Phosphorylation of ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) by Akt promotes stability and mitogenic function of S-phase kinase-associated protein-2 (Skp2)

The molecular sociology of NHERF1 PDZ proteins controlling renal hormone-regulated phosphate transport

Parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) control extracellular phosphate levels by regulating renal NPT2A-mediated phosphate transport by a process requiring the PDZ scaffold protein NHERF1. NHERF1 possesses two PDZ domains, PDZ1 and PDZ2, with identical core-binding GYGF motifs explicitly recognizing distinct binding partners that play different and specific roles in hormone-regulated phosphate transport. The interaction of PDZ1 and the carboxy-terminal PDZ-binding motif of NPT2A (C-TRL) is required for basal phosphate transport. PDZ2 is a regulatory domain that scaffolds multiple biological targets, including kinases and phosphatases involved in FGF23 and PTH signaling. FGF23 and PTH trigger disassembly of the NHERF1-NPT2A complex through reversible hormone-stimulated phosphorylation with ensuing NPT2A sequestration, down-regulation, and cessation of phosphate absorption. In the absence of NHERF1-NPT2A interaction, inhibition of FGF23 or PTH signaling results in disordered phosphate homeostasis and phosphate wasting. Additional studies are crucial to elucidate how NHERF1 spatiotemporally coordinates cellular partners to regulate extracellular phosphate levels.

EBP50 is a key molecule for the Schwann cell-axon interaction in peripheral nerves

Peripheral nerve injury disrupts the Schwann cell-axon interaction and the cellular communication between them. The peripheral nervous system has immense potential for regeneration extensively due to the innate plastic potential of Schwann cells (SCs) that allows SCs to interact with the injured axons and exert specific repair functions essential for peripheral nerve regeneration. In this study, we show that EBP50 is essential for the repair function of SCs and regeneration following nerve injury. The increased expression of EBP50 in the injured sciatic nerve of control mice suggested a significant role in regeneration. The ablation of EBP50 in mice resulted in delayed nerve repair, recovery of behavioral function, and remyelination following nerve injury. EBP50 deficiency led to deficits in SC functions, including proliferation, migration, cytoskeleton dynamics, and axon interactions. The adeno-associated virus (AAV)-mediated local expression of EBP50 improved SCs migration, functional recovery, and remyelination. ErbB2-related proteins were not differentially expressed in EBP50-deficient sciatic nerves following injury. EBP50 binds and stabilizes ErbB2 and activates the repair functions to promote regeneration. Thus, we identified EBP50 as a potent SC protein that can enhance the regeneration and functional recovery driven by NRG1-ErbB2 signaling, as well as a novel regeneration modulator capable of potential therapeutic effects.

Cotranslational interaction of human EBP50 and ezrin overcomes masked binding site during complex assembly

Multiprotein assemblages are the intracellular workhorses of many physiological processes. Assembly of constituents into complexes can be driven by stochastic, domain-dependent, posttranslational events in which mature, folded proteins specifically interact. However, inaccessibility of interacting surfaces in mature proteins (e.g., due to "buried" domains) can obstruct complex formation. Mechanisms by which multiprotein complex constituents overcome topological impediments remain enigmatic. For example, the heterodimeric complex formed by EBP50 and ezrin must address this issue as the EBP50-interacting domain in ezrin is obstructed by a self-interaction that occupies the EBP50 binding site. Here, we show that the EBP50-ezrin complex is formed by a cotranslational mechanism in which the C terminus of mature, fully formed EBP50 binds the emerging, ribosome-bound N-terminal FERM domain of ezrin during EZR mRNA translation. Consistent with this observation, a C-terminal EBP50 peptide mimetic reduces the cotranslational interaction and abrogates EBP50-ezrin complex formation. Phosphorylation of EBP50 at Ser³³⁹ and Ser³⁴⁰ abrogates the cotranslational interaction and inhibits complex formation. In summary, we show that the function of eukaryotic mRNA translation extends beyond "simple" generation of a linear peptide chain that folds into a tertiary structure, potentially for subsequent complex assembly; importantly, translation can facilitate interactions with sterically inaccessible domains to form functional multiprotein complexes.

