APPL suppresses androgen receptor transactivation via potentiating Akt activity

Regulation of Phosphoinositide Signaling by Scaffolds at Cytoplasmic Membranes

Cytoplasmic phosphoinositides (PI) are critical regulators of the membrane-cytosol interface that control a myriad of cellular functions despite their low abundance among phospholipids. The metabolic cycle that generates different PI species is crucial to their regulatory role, controlling membrane dynamics, vesicular trafficking, signal transduction, and other key cellular events. The synthesis of phosphatidylinositol (3,4,5)-triphosphate (PI3,4,5P3) in the cytoplamic PI3K/Akt pathway is central to the life and death of a cell. This review will focus on the emerging evidence that scaffold proteins regulate the PI3K/Akt pathway in distinct membrane structures in response to diverse stimuli, challenging the belief that the plasma membrane is the predominant site for PI3k/Akt signaling. In addition, we will discuss how PIs regulate the recruitment of specific scaffolding complexes to membrane structures to coordinate vesicle formation, fusion, and reformation during autophagy as well as a novel lysosome repair pathway.

Akt scaffold proteins: the key to controlling specificity of Akt signaling

The phosphatidylinositol 3-kinase-Akt signaling pathway plays an essential role in regulating cell proliferation and apoptosis. Akt kinase is at the center of this signaling pathway and interacts with a variety of proteins. Akt is overexpressed in almost 80% of tumors. However, inhibiting Akt has serious clinical side effects so is not a suitable treatment for cancer. During recent years, Akt scaffold proteins have received increasing attention for their ability to regulate Akt signaling and have emerged as potential targets for cancer therapy. In this paper, we categorize Akt kinase scaffold proteins into four groups based on their cellular location: membrane-bound activator and inhibitor, cytoplasm, and endosome. We describe how these scaffolds interact with Akt kinase, how they affect Akt activity, and how they regulate the specificity of Akt signaling. We also discuss the clinical application of Akt scaffold proteins as targets for cancer therapy.

APPL1 is a multifunctional endosomal signaling adaptor protein

Endosomal adaptor proteins are important regulators of signaling pathways underlying many biological processes. These adaptors can integrate signals from multiple pathways via localization to specific endosomal compartments, as well as through multiple protein-protein interactions. One such adaptor protein that has been implicated in regulating signaling pathways is the adaptor protein containing a pleckstrin homology (PH) domain, phosphotyrosine-binding (PTB) domain, and leucine zipper motif 1 (APPL1). APPL1 localizes to a subset of Rab5-positive endosomes through its Bin-Amphiphysin-Rvs and PH domains, and it coordinates signaling pathways through its interaction with many signaling receptors and proteins through its PTB domain. This review discusses our current understanding of the role of APPL1 in signaling and trafficking, as well as highlights recent work into the function of APPL1 in cell migration and adhesion.

APPLs: More than just adiponectin receptor binding proteins

APPLs (adaptor proteins containing the pleckstrin homology domain, phosphotyrosine binding domain and leucine zipper motif) are multifunctional adaptor proteins that bind to various membrane receptors, nuclear factors and signaling proteins to regulate many biological activities and processes, such as cell proliferation, chromatin remodeling, endosomal trafficking, cell survival, cell metabolism and apoptosis. APPL1, one of the APPL isoforms, was the first identified protein and interacts directly with adiponectin receptors to mediate adiponectin signaling to enhance lipid oxidation and glucose uptake. APPLs also act on insulin signaling pathways and are important mediators of insulin sensitization. Based on recent findings, this review highlights the critical roles of APPLs, particularly APPL1 and its isoform partner APPL2, in mediating adiponectin, insulin, endosomal trafficking and other signaling pathways. A deep understanding of APPLs and their related signaling pathways may potentially lead to therapeutic and interventional treatments for obesity, diabetes, cancer and neurodegenerative diseases.

APPL proteins promote TGFβ-induced nuclear transport of the TGFβ type I receptor intracellular domain

The multifunctional cytokine transforming growth factor-β (TGFβ) is produced by several types of cancers, including prostate cancer, and promote tumour progression in autocrine and paracrine manners. In response to ligand binding, the TGFβ type I receptor (TβRI) activates Smad and non-Smad signalling pathways. The ubiquitin-ligase tumour necrosis factor receptor-associated factor 6 (TRAF6) was recently linked to regulate intramembrane proteolytic cleavage of the TβRI in cancer cells. Subsequently, the intracellular domain (ICD) of TβRI enters in an unknown manner into the nucleus, where it promotes the transcription of pro-invasive genes, such as MMP2 and MMP9. Here we show that the endocytic adaptor molecules APPL1 and APPL2 are required for TGFβ-induced nuclear translocation of TβRI-ICD and for cancer cell invasiveness of human prostate and breast cancer cell lines. Moreover, APPL proteins were found to be expressed at high levels in aggressive prostate cancer tissues, and to be associated with TβRI in a TRAF6-dependent manner. Our results suggest that the APPL–TβRI complex promotes prostate tumour progression, and may serve as a prognostic marker.

A novel function of TBK1 as a target of Cdon in oligodendrocyte differentiation and myelination

During central nervous system development, oligodendrocyte progenitors elaborate multiple branched processes to contact axons and initiate myelination. Using cultured primary rat oligodendrocytes (OLGs), we have recently demonstrated that a cell surface protein belonging to the immunoglobulin superfamily, cell adhesion molecule-related, down-regulated by oncogenes (Cdon), is important in initiating OLG differentiation and axon myelination by promoting the formation of branched cellular processes; however, the molecular mechanism by which Cdon regulates OLG differentiation is not known. Here, using Cdon immunoprecipitation (IP) and liquid chromatography-tandem mass spectrometry analysis, we identified serine/threonine kinase TANK-binding kinase 1 (TBK1) as a candidate novel target of Cdon. We confirmed this interaction using co-IP and immunofluorescence with TBK1 antibodies, showing that TBK1 partly co-localizes with Cdon along cellular processes in puncta-like structures. We show that TBK1 is expressed throughout OLG differentiation, and surprisingly, that levels of phosphorylated TBK1 (ser172) increase during OLG maturation, while total levels of TBK1 protein decrease. To investigate function, TBK1 expression was knocked down using siRNA in OLG primary cultures, reducing protein levels by 69%. Two myelin-specific proteins, myelin basic protein and myelin-associated glycoprotein, were similarly reduced when examined at day 2 and day 4 of OLG differentiation. Reduced Cdon or TBK1 expression also decreased Akt phosphorylation at Threonine 308 in OLG. Our findings provide evidence that a Cdon-TBK1 complex is associated with Akt phosphorylation and early OLG differentiation.

Experimental verification of a predicted novel microRNA located in human PIK3CA gene with a potential oncogenic function in colorectal cancer

PI3K/AKT signaling is involved in cell survival, proliferation, and migration. In this pathway, PI3Kα enzyme is composed of a regulatory protein encoded by p85 gene and a catalytic protein encoded by PIK3CA gene. Human PIK3CA locus is amplified in several cancers including lung and colorectal cancer (CRC). Therefore, microRNAs (miRNAs) that are encoded within the PIK3CA gene might have a role in cancer development. Here, we report a novel microRNA named PIK3CA-miR1 (EBI accession no. LN626315), which is located within PIK3CA gene. A DNA segment corresponding to PIK3CA-premir1 sequence was transfected in human cell lines that resulted in generation of mature exogenous PIK3CA-miR1. Following the overexpression of PIK3CA-miR1, its predicted target genes (APPL1 and TrkC) were significantly downregulated in the CRC-originated HCT116 and SW480 cell lines, detected by qRT-PCR. Then, dual luciferase assay supported the interaction of PIK3CA-miR1 with APPL1 and TrkC transcripts. Endogenous PIK3CA-miR1 expression was also detected in several cell lines (highly in HCT116 and SW480) and highly in CRC specimens. Consistently, overexpression of PIK3CA-premir1 in HCT116 and SW480 cells resulted in significant reduction of the sub-G1 cell distribution and apoptotic cell rate, as detected by flowcytometry, and resulted in increased cell proliferation, as detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. PIK3CA-miR1 overexpression also resulted in Wnt signaling upregulation detected by Top/Fop assay. Overall, accumulative evidences indicated the presence of a bona fide novel onco-miRNA encoded within the PIK3CA oncogene, which is highly expressed in colorectal cancer and has a survival effect in CRC-originated cells.

