Flightless I homolog negatively regulates ChREBP activity in cancer cells

IRF3-binding lncRNA-ISIR strengthens interferon production in viral infection and autoinflammation

Interferon regulatory factor 3 (IRF3) is an essential transductor for initiation of many immune responses. Here, we show that lncRNA-ISIR directly binds IRF3 to promote its phosphorylation, dimerization, and nuclear translocation, along with enhanced target gene productions. In vivo lncRNA-ISIR deficiency results in reduced IFN production, uncontrolled viral replication, and increased mortality. The human homolog, AK131315, also binds IRF3 and promotes its activation. More important, AK131315 expression is positively correlated with type I interferon (IFN-I) level and severity in patients with lupus. Mechanistically, in resting cells, IRF3 is bound to suppressor protein Flightless-1 (Fli-1), which keeps its inactive state. Upon infection, IFN-I-induced lncRNA-ISIR binds IRF3 at DNA-binding domain in cytoplasm and removes Fli-1's association from IRF3, consequently facilitating IRF3 activation. Our results demonstrate that IFN-I-inducible lncRNA-ISIR feedback strengthens IRF3 activation by removing suppressive Fli-1 in immune responses, revealing a method of lncRNA-mediated modulation of transcription factor (TF) activation.

Flightless I Homolog Reverses Enzalutamide Resistance through PD-L1-Mediated Immune Evasion in Prostate Cancer

Tumor cells can evade immune surveillance and immune killing during the emergence of endocrine therapy resistance in prostate cancer, but the mechanisms underlying this phenomenon are still unclear. Flightless I homolog (FLII) is a coregulator for transcription factors in several malignancies. Here, we have demonstrated that endocrine therapy resistance can induce an immunosuppressive prostate tumor microenvironment and immune evasion through FLII downregulation, which leads to activation of the YBX1/PD-L1 signaling pathway. FLII expression negatively correlated with expression of PD-L1 in tumors. Mechanism studies demonstrated that FLII physically interacted with YBX1 to inhibit nuclear localization of YBX1 and thereby suppress transcription of PDL1 in enzalutamide-resistant tumors. Restoration of FLII expression reversed enzalutamide resistance through activation of T-cell responses in the tumor microenvironment through inhibition of the YBX1/PD-L1 pathway. We also found that reversal of endocrine therapy resistance and immune evasion was mediated by proliferation of effector CD8⁺ T cells and inhibition of tumor infiltration by regulatory T cells and myeloid-derived suppressor cells. Taken together, our results demonstrate a functional and biological interaction between endocrine therapy resistance and immune evasion mediated through the FLII/YBX1/PD-L1 cascade. Combination therapy with FLII expression and endocrine therapy may benefit patients with prostate cancer by preventing tumor immune evasion.

ERα down-regulates carbohydrate responsive element binding protein and decreases aerobic glycolysis in liver cancer cells

Deregulated metabolism is one of the characteristics of hepatocellular carcinoma. Sex hormone receptor signalling has been involved in the marked gender dimorphism of hepatocellular carcinoma pathogenesis. Oestrogen receptor (ER) has been reported to reduce the incidence of liver cancer. However, it remains unclear how oestrogen and ER regulate metabolic alterations in liver tumour cells. Our previous work revealed that ERα interacted with carbohydrate responsive element binding protein (ChREBP), which is a transcription factor promoting aerobic glycolysis and proliferation of hepatoma cells. Here, the data showed that ERα overexpression with E2 treatment reduced aerobic glycolysis and cell proliferation of hepatoma cells. In addition to modestly down-regulating ChREBP transcription, ERα promoted ChREBP degradation. ERα co-immunoprecipitated with both ChREBP-α and ChREBP-β, the two known subtypes of ChREBP. Although E2 promoted ERα to translocate to the nucleus, it did not change subcellular localization of ChREBP. In addition to interacting with ChREBP-β and promoting its degradation, ERα decreased ChREBP-α-induced ChREBP-β transcription. Taken together, we confirmed an original role of ERα in suppressing aerobic glycolysis in liver cancer cells and elucidated the mechanism by which ERα and ChREBP-α together regulated ChREBP-β expression.