Cytoplasmic EBP50 and elevated PARP1 are unfavorable prognostic factors in ovarian clear cell carcinoma

Patients with ovarian clear cell carcinoma (OCCC) experience frequent recurrence, which is most likely due to chemoresistance. We used shotgun proteomics analysis and identified upregulation of ezrin-binding phosphoprotein 50 (EBP50) in recurrent OCCC samples. Cytoplasmic and/or nuclear (Cyt/N), but not membranous, EBP50 immunoreactivity was significantly higher in recurrent OCCC as compared to that of primary tumors. OCCC cells expressing cytoplasmic EBP50 were significantly less susceptible to cisplatin (CDDP)-induced apoptosis compared to cells expressing membranous EBP50. Abrogation of resistance following knockdown of cytoplasmic EBP50 was accompanied by decreased XIAP and BCL2, increased BAX and increased caspase-3 cleavage. We found that poly (ADP-ribose) polymerase1 (PARP1), which is involved in DNA damage detection and repair, binds to EBP50 through its PDZ1 domain. CDDP treatment of cells expressing cytoplasmic (but not membranous) EBP50 increased nuclear PARP1 expression, whereas knockdown of EBP50 cells decreased PARP1 expression and activity following CDDP treatment. Finally, OCCC patients with a combination of Cyt/N EBP50 and high PARP1 score had worst the prognosis for overall and progression-free survival. Together, our data suggest that cytoplasmic EBP50 inhibits apoptosis and promotes OCCC survival through stabilization of PARP1 activity and modulation of the XIAP/BCL2/BAX axis. This may increase the likelihood of tumor recurrence, and we therefore suggest a combined analysis for EBP50 and PARP1 may have great utility in OCCC prediction and prognosis.

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.

The Tubulin Inhibitor VERU-111 in Combination With Vemurafenib Provides an Effective Treatment of Vemurafenib-Resistant A375 Melanoma

Melanoma is one of the deadliest skin cancers having a five-year survival rate around 15–20%. An overactivated MAPK/AKT pathway is well-established in BRAF mutant melanoma. Vemurafenib (Vem) was the first FDA-approved BRAF inhibitor and gained great clinical success in treating late-stage melanoma. However, most patients develop acquired resistance to Vem within 6–9 months. Therefore, developing a new treatment strategy to overcome Vem-resistance is highly significant. Our previous study reported that the combination of a tubulin inhibitor ABI-274 with Vem showed a significant synergistic effect to sensitize Vem-resistant melanoma both in vitro and in vivo. In the present study, we unveiled that VERU-111, an orally bioavailable inhibitor of α and β tubulin that is under clinical development, is highly potent against Vem-resistant melanoma cells. The combination of Vem and VERU-111 resulted in a dramatically enhanced inhibitory effect on cancer cells in vitro and Vem-resistant melanoma tumor growth in vivo compared with single-agent treatment. Further molecular signaling analyses demonstrated that in addition to ERK/AKT pathway, Skp2 E3 ligase also plays a critical role in Vem-resistant mechanisms. Knockout of Skp2 diminished oncogene AKT expression and contributed to the synergistic inhibitory effect of Vem and VERU-111. Our results indicate a treatment combination of VERU-111 and Vem holds a great promise to overcome Vem-resistance for melanoma patients harboring BRAF (V600E) mutation.

