Filamin A modulates kinase activation and intracellular trafficking of epidermal growth factor receptors in human melanoma cells

Filamin A forms a complex with EphA2 and regulates EphA2 serine 897 phosphorylation and glioblastoma cell proliferation

EphA2 is phosphorylated on serine 897 (S897) in response to growth factors such as epidermal growth factor (EGF) and on tyrosine 588 (Y588) in response to its ligand ephrinA1, causing different cellular responses. In this study, we show that the actin-binding protein Filamin A forms a complex with EphA2 and promotes its S897 phosphorylation and glioblastoma cell proliferation. Suppression of Filamin A expression by siRNAs inhibited glioblastoma cell proliferation induced by EGF stimulation or overexpression of EphA2. Knockdown of Filamin A inhibited EGF-induced S897 phosphorylation of EphA2, whereas it had little effect on ephrinA1-induced Y588 phosphorylation of EphA2. Furthermore, Filamin A expression affected the subcellular localization of EphA2. This study suggests that Filamin A selectively promotes EphA2 S897 phosphorylation and plays an important role in glioblastoma cell proliferation.

Synthesis and characterization of a novel pH-responsive drug-releasing nanocomposite hydrogel for skin cancer therapy and wound healing

Local skin cancer recurrence occurs in ∼12% of the patients post-surgery due to persistent growth of residual cancer cells. Wound infection is another significant complication following surgery. We report a novel in situ-forming nanocomposite hydrogel (NCH) containing PLGA-carboxymethyl chitosan nanoparticles (186 nm) for localized pH-responsive skin cancer therapy and wound healing. This injectable hydrogel, comprising of a citric acid-derived polymer backbone, gelled within 5 minutes, and demonstrated excellent swelling (283% of dry weight) and compressive strengths (∼5.34 MPa). Nanoparticle incorporation did not significantly affect hydrogel properties. The NCH effluents were cytocompatible with human dermal fibroblasts at 500 μg ml-1 concentration and demonstrated pH-dependent drug release and promising therapeutic efficacy against A431 and G361 skin cancer cells in vitro. Significant zones of inhibition were observed in S. aureus and E. coli cultures on NCH treatment, confirming its antibacterial properties. Our studies show that the pH-responsive NCH can be potentially used for adjuvant skin cancer treatment and wound healing.

The cytoskeleton actin binding protein filamin A impairs both IGF2 mitogenic effects and the efficacy of IGF1R inhibitors in adrenocortical cancer cells

Adrenocortical carcinomas (ACCs) overexpress insulin-like growth factor 2 (IGF2), that drives a proliferative autocrine loop by binding to IGF1R and IR, but IGF1R/IR-targeted therapies failed in ACC patients. The cytoskeleton actin-binding protein filamin A (FLNA) impairs IR signalling in melanoma cells. Aims of this study were to test FLNA involvement in regulating IGF1R and IR responsiveness to both IGF2 and inhibitors in ACC. In ACC cells H295R and SW13 and primary cultures (1ACC, 4 adenomas) we found that IGF1R and IR interacted with FLNA, and FLNA silencing increased IGF1R and reduced IR expression, with a downstream effect of increased cell proliferation and ERK phosphorylation. In addition, FLNA knockdown potentiated antiproliferative effects of IGF1R/IR inhibitor Linsitinib and IGF1R inhibitor NVP-ADW742 in H295R. Finally, Western blot showed lower FLNA expression in ACCs (n = 10) than in ACAs (n = 10) and an inverse correlation of FLNA/IGF1R ratio with ERK phosphorylation in ACCs only. In conclusion, we demonstrated that low FLNA levels enhance both IGF2 proliferative effects and IGF1R/IR inhibitors efficacy in ACC cells, suggesting FLNA as a new factor influencing tumor clinical behavior and the response to the therapy with IGF1R/IR-targeted drugs.

Composite regulation of ERK activity dynamics underlying tumour-specific traits in the intestine

Acting downstream of many growth factors, extracellular signal-regulated kinase (ERK) plays a pivotal role in regulating cell proliferation and tumorigenesis, where its spatiotemporal dynamics, as well as its strength, determine cellular responses. Here, we uncover the ERK activity dynamics in intestinal epithelial cells (IECs) and their association with tumour characteristics. Intravital imaging identifies two distinct modes of ERK activity, sustained and pulse-like activity, in IECs. The sustained and pulse-like activities depend on ErbB2 and EGFR, respectively. Notably, activation of Wnt signalling, the earliest event in intestinal tumorigenesis, augments EGFR signalling and increases the frequency of ERK activity pulses through controlling the expression of EGFR and its regulators, rendering IECs sensitive to EGFR inhibition. Furthermore, the increased pulse frequency is correlated with increased cell proliferation. Thus, ERK activity dynamics are defined by composite inputs from EGFR and ErbB2 signalling in IECs and their alterations might underlie tumour-specific sensitivity to pharmacological EGFR inhibition.