Interception Targets of Angelica Gigas Nakai Root Extract versus Pyranocoumarins in Prostate Early Lesions and Neuroendocrine Carcinomas in TRAMP Mice

We reported efficacy of Angelica gigas Nakai (AGN) root ethanol extract and equimolar decursin (D)/decursinol angelate (DA) through daily gavage starting at 8 weeks of age (WOA) to male transgenic adenocarcinoma of mouse prostate (TRAMP) mice such that these modalities suppressed precancerous epithelial lesions in their dorsolateral prostate (DLP) to similar extent, but AGN extract was better than the D/DA mixture at promoting the survival of mice bearing prostate neuroendocrine carcinomas to 28 WOA. Here, we compared by microarray hybridization the mRNA levels in pooled DLP tissues and individual neuroendocrine carcinomas to characterize potential molecular targets of AGN extract and D/DA. Clustering and principal component analyses supported distinct gene expression profiles of TRAMP DLP versus neuroendocrine carcinomas. Pathway Enrichment, Gene Ontology, and Ingenuity Pathway Analyses of differential genes indicated that AGN and D/DA affected chiefly processes of lipid and mitochondrial energy metabolism and oxidation-reduction in TRAMP DLP, while AGN affected neuronal signaling, immune systems and cell cycling in neuroendocrine carcinomas. Protein-Protein Interaction Network analysis predicted and reverse transcription-PCR verified multiple hub genes common in the DLP of AGN- and D/DA-treated TRAMP mice at 28 WOA and select hub genes attributable to the non-D/DA AGN components. The vast majority of hub genes in the AGN-treated neuroendocrine carcinomas differed from those in TRAMP DLP. In summary, the transcriptomic approach illuminated vastly different signaling pathways and networks, cellular processes, and hub genes of two TRAMP prostate malignancy lineages and their associations with the interception efficacy of AGN and D/DA. PREVENTION RELEVANCE: This study explores potential molecular targets associated with in vivo activity of AGN root alcoholic extract and its major pyranocoumarins to intercept precancerous epithelial lesions and early malignancies of the prostate. Without an ethically-acceptable, clearly defined cancer initiation risk reduction strategy available for the prostate, using natural products like AGN to delay formation of malignant tumors could be a plausible approach for prostate cancer prevention.

Multifunctional Roles of the Actin-Binding Protein Flightless I in Inflammation, Cancer and Wound Healing

Flightless I is an actin-binding member of the gelsolin family of actin-remodeling proteins that inhibits actin polymerization but does not possess actin severing ability. Flightless I functions as a regulator of many cellular processes including proliferation, differentiation, apoptosis, and migration all of which are important for many physiological processes including wound repair, cancer progression and inflammation. More than simply facilitating cytoskeletal rearrangements, Flightless I has other important roles in the regulation of gene transcription within the nucleus where it interacts with nuclear hormone receptors to modulate cellular activities. In conjunction with key binding partners Leucine rich repeat in the Flightless I interaction proteins (LRRFIP)1/2, Flightless I acts both synergistically and competitively to regulate a wide range of cellular signaling including interacting with two of the most important inflammatory pathways, the NLRP3 inflammasome and the MyD88-TLR4 pathways. In this review we outline the current knowledge about this important cytoskeletal protein and describe its many functions across a range of health conditions and pathologies. We provide perspectives for future development of Flightless I as a potential target for clinical translation and insights into potential therapeutic approaches to manipulate Flightless I functions.

Flightless-1 inhibits ER stress-induced apoptosis in colorectal cancer cells by regulating Ca2+ homeostasis

The endoplasmic reticulum (ER) stress response is an adaptive mechanism that is activated upon disruption of ER homeostasis and protects the cells against certain harmful environmental stimuli. However, critical and prolonged cell stress triggers cell death. In this study, we demonstrate that Flightless-1 (FliI) regulates ER stress-induced apoptosis in colon cancer cells by modulating Ca²⁺ homeostasis. FliI was highly expressed in both colon cell lines and colorectal cancer mouse models. In a mouse xenograft model using CT26 mouse colorectal cancer cells, tumor formation was slowed due to elevated levels of apoptosis in FliI-knockdown (FliI-KD) cells. FliI-KD cells treated with ER stress inducers, thapsigargin (TG), and tunicamycin exhibited activation of the unfolded protein response (UPR) and induction of UPR-related gene expression, which eventually triggered apoptosis. FliI-KD increased the intracellular Ca²⁺ concentration, and this upregulation was caused by accelerated ER-to-cytosolic efflux of Ca²⁺. The increase in intracellular Ca²⁺ concentration was significantly blocked by dantrolene and tetracaine, inhibitors of ryanodine receptors (RyRs). Dantrolene inhibited TG-induced ER stress and decreased the rate of apoptosis in FliI-KD CT26 cells. Finally, we found that knockdown of FliI decreased the levels of sorcin and ER Ca²⁺ and that TG-induced ER stress was recovered by overexpression of sorcin in FliI-KD cells. Taken together, these results suggest that FliI regulates sorcin expression, which modulates Ca²⁺ homeostasis in the ER through RyRs. Our findings reveal a novel mechanism by which FliI influences Ca²⁺ homeostasis and cell survival during ER stress.

A cytoskeletal protein that helps tumors avoid cell death offers a promising new drug target for fighting cancer. A team led by Jang Hyun Choi and Sun Sil Choi of the Ulsan National Institute of Science and Technology, South Korea, detailed how a protein called Flightless I (FliI) that normally regulates the remodeling of structural filaments in the cell can, in colorectal cancer cells, serve as a tumor promoter through its action on calcium levels. Typically, cells respond to chronic stress by altering calcium signaling to promote their own death. In tumors, however, FliI maintains normal calcium levels to enhance cell survival even in the face of chemotherapy and other stressful stimuli. Suppressing FliI activity could thus help sensitize cancer cells to other stress- and death-inducing drug regimens.

Potential involvement of Drosophila flightless-1 in carbohydrate metabolism

A previous study of ours indicated that Drosophila flightless-1 controls lipid metabolism, and that there is an accumulation of triglycerides in flightless-1 (fliI)-mutant flies, where this mutation triggers metabolic stress and an obesity phenotype. Here, with the aim of characterizing the function of FliI in metabolism, we analyzed the levels of gene expression and metabolites in fliI-mutant flies. The levels of enzymes related to glycolysis, lipogenesis, and the pentose phosphate pathway increased in fliI mutants; this result is consistent with the levels of metabolites corresponding to a metabolic pathway. Moreover, high-throughput RNA sequencing revealed that Drosophila FliI regulates the expression of genes related to biological processes such as chromosome organization, carbohydrate metabolism, and immune reactions. These results showed that Drosophila FliI regulates the expression of metabolic genes, and that dysregulation of the transcription controlled by FliI gives rise to metabolic stress and problems in the development and physiology of Drosophila.

Flightless-I Blocks p62-Mediated Recognition of LC3 to Impede Selective Autophagy and Promote Breast Cancer Progression