Mutated SPOP E3 Ligase Promotes 17βHSD4 Protein Degradation to Drive Androgenesis and Prostate Cancer Progression

Molecular mechanisms underlying intratumoral androgenesis and aberrant androgen receptor (AR) activation in prostate cancer remain poorly understood. Here we demonstrate that ectopic expression of the E3 ubiquitin ligase adaptor speckle-type poxvirus and zinc finger domain protein (SPOP) stabilizes 17βHSD4. SPOP bound a functional substrate-binding consensus (SBC) motif 315RATST319 in 17βHSD4 and promoted nondegradable K27- and K29-linked polyubiquitination of 17βHSD4. The effect of SPOP was antagonized by serum- and glucocorticoid kinase-3 (SGK3)-mediated phosphorylation of serine 318 (S318) in the SBC and S318 phosphorylation-dependent binding of SKP2 E3 ligase and subsequent K48-linked polyubiquitination and proteasomal degradation of 17βHSD4. Prostate cancer-associated SPOP mutations impaired the SPOP-17βHSD4 interaction, caused 17βHSD4 protein destruction in prostate cancer cells in culture and patient specimens, and increased testosterone production and prostate cancer cell growth in vitro and in mouse models. Thus, we have identified SPOP and SKP2 as two essential E3 ubiquitin ligases that exert opposite effects on 17βHSD4 protein degradation and intratumoral androgenesis in prostate cancer cells. We further demonstrate that SPOP mutations or SKP2 overexpression contribute to prostate cancer progression by decreasing 17βHSD4 expression and increasing intratumoral androgen synthesis. SIGNIFICANCE: This study reveals a novel mechanism of aberrant AR activation in SPOP-mutated prostate cancer and uncovers putative biomarkers for effective treatment by AR-targeted therapies.

A Critical Role for Na+/H+ Exchanger Regulatory Factor 1 in Modulating FcεRI-Mediated Mast Cell Activation

Mast cells are tissue-resident immune cells that play pivotal roles in initiating and amplifying allergic/anaphylactic reactions in humans. Their activation occurs via multiple mechanisms, which include cross-linking of the IgE-bound, high-affinity IgE receptors (FcεRI) by allergens or Ags and the binding of anaphylatoxins such as C3a to its receptor, C3aR. We have previously demonstrated that the Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) promotes C3aR functions in human mast cells. In the current study, we show that NHERF1 regulates mast cell response following FcεRI stimulation. Specifically, intracellular Ca²⁺ mobilization, activation of the MAPKs (ERK1/2 and P38), and production of cytokines (IL-13 and IL-6) following exposure to IgE/Ag were significantly reduced in mast cells from NHERF1^+/‒ mice. In agreement with our in vitro data, mast cell-mediated passive cutaneous anaphylaxis and passive systemic anaphylaxis were reduced in NHERF1^+/‒ mice and mast cell-deficient Kit^W-sh/W-sh mice engrafted with NHERF1^+/‒ mast cells. Mechanistically, the levels of microRNAs (miRNAs) that regulate mast cell responses, miRNA 155-3p and miRNA 155-5p, were altered in mast cells from NHERF1^+/‒ mice. Moreover, NHERF1 rapidly localized to the nucleus of mast cells following FcεRI stimulation. In summary, our results suggest that the NHERF1 acts as an adapter molecule and promotes IgE/Ag-induced mast cell activation. Further elucidating the mechanisms through which NHERF1 modulates mast cell responses will lend insights into the development of new therapeutic strategies to target mast cells during anaphylaxis or other allergic diseases.

Loss-of-function of EBP50 is a new cause of hereditary peripheral neuropathy: EBP50 functions in peripheral nerve system

Finding causative genetic mutations is important in the diagnosis and treatment of hereditary peripheral neuropathies. This study was conducted to find new genes involved in the pathophysiology of hereditary peripheral neuropathy. We identified a new mutation in the EBP50 gene, which is co-segregated with neuropathic phenotypes, including motor and sensory deficit in a family with Charcot-Marie-Tooth disease. EBP50 is known to be important for the formation of microvilli in epithelial cells, and the discovery of this gene mutation allowed us to study the function of EBP50 in the nervous system. EBP50 was strongly expressed in the nodal and paranodal regions of sciatic nerve fibers, where Schwann cell microvilli contact the axolemma, and at the growth tips of primary Schwann cells. In addition, EBP50 expression was decreased in mouse models of peripheral neuropathy. Knockout mice were used to study EBP50 function in the peripheral nervous system. Interestingly motor function deficit and abnormal histology of nerve fibers were observed in EBP50^+/- heterozygous mice at 12 months of age, but not 3 months. in vitro studies using Schwann cells showed that NRG1-induced AKT activation and migration were significantly reduced in cells overexpressing the I325V mutant of EBP50 or cells with knocked-down EBP50 expression. In conclusion, we show for the first time that loss of function due to EBP50 gene deficiency or mutation can cause peripheral neuropathy.

