Phosphoinositide-3-kinase signaling controls S-phase kinase-associated protein 2 transcription via E2F1 in pancreatic ductal adenocarcinoma cells

Fatty acid synthase: A key driver of ovarian cancer metastasis and a promising therapeutic target

Fatty acid synthase (FASN) is a critical enzyme essential for the production of fats in the body. The abnormal expression of FASN is associated with different types of malignancies, including ovarian cancer. FASN plays a crucial role in cell growth and survival as a metabolic oncogene, although the specific processes that cause its dysregulation are still unknown. FASN interacts with signaling pathways linked to the progression of cancer. Pharmacologically inhibiting or inactivating the FASN gene has shown potential in causing the death of cancer cells, offering a possible treatment approach. This review examines the function of FASN in ovarian cancer, namely its level of expression, influence on the advancement of the disease, and its potential as a target for therapeutic interventions.

Skp2: A critical molecule for ubiquitination and its role in cancer

The ubiquitin-proteasome system (UPS) is a multi-step process that serves as the primary pathway for protein degradation within cells. UPS activity also plays a crucial role in regulating various life processes, including the cell cycle, signal transduction, DNA repair, and others. The F-box protein Skp2, a crucial member of the UPS, plays a central role in the development of various diseases. Skp2 controls cancer cell growth and drug resistance by ubiquitinating modifications to a variety of proteins. This review emphasizes the multifaceted role of Skp2 in a wide range of cancers and the mechanisms involved, highlighting the potential of Skp2 as a therapeutic target in cancer. Additionally, we describe the impactful influence exerted by Skp2 in various other diseases beyond cancer.

Intact regulation of G1/S transition renders esophageal squamous cell carcinoma sensitive to PI3Kα inhibitors

Phosphatidylinositol 3-kinase alpha (PI3Kα) inhibitors are currently evaluated for the therapy of esophageal squamous cell carcinoma (ESCC). It is of great importance to identify potential biomarkers to predict or monitor the efficacy of PI3Kα inhibitors in an aim to improve the clinical responsive rate in ESCC. Here, ESCC PDXs with CCND1 amplification were found to be more sensitive to CYH33, a novel PI3Kα-selective inhibitor currently in clinical trials for the treatment of advanced solid tumors including ESCC. Elevated level of cyclin D1, p21 and Rb was found in CYH33-sensitive ESCC cells compared to those in resistant cells. CYH33 significantly arrested sensitive cells but not resistant cells at G1 phase, which was associated with accumulation of p21 and suppression of Rb phosphorylation by CDK4/6 and CDK2. Hypo-phosphorylation of Rb attenuated the transcriptional activation of SKP2 by E2F1, which in turn hindered SKP2-mediated degradation of p21 and reinforced accumulation of p21. Moreover, CDK4/6 inhibitors sensitized resistant ESCC cells and PDXs to CYH33. These findings provided mechanistic rationale to evaluate PI3Kα inhibitors in ESCC patients harboring amplified CCND1 and the combined regimen with CDK4/6 inhibitors in ESCC with proficient Rb.

Small-molecule compounds inhibiting S-phase kinase-associated protein 2: A review

S-phase kinase-associated protein 2 (Skp2) is a substrate-specific adaptor in Skp1-CUL1-ROC1-F-box E3 ubiquitin ligases and widely regarded as an oncogene. Therefore, Skp2 has remained as an active anticancer research topic since its discovery. Accordingly, the structure of Skp2 has been solved and numerous Skp2 inhibiting compounds have been identified. In this review, we would describe the structural features of Skp2, introduce the ubiquitination function of SCF^Skp2, and summarize the diverse natural and synthetic Skp2 inhibiting compounds reported to date. The IC₅₀ data of the Skp2 inhibitors or inhibiting compounds in various kinds of tumors at cellular levels implied that the cancer type, stage and pathological mechanisms should be taken into consideration when selecting Skp2-inhibiting compound for cancer treatment.

MicroRNA-205-5p targets E2F1 to promote autophagy and inhibit pulmonary fibrosis in silicosis through impairing SKP2-mediated Beclin1 ubiquitination

Silicosis is an occupational disease characterized by extensive pulmonary fibrosis, and the underlying pathological process remains uncertain. Herein, we explored the molecular mechanism by which microRNA‐205‐5p (miR‐205‐5p) affects the autophagy of alveolar macrophages (AMs) and pulmonary fibrosis in mice with silicosis through the E2F transcription factor 1 (E2F1)/S‐phase kinase‐associated protein 2 (SKP2)/Beclin1 axis. Alveolar macrophages (MH‐S cells) were exposed to crystalline silica (CS) to develop an in vitro model, and mice were treated with CS to establish an in vivo model. Decreased Beclin1 and increased SKP2 and E2F1 were identified in mice with silicosis. We silenced or overexpressed miR‐205‐5p, E2F1, SKP2 and Beclin1 to investigate their potential roles in pulmonary fibrosis in vivo and autophagy in vitro. Recombinant adenovirus mRFP‐GFP‐LC3 was transduced into the MH‐S cells to assay autophagic flow. Knocking down Beclin1 promoted pulmonary fibrosis and suppressed the autophagy. Co‐immunoprecipitation and ubiquitination assays suggested that SKP2 induced K48‐linked ubiquitination of Beclin1. Furthermore, chromatin immunoprecipitation‐PCR revealed the site where E2F1 bound to the SKP2 promoter between 1638 bp and 1645 bp. As shown by dual‐luciferase reporter gene assay, the transfection with miR‐205‐5p mimic inhibited the luciferase activity of the wild‐type E2F1 3′untranslated region, suggesting that miR‐205‐5p targeted E2F1. Additionally, miR‐205‐5p overexpression increased autophagy and reduced the pulmonary fibrosis, while overexpression of E2F1 or SKP2 or inhibition of Beclin1 could annul this effect. The current study elucidated that miR‐205‐5p targeted E2F1, thereby inhibiting SKP2‐mediated Beclin1 ubiquitination to promote macrophage autophagy and inhibit pulmonary fibrosis in mice with silicosis.

Emerging Roles of SKP2 in Cancer Drug Resistance

More than half of all cancer patients receive chemotherapy, however, some of them easily acquire drug resistance. Resistance to chemotherapy has become a massive obstacle to achieve high rates of pathological complete response during cancer therapy. S-phase kinase-associated protein 2 (Skp2), as an E3 ligase, was found to be highly correlated with drug resistance and poor prognosis. In this review, we summarize the mechanisms that Skp2 confers to drug resistance, including the Akt-Skp2 feedback loop, Skp2-p27 pathway, cell cycle and mitosis regulation, EMT (epithelial-mesenchymal transition) property, enhanced DNA damage response and repair, etc. We also addressed novel molecules that either inhibit Skp2 expression or target Skp2-centered interactions, which might have vast potential for application in clinics and benefit cancer patients in the future.

Safrana l Prevents Prostate Cancer Recurrence by Blocking the Re-activation of Quiescent Cancer Cells via Downregulation of S-Phase Kinase-Associated Protein 2

The re-proliferation of quiescent cancer cells is considered to be the primary contributor to prostate cancer (Pca) recurrence and progression. In this study, we investigated the inhibitory effect of safranal, a monoterpene aldehyde isolated from Crocus sativus (saffron), on the re-proliferation of quiescent Pca cells in vitro and in vivo. The results showed that safranal efficiently blocked the re-activation of quiescent Pca cells by downregulating the G₀/G₁ cell cycle regulatory proteins CDK2, CDK4, CDK6, and phospho-Rb at Ser807/811 and elevating the levels of cyclin-dependent kinase inhibitors, p21 and p27. Further investigation on the underlying mechanisms revealed that safranal suppressed the mRNA and protein expression levels of Skp2, possibly through the deregulation of the transcriptional activity of two major transcriptional factors, E2F1 and NF-κB subunits. Moreover, safranal inhibited AKT phosphorylation at Ser473 and deregulated both canonical and non-canonical NF-κB signaling pathways. Safranal suppressed the tumor growth of quiescent Pca cell xenografts in vivo. Furthermore, safranal-treated tumor tissues exhibited a reduction in Skp2, E2F1, NF-κB p65, p-IκBα (Ser32), c-MYC, p-Rb (Ser807), CDK4, CDK6, and CDK2 and an elevation of p27 and p21 protein levels. Therefore, our findings demonstrate that safranal suppresses cell cycle re-entry of quiescent Pca cells in vitro and in vivo plausibly by repressing the transcriptional activity of two major transcriptional activators of Skp2, namely, E2F1 and NF-κB, through the downregulation of AKT phosphorylation and NF-κB signaling pathways, respectively.

The Impact of the Ubiquitin System in the Pathogenesis of Squamous Cell Carcinomas

The ubiquitin system is a dynamic regulatory pathway controlling the activity, subcellular localization and stability of a myriad of cellular proteins, which in turn affects cellular homeostasis through the regulation of a variety of signaling cascades. Aberrant activity of key components of the ubiquitin system has been functionally linked with numerous human diseases including the initiation and progression of human tumors. In this review, we will contextualize the importance of the two main components of the ubiquitin system, the E3 ubiquitin ligases (E3s) and deubiquitinating enzymes (DUBs), in the etiology of squamous cell carcinomas (SCCs). We will discuss the signaling pathways regulated by these enzymes, emphasizing the genetic and molecular determinants underlying their deregulation in SCCs.

Skp2 in the ubiquitin-proteasome system: A comprehensive review

The ubiquitin-proteasome system (UPS) is a complex process that regulates protein stability and activity by the sequential actions of E1, E2 and E3 enzymes to influence diverse aspects of eukaryotic cells. However, due to the diversity of proteins in cells, substrate selection is a highly critical part of the process. As a key player in UPS, E3 ubiquitin ligases recruit substrates for ubiquitination specifically. Among them, RING E3 ubiquitin ligases which are the most abundant E3 ubiquitin ligases contribute to diverse cellular processes. The multisubunit cullin-RING ligases (CRLs) are the largest family of RING E3 ubiquitin ligases with tremendous plasticity in substrate specificity and regulate a vast array of cellular functions. The F-box protein Skp2 is a component of CRL1 (the prototype of CRLs) which is expressed in many tissues and participates in multiple cellular functions such as cell proliferation, metabolism, and tumorigenesis by contributing to the ubiquitination and subsequent degradation of several specific tumor suppressors. Most importantly, Skp2 plays a pivotal role in a plethora of cancer-associated signaling pathways. It enhances cell growth, accelerates cell cycle progression, promotes migration and invasion, and inhibits cell apoptosis among others. Hence, targeting Skp2 may represent a novel and attractive strategy for the treatment of different human cancers overexpressing this oncogene. In this review article, we summarized the known roles of Skp2 both in health and disease states in relation to the UPS.

The Skp2 Pathway: A Critical Target for Cancer Therapy

Strictly regulated protein degradation by ubiquitin-proteasome system (UPS) is essential for various cellular processes whose dysregulation is linked to serious diseases including cancer. Skp2, a well characterized component of Skp2-SCF E3 ligase complex, is able to conjugate both K48-linked ubiquitin chains and K63-linked ubiquitin chains on its diverse substrates, inducing proteasome mediated proteolysis or modulating the function of tagged substrates respectively. Overexpression of Skp2 is observed in various human cancers associated with poor survival and adverse therapeutic outcomes, which in turn suggests that Skp2 engages in tumorigenic activity. To that end, the oncogenic properties of Skp2 are demonstrated by various genetic mouse models, highlighting the potential of Skp2 as a target for tackling cancer. In this article, we will describe the downstream substrates of Skp2 as well as upstream regulators for Skp2-SCF complex activity. We will further summarize the comprehensive oncogenic functions of Skp2 while describing diverse strategies and therapeutic platforms currently available for developing Skp2 inhibitors.

Irisin functions to inhibit malignant growth of human pancreatic cancer cells via downregulation of the PI3K/AKT signaling pathway

Introduction

Irisin is a newly identified cytokine that has gained increasing attention because of its potential therapeutic applications in metabolic diseases and human cancers. Recently, accumulating evidence indicates that irisin plays an important role in the development and metastasis of various tumors. The aim of this study was to evaluate the effects and underlying mechanisms of irisin on malignant growth of pancreatic cancer cells.

Materials and methods

The anti-proliferative effect of irisin was examined using the CCK-8 assay. Irisin-induced apoptosis was determined by the annexin V-FITC/PI staining assay. The effects of irisin on cell migration and invasion were assessed using the scratch-induced wound healing assay and transwell invasion assay, respectively. The expression and phosphorylation of signaling proteins were detected by Western blot analysis.

Results

Our results showed that irisin inhibited cell proliferation and induced apoptosis of pancreatic cancer cells in a dose-dependent manner. In addition, irisin decreased the migration and invasion of pancreatic cancer cells. Finally, Western blot analysis revealed that irisin downregulated the PI3K/AKT signaling pathway.

Conclusion

Our findings suggest that irisin is a novel therapeutic agent for pancreatic cancer.

A Derivate of Benzimidazole-Isoquinolinone Induces SKP2 Transcriptional Inhibition to Exert Anti-Tumor Activity in Glioblastoma Cells

We have previously shown that compound-7g inhibits colorectal cancer cell proliferation and survival by inducing cell cycle arrest and PI3K/AKT/mTOR pathway blockage. However, whether it has the ability to exert antitumor activity in other cancer cells and what is the exact molecular mechanism for its antiproliferation effect remained to be determined. In the present study, compound-7g exhibited strong activity in suppressing proliferation and growth of glioblastoma cells. The inhibitor selectively downregulated F-box protein SKP2 expression and upregulated cell cycle inhibitor p27, and then resulted in G1 cell cycle arrest. Mechanism analysis revealed that compound-7g also provokes the down-regulation of E2F-1, which acts as a transcriptional factor of SKP2. Further results indicated that compound-7g induced an increase of LC3B-II and p62, which causes a suppression of fusion between autophagosome and lysosome. Moreover, compound-7g mediated autophagic flux blockage promoted accumulation of ubiquitinated proteins and then led to endoplasmic reticulum stress. Our study thus demonstrated that pharmacological inactivation of E2F-1-SKP2-p27 axis is a promising target for restricting cancer progression.

PHD-finger domain protein 5A functions as a novel oncoprotein in lung adenocarcinoma

BACKGROUND

PHD-finger domain protein 5A (PHF5A) is a highly conserved small transcriptional regulator also involved in pre-mRNA splicing; however, its biological functions and molecular mechanisms in non-small cell lung cancer (NSCLC) have not yet been investigated. The purpose of this study was to determine the functional relevance and therapeutic potential of PHF5A in lung adenocarcinoma (LAC).

METHODS

The expression of PHF5A in LAC tissues and adjacent non-tumor (ANT) tissues was investigated using immunohistochemistry of a tissue microarray, qRT-PCR, western blot and bioinformatics. The function of PHF5A was determined using several in vitro assays and also in vivo assay by lentiviral vector-mediated PHF5A depletion in LAC cell lines.

RESULTS

PHF5A was highly upregulated in LAC tissues compared with the ANT counterparts, and closely associated with tumor progression and poor patient prognosis. These results were further confirmed by findings of the TCGA database. Moreover, functional studies demonstrated that PHF5A knockdown not only resulted in reduced cell proliferation, increased cell apoptosis, and cell cycle arrest, but also suppressed migration and invasion in LAC cells. PHF5A silencing was also found to inhibit LAC tumor growth in nude mice. Microarray and bioinformatics analyses revealed that PHF5A depletion led to dysregulation of multiple tumor signaling pathways; selected factors in key signaling pathways were verified in vitro.

CONCLUSIONS

The data suggest for the first time that PHF5A is an oncoprotein that contributes to LAC progression by regulating multiple signaling pathways, and may constitute a prognostic factor and potential new therapeutic target in NSCLC.

Skp2-dependent reactivation of AKT drives resistance to PI3K inhibitors

The PI3K-AKT kinase signaling pathway is frequently deregulated in human cancers, particularly breast cancer, where amplification and somatic mutations of PIK3CA occur with high frequency in patients. Numerous small-molecule inhibitors targeting both PI3K and AKT are under clinical evaluation, but dose-limiting toxicities and the emergence of resistance limit therapeutic efficacy. Various resistance mechanisms to PI3K inhibitors have been identified, including de novo mutations, feedback activation of AKT, or cross-talk pathways. We found a previously unknown resistance mechanism to PI3K pathway inhibition that results in AKT rebound activation. In a subset of triple-negative breast cancer cell lines, treatment with a PI3K inhibitor or depletion of PIK3CA expression ultimately promoted AKT reactivation in a manner dependent on the E3 ubiquitin ligase Skp2, the kinases IGF-1R (insulin-like growth factor 1 receptor) and PDK-1 (phosphoinositide-dependent kinase-1), and the cell growth and metabolism-regulating complex mTORC2 (mechanistic target of rapamycin complex 2), but was independent of PI3K activity or PIP₃ production. Resistance to PI3K inhibitors correlated with the increased abundance of Skp2, ubiquitylation of AKT, cell proliferation in culture, and xenograft tumor growth in mice. These findings reveal a ubiquitin signaling feedback mechanism by which PI3K inhibitor resistance may emerge in aggressive breast cancer cells.

The long non-coding RNA FOXD2-AS1 promotes bladder cancer progression and recurrence through a positive feedback loop with Akt and E2F1

Long non-coding RNAs (lncRNAs) have been identified as significant regulators in cancer progression. Positive feedback loops between lncRNAs and transcription factors have attracted increasing attention. Akt pathway plays a crucial role in bladder cancer growth and recurrence. In the present study, we demonstrate a novel regulatory pattern involving FOXD2-AS1, Akt, and E2F1. FOXD2-AS1 is highly expressed in bladder cancer and is associated with tumor stage, recurrence, and poor prognosis. Further experiments showed that FOXD2-AS1 promotes bladder cancer cell proliferation, migration, and invasion in vitro and in vivo. Microarray analysis demonstrated that FOXD2-AS1 negatively regulates the expression of Tribbles pseudokinase 3 (TRIB3), a negative regulator of Akt. Mechanistically, FOXD2-AS1 forms an RNA-DNA complex with the promoter of TRIB3, the transcriptional activity of which is subsequently repressed, and leads to the activation of Akt, which further increases the expression of E2F1, a vital transcription factor involved in the G/S transition. Interestingly, E2F1 could bind to the FOXD2-AS1 promoter region and subsequently enhance its transcriptional activity, indicating that FOXD2-AS1/Akt/E2F1 forms a feedback loop. In summary, this regulatory pattern of positive feedback may be a novel target for the treatment of bladder cancer and FOXD2-AS1 has the potential to be a new recurrence predictor.

BAG3 promotes proliferation of ovarian cancer cells via post-transcriptional regulation of Skp2 expression

Bcl-2 associated athanogene 3 (BAG3) contains a modular structure, through which BAG3 interacts with a wide range of proteins, thereby affording its capacity to regulate multifaceted biological processes. BAG3 is often highly expressed and functions as a pro-survival factor in many cancers. However, the oncogenic potential of BAG3 remains not fully understood. The cell cycle regulator, S-phase kinase associated protein 2 (Skp2) is increased in various cancers and plays an important role in tumorigenesis. The current study demonstrated that BAG3 promoted proliferation of ovarian cancer cells via upregulation of Skp2. BAG3 stabilized Skp2 mRNA via its 3'-untranslated region (UTR). The current study demonstrated that BAG3 interacted with Skp2 mRNA. In addition, miR-21-5p suppressed Skp2 expression, which was compromised by forced BAG3 expression. These results indicated that at least some oncogenic functions of BAG3 were mediated through posttranscriptional regulation of Skp2 via antagonizing suppressive action of miR-21-5p in ovarian cancer cells.

Simvastatin-induced cell cycle arrest through inhibition of STAT3/SKP2 axis and activation of AMPK to promote p27 and p21 accumulation in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is characterized by a poor prognosis and is one of the leading causes of cancer-related death worldwide. Simvastatin, an HMG-CoA reductase inhibitor, which decreases cholesterol synthesis by inhibiting mevalonate pathways and is widely used to treat cardiovascular diseases. Simvastatin exhibits anticancer effects against several malignancies. However, the molecular mechanisms underlying the anticancer effects of simvastatin on HCC are still not well understood. In this study, we demonstrated simvastatin-induced G0/G1 arrest by inducing p21 and p27 accumulation in HepG2 and Hep3B cells. Simvastatin also promoted AMP-activated protein kinase (AMPK) activation, which induced p21 upregulation by increasing its transcription. Consistent with this finding, we found genetic silencing of AMPK reduced p21 expression; however, AMPK silencing had no effect on p27 expression in HCC cells. Simvastatin decreased Skp2 expression at the transcriptional level, which resulted in p27 accumulation by preventing proteasomal degradation, an effect mediated by signal transducer and activator of transcription 3 (STAT3) inhibition. Constitutive STAT3 activation maintained high-level Skp2 expression and lower level p27 expression and significantly prevented G0/G1 arrest in simvastatin-treated HCC cells. Mevalonate decreased simvastatin-induced AMPK activation and rescued phospho-STAT3 and Skp2 expression in HCC cells, which resulted in the prevention of G0/G1 arrest through inhibition of p21 and p27 accumulation. Moreover, simvastatin significantly decreased tumor growth in HepG2 xenograft mice. Consistently, we found that simvastatin also increased p21 and p27 expression in tumor sections by reducing Skp2 expression and inducing AMPK activation and STAT3 suppression in the same tumor tissues. Taken together, these findings are demonstrative of the existence of a novel pathway in which simvastatin induces G0/G1 arrest by upregulating p21 and p27 by activating AMPK and inhibiting the STAT3–Skp2 axis, respectively. The results identify novel targets that explain the beneficial anticancer effects of simvastatin treatment on HCC in vitro and in vivo.

14-3-3β Depletion Drives a Senescence Program in Glioblastoma Cells Through the ERK/SKP2/p27 Pathway

The induction of senescence in cancer cells has recently been implicated as a mechanism of tumor regression in response to various modes of stress. 14-3-3 proteins are conserved scaffolding molecules that are involved in various cellular functions. Among the seven isoforms, 14-3-3β is specifically expressed in astrocytoma in correlation with the malignancy grade. We investigated the possible role of 14-3-3β in the regulation of senescence induction in A172 glioblastoma cells. The knockdown of 14-3-3β by specific small interfering RNA resulted in a significant change in cellular phenotypes and an increase in cells staining positive for senescence-associated β-galactosidase. Western blotting of the 14-3-3β-depleted A172 cells revealed increased p27 expression and decreased SKP2 expression, while the expression of p53 and p21 was not altered. Subsequently, we demonstrated that ERK is a key modulator of SKP2/p27 axis activity in 14-3-3β-mediated senescence based on the following: (1) 14-3-3β knockdown decreased p-ERK levels; (2) treatment with U0126, an MEK inhibitor, completely reproduced the senescence morphology as well as the expression profiles of p27 and SKP2; and (3) the senescence phenotypes induced by 14-3-3β depletion were considerably recovered by constitutively active ERK expression. Our results indicate that 14-3-3β negatively regulates senescence in glioblastoma cells via the ERK/SKP2/p27 pathway. Furthermore, 14-3-3β depletion also resulted in senescence phenotypes in U87 glioblastoma cells, suggesting that 14-3-3β could be targeted to induce premature senescence as a therapeutic strategy against glioblastoma progression.

Involvement of F-box proteins in esophageal cancer (Review)

The F-box proteins (FBPs) in esophageal tumorigenesis are pivotal as they govern a broad array of basic physiological responses including cell growth, cell death and DNA damage repair. Esophageal cancer (EC) is a common and highly aggressive cancer worldwide. Aberrant stabilization of crucial proteins participates in esophageal tumorigenesis. Recently, growing evidence has shown that FBPs play a critical role in oncogenesis, invasion, metastasis and prognosis assessment of EC. In this review we summarized published data on the roles of known FBPs, their respective substrates and the key signaling pathways, in the development of EC, aiming to uncover new ways for the rational design of targeted therapies in EC.

MicroRNA Profiling Implies New Markers of Gemcitabine Chemoresistance in Mutant p53 Pancreatic Ductal Adenocarcinoma

BACKGROUND

No reliable predictors of susceptibility to gemcitabine chemotherapy exist in pancreatic ductal adenocarcinoma (PDAC). MicroRNAs (miR) are epigenetic gene regulators with tumorsuppressive or oncogenic roles in various carcinomas. This study assesses chemoresistant PDAC for its specific miR expression pattern.

METHODS

Gemcitabine-resistant variants of two mutant p53 human PDAC cell lines were established. Survival rates were analyzed by cytotoxicity and apoptosis assays. Expression of 1733 human miRs was investigated by microarray and validated by qRT-PCR. After in-silico analysis of specific target genes and proteins of dysregulated miRs, expression of MRP-1, Bcl-2, mutant p53, and CDK1 was quantified by Western blot.

RESULTS

Both established PDAC clones showed a significant resistance to gemcitabine (p<0.02) with low apoptosis rate (p<0.001) vs. parental cells. MiR-screening revealed significantly upregulated (miR-21, miR-99a, miR-100, miR-125b, miR-138, miR-210) and downregulated miRs (miR-31*, miR-330, miR-378) in chemoresistant PDAC (p<0.05). Bioinformatic analysis suggested involvement of these miRs in pathways controlling cell death and cycle. MRP-1 (p<0.02) and Bcl-2 (p<0.003) were significantly overexpressed in both resistant cell clones and mutant p53 (p = 0.023) in one clone.

CONCLUSION

Consistent miR expression profiles, in part regulated by mutant TP53 gene, were identified in gemcitabine-resistant PDAC with significant MRP-1 and Bcl-2 overexpression. These results provide a basis for further elucidation of chemoresistance mechanisms and therapeutic approaches to overcome chemoresistance in PDAC.

Dysregulation of ubiquitin ligases in cancer

Ubiquitin ligases (UBLs) are critical components of the ubiquitin proteasome system (UPS), which governs fundamental processes regulating normal cellular homeostasis, metabolism, and cell cycle in response to external stress signals and DNA damage. Among multiple steps of the UPS system required to regulate protein ubiquitination and stability, UBLs define specificity, as they recognize and interact with substrates in a temporally- and spatially-regulated manner. Such interactions are required for substrate modification by ubiquitin chains, which marks proteins for recognition and degradation by the proteasome or alters their subcellular localization or assembly into functional complexes. UBLs are often deregulated in cancer, altering substrate availability or activity in a manner that can promote cellular transformation. Such deregulation can occur at the epigenetic, genomic, or post-translational levels. Alterations in UBL can be used to predict their contributions, affecting tumor suppressors or oncogenes in select tumors. Better understanding of mechanisms underlying UBL expression and activities is expected to drive the development of next generation modulators that can serve as novel therapeutic modalities. This review summarizes our current understanding of UBL deregulation in cancer and highlights novel opportunities for therapeutic interventions.

Implication of PI3K/Akt pathway in pancreatic cancer: When PI3K isoforms matter?

Pancreatic cancer belongs to the incurable family of solid cancers. Despite of a recent better understanding its molecular biology, and an increased number of clinical trials, there is still a lack for innovative targeted therapies to fight this deadly malignancy. PI3K/Akt signalling is one of the most commonly deregulated signalling pathways in cancer, which explains the massive attention from many pharmaceutical companies over the ten past years on these signalling molecules. The already developed small molecule inhibitors are currently under clinical trial in various cancer types. Class I PI3Ks have 4 isoforms for which the role in physiology starts to be well described in the literature. Data are more unclear for their differential involvement in oncogenesis. In this review, we will discuss about the cognitive and therapeutic potential of targeting this signalling pathway and in particular Class I PI3K isoforms for pancreatic cancer treatment. Isoform-specificity of PI3K inhibitors are currently designed to achieve the same goal as pan-PI3K inhibitors but without potential adverse effects. We will discuss if such strategy is relevant in pancreatic adenocarcinoma.

High expression of ErbB3 binding protein 1 (EBP1) predicts poor prognosis of pancreatic ductal adenocarcinoma (PDAC)

Recent studies have identified that ErbB3 binding protein 1 (EBP1) is broadly expressed in various cancer tissues and critically involved in plenty of biological processes in this regard. However, the functional role of EBP1 in pancreatic ductal adenocarcinoma (PDAC) has never been elucidated. In this study, we found that EBP1 could serve as a prognostic biomarker of PDAC. Western blot analysis revealed that EBP1 was remarkably upregulated in PDAC tissues and cell lines. Using immunohistochemical analysis, we showed that the expression of EBP1 was correlated with tumor size (P = 0.004), histological differentiation (P = 0.041), and tumor node metastasis (TNM) stage (P = 0.000). Notably, Kaplan-Meier curve showed that high expression of EBP1 predicted significantly worsened prognosis of PDAC patients (P = 0.001). In addition, knockdown of EBP1 expression suppressed PDAC cell proliferation and retarded cell cycle progression. Furthermore, depletion of EBP1 induced the apoptosis of Panc-1 cells. Of great interest, we found that EBP1 interacted with anti-apoptotic protein, Bcl-xL, and promoted its accumulation. In summary, our results suggest that EBP1 is a novel prognostic indicator and potential therapeutic target of PDAC, shedding new insights into the important role of EBP1 in cancer development.

PTEN plays an important role in thrombin-mediated lung cancer cell functions

Thrombin and its membrane receptor, protease-activated receptor 1 (PAR1), have been reported to promote the development of lung cancer in vitro and in vivo. However, the intracellular molecular mechanism or signaling pathway that mediates the cytological effects after the thrombin-receptor interaction is poorly understood. Our previous study observed that the expression of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) was downregulated in thrombin-stimulated lung cancer. In this study, the role of PTEN in thrombin-mediated cell function and the corresponding cell signaling pathway were studied in lung cancer cell Glc-82. The results indicated that thrombin downregulates the PTEN expression level and that PTEN plays an important role in thrombin-mediated Glc-82 functions, including cell cycle progression, cell apoptosis, and cell migration. The PI3K/AKT signaling pathway and its related proteins, including p27 and S phase kinase associated protein 2 (Skp2), are involved in the effects induced by PTEN downregulation. PAR1 plays a role in thrombin-mediated reduction of PTEN expression. This study suggested that the PTEN/PI3K/AKT signaling pathway plays an important role in thrombin/PAR1-mediated lung cancer cell growth and migration.

Heterogeneous nuclear ribonucleoprotein A2/B1 regulates the self-renewal and pluripotency of human embryonic stem cells via the control of the G1/S transition

Self-renewal and pluripotency of human embryonic stem cells (hESCs) are a complex biological process for maintaining hESC stemness. However, the molecular mechanisms underlying these special properties of hESCs are not fully understood. Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP A2/B1) is a multifunctional RNA-binding protein whose expression is related to cell proliferation and carcinogenesis. In this study, we found that hnRNP A2/B1 expression was localized to undifferentiated hESCs and decreased upon differentiation of hESCs. hnRNP A2/B1 knockdown reduced the number of alkaline phosphatase-positive colonies in hESCs and led to a decrease in the expression of pluripotency-associated transcription factors OCT4, NANOG, and SOX2, indicating that hnRNP A2/B1 is essential for hESC self-renewal and pluripotency. hnRNP A2/B1 knockdown increased the expression of gene markers associated with the early development of three germ layers, and promoted the process of epithelial-mesenchymal transition, suggesting that hnRNP A2/B1 is required for maintaining the undifferentiated and epithelial phenotypes of hESCs. hnRNP A2/B1 knockdown inhibited hESC proliferation and induced cell cycle arrest in the G0/G1 phase before differentiation via degradation of cyclin D1, cyclin E, and Cdc25A. hnRNP A2/B1 knockdown increased p27 expression and induced phosphorylation of p53 and Chk1, suggesting that hnRNP A2/B1 also regulates the G1/S transition of hESC cell cycle through the control of p27 expression and p53 and Chk1 activity. Analysis of signaling molecules further revealed that hnRNP A2/B1 regulated hESC proliferation in a PI3K/Akt-dependent manner. These findings provide for the first time mechanistic insights into how hnRNP A2/B1 regulates hESC self-renewal and pluripotency.

The use of multidimensional data to identify the molecular biomarker for pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease, and the patient has an extremely poor overall survival with a less than 5% 5-year survival rate. Development of potential biomarkers provides a critical foundation for the diagnosis of PDAC. In this project, we have adopted an integrative approach to simultaneously identify biomarker and generate testable hypothesis from multidimensional omics data. We first examine genes for which expression levels are correlated with survival data. The gene list was screened with TF regulation, predicted miRNA targets information, and KEGG pathways. We identified that 273 candidate genes are correlated with patient survival data. 12 TF regulation gene sets, 11 miRNAs targets gene sets, and 15 KEGG pathways are enriched with these survival genes. Notably, CEBPA/miRNA32/PER2 signaling to the clock rhythm qualifies this pathway as a suitable target for therapeutic intervention in PDAC. PER2 expression was highly associated with survival data, thus representing a novel biomarker for earlier detection of PDAC.

Genetic determinants and potential therapeutic targets for pancreatic adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer deaths in both men and women in the United States, carrying a 5-year survival rate of approximately 5%, which is the poorest prognosis of any solid tumor type. Given the dismal prognosis associated with PDAC, a more thorough understanding of risk factors and genetic predisposition has important implications not only for cancer prevention, but also for screening techniques and the development of personalized therapies. While screening of the general population is not recommended or practicable with current diagnostic methods, studies are ongoing to evaluate its usefulness in people with at least 5- to 10-fold increased risk of PDAC. In order to help identify high-risk populations who would be most likely to benefit from early detection screening tests for pancreatic cancer, discovery of additional pancreatic cancer susceptibility genes is crucial. Thus, specific gene-based, gene-product, and marker-based testing for the early detection of pancreatic cancer are currently being developed, with the potential for these to be useful as potential therapeutic targets as well. The goal of this review is to provide an overview of the genetic basis for PDAC with a focus on germline and familial determinants. A discussion of potential therapeutic targets and future directions in screening and treatment is also provided.

Efemp1 and p27(Kip1) modulate responsiveness of pancreatic cancer cells towards a dual PI3K/mTOR inhibitor in preclinical models

Pancreatic ductal adenocarcinoma (PDAC) remains a dismal disease with a poor prognosis and targeted therapies have failed in the clinic so far. Several evidences point to the phosphatidylinositol 3-kinase (PI3K)-mTOR pathway as a promising signaling node for targeted therapeutic intervention. Markers, which predict responsiveness of PDAC cells towards PI3K inhibitors are unknown. However, such markers are needed and critical to better stratify patients in clinical trials.

We used a large murine Kras^G12D- and PI3K (p110α^H1047R)-driven PDAC cell line platform to unbiased define modulators of responsiveness towards the dual PI3K-mTOR inhibitor Bez235. In contrast to other tumor models, we show that Kras^G12D- and PI3K (p110α^H1047R)-driven PDAC cell lines are equally sensitive towards Bez235. In an unbiased approach we found that the extracellular matrix protein Efemp1 controls sensitivity of murine PDAC cells towards Bez235. We show that Efemp1 expression is connected to the cyclin-dependent kinase inhibitor p27^Kip1. In a murine Kras^G12D- driven PDAC model, p27^Kip1 haploinsufficiency accelerates cancer development in vivo. Furthermore, p27^Kip1 controls Bez235 sensitivity in a gene dose-dependent fashion in murine PDAC cells and lowering of p27^Kip1 decreases Bez235 responsiveness in murine PDAC models.

Together, we define the Efemp1-p27^Kip1 axis as a potential marker module of PDAC cell sensitivity towards dual PI3K-mTOR inhibitors, which might help to better stratify patients in clinical trials.

Epidermal growth factor upregulates Skp2/Cks1 and p27kip1 in human extrahepatic cholangiocarcinoma cells

AIM: To evaluate the expression status of S-phase kinase-associated protein 2 (Skp2)/cyclin-dependent kinases regulatory subunit 1 (Cks1) and p27^kip1, and assess the prognostic significance of Skp2/Cks1 expression with p27^kip1 in patients with extrahepatic cholangiocarcinoma.

METHODS: Seventy-six patients who underwent curative resection for histologically confirmed extrahepatic cholangiocarcinoma at our institution from December 1994 to March 2008 were enrolled. Immunohistochemical staining for Skp2, Cks1, p27^kip1, and Ki67, along with other relevant molecular biologic experiments, were performed.

RESULTS: By Cox regression analyses, advanced age (> 65 years), advanced AJCC tumor stage, poorly differentiated histology, and higher immunostaining intensity of Skp2 were identified as independent prognostic factors in patients with extrahepatic cholangiocarcinoma. Exogenous epidermal growth factor (EGF, especially 0.1-10 ng/mL) significantly increased the proliferation indices by MTT assay and the mRNA levels of Skp2/Cks1 and p27^kip1 in SNU-1196, SNU-1079, and SNU-245 cells. The protein levels of Skp2/Cks1 (from nuclear lysates) and p27^kip1 (from cytosolic lysate) were also significantly increased in these cells. There were significant reductions in the protein levels of Skp2/Cks1 and p27^kip1 (from nuclear lysate) after the treatment of LY294002. By chromatin immunoprecipitation assay, we found that E2F1 transcription factor directly binds to the promoter site of Skp2.

CONCLUSION: Higher immunostaining intensity of Skp2/Cks1 was an independent prognostic factor for patients with extrahepatic cholangiocarcinoma. EGF upregulates the mRNA and protein levels of Skp2/Cks1 and p27^kip1via the PI3K/Akt pathway and direct binding of E2F1 transcription factor with the Skp2 promoter.

ATM regulates Cdt1 stability during the unperturbed S phase to prevent re-replication

Ataxia-telangiectasia mutated (ATM) plays crucial roles in DNA damage responses, especially with regard to DNA double-strand breaks (DSBs). However, it appears that ATM can be activated not only by DSB, but also by some changes in chromatin architecture, suggesting potential ATM function in cell cycle control. Here, we found that ATM is involved in timely degradation of Cdt1, a critical replication licensing factor, during the unperturbed S phase. At least in certain cell types, degradation of p27^Kip1 was also impaired by ATM inhibition. The novel ATM function for Cdt1 regulation was dependent on its kinase activity and NBS1. Indeed, we found that ATM is moderately phosphorylated at Ser1981 during the S phase. ATM silencing induced partial reduction in levels of Skp2, a component of SCF^Skp2 ubiquitin ligase that controls Cdt1 degradation. Furthermore, Skp2 silencing resulted in Cdt1 stabilization like ATM inhibition. In addition, as reported previously, ATM silencing partially prevented Akt phosphorylation at Ser473, indicative of its activation, and Akt inhibition led to modest stabilization of Cdt1. Therefore, the ATM-Akt-SCF^Skp2 pathway may partly contribute to the novel ATM function. Finally, ATM inhibition rendered cells hypersensitive to induction of re-replication, indicating importance for maintenance of genome stability.

A signal transduction pathway from TGF-β1 to SKP2 via Akt1 and c-Myc and its correlation with progression in human melanoma

Both SKP2 (S-phase kinase-associated protein 2) and transforming growth factor-β1 (TGF-β1) play important roles in cancer metastasis through different mechanisms: TGF-β1 via induction of epithelial-mesenchymal transition (EMT) and SKP2 via downregulating p27(kip1). Recent studies indicated that c-Myc and Akt1 were active players in metastasis. In this study we demonstrated a crosstalk between these pathways. Specifically, we found that TGF-β1 treatment increased SKP2 expression accompanied with increased phosphorylation of Akt1 and c-Myc protein accumulation during EMT. We demonstrated that Akt1 was required for TGF-β1-mediated SKP2 upregulation and that c-Myc transcription factor specifically bound to the promoter of SKP2 for its enhanced transcription. Analysis of 25 samples of normal human skin, nevi, and melanomas revealed a positive correlation between c-Myc and SKP2 accumulation. Furthermore, accumulation of SKP2 and c-Myc proteins was significantly higher in metastatic melanoma samples as compared with that in primary melanomas, which again was higher than that in normal skin or nevi. In summary, our results integrated TGF-β1 signals to SKP2 via Akt1 and c-Myc during EMT, and provided, to our knowledge, a previously unreported mechanistic molecular event for TGF-β1-induced metastasis in human melanoma.

A new function of the splicing factor SRSF2 in the control of E2F1-mediated cell cycle progression in neuroendocrine lung tumors

The transcription factor E2F1 belongs to the E2F family and plays a crucial role during cell cycle progression and apoptosis. Ser/Arg-Rich (SR) proteins are a family of RNA-binding phosphoproteins that control both constitutive and alternative pre-mRNA splicing events. We previously identified the SR protein SRSF2 as a new transcriptional target of E2F1 and demonstrated that both proteins cooperate to induce apoptosis in non-small cell lung carcinoma. In this study, we postulated that SRSF2 is also involved in the proliferative functions of E2F1. Using IHC, we first demonstrate that SRSF2 and its phosphorylated form (P-SRSF2) are overexpressed in neuroendocrine lung tumors that are highly proliferative tumors expressing high levels of E2F1. Importantly, we show a direct correlation between cyclin E, an E2F1-target gene controlling S phase, and P-SRSF2 proteins levels (p = 0.0083), suggesting a role of SRSF2 in E2F1-mediated cellular proliferation. Accordingly, using neuroendocrine lung carcinoma cell lines, we demonstrate that SRSF2 is a cell cycle-regulated protein involved in entry and progression into S phase. We also provide evidence that SRSF2 interacts with E2F1 and stimulates its transcriptional control of cell cycle target genes such as cyclin E. Finally, we show that inhibition of AKT signaling pathway prevents SRSF2 phosphorylation and activity toward E2F1 transcriptional function. Taken together, these results identify a new role of SRSF2 in the control of cell cycle progression and reinforce the functional link between SRSF2 and E2F1 proteins.

Selective requirement of PI3K/PDK1 signaling for Kras oncogene-driven pancreatic cell plasticity and cancer

Oncogenic Kras activates a plethora of signaling pathways, but our understanding of critical Ras effectors is still very limited. We show that cell-autonomous phosphoinositide 3-kinase (PI3K) and 3-phosphoinositide-dependent protein kinase 1 (PDK1), but not Craf, are key effectors of oncogenic Kras in the pancreas, mediating cell plasticity, acinar-to-ductal metaplasia (ADM), and pancreatic ductal adenocarcinoma (PDAC) formation. This contrasts with Kras-driven non-small cell lung cancer, where signaling via Craf, but not PDK1, is an essential tumor-initiating event. These in vivo genetic studies together with pharmacologic treatment studies in models of human ADM and PDAC demonstrate tissue-specific differences of oncogenic Kras signaling and define PI3K/PDK1 as a suitable target for therapeutic intervention specifically in PDAC.

LY294002 enhances inhibitory effect of gemcitabine on proliferation of human pancreatic carcinoma PANC-1 cells

Phosphatidylinositide 3-kinase (PI3K)/protein kinase B (PKB, Akt) pathway plays a major role in proliferation and survival of many types of cells. The inhibitory effect of LY294002, widely applied as an inhibitor of PI3K, in combination with gemcitabine on proliferation of PANC-1 cells was investigated. The expression of PI3K, phosphorylated Akt (p-Akt) and multidrug-resistance like protein (MRP) in normal pancreas tissues, chronic pancreatitis tissues and pancreatic carcinoma tissues was detected. The effects of LY294002 combined with gemcitabine on proliferation of PANC-1 cells and protein levels of p-Akt and MRP were detected. The results showed that the positive expression rate of PI3K, p-Akt and MRP in pancreatic carcinoma tissues was significantly higher than that in normal pancreas tissues and chronic pancreatitis tissues (P<0.01 and P<0.05 respectively). LY294002 could effectively enhance the inhibitory effect of gemcitabine on proliferation of PANC-1 cells. Furthermore, Western blotting revealed that LY294002 combined with gemcitabine reduced the protein levels of p-Akt and MRP, which contributed to the inhibition of proliferation. It is concluded that LY294002 in combination with gemcitabine may represent an alternative therapy for pancreatic carcinoma.

Prodigiosin down-regulates SKP2 to induce p27(KIP1) stabilization and antiproliferation in human lung adenocarcinoma cells

BACKGROUND AND PURPOSE

High levels of SKP2 are a poor prognostic factor in multiple human cancers and mostly correlate with low p27(KIP1) levels. Prodigiosin is a bacterial tripyrrole pigment with strong pro-apoptotic activity. Induction of cell cycle blockade underlies one of its anticancer actions but the mechanisms involved are unclear. The aim of this study was to explore the role of the SKP2-p27(KIP1) axis in prodigiosin's cytostatic effect on human lung adenocarcinoma cells.

EXPERIMENTAL APPROACH

Prodigiosin's effects on cell cycle progression and long-term cell proliferation of human lung adenocarcinoma cells were characterized by flow cytometry and colony formation assay, respectively. Real-time RT-PCR and promoter activity analyses were performed for assessing transcriptional control, while cycloheximide chase analysis evaluated protein stability. Immunoblotting was employed for mechanistic study.

KEY RESULTS

Prodigiosin increased p27(KIP1) expression mainly by stabilizing p27(KIP1) through transcriptional repression of SKP2. Importantly, SKP2 overexpression or p27(KIP1) depletion restored the colony forming capacity of prodigiosin-treated cells. Furthermore, prodigiosin induced PKB dephosphorylation, leading to PKB inhibition as revealed by decreased serine 9 phosphorylation of GSK-3β. Constitutive PKB activation reduced prodigiosin-induced SKP2 repression. Prodigiosin also down-regulated E2F1 (mediates PI3K/PKB-induced SKP2 transcription), but E2F1 overexpression failed to restore SKP2 expression in prodigiosin-treated cells.

CONCLUSIONS AND IMPLICATIONS

Transcriptional repression of SKP2 and the consequent accumulation of p27(KIP1) are essential for prodigiosin's antiproliferative action. Mechanistically, prodigiosin induces PKB inhibition to down-regulate SKP2 in a GSK-3β- and E2F1-independent manner. Our findings further implicate the potential for developing prodigiosin as a novel class of SKP2-targeting anticancer agent.

Sulforaphane inhibits pancreatic cancer through disrupting Hsp90-p50(Cdc37) complex and direct interactions with amino acids residues of Hsp90

Sulforaphane [1-isothiocyanato-4-(methyl-sulfinyl) butane)], an isothiocyanate derived from cruciferous vegetables, has been shown to possess potent chemopreventive activity. We analyzed the effect of sulforaphane on the proliferation of pancreatic cancer cells. Sulforaphane inhibited pancreatic cancer cell growth in vitro with IC(50)s of around 10-15 μM and induced apoptosis. In pancreatic cancer xenograft mouse model, administration of sulforaphane showed remarkable inhibition of tumor growth without apparent toxicity noticed. We found that sulforaphane induced the degradation of heat shock protein 90 (Hsp90) client proteins and blocked the interaction of Hsp90 with its cochaperone p50(Cdc37) in pancreatic cancer cells. Using nuclear magnetic resonance spectroscopy (NMR) with an isoleucine-specific labeling strategy, we overcame the protein size limit of conventional NMR and studied the interaction of sulforaphane with full-length Hsp90 dimer (170 kDa) in solution. NMR revealed multiple chemical shifts in sheet 2 and the adjacent loop in Hsp90 N-terminal domain after incubation of Hsp90 with sulforaphane. Liquid chromatography coupled to mass spectrometry further mapped a short peptide in this region that was tagged with sulforaphane. These data suggest a new mechanism of sulforaphane that disrupts protein-protein interaction in Hsp90 complex for its chemopreventive activity.

Gene expression of the tumour suppressor LKB1 is mediated by Sp1, NF-Y and FOXO transcription factors

The serine/threonine kinase LKB1 is a tumour suppressor that regulates multiple biological pathways, including cell cycle control, cell polarity and energy metabolism by direct phosphorylation of 14 different AMP-activated protein kinase (AMPK) family members. Although many downstream targets have been described, the regulation of LKB1 gene expression is still poorly understood. In this study, we performed a functional analysis of the human LKB1 upstream regulatory region. We used 200 base pair deletion constructs of the 5'-flanking region fused to a luciferase reporter to identify the core promoter. It encompasses nucleotides -345 to +52 relative to the transcription start site and coincides with a DNase I hypersensitive site. Based on extensive deletion and substitution mutant analysis of the LKB1 promoter, we identified four cis-acting elements which are critical for transcriptional activation. Using electrophoretic mobility shift assays as well as chromatin immunoprecipitations, we demonstrate that the transcription factors Sp1, NF-Y and two forkhead box O (FOXO) family members FOXO3 and FOXO4 bind to these elements. Overexpression of these factors significantly increased the LKB1 promoter activity. Conversely, small interfering RNAs directed against NF-Y alpha and the two FOXO proteins greatly reduced endogenous LKB1 expression and phosphorylation of LKB1's main substrate AMPK in three different cell lines. Taken together, these results demonstrate that Sp1, NF-Y and FOXO transcription factors are involved in the regulation of LKB1 transcription.

Foxo3a transcription factor is a negative regulator of Skp2 and Skp2 SCF complex

Skp2 (S-phase kinase-associated protein-2) SCF complex displays E3 ligase activity and oncogenic activity by regulating protein ubiquitination and degradation, in turn regulating cell cycle entry, senescence and tumorigenesis. The maintenance of the integrity of Skp2 SCF complex is critical for its E3 ligase activity. The Skp2 F-box protein is a rate-limiting step and key factor in this complex, which binds to its protein substrates and triggers ubiquitination and degradation of its substrates. Skp2 is found to be overexpressed in numerous human cancers, which has an important role in tumorigenesis. The molecular mechanism by which the function of Skp2 and Skp2 SCF complex is regulated remains largely unknown. Here we show that Foxo3a transcription factor is a novel and negative regulator of Skp2 SCF complex. Foxo3a is found to be a transcriptional repressor of Skp2 gene expression by directly binding to the Skp2 promoter, thereby inhibiting Skp2 protein expression. Surprisingly, we found for the first time that Foxo3a also displays a transcription-independent activity by directly interacting with Skp2 and disrupting Skp2 SCF complex formation, in turn inhibiting Skp2 SCF E3 ligase activity and promoting p27 stability. Finally, we show that the oncogenic activity of Skp2 is repressed by Foxo3a overexpression. Our results not only reveal novel insights into how Skp2 SCF complex is regulated, but also establish a new role for Foxo3a in tumor suppression through a transcription-dependent and independent manner.

Rapamycin resistance is linked to defective regulation of Skp2

The mammalian target of rapamycin (mTOR) plays a role in controlling malignant cellular growth. mTOR inhibitors, including rapamycin (sirolimus), are currently being evaluated in cancer trials. However, a significant number of tumors are rapamycin resistant. In this study, we report that the ability of rapamycin to downregulate Skp2, a subunit of the ubiquitin protein ligase complex, identifies tumors that are sensitive to rapamycin. RNA interference (RNAi)-mediated silencing of Skp2 in human tumor cells increased their sensitivity to rapamycin in vitro and inhibited the growth of tumor xenografts in vivo. Our findings suggest that Skp2 levels are a key determinant of antitumor responses to mTOR inhibitors, highlighting a potentially important pharmacogenomic marker to predict sensitivity to rapamycin as well as Skp2 silencing strategies for therapeutic purposes.

Stat3 induces oncogenic Skp2 expression in human cervical carcinoma cells

Dysregulated Skp2 function promotes cell proliferation, which is consistent with observations of Skp2 over-expression in many types of human cancers, including cervical carcinoma (CC). However, the molecular mechanisms underlying elevated Skp2 expression have not been fully explored. Interleukin-6 (IL-6) induced Stat3 activation is viewed as crucial for multiple tumor growth and metastasis. Here, we demonstrate that Skp2 is a direct transcriptional target of Stat3 in the human cervical carcinoma cells. Our data show that IL-6 administration or transfection of a constitutively activated Stat3 in HeLa cells activates Skp2 mRNA transcription. Using luciferase reporter and ChIP assays, we show that Stat3 binds to the promoter region of Skp2 and promotes its activity through recruiting P300. As a result of the increase of Skp2 expression, endogenous p27 protein levels are markedly decreased. Thus, our results suggest a previously unknown Stat3-Skp2 molecular network controlling cervical carcinoma development.

LY294002 combined with gemcitabine inhibits p-Akt and MRP expression in human pancreatic carcinoma PANC-1 cells

AIM: To investigate the effect of LY294002 (a PI3K inhibitor) combined with gemcitabine on p-Akt and multidrug resistance-associated protein (MRP) expression in human pancreatic carcinoma PANC-1 cells.

METHODS: After PANC-1 cells were treated with different concentrations of LY294002 and gemcitabine, the expression of MRP mRNA and p-Akt and MRP proteins was detected by semi-quantitative RT-PCR and Western blot, respectively.

RESULTS: Compared to untreated control cells, treatment with LY294002 combined with gemcitabine significantly decreased the expression of MRP mRNA (1.47 ± 0.03, 1.31 ± 0.05, 1.02 ± 0.04, 0.76 ± 0.06, 0.37 ± 0.02, P < 0.05) and p-Akt and MRP proteins (p-Akt: 0.80±0.02, 0.63±0.01, 0.52±0.01, 0.41 ± 0.02, 0.35 ± 0.02, P < 0.05; MRP: 0.93 ± 0.05, 0.87 ± 0.03, 0.81 ± 0.03, 0.71 ± 0.02, 0.40 ± 0.03, both P < 0.05) in a concentration-dependent manner.

CONCLUSION: LY294002 could effectively strengthen the sensitivity of human pancreatic carcinoma PANC-1 cells to gemcitabine. LY294002 may down-regulate MRP transcription by inhibiting p-Akt expression and therefore reverse resistance of PANC-1 cells to gemcitabine.

Effects of E2F1-cyclin E1-E2 circuit down regulation in hepatocellular carcinoma cells

BACKGROUND

No effective therapy is available for hepatocellular carcinoma. To identify novel therapeutic strategies, we explored the effects of the depletion of E2F1, cyclin E1-E2 whose inter-relationships in hepatocellular carcinoma cell proliferation have never been defined.

METHODS

siRNA-mediated depletion of the targets was studied in the hepatocellular carcinoma cells HepG2, HuH7 and JHH6 characterized by high, medium and low hepatocyte differentiation grade, respectively; a model of normal human hepatocytes was also considered.

RESULTS

The depletion of each target mRNA reduced the levels of the other two mRNAs, thus demonstrating a close regulatory control, also confirmed by over-expression experiments. At the protein level, an exception to this trend was observed for cyclinE1 whose amount increased upon cyclin E2 (HepG2, HuH7, JHH6) and E2F1 (HepG2) depletion. In HepG2, reduced cyclinE1 proteolysis accounted for this observation. Additionally, cyclin E1-E2-E2F1 targeting decreased the levels of cyclin A2 mRNA and of the hyper-phosphorylated form of pRb thus leading to an S-phase cell decrease; migration was impaired as well. Finally, the model of human hepatocytes used was clearly less affected by target mRNAs depletion than hepatocellular carcinoma cells.

CONCLUSION

Our data provide novel mutual relationships amongst cyclin E1-E2-E2F1 and indicate their role in sustaining hepatocellular carcinoma cell proliferation/migration, validating the concept of an anti-cyclin E1-E2-E2F1 therapeutic approach for hepatocellular carcinoma.

Skp2: a novel potential therapeutic target for prostate cancer

Prostate cancer is the most frequently diagnosed tumor in men and the second most common cause of cancer-related death for males in the United States. It has been shown that multiple signaling pathways are involved in the pathogenesis of prostate cancer, such as androgen receptor (AR), Akt, Wnt, Hedgehog (Hh) and Notch. Recently, burgeoning amounts of evidence have implicated that the F-box protein Skp2 (S-phase kinase associated protein 2), a well-characterized oncoprotein, also plays a critical role in the development and progression of prostate cancer. Therefore, this review discusses the recent literature regarding the function and regulation of Skp2 in the pathogenesis of prostate cancer. Furthermore, we highlight that Skp2 may represent an attractive therapeutic target, thus warrants further development of agents to target Skp2, which could have significant therapeutic impact on prostate cancer.

Molecular imaging of Cathepsin E-positive tumors in mice using a novel protease-activatable fluorescent probe

The purpose of this study is to demonstrate the ability of imaging Cathepsin E (Cath E) positive tumors in living animals through selective targeting of Cath E proteolytic activity using a sensitive molecular imaging agent.

METHODS

A peptide-based Cath E imaging probe and a control probe were synthesized for this study. Human Cath E-positive cancer cells (MPanc96-E) were implanted subcutaneously in nude mice. Tumor-bearing mice were examined in vivo with near-infrared fluorescence (NIRF) imaging at various time points after intravenous injection of the Cath E sensing imaging probe. Excised organs and tissues of interest were further imaged ex vivo.

RESULTS

Upon specific Cath E proteolytic activation, the NIRF signal of the imaging probe a was converted from an optically quenched initial state to a highly fluorescent active state. Imaging probe a was able to highlight the Cath E-positive tumors as early as 24 h post injection. Fluorescent signal in tumor was 3-fold higher than background. The confined specificity of imaging probe a to tumor associated Cath E was verified by using control imaging probe b. Both in vivo and ex vivo imaging results confirmed the superior selectivity and sensitivity of imaging probe a in Cath E imaging.

CONCLUSIONS

The small animal studies demonstrated the capability of probe a for imaging Cath E-positive tumors. The developed optical probe could be applied in early diagnostic imaging and guiding subsequent surgical procedure.

Pancreatic ductal adenocarcinoma and transcription factors: role of c-Myc

INTRODUCTION

Deregulated expression/activation of transcription factors is a key event in the establishment and progression of human cancer. Furthermore, most oncogenic signaling pathways converge on sets of transcription factors that ultimately control gene expression patterns resulting in cancer development, progression, and metastasis.

METHODS

Ductal pancreatic adenocarcinoma (PDA) is the main type of pancreatic cancer and the fourth leading cause of cancer mortality in the Western world. The early stage of the disease is characterized by pancreatic intraepithelial neoplasia lesions bearing mutations in the K-RAS proto-oncogene, which progress to malignant PDA by accumulating additional mutations in the tumor suppressor gene CDKN2A (p16) and in SMAD4 and TP53 transcription factors. The involvement of other signaling pathways in PDA development and progression is an active area of research which may help to clarify the critical steps of this devastating disease.

RESULTS

In this regard, several in vitro and in vivo data have demonstrated the contribution of the transcription factor c-Myc to pancreatic carcinogenesis although the molecular mechanisms are poorly understood. c-Myc is a proto-oncogene which has a pivotal function in growth control, differentiation and apoptosis and is known to act as a downstream transcriptional effector of many signaling pathways involved in these processes. It is regulated at multiple levels and its abnormal expression contributes to the genesis of many human tumors.

CONCLUSIONS

This review focuses on the role of c-Myc in pancreatic embryonic development and homeostasis as well as its involvement on pancreatic tumorigenesis. Evidences showing that c-Myc function is highly dose and cell context dependent, together with its recently demonstrated ability to reprogram somatic cells towards a pluripotent stem cell-like state, indicate that the role of c-Myc in pancreas pathophysiology might have been previously underscored.