Src homology 3 domain-dependent interaction of Nck-2 with insulin receptor substrate-1

Genome-wide association study for systemic lupus erythematosus in an egyptian population

Systemic lupus erythematosus (SLE) susceptibility has a strong genetic component. Genome-wide association studies (GWAS) across trans-ancestral populations show both common and distinct genetic variants of susceptibility across European and Asian ancestries, while many other ethnic populations remain underexplored. We conducted the first SLE GWAS on Egyptians-an admixed North African/Middle Eastern population-using 537 patients and 883 controls. To identify novel susceptibility loci and replicate previously known loci, we performed imputation-based association analysis with 6,382,276 SNPs while accounting for individual admixture. We validated the association analysis using adaptive permutation tests (n = 109). We identified a novel genome-wide significant locus near IRS1/miR-5702 (Pcorrected = 1.98 × 10-8) and eight novel suggestive loci (Pcorrected < 1.0 × 10-5). We also replicated (Pperm < 0.01) 97 previously known loci with at least one associated nearby SNP, with ITGAM, DEF6-PPARD and IRF5 the top three replicated loci. SNPs correlated (r 2 > 0.8) with lead SNPs from four suggestive loci (ARMC9, DIAPH3, IFLDT1, and ENTPD3) were associated with differential gene expression (3.5 × 10-95 < p < 1.0 × 10-2) across diverse tissues. These loci are involved in cellular proliferation and invasion-pathways prominent in lupus and nephritis. Our study highlights the utility of GWAS in an admixed Egyptian population for delineating new genetic associations and for understanding SLE pathogenesis.

IRTKS (BAIAP2L1) Elongates Epithelial Microvilli Using EPS8-Dependent and Independent Mechanisms

Transporting epithelial cells like those that line the gut build large arrays of actin-supported protrusions called microvilli, which extend from the apical surface into luminal spaces to increase functional surface area. Although critical for maintaining physiological homeostasis, mechanisms controlling the formation of microvilli remain poorly understood. Here, we report that the inverse-bin-amphiphysin-Rvs (I-BAR)-domain-containing protein insulin receptor tyrosine kinase substrate (IRTKS) (also known as BAIAP2L1) promotes the growth of epithelial microvilli. Super-resolution microscopy and live imaging of differentiating epithelial cells revealed that IRTKS localizes to the distal tips of actively growing microvilli via a mechanism that requires its N-terminal I-BAR domain. At microvillar tips, IRTKS promotes elongation through a mechanism involving its C-terminal actin-binding WH2 domain. IRTKS can also drive microvillar elongation using its SH3 domain to recruit the bundling protein EPS8 to microvillar tips. These results provide new insight on mechanisms that control microvillar growth during the differentiation of transporting epithelial cells and help explain why IRTKS is targeted by enteric pathogens that disrupt microvillar structure during infection of the intestinal epithelium.

Meta-fibrosis links positive energy balance and mitochondrial metabolism to insulin resistance

Obesity and insulin resistance often emerge from positive energy balance and generally are linked to low-grade inflammation. This low-grade inflammation has been called “meta-inflammation” because it is a consequence of the metabolic dysregulation that can accompany overnutrition. One means by which meta-inflammation is linked to insulin resistance is extracellular matrix expansion secondary to meta-inflammation, which we define here as “meta-fibrosis”. The significance of meta-fibrosis is that it reflects a situation in which the extracellular matrix functions as a multi-level integrator of local (for example, mitochondrial reactive oxygen species production) and systemic (for example, inflammation) inputs that couple to cellular processes creating insulin resistance. While adipose tissue extracellular matrix remodeling has received considerable attention, it is becoming increasingly apparent that liver and skeletal muscle extracellular matrix remodeling also contributes to insulin resistance. In this review, we address recent advances in our understanding of energy balance, mitochondrial energetics, meta-inflammation, and meta-fibrosis in the development of insulin resistance.

Neutrophil elastase-mediated degradation of IRS-1 accelerates lung tumor growth

Lung cancer is the leading cause of cancer death worldwide1. Recent data suggest that tumor-associated inflammatory cells may modify lung tumor growth and invasiveness2-3. To determine the role of neutrophil elastase (NE or Elane) on tumor progression, we utilized the LSL-K-ras model of murine lung adenocarcinoma4 to generate LSL-K-ras/Elane^−/− mice. Tumor burden was markedly reduced in LSL-K-ras/Elane^−/− mice at all time points following induction of mutant K-ras expression. Kaplan-Meier life survival analysis demonstrated that while 100% of LSL-K-ras/Elane^+/+ mice died, none of the mice lacking NE died. NE directly induced tumor cell proliferation in both human and mouse lung adenocarcinomas by gaining access to an endosomal compartment within tumor cells where it degraded insulin receptor substrate-1 (IRS1). Co-immunoprecipitation studies showed that as NE degraded IRS1, there was increased interaction between PI3K and the potent mitogen platelet derived growth factor receptor (PDGFR) thereby skewing the PI3K axis toward tumor cell proliferation. The inverse relationship identified between NE and IRS1 in LSL-K-ras mice was also identified in human lung adenocarcinomas, thus translating these findings to human disease. This study identifies IRS1 as a key regulator of PI3K within malignant cells. Additionally, this is the first description of a secreted proteinase gaining access to a cell beyond its plasma membrane and altering intracellular signaling.

