dynamic feature of the ARF–p53 pathway and suggest that transcription-independent mechanisms are critically involved in ARF regulation during responses to oncogenic stress.
Autophagy is activated to maintain cellular energy homeostasis in response to nutrient starvation. However, autophagy is not persistently activated, which is poorly understood at a mechanistic level. Here, we report that turnover of FoxO1 is involved in the dynamic autophagic process caused by glutamine starvation. X-box-binding protein-1u (XBP-1u) has a critical role in FoxO1 degradation by recruiting FoxO1 to the 20S proteasome. In addition, the phosphorylation of XBP-1u by extracellular regulated protein kinases1/2 (ERK1/2) on Ser61 and Ser176 was found to be critical for the increased interaction between XBP-1u and FoxO1 upon glutamine starvation. Furthermore, knockdown of XBP-1u caused the sustained level of FoxO1 and the persistent activation of autophagy, leading to a significant decrease in cell viability. Finally, the inverse correlation between XBP-1u and FoxO1 expression agrees well with the expression profiles observed in many human cancer tissues. Thus, our findings link the dynamic process of autophagy to XBP-1u-induced FoxO1 degradation.
FoxO1; XBP-1u; ERK; autophagy; cancer
Adiponectin, an abundant adipose tissue-derived protein, exerts protective effect against cardiovascular disease. Adiponectin receptors (AdipoR1 and AdipoR2) mediate the beneficial effects of adiponectin on the cardiovascular system. However, the alteration of AdipoRs in cardiac remodeling is not fully elucidated. Here, we investigated the effect of angiotensin II (AngII) on cardiac AdipoRs expression and explored the possible molecular mechanism. AngII infusion into rats induced cardiac hypertrophy, reduced AdipoR1 but not AdipoR2 expression, and attenuated the phosphorylations of adenosine monophosphate-activated protein kinase and acetyl coenzyme A carboxylase, and those effects were all reversed by losartan, an AngII type 1 (AT1) receptor blocker. AngII reduced expression of AdipoR1 mRNA and protein in cultured neonatal rat cardiomyocytes, which was abolished by losartan, but not by PD123319, an AT2 receptor antagonist. The antioxidants including reactive oxygen species (ROS) scavenger NAC, NADPH oxidase inhibitor apocynin, Nox2 inhibitor peptide gp91 ds-tat, and mitochondrial electron transport chain complex I inhibitor rotenone attenuated AngII-induced production of ROS and phosphorylation of extracellular signal-regulated kinase (ERK) 1/2. AngII-reduced AdipoR1 expression was reversed by pretreatment with NAC, apocynin, gp91 ds-tat, rotenone, and an ERK1/2 inhibitor PD98059. Chromatin immunoprecipitation assay demonstrated that AngII provoked the recruitment of c-Myc onto the promoter region of AdipoR1, which was attenuated by PD98059. Moreover, AngII-induced DNA binding activity of c-Myc was inhibited by losartan, NAC, apocynin, gp91 ds-tat, rotenone, and PD98059. c-Myc small interfering RNA abolished the inhibitory effect of AngII on AdipoR1 expression. Our results suggest that AngII inhibits cardiac AdipoR1 expression in vivo and in vitro and AT1 receptor/ROS/ERK1/2/c-Myc pathway is required for the downregulation of AdipoR1 induced by AngII.
Autophagy has attracted a lot of attention in recent years. More and more proteins and signaling pathways have been discovered that somehow feed into the autophagy regulatory pathways. Regulation of autophagy is complex and condition-specific, and in several diseases, autophagic fluxes are changed. Here, we review the most well-established concepts in this field as well as the reported signaling pathways or components which steer the autophagy machinery. Furthermore, we will highlight how autophagic fluxes are changed in various diseases either as cause for or as response to deal with an altered cellular homeostasis and how modulation of autophagy might be used as potential therapy for such diseases.
Autophagy; ATGs; mTOR; UPR; FOXO1; HSF1; Heat Shock Proteins (HSP); Human diseases.
Among the human genome, p53 is one of the first tumor suppressor genes to be discovered. It has a wide range of functions covering cell cycle control, apoptosis, genome integrity maintenance, metabolism, fertility, cellular reprogramming and autophagy. Although different possible underlying mechanisms for p53 regulation have been proposed for decades, none of them is conclusive. While much literature focuses on the importance of individual post-translational modifications, further explorations indicate a new layer of p53 coordination through the interplay of the modifications, which builds up a complex 'network'. This review focuses on the necessity, characteristics and mechanisms of the crosstalk among post-translational modifications and its effects on the precise and selective behavior of p53.
p53; post-translational modification; crosstalk; protein-protein interaction; semiotic system.