The ERK signalling pathway regulates homeostasis of the intestinal epithelium. Here the authors identify two modes of ERK activity generated independently from EGFR and ErbB2 receptor and whose balance in cancer is shifted by Wnt pathway activation, resulting in enhanced sensitivity to EGFR inhibitors.

Caveolin-1 Inhibits Proliferation, Migration, and Invasion of Human Colorectal Cancer Cells by Suppressing Phosphorylation of Epidermal Growth Factor Receptor

Background

Although downregulation of caveolin-1 (Cav-1), which is a key constituent of membrane caveolae and a regulator of cellular processes, is associated with colorectal cancer (CRC), its involvement in the disease progression is largely unknown. This study aimed to explore the role of Cav-1 in CRC and the associated mechanisms.

Material/Methods

Fresh tissues from patients with CRC and human CRC SW480 cells were used to evaluate Cav-1 and Ki-67 expression using immunohistochemistry and Western blotting. The MTS and Transwell assays were performed to determine the effects of Cav-1 overexpression via pcDNA3.1/Cav-1 plasmid on cell proliferation and metastasis. The effect of Cav-1 on the epidermal growth factor receptor (EGFR) activation was evaluated by Western blotting. The correlation of Cav-1 expression with clinicopathological factors was statistically analyzed.

Results

Overexpression of Cav-1 significantly reduced proliferation, migration, and invasion of SW480 cancer cells in vitro. The EGF-induced phosphorylation of EGFR and activations of the RAF-MEK-ERK and PI3K-AKT pathways were adversely regulated by Cav-1 overexpression in vitro. In 76 cases of CRC patients with EGFR expression, a negative correlation was observed between the level of Cav-1 and tumor-node-metastasis stage, lymph node metastasis, and distant metastasis (All p<0.05). Finally, the expression level of Cav-1 was negatively correlated with that of Ki-67.

Conclusions

This report is the first to show that overexpression of Cav-1significantly inhibits the proliferation, migration, and invasion potential of SW480 cells, possibly through reducing EGF-induced EGFR activation. High Cav-1 expression level may be a predictor of positive outcomes in patients with colorectal cancer.

Somatostatin Receptor Type 2 (SSTR2) Internalization and Intracellular Trafficking in Pituitary GH-Secreting Adenomas: Role of Scaffold Proteins and Implications for Pharmacological Resistance

Somatostatin receptor type 2 (SSTR2), together with SSTR5, represents the main target of medical treatment for growth hormone (GH)-secreting pituitary tumors, since it is expressed in most of these tumors and exerts both antiproliferative and cytostatic effects, and reduces hormone secretion, as well. However, clinical practice indicates a great variability in the frequency and entity of favorable responses of acromegalic patients to long-acting somatostatin analogues (SSAs), but the molecular mechanisms regulating this pharmacological resistance are not completely understood. So far, several potentially implied mechanisms have been suggested, including impaired expression of SSTRs, or post-receptor signal transduction alterations. More recently, new studies exploited the molecular factors involved in SSTRs intracellular trafficking regulation, this being a critical point for the modulation of the available active G-coupled receptors (GPCRs) amount at the cell surface. In this respect, the role of the scaffold proteins such as β-arrestins, and the cytoskeleton protein Filamin A (FLNA), have become of relevant importance for GH-secreting pituitary tumors. In fact, β-arrestins are linked to SSTR2 desensitization and internalization, and FLNA is able to regulate SSTR2 trafficking and stability at the plasma membrane. Therefore, the present review will summarize emerging evidence highlighting the role of β-arrestins and FLNA, as possible novel players in the modulation of agonist activated-SSTR2 receptor trafficking and response in GH-secreting pituitary tumors.

Ectopic overexpression of filamin C scaffolds MEK1/2 and ERK1/2 to promote the progression of human hepatocellular carcinoma

Hepatocellular carcinoma (HCC) invasion and metastasis are mediated by a complicated signal transduction network and downstream cytoskeletal and adhesion molecules. In this study, a microarray-based analysis revealed a dramatic increase in filamin C (FLNC), which is commonly expressed in muscle rather than in liver cells, in the two metastatic HCC cell lines MHCC97L and HCCLM3. Clinicopathological studies showed that increased FLNC expression was associated with microvascular invasion and poor prognosis. Specific hypomethylation was identified within the FLNC promoter region in HCC cell lines and patient tumor samples, which might contribute to the ectopic overexpression of FLNC. FLNC downregulation inhibited cell migration and impaired cell proliferation and promoted apoptosis. Mechanistic studies suggested that FLNC interacts with mitogen-activated extracellular signal-regulated kinase 1/2 (MEK1/2) and extracellular signal-regulated kinase 1/2 (ERK1/2) and that FLNC downregulation inhibited MEK1/2 and ERK1/2 activation. Xenographic tumor transplantation experiments in nude mice further confirmed the role of FLNC in HCC progression and metastasis. Our results reveal a novel mechanism by which the cytoskeletal protein FLNC enhances the mitogen-activated protein kinase signaling pathway during tumorigenesis.