Parallel Post-Translational Modification Scanning Enhancing Hydrogen-Deuterium Exchange-Mass Spectrometry Coverage of Key Structural Regions

Hydrogen-deuterium exchange-mass spectrometry (HDXMS) is a powerful technology to characterize conformations and conformational dynamics of proteins and protein complexes. HDXMS has been widely used in the field of therapeutics for the development of protein drugs. Although sufficient sequence coverage is critical to the success of HDXMS, it is sometimes difficult to achieve. In this study, we developed a HDXMS data analysis strategy that includes parallel post-translational modification (PTM) scanning in HDXMS analysis. Using a membrane-delimited G protein-coupled receptor (vasopressin type 2 receptor; V₂R) and a cytosolic protein (Na⁺/H⁺ exchanger regulatory factor-1; NHERF1) as examples, we demonstrate that this strategy substantially improves protein sequence coverage, especially in key structural regions likely including PTMs themselves that play important roles in protein conformational dynamics and function.

Parathyroid hormone initiates dynamic NHERF1 phosphorylation cycling and conformational changes that regulate NPT2A-dependent phosphate transport

Na⁺-H⁺ exchanger regulatory factor-1 (NHERF1) is a PDZ protein that scaffolds membrane proteins, including sodium-phosphate co-transport protein 2A (NPT2A) at the plasma membrane. NHERF1 is a phosphoprotein with 40 Ser and Thr residues. Here, using tandem MS analysis, we characterized the sites of parathyroid hormone (PTH)-induced NHERF1 phosphorylation and identified 10 high-confidence phosphorylation sites. Ala replacement at Ser⁴⁶, Ser¹⁶², Ser¹⁸¹, Ser²⁶⁹, Ser²⁸⁰, Ser²⁹¹, Thr²⁹³, Ser²⁹⁹, and Ser³⁰² did not affect phosphate uptake, but S290A substitution abolished PTH-dependent phosphate transport. Unexpectedly, Ser²⁹⁰ was rapidly dephosphorylated and rephosphorylated after PTH stimulation, and we found that protein phosphatase 1α (PP1α), which binds NHERF1 through a conserved VxF/W PP1 motif, dephosphorylates Ser²⁹⁰ Mutating ²⁵⁷VPF²⁵⁹ eliminated PP1 binding and blunted dephosphorylation. Tautomycetin blocked PP1 activity and abrogated PTH-sensitive phosphate transport. Using fluorescence lifetime imaging (FLIM), we observed that PTH paradoxically and transiently elevates intracellular phosphate. Added phosphate blocked PP1α-mediated Ser²⁹⁰ dephosphorylation of recombinant NHERF1. Hydrogen-deuterium exchange MS revealed that β-sheets in NHERF1's PDZ2 domain display lower deuterium uptake than those in the structurally similar PDZ1, implying that PDZ1 is more cloistered. Dephosphorylated NHERF1 exhibited faster exchange at C-terminal residues suggesting that NHERF1 dephosphorylation precedes Ser²⁹⁰ rephosphorylation. Our results show that PP1α and NHERF1 form a holoenzyme and that a multiprotein kinase cascade involving G protein-coupled receptor kinase 6A controls the Ser²⁹⁰ phosphorylation status of NHERF1 and regulates PTH-sensitive, NPT2A-mediated phosphate uptake. These findings reveal how reversible phosphorylation modifies protein conformation and function and the biochemical mechanisms underlying PTH control of phosphate transport.

A novel thyroid hormone receptor isoform, TRβ2-46, promotes SKP2 expression and retinoblastoma cell proliferation

Retinoblastoma is a childhood retinal tumor that develops from cone photoreceptor precursors in response to inactivating RB1 mutations and loss of functional RB protein. The cone precursor's response to RB loss involves cell type-specific signaling circuitry that helps to drive tumorigenesis. One component of the cone precursor circuitry, the thyroid hormone receptor β2 (TRβ2), enables the aberrant proliferation of diverse RB-deficient cells in part by opposing the down-regulation of S-phase kinase-associated protein 2 (SKP2) by the more widely expressed and tumor-suppressive TRβ1. However, it is unclear how TRβ2 opposes TRβ1 to enable SKP2 expression and cell proliferation. Here, we show that in human retinoblastoma cells TRβ2 mRNA encodes two TRβ2 protein isoforms: a predominantly cytoplasmic 54-kDa protein (TRβ2-54) corresponding to the well-characterized full-length murine Trβ2 and an N-terminally truncated and exclusively cytoplasmic 46-kDa protein (TRβ2-46) that starts at Met-79. Whereas TRβ2 knockdown decreased SKP2 expression and impaired retinoblastoma cell cycle progression, re-expression of TRβ2-46 but not TRβ2-54 stabilized SKP2 and restored proliferation to an extent similar to that of ectopic SKP2 restoration. We conclude that TRβ2-46 is an oncogenic thyroid hormone receptor isoform that promotes SKP2 expression and SKP2-dependent retinoblastoma cell proliferation.

Phosphorylation of NHERF1 S279 and S301 differentially regulates breast cancer cell phenotype and metastatic organotropism

Metastatic cancer cells are highly plastic for the expression of different tumor phenotype hallmarks and organotropism. This plasticity is highly regulated but the dynamics of the signaling processes orchestrating the shift from one cell phenotype and metastatic organ pattern to another are still largely unknown. The scaffolding protein NHERF1 has been shown to regulate the expression of different neoplastic phenotypes through its PDZ domains, which forms the mechanistic basis for metastatic organotropism. This reprogramming activity was postulated to be dependent on its differential phosphorylation patterns. Here, we show that NHERF1 phosphorylation on S279/S301 dictates several tumor phenotypes such as in vivo invasion, NHE1-mediated matrix digestion, growth and vasculogenic mimicry. Remarkably, injecting mice with cells having differential NHERF1 expression and phosphorylation drove a shift from the predominantly lung colonization (WT NHERF1) to predominately bone colonization (double S279A/S301A mutant), indicating that NHERF1 phosphorylation also acts as a signaling switch in metastatic organotropism.

β-catenin knockdown promotes NHERF1-mediated survival of colorectal cancer cells: implications for a double-targeted therapy

Nuclear activated β-catenin plays a causative role in colorectal cancers (CRC) but remains an elusive therapeutic target. Using human CRC cells harboring different Wnt/β-catenin pathway mutations in APC/KRAS or β-catenin/KRAS genes, and both genetic and pharmacological knockdown approaches, we show that oncogenic β-catenin signaling negatively regulates the expression of NHERF1 (Na⁺/H⁺ exchanger 3 regulating factor 1), a PDZ-adaptor protein that is usually lost or downregulated in early dysplastic adenomas to exacerbate nuclear β-catenin activity. Chromatin immunoprecipitation (ChIP) assays demonstrated that β-catenin represses NHERF1 via TCF4 directly, while the association between TCF1 and the Nherf1 promoter increased upon β-catenin knockdown. To note, the occurrence of a cytostatic survival response in settings of single β-catenin-depleted CRC cells was abrogated by combining NHERF1 inhibition via small hairpin RNA (shRNA) or RS5517, a novel PDZ1-domain ligand of NHERF1 that prevented its ectopic nuclear entry. Mechanistically, dual NHERF1/β-catenin targeting promoted an autophagy-to-apoptosis switch consistent with the activation of Caspase-3, the cleavage of PARP and reduced levels of phospho-ERK1/2, Beclin-1, and Rab7 autophagic proteins compared with β-catenin knockdown alone. Collectively, our data unveil novel β-catenin/TCF-dependent mechanisms of CRC carcinogenesis, also offering preclinical proof of concept for combining β-catenin and NHERF1 pharmacological inhibitors as a mechanism-based strategy to augment apoptotic death of CRC cells refractory to current Wnt/β-catenin-targeted therapeutics.