Histone deacetylase inhibitor (HDACi) has been shown to demethylate the mammalian genome, which further strengthens the concept that DNA methylation and histone modifications interact in regulation of gene expression. Here, we report that an HDAC inhibitor, depsipeptide, exhibited significant demethylating activity on the promoters of several genes, including p16, SALL3, and GATA4 in human lung cancer cell lines H719 and H23, colon cancer cell line HT-29, and pancreatic cancer cell line PANC1. Although expression of DNA methyltransferase 1 (DNMT1) was not affected by depsipeptide, a decrease in binding of DNMT1 to the promoter of these genes played a dominant role in depsipeptide-induced demethylation and reactivation. Depsipeptide also suppressed expression of histone methyltransferases G9A and SUV39H1, which in turn resulted in a decrease of di- and trimethylated H3K9 around these genes' promoter. Furthermore, both loading of heterochromatin-associated protein 1 (HP1α and HP1β) to methylated H3K9 and binding of DNMT1 to these genes' promoter were significantly reduced in depsipeptide-treated cells. Similar DNA demethylation was induced by another HDAC inhibitor, apicidin, but not by trichostatin A. Our data describe a novel mechanism of HDACi-mediated DNA demethylation via suppression of histone methyltransferases and reduced recruitment of HP1 and DNMT1 to the genes' promoter.
5-aza-2′-deoxycytidine (5-aza-CdR) is used extensively as a demethylating agent and acts in concert with histone deacetylase inhibitors (HDACI) to induce apoptosis or inhibition of cell proliferation in human cancer cells. Whether the action of 5-aza-CdR in this synergistic effect results from demethylation by this agent is not yet clear. In this study we found that inhibition of cell proliferation was not observed when cells with knockdown of DNA methyltransferase 1 (DNMT1), or double knock down of DNMT1-DNMT3A or DNMT1-DNMT3B were treated with HDACI, implying that the demethylating function of 5-aza-CdR may be not involved in this synergistic effect. Further study showed that there was a causal relationship between 5-aza-CdR induced DNA damage and the amount of [3H]-5-aza-CdR incorporated in DNA. However, incorporated [3H]-5-aza-CdR gradually decreased when cells were incubated in [3H]-5-aza-CdR free medium, indicating that 5-aza-CdR, which is an abnormal base, may be excluded by the cell repair system. It was of interest that HDACI significantly postponed the removal of the incorporated [3H]-5-aza-CdR from DNA. Moreover, HDAC inhibitor showed selective synergy with nucleoside analog-induced DNA damage to inhibit cell proliferation, but showed no such effect with other DNA damage stresses such as γ-ray and UV, etoposide or cisplatin. This study demonstrates that HDACI synergistically inhibits cell proliferation with nucleoside analogs by suppressing removal of incorporated harmful nucleotide analogs from DNA.
Deregulated expression of E2F1 not only promotes S-phase entry but also induces apoptosis. Although it has been well documented that E2F1 is able to induce p53-dependent apoptosis via raising ARF activity, the mechanism by which E2F induces p53-independent apoptosis remains unclear. Here we report that E2F1 can directly bind to and activate the promoter of Smac/DIABLO, a mitochondrial proapoptotic gene, through the E2F1-binding sites BS2 (−542 ∼ −535 bp) and BS3 (−200 ∼ −193 bp). BS2 and BS3 appear to be utilized in combination rather than singly by E2F1 in activation of Smac/DIABLO. Activation of BS2 and BS3 are E2F1-specific, since neither E2F2 nor E2F3 is able to activate BS2 or BS3. Using the H1299 ER-E2F1 cell line where E2F1 activity can be conditionally induced, E2F1 has been shown to upregulate the Smac/DIABLO expression at both mRNA and protein levels upon 4-hydroxytamoxifen treatment, resulting in an enhanced mitochondria-mediated apoptosis. Reversely, reducing the Smac/DIABLO expression by RNA interference significantly diminishes apoptosis induced by E2F1. These results may suggest a novel mechanism by which E2F1 promotes p53-independent apoptosis through directly regulating its downstream mitochondrial apoptosis-inducing factors, such as Smac/DIABLO.