The adaptor protein APPL2 inhibits insulin-stimulated glucose uptake by interacting with TBC1D1 in skeletal muscle

Insulin stimulates glucose uptake by promoting the trafficking of GLUT4 to the plasma membrane in muscle cells, and impairment of this insulin action contributes to hyperglycemia in type 2 diabetes. The adaptor protein APPL1 potentiates insulin-stimulated Akt activation and downstream actions. However, the physiological functions of APPL2, a close homolog of APPL1, in regulating glucose metabolism remain elusive. We show that insulin-evoked plasma membrane recruitment of GLUT4 and glucose uptake are impaired by APPL2 overexpression but enhanced by APPL2 knockdown. Likewise, conditional deletion of APPL2 in skeletal muscles enhances insulin sensitivity, leading to an improvement in glucose tolerance. We identified the Rab-GTPase-activating protein TBC1D1 as an interacting partner of APPL2. Insulin stimulates TBC1D1 phosphorylation on serine 235, leading to enhanced interaction with the BAR domain of APPL2, which in turn suppresses insulin-evoked TBC1D1 phosphorylation on threonine 596 in cultured myotubes and skeletal muscle. Substitution of serine 235 with alanine diminishes APPL2-mediated inhibition on insulin-dependent TBC1D1 phosphorylation on threonine 596 and the suppressive effects of TBC1D1 on insulin-induced glucose uptake and GLUT4 translocation to the plasma membrane in cultured myotubes. Therefore, the APPL2-TBC1D1 interaction is a key step to fine tune insulin-stimulated glucose uptake by regulating the membrane recruitment of GLUT4 in skeletal muscle.

AdipoR1/APPL1 potentiates the protective effects of globular adiponectin on angiotensin II-induced cardiac hypertrophy and fibrosis in neonatal rat atrial myocytes and fibroblasts

Atrial hypertrophy and fibrosis are essential pathological features of atrial fibrillation. Recently, adiponectin has become a protein of interest due to its beneficial effects on cardiovascular diseases. However, the molecular mechanism of atrial structural remodeling and signaling pathways evoked by adiponectin remain unclear. In the present study, we investigated the cardioprotective effect of globular adiponectin (gAcrp) on angiotensin II-induced atrial hypertrophy and fibrosis in neonatal Sprague-Dawley rat. To further investigate the molecular mechanisms underlying the preventive effect of gAcrp, transfection of cells with siRNA was used to suppress the mRNA expression of adiponectin receptor 1 (AdipoR1) and its downstream adaptor protein APPL1. Non-silencing-Cy-3 labelled siRNA was used to determine transfection efficiency using fluorescence microscopy. The expression of atrial natriuretic peptide and procollagen type1 α-1, hypertrophy marker and fibrosis one, respectively, was detected by real-time PCR. Furthermore, the expression of adenosine monophosphate-activated protein kinase (AMPK), phosphatidylinositol 3-kinase (PI3K) and Akt was detected by western blotting. In addition, nuclear p65 translocation activity was analyzed by EMSA supershift assay. Our results showed that AdipoR1 and the adaptor protein APPL1 mediated the protective effects of gAcrp. In addition, the function of adiponectin and phosphorylation of AMPK were prominently diminished by inhibition of PI3K. Furthermore, nuclear factor-κB (NF-κB) transcription was diminished by the specific inhibition of AMPK. Taken together, AMPK pivotally interacts with NF-κB and PI3K, mediating the cardioprotective effect of adiponectin, and may serve as a therapeutic target for preventing atrial hypertrophy and fibrosis. Our present study suggests that gAcrp could ameliorate AngII-induced cardiac hypertrophy and fibrosis in rat atrial cells, which is mediated by the activation of AMPK signaling pathways. APPL1 and AdipoR1 are the key factors involved in the downstream of gAcrp approach.

Altered endosome biogenesis in prostate cancer has biomarker potential

Prostate cancer is the second most common form of cancer in males, affecting one in eight men by the time they reach the age of 70 years. Current diagnostic tests for prostate cancer have significant problems with both false negatives and false positives, necessitating the search for new molecular markers. A recent investigation of endosomal and lysosomal proteins revealed that the critical process of endosomal biogenesis might be altered in prostate cancer. Here, a panel of endosomal markers was evaluated in prostate cancer and nonmalignant cells and a significant increase in gene and protein expression was found for early, but not late endosomal proteins. There was also a differential distribution of early endosomes, and altered endosomal traffic and signaling of the transferrin receptors (TFRC and TFR2) in prostate cancer cells. These findings support the concept that endosome biogenesis and function are altered in prostate cancer. Microarray analysis of a clinical cohort confirmed the altered endosomal gene expression observed in cultured prostate cancer cells. Furthermore, in prostate cancer patient tissue specimens, the early endosomal marker and adaptor protein APPL1 showed consistently altered basement membrane histology in the vicinity of tumors and concentrated staining within tumor masses. These novel observations on altered early endosome biogenesis provide a new avenue for prostate cancer biomarker investigation and suggest new methods for the early diagnosis and accurate prognosis of prostate cancer.

IMPLICATIONS

This discovery of altered endosome biogenesis in prostate cancer may lead to novel biomarkers for more precise cancer detection and patient prognosis.

Increased gene copy number of VAMP7 disrupts human male urogenital development through altered estrogen action

Vesicle transport is intimately connected with key nuclear functions and transcriptional regulation. Here, children born with congenital genitourinary tract masculinization disorders were analyzed by array-Comparative Genomic Hybridization, which revealed the presence of de novo copy number gains on Xq28 encompassing the VAMP7 gene encoding a vesicle-trafficking protein. Humanized VAMP7 BAC transgenic mice displayed cryptorchidism, urethral defects, and hypospadias. Mutant mice exhibited reduced penile length, focal spermatogenic anomalies, diminished sperm motility, and subfertility. VAMP7 colocalized with estrogen receptor alpha (ESR1) in the presence of ligand. Elevated levels of VAMP7 markedly intensified ESR1 transcriptional activity by increasing ESR1 protein cellular content upon ligand stimulation and up-regulated the expression of estrogen-responsive genes including ATF3, CYR61, and CTGF, all of which are implicated in human hypospadias. Hence, increased gene dosage of the SNARE protein, VAMP7, enhances estrogen receptor action in male genitourinary tissues, affects the virilization of the reproductive tract, and results in genitourinary birth defects in humans.

APPL1 potentiates insulin sensitivity by facilitating the binding of IRS1/2 to the insulin receptor

Binding of insulin receptor substrate proteins 1 and 2 (IRS1/2) to the insulin receptor (IR) is essential for the regulation of insulin sensitivity and energy homeostasis. However, the mechanism of IRS1/2 recruitment to the IR remains elusive. Here, we identify adaptor protein APPL1 as a critical molecule that promotes IRS1/2-IR interaction. APPL1 forms a complex with IRS1/2 under basal conditions, and this complex is then recruited to the IR in response to insulin or adiponectin stimulation. The interaction between APPL1 and IR depends on insulin- or adiponectin-stimulated APPL1 phosphorylation, which is greatly reduced in insulin target tissues in obese mice. appl1 deletion in mice consistently leads to systemic insulin resistance and a significant reduction in insulin-stimulated IRS1/2, but not IR, tyrosine phosphorylation, indicating that APPL1 sensitizes insulin signaling by acting at a site downstream of the IR. Our study uncovers a mechanism regulating insulin signaling and crosstalk between the insulin and adiponectin pathways.