Inhibition of non-small cell lung cancer (NSCLC) growth by a novel small molecular inhibitor of EGFR

The epidermal growth factor receptor (EGFR) is a therapeutic target (oncotarget) in NSCLC. Using in vitro EGFR kinase activity system, we identified a novel small molecule, WB-308, as an inhibitor of EGFR. WB-308 decreased NSCLC cell proliferation and colony formation, by causing G2/M arrest and apoptosis. Furthermore, WB-308 inhibited the engraft tumor growths in two animal models in vivo (lung orthotopic transplantation model and patient-derived engraft mouse model). WB-308 impaired the phosphorylation of EGFR, AKT, and ERK1/2 protein. WB-308 was less cytotoxic than Gefitinib. Our study suggests that WB-308 is a novel EGFR-TKI and may be considered to substitute for Gefitinib in clinical therapy for NSCLC.

Filamin A Is Involved in HIV-1 Vpu-mediated Evasion of Host Restriction by Modulating Tetherin Expression

Tetherin, also known as bone marrow stromal antigen 2 (BST-2), inhibits the release of a wide range of enveloped viruses, including human immunodeficiency virus, type 1 (HIV-1) by directly tethering nascent virions to the surface of infected cells. The HIV-1 accessary protein Vpu counteracts tetherin restriction via sequestration, down-regulation, and/or displacement mechanisms to remove tetherin from sites of virus budding. However, the exact mechanism of Vpu-mediated antagonism of tetherin restriction remains to be fully understood. Here we report a novel role for the actin cross-linking regulator filamin A (FLNa) in Vpu anti-tetherin activities. We demonstrate that FLNa associates with tetherin and that FLNa modulates tetherin turnover. FLNa deficiency was found to enhance cell surface and steady-state levels of tetherin expression. In contrast, we observed that overexpression of FLNa reduced tetherin expression levels both on the plasma membrane and in intracellular compartments. Although FLNb shows high amino acid sequence similarity with FLNa, we reveal that only FLNa, but not FLNb, plays an essential role in tetherin turnover. We further showed that FLNa deficiency inhibited Vpu-mediated enhancement of virus release through interfering with the activity of Vpu to down-regulate cellular tetherin. Taken together, our studies suggest that Vpu hijacks the FLNa function in the modulation of tetherin to neutralize the antiviral factor tetherin. These findings may provide novel strategies for the treatment of HIV-1 infection.

Expression of FLNa in human melanoma cells regulates the function of integrin α1β1 and phosphorylation and localisation of PKB/AKT/ERK1/2 kinases

FLNa is a ubiquitous cytoskeletal protein that links transmembrane receptors, including integrins, to F-actin and functions as a signalling intermediate. We investigated FLNa's role in the function of integrin-type collagen receptors, EGF-EGFR signalling and regulation of PKB/Akt and ERK1/2. Using FLNa-deficient M2 human melanoma cells, and same cells expressing EGFP-FLNa (M2F) or its Ig-like repeats 1-8+24, 8-15+24 and 16-24, we found that in M2F and M2 8-15+24 cells, EGF induced the increased phosphorylation of PKB/Akt and ERK1/2. In M2F cells EGF induced the localisation of these kinases to cell nucleus and lamellipodia, respectively, and the ERK1/2 phosphorylation-dependent co-immunoprecipitation of FLNa with ERK1/2. Only M2F and M2 8-15+24 cells adhered to and spread on type I collagen whereas on fibronectin all cells behaved similarly. α1β1 and α2β1 were the integrin-type collagen receptors expressed on these cells with primarily α1β1 localising to focal contacts and affecting cell adhesion and migration in a manner dependent on FLNa or its Ig-like repeats 8-15. Our results suggest a role for FLNa repeats 8-15 in the α1-subunit-dependent regulation of integrin α1β1 function, EGF-EGFR signalling to PKB/Akt and ERK1/2, identify ERK1/2 in EGF-induced FLNa-associated protein complexes, and show that the function of different integrins is subjected to differential regulation by FLNa.

Expression of caveolin-1 is correlated with lung adenocarcinoma proliferation, migration, and invasion

Both tumor suppressor and tumor promoter roles, which are dependent on the tumor type, have been described for caveolin-1 (CAV-1). Because CAV-1 can modulate cell signaling, we tested the hypothesis that it regulates lung adenocarcinoma cell proliferation and metastasis via modulation of epidermal growth factor receptor (EGFR) activity. The lung adenocarcinoma cell line, GLC-82, was transfected with pcDNA3.1CAV-1 plasmid, before cell proliferation, migration, and invasion were analyzed. In the in vivo xenograft model, the relationship between the CAV-1 expression and EGFR phosphorylation and signaling was assessed by western blot analysis. The relationship between the CAV-1 as well as Ki67 expression and the clinicopathological characteristics of 68 lung adenocarcinoma patients was also examined using immunohistochemistry. Overexpression of CAV-1 significantly increased GLC-82 proliferation (p < 0.001), migration (p < 0.001), and invasion (p = 0.002) as well as EGFR and ERK phosphorylation (p < 0.05). The GLC-82/CAV-1 cell tumors were also significantly larger than those of control cells (all p ≤ 0.05). In lung adenocarcinoma patients, CAV-1 expression was positively correlated with lymph node metastasis and cancer stage. Finally, CAV-1 expression was associated with the expression of Ki-67, a marker of cell proliferation. CAV-1 enhanced GLC-82 cell proliferation, migration, and invasion possibly through EGFR and ERK signaling. Furthermore, the relationship of CAV-1 with Ki67 expression, a marker of proliferative capacity, in lung adenocarcinoma samples is suggestive of its role in disease progression. Further studies are required to confirm the role of CAV-1 in the metastasis of lung adenocarcinoma as well as its potential prognostic and therapeutic value.