p62 is a receptor that facilitates selective autophagy by interacting simultaneously with cargoes and LC3 protein on the autophagosome to maintain cellular homeostasis. However, the regulatory mechanism(s) behind this process and its association with breast cancer remain to be elucidated. Here, we report that Flightless-I (FliI), a novel p62-interacting protein, promotes breast cancer progression by impeding selective autophagy. FliI was highly expressed in clinical breast cancer samples, and heterozygous deletion of FliI retarded the development of mammary tumors in PyVT mice. FliI induced p62-recruited cargoes into Triton X-100 insoluble fractions (TI) to form aggregates, thereby blocking p62 recognition of LC3 and hindering p62-dependent selective autophagy. This function of Flil was reinforced by Akt-mediated phosphorylation at Ser436 and inhibited by phosphorylation of Ulk1 at Ser64. Obstruction of autophagic clearance of p62-recruited cargoes by FliI was associated with the accumulation of oxidative damage on proteins and DNA, which could contribute to the development of cancer. Heterozygous knockout of FliI facilitated selectively autophagic clearance of aggregates, abatement of ROS levels, and protein oxidative damage, ultimately retarding mammary cancer progression. In clinical breast cancer samples, Akt-mediated phosphorylation of FliI at Ser436 negatively correlated with long-term prognosis, while Ulk1-induced FliI phosphorylation at Ser64 positively correlated with clinical outcome. Together, this work demonstrates that FliI functions as a checkpoint protein for selective autophagy in the crosstalk between FliI and p62-recruited cargoes, and its phosphorylation may serve as a prognostic marker for breast cancer.Significance: Flightless-I functions as a checkpoint protein for selective autophagy by interacting with p62 to block its recognition of LC3, leading to tumorigenesis in breast cancer.Cancer Res; 78(17); 4853-64. ©2018 AACR.

Flightless-I Controls Fat Storage in Drosophila

Triglyceride homeostasis is a key process of normal development and is essential for the maintenance of energy metabolism. Dysregulation of this process leads to metabolic disorders such as obesity and hyperlipidemia. Here, we report a novel function of the Drosophila flightless-I (fliI) gene in lipid metabolism. Drosophila fliI mutants were resistant to starvation and showed increased levels of triglycerides in the fat body and intestine, whereas fliI overexpression decreased triglyceride levels. These flies suffered from metabolic stress indicated by increased levels of trehalose in hemolymph and enhanced phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α). Moreover, upregulation of triglycerides via a knockdown of fliI was reversed by a knockdown of desat1 in the fat body of flies. These results indicate that fliI suppresses the expression of desat1, thereby inhibiting the development of obesity; fliI may, thus, serve as a novel therapeutic target in obesity and metabolic diseases.

Stearoyl-CoA desaturase-1 promotes colorectal cancer metastasis in response to glucose by suppressing PTEN

BACKGROUND

Diabetic patients have a higher risk factor for colorectal cancer (CRC) metastasis. Stearoyl-CoA desaturase 1 (SCD1), the main enzyme responsible for producing monounsaturated fatty acids(MUFA) from saturated fatty acids, is frequently deregulated in both diabetes and CRC. The function and mechanism of SCD1 in metastasis of CRC and its relevance to glucose remains largely unknown.

METHODS

SCD1 expression levels were analyzed in human CRC tissues and the Cancer Browser database ( https://genome-cancer.ucsc.edu/ ). CRC cell lines stably transfected with SCD1 shRNAs or vector were established to investigate the role of SCD1 in modulating migration and invasion of CRC cells. A glucose concentration gradient was set to investigate regulation of SCD1 in CRC relevant to diabetic conditions.

RESULTS

The clinical data analysis showed high expression of SCD1 in CRC tissues with a negative correlation with the prognosis of CRC. In vitro experiments revealed that SCD1 increased CRC progression through promoting epithelial-mesenchymal transition (EMT). Lipidomic analysis demonstrated that SCD1 increased MUFA levels and MUFA administration could rescue migration and invasion defect of CRC cells induced by SCD1 knockdown. Furthermore, SCD1-mediated progression of CRC was promoted by carbohydrate response-element binding protein (ChREBP) in response to high glucose. Mechanistically, hyperglycemia-SCD1-MUFA induced CRC cell migration and invasion by regulating PTEN.

CONCLUSIONS

Our findings show that SCD1 promotes metastasis of CRC cells through MUFA production and suppressing PTEN in response to glucose, which may be a novel mechanism for diabetes-induced CRC metastasis.

