Molecular pathways: harnessing E2F1 regulation for prosenescence therapy in p53-defective cancer cells

circE2F1-encoded peptide inhibits circadian machinery essential for nucleotide biosynthesis and tumor progression via repressing SPIB/E2F1 axis

Circadian clock dominates a variety of biological activities, while its roles and regulatory mechanisms in neuroblastoma (NB), a pediatric extracranial malignancy, still remain largely elusive. Herein, through comprehensive analyses of public datasets, E2F transcription factor 1 (E2F1) and its circular RNA (circE2F1)-encoded 99-amino acid peptide (E2F1-99aa) were identified as vital regulators of circadian machinery essential for purine and pyrimidine biosynthesis during NB progression. Mechanistically, through interaction with Spi-B transcription factor (SPIB), E2F1 was transactivated to up-regulate circadian machinery genes (CRY1 and TIMELESS), resulting in relief of CLOCK/BMAL1-repressed transcription of enzymes (DHODH, PAICS, or PPAT) essential for de novo purine and pyrimidine biosynthesis. The biogenesis of circE2F1 was repressed by eukaryotic translation initiation factor 4A3 (EIF4A3), while E2F1-99aa or its truncated peptide competitively bound to SPIB, leading to decrease in SPIB-E2F1 interaction, circadian machinery and nucleotide biosynthetic gene expression, purine or pyrimidine biosynthesis, tumorigenesis, and aggresiveness of NB cells. In clinical NB cases, high EIF4A3, E2F1 or SPIB expression was correlated with low survival possibility of patients, while lower circE2F1 or E2F1-99aa levels were associated with advanced stages and tumor progression. These results indicate that circE2F1-encoded peptide inhibits circadian machinery essential for nucleotide biosynthesis and tumor progression via repressing SPIB/E2F1 axis.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

Deregulated E2F Activity as a Cancer-Cell Specific Therapeutic Tool

The transcription factor E2F, the principal target of the tumor suppressor pRB, plays crucial roles in cell proliferation and tumor suppression. In almost all cancers, pRB function is disabled, and E2F activity is enhanced. To specifically target cancer cells, trials have been undertaken to suppress enhanced E2F activity to restrain cell proliferation or selectively kill cancer cells, utilizing enhanced E2F activity. However, these approaches may also impact normal growing cells, since growth stimulation also inactivates pRB and enhances E2F activity. E2F activated upon the loss of pRB control (deregulated E2F) activates tumor suppressor genes, which are not activated by E2F induced by growth stimulation, inducing cellular senescence or apoptosis to protect cells from tumorigenesis. Deregulated E2F activity is tolerated in cancer cells due to inactivation of the ARF-p53 pathway, thus representing a feature unique to cancer cells. Deregulated E2F activity, which activates tumor suppressor genes, is distinct from enhanced E2F activity, which activates growth-related genes, in that deregulated E2F activity does not depend on the heterodimeric partner DP. Indeed, the ARF promoter, which is specifically activated by deregulated E2F, showed higher cancer-cell specific activity, compared to the E2F1 promoter, which is also activated by E2F induced by growth stimulation. Thus, deregulated E2F activity is an attractive potential therapeutic tool to specifically target cancer cells.

The role of transcription factors in the acquisition of the four latest proposed hallmarks of cancer and corresponding enabling characteristics

With the recent description of the molecular and cellular characteristics that enable acquisition of both core and new hallmarks of cancer, the consequences of transcription factor dysregulation in the hallmarks scheme has become increasingly evident. Dysregulation or mutation of transcription factors has long been recognized in the development of cancer where alterations in these key regulatory molecules can result in aberrant gene expression and consequential blockade of normal cellular differentiation. Here, we provide an up-to-date review of involvement of dysregulated transcription factor networks with the most recently reported cancer hallmarks and enabling characteristic properties. We present some illustrative examples of the impact of dysregulated transcription factors, specifically focusing on the characteristics of phenotypic plasticity, non-mutational epigenetic reprogramming, polymorphic microbiomes, and senescence. We also discuss how new insights into transcription factor dysregulation in cancer is contributing to addressing current therapeutic challenges.