Activation of β2-adrenergic receptor by (R,R')-4'-methoxy-1-naphthylfenoterol inhibits proliferation and motility of melanoma cells

(R,R')-4'-methoxy-1-naphthylfenoterol [(R,R')-MNF] is a highly-selective β2 adrenergic receptor (β2-AR) agonist. Incubation of a panel of human-derived melanoma cell lines with (R,R')-MNF resulted in a dose- and time-dependent inhibition of motility as assessed by in vitro wound healing and xCELLigence migration and invasion assays. Activity of (R,R')-MNF positively correlated with the β2-AR expression levels across tested cell lines. The anti-motility activity of (R,R')-MNF was inhibited by the β2-AR antagonist ICI-118,551 and the protein kinase A inhibitor H-89. The adenylyl cyclase activator forskolin and the phosphodiesterase 4 inhibitor Ro 20-1724 mimicked the ability of (R,R')-MNF to inhibit migration of melanoma cell lines in culture, highlighting the importance of cAMP for this phenomenon. (R,R')-MNF caused significant inhibition of cell growth in β2-AR-expressing cells as monitored by radiolabeled thymidine incorporation and xCELLigence system. The MEK/ERK cascade functions in cellular proliferation, and constitutive phosphorylation of MEK and ERK at their active sites was significantly reduced upon β2-AR activation with (R,R')-MNF. Protein synthesis was inhibited concomitantly both with increased eEF2 phosphorylation and lower expression of tumor cell regulators, EGF receptors, cyclin A and MMP-9. Taken together, these results identified β2-AR as a novel potential target for melanoma management, and (R,R')-MNF as an efficient trigger of anti-tumorigenic cAMP/PKA-dependent signaling in β2-AR-expressing lesions.

New cancer suppressor gene for colorectal adenocarcinoma: Filamin A

AIM: To determine the expression and significance of filamin A (FLNa) in colorectal adenocarcinoma tissue.

METHODS: The expression of FLNa in 46 colorectal cancer tissues and normal tissues was detected by immunohistochemistry, reverse transcription polymerase chain reaction (RT-PCR) and Western blotting, and its relationship with clinical parameters and prognosis was analyzed.

RESULTS: The positive expression of FLNa in cancer tissues was lower than that in normal mucosa, and the difference was statistically significant. The expression of FLNa correlated with liver metastasis, lymph node metastasis and rectal invasion depth, regardless of sex, age, tumor location, tumor size, gross shape and histological type of colorectal carcinoma. Multivariate analysis showed that FLNa was an independent risk factor for postoperative survival of patients with colorectal adenocarcinoma. Moreover, survival analysis showed that the expression level of FLNa was closely related with survival of patients with colorectal adenocarcinoma. The results of RT-PCR and Western blotting were consistent with those of immunohistochemistry.

CONCLUSION: FLNa showed low expression in colorectal adenocarcinoma, high correlation with the incidence and development of colorectal cancer, and was considered an indicator of prognosis.

Filamin A expression correlates with proliferation and invasive properties of human metastatic melanoma tumors: implications for survival in patients

PURPOSE

Filamin A (FLNa) cross-links actin filaments into dynamic orthogonal networks and interacts with binding proteins of diverse cellular functions that are implicated in cell growth and motility regulation. Here, we tested the hypothesis that FLNa plays a role in cancer proliferation and metastasis via the regulation of epidermal growth factor receptor (EGFR) function.

METHODS

Ectopic expression and knockdown of FLNa in human melanoma cell lines was performed to investigate changes in cellular proliferation, migration and invasion in vitro and tumor growth in a xenograft model in the mouse. The role of FLNa in EGFR expression and signaling was evaluated by Western blot. Immunohistochemistry was performed on histological sections of human melanoma tumors to determine whether an association existed between FLNa and overall survival.

RESULTS

The depletion of FLNa significantly reduced the proliferation, migration and invasion of two melanoma cell lines in vitro and was associated with smaller tumors in a xenograft model in vivo. EGF-induced phosphorylation of EGFR and activation of the Raf-MEK-ERK cascade was negatively affected by the silencing of FLNa both in vitro and in vivo. Cancer patients with low melanoma tumor FLNa expression have improved survival benefit.

CONCLUSION

These data indicate that enhanced tumorigenesis occurs through increase in EGF-induced EGFR activation in FLNa-expressing melanoma cells and that high FLNa levels are predictors of negative outcome for patients with melanoma tumors.