Advanced glycation end products promote ChREBP expression and cell proliferation in liver cancer cells by increasing reactive oxygen species

The aim of the study was to elucidate the mechanism by which advanced glycation end products (AGEs) promote cell proliferation in liver cancer cells.We treated liver cancer HepG2 cells with 200 mg/L AGEs or bovine serum albumin (BSA) and assayed for cell viability, cell cycle, and apoptosis. We performed real-time PCR and Western blot analysis for RNA and protein levels of carbohydrate responsive element-binding protein (ChREBP) in AGEs- or BSA-treated HepG2 cells. We analyzed the level of reactive oxygen species (ROS) in HepG2 cells treated with AGEs or BSA.We found that increased S-phase cell percentage and decreased apoptosis contributed to AGEs-induced liver cancer cell proliferation. Real-time PCR and Western blot analysis showed that AGEs stimulated RNA and protein levels of ChREBP, a transcription factor promoting glycolysis and maintaining cell proliferation in liver cancer cells. Intriguingly, the level of ROS was higher in AGEs-treated liver cancer cells. Treating liver cancer cells with antioxidant N-acetyl cystein (NAC) partly blocked AGEs-induced ChREBP expression and cell proliferation.Our results suggest that the AGEs-ROS-ChREBP pathway plays a critical role in promoting ChREBP expression and liver cancer cell proliferation.

The flightless I protein interacts with RNA-binding proteins and is involved in the genome-wide mRNA post-transcriptional regulation in lung carcinoma cells

The flightless I protein (FLII) belongs to the gelsolin family. Its function has been associated with actin remodeling, embryonic development, wound repair, and more recently with cancer. The structure of FLII is characterized by the N-terminal leucine-rich repeats (LRR) and C-terminal gesolin related repeated units that are both protein-protein inter-action domains, suggesting that FLII may exert its function by interaction with other proteins. Therefore, systematic study of protein interactions of FLII in cells is important for the understanding of FLII functions. In this study, we found that FLII was downregulated in lung carcinoma cell lines H1299 and A549 as compared with normal HBE (human bronchial epithelial) cell line. The investigation of FLII interactome in H1299 cells revealed that 74 of the total 132 putative FLII interactors are involved in RNA post-transcriptional modification and trafficking. Furthermore, by using high-throughput transcriptome and translatome sequencing combined with cell fractionation, we showed that the overexpression or knockdown of FLII impacts on the overall nuclear export, and translation of mRNAs. IPA analysis revealed that the majority of these target mRNAs encode the proteins whose functions are reminiscent of those previously reported for FLII, suggesting that the post-transcriptional regulation of mRNA might be a major mechanism of action for FLII.

MondoA/ChREBP: The usual suspects of transcriptional glucose sensing; Implication in pathophysiology

Identification of the Mondo glucose-responsive transcription factors family, including the MondoA and MondoB/ChREBP paralogs, has shed light on the mechanism whereby glucose affects gene transcription. They have clearly emerged, in recent years, as key mediators of glucose sensing by multiple cell types. MondoA and ChREBP have overlapping yet distinct expression profiles, which underlie their downstream targets and separate roles in regulating genes involved in glucose metabolism. MondoA can restrict glucose uptake and influences energy utilization in skeletal muscle, while ChREBP signals energy storage through de novo lipogenesis in liver and white adipose tissue. Because Mondo proteins mediate metabolic adaptations to changing glucose levels, a better understanding of cellular glucose sensing through Mondo proteins will likely uncover new therapeutic opportunities in the context of the imbalanced glucose homeostasis that accompanies metabolic diseases such as type 2 diabetes and cancer. Here, we provide an overview of structural homologies, transcriptional partners as well as the nutrient and hormonal mechanisms underlying Mondo proteins regulation. We next summarize their relative contribution to energy metabolism changes in physiological states and the evolutionary conservation of these pathways. Finally, we discuss their possible targeting in human pathologies.