Negative regulators of insulin signaling revealed in a genome-wide functional screen

Background

Type 2 diabetes develops due to a combination of insulin resistance and β-cell failure and current therapeutics aim at both of these underlying causes. Several negative regulators of insulin signaling are known and are the subject of drug discovery efforts. We sought to identify novel contributors to insulin resistance and hence potentially novel targets for therapeutic intervention.

Methodology

An arrayed cDNA library encoding 18,441 human transcripts was screened for inhibitors of insulin signaling and revealed known inhibitors and numerous potential novel regulators. The novel hits included proteins of various functional classes such as kinases, phosphatases, transcription factors, and GTPase associated proteins. A series of secondary assays confirmed the relevance of the primary screen hits to insulin signaling and provided further insight into their modes of action.

Conclusion/Significance

Among the novel hits was PALD (KIAA1274, paladin), a previously uncharacterized protein that when overexpressed led to inhibition of insulin's ability to down regulate a FOXO1A-driven reporter gene, reduced upstream insulin-stimulated AKT phosphorylation, and decreased insulin receptor (IR) abundance. Conversely, knockdown of PALD gene expression resulted in increased IR abundance, enhanced insulin-stimulated AKT phosphorylation, and an improvement in insulin's ability to suppress FOXO1A-driven reporter gene activity. The present data demonstrate that the application of arrayed genome-wide screening technologies to insulin signaling is fruitful and is likely to reveal novel drug targets for insulin resistance and the metabolic syndrome.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Phosphatidylinositol 3-phosphate-dependent and -independent functions of p40phox in activation of the neutrophil NADPH oxidase

In response to bacterial infection, the neutrophil NADPH oxidase assembles on phagolysosomes to catalyze the transfer of electrons from NADPH to oxygen, forming superoxide and downstream reactive oxygen species (ROS). The active oxidase is composed of a membrane-bound cytochrome together with three cytosolic phox proteins, p40(phox), p47(phox), and p67(phox), and the small GTPase Rac2, and is regulated through a process involving protein kinase C, MAPK, and phosphatidylinositol 3-kinase. The role of p40(phox) remains less well defined than those of p47(phox) and p67(phox). We investigated the biological role of p40(phox) in differentiated PLB-985 neutrophils, and we show that depletion of endogenous p40(phox) using lentiviral short hairpin RNA reduces ROS production and impairs bacterial killing under conditions where p67(phox) levels remain constant. Biochemical studies using a cytosol-reconstituted permeabilized human neutrophil cores system that recapitulates intracellular oxidase activation revealed that depletion of p40(phox) reduces both the maximal rate and total amount of ROS produced without altering the K(M) value of the oxidase for NADPH. Using a series of mutants, p47PX-p40(phox) chimeras, and deletion constructs, we found that the p40(phox) PX domain has phosphatidylinositol 3-phosphate (PtdIns(3)P)-dependent and -independent functions. Translocation of p67(phox) requires the PX domain but not 3-phosphoinositide binding. Activation of the oxidase by p40(phox), however, requires both PtdIns(3)P binding and an Src homology 3 (SH3) domain competent to bind to poly-Pro ligands. Mutations that disrupt the closed auto-inhibited form of full-length p40(phox) can increase oxidase activity approximately 2.5-fold above that of wild-type p40(phox) but maintain the requirement for PX and SH3 domain function. We present a model where p40(phox) translocates p67(phox) to the region of the cytochrome and subsequently switches the oxidase to an activated state dependent upon PtdIns(3)P and SH3 domain engagement.

Intramolecular interaction in the tail of Acanthamoeba myosin IC between the SH3 domain and a putative pleckstrin homology domain

The 466-aa tail of the heavy chain of Acanthamoeba myosin IC (AMIC) comprises an N-terminal 220-residue basic region (BR) followed by a 56-residue Gly/Pro/Ala-rich region (GPA1), a 55-residue Src homology 3 (SH3) domain, and a C-terminal 135-residue Gly/Pro/Ala-rich region (GPA2). Cryo-electron microscopy of AMIC had shown previously that the AMIC tail is folded back on itself, suggesting the possibility of interactions between its N- and C-terminal regions. We now show specific differences between the NMR spectrum of bacterially expressed full-length tail and the sum of the spectra of individually expressed BR and GPA1-SH3-GPA2 (GSG) regions. These results are indicative of interactions between the two subdomains in the full-length tail. From the NMR data, we could assign many of the residues in BR and GSG that are involved in these interactions. By combining homology modeling with the NMR data, we identify a putative pleckstrin homology (PH) domain within BR, and show that the PH domain interacts with the SH3 domain.