E2F1 Expression and Apoptosis Initiation in Crayfish and Rat Peripheral Neurons and Glial Cells after Axonal Injury

Neurotrauma is among the main causes of human disability and mortality. The transcription factor E2F1 is one of the key proteins that determine the fate of cells. The involvement of E2F1 in the regulation of survival and death of peripheral nerve cells after axotomy has not been previously studied. We, for the first time, studied axotomy-induced changes in the expression and localization of E2F1 following axonal injury in rats and crayfish. Immunoblotting and immunofluorescence microscopy were used for the analysis of the expression and intracellular localization of E2F1 and its changes after axotomy. To evaluate whether this transcription factor promotes cell apoptosis, we examined the effect of pharmacological inhibition of E2F activity in axotomized rat models. In this work, axotomy caused increased expression of E2F1 as early as 4 h and even 1 h after axotomy of mechanoreceptor neurons and ganglia of crayfish ventral nerve cord (VNC), as well as rat dorsal root ganglia (DRG). The level of E2F1 expression increased both in the cytoplasm and the nuclei of neurons. Pharmacological inhibition of E2F demonstrated a pronounced neuroprotective activity against axotomized DRGs. E2F1 and downstream targets could be considered promising molecular targets for the development of potential neuroprotective agents.

c-Myc-activated USP2-AS1 suppresses senescence and promotes tumor progression via stabilization of E2F1 mRNA

The c-Myc oncoprotein plays a prominent role in cancer initiation, progression, and maintenance. Long noncoding RNAs (lncRNAs) are recently emerging as critical regulators of the c-Myc signaling pathway. Here, we report the lncRNA USP2-AS1 as a direct transcriptional target of c-Myc. Functionally, USP2-AS1 inhibits cellular senescence and acts as an oncogenic molecule by inducing E2F1 expression. Mechanistically, USP2-AS1 associates with the RNA-binding protein G3BP1 and facilitates the interaction of G3BP1 to E2F1 3′-untranslated region, thereby leading to the stabilization of E2F1 messenger RNA. Furthermore, USP2-AS1 is shown as a mediator of the oncogenic function of c-Myc via the regulation of E2F1. Together, these findings suggest that USP2-AS1 is a negative regulator of cellular senescence and also implicates USP2-AS1 as an important player in mediating c-Myc function.

[Exploratory screening of potential pan-cancer biomarkers based on The Cancer Genome Atlas database]

OBJECTIVE

To screen potential pan-cancer biomarkers based on The Cancer Genome Atlas (TCGA) database, and to provide help for the diagnosis and prognosis assessment of a variety of cancers.

METHODS

"GDC Data Transfer Tool" and "GDCRNATools" packages were used to obtain TCGA database. After data sorting, a total of 13 cancers were selected for further analysis. False disco-very rate (FDR) < 0.05 and fold change (FC) >1.5 were used as the differential expression criteria to screen genes and miRNAs that were up- or down-regulated in all the 13 cancers. In the receiver operating characteristic curve (ROC curve), the area under the curve (AUC), the best cut-off value and the corresponding sensitivity and specificity were used to reflect diagnostic significance. The Kaplan-Meier method was used to calculate the survival probability and then the log-rank test was performed. Hazard ratio (HR) was calculated to reflect prognostic evaluation significance. DAVID tool were used to perform GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis for differentially expressed genes. STRING and TargetScan tools were used to analyze the regulatory network of differentially expressed genes and miRNAs.

RESULTS

A total of 48 genes and 2 miRNAs were differentially expressed in all the 13 cancers. Among them, 25 genes were up-regulated, 23 genes and 2 miRNAs were down-regulated. Most differentially expressed genes and miRNAs had good ability to distinguish between the cases and controls, with AUC, sensitivity and specificity up to 0.8-0.9. Survival analysis results show that differentially expressed genes and miRNAs were significantly associated with patient survival in a variety of cancers. Most up-regulated genes were risk factors for patient survival (HR>1), while most down-regulated genes were protective factors for patient survival (0 < HR < 1). The enrichment analysis of GO and KEGG showed that the differentially expressed genes were mostly enriched in biological events related to cell proliferation. In the regulatory network analysis, a total of 13 differentially expressed genes and 2 differentially expressed miRNAs had regulatory and interaction relationships.

CONCLUSION

The 48 genes and 2 miRNAs that were differentially expressed in 13 cancers may serve as potential pan-cancer biomarkers, providing help for the diagnosis and prognosis evaluation of a variety of cancers, and providing clues for the development of broad-spectrum tumor therapeutic targets.

Association of a novel endometrial cancer biomarker panel with prognostic risk, platinum insensitivity, and targetable therapeutic options

During the past decade, the age-adjusted mortality rate for endometrial cancer (EC) increased 1.9% annually with TP53 mutant (TP53-mu) EC disproportionally represented in advanced disease and deaths. Therefore, we aimed to assess pivotal molecular parameters that differentiate clinical outcomes in high- and low-risk EC. Using the Cancer Genome Atlas, we analyzed EC specimens with available DNA sequences and quantitative gene-specific RNA expression data. After polymerase ɛ (POLE)-mutant specimens were excluded, differential gene-specific mutations and mRNA expressions were annotated and integrated. Consequent to TP53-mu failure to induce p21, derepression of multiple oncogenes harboring promoter p21 repressive sites was observed, including CCNA2 and FOXM1 (P < .001 compared with TP53 wild type [TP53-wt]). TP53-wt EC with high CCNA2 expression (CCNA2-H) had a targeted transcriptomic profile similar to that of TP53-mu EC, suggesting CCNA2 is a seminal determinant for both TP53-wt and TP53-mu EC. CCNA2 enhances E2F1 function, upregulating FOXM1 and CIP2A, as observed in TP53-mu and CCNA2-H TP53-wt EC (P < .001). CIP2A inhibits protein phosphatase 2A, leading to AKT inactivation of GSK3β and restricted oncoprotein degradation; PPP2R1A and FBXW7 mutations yield similar results. Upregulation of FOXM1 and failed degradation of FOXM1 is evidenced by marked upregulation of multiple homologous recombination genes (P < .001). Integrating these molecular aberrations generated a molecular biomarker panel with significant prognostic discrimination (P = 5.8×10⁻⁷); adjusting for age, histology, grade, myometrial invasion, TP53 status, and stage, only CCNA2-H/E2F1-H (P = .0003), FBXW7-mu/PPP2R1A-mu (P = .0002), and stage (P = .017) were significant. The generated prognostic molecular classification system identifies dissimilar signaling aberrations potentially amenable to targetable therapeutic options.

Effects of Cellular Senescence on Dental Follicle Cells

The dental follicle is part of the tooth germ, and isolated stem cells from this tissue (dental follicle cells; DFCs) are considered, for example, for regenerative medicine and immunotherapies. However somatic stem cells can also improve pharmaceutical research. Cell proliferation is limited by the induction of senescence, which, while reducing the therapeutic potential of DFCs for cell therapy, can also be used to study aging processes at the cellular level that can be used to test anti-aging pharmaceuticals. Unfortunately, very little is known about cellular senescence in DFCs. This review presents current knowledge about cellular senescence in DFCs.

p53 inhibits the osteogenic differentiation but does not induce senescence in human dental follicle cells

Replicative senescence causes a reduced osteogenic differentiation potential of senescent dental follicle cells (DFCs). The transcription factor p53 is often involved in the induction of cellular senescence, but little is known about its role in DFCs. This study examined for the first time the role of p53 compared to its pro-proliferative antagonist E2F-1 in terms of osteogenic differentiation potential and induction of senescence. Protein expression of E2F-1 decreased during cell aging, while p53 was expressed constitutively. Gene silencing of E2F1 (E2F-1) inhibited the proliferation rate of DFCs and increased the induction of cellular senescence. The induction of cellular senescence is regulated independently of the gene expression of TP53 (p53), since its gene expression depends on the expression of E2F1. Moreover, gene silencing of TP53 induced E2F1 gene expression and increased cell proliferation, but did not affect the rate of induction of cellular senescence. TP53 knockdown further induced the alkaline phosphatase and mineralization in DFCs. However, the simultaneous silencing of TP53 and E2F1 did not inhibit the inductive effect of TP53 knockdown on osteogenic differentiation, indicating that this effect is independent of E2F-1. In summary, our results suggest that p53 inhibits osteogenic differentiation and cell proliferation in senescent DFCs, but is not significantly involved in senescence induction.

A Photocaged, Water-Oxidizing, and Nucleolus-Targeted Pt(IV) Complex with a Distinct Anticancer Mechanism

Targeted anticancer prodrugs that can be controllably activated are highly desired for personalized precision medicine in cancer therapy. Such prodrugs with unique action modes are also promising to overcome drug resistance. Herein, we report coumaplatin, an oxaliplatin-based and photocaged Pt(IV) prodrug, to realize nuclear accumulation along with "on-demand" activation. This prodrug is based on a Pt(IV) complex that can be efficiently photoactivated via water oxidation without the requirement of a reducing agent. Coumaplatin accumulates very efficiently in the nucleoli, and upon photoactivation, this prodrug exhibits a level of photocytotoxicity up to 2 orders of magnitude higher than that of oxaliplatin. Unexpectedly, this prodrug presents strikingly enhanced tumor penetration ability and utilizes a distinct action mode to overcome drug resistance; i.e., coumaplatin but not oxaliplatin induces cell senescence, p53-independent cell death, and immunogenic cell death along with T cell activation. Our findings not only provide a novel strategy for the rational design of controllably activated and nucleolus-targeted Pt(IV) anticancer prodrugs but also demonstrate that accumulating conventional platinum drugs to the nucleus is a practical way to change its canonical mechanism of action and to achieve reduced resistance.

E2F1-Ror2 signaling mediates coordinated transcriptional regulation to promote G1/S phase transition in bFGF-stimulated NIH/3T3 fibroblasts

Ror2 signaling has been shown to regulate the cell cycle progression in normal and cancer cells. However, the molecular mechanism of the cell cycle progression upon activation of Ror2 signaling still remains unknown. Here, we found that the expression levels of Ror2 in G1-arrested NIH/3T3 fibroblasts are low and are rapidly increased following the cell cycle progression induced by basic fibroblast growth factor (bFGF) stimulation. By expressing wild-type or a dominant negative mutant of E2F1, we show that E2F1 mediates bFGF-induced expression of Ror2, and that E2F1 binds to the promoter of the Ror2 gene to activate its expression. We also found that G1/S phase transition of bFGF-stimulated NIH/3T3 cells is delayed by the suppressed expression of Ror2. RNA-seq analysis revealed that the suppressed expression of Ror2 results in the decreased expression of various E2F target genes concomitantly with increased expression of Forkhead box O (FoxO) target genes, including p21^Cip1 , and p27^Kip1 . Moreover, the inhibitory effect of Ror2 knockdown on the cell cycle progression can be restored by suppressed expression of p21^Cip1 , p27^Kip1 ,or FoxO3a. Collectively, these findings indicate that E2F1-Ror2 signaling mediates the transcriptional activation and inhibition of E2F1-driven and FoxO3a-driven cell cycle-regulated genes, respectively, thereby promoting G1/S phase transition of bFGF-stimulated NIH/3T3 cells.

Variable DNA methylation of aging-related genes is associated with male COPD

Background

Chronic obstructive pulmonary disease (COPD) is a chronic lung inflammatory disease which has a close relationship with aging. Genome-wide analysis reveals that DNA methylation markers vary obviously with age. DNA methylation variations in peripheral blood have the potential to be biomarkers for COPD. However, the specific DNA methylation of aging-related genes in the peripheral blood of COPD patients remains largely unknown.

Methods

Firstly, 9 aging-related differentially expressed genes (DEGs) in COPD patients were screened out from the 25 aging-related genes profile through a comprehensive screening strategy. Secondly, qPCR and multiple targeted bisulfite enrichment sequencing (MethTarget) were used to detect the mRNA level and DNA methylation level of the 9 differentially expressed genes in the peripheral blood of 60 control subjects and 45 COPD patients. The candidate functional CpG sites were selected on the basis of the regulation ability of the target gene expression. Thirdly, the correlation was evaluated between the DNA methylation level of the key CpG sites and the clinical parameters of COPD patients, including forced expiratory volume in one second (FEV1), forced expiratory volume in one second as percentage of predicted volume (FEV1%), forced expiratory volume/ forced vital capacity (FEV/FVC), modified British medical research council (mMRC) score, acute exacerbation frequency and the situation of frequent of acute aggravation (CAT) score. Lastly, differentially methylated CpG sites unrelated to smoking were also determined in COPD patients.

Results

Of the 9 differentially expressed aging-related genes, the mRNA expression of 8 genes were detected to be significantly down-regulated in COPD group, compared with control group. Meanwhile, the methylated level of all aging-related genes was changed in COPD group containing 219 COPD-related CpG sites in total. Notably, 27 CpG sites of FOXO3 gene showed a lower False Discovery Rate (FDR) and higher methylation difference values. Also, some variable DNA methylation is associated with the severity of COPD. Additionally, of the 219 COPD-related CpG sites, 147 CpG sites were not related to smoking.

Conclusion

These results identified that the mRNA expression and DNA methylation level of aging-related genes were changed in male COPD patients, which provides a molecular link between aging and COPD. The identified CpG markers are associated with the severity of COPD and provide new insights into the prediction and identification of COPD.

Targeting LINC00673 expression triggers cellular senescence in lung cancer

Aberrant expression of noncoding RNAs plays a critical role during tumorigenesis. To uncover novel functions of long non-coding RNA (lncRNA) in lung adenocarcinoma, we used a microarray-based screen identifying LINC00673 with elevated expression in matched tumor versus normal tissue. We report that loss of LINC00673 is sufficient to trigger cellular senescence, a tumor suppressive mechanism associated with permanent cell cycle arrest, both in lung cancer and normal cells in a p53-dependent manner. LINC00673-depleted cells fail to efficiently transit from G1- to S-phase. Using a quantitative proteomics approach, we confirm the modulation of senescence-associated genes as a result of LINC00673 knockdown. In addition, we uncover that depletion of p53 in normal and tumor cells is sufficient to overcome LINC00673-mediated cell cycle arrest and cellular senescence. Furthermore, we report that overexpression of LINC00673 reduces p53 translation and contributes to the bypass of Ras-induced senescence. In summary, our findings highlight LINC00673 as a crucial regulator of proliferation and cellular senescence in lung cancer.

The E2F1-miR-520/372/373-SPOP Axis Modulates Progression of Renal Carcinoma

: Although renal cell carcinoma (RCC) is the most malignant urologic cancer, its pathogenesis remains unclear, and effective treatments for advanced RCC are still lacking. Here, we report that a novel E2F1-miR-520/372/373-SPOP axis controls RCC carcinogenesis. Speckle-type POZ protein (SPOP) was upregulated in over 90% of RCC tissues, whereas the miR-520/372/373 family was downregulated and correlated inversely with SPOP protein levels in RCC tissues. The miR-520/372/373 family targeted the SPOP 3'-UTR and suppressed SPOP protein expression, leading to elevation of PTEN and DUSP7 levels and, consequently, decreased proliferation, invasion/migration, and metastasis of RCC cells in vitro and in vivo. Tail-vein delivery of therapeutic miR-520/372/373 family significantly decreased both tumor size and lung metastasis ratio in mice bearing orthotopic xenograft tumors. Decreased expression of miR-520/372/373 family was mediated by transcription factor E2F1. In conclusion, our results demonstrate that the E2F1-miR-520/372/373-SPOP axis functions as a key signaling pathway in RCC progression and metastasis and represents a promising opportunity for targeted therapies. SIGNIFICANCE: These findings show that the E2F1-miR-520/372/373 family-SPOP axis promotes RCC progression, thereby contributing to our understanding of RCC pathogenesis and unveiling new avenues for more effective targeted therapies.

Tris (2-chloroethyl) phosphate induces senescence-like phenotype of hepatocytes via the p21Waf1/Cip1-Rb pathway in a p53-independent manner

Tris (2-chloroethyl) phosphate (TCEP) has been widely used as a plasticizer and flame retardant. TCEP as a potential carcinogen is often detected in the occupational and nature environments. To investigate effects of TCEP on human hepatocytes, we assessed cell growth rate, cellular membrane integrity, senescence-associated β-galactosidase (SA-β-Gal) activity and analyzed expression of regulators involved in the p53-p21^Waf1/Cip1-Rb pathway in TCEP-treated L02 cells. The results showed TCEP increased the percentage of SA-β-Gal positive cells, decreased IL-6 levels, down-regulated the regulators of p38MAPK-NF-κB pathways, but up-regulated the regulators of p21^Waf1/Cip1-Rb pathway in L02 cells. Furthermore, we measured the SA-β-Gal activity and expression of regulators involved in the p53-p21^Waf1/Cip1-Rb pathway in L02^-p53 cells and p53-null Hep3B cells. Similar results were found in L02^-p53 cells and Hep3B cells. The findings demonstrated that TCEP induced senescence-like growth arrest via the p21^Waf1/Cip1-Rb pathway in a p53-independent manner, without activation of the IL-6/IL6R, p38MAPK-NF-κB pathways in hepatocytes.

Tumor suppressive protein phosphatases in human cancer: Emerging targets for therapeutic intervention and tumor stratification

Aberrant protein phosphorylation is one of the hallmarks of cancer cells, and in many cases a prerequisite to sustain tumor development and progression. Like protein kinases, protein phosphatases are key regulators of cell signaling. However, their contribution to aberrant signaling in cancer cells is overall less well appreciated, and therefore, their clinical potential remains largely unexploited. In this review, we provide an overview of tumor suppressive protein phosphatases in human cancer. Along their mechanisms of inactivation in defined cancer contexts, we give an overview of their functional roles in diverse signaling pathways that contribute to their tumor suppressive abilities. Finally, we discuss their emerging roles as predictive or prognostic markers, their potential as synthetic lethality targets, and the current feasibility of their reactivation with pharmacologic compounds as promising new cancer therapies. We conclude that their inclusion in clinical practice has obvious potential to significantly improve therapeutic outcome in various ways, and should now definitely be pushed forward.

Gambogic acid sensitizes gemcitabine efficacy in pancreatic cancer by reducing the expression of ribonucleotide reductase subunit-M2 (RRM2)

Background

Pancreatic cancer is susceptible to gemcitabine resistance, and patients receive less benefit from gemcitabine chemotherapy. Previous studies report that gambogic acid possesses antineoplastic properties; however, to our knowledge, there have been no specific studies on its effects in pancreatic cancer. Therefore, the purpose of this study was to explore whether increases the sensitivity of pancreatic cancer to gemcitabine, and determine the synergistic effects of gambogic acid and gemcitabine against pancreatic cancer.

Methods

The effects of gambogic acid on cell viability, the cell cycle, and apoptosis were assessed using 4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT) and flow cytometry in pancreatic cancer cell lines. Protein expression was detected by western blot analysis and mRNA expression was detected using q-PCR. A xenograft tumor model of pancreatic cancer was used to investigate the synergistic effects of gambogic acid and gemcitabine.

Results

Gambogic acid effectively inhibited the growth of pancreatic cancer cell lines by inducing S-phase cell cycle arrest and apoptosis. Synergistic activity of gambogic acid combined with gemcitabine was observed in PANC-1 and BxPC-3 cells based on the results of MTT, colony formation, and apoptosis assays. Western blot results demonstrated that gambogic acid sensitized gemcitabine-induced apoptosis by enhancing the expression of cleaved caspase-3, cleaved caspase-9, cleaved-PARP, and Bax, and reducing the expression of Bcl-2. In particular, gambogic acid reduced the expression of the ribonucleotide reductase subunit-M2 (RRM2) protein and mRNA, a trend that correlated with resistance to gemcitabine through inhibition of the extracellular signal-regulated kinase (ERK)/E2F1 signaling pathway. Treatment with gambogic acid and gemcitabine significantly repressed tumor growth in the xenograft pancreatic cancer model. Immunohistochemistry results demonstrated a downregulation of p-ERK, E2F1, and RRM2 in mice receiving gambogic acid treatment and combination treatment.

Conclusions

These results demonstrate that gambogic acid sensitizes pancreatic cancer cells to gemcitabine in vitro and in vivo by inhibiting the activation of the ERK/E2F1/RRM2 signaling pathway. The results also indicate that gambogic acid treatment combined with gemcitabine might be a promising chemotherapy strategy for pancreatic cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-017-0579-0) contains supplementary material, which is available to authorized users.

DNA damage and S phase-dependent E2F1 stabilization requires the cIAP1 E3-ubiquitin ligase and is associated with K63-poly-ubiquitination on lysine 161/164 residues

The E2F transcription factor 1 is subtly regulated along the cell cycle progression and in response to DNA damage by post-translational modifications. Here, we demonstrated that the E3-ubiquitin ligase cellular inhibitor of apoptosis 1 (cIAP1) increases E2F1 K63-poly-ubiquitination on the lysine residue 161/164 cluster, which is associated with the transcriptional factor stability and activity. Mutation of these lysine residues completely abrogates the binding of E2F1 to CCNE, TP73 and APAF1 promoters, thus inhibiting transcriptional activation of these genes and E2F1-mediated cell proliferation control. Importantly, E2F1 stabilization in response to etoposide-induced DNA damage or during the S phase of cell cycle, as revealed by cyclin A silencing, is associated with K63-poly-ubiquitinylation of E2F1 on lysine 161/164 residues and involves cIAP1. Our results reveal an additional level of regulation of the stability and the activity of E2F1 by a non-degradative K63-poly-ubiquitination and uncover a novel function for the E3-ubiquitin ligase cIAP1.

lncRNA-HIT promotes cell proliferation of non-small cell lung cancer by association with E2F1

Lung cancer is the leading cause of cancer-related death around the world. Long noncoding RNA (lncRNA) has pivotal roles in cancer occurrence and development. However, only a few lncRNAs have been functionally characterized. In the present study, we investigated the effects of lncRNA-HIT (HOXA transcript induced by TGFβ) expression on non-small cell lung cancer (NSCLC) cell phenotype with the gain-of-function and loss-of-function assays. We found that ectopic expression or knockdown of lncRNA-HIT markedly increased or decreased NSCLC cell proliferation, respectively. Moreover, we also showed that lncRNA-HIT interacted with E2F1 to regulate its target genes, such as Survivin, FOXM1, SKP2, NELL2 and DOK1. Collectively, our findings indicated that lncRNA-HIT affected the proliferation of NSCLC cells at least in part via regulating the occupancy of E2F1 in the promoter regions of its target genes. The lncRNA-HIT-E2F1 complex may be a potential target for NSCLC treatment.

Long noncoding RNA CRNDE functions as a competing endogenous RNA to promote metastasis and oxaliplatin resistance by sponging miR-136 in colorectal cancer

Colorectal neoplasia differentially expressed (CRNDE) is a novel gene recognized as a long noncoding RNA (lncRNA) that is highly elevated in colorectal cancer and many other solid tumors but its functions on metastasis and oxaliplatin (OXA) resistance are unknown. In our study, we confirmed the upregulation of CRNDE in both primary specimens from colorectal cancer patients and colorectal cancer cell lines. Knockdown of CRNDE expression inhibited the migration and invasion potency of colorectal cancer cells with no effect on cell apoptosis. Overexpression of CRNDE promoted the migration and invasion potency of colorectal cancer cells. Furthermore, we found that CRNDE conferred chemoresistance in colorectal cancer cells. Knockdown of CRNDE with OXA treatment decreased cell viability and promoted DNA damage and cell apoptosis, while the overexpression of CRNDE with OXA treatment reduced DNA damage and cell apoptosis. Further in-depth mechanistic studies revealed that CRNDE functioned as a competing endogenous RNA for miR-136, led to the de-repression of its endogenous target, E2F transcription factor 1 (E2F1). Overall, our findings demonstrate that CRNDE functions as a competing endogenous RNA to promote metastasis and OXA resistance by sponging miR-136 in colorectal cancer.

TM7SF3, a novel p53-regulated homeostatic factor, attenuates cellular stress and the subsequent induction of the unfolded protein response

Earlier reported small interfering RNA (siRNA) high-throughput screens, identified seven-transmembrane superfamily member 3 (TM7SF3) as a novel inhibitor of pancreatic β-cell death. Here we show that TM7SF3 maintains protein homeostasis and promotes cell survival through attenuation of ER stress. Overexpression of TM7SF3 inhibits caspase 3/7 activation. In contrast, siRNA-mediated silencing of TM7SF3 accelerates ER stress and activation of the unfolded protein response (UPR). This involves inhibitory phosphorylation of eukaryotic translation initiation factor 2α activity and increased expression of activating transcription factor-3 (ATF3), ATF4 and C/EBP homologous protein, followed by induction of apoptosis. This process is observed both in human pancreatic islets and in a number of cell lines. Some of the effects of TM7SF3 silencing are evident both under basal conditions, in otherwise untreated cells, as well as under different stress conditions induced by thapsigargin, tunicamycin or a mixture of pro-inflammatory cytokines (tumor necrosis factor alpha, interleukin-1 beta and interferon gamma). Notably, TM7SF3 is a downstream target of p53: activation of p53 by Nutlin increases TM7SF3 expression in a time-dependent manner, although silencing of p53 abrogates this effect. Furthermore, p53 is found in physical association with the TM7SF3 promoter. Interestingly, silencing of TM7SF3 promotes p53 activity, suggesting the existence of a negative-feedback loop, whereby p53 promotes expression of TM7SF3 that acts to restrict p53 activity. Our findings implicate TM7SF3 as a novel p53-regulated pro-survival homeostatic factor that attenuates the development of cellular stress and the subsequent induction of the UPR.

Who watches the watchmen? Regulation of the expression and activity of sirtuins

Sirtuins (SIRT1-7) are a family of nicotine adenine dinucleotide (NAD⁺)-dependent enzymes that catalyze post-translational modifications of proteins. Together, they regulate crucial cellular functions and are traditionally associated with aging and longevity. Dysregulation of sirtuins plays an important role in major diseases, including cancer and metabolic, cardiac, and neurodegerative diseases. They are extensively regulated in response to a wide range of stimuli, including nutritional and metabolic challenges, inflammatory signals or hypoxic and oxidative stress. Each sirtuin is regulated individually in a tissue- and cell-specific manner. The control of sirtuin expression involves all the major points of regulation, including transcriptional and post-translational mechanisms and microRNAs. Collectively, these mechanisms control the protein levels, localization, and enzymatic activity of sirtuins. In many cases, the regulators of sirtuin expression are also their substrates, which lead to formation of intricate regulatory networks and extensive feedback loops. In this review, we highlight the mechanisms mediating the physiologic and pathologic regulation of sirtuin expression and activity. We also discuss the consequences of this regulation on sirtuin function and cellular physiology.-Buler, M., Andersson, U., Hakkola, J. Who watches the watchmen? Regulation of the expression and activity of sirtuins.

Genistein targets the cancerous inhibitor of PP2A to induce growth inhibition and apoptosis in breast cancer cells

Genistein is a soy isoflavone with phytoestrogen and tyrosine kinase inhibitory properties. High intake of soy/genistein has been associated with reduced breast cancer risk. Despite the advances in genistein-mediated antitumor studies, the underlying mechanisms remain unclear. In the present study, we investigated genistein-induced regulation of the cancerous inhibitor of protein phosphatase 2A (CIP2A), a novel oncogene frequently overexpressed in breast cancer, and its functional impact on genistein-induced growth inhibition and apoptosis. We demonstrated that genistein induced downregulation of CIP2A in MCF-7-C3 and T47D breast cancer cells, which was correlated with its growth inhibition and apoptotic activities. Overexpression of CIP2A attenuated, whereas CIP2A knockdown sensitized, genistein-induced growth inhibition and apoptosis. We further showed that genistein-induced downregulation of CIP2A involved both transcriptional suppression and proteasomal degradation. In particular, genistein at higher concentrations induced concurrent downregulation of E2F1 and CIP2A. Overexpression of E2F1 attenuated genistein-induced downregulation of CIP2A mRNA, indicating the role of E2F1 in genistein-induced transcriptional suppression of CIP2A. Taken together, our results identified CIP2A as a functional target of genistein and demonstrated that modulation of E2F1-mediated transcriptional regulation of CIP2A contributes to its downregulation. These data advance our understanding of genistein-induced growth inhibition and apoptosis, and support further investigation on CIP2A as a therapeutic target of relevant anticancer agents.

Tumor suppressor SET9 guides the epigenetic plasticity of breast cancer cells and serves as an early-stage biomarker for predicting metastasis

During the course of cancer progression, neoplastic cells undergo dynamic and reversible transitions between multiple phenotypic states, and this plasticity is enabled by underlying shifts in epigenetic regulation. Our results identified a negative feedback loop in which SET9 controls DNA methyltransferase-1 protein stability, which represses the transcriptional activity of the SET9 promoter in coordination with Snail. The modulation of SET9 expression in breast cancer cells revealed a connection with E2F1 and the silencing of SET9 was sufficient to complete an epigenetic program that favored epithelial-mesenchymal transition and the generation of cancer stem cells, indicating that SET9 plays a role in modulating breast cancer metastasis. SET9 expression levels were significantly higher in samples from patients with pathological complete remission than in samples from patients with disease recurrence, which indicates that SET9 acts as a tumor suppressor in breast cancer and that its expression may serve as a prognostic marker for malignancy.

Oncogenic nexus of cancerous inhibitor of protein phosphatase 2A (CIP2A): an oncoprotein with many hands

Oncoprotein CIP2A a Cancerous Inhibitor of PP2A forms an "oncogenic nexus" by virtue of its control on PP2A and MYC stabilization in cancer cells. The expression and prognostic function of CIP2A in different solid tumors including colorectal carcinoma, head and neck cancers, gastric cancers, lung carcinoma, cholangiocarcinoma, esophageal cancers, pancreatic carcinoma, brain cancers, breast carcinoma, bladder cancers, ovarian carcinoma, renal cell carcinomas, tongue cancers, cervical carcinoma, prostate cancers, and oral carcinoma as well as a number of hematological malignancies are just beginning to emerge. Herein, we reviewed the recent progress in our understanding of (1) how an "oncogenic nexus" of CIP2A participates in the tumorigenic transformation of cells and (2) how we can prospect/view the clinical relevance of CIP2A in the context of cancer therapy. The review will try to understand the role of CIP2A (a) as a biomarker in cancers and evaluate the prognostic value of CIP2A in different cancers (b) as a therapeutic target in cancers and (c) in drug response and developing chemo-resistance in cancers.