Cytoskeletal protein Flightless I inhibits apoptosis, enhances tumor cell invasion and promotes cutaneous squamous cell carcinoma progression

Flightless I (Flii) is an actin remodeling protein that affects cellular processes including adhesion, proliferation and migration. In order to determine the role of Flii during carcinogenesis, squamous cell carcinomas (SCCs) were induced in Flii heterozygous (Flii+/-), wild-type and Flii overexpressing (FliiTg/Tg) mice by intradermal injection of 3-methylcholanthrene (MCA). Flii levels were further assessed in biopsies from human SCCs and the human SCC cell line (MET-1) was used to determine the effect of Flii on cellular invasion. Flii was highly expressed in human SCC biopsies particularly by the invading cells at the tumor edge. FliiTg/Tg mice developed large, aggressive SCCs in response to MCA. In contrast Flii+/- mice had significantly smaller tumors that were less invasive. Intradermal injection of Flii neutralizing antibodies during SCC initiation and progression significantly reduced the size of the tumors and, in vitro, decreased cellular sphere formation and invasion. Analysis of the tumors from the Flii overexpressing mice showed reduced caspase I and annexin V expression suggesting Flii may negatively regulate apoptosis within these tumors. These studies therefore suggest that Flii enhances SCC tumor progression by decreasing apoptosis and enhancing tumor cell invasion. Targeting Flii may be a potential strategy for reducing the severity of SCCs.

Cytoskeletal Regulation of Inflammation and Its Impact on Skin Blistering Disease Epidermolysis Bullosa Acquisita

Actin remodelling proteins regulate cytoskeletal cell responses and are important in both innate and adaptive immunity. These responses play a major role in providing a fine balance in a cascade of biological events that results in either protective acute inflammation or chronic inflammation that leads to a host of diseases including autoimmune inflammation mediated epidermolysis bullosa acquisita (EBA). This review describes the role of the actin cytoskeleton and in particular the actin remodelling protein called Flightless I (Flii) in regulating cellular inflammatory responses and its subsequent effect on the autoimmune skin blistering disease EBA. It also outlines the potential of an antibody based therapy for decreasing Flii expression in vivo to ameliorate the symptoms associated with EBA.

Identification of HNF-4α as a key transcription factor to promote ChREBP expression in response to glucose

Transcription factor carbohydrate responsive element binding protein (ChREBP) promotes glycolysis and lipogenesis in metabolic tissues and cancer cells. ChREBP-α and ChREBP-β, two isoforms of ChREBP transcribed from different promoters, are both transcriptionally induced by glucose. However, the mechanism by which glucose increases ChREBP mRNA levels remains unclear. Here we report that hepatocyte nuclear factor 4 alpha (HNF-4α) is a key transcription factor for glucose-induced ChREBP-α and ChREBP-β expression. Ectopic HNF-4α expression increased ChREBP transcription while knockdown of HNF-4α greatly reduced ChREBP mRNA levels in liver cancer cells and mouse primary hepatocytes. HNF-4α not only directly bound to an E-box-containing region in intron 12 of the ChREBP gene, but also promoted ChREBP-β transcription by directly binding to two DR1 sites and one E-box-containing site of the ChREBP-β promoter. Moreover, HNF-4α interacted with ChREBP-α and synergistically promoted ChREBP-β transcription. Functionally, HNF-4α suppression reduced glucose-dependent ChREBP induction. Increased nuclear abundance of HNF-4α and its binding to cis-elements of ChREBP gene in response to glucose contributed to glucose-responsive ChREBP transcription. Taken together, our results not only revealed the novel mechanism by which HNF-4α promoted ChREBP transcription in response to glucose, but also demonstrated that ChREBP-α and HNF-4α synergistically increased ChREBP-β transcription.

High glucose induces platelet-derived growth factor-C via carbohydrate response element-binding protein in glomerular mesangial cells

Persistent high concentration of glucose causes cellular stress and damage in diabetes via derangement of gene expressions. We previously reported high glucose activates hypoxia‐inducible factor‐1α and downstream gene expression in mesangial cells, leading to an extracellular matrix expansion in the glomeruli. A glucose‐responsive transcription factor carbohydrate response element‐binding protein (ChREBP) is a key mediator for such perturbation of gene regulation. To provide insight into glucose‐mediated gene regulation in mesangial cells, we performed chromatin immunoprecipitation followed by DNA microarray analysis and identified platelet‐derived growth factor‐C (PDGF‐C) as a novel target gene of ChREBP. In streptozotocin‐induced diabetic mice, glomerular cells showed a significant increase in PDGF‐C expression; the ratio of PDGF‐C‐positive cells to the total number glomerular cells demonstrated more than threefold increase when compared with control animals. In cultured human mesangial cells, high glucose enhanced expression of PDGF‐C protein by 1.9‐fold. Knock‐down of ChREBP abrogated this induction response. Upregulated PDGF‐C contributed to the production of type IV and type VI collagen, possibly via an autocrine mechanism. Interestingly, urinary PDGF‐C levels in diabetic model mice were significantly elevated in a fashion similar to urinary albumin. Taken together, we hypothesize that a high glucose‐mediated induction of PDGF‐C via ChREBP in mesangial cells contributes to the development of glomerular mesangial expansion in diabetes, which may provide a platform for novel predictive and therapeutic strategies for diabetic nephropathy.

Flightless I Homolog Represses Prostate Cancer Progression through Targeting Androgen Receptor Signaling

PURPOSE

Flightless I (FLII), member of the gelsolin superfamily of actin-remodeling proteins, functions as a transcriptional coregulator. We aim to evaluate a tumor-suppressive function of FLII in regulating androgen receptor (AR) in prostate cancer progression.

EXPERIMENTAL DESIGN

We examined FLII protein and mRNA expression in clinical prostate cancer specimens by immunohistochemistry. Kaplan-Meier analysis was conducted to evaluate the difference in disease-overall survival associated with the expression levels of FLII and AR. Prostate cancer cells stably expressing FLII or shRNA knockdown were used for functional analyses. Immunoprecipitation, Luciferase reporter, and immunofluorescence staining assays were performed to examine the functional interaction between FLII and AR.

RESULTS

Our analysis of the expression levels of FLII in a clinical gene expression array dataset showed that the expression of FLII was positively correlated with the overall survival of prostate cancer patients exhibiting high levels of AR expression. Examination of protein and mRNA levels of FLII showed a significant decrease of FLII expression in human prostate cancers. AR and FLII formed a complex in a ligand-dependent manner through the ligand-binding domain (LBD) of AR. Subsequently, we observed a competitive binding to AR between FLII and the ligand. FLII inhibited AR transactivation and decreased AR nuclear localization. Furthermore, FLII contributed to castration-sensitive and castration-resistant prostate cancer cell growth through AR-dependent signaling, and reintroduction of FLII in prostate cancer cells sensitized the cells to bicalutamide and enzalutamide treatment.

CONCLUSIONS

FLII plays a tumor-suppressive role and serves as a crucial determinant of resistance of prostate cancer to endocrine therapies.

Flightless-1, a novel transcriptional modulator of PPARγ through competing with RXRα

Peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor family and plays key roles in glucose and lipid metabolism. Its transcriptional control of target genes is mediated by ligand-dependent recruitment of coactivators. In this study, we demonstrate that a novel transcriptional modulator of PPARγ, Flightless-I (FLII) binds directly to and suppresses the transcriptional activity of PPARγ. The LXXLL motif within the leucine-rich repeat (LRR) domain of FLII interacts directly with the DNA-binding domain of PPARγ. Interestingly, in the presence of PPARγ ligands, such as rosiglitazone and SR1664, this interaction was abolished in vitro. When FLII was overexpressed, both the transcriptional activity of PPARγ and adipogenesis were suppressed significantly, whereas specific knockdown of FLII reversed these effects. Furthermore, DNA occupancy of PPARγ on its target gene promoters was enhanced by FLII knockdown, and the interaction between PPARγ and retinoid X receptor α (RXRα) was blocked by FLII. Together, these findings strongly suggest that FLII functions in PPARγ activation as a molecular switch to repress transcriptional activity by interrupting formation of the PPARγ/RXRα complex, and FLII may serve as a novel therapeutic target in the treatment of adiposity-related metabolic syndromes.