(R,R')-4'-methoxy-1-naphthylfenoterol targets GPR55-mediated ligand internalization and impairs cancer cell motility

(R,R')-4'-Methoxy-1-naphthylfenoterol (MNF) promotes growth inhibition and apoptosis of human HepG2 hepatocarcinoma cells via cannabinoid receptor (CBR) activation. The synthetic CB1R inverse agonist, AM251, has been shown to block the anti-mitogenic effect of MNF in these cells; however, AM251 is also an agonist of the recently deorphanized, lipid-sensing receptor, GPR55, whose upregulation contributes to carcinogenesis. Here, we investigated the role of MNF in GPR55 signaling in human HepG2 and PANC-1 cancer cell lines in culture by focusing first on internalization of the fluorescent ligand Tocrifluor 1117 (T1117). Initial results indicated that cell pretreatment with GPR55 agonists, including the atypical cannabinoid O-1602 and l-α-lysophosphatidylinositol, dose-dependently reduced the rate of cellular T1117 uptake, a process that was sensitive to MNF inhibition. GPR55 internalization and signaling mediated by O-1602 was blocked by MNF in GPR55-expressing HEK293 cells. Pretreatment of HepG2 and PANC-1 cells with MNF significantly abrogated the induction of ERK1/2 phosphorylation in response to AM251 and O-1602. Moreover, MNF exerted a coordinated negative regulation of AM251 and O-1602 inducible processes, including changes in cellular morphology and cell migration using scratch wound healing assay. This study shows for the first time that MNF impairs GPR55-mediated signaling and, therefore, may have therapeutic potential in the management of cancer.

The dual role of filamin A in cancer: can't live with (too much of) it, can't live without it

Filamin A (FlnA) has been associated with actin as cytoskeleton regulator. Recently its role in the cell has come under scrutiny for FlnA's involvement in cancer development. FlnA was originally revealed as a cancer-promoting protein, involved in invasion and metastasis. However, recent studies have also found that under certain conditions, it prevented tumor formation or progression, confusing the precise function of FlnA in cancer development. Here, we try to decipher the role of FlnA in cancer and the implications for its dual role. We propose that differences in subcellular localization of FlnA dictate its role in cancer development. In the cytoplasm, FlnA functions in various growth signaling pathways, such as vascular endothelial growth factor, in addition to being involved in cell migration and adhesion pathways, such as R-Ras and integrin signaling. Involvement in these pathways and various others has shown a correlation between high cytoplasmic FlnA levels and invasive cancers. However, an active cleaved form of FlnA can localize to the nucleus rather than the cytoplasm and its interaction with transcription factors has been linked to a decrease in invasiveness of cancers. Therefore, overexpression of FlnA has a tumor-promoting effect, only when it is localized to the cytoplasm, whereas if FlnA undergoes proteolysis and the resulting C-terminal fragment localizes to the nucleus, it acts to suppress tumor growth and inhibit metastasis. Development of drugs to target FlnA and cause cleavage and subsequent localization to the nucleus could be a new and potent field of research in treating cancer.

Spatio-temporal profiling of Filamin A RNA-editing reveals ADAR preferences and high editing levels outside neuronal tissues

RNA editing by ADARs can change the coding potential of protein-coding mRNAs. So far, this type of RNA editing has mainly been shown to affect RNAs expressed in the nervous system with much lower editing levels being observed in other tissues.

The actin crosslinking proteins filamin α and filamin β are widely expressed in most tissues. The mRNAs encoding either protein are edited at the same position leading to a conserved Q to R exchange in both proteins. Using bar-coded next generation sequencing, we show that editing of filamin α is most abundant in the gastrointestinal tract and only to a lesser extent in the nervous system. Using knockout mice, we show that ADARB1 (ADAR2) is responsible for the majority of FLNA editing, while ADAR1 can edit filamin α mRNA in some tissues quite efficiently. Interestingly, editing levels of filamin α and β do not follow the same trend across tissues, suggesting a substrate-specific regulation of editing.

Complex roles of filamin-A mediated cytoskeleton network in cancer progression

Filamin-A (FLNA), also called actin-binding protein 280 (ABP-280), was originally identified as a non-muscle actin binding protein, which organizes filamentous actin into orthogonal networks and stress fibers. Filamin-A also anchors various transmembrane proteins to the actin cytoskeleton and provides a scaffold for a wide range of cytoplasmic and nuclear signaling proteins. Intriguingly, several studies have revealed that filamin-A associates with multiple non-cytoskeletal proteins of diverse function and is involved in several unrelated pathways. Mutations and aberrant expression of filamin-A have been reported in human genetic diseases and several types of cancer. In this review, we discuss the implications of filamin-A in cancer progression, including metastasis and DNA damage response.