Signalling via integrins: implications for cell survival and anticancer strategies

Integrin-associated signalling renders cells more resistant to genotoxic anti-cancer agents like ionizing radiation and chemotherapeutic substances, a phenomenon termed cell adhesion-mediated radioresistance/drug resistance (CAM-RR, CAM-DR). Integrins are heterodimeric cell-surface molecules that on one side link the actin cytoskeleton to the cell membrane and on the other side mediate cell-matrix interactions. In addition to their structural functions, integrins mediate signalling from the extracellular space into the cell through integrin-associated signalling and adaptor molecules such as FAK (focal adhesion kinase), ILK (integrin-linked kinase), PINCH (particularly interesting new cysteine-histidine rich protein) and Nck2 (non-catalytic (region of) tyrosine kinase adaptor protein 2). Via these molecules, integrin signalling tightly and cooperatively interacts with receptor tyrosine kinase signalling to regulate survival, proliferation and cell shape as well as polarity, adhesion, migration and differentiation. In tumour cells of diverse origin like breast, colon or skin, the function and regulation of these molecules is partly disturbed and thus might contribute to the malignant phenotype and pre-existent and acquired multidrug resistance. These issues as well as a variety of therapeutic options envisioned to influence tumour cell growth, metastasis and resistance, including kinase inhibitors, anti-integrin antibodies or RNA interference, will be summarized and discussed in this review.

The subcellular fractionation properties and function of insulin receptor substrate-1 (IRS-1) are independent of cytoskeletal integrity

Efficient insulin action requires spatial and temporal coordination of signaling cascades. The prototypical insulin receptor substrate, IRS-1 plays a central role in insulin signaling. By subcellular fractionation IRS-1 is enriched in a particulate fraction, termed the high speed pellet (HSP), and its redistribution from this fraction is associated with signal attenuation and insulin resistance. Anecdotal evidence suggests the cytoskeleton may underpin the localization of IRS-1 to the HSP. In the present study we have taken a systematic approach to examine whether the cytoskeleton contributes to the subcellular fractionation properties and function of IRS-1. By standard microscopy or immunoprecipitation we were unable to detect evidence to support a specific interaction between IRS-1 and the major cytoskeletal components actin (microfilaments), vimentin (intermediate filaments), and tubulin (microtubules) in 3T3-L1 adipocytes or in CHO.IR.IRS-1 cells. Pharmacological disruption of microfilaments and microtubules, individually or in combination, was without effect on the subcellular distribution of IRS-1 or insulin-stimulated tyrosine phosphorylation in either cell type. Phosphorylation of Akt was modestly reduced (20-35%) in 3T3-L1 adipocytes but not in CHO.IR.IRS-1 cells. In cells lacking intermediate filaments (Vim(-/-)) IRS-1 expression, distribution and insulin-stimulated phosphorylation appeared normal. Even after depolymerisation of microfilaments and microtubules, insulin-stimulated phosphorylation of IRS-1 and Akt were maintained in Vim(-/-) cells. Taken together these data indicate that the characteristic subcellular fractionation properties and function of IRS-1 are unlikely to be mediated by cytoskeletal networks and that proximal insulin signaling does not require an intact cytoskeleton.

The PINCH-ILK-parvin complexes: assembly, functions and regulation

Cell-extracellular matrix (ECM) adhesion is mediated by transmembrane cell adhesion receptors (e.g., integrins) and receptor proximal cytoplasmic proteins. Over the past several years, studies using biochemical, structural, cell biological and genetic approaches have provided important evidence suggesting crucial roles of integrin-linked kinase (ILK), PINCH and CH-ILKBP/actopaxin/affixin/parvin (abbreviated as parvin herein) in ECM control of cell behavior. One general theme emerging from these studies is that the formation of ternary protein complexes consisting of ILK, PINCH and parvin is pivotal to the functions of PINCH, ILK and parvin proteins. In addition, recent studies have begun to uncover the molecular mechanisms underlying the assembly, functions and regulation of the PINCH-ILK-parvin (PIP) complexes. The PIP complexes provide crucial physical linkages between integrins and the actin cytoskeleton and transduce diverse signals from ECM to intracellular effectors. Among the challenges of future studies are to define the functions of different PIP complexes in various cellular processes, identify additional partners of the PIP complexes that regulate and/or mediate the functions of the PIP complexes, and determine the roles of the PIP complexes in the pathogenesis of human diseases involving abnormal cell-ECM adhesion and signaling.

Nck-2 interacts with focal adhesion kinase and modulates cell motility

Nck-2 is a ubiquitously expressed adaptor protein comprising primarily three N-terminal SH3 domains and one C-terminal SH2 domain. We report here that Nck-2 interacts with focal adhesion kinase (FAK), a cