Thiazolidinediones modulate the expression of beta-catenin and other cell-cycle regulatory proteins by targeting the F-box proteins of Skp1-Cul1-F-box protein E3 ubiquitin ligase independently of peroxisome proliferator-activated receptor gamma

Medicinal Perspective of 2,4-Thiazolidinediones Derivatives: An Insight into Recent Advancements

2,4-Thiazolidinedione derivatives represent nitrogen-containing heterocyclic compounds utilized in type 2 diabetes mellitus management. Recent advances in medicinal chemistry have unveiled diverse therapeutic potentials and structural modifications of these derivatives. This review delves into novel TZD derivatives, encompassing their synthesis, structure-activity relationships, and pharmacokinetic profiles. Various therapeutic potentials of TZDs are explored, including anticancer, antimicrobial, anti-inflammatory, antioxidant, anticonvulsant, antihyperlipidemic, anticorrosive, and antitubercular activities. Additionally, it addresses mitigating side effects associated with marketed TZD derivatives such as weight gain, oedema, fractures, and congestive heart failure in type 2 diabetes mellitus management. The review elaborates on in vivo, in vitro, and ex vivo studies supporting different biological activities, alongside predicting ADME and drug-likeness properties of TZDs. Computational studies are also integrated to elucidate binding modes and affinities of novel TZD derivatives. Furthermore, a plethora of novel TZD derivatives with varied and enhanced therapeutic potentials are presented, warranting further evaluation of their biological activities.

Inhibitors Targeting the F-BOX Proteins

F-box proteins are involved in multiple cellular processes through ubiquitylation and consequent degradation of targeted substrates. Any significant mutation in F-box protein-mediated proteolysis can cause human malformations. The various cellular processes F-box proteins involved include cell proliferation, apoptosis, invasion, angiogenesis, and metastasis. To target F-box proteins and their associated signaling pathways for cancer treatment, researchers have developed thousands of F-box inhibitors. The most advanced inhibitor of FBW7, NVD-BK M120, is a powerful P13 kinase inhibitor that has been proven to bring about apoptosis in cancerous human lung cells by disrupting levels of the protein known as MCL1. Moreover, F-box Inhibitors have demonstrated their efficacy for treating certain cancers through targeting particular mutated proteins. This paper explores the key studies on how F-box proteins act and their contribution to malignancy development, which fabricates an in-depth perception of inhibitors targeting the F-box proteins and their signaling pathways that eventually isolate the most promising approach to anti-cancer treatments.

Beta-Transducin Repeats-Containing Proteins as an Anticancer Target

Beta-transducin repeat-containing proteins (β-TrCPs) are E3-ubiquitin-ligase-recognizing substrates and regulate proteasomal degradation. The degradation of β-TrCPs' substrates is tightly controlled by various external and internal signaling and confers diverse cellular processes, including cell cycle progression, apoptosis, and DNA damage response. In addition, β-TrCPs function to regulate transcriptional activity and stabilize a set of substrates by distinct mechanisms. Despite the association of β-TrCPs with tumorigenesis and tumor progression, studies on the mechanisms of the regulation of β-TrCPs' activity have been limited. In this review, we studied publications on the regulation of β-TrCPs themselves and analyzed the knowledge gaps to understand and modulate β-TrCPs' activity in the future.

Targeting CSC-related transcription factors by E3 ubiquitin ligases for cancer therapy

Evidence has revealed that transcription factors play essential roles in regulation of multiple cellular processes, including cell proliferation, metastasis, EMT, cancer stem cells and chemoresistance. Dysregulated expression levels of transcription factors contribute to tumorigenesis and malignant progression. The expression of transcription factors is tightly governed by several signaling pathways, noncoding RNAs and E3 ubiquitin ligases. Cancer stem cells (CSCs) have been validated in regulation of tumor metastasis, reoccurrence and chemoresistance in human cancer. Transcription factors have been verified to participate in regulation of CSC formation, including Oct4, SOX2, KLF4, c-Myc, Nanog, GATA, SALL4, Bmi-1, OLIG2, POU3F2 and FOX proteins. In this review article, we will describe the critical role of CSC-related transcription factors. We will further discuss which E3 ligases regulate the degradation of these CSC-related transcription factors and their underlying mechanisms. We also mentioned the functions and mechanisms of EMT-associated transcription factors such as ZEB1, ZEB2, Snail, Slug, Twist1 and Twist2. Furthermore, we highlight the therapeutic potential via targeting E3 ubiquitin ligases for modulation of these transcription factors.

Case Report: Congenital Brain Dysplasia, Developmental Delay and Intellectual Disability in a Patient With a 7q35-7q36.3 Deletion

7q terminal deletion syndrome is a rare condition presenting with multiple congenital malformations, including abnormal brain and facial structures, developmental delay, intellectual disability, abnormal limbs, and sacral anomalies. At least 40 OMIM genes located in the 7q34-7q36.3 region act as candidate genes for these phenotypes, of which SHH, EN2, KCNH2, RHEB, HLXB9, EZH2, MNX1 and LIMR1 may be the most important. In this study, we discuss the case of a 2.5-year-old male patient with multiple malformations, congenital brain dysplasia, developmental delay, and intellectual disability. A high-resolution genome-wide single nucleotide polymorphism array and real-time polymerase chain reaction were performed to detect genetic lesions. A de novo 9.4 Mb deletion in chromosome region 7q35-7q36.3 (chr7:147,493,985-156,774,460) was found. This chromosome region contains 68 genes, some of which are candidate genes for each phenotype. To the best of our knowledge, this is a rare case report of 7q terminal deletion syndrome in a Chinese patient. Our study identifies a rare phenotype in terms of brain structure abnormalities and cerebellar sulcus widening in patients with deletion in 7q35-7q36.3.

F-Box Proteins and Cancer

Controlled protein degradation is essential for the operation of a variety of cellular processes including cell division, growth, and differentiation. Identification of the relations between ubiquitin ligases and their substrates is key to understanding the molecular basis of cancer development and to the discovery of novel targets for cancer therapeutics. F-box proteins function as the substrate recognition subunits of S-phase kinase-associated protein 1 (SKP1)−Cullin1 (CUL1)−F-box protein (SCF) ubiquitin ligase complexes. Here, we summarize the roles of specific F-box proteins that have been shown to function as tumor promoters or suppressors. We also highlight proto-oncoproteins that are targeted for ubiquitylation by multiple F-box proteins, and discuss how these F-box proteins are deployed to regulate their cognate substrates in various situations.

F-box proteins and cancer: an update from functional and regulatory mechanism to therapeutic clinical prospects

E3 ubiquitin ligases play a critical role in cellular mechanisms and cancer progression. F-box protein is the core component of the SKP1-cullin 1-F-box (SCF)-type E3 ubiquitin ligase and directly binds to substrates by various specific domains. According to the specific domains, F-box proteins are further classified into three sub-families: 1) F-box with leucine rich amino acid repeats (FBXL); 2) F-box with WD 40 amino acid repeats (FBXW); 3) F-box only with uncharacterized domains (FBXO). Here, we summarize the substrates of F-box proteins, discuss the important molecular mechanism and emerging role of F-box proteins especially from the perspective of cancer development and progression. These findings will shed new light on malignant tumor progression mechanisms, and suggest the potential role of F-box proteins as cancer biomarkers and therapeutic targets for future cancer treatment.

Regulation of Transcription Factor SP1 by the β-Catenin Destruction Complex Modulates Wnt Response

The ubiquitous transcription factor specificity protein 1 (SP1) is heavily modified posttranslationally. These modifications are critical for switching its functions and modulation of its transcriptional activity and DNA binding and stability. However, the mechanism governing the stability of SP1 by cellular signaling pathways is not well understood. Here, we provide biochemical and functional evidence that SP1 is an integral part of the Wnt signaling pathway. We identified a phosphodegron motif in SP1 that is specific to mammals. In the absence of Wnt signaling, glycogen synthase kinase 3β (GSK3β)-mediated phosphorylation and β-TrCP E3 ubiquitin ligase-mediated ubiquitination are required to induce SP1 degradation. When Wnt signaling is on, SP1 is stabilized in a β-catenin-dependent manner. SP1 directly interacts with β-catenin, and Wnt signaling induces the stabilization of SP1 by impeding its interaction with β-TrCP and axin1, components of the destruction complex. Wnt signaling suppresses ubiquitination and subsequent proteosomal degradation of SP1. Furthermore, SP1 regulates Wnt-dependent stability of β-catenin and their mutual stabilization is critical for target gene expression, suggesting a feedback mechanism. Upon stabilization, SP1 and β-catenin cooccupy the promoters of TCFL2/β-catenin target genes. Collectively, this study uncovers a direct link between SP1 and β-catenin in the Wnt signaling pathway.

Crosstalk between the Androgen Receptor and PPAR Gamma Signaling Pathways in the Prostate

Nuclear receptors are a superfamily of ligand-activated transcription factors that play critical roles in the regulation of normal biological processes and several disease states. Of the nuclear receptors expressed within the prostate, the androgen receptor (AR) promotes the differentiation of prostatic epithelial cells and stimulates production of enzymes needed for liquefaction of semen. Multiple forms of AR also promote the growth of both early and late stage prostate cancers. As a result, drugs that target the AR signaling pathway are routinely used to treat patients with advanced forms of prostate cancer. Data also suggest that a second member of the nuclear receptor superfamily, the peroxisome proliferator activated receptor gamma (PPARγ), is a tumor suppressor that regulates growth of normal prostate and prostate cancers. Recent studies indicate there is a bidirectional interaction between AR and PPARγ, with each receptor influencing the expression and/or activity of the other within prostatic tissues. In this review, we examine how AR and PPARγ each regulate the growth and development of normal prostatic epithelial cells and prostate cancers. We also discuss interactions between the AR and PPARγ signaling pathways and how those interactions may influence prostate biology.

In vitro and in vivo cytotoxicity of troglitazone in pancreatic cancer

Background

Troglitazone (TGZ) is a peroxisome proliferator-activated receptor gamma (PPARγ) agonist that has been investigated as a potential chemopreventive and chemotherapeutic agent. However, the antitumor efficacy and mechanisms of TGZ in pancreatic cancer have not been extensively investigated. This study was performed to investigate the in vitro and in vivo effects of TGZ against pancreatic cancer cell lines, as well as its action mechanisms in terms of PPARγ dependency and the Akt and mitogen-activated protein kinase (MAPK) pathways. We also evaluated the effects of TGZ on cell invasion and migration.

Methods

MIA Paca2 and PANC-1 human pancreatic cancer cell lines were used. Cell viability and caspase-3 activity were detected using fluorescent reagents, and chromatin condensation was observed after staining the cells with Hoechst 33342. Protein expression levels were detected by western blot analysis. Invasion and migration assays were performed using 24-well chambers. The in vivo antitumor effects of TGZ were investigated in nude mice inoculated with MIA Paca2 cells. Mice were orally administered TGZ (200 mg/kg) every day for 5 weeks, and tumor volumes were measured bi-dimensionally.

Results

TGZ showed dose-dependent cytotoxicity against both cell lines, which was not attenuated by a PPARγ inhibitor. Further, TGZ induced chromatin condensation, elevated caspase-3 activity, and increased Bax/Bcl-2 relative expression in MIA Paca2 cells. TGZ also increased phosphorylation of Akt and MAPK (ERK/p38/JNK) in both cell lines, and a JNK inhibitor significantly increased the viability of MIA Paca2 cells. TGZ moderately inhibited cell migration. Tumor growth in the MIA Paca2 xenograft model was inhibited by TGZ administration, while mouse body weights in the treated group were not different from those of the vehicle administration group.

Conclusion

We demonstrated for the first time the in vivo antitumor effects of TGZ in pancreatic cancer without marked adverse effects. TGZ induced mitochondria-mediated apoptosis in MIA Paca2 cells, and its cytotoxic effects were PPARγ-independent and occurred via the JNK pathway. Our results indicate that TGZ is a potential approach for the treatment of pancreatic cancer and warrants further studies regarding its detailed mechanisms and clinical efficacy.

Cyclin D1, cancer progression, and opportunities in cancer treatment

Mammalian cells encode three D cyclins (D1, D2, and D3) that coordinately function as allosteric regulators of cyclin-dependent kinase 4 (CDK4) and CDK6 to regulate cell cycle transition from G1 to S phase. Cyclin expression, accumulation, and degradation, as well as assembly and activation of CDK4/CDK6 are governed by growth factor stimulation. Cyclin D1 is more frequently dysregulated than cyclin D2 or D3 in human cancers, and as such, it has been more extensively characterized. Overexpression of cyclin D1 results in dysregulated CDK activity, rapid cell growth under conditions of restricted mitogenic signaling, bypass of key cellular checkpoints, and ultimately, neoplastic growth. This review discusses cyclin D1 transcriptional, translational, and post-translational regulations and its biological function with a particular focus on the mechanisms that result in its dysregulation in human cancers.

Inhibition of ERK activity enhances the cytotoxic effect of peroxisome proliferator-activated receptor γ (PPARγ) agonists in HeLa cells

In this study, we examined whether the peroxisome proliferator-activated receptor γ (PPARγ) agonists, ciglitazone (CGZ) and troglitazone (TGZ), induce cell death in human cervical cancer HeLa cells. The cells were treated with a range of CGZ or TGZ doses for 24 or 48 h. Low concentrations of CGZ (≤10 μM) or TGZ (≤20 μM) had no effect on cell viability whereas higher doses induced cell death in a time- and dose-dependent manner as evidenced by the detection of activated caspase-3 and PARP cleavage. Treatment with the PPARγ antagonist GW9662 followed by PPARγ agonists did not increase CGZ- or TGZ-induced cell death, indicating that PPARγ agonists induced HeLa cell death independently of PPARγ. Moreover, ERK1/2 activation was observed at a CGZ concentration of 25 μM and a TGZ concentration of 35 μM, both of which induced cell death. To elucidate the role of ERK1/2 activated by the two PPARγ agonists, the effect of U0126, an inhibitor of ERK1/2, on PPARγ-agonist-induced cell death was examined. Treatment with 10 or 20 μM U0126 followed by CGZ or TGZ induced the down-regulation of ERK1/2 activity and a decrease in Bcl-2 expression accompanied by the collapse of mitochondrial membrane potential, which in turn significantly enhanced CGZ- or TGZ-induced apoptotic cell death. Our results suggest that PPARγ agonists are capable of inducing apoptotic cell death in HeLa cells independently of PPARγ and that inhibition of ERK1/2 activity offers a strategy to enhance the cytotoxicity of PPARγ agonists in the treatment of cervical cancer.

Δ2-Troglitazone promotes cytostatic rather than pro-apoptotic effects in breast cancer cells cultured in high serum conditions

We have previously shown that Δ2-Troglitazone (Δ2-TGZ) displayed anticancer effects on breast cancer cell lines grown in low serum conditions (1% fetal calf serum (FCS)). The present study was performed in order to characterize the effects of Δ2-TGZ in high serum containing medium and to determine if starvation could influence the response of breast cancer cells to this compound, keeping in mind the potential interest for breast cancer therapy. We observed that in high serum conditions (10% FCS), a 48 h treatment with Δ2-TGZ induced a decrease in cell numbers in MDA-MB-231 and MCF-7 breast cancer cell lines. The IC₅₀ values were higher than in low serum conditions. Furthermore, in contrast to our previous results obtained in 1% FCS conditions, we observed that in 10% FCS-containing medium, MCF-7 cells were more sensitive to Δ2-TGZ than MDA-MB-231 cells. Δ2-TGZ also induced endoplasmic reticulum (ER) stress mainly in MDA-MB-231 cells. Besides, in high serum conditions, Δ2-TGZ induced a G₀/G₁ cell cycle arrest, an inhibition of BrdU incorporation and a reduced level of cyclin D1. We observed a limited cleavage of PARP and a limited proportion of cells in sub-G₁ phase. Thus, in high serum conditions, Δ2-TGZ displayed cytostatic effects rather than apoptosis as previously reported in 1% FCS-containing medium. Our results are in accordance with studies suggesting that serum starvation could potentiate the action of diverse anti-cancer agents.

Modulation of glycogen synthase kinase-3β following TRAIL combinatorial treatment in cancer cells

Glycogen Synthase Kinase-3β (GSK3β) is a serine/threonine kinase, known to regulate various cellular processes including proliferation, differentiation, survival, apoptosis as well as TRAIL-resistance. Thus pathways that can modulate GSK3β axis are important targets for cancer drug development. Our earlier studies have shown that combinatorial treatment with Troglitazone (TZD) and TRAIL can induce apoptosis in TRAIL-resistant cancer cells. The current studies were undertaken to investigate whether GSK3β pathway was modulated during this apoptosis. Our results indicated an increase in inhibitory GSK3β^Ser9 phosphorylation during apoptosis, mediated via AKT. At a later time, however, TZD alone and TRAIL-TZD combination produced a dramatic reduction of GSK3β expression, which was abolished by cycloheximide. Luciferase assays with GSK3β-luc promoter reporter showed that TZD can effectively antagonize GSK3β promoter activity. Since TZD is a ligand for transcription factor PPARγ and can activate AMPK, we determined their roles on antagonism of GSK3β. Knockdown of PPARγ was unable to restore GSK3β expression or antagonize GSK3β^Ser9 phosphorylation. Although pretreatment with Compound C (pharmacological inhibitor of AMPK) partially rescued GSK3β expression, knockdown of AMPKα1 or α2 alone or in combination were ineffective. These studies suggested a novel PPARγ-AMPK-independent mechanism of targeting GSK3β by TZD, elucidation of which might provide newer insights to improve our understanding of TRAIL-resistance.

A novel HIF-1α-integrin-linked kinase regulatory loop that facilitates hypoxia-induced HIF-1α expression and epithelial-mesenchymal transition in cancer cells

Here, we described a novel regulatory feedback loop in which hypoxia induces integrin-linked kinase (ILK) expression through a HIF-1α-dependent mechanism and ILK, in turn, stimulates HIF-1α expression through cell type- and cell context-dependent pathways. HIF-1α increased ILK via transcriptional activation. ILK increased HIF-1α levels by promoting mTOR-mediated translation in PC-3 and MCF-7 cells, and by blocking GSK3β-mediated degradation in LNCaP cells, consistent with the cell line-/cellular context-specific functions of ILK as a Ser473-Akt kinase. We show that ILK can account for the effects of hypoxia on Akt, mTOR, and GSK3β phosphorylation. Also, ILK can de-repress HIF-1α signaling through the YB-1-mediated inhibition of Foxo3a expression. In concert with HIF-1α, these downstream effectors promote epithelial-mesenchymal transition (EMT) through modulation of Snail and Zeb1. Thus, the ILK-HIF-1α regulatory loop could underlie the maintenance of high HIF-1α expression levels and the promotion of EMT under hypoxic conditions. Finally, we show that the small-molecule ILK inhibitor T315 can disrupt this regulatory loop in vivo and suppress xenograft tumor growth, thereby providing proof-of-concept that targeting ILK represents an effective strategy to block HIF-1α expression and aggressive phenotype in cancer cells.

The role of PPARγ-mediated signalling in skin biology and pathology: new targets and opportunities for clinical dermatology

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that modulate the expression of multiple different genes involved in the regulation of lipid, glucose and amino acid metabolism. PPARs and cognate ligands also regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. This includes a role in mediating skin and pilosebaceous unit homoeostasis: PPARs appear to be essential for maintaining skin barrier permeability, inhibit keratinocyte cell growth, promote keratinocyte terminal differentiation and regulate skin inflammation. They also may have protective effects on human hair follicle (HFs) epithelial stem cells, while defects in PPARγ-mediated signalling may promote the death of these stem cells and thus facilitate the development of cicatricial alopecia (lichen planopilaris). Overall, however, selected PPARγ modulators appear to act as hair growth inhibitors that reduce the proliferation and promote apoptosis of hair matrix keratinocytes. The fact that commonly prescribed PPARγ-modulatory drugs of the thiazolidine-2,4-dione class can exhibit a battery of adverse cutaneous effects underscores the importance of distinguishing beneficial from clinically undesired cutaneous activities of PPARγ ligands and to better understand on the molecular level how PPARγ-regulated cutaneous lipid metabolism and PPARγ-mediated signalling impact on human skin physiology and pathology. Surely, the therapeutic potential that endogenous and exogenous PPARγ modulators may possess in selected skin diseases, ranging from chronic inflammatory hyperproliferative dermatoses like psoriasis and atopic dermatitis, via scarring alopecia and acne can only be harnessed if the complexities of PPARγ signalling in human skin and its appendages are systematically dissected.

Downregulation of vascular endothelial growth factor receptor-2 under oxidative stress conditions is mediated by β-transduction repeat-containing protein via glycogen synthase kinase-3β signaling

Vascular endothelial growth factor receptor-2 (VEGFR-2), which is a key determinant of the angiogenecic response, is decreased in diabetic mice under oxidative stress. β-transduction repeat-containing protein (β-TrCP) has been reported to participate in VEGFR-2 degradation in thyroid cancer cells. Additionally, glycogen synthase kinase-3β (GSK-3β) acts as a mediator in the β-TrCP-induced degradation of several proteins. However, the role played by β-TrCP and GSK-3β in the degradation of VEGFR-2 in endothelial cells where hyperglycemia had been induced was not fully understood. In the present study, we aimed to analyze the mechanisms of VEGFR-2 degradation by studying excess reactive oxygen species (ROS) induced by hyperglycemia or glucose oxidase (GO). Human umbilical vein endothelial cells (HUVECs) were treated with different concentrations of glucose (6.6, 19.8 and 33 mM), mannitol (33 mM) and GO (1 U/ml). Angiogenic function, ROS levels, the co-location of VEGFR-2 and β-TrCP were evaluated. Cells were collected for RT-qPCR and western blot analysis. We noted that angiogenesis was impaired upon increasing the glucose concentration. When HUVECs were in a hyperglycemic state, ROS production increased, comparable to exposure to GO; GO catalyzes oxidation of glucose into H₂O₂ and D-glucono-δ-lactone. Phosphorylated VEGFR-2 was reduced by hyperglycemia while total VEGFR-2 was almost unaltered. However, VEGFR-2 was reduced when directly exposed to ROS, with resultant co-location of β-TrCP and VEGFR-2. Through a co-immunoprecipitation assay, we noted that ubiquitinated VEGFR-2 was significantly augmented by excess ROS. Decreased VEGFR-2 caused by ROS was ameliorated by β-TrCP siRNA, proteasome inhibitor MG132 and GSK-3β activity inhibitor (lithium chloride and SB216763). We suggest that redundant ROS reduces VEGFR-2 through β-TrCP-mediated VEGFR-2 degradation, which is postulated to be regulated by GSK-3β.

AKT1 Inhibits Epithelial-to-Mesenchymal Transition in Breast Cancer through Phosphorylation-Dependent Twist1 Degradation

Epithelial-to-mesenchymal transition (EMT) is an essential physiologic process that promotes cancer cell migration, invasion, and metastasis. Several lines of evidence from both cellular and genetic studies suggest that AKT1/PKBα, but not AKT2 or AKT3, serves as a negative regulator of EMT and breast cancer metastasis. However, the underlying mechanism by which AKT1 suppresses EMT remains poorly defined. Here, we demonstrate that phosphorylation of Twist1 by AKT1 is required for β-TrCP-mediated Twist1 ubiquitination and degradation. The clinically used AKT inhibitor MK-2206, which possesses higher specificity toward AKT1, stabilized Twist1 and enhanced EMT in breast cancer cells. However, we discovered that resveratrol, a naturally occurring compound, induced β-TrCP-mediated Twist1 degradation to attenuate MK-2206-induced EMT in breast cancer cells. Taken together, our findings demonstrate that resveratrol counteracts the unexpected metastatic potential induced by anti-AKT therapy and therefore suggest that the addition of resveratrol to an anti-AKT therapeutic regimen may provide extra support for limiting EMT.

Energy restriction mimetic agents to target cancer cells: comparison between 2-deoxyglucose and thiazolidinediones

The use of energy restriction mimetic agents (ERMAs) to selectively target cancer cells addicted to glycolysis could be a promising therapeutic approach. Thiazolidinediones (TZDs) are synthetic agonists of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)γ that were developed to treat type II diabetes. These compounds also display anticancer effects which appear mainly to be independent of their PPARγ agonist activity but the molecular mechanisms involved in the anticancer action are not yet well understood. Results obtained on ciglitazone derivatives, mainly in prostate cancer cell models, suggest that these compounds could act as ERMAs. In the present paper, we introduce how compounds like 2-deoxyglucose target the Warburg effect and then we discuss the possibility that the PPARγ-independent effects of various TZD could result from their action as ERMAs.

SIRT1 suppresses cellular accumulation of β-TrCP E3 ligase via protein degradation

β-Transducin repeat-containing protein (β-TrCP), an E3 ligase, promotes the degradation of substrate proteins in response to various stimuli. Even though several β-TrCP substrates have been identified to date, limited information of its upstream regulators is available. Here, we showed that SIRT1 suppresses β-TrCP protein synthesis via post-translational degradation. SIRT1 depletion led to a significant increase in the β-TrCP accumulation without affecting the mRNA level. Consistently, β-TrCP protein accumulation induced by resveratrol was further enhanced upon SIRT1 depletion. Rescue of SIRT1 reversed the effect of resveratrol, leading to reduced β-TrCP protein levels. Proteasomal inhibition led to recovery of β-TrCP in cells with SIRT1 overexpression. Notably, the recovered β-TrCP colocalized mostly with SIRT1. Thus, SIRT1 acts as a negative regulator of β-TrCP synthesis via promoting protein degradation.

Rescue of the genetically engineered Cul4b mutant mouse as a potential model for human X-linked mental retardation

Mutation in CUL4B, which encodes a scaffold protein of the E3 ubiquitin ligase complex, has been found in patients with X-linked mental retardation (XLMR). However, early deletion of Cul4b in mice causes prenatal lethality, which has frustrated attempts to characterize the phenotypes in vivo. In this report, we successfully rescued Cul4b mutant mice by crossing female mice in which exons 4-5 of Cul4b were flanked by loxP sequences with Sox2-Cre male mice. In Cul4b-deficient (Cul4b(Δ)/Y) mice, no CUL4B protein was detected in any of the major organs, including the brain. In the hippocampus, the levels of CUL4A, CUL4B substrates (TOP1, β-catenin, cyclin E and WDR5) and neuronal markers (MAP2, tau-1, GAP-43, PSD95 and syn-1) were not sensitive to Cul4b deletion, whereas the number of parvalbumin (PV)-positive GABAergic interneurons was decreased in Cul4b(Δ)/Y mice, especially in the dentate gyrus (DG). Some dendritic features, including the complexity, diameter and spine density in the CA1 and DG hippocampal neurons, were also affected by Cul4b deletion. Together, the decrease in the number of PV-positive neurons and altered dendritic properties in Cul4b(Δ)/Y mice imply a reduction in inhibitory regulation and dendritic integration in the hippocampal neural circuit, which lead to increased epileptic susceptibility and spatial learning deficits. Our results identify Cul4b(Δ)/Y mice as a potential model for the non-syndromic model of XLMR that replicates the CUL4B-associated MR and is valuable for the development of a therapeutic strategy for treating MR.

The PPAR Gamma Agonist Troglitazone Regulates Erk 1/2 Phosphorylation via a PPARγ-Independent, MEK-Dependent Pathway in Human Prostate Cancer Cells

Thiazolidinediones (TZDs) dramatically reduce the growth of human prostate cancer cells in vitro and in vivo. To determine whether the antitumor effects of TZDs were due in part to changes in the MEK/Erk signaling pathway, we examined the regulation of Erk phosphorylation by the TZD troglitazone within the PC-3 and C4-2 human prostate cancer cell lines. Western blot analysis revealed troglitazone-induced phosphorylation of Erk in both PC-3 and C4-2 cells. Troglitazone-induced increases in Erk phosphorylation were suppressed by the MEK inhibitor U0126 but not by the PPARγ antagonist GW9662. Pretreatment with U0126 did not alter the ability of troglitazone to regulate expression of two proteins that control cell cycle, p21, and c-Myc. Troglitazone was also still effective at reducing PC-3 proliferation in the presence of U0126. Therefore, our data suggest that troglitazone-induced Erk phosphorylation does not significantly contribute to the antiproliferative effect of troglitazone.

The role of peroxisome proliferator-activated receptors in carcinogenesis and chemoprevention

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.

Skp2 is a promising therapeutic target in breast cancer

Breast cancer is the most common type of cancer among American women, and remains the second leading cause of cancer-related death for female in the United States. It has been known that several signaling pathways and various factors play critical roles in the development and progression of breast cancer, such as estrogen receptor, Notch, PTEN, human epidermal growth factor receptor 2, PI3K/Akt, BRCA1, and BRCA2. Emerging evidence has shown that the F-box protein S-phase kinase associated protein 2 (Skp2) also plays an important role in the pathogenesis of breast cancer. Therefore, in this brief review, we summarize the novel functions of Skp2 in the pathogenesis of breast cancer. Moreover, we provide further evidence regarding the state of our knowledge toward the development of novel Skp2 inhibitors especially natural "chemopreventive agents" as targeted approach for the prevention and/or treatment of breast cancer.

Inhibition of GSK3β improves hippocampus-dependent learning and rescues neurogenesis in a mouse model of fragile X syndrome

Fragile X syndrome (FXS), a common inherited form of intellectual disability with learning deficits, results from a loss of fragile X mental retardation protein (FMRP). Despite extensive research, treatment options for FXS remain limited. Since FMRP is known to play an important role in adult hippocampal neurogenesis and hippocampus-dependent learning and FMRP regulates the adult neural stem cell fate through the translational regulation of glycogen synthase kinase 3β (GSK3β), we investigated the effects of a GSK3β inhibitor, SB216763, on Fmr1 knockout mice (Fmr1 KO). We found that the inhibition of GSK3β could reverse the hippocampus-dependent learning deficits and rescue adult hippocampal neurogenesis at multiple stages in Fmr1 KO mice. Our results point to GSK3β inhibition as a potential treatment for the learning deficits seen in FXS.

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.

Cytotoxicity of troglitazone through PPARγ-independent pathway and p38 MAPK pathway in renal cell carcinoma

Agonists of peroxisome proliferator-activated receptor gamma (PPARγ) have been examined as chemopreventive and chemotherapeutic agents. The aim was to investigate the cytotoxicity of troglitazone (TGZ) and its mechanisms in terms of PPARγ dependency and the p38 mitogen-activated protein kinase (MAPK) pathway in three human renal cell carcinoma (RCC) cell lines, 786-O, Caki-2 and ACHN cells. TGZ induced apoptosis and exerted cytotoxicity in a PPARγ-independent manner. We demonstrated that TGZ activated the p38 MAPK pathway and was involved in the cytotoxicity of TGZ. It was also revealed that TGZ induced G(2)/M cell cycle arrest through activation of p38 MAPK.

PPARgamma and MEK Interactions in Cancer

Peroxisome proliferator-activated receptor-gamma (PPARγ) exerts multiple functions in determination of cell fate, tissue metabolism, and host immunity. Two synthetic PPARγ ligands (rosiglitazone and pioglitazone) were approved for the therapy of type-2 diabetes mellitus and are expected to serve as novel cures for inflammatory diseases and cancer. However, PPARγ and its ligands exhibit a janus-face behaviour as tumor modulators in various systems, resulting in either tumor suppression or tumor promotion. This may be in part due to signaling crosstalk to the mitogen-activated protein kinase (MAPK) cascades. The genomic activity of PPARγ is modulated, in addition to ligand binding, by phosphorylation of a serine residue by MAPKs, such as extracellular signal-regulated protein kinases-1/2 (ERK-1/2), or by nucleocytoplasmic compartmentalization through the ERK activators MAPK kinases-1/2 (MEK-1/2). PPARγ ligands themselves activate the ERK cascade through nongenomic and often PPARγ-independent signaling. In the current review, we discuss the molecular mechanisms and physiological implications of the crosstalk of PPARγ with MEK-ERK signaling and its potential as a novel drug target for cancer therapy in patients.

Multiple Interactions between Peroxisome Proliferators-Activated Receptors and the Ubiquitin-Proteasome System and Implications for Cancer Pathogenesis

The peroxisome proliferator-activated receptors (PPAR) α, β/δ, and γ are ligand-activated nuclear receptors involved in a number of physiological processes, including lipid and glucose homeostasis, inflammation, cell growth, differentiation, and death. PPAR agonists are used in the treatment of human diseases, like type 2 diabetes and dyslipidemia, and PPARs appear as promising therapeutic targets in other conditions, including cancer. A better understanding of the functions and regulation of PPARs in normal and pathological processes is of primary importance to devise appropriate therapeutic strategies. The ubiquitin-proteasome system (UPS) plays an important role in controlling level and activity of many nuclear receptors and transcription factors. PPARs are subjected to UPS-dependent regulation. Interestingly, the three PPAR isotypes are differentially regulated by the UPS in response to ligand-dependent activation, a phenomenon that may be intrinsically connected to their distinct cellular functions and behaviors. In addition to their effects ongene expression, PPARs appear to affect protein levels and downstream pathways also by modulating the activity of the UPS in target-specific manners. Here we review the current knowledge of the interactions between the UPS and PPARs in light of the potential implications for their effects on cell fate and tumorigenesis.

Troglitazone suppresses c-Myc levels in human prostate cancer cells via a PPARγ-independent mechanism

Troglitazone is a ligand for the peroxisome proliferator activated receptor gamma (PPARγ) that decreases growth of human prostate cancer cells in vitro and in vivo. However, the mechanism by which troglitazone reduces prostate cancer cell growth is not fully understood. To understand the signaling pathways involved in troglitazone-induced decreases in prostate cancer growth, we examined the effect of troglitazone on androgen-independent C4-2 human prostate cancer cells. Initial experiments revealed troglitazone inhibited C4-2 cell proliferation by arresting cells in the G(0)/G(1) phase of the cell cycle and inducing apoptosis. Since the proto-oncogene product c-Myc regulates both apoptosis and cell cycle progression, we next examined whether troglitazone altered expression of c-Myc. Troglitazone decreased c-Myc protein levels as well as expression of downstream targets of c-Myc in a dose-dependent manner. In C4-2 cells, troglitazone-induced decreases in c-Myc protein involve proteasome-mediated degradation of c-Myc protein as well as reductions in c-Myc mRNA levels. It appears that troglitazone stimulates degradation of c-Myc by increasing c-Myc phosphorylation, for the level of phosphorylated c-Myc was elevated in prostate cancer cells exposed to troglitazone. While troglitazone dramatically decreased the amount of c-Myc within C4-2 cells, the PPARγ ligands ciglitazone, rosiglitazone and pioglitazone did not reduce c-Myc protein levels. Furthermore the down-regulation of c-Myc by troglitazone was not blocked by the PPARγ antagonist GW9662 and siRNA-mediated decreases in PPARγ protein. Thus, our data suggest that troglitazone reduces c-Myc protein independently of PPARγ.

An RNAi-based chemical genetic screen identifies three small-molecule inhibitors of the Wnt/wingless signaling pathway

Misregulated β-catenin responsive transcription (CRT) has been implicated in the genesis of various malignancies, including colorectal carcinomas, and it is a key therapeutic target in combating various cancers. Despite significant effort, successful clinical implementation of CRT inhibitory therapeutics remains a challenging goal. This is, in part, because of the challenge of identifying inhibitory compounds that specifically modulate the nuclear transcriptional activity of β-catenin while not affecting its cytoskeletal function in stabilizing adherens junctions at the cell membrane. Here, we report an RNAi-based modifier screening strategy for the identification of CRT inhibitors. Our data provide support for the specificity of these inhibitory compounds in antagonizing the transcriptional function of nuclear β-catenin. We show that these inhibitors efficiently block Wnt/β-catenin-induced target genes and phenotypes in various mammalian and cancer cell lines. Importantly, these Wnt inhibitors are specifically cytotoxic to human colon tumor biopsy cultures as well as colon cancer cell lines that exhibit deregulated Wnt signaling.

Thiazolidinediones alter growth and epithelial cell integrity, independent of PPAR-γ and MAPK activation, in mouse M1 cortical collecting duct cells

Peroxisome proliferator-activated receptor (PPAR)-γ is highly expressed in the collecting duct (CD), yet little is known about the effects of PPAR-γ ligands, thiazolidinediones (TZDs), on CD cell structure and function. M1 mouse cortical CD cells were treated with 5 μM troglitazone (TRO) and rosiglitazone (ROSI). First, growth was measured by [(3)H]thymidine and [(3)H]leucine incorporation, as well as analysis of cyclin D1 and the CDK inhibitor p27 by Western blot. [(3)H]thymidine incorporation was reduced by 56 and 24% by TRO and ROSI at 6 h, and [(3)H]leucine by 21 and 10%. A similar growth inhibition was also observed after 24 h for thymidine, but leucine was reduced by 48 and 24%, respectively. Likewise, cyclin D1 was diminished 60% by TRO, and p27 was elevated 1.6- and 1.7-fold in response to TRO and ROSI. Next, epithelial cell integrity was assessed by measuring different markers by Western blot analysis. While fibronectin and α-smooth muscle actin levels were unchanged, by 24 h E-cadherin was decreased by 50%, and β-catenin levels were reduced 2- and 1.5-fold in response to TRO and ROSI, respectively. GW9662, a PPAR-γ antagonist, did not reverse any of the TZD responses in M1 cells. Of interest, phosho-p38 levels were also elevated 2-fold in response to TRO and 2.3-fold to ROSI, but MAPK inhibition by PD98059 or SB203580 caused an additive inhibition of cell growth and did not alter E-cadherin or β-catenin in response to TZDs. Finally, apoptotic death was assessed by Western blot, but cleaved caspase-3 levels were unchanged from 15 min to 24 h in response to TZDs, and TRO did not affect cell viability or reactive oxygen species generation. Our data suggest that TZDs cause a disruption of M1 cell integrity that is preceded by an inhibition of cell growth. This response is independent of p38 or PPAR-γ activation.

Energy restriction as an antitumor target of thiazolidinediones

Cancer cells gain growth advantages in the microenvironment by shifting cellular metabolism to aerobic glycolysis, the so-called Warburg effect. There is a growing interest in targeting aerobic glycolysis for cancer therapy by exploiting the differential susceptibility of malignant versus normal cells to glycolytic inhibition, of which the proof-of-concept is provided by the in vivo efficacy of dietary caloric restriction and natural product-based energy restriction-mimetic agents (ERMAs) such as resveratrol and 2-deoxyglucose in suppressing carcinogenesis in animal models. Here, we identified thiazolidinediones as a novel class of ERMAs in that they elicited hallmark cellular responses characteristic of energy restriction, including transient induction of Sirt1 (silent information regulator 1) expression, activation of the intracellular fuel sensor AMP-activated protein kinase, and endoplasmic reticulum stress, the interplay among which culminated in autophagic and apoptotic death. The translational implications of this finding are multifold. First, the novel function of troglitazone and ciglitazone in targeting energy restriction provides a mechanistic basis to account for their peroxisome proliferator-activated receptor gamma-independent effects on a broad spectrum of signaling targets. Second, we demonstrated that Sirt1-mediated up-regulation of beta-transducin repeat-containing protein-facilitated proteolysis of cell cycle- and apoptosis-regulatory proteins is an energy restriction-elicited signaling event and is critical for the antitumor effects of ERMAs. Third, it provides a molecular rationale for using thiazolidinediones as scaffolds to develop potent ERMAs, of which the proof-of-principle is demonstrated by OSU-CG12. OSU-CG12, a peroxisome proliferator-activated receptor gamma-inactive ciglitazone derivative, exhibits 1- and 3-order of magnitude higher potency in eliciting starvation-like cellular responses relative to resveratrol and 2-deoxyglucose, respectively.

DH9, a novel PPARγ agonist suppresses the proliferation of ADPKD epithelial cells: An association with an inhibition of β-catenin signaling

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disease that exclusively progresses to renal failure. An important target for the treatment of ADPKD is to reduce cystic cell proliferation. PPARγ agonists such as TZDs are insulin sensitizing agents that have also been reported to decrease tumor growth. Here we tested DH9, a newly synthesized PPARγ agonist on the proliferation of an ADPKD cell line, WT9-12. DH9 showed a potent anti-proliferative activity against ADPKD cells. At high concentration, DH9 also induced apoptotic cell death. The effect of DH9 on cell proliferation was mediated by a PPARγ independent mechanism. Since DH9 decreased the levels of β-catenin in cells via a GSK3β mediated degradation pathway, this acts as a mechanism for growth inhibition by DH9.

Thiazolidinediones mimic glucose starvation in facilitating Sp1 degradation through the up-regulation of beta-transducin repeat-containing protein

This study investigated the mechanism by which the transcription factor Sp1 is degraded in prostate cancer cells. We recently developed a thiazolidinedione derivative, (Z)-5-(4-hydroxy-3-trifluoromethylbenzylidene)-3-(1-methylcyclohexyl)-thiazolidine-2,4-dione (OSU-CG12), that induces Sp1 degradation in a manner paralleling that of glucose starvation. Based on our finding that thiazolidinediones suppress beta-catenin and cyclin D1 by up-regulating the E3 ligase SCF(beta-TrCP), we hypothesized that beta-transducin repeat-containing protein (beta-TrCP) targets Sp1 for proteasomal degradation in response to glucose starvation or OSU-CG12. Here we show that either treatment of LNCaP cells increased specific binding of Sp1 with beta-TrCP. This direct binding was confirmed by in vitro pull-down analysis with bacterially expressed beta-TrCP. Although ectopic expression of beta-TrCP enhanced the ability of OSU-CG12 to facilitate Sp1 degradation, suppression of endogenous beta-TrCP function by a dominant-negative mutant or small interfering RNA-mediated knockdown blocked OSU-CG12-facilitated Sp1 ubiquitination and/or degradation. Sp1 contains a C-terminal conventional DSG destruction box ((727)DSGAGS(732)) that mediates beta-TrCP recognition and encompasses a glycogen synthase kinase 3beta (GSK3beta) phosphorylation motif (SXXXS). Pharmacological and molecular genetic approaches and mutational analyses indicate that extracellular signal-regulated kinase-mediated phosphorylation of Thr739 and GSK3beta-mediated phosphorylation of Ser728 and Ser732 were critical for Sp1 degradation. The ability of OSU-CG12 to mimic glucose starvation to activate beta-TrCP-mediated Sp1 degradation has translational potential to foster novel strategies for cancer therapy.

PPARgamma-independent antitumor effects of thiazolidinediones

The thiazolidinedione (TZD) family of PPARgamma agonists, especially troglitazone and ciglitazone, induce cell cycle arrest, differentiation, and apoptosis in cancer cells. Mounting evidence indicates that TZDs interfere with multiple signaling mechanisms independently of PPARgamma activation, which affect many aspects of cellular functions governing cell cycle progression and survival of cancer cells. Here, we review the "off-target" mechanisms that underlie the antitumor effects of TZDs with emphasis on three key pathways, namely, inhibition of Bcl-2/Bcl-xL function, proteasomal degradation of cell cycle- and apoptosis-regulatory proteins, and transcriptional repression of androgen receptor (AR) through Sp1 degradation. Relative to tumor cells, nonmalignant cells are resistant to these PPARgamma-independent antitumor effects, which underscores the translational potential of these agents. Furthermore, dissociation of these antitumor effects from their PPARgamma agonist activity provides a rationale for using TZDs as scaffolds for lead optimization to develop a novel class of antitumor agents with a unique mode of mechanism.

A novel mechanism by which thiazolidinediones facilitate the proteasomal degradation of cyclin D1 in cancer cells

This study identifies a novel mechanism by which thiazolidinediones mediate cyclin D1 repression in prostate cancer cells. Based on the finding that the thiazolidinedione family of peroxisome proliferator-activated receptor gamma (PPARgamma) agonists mediated PPARgamma-independent cyclin D1 degradation, we developed a novel PPARgamma-inactive troglitazone derivative, STG28, with high potency in cyclin D1 ablation. STG28-mediated cyclin D1 degradation was preceded by Thr-286 phosphorylation and nuclear export, which however, were independent of glycogen synthase kinase 3beta. Mutational analysis further confirmed the pivotal role of Thr-286 phosphorylation in STG28-induced nuclear export and proteolysis. Of several kinases examined, inhibition of IkappaB kinase alpha blocked STG28-mediated cytoplasmic sequestration and degradation of cyclin D1. Pulldown of ectopically expressed Cul1, the scaffold protein of the Skp-Cullin-F-box E3 ligase, in STG28-treated cells revealed an increased association of cyclin D1 with beta-TrCP, whereas no specific binding was noted with other F-box proteins examined, including Skp2, Fbw7, Fbx4, and Fbxw8. This finding represents the first evidence that cyclin D1 is targeted by beta-TrCP. Moreover, beta-TrCP expression was up-regulated in response to STG28, and ectopic expression and small interfering RNA-mediated knock-down of beta-TrCP enhanced and protected against STG28-facilitated cyclin D1 degradation, respectively. Because cyclin D1 lacks the DSG destruction motif, mutational and modeling analyses indicate that cyclin D1 was targeted by beta-TrCP through an unconventional recognition site, (279)EEVDLACpT(286), reminiscent to that of Wee1. Moreover, we obtained evidence that this beta-TrCP-dependent degradation takes part in controlling cyclin D1 turnover when cancer cells undergo glucose starvation, which endows physiological relevance to this novel mechanism.

Pharmacological exploitation of the peroxisome proliferator-activated receptor gamma agonist ciglitazone to develop a novel class of androgen receptor-ablative agents

On the basis of our finding that the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist ciglitazone at high doses was able to mediate PPARgamma-independent transcriptional repression of androgen receptor (AR) in a tumor cell-specific manner, we used Delta2CG, a PPARgamma-inactive analogue of ciglitazone, to conduct lead optimization to develop a novel class of AR-ablative agents. Structure-activity analysis indicates a high degree of flexibility in realigning Delta2CG's structural moieties without compromising potency in AR repression, as evidenced by the higher AR-ablative activity of the permuted isomer 9 [( Z)-5-(4-hydroxybenzylidene)-3-(1-methylcyclohexylmethyl)thiazolidine-2,4-dione]. Further modifications of 9 gave rise to 12 [( Z)-5-(4-hydroxy-3-trifluoromethylbenzylidene)-3-(1-methylcyclohexylmethyl)thiazolidine-2,4-dione], which completely inhibited AR expression in LNCaP cells at low micromolar concentrations. This AR down-regulation led to growth inhibition in LNCaP cells through apoptosis induction. Moreover, the role of AR repression in the antiproliferative effect of compound 12 was validated by the differential inhibition of cell viability between androgen-responsive and androgen-nonresponsive cells.

A Role for the PPARgamma in Cancer Therapy

In 1997, the first published reports highlighted PPARgamma as a novel cancer therapeutic target regulating differentiation of cancer cells. A subsequent flurry of papers described these activities more widely and fuelled further enthusiasm for differentiation therapy, as the ligands for the PPARgamma were seen as well tolerated and in several cases well-established in other therapeutic contexts. This initial enthusiasm and promise was somewhat tempered by contradictory findings in several murine cancer models and equivocal trial findings. As more understanding has emerged in recent years, a renaissance has occurred in targeting PPARgamma within the context of either chemoprevention or chemotherapy. This clarity has arisen in part through a clearer understanding of PPARgamma biology, how the receptor interacts with other proteins and signaling events, and the mechanisms that modulate its transcriptional actions. Equally greater translational understanding of this target has arisen from a clearer understanding of in vivo murine cancer models. Clinical exploitation will most likely require precise and quantifiable description of PPARgamma actions, and resolution of which targets are the most beneficial to target combined with an understanding of the mechanisms that limits its anticancer effectiveness.