Periventricular Heterotopia: Shuttling of Proteins through Vesicles and Actin in Cortical Development and Disease

During cortical development, proliferating neural progenitors exhibit polarized apical and basolateral membranes that are maintained by tightly controlled and membrane-specific vesicular trafficking pathways. Disruption of polarity through impaired delivery of proteins can alter cell fate decisions and consequent expansion of the progenitor pool, as well as impact the integrity of the neuroependymal lining. Loss of neuroependymal integrity disrupts radial glial scaffolding and alters initial neuronal migration from the ventricular zone. Vesicle trafficking is also required for maintenance of lipid and protein cycling within the leading and trailing edge of migratory neurons, as well as dendrites and synapses of mature neurons. Defects in this transport machinery disrupt neuronal identity, migration, and connectivity and give rise to a malformation of cortical development termed as periventricular heterotopia (PH). PH is characterized by a reduction in brain size, ectopic clusters of neurons localized along the lateral ventricle, and epilepsy and dyslexia. These anatomical anomalies correlate with developmental impairments in neural progenitor proliferation and specification, migration from loss of neuroependymal integrity and neuronal motility, and aberrant neuronal process extension. Genes causal for PH regulate vesicle-mediated endocytosis along an actin cytoskeletal network. This paper explores the role of these dynamic processes in cortical development and disease.

Structural interaction and functional regulation of polycystin-2 by filamin

Filamins are important actin cross-linking proteins implicated in scaffolding, membrane stabilization and signal transduction, through interaction with ion channels, receptors and signaling proteins. Here we report the physical and functional interaction between filamins and polycystin-2, a TRP-type cation channel mutated in 10–15% patients with autosomal dominant polycystic kidney disease. Yeast two-hybrid and GST pull-down experiments demonstrated that the C-termini of filamin isoforms A, B and C directly bind to both the intracellular N- and C-termini of polycystin-2. Reciprocal co-immunoprecipitation experiments showed that endogenous polycystin-2 and filamins are in the same complexes in renal epithelial cells and human melanoma A7 cells. We then examined the effect of filamin on polycystin-2 channel function by electrophysiology studies with a lipid bilayer reconstitution system and found that filamin-A substantially inhibits polycystin-2 channel activity. Our study indicates that filamins are important regulators of polycystin-2 channel function, and further links actin cytoskeletal dynamics to the regulation of this channel protein.

A chemical cross-linking method for the analysis of binding partners of heat shock protein-90 in intact cells

Members of the heat shock protein-90 (Hsp90) family are key regulators of biological processes through dynamic interaction with a multitude of protein partners. However, the transient nature of these interactions hinders the identification of Hsp90 interactors. Here we show that chemical cross-linking with ethylene glycolbis (succinimidylsuccinate), but not shorter cross-linkers, generated an abundant 240-kDa heteroconjugate of the molecular chaperone Hsp90 in different cell types. The combined use of pharmacological and genetic approaches allowed the characterization of the subunit composition and subcellular compartmentalization of the multimeric protein complex, termed p240. The in situ formation of p240 did not require the N-terminal domain or the ATPase activity of Hsp90. Utilizing subcellular fractionation techniques and a cell-impermeant cross-linker, subpopulations of p240 were found to be present in both the plasma membrane and the mitochondria. The Hsp90-interacting proteins, including Hsp70, p60Hop and the scaffolding protein filamin A, had no role in governing the formation of p240. Therefore, chemical cross-linking combined with proteomic methods has the potential to unravel the protein components of this p240 complex and, more importantly, may provide an approach to expand the range of tools available to the study of the Hsp90 interactome.

Prognostic values of filamin-A status for topoisomerase II poison chemotherapy

Filamin-A, also called Actin Binding Protein-280, is not only an essential component of the cytoskeleton networks, but also serves as the scaffold in various signaling networks. It has been shown that filamin-A facilitates DNA repair and filamin-A proficient cells are more resistant to ionizing radiation, bleomycin, and cisplatin. In this study, we assessed the role of filamin-A in modulating cancer cell sensitivity to Topo II poisons, including etoposide and doxorubicin. Intriguingly, we found that cells with filamin-A expression are more sensitive to Topo II poisons than those with defective filamin-A, and filamin-A proficient xenograft melanomas have better response to etoposide treatment than the filamin-A deficient tumors. This is associated with more potent induction of DNA double strand breaks (DSBs) by Topo II poisons in filamin-A proficient cells than the deficient cells. Although the expression of filamin-A enables cells a slightly stronger capability to repair DSB, the net outcome is that filamin-A proficient cells bear more DSBs due to the significantly enhanced DSB induction by Topo II poisons in these cells. We further found that filamin-A proficient cells have increased drug influx and decreased drug efflux, suggesting that filamin-A modulates the intra-cellular drug kinetics of Topo II poisons to facilitate the generation of DSB after Topo II poison exposure. These data suggest a novel function of filamin-A in regulating the pharmacokinetics of Topo II poisons, and that the status of filamin-A may be used as a prognostic marker for Topo II poisons based cancer treatments.