Role of the PDZ-scaffold protein NHERF1/EBP50 in cancer biology: from signaling regulation to clinical relevance

The transmission of cellular information requires fine and subtle regulation of proteins that need to interact in a coordinated and specific way to form efficient signaling networks. The spatial and temporal coordination relies on scaffold proteins. Thanks to protein interaction domains such as PDZ domains, scaffold proteins organize multiprotein complexes enabling the proper transmission of cellular information through intracellular networks. NHERF1/EBP50 is a PDZ-scaffold protein that was initially identified as an organizer and regulator of transporters and channels at the apical side of epithelia through actin-binding ezrin-moesin-radixin proteins. Since, NHERF1/EBP50 has emerged as a major regulator of cancer signaling network by assembling cancer-related proteins. The PDZ-scaffold EBP50 carries either anti-tumor or pro-tumor functions, two antinomic functions dictated by EBP50 expression or subcellular localization. The dual function of NHERF1/EBP50 encompasses the regulation of several major signaling pathways engaged in cancer, including the receptor tyrosine kinases PDGFR and EGFR, PI3K/PTEN/AKT and Wnt-β-catenin pathways.

Adenosine Attenuates Human Coronary Artery Smooth Muscle Cell Proliferation by Inhibiting Multiple Signaling Pathways That Converge on Cyclin D

The goal of this study was to determine whether and how adenosine affects the proliferation of human coronary artery smooth muscle cells (HCASMCs). In HCASMCs, 2-chloroadenosine (stable adenosine analogue), but not N(6)-cyclopentyladenosine, CGS21680, or N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide, inhibited HCASMC proliferation (A2B receptor profile). 2-Chloroadenosine increased cAMP, reduced phosphorylation (activation) of ERK and Akt (protein kinases known to increase cyclin D expression and activity, respectively), and reduced levels of cyclin D1 (cyclin that promotes cell-cycle progression in G1). Moreover, 2-chloroadenosine inhibited expression of S-phase kinase-associated protein-2 (Skp2; promotes proteolysis of p27(Kip1)) and upregulated levels of p27(Kip1) (cell-cycle regulator that impairs cyclin D function). 2-Chloroadenosine also inhibited signaling downstream of cyclin D, including hyperphosphorylation of retinoblastoma protein and expression of cyclin A (S phase cyclin). Knockdown of A2B receptors prevented the effects of 2-chloroadenosine on ERK1/2, Akt, Skp2, p27(Kip1), cyclin D1, cyclin A, and proliferation. Likewise, inhibition of adenylyl cyclase and protein kinase A abrogated 2-chloroadenosine's inhibitory effects on Skp2 and stimulatory effects on p27(Kip1) and rescued HCASMCs from 2-chloroadenosine-mediated inhibition. Knockdown of p27(Kip1) also reversed the inhibitory effects of 2-chloroadenosine on HCASMC proliferation. In vivo, peri-arterial (rat carotid artery) 2-chloroadenosine (20 μmol/L for 7 days) downregulated vascular expression of Skp2, upregulated vascular expression of p27(Kip1), and reduced neointima hyperplasia by 71% (P<0.05; neointimal thickness: control, 37 424±18 371 pixels; treated, 10 352±2824 pixels). In conclusion, the adenosine/A2B receptor/cAMP/protein kinase A axis inhibits HCASMC proliferation by blocking multiple signaling pathways (ERK1/2, Akt, and Skp2) that converge at cyclin D, a key G1 cyclin that controls cell-cycle progression.