Carboxyl-terminal modulator protein positively regulates Akt phosphorylation and acts as an oncogenic driver in breast cancer

Akt activation has been implicated broadly in tumorigenesis, but the basis for its dysregulation in cancer cells is incompletely understood. In this study, we sought to clarify a regulatory role for the Akt-binding carboxy-terminal modulator protein (CTMP), which has been controversial. In evaluating CTMP expression in paired normal-tumor specimens of 198 patients with breast cancer, we found that CTMP was upregulated in breast tumors, where it was associated with poor patient survival. Notably, CTMP expression also correlated positively with Akt phosphorylation in breast cancer clinical specimens and cell lines. Furthermore, ectopic expression of CTMP promoted cell proliferation and enhanced the tumorigenic properties of estrogen-dependent breast cancer cells. This effect was correlated with increased sensitivity to insulin-induced Akt phosphorylation, which is mediated primarily by the phosphoinositide 3-kinase-Akt pathway. In contrast, short hairpin RNA-mediated silencing of endogenous CTMP decreased the proliferation of estrogen-dependent or estrogen-independent breast cancer cells. Mechanistic investigations defined the N-terminal domain of CTMP at amino acids 1 to 64 as responsible for Akt binding. Taken together, our results firmly corroborate the concept that CTMP promotes Akt phosphorylation and functions as an oncogenic molecule in breast cancer.

Multifunctional protein APPL2 contributes to survival of human glioma cells

Some endocytic proteins have recently been shown to play a role in tumorigenesis. In this study, we demonstrate that APPL2, an adapter protein with known endocytic functions, is upregulated in 40% cases of glioblastoma multiforme, the most common and aggressive cancer of the central nervous system. The silencing of APPL2 expression by small interfering RNAs (siRNAs) in glioma cells markedly reduces cell survival under conditions of low growth factor availability and enhances apoptosis (measured by executor caspase activity). Long‐term depletion of APPL2 by short hairpin RNAs (shRNAs), under regular growth factor availability, suppresses the cell transformation abilities, assessed by inhibited colony formation in soft agar and by reduced xenograft tumor growth in vivo. At the molecular level, the negative effect of APPL2 knockdown on cell survival is not due to the alterations in AKT or GSK3β activities which were reported to be modulated by APPL proteins. Instead, we attribute the reduced cell survival upon APPL2 depletion to the changes in gene expression, in particular to the upregulation of apoptosis‐related genes, such as UNC5B (a proapoptotic dependence receptor) and HRK (harakiri, an activator of apoptosis, which antagonizes anti‐apoptotic function of Bcl2). In support of this notion, the loss of glioma cell survival upon APPL2 knockdown can be rescued either by an excess of netrin‐1, the prosurvival ligand of UNC5B or by simultaneous silencing of HRK. Consistently, APPL2 overexpression reduces expression of HRK and caspase activation in cells treated with apoptosis inducers, resulting in the enhancement of cell viability. This prosurvival activity of APPL2 is independent of its endosomal localization. Cumulatively, our data indicate that a high level of APPL2 protein might enhance glioblastoma growth by maintaining low expression level of genes responsible for cell death induction.

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APPL2 protein levels are elevated in 40% cases of glioblastoma multiforme.

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Overexpression of APPL2 exhibits cytoprotective effects in glioma cells.

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APPL2 depletion reduces survival and transformation abilities of glioma cells.

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Silencing of APPL2 promotes expression of proapoptotic genes HRK and UNC5B.

SGEF is overexpressed in prostate cancer and contributes to prostate cancer progression

The purpose of this study was to investigate the potential roles of the SH3-containing guanine nucleotide exchange factor (SGEF) in human prostate cancer. Experimental data showed that SGEF was overexpressed in human prostate cancer cells and specimens. The reduction of SGEF expression through an SGEF-targeting siRNA in androgen-independent C4-2 and C4-2B cells suppressed both anchorage-dependent and anchorage-independent growth. In addition, the androgen receptor (AR) antagonist bicalutamide further strengthened this inhibitory effect due to the suppression of the elevated AR transactivation after knockdown of SGEF. Collectively, our results provide the first demonstration that SGEF is a novel promoter of human prostate cancer progression and development.

Expression of adaptor protein containing PH domain, PTB domain and leucine zipper motif 1 in colorectal carcinogenesis

AIM: To investigate the relationship between expression of adaptor protein containing PH domain, PTB domain and leucine zipper motif 1(APPL1) and clinicopathological parameters of colorectal cancer.

METHODS: Expression of APPL1 protein and mRNA in 35 surgical specimens of colorectal carcinoma (CRC) and 27 normal colorectal tissue specimens was detected by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR), respectively.

RESULTS: The expression of APPL1 in CRC was significantly higher than that in normal mucosa. APPL1 expression was correlated with histological differentiation, lymph node metastasis and TNM stage (all P < 0.05), but not with sex, age, or tumor size (all P > 0.05) in patients with CRC.

CONCLUSION: The expression of APPL1 protein is increased in CRC. APPL1 protein expression is closely related with tumor differentiation, lymph node metastasis and TNM stage in patients with colorectal cancer. APPL1 may be a novel therapeutic target for CRC.

Retrolinkin cooperates with endophilin A1 to mediate BDNF-TrkB early endocytic trafficking and signaling from early endosomes

Both retrolinkin and its interaction partner endophilin A1 are required for BDNF-induced dendrite outgrowth of cultured hippocampal neurons. They function sequentially in an early endocytic trafficking pathway for BDNF-activated TrkB, which provides spatiotemporal control of downstream ERK signaling from endosomes.

Brain-derived neurotrophic factor (BDNF) binds to its cell surface receptor TrkB to regulate differentiation, development, synaptic plasticity, and functional maintenance of neuronal cells. Binding of BDNF triggers TrkB dimerization and autophosphorylation, which provides docking sites for adaptor proteins to recruit and activate downstream signaling molecules. The molecular mechanisms underlying BDNF–TrkB endocytic trafficking crucial for spatiotemporal control of signaling pathways remain to be elucidated. Here we show that retrolinkin, a transmembrane protein, interacts with endophilin A1 and mediates BDNF-activated TrkB (pTrk) trafficking and signaling in CNS neurons. We find that activated TrkB colocalizes and interacts with the early endosome marker APPL1. Both retrolinkin and endophilin A1 are required for BDNF-induced dendrite development and acute extracellular signal-regulated kinase activation from early endosomes. Suppression of retrolinkin expression not only blocks BDNF-triggered TrkB internalization, but also prevents recruitment of endophilin A1 to pTrk vesicles trafficking through APPL1-positive endosomes. These findings reveal a novel mechanism for BDNF–TrkB to regulate signaling both in time and space through a specific membrane trafficking pathway.

Increased abundance of the adaptor protein containing pleckstrin homology domain, phosphotyrosine binding domain and leucine zipper motif (APPL1) in patients with obesity and type 2 diabetes: evidence for altered adiponectin signalling

AIMS/HYPOTHESIS

The adiponectin signalling pathway is largely unknown, but recently the adaptor protein containing pleckstrin homology domain, phosphotyrosine binding domain and leucine zipper motif (APPL1), has been shown to interact directly with adiponectin receptor (ADIPOR)1. APPL1 is present in C2C12 myoblasts and mouse skeletal muscle, but its presence in human skeletal muscle has not been investigated.

METHODS

Samples from type 2 diabetic, and lean and non-diabetic obese participants were analysed by: immunoprecipitation and western blot; HPLC-electrospray ionisation (ESI)-mass spectrometry (MS) analysis; peak area analysis by MS; HPLC-ESI-MS/MS/MS analysis; and RT-PCR analysis of APPL1 mRNA.

RESULTS

Immunoprecipitation and western blot indicated a band specific to APPL1. Tryptic digestion and HPLC-ESI-MS analysis of whole-muscle homogenate APPL1 unambiguously identified APPL1 with 56% sequence coverage. Peak area analysis by MS validated western blot results, showing APPL1 levels to be significantly increased in type 2 diabetic and obese as compared with lean participants. Targeted phosphopeptide analysis by HPLC-ESI-MS/MS/MS showed that APPL1 was phosphorylated specifically on Ser(401). APPL1 mRNA expression was significantly increased in obese and type 2 diabetic participants as compared with lean participants. After bariatric surgery in morbidly obese participants with subsequent weight loss, skeletal muscle APPL1 abundance was significantly reduced (p < 0.05) in association with an increase in plasma adiponectin (p < 0.01), increased levels of ADIPOR1 (p < 0.05) and increased muscle AMP-activated protein kinase (AMPK) phosphorylation (p < 0.05).

CONCLUSIONS/INTERPRETATION

APPL1 abundance is significantly higher in type 2 diabetic muscle; APPL1 is phosphorylated in vivo on Ser(401). Improvements in hyperglycaemia and hypoadiponectinaemia following weight loss are associated with reduced skeletal muscle APPL1, and increased plasma adiponectin levels and muscle AMPK phosphorylation.

Adiponectin Inhibits Spontaneous and Catecholamine-Induced Lipolysis in Human Adipocytes of Non-Obese Subjects Through AMPK-Dependent Mechanisms

Adiponectin is an adipokine increasing glucose and fatty acid metabolism and improving insulin sensitivity. The aim of this study was to investigate the role of adiponectin in the regulation of adipocyte lipolysis. Human adipocytes isolated from biopsies obtained during surgical operations from 16 non-obese and 17 obese subjects were incubated with 1) human adiponectin (20 µg/ml) or 2) 0.5 mM AICAR – activator of AMPK (adenosine monophosphate activated protein kinase). Following these incubations, isoprenaline was added (10-6 M) to investigate the influence of adiponectin and AICAR on catecholamine-induced lipolysis. Glycerol concentration was measured as lipolysis marker. We observed that adiponectin suppressed spontaneous lipolysis by 21 % and isoprenaline-induced lipolysis by 14 % in non-obese subjects. These effects were not detectable in obese individuals, but statistically significant differences in the effect of adiponectin between obese and non-obese were not revealed by two way ANOVA test. The inhibitory effect of AICAR and adiponectin on lipolysis was reversed by Compound C. Our results suggest, that adiponectin in physiological concentrations inhibits spontaneous as well as catecholamine-induced lipolysis. This effect might be lower in obese individuals and this regulation seems to involve AMPK.

An APPL1/Akt signaling complex regulates dendritic spine and synapse formation in hippocampal neurons

The formation and plasticity of dendritic spines and synapses, which are poorly understood on a molecular level, are critical for cognitive functions, such as learning and memory. The adaptor protein containing a PH domain, PTB domain, and leucine zipper motif (APPL1) is emerging as a critical regulator of various cellular processes in non-neuronal cells, but its function in the nervous system is not well understood. Here, we show that APPL1 localizes to dendritic spines and synapses and regulates the development of these structures in hippocampal neurons. Knockdown of endogenous APPL1 using siRNA led to a significant decrease in the number of spines as well as synapses and this defect could be rescued by expression of siRNA-resistant APPL1. Expression of exogenous APPL1 increased the spine and synaptic density and the amount of surface GluR1-containing α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). Deletion of the C-terminal phosphotyrosine binding domain of APPL1, which binds the serine/threonine kinase Akt, resulted in a significant decrease in the spine and synaptic density, suggesting a role for Akt in regulating the development of these structures. Consistent with this, knockdown of Akt with siRNA or expression of dominant negative Akt led to a dramatic decrease in spine and synapse formation. In addition, APPL1 increased the amount of active Akt in spines and synapses and the effects of APPL1 on these structures were dependent on Akt, indicating that Akt is an effector of APPL1 in the regulation of these processes. Moreover, APPL1 signaling modulates spine and synapse formation through p21-activated kinase (PAK). Thus, our results indicate that APPL1 signaling through Akt and PAK is critical for spine and synaptic development and point to a role for APPL1 and its effectors in regulating cognitive function.

Adiponectin lowers glucose production by increasing SOGA

Adiponectin is a hormone that lowers glucose production by increasing liver insulin sensitivity. Insulin blocks the generation of biochemical intermediates for glucose production by inhibiting autophagy. However, autophagy is stimulated by an essential mediator of adiponectin action, AMPK. This deadlock led to our hypothesis that adiponectin inhibits autophagy through a novel mediator. Mass spectrometry revealed a novel protein that we call suppressor of glucose by autophagy (SOGA) in adiponectin-treated hepatoma cells. Adiponectin increased SOGA in hepatocytes, and siRNA knockdown of SOGA blocked adiponectin inhibition of glucose production. Furthermore, knockdown of SOGA increased late autophagosome and lysosome staining and the secretion of valine, an amino acid that cannot be synthesized or metabolized by liver cells, suggesting that SOGA inhibits autophagy. SOGA decreased in response to AICAR, an activator of AMPK, and LY294002, an inhibitor of the insulin signaling intermediate, PI3K. AICAR reduction of SOGA was blocked by adiponectin; however, adiponectin did not increase SOGA during PI3K inhibition, suggesting that adiponectin increases SOGA through the insulin signaling pathway. SOGA contains an internal signal peptide that enables the secretion of a circulating fragment of SOGA, providing a surrogate marker for intracellular SOGA levels. Circulating SOGA increased in parallel with adiponectin and insulin activity in both humans and mice. These results suggest that adiponectin-mediated increases in SOGA contribute to the inhibition of glucose production.

Appl1 is dispensable for mouse development, and loss of Appl1 has growth factor-selective effects on Akt signaling in murine embryonic fibroblasts

The adaptor protein APPL1 (adaptor protein containing pleckstrin homology (PH), phosphotyrosine binding (PTB), and leucine zipper motifs) was first identified as a binding protein of AKT2 by yeast two-hybrid screening. APPL1 was subsequently found to bind to several membrane-bound receptors and was implicated in their signal transduction through AKT and/or MAPK pathways. To determine the unambiguous role of Appl1 in vivo, we generated Appl1 knock-out mice. Here we report that Appl1 knock-out mice are viable and fertile. Appl1-null mice were born at expected Mendelian ratios, without obvious phenotypic abnormalities. Moreover, Akt activity in various fetal tissues was unchanged compared with that observed in wild-type littermates. Studies of isolated Appl1(-/-) murine embryonic fibroblasts (MEFs) showed that Akt activation by epidermal growth factor, insulin, or fetal bovine serum was similar to that observed in wild-type MEFs, although Akt activation by HGF was diminished in Appl1(-/-) MEFs. To rule out a possible redundant role played by the related Appl2, we used small interfering RNA to knock down Appl2 expression in Appl1(-/-) MEFs. Unexpectedly, cell survival was unaffected under normal culture conditions, and activation of Akt was unaltered following epidermal growth factor stimulation, although Akt activity did decrease further after HGF stimulation. Furthermore, we found that Appl proteins are required for HGF-induced cell survival and migration via activation of Akt. Our studies suggest that Appl1 is dispensable for development and only participate in Akt signaling under certain conditions.

Functional characterization of the interactions between endosomal adaptor protein APPL1 and the NuRD co-repressor complex

Multifunctional adaptor protein APPL1 [adaptor protein containing PH (pleckstrin homology) domain, PTB (phosphotyrosine binding) domain and leucine zipper motif] belongs to a growing group of endocytic proteins which actively participate in various stages of signalling pathways. Owing to its interaction with the small GTPase Rab5, APPL1 localizes predominantly to a subpopulation of early endosomes but is also capable of nucleocytoplasmic shuttling. Among its various binding partners, APPL1 was reported to associate with the nuclear co-repressor complex NuRD (nucleosome remodelling and deacetylase), containing both nucleosome remodelling and HDAC (histone deacetylase) activities, but the biochemical basis or functional relevance of this interaction remained unknown. Here we characterized the binding between APPL1 and NuRD in more detail, identifying HDAC2 as the key NuRD subunit responsible for this association. APPL1 interacts with the NuRD complex containing enzymatically active HDAC2 but not HDAC1 as the only deacetylase. However, the cellular levels of HDAC1 can regulate the extent of APPL1–NuRD interactions, which in turn modulates the nucleocytoplasmic distribution of APPL1. Increased binding of APPL1 to NuRD upon silencing of HDAC1 promotes the nuclear localization of APPL1, whereas HDAC1 overexpression exerts an opposite effect. Moreover, we also uncovered a NuRD-independent interaction of APPL1 with HDAC1. APPL1 overexpression affects the composition of the HDAC1-containing NuRD complex and the expression of HDAC1 target p21^WAF1/CIP1. Cumulatively, these data reveal a surprising complexity of APPL1 interactions with HDACs, with functional consequences for the modulation of gene expression. In a broader sense, these results contribute to an emerging theme of endocytic proteins playing alternative roles in the cell nucleus.

Yin-Yang regulation of adiponectin signaling by APPL isoforms in muscle cells

APPL1 is a newly identified adiponectin receptor-binding protein that positively mediates adiponectin signaling in cells. Here we report that APPL2, an isoform of APPL1 that forms a dimer with APPL1, can interacts with both AdipoR1 and AdipoR2 and acts as a negative regulator of adiponectin signaling in muscle cells. Overexpression of APPL2 inhibits the interaction between APPL1 and AdipoR1, leading to down-regulation of adiponectin signaling in C2C12 myotubes. In contrast, suppressing APPL2 expression by RNAi significantly enhances adiponectin-stimulated glucose uptake and fatty acid oxidation. In addition to targeting directly to and competing with APPL1 in binding with the adiponectin receptors, APPL2 also suppresses adiponectin and insulin signaling by sequestrating APPL1 from these two pathways. In addition to adiponectin, metformin also induces APPL1-APPL2 dissociation. Taken together, our results reveal that APPL isoforms function as an integrated Yin-Yang regulator of adiponectin signaling and mediate the cross-talk between adiponectin and insulin signaling pathways in muscle cells.

Endocytic proteins in the regulation of nuclear signaling, transcription and tumorigenesis

Accumulating evidence argues that many proteins governing membrane sorting during endocytosis participate also in nuclear signaling and transcriptional regulation, mostly by modulating the activity of various nuclear factors. Some adaptors and accessory proteins acting in clathrin-mediated internalization, as well as endosomal sorting proteins can undergo nuclear translocation and affect gene expression directly, while for others the effects may be more indirect. Although it is often unclear to what extent the endocytic and nuclear functions are interrelated, several of such proteins are implicated in the regulation of cell proliferation and tumorigenesis, arguing that their dual-function nature may be of physiological importance.

APPL1 potentiates insulin-mediated inhibition of hepatic glucose production and alleviates diabetes via Akt activation in mice

Hepatic insulin resistance is the major contributor to fasting hyperglycemia in type 2 diabetes. Here we report that the endosomal adaptor protein APPL1 increases hepatic insulin sensitivity by potentiating insulin-mediated suppression of the gluconeogenic program. Insulin-stimulated activation of Akt and suppression of gluconeogenesis in hepatocytes are enhanced by APPL1 overexpression, but are attenuated by APPL1 knockdown. APPL1 interacts with Akt and blocks the association of Akt with its endogenous inhibitor tribble 3 (TRB3) through direct competition, thereby promoting Akt translocation to the plasma membrane and the endosomes for further activation. In db/db diabetic mice, the blockage of the augmented interaction between Akt and TRB3 by hepatic overexpression of APPL1 is accompanied by a marked attenuation of hyperglycemia and insulin resistance. These results suggest that the potentiating effects of APPL1 on insulin-stimulated suppression of hepatic glucose production are attributed to its ability in counteracting the inhibition of Akt activation by TRB3.

APPL1: role in adiponectin signaling and beyond

Adiponectin, an adipokine secreted by the white adipose tissue, plays an important role in regulating glucose and lipid metabolism and controlling energy homeostasis in insulin-sensitive tissues. A decrease in the circulating level of adiponectin has been linked to insulin resistance, type 2 diabetes, atherosclerosis, and metabolic syndrome. Adiponectin exerts its effects through two membrane receptors, AdipoR1 and AdipoR2. APPL1 is the first identified protein that interacts directly with adiponectin receptors. APPL1 is an adaptor protein with multiple functional domains, the Bin1/amphiphysin/rvs167, pleckstrin homology, and phosphotyrosine binding domains. The PTB domain of APPL1 interacts directly with the intracellular region of adiponectin receptors. Through this interaction, APPL1 mediates adiponectin signaling and its effects on metabolism. APPL1 also functions in insulin-signaling pathway and is an important mediator of adiponectin-dependent insulin sensitization in skeletal muscle. Adiponectin signaling through APPL1 is necessary to exert its anti-inflammatory and cytoprotective effects on endothelial cells. APPL1 also acts as a mediator of other signaling pathways by interacting directly with membrane receptors or signaling proteins, thereby playing critical roles in cell proliferation, apoptosis, cell survival, endosomal trafficking, and chromatin remodeling. This review focuses mainly on our current understanding of adiponectin signaling in various tissues, the role of APPL1 in mediating adiponectin signaling, and also its role in the cross-talk between adiponectin/insulin-signaling pathways.

BAR the door: cancer suppression by amphiphysin-like genes

The evolutionarily conserved amphiphysin-like genes Bin1 and Bin3 function in membrane and actin dynamics, cell polarity, and stress signaling. Recent genetic studies in mice discriminate non-essential roles in endocytic processes commonly ascribed to amphiphysins from essential roles in cancer suppression. Bin1 acts in default pathways of apoptosis and senescence that are triggered by the Myc and Raf oncogenes in primary cells, and Bin1 gene products display a 'moonlighting function' in the nucleus where a variety of other 'endocytic' proteins are also found. Together, genetic investigations in yeast, flies, and mice suggest that amphiphysin-like adapter proteins may suppress cancer by helping integrate cell polarity signals generated by actin and vesicle dynamics with central regulators of cell cycle arrest, apoptosis, and immune surveillance.

The endosomal protein Appl1 mediates Akt substrate specificity and cell survival in vertebrate development

During development of multicellular organisms, cells respond to extracellular cues through nonlinear signal transduction cascades whose principal components have been identified. Nevertheless, the molecular mechanisms underlying specificity of cellular responses remain poorly understood. Spatial distribution of signaling proteins may contribute to signaling specificity. Here, we tested this hypothesis by investigating the role of the Rab5 effector Appl1, an endosomal protein that interacts with transmembrane receptors and Akt. We show that in zebrafish, Appl1 regulates Akt activity and substrate specificity, controlling GSK-3beta but not TSC2. Consistent with this pattern, Appl1 is selectively required for cell survival, most critically in highly expressing tissues. Remarkably, Appl1 function requires its endosomal localization. Indeed, Akt and GSK-3beta, but not TSC2, dynamically associate with Appl1 endosomes upon growth factor stimulation. We propose that partitioning of Akt and selected effectors onto endosomal compartments represents a key mechanism contributing to the specificity of signal transduction in vertebrate development.

Androgen receptor (AR) coregulators: a diversity of functions converging on and regulating the AR transcriptional complex

Androgens, acting through the androgen receptor (AR), are responsible for the development of the male phenotype during embryogenesis, the achievement of sexual maturation at puberty, and the maintenance of male reproductive function and behavior in adulthood. In addition, androgens affect a wide variety of nonreproductive tissues. Moreover, aberrant androgen action plays a critical role in multiple pathologies, including prostate cancer and androgen insensitivity syndromes. The formation of a productive AR transcriptional complex requires the functional and structural interaction of the AR with its coregulators. In the last decade, an overwhelming and ever increasing number of proteins have been proposed to possess AR coactivating or corepressing characteristics. Intriguingly, a vast diversity of functions has been ascribed to these proteins, indicating that a multitude of cellular functions and signals converge on the AR to regulate its function. The current review aims to provide an overview of the AR coregulator proteins identified to date and to propose a classification of these AR coregulator proteins according to the function(s) ascribed to them. Taken together, this approach will increase our understanding of the cellular pathways that converge on the AR to ensure an appropriate transcriptional response to androgens.

Multiple facets of follicle-stimulating hormone receptor function

Follicle-stimulating hormone (FSH) is a glycoprotein hormone produced by the anterior pituitary gland. This gonadotropin plays an essential role in reproduction. Its receptor (FSHR) belongs to the superfamily of G protein-coupled receptors (GPCR), specifically the family of rhodopsin-like receptors. Agonist binding to the FSHR triggers the rapid activation of multiple signaling cascades, mainly the cAMP-adenylyl cyclase-protein kinase A cascade, that impact diverse biological effects of FSH in the gonads. As in other G protein-coupled receptors, the several cytoplasmic domains of the FSHR are involved in signal transduction and termination of the FSH signal. Here we summarize some recent information on the signaling cascades activated by FSH as well as on the role of the intracytoplasmic domains of the FSHR in coupling to membrane and cytosolic proteins linked to key biological functions regulated by the FSH-FSHR system.

The NF-kappaB/IL-6 pathway in metastatic androgen-independent prostate cancer: new therapeutic approaches?

The nuclear factor of kappa beta (NF-kappaB) transcription factor regulates the transcription of numerous genes including that of interleukin 6 (IL-6). The IL-6 acts as an autocrine and paracrine growth factor of androgen-independent prostate cancer. An aberrant expression of the IL-6 gene and an increase in IL-6 expression are detected in bone metastatic and hormone-refractory prostate cancer. IL-6 has been suggested to have a crucial role in the resistance to chemotherapy or hormonal therapy involving apoptotic cell death. The NF-kappaB/IL-6 dependent pathways promote tumour-cell survival and in most situations protect cells against apoptotic stimuli. These data provide a rational framework for targeting NF-kappaB and IL-6 activity in novel biologically based therapies for aggressive and androgen independent prostate cancers.

Adiponectin-induced endothelial nitric oxide synthase activation and nitric oxide production are mediated by APPL1 in endothelial cells

Adiponectin protects the vascular system partly through stimulation of endothelial nitric oxide (NO) production and endothelium-dependent vasodilation. The current study investigated the role of two recently identified adiponectin receptors, AdipoR1 and -R2, and their downstream effectors in mediating the endothelium actions of adiponectin. In human umbilical vein endothelial cells, adiponectin-induced phosphorylation of endothelial NO synthase (eNOS) at Ser(1177) and NO production were abrogated when expression of AdipoR1 and -R2 were simultaneously suppressed. Proteomic analysis demonstrated that the cytoplasmic tails of both AdipoR1 and -R2 interacted with APPL1, an adaptor protein that contains a PH (pleckstrin homology) domain, a PTB (phosphotyrosine-binding) domain, and a Leucine zipper motif. Suppression of APPL1 expression by RNA interference significantly attenuated adiponectin-induced phosphorylation of AMP-activated protein kinase (AMPK) at Thr(172) and eNOS at Ser(1177), and the complex formation between eNOS and heat shock protein 90, resulting in a marked reduction of NO production. Adenovirus-mediated overexpression of a constitutively active version of AMPK reversed these changes. In db/db diabetic mice, both APPL1 expression and adiponectin-induced vasodilation were significantly decreased compared with their lean littermates. Taken together, these results suggest that APPL1 acts as a common downstream effector of AdipoR1 and -R2, mediating adiponectin-evoked endothelial NO production and endothelium-dependent vasodilation.

Expression of human AR cDNA driven by its own promoter results in mild promotion, but not suppression, of growth in human prostate cancer PC-3 cells

AIM

To examine the physiological role of the androgen receptor (AR) in the PC-3 cell line by transfecting full-length functional AR cDNA driven by its natural human AR promoter.

METHODS

We generated an AR-expressing PC-3(AR)9 stable clone that expresses AR under the control of the natural human AR promoter and compared its proliferation to that of the PC-3(AR)2 (stable clone that expresses AR under the control of the cytomegalovirus (CMV) promoter, established by Heisler et al.) after androgen treatment.

RESULTS

We found that dihydrotestosterone (DHT) from 0.001 nmol/L to 10 nmol/L induces cell cycle arrest or inhibits proliferation of PC-3(AR)2 compared with its vector control, PC-3(pIRES). In contrast, PC-3(AR)9 cell growth slightly increased or did not change when treated with physiological concentrations of 1 nmol/L DHT.

CONCLUSION

These data suggest that intracellular control of AR expression levels through the natural AR promoter might be needed for determining AR function in androgen-independent prostate cancer (AIPC) PC-3 cells. Unlike previous publications that showed DHT mediated suppression of PC-3 growth after transfection of viral promoter-driven AR overexpression, we report here that DHT-mediated PC-3 proliferation is slightly induced or does not change compared with its baseline after reintroducing AR expression driven by its own natural promoter, as shown in PC-3(AR)9 prostate cancer cells.

APPL1, APPL2, Akt2 and FOXO1a interact with FSHR in a potential signaling complex

A number of signaling proteins have been demonstrated to interact with follicle stimulating hormone (FSH) receptor (FSHR), including APPL1, 14-3-3tau and Akt2. To further define the repertoire of proteins involved in FSH-induced signal transduction, several signaling and adapter proteins were examined for the ability to associate with FSHR. This report shows that, in addition to APPL1, FSHR interacts with FOXO1a and APPL2. Moreover, APPL1 and APPL2 associate with one another via the N-terminus of APPL1, presumably via the Bin-Amphiphysin-Rvs (BAR) domain. The interactions between FSHR and APPL2 and between FSHR and FOXO1a evidently are distinct since FOXO1a does not associate with either APPL1 or with APPL2. Though APPL1 and APPL2 show some similarity in primary sequence, APPL1 associates with Akt2, whereas APPL2 does not. This is the first documented difference in function between APPL1 and APPL2. These results suggest that FSHR, APPL1, APPL2, Akt2 and FOXO1a are organized into distinct scaffolding networks in the cell. Accordingly, the spatial organization of signaling and adapter proteins with FSHR likely facilitates and finely regulates the signal transduction induced by FSH.

APPL1 associates with TrkA and GIPC1 and is required for nerve growth factor-mediated signal transduction

The neurotrophin receptor TrkA plays critical roles in the nervous system by recruiting signaling molecules that activate pathways required for the growth and survival of neurons. Here, we report APPL1 as a TrkA-associated protein. APPL1 and TrkA co-immunoprecipitated in sympathetic neurons. We have identified two routes through which this association can occur. APPL1 was isolated as a binding partner for the TrkA-interacting protein GIPC1 from rat brain lysate by mass spectrometry. The PDZ domain of GIPC1 directly engaged the C-terminal sequence of APPL1. This interaction provides a means through which APPL1 may be recruited to TrkA. In addition, the APPL1 PTB domain bound to TrkA, indicating that APPL1 may associate with TrkA independently of GIPC1. Isolation of endosomal fractions by high-resolution centrifugation determined that APPL1, GIPC1, and phosphorylated TrkA are enriched in the same fractions. Reduction of APPL1 or GIPC1 protein levels suppressed nerve growth factor (NGF)-dependent MEK, extracellular signal-regulated kinase, and Akt activation and neurite outgrowth in PC12 cells. Together, these results indicate that GIPC1 and APPL1 play a role in TrkA function and suggest that a population of endosomes bearing a complex of APPL1, GIPC1, and activated TrkA may transmit NGF signals.

APPLied mechanics: uncovering how adiponectin modulates insulin action

Adiponectin, an adipocyte secretory hormone, has been causally linked to insulin resistance in the metabolic syndrome and diabetes. A recent paper (Mao et al., 2006) shows that the APPL1 adaptor protein binds to the intracellular domain of adiponectin receptors and mediates some of adiponectin's actions, identifying a novel mechanism linking adiponectin to insulin sensitization.

Regulatory processes affecting androgen receptor expression, stability, and function: Potential targets to treat hormone-refractory prostate cancer

Prostate cancer cells rely on androgen receptor (AR) for proliferation and survival. Therefore, curing prostate cancer will require elimination of AR. Although androgen is the natural ligand that activates AR, AR activity is also subject to regulation by growth factor/growth factor receptor-stimulated signaling pathways that control the cell cycle. Cell cycle regulatory proteins and protein kinases in signaling pathways affected by growth factors can lead to AR activation in the absence of androgen. While downstream signaling proteins such as cyclins, cyclin-dependent kinases (CDKs), and pRB can modulate AR activity, upstream signaling pathways involving protein kinases such as mitogen-activated protein kinases, protein kinase A, and protein kinase B/Akt can affect post-translational modification of AR to affect not only AR function but also AR stability. Calcium and calmodulin (CaM), essential for proliferation and viability of a number of cells, including prostate cancer cells, play an important role in AR expression, stability, and function. CaM affects AR partly by interacting directly with AR and partly by activating protein kinases such as Akt and DNA-PK that can phosphorylate AR. The ubiquitin/26S proteasome pathway responsible for timely destruction of cell cycle regulatory proteins whose levels impede cell cycle progression also induces AR expression by activating NF-kappaB, and promotes AR activity by participating in the assembly of an AR transcription complex. Maspin, a serine protease inhibitor that is known mostly for its role as a tumor suppressor can also regulate AR intracellular localization and function by competing with AR for binding to the chaperone protein Hsp90 and co-repressor HDAC1, respectively. This perspective reviews the experimental evidence implicating these diverse cellular processes in AR expression, stability, and/or function, and presents a rationale for disrupting these cellular processes as a viable option for the treatment of both the hormone-sensitive and the hormone-insensitive prostate cancer.

Regulation of the Akt kinase by interacting proteins

Ten years ago, it was observed that the Akt kinase is activated by phosphorylation via a phosphoinositide 3-kinase (PI-3K)-dependent process. This discovery generated enormous interest because it provided a link between PI-3K, an enzyme known to play a critical role in cellular physiology, and its downstream targets. Subsequently, it was shown that the activity of the core components of the 'PI-3K/Akt pathway' is modulated by a complex network of regulatory proteins and pathways. Some of the Akt-binding partners modulate its activation by external signals by interacting with different domains of the Akt protein. This review focuses on the Akt interacting proteins and the mechanisms by which they regulate Akt activation.

Induction of androgen receptor expression by phosphatidylinositol 3-kinase/Akt downstream substrate, FOXO3a, and their roles in apoptosis of LNCaP prostate cancer cells

The phosphatidylinositol 3-kinase (PI3K)/Akt pathway plays important roles for prostate cancer cell survival, and the androgen receptor (AR) plays essential roles for prostate cancer cell proliferation. How these two signals cooperate to control cell growth and death, however, remains unclear and debated. Here we provide the first linkage by the identification of Forkhead transcription factor FOXO3a, the PI3K/Akt downstream substrate, as a positive regulator for the induction of AR gene expression. Both Western blot and real time PCR assays demonstrate that FOXO3a can induce AR expression at the protein and mRNA levels, and gel shift and chromatin immunoprecipitation assays further demonstrate that FOXO3a can induce 5' AR promoter activity via binding to the consensus DNA-binding sequence in the AR 5' promoter -1290 to -1297 (5'-TTGTTTCA-3'). Under normal growth conditions, blocking PI3K/Akt signals by LY294002 causes LNCaP cell arrest in G1 phase rather than apoptosis. However, further blocking of AR functions by AR small interfering RNA leads to dramatic LNCaP cell death, suggesting that AR may play important protective roles when the PI3K/Akt signal pathway is blocked by LY294002. Together, our data provide the first model to explain how PI3K/Akt and AR can cooperate to control LNCaP cell growth and death under normal conditions.

Regulation of interleukin-6-mediated PI3K activation and neuroendocrine differentiation by androgen signaling in prostate cancer LNCaP cells

BACKGROUND

Neuroendocrine (NE) differentiation in prostate cancer has been suggested to be one of the early events in the development of androgen independence. In the human prostate cancer LNCaP cell line, treatment with interleukin-6 (IL-6) induces NE-like differentiation, which is similar to the phenomena observed in advanced stages of prostate cancer progression. In this study, we investigate how androgen plays a role in IL-6-mediated NE differentiation in LNCaP cell line.

METHODS

Western blot, co-immunoprecipitation (co-IP), and GST pull-down assays were performed to detect the protein expression and protein-protein interaction. PI3K kinase assay was used to measure PI3K activity.

RESULTS

Addition of androgen blocks IL-6-mediated PI3K activation and NE differentiation in LNCaP cells. In vivo and in vitro protein interaction assays suggested that androgen receptor (AR) can directly interact with IL-6 transducer gp130. In addition, androgen treatment enhances the interaction between AR and gp130, interrupts the IL-6-induced gp130-mediated PI3K activation, which may lead to inhibition of IL-6-mediated NE differentiation in LNCaP cells.

CONCLUSIONS

Our results suggest androgen and AR can regulate IL-6-mediated LNCaP cell NE differentiation via directly modulating the IL-6-PI3K pathway.

Environmental, genetic, and molecular features of prostate cancer

Prostate cancer is the sixth most common cancer in the world and the third leading cause of cancer in men. The increase in the understanding of prostate carcinogenesis over the past 15 years has helped to define crucial steps in the natural history of the disease, namely initiation and progression to androgen independence. This heterogeneous disease encompasses a range of environmental and familial factors, which provides strong support for the use of chemopreventive strategies. Most patients with advanced prostate cancer are treated with androgen-deprivation therapy, which leads to a striking regression of androgen-responsive cancer cells. A transition from an androgen-responsive to an androgen-unresponsive stage is seen during the clinical course in almost all patients with prostate cancer. This transition also signals a substantial worsening of prognosis. Here, we review the most important findings in prostate carcinogenesis and the molecular anomalies associated with the androgen-refractory stage.

Human follicle-stimulating hormone (FSH) receptor interacts with the adaptor protein APPL1 in HEK 293 cells: potential involvement of the PI3K pathway in FSH signaling

Selection of a dominant follicle that will ovulate likely occurs by activation of cell survival pathways and suppression of death-promoting pathways in a mechanism involving FSH and its cognate receptor (FSHR). A yeast two-hybrid screen of an ovarian cDNA library was employed to identify potential interacting partners with human FSHR intracellular loops 1 and 2. Among eight cDNA clones identified in the screen, APPL1 (adaptor protein containing PH domain, PTB domain, and leucine zipper motif; also known as APPL or DIP13alpha) was chosen for further analysis. APPL1 appears to coimmunoprecipitate with FSHR in HEK 293 cells stably expressing FSHR (293/FSHR cells), confirming APPL1 as a potential FSHR-interacting partner. The phosphorylation status of members of the phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway was also examined because of the proposed role of APPL1 in the antiapoptotic PI3K/Akt pathway. FOXO1a, also referred to as forkhead homologue in rhabdomyosarcoma, is a downstream effector in the pathway and tightly linked to expression of proapoptotic genes. FOXO1a, but not the upstream kinase Akt, is rapidly phosphorylated, and FOXO1a is thereby inactivated when 293/FSHR cells are treated with FSH. In addition, FSHR coimmunoprecipitates with Akt. The identification of APPL1 as a potential interactor with FSHR and the finding that FOXO1a is phosphorylated in response to FSH provide a possible link between FSH and PI3K/Akt signaling, which may help to delineate a survival mechanism whereby FSH selects the dominant follicle to survive.

Human checkpoint protein hRad9 functions as a negative coregulator to repress androgen receptor transactivation in prostate cancer cells

Positive responses to combined androgen elimination therapy and radiation therapy have been well documented in the treatment of prostate cancer patients. The detailed mechanisms how androgen-androgen receptor (AR) cross talks to the radiation-related signal pathways, however, remain largely unknown. Here we report the identification of hRad9, a key member of the checkpoint Rad protein family, as a coregulator to suppress androgen-AR transactivation in prostate cancer cells. In vivo and in vitro interaction assays using Saccharomyces cerevisiae two-hybrid, mammalian two-hybrid, glutathione S-transferase pull-down, and coimmunoprecipitation methods prove that AR can interact with the C terminus of hRad9 via its ligand binding domain. The FXXLF motif within the C terminus of hRad9 interrupts the androgen-induced interaction between the N terminus and C terminus of AR. This interaction between AR and hRad9 may result in the suppression of AR transactivation, demonstrated by the repressed AR transactivation in androgen-induced luciferase reporter assay and the reduced endogenous prostate-specific antigen expression in Western blot assay. Addition of small interfering RNA of hRad9 can reverse hRad9 suppression effects, which suggests that hRad9 functions as a repressor of AR transactivation in vivo. Together, our data provide the first linkage between androgen-AR signals and radiation-induced responses. Further studies of the influence of hRad9 on prostate cancer growth may provide potential new therapeutic approaches.

Androgen receptor in prostate cancer

The normal development and maintenance of the prostate is dependent on androgen acting through the androgen receptor (AR). AR remains important in the development and progression of prostate cancer. AR expression is maintained throughout prostate cancer progression, and the majority of androgen-independent or hormone refractory prostate cancers express AR. Mutation of AR, especially mutations that result in a relaxation of AR ligand specificity, may contribute to the progression of prostate cancer and the failure of endocrine therapy by allowing AR transcriptional activation in response to antiandrogens or other endogenous hormones. Similarly, alterations in the relative expression of AR coregulators have been found to occur with prostate cancer progression and may contribute to differences in AR ligand specificity or transcriptional activity. Prostate cancer progression is also associated with increased growth factor production and an altered response to growth factors by prostate cancer cells. The kinase signal transduction cascades initiated by mitogenic growth factors modulate the transcriptional activity of AR and the interaction between AR and AR coactivators. The inhibition of AR activity through mechanisms in addition to androgen ablation, such as modulation of signal transduction pathways, may delay prostate cancer progression.

Androgen suppresses PML protein expression in prostate cancer CWR22R cells

The ability of PML to modulate key suppressive pathways in tumor cells suggests that PML may act as a tumor suppressor. The detailed mechanism of how PML functions in prostate cancer progression, however, remains unknown. Here we demonstrate that in the presence of androgen, PML protein expression can be suppressed in CWR22R prostate cancer cells. Further studies reveal that PML can selectively suppress AR transactivation and PML protein expression positively correlates with increased p21 protein level and enhances p53 transcription ability in CWR22R cells. We also found that PML strongly inhibits CWR22R cell colony formation, while PML siRNA enhances AR activity and CWR22R cell colony formation. Together our results suggest that PML may suppress prostate cancer cell growth by inhibiting AR transactivation and/or enhancing p53 activity.

The scaffolding protein RACK1 interacts with androgen receptor and promotes cross-talk through a protein kinase C signaling pathway

The androgen receptor (AR), a member of the nuclear hormone receptor superfamily, functions as a ligand-dependent transcription factor that regulates genes involved in cell proliferation and differentiation. Using a C-terminal region of the human AR in a yeast two-hybrid screen, we have identified RACK1 (receptor for activated C kinase-1) as an AR-interacting protein. In this report we found that RACK1, which was previously shown to be a protein kinase C (PKC)-anchoring protein that determines the localization of activated PKCbetaII isoform, facilitates ligand-independent AR nuclear translocation upon PKC activation by indolactam V. We also observed RACK1 to suppress ligand-dependent and -independent AR transactivation through PKC activation. In chromatin immunoprecipitation assays, we demonstrate a decrease in AR recruitment to the AR-responsive prostate-specific antigen (PSA) promoter following stimulation of PKC. Furthermore, prolonged exposure to indolactam V, a PKC activator, caused a reduction in PSA mRNA expression in prostate cancer LNCaP cells. Finally, we found PKC activation to have a repressive effect on AR and PSA protein expression in androgen-treated LNCaP cells. Our data suggest that RACK1 may function as a scaffold for the association and modification of AR by PKC enabling translocation of AR to the nucleus but rendering AR unable to activate transcription of its target genes.

Hepatocyte growth factor suppresses renal interstitial myofibroblast activation and intercepts Smad signal transduction

Interstitial myofibroblasts are alpha-smooth muscle actin-positive cells that play a crucial role in the accumulation of excess extracellular matrix during renal interstitial fibrogenesis. Despite their importance in the pathogenesis of renal fibrosis, relatively little is known about the regulators and the mechanism controlling the activation of renal interstitial myofibroblasts in disease conditions. Here, we show that hepatocyte growth factor (HGF) acts as a potent inhibitor of the transforming growth factor (TGF)-beta1-mediated myofibroblastic activation from normal rat renal interstitial fibroblasts (NRK-49F). Simultaneous incubation of HGF abolished TGF-beta1-induced de novo alpha-smooth muscle actin expression, F-actin reorganization, and interstitial collagen I overproduction in a dose-dependent manner. To decipher the mechanism underlying HGF antagonizing TGF-beta1's action, we examined the effects of HGF on TGF-beta1-mediated Smad signaling. HGF neither inhibited Smad-2/3 phosphorylation and their association with Smad-4 induced by TGF-beta1, nor significantly affected inhibitory Smad-6 and -7 expression and cellular abundance of Smad transcriptional co-repressors in NRK-49F cells. However, pretreatment with HGF markedly attenuated activated Smad-2/3 nuclear translocation and accumulation. This action of HGF was apparently dependent on HGF-mediated extracellular signal-regulated kinase-1 and -2 (Erk-1/2) phosphorylation and activation. Inhibition of Erk-1/2 activation by Mek kinase inhibitor PD98059 restored TGF-beta1-mediated Smad-2/3 nuclear accumulation and myofibroblast activation. In vivo, HGF selectively blocked Smad-2/3 nuclear accumulation in renal interstitial cells in the fibrotic kidneys induced by unilateral ureteral obstruction. Therefore, HGF suppresses TGF-beta1-mediated renal interstitial myofibroblastic activation; and this action of HGF is likely related to a mitogen-activated protein kinase-dependent blockade of Smad nuclear translocation.