Filamin A protein interacts with human immunodeficiency virus type 1 Gag protein and contributes to productive particle assembly

HIV-1 Gag precursor directs virus particle assembly and release. In a search for Gag-interacting proteins that are involved in late stages of the HIV-1 replication cycle, we performed yeast two-hybrid screening against a human cDNA library and identified the non-muscle actin filament cross-linking protein filamin A as a novel Gag binding partner. The 280-kDa filamin A regulates cortical actin network dynamics and participates in the anchoring of membrane proteins to the actin cytoskeleton. Recent studies have shown that filamin A facilitates HIV-1 cell-to-cell transmission by binding to HIV receptors and coreceptors and regulating their clustering on the target cell surface. Here we report a novel role for filamin A in HIV-1 Gag intracellular trafficking. We demonstrate that filamin A interacts with the capsid domain of HIV-1 Gag and that this interaction is involved in particle release in a productive manner. Disruption of this interaction eliminated Gag localization at the plasma membrane and induced Gag accumulation within internal compartments. Moreover, blocking clathrin-dependent endocytic pathways did not relieve the restriction to particle release induced by filamin A depletion. These results suggest that filamin A is involved in the distinct step of the Gag trafficking pathway. The discovery of the Gag-filamin A interaction may provide a new therapeutic target for the treatment of HIV infection.

Invasion of Cryptococcus neoformans into human brain microvascular endothelial cells is mediated through the lipid rafts-endocytic pathway via the dual specificity tyrosine phosphorylation-regulated kinase 3 (DYRK3)

Cryptococcus neoformans is a neurotropic fungal pathogen, which provokes the onset of devastating meningoencephalitis. We used human brain microvascular endothelial cells (HBMEC) as the in vitro model to investigate how C. neoformans traverses across the blood-brain barrier. In this study, we present several lines of evidence indicating that C. neoformans invasion is mediated through the endocytic pathway via lipid rafts. Human CD44 molecules from lipid rafts can directly interact with hyaluronic acid, the C. neoformans ligand. Bikunin, which perturbs CD44 function in the lipid raft, can block C. neoformans adhesion and invasion of HBMEC. The lipid raft marker, ganglioside GM1, co-localizes with CD44 on the plasma membrane, and C. neoformans cells can adhere to the host cell in areas where GM1 is enriched. These findings suggest that C. neoformans entry takes place on the lipid rafts. Upon C. neoformans engagement, GM1 is internalized through vesicular structures to the nuclear membrane. This endocytic redistribution process is abolished by cytochalasin D, nocodazole, or anti-DYRK3 (dual specificity tyrosine-phosphorylation-regulated kinase 3) siRNA. Concomitantly, the knockdown of DYRK3 significantly reduces C. neoformans invasion across the HBMEC monolayer in vitro. Our data demonstrate that the lipid raft-dependent endocytosis process mediates C. neoformans internalization into HBMEC and that the CD44 protein of the hosts, cytoskeleton, and intracellular kinase-DYRK3 are involved in this process.

Adapter protein SH2B1beta binds filamin A to regulate prolactin-dependent cytoskeletal reorganization and cell motility

Prolactin (PRL) regulates cytoskeletal rearrangement and cell motility. PRL-activated Janus tyrosine kinase 2 (JAK2) phosphorylates the p21-activated serine-threonine kinase (PAK)1 and the Src homology 2 (SH2) domain-containing adapter protein SH2B1β. SH2B1β is an actin-binding protein that cross-links actin filaments, whereas PAK1 regulates the actin cytoskeleton by different mechanisms, including direct phosphorylation of the actin-binding protein filamin A (FLNa). Here, we have used a FLNa-deficient human melanoma cell line (M2) and its derivative line (A7) that stably expresses FLNa to demonstrate that SH2B1β and FLNa are required for maximal PRL-dependent cell ruffling. We have found that in addition to two actin-binding domains, SH2B1β has a FLNa-binding domain (amino acids 200-260) that binds directly to repeats 17-23 of FLNa. The SH2B1β-FLNa interaction participates in PRL-dependent actin rearrangement. We also show that phosphorylation of the three tyrosines of PAK1 by JAK2, as well as the presence of FLNa, play a role in PRL-dependent cell ruffling. Finally, we show that the actin- and FLNa-binding-deficient mutant of SH2B1β (SH2B1β 3Δ) abolished PRL-dependent ruffling and PRL-dependent cell migration when expressed along with PAK1 Y3F (JAK2 tyrosyl-phosphorylation-deficient mutant). Together, these data provide insight into a novel mechanism of PRL-stimulated regulation of the actin cytoskeleton and cell motility via JAK2 signaling through FLNa, PAK1, and SH2B1β. We propose a model for PRL-dependent regulation of the actin cytoskeleton that integrates our findings with previous studies.

AFM stiffness nanotomography of normal, metaplastic and dysplastic human esophageal cells

The mechanical stiffness of individual cells is important in tissue homeostasis, cell growth, division and motility, and the epithelial-mesenchymal transition in the initiation of cancer. In this work, a normal squamous cell line (EPC2) and metaplastic (CP-A) as well as dysplastic (CP-D) Barrett's Esophagus columnar cell lines are studied as a model of pre-neoplastic progression in the human esophagus. We used the combination of an atomic force microscope (AFM) with a scanning confocal fluorescence lifetime imaging microscope to study the mechanical properties of single adherent cells. Sixty four force indentation curves were taken over the nucleus of each cell in an 8 x 8 grid pattern. Analyzing the force indentation curves, indentation depth-dependent Young's moduli were found for all cell lines. Stiffness tomograms demonstrate distinct differences between the mechanical properties of the studied cell lines. Comparing the stiffness for indentation forces of 1 nN, most probable Young's moduli were calculated to 4.7 kPa for EPC2 (n = 18 cells), 3.1 kPa for CP-A (n = 10) and 2.6 kPa for CP-D (n = 19). We also tested the influence of nuclei and nucleoli staining organic dyes on the mechanical properties of the cells. For stained EPC2 cells (n = 5), significant stiffening was found (9.9 kPa), while CP-A cells (n = 5) showed no clear trend (2.9 kPa) and a slight softening was observed (2.1 kPa) in the case of CP-D cells (n = 16). Some force-indentation curves show non-monotonic discontinuities with segments of negative slope, resembling a sawtooth pattern. We found the incidence of these 'breakthrough events' to be highest in the dysplastic CP-D cells, intermediate in the metaplastic CP-A cells and lowest in the normal EPC2 cells. This observation suggests that the microscopic explanation for the increased compliance of cancerous and pre-cancerous cells may lie in their susceptibility to 'crumble and yield' rather than their ability to 'bend and flex'.

Alternative splicing enriched cDNA libraries identify breast cancer-associated transcripts

Background

Alternative splicing (AS) is a central mechanism in the generation of genomic complexity and is a major contributor to transcriptome and proteome diversity. Alterations of the splicing process can lead to deregulation of crucial cellular processes and have been associated with a large spectrum of human diseases. Cancer-associated transcripts are potential molecular markers and may contribute to the development of more accurate diagnostic and prognostic methods and also serve as therapeutic targets. Alternative splicing-enriched cDNA libraries have been used to explore the variability generated by alternative splicing. In this study, by combining the use of trapping heteroduplexes and RNA amplification, we developed a powerful approach that enables transcriptome-wide exploration of the AS repertoire for identifying AS variants associated with breast tumor cells modulated by ERBB2 (HER-2/neu) oncogene expression.

Results

The human breast cell line (C5.2) and a pool of 5 ERBB2 over-expressing breast tumor samples were used independently for the construction of two AS-enriched libraries. In total, 2,048 partial cDNA sequences were obtained, revealing 214 alternative splicing sequence-enriched tags (ASSETs). A subset with 79 multiple exon ASSETs was compared to public databases and reported 138 different AS events. A high success rate of RT-PCR validation (94.5%) was obtained, and 2 novel AS events were identified. The influence of ERBB2-mediated expression on AS regulation was evaluated by capillary electrophoresis and probe-ligation approaches in two mammary cell lines (Hb4a and C5.2) expressing different levels of ERBB2. The relative expression balance between AS variants from 3 genes was differentially modulated by ERBB2 in this model system.

Conclusions

In this study, we presented a method for exploring AS from any RNA source in a transcriptome-wide format, which can be directly easily adapted to next generation sequencers. We identified AS transcripts that were differently modulated by ERBB2-mediated expression and that can be tested as molecular markers for breast cancer. Such a methodology will be useful for completely deciphering the cancer cell transcriptome diversity resulting from AS and for finding more precise molecular markers.

Binding of pro-prion to filamin A: by design or an unfortunate blunder

Over the last decades, cancer research has focused on tumor suppressor genes and oncogenes. Genes in other cellular pathways has received less attention. Between 0.5% to 1% of the mammalian genome encodes for proteins that are tethered on the cell membrane via a glycosylphosphatidylinositol (GPI)-anchor. The GPI modification pathway is complex and not completely understood. Prion (PrP), a GPI-anchored protein, is infamous for being the only normal protein that when misfolded can cause and transmit a deadly disease. Though widely expressed and highly conserved, little is known about the functions of PrP. Pancreatic cancer and melanoma cell lines express PrP. However, in these cell lines the PrP exists as a pro-PrP as defined by retaining its GPI anchor peptide signal sequence (GPI-PSS). Unexpectedly, the GPI-PSS of PrP has a filamin A (FLNA) binding motif and binds FLNA. FLNA is a cytolinker protein, and an integrator of cell mechanics and signaling. Binding of pro-PrP to FLNA disrupts the normal FLNA functions. Although normal pancreatic ductal cells lack PrP, about 40% of patients with pancreatic ductal cell adenocarcinoma express PrP in their cancers. These patients have significantly shorter survival time compared with patients whose cancers lack PrP. Pro-PrP is also detected in melanoma in situ but is undetectable in normal melanocyte, and invasive melanoma expresses more pro-PrP. In this review, we will discuss the underlying mechanisms by which binding of pro-PrP to FLNA disrupts normal cellular physiology and contributes to tumorigenesis, and the potential mechanisms that cause the accumulation of pro-PrP in cancer cells.