Generally, histone deacetylase (HDAC) inhibitor-induced p21Waf1/Cip1 expression is thought to be p53 independent. Here we found that an inhibitor of HDAC, depsipeptide (FR901228), but not trichostatin A (TSA), induces p21Waf1/Cip1 expression through both p53 and Sp1/Sp3 pathways in A549 cells (which retain wild-type p53). This is demonstrated by measuring relative luciferase activities of p21 promoter constructs with p53 or Sp1 binding site mutagenesis and was further confirmed by transfection of wild-type p53 into H1299 cells (p53 null). That p53 was acetylated after depsipeptide treatment was tested by sequential immunoprecipitation/Western immunoblot analysis with anti-acetylated lysines and anti-p53 antibodies. The acetylated p53 has a longer half-life due to a significant decrease in p53 ubiquitination. Further study using site-specific antiacetyllysine antibodies and transfection of mutated p53 vectors (K319/K320/K321R mutated and K373R/K382R mutations) into H1299 cells revealed that depsipeptide specifically induces p53 acetylation at K373/K382, but not at K320. As assayed by coimmunoprecipitation, the K373/K382 acetylation is accompanied by a recruitment of p300, but neither CREB-binding protein (CBP) nor p300/CBP-associated factor (PCAF), to the p53 C terminus. Furthermore, activity associated with the binding of the acetylated p53 at K373/K382 to the p21 promoter as well as p21Waf1/Cip1 expression is significantly increased after depsipeptide treatment, as tested by chromatin immunoprecipitations and Western blotting, respectively. In addition, p53 acetylation at K373/K382 is confirmed to be required for recruitment of p300 to the p21 promoter, and the depsipeptide-induced p53 acetylation at K373/K382 is unlikely to be dependent on p53 phosphorylation at Ser15, Ser20, and Ser392 sites. Our data suggest that p53 acetylation at K373/K382 plays an important role in depsipeptide-induced p21Waf1/Cip1 expression.
DNA methylation in the promoter of certain genes is associated with transcriptional silencing. Methylation affects gene expression directly by interfering with transcription factor binding and/or indirectly by recruiting histone deacetylases through methyl-DNA-binding proteins. In this study, we demonstrate that the human lung cancer cell line H719 lacks p53-dependent and -independent p21Cip1 expression. p53 response to treatment with gamma irradiation or etoposide is lost due to a mutation at codon 242 of p53 (C→W). Treatment with depsipeptide, an inhibitor of histone deacetylase, was unable to induce p53-independent p21Cip1 expression because the promoter of p21Cip1 in these cells is hypermethylated. By analyzing luciferase activity of transfected p21Cip1 promoter vectors, we demonstrate that depsipeptide functions on Sp1-binding sites to induce p21Cip1 expression. We hypothesize that hypermethylation may interfere with Sp1/Sp3 binding. By using an electrophoretic mobility shift assay, we show that, although methylation within the consensus Sp1-binding site did not reduce Sp1/Sp3 binding, methylation outside of the consensus Sp1 element induced a significant decrease in Sp1/Sp3 binding. Depsipeptide induced p21Cip1 expression was reconstituted when cells were pretreated with 5-aza-2′-deoxycytidine. Our data suggest, for the first time, that hypermethylation around the consensus Sp1-binding sites may directly reduce Sp1/Sp3 binding, therefore leading to a reduced p21Cip1 expression in response to depsipeptide treatment.
Epigenetic changes, including DNA methylation, are a common finding in cancer. In lung cancers methylation of cytosine residues may affect tumor initiation and progression in several ways, including the silencing of tumor suppressor genes through promoter methylation and by providing the targets for adduct formation of polycyclic aromatic hydrocarbons present in combustion products of cigarette smoke. Although the importance of aberrant DNA methylation is well established, the extent of DNA methylation in lung cancers has never been determined. Restriction landmark genomic scanning (RLGS) is a highly reproducible two-dimensional gel electrophoresis that allows the determination of the methylation status of up to 2000 promoter sequences in a single gel. We selected 1184 CpG islands for RLGS analysis and determined their methylation status in 16 primary non-small cell lung cancers. Some tumors did not show methylation whereas others showed up to 5.3% methylation in all CpG islands of the profile. Cloning of 21 methylated loci identified 11 genes and 6 ESTs. We demonstrate that methylation is part of the silencing process of BMP3B in primary tumors and lung cancer cell lines.
non-small cell lung cancer; DNA methylation; RLGS; genome scanning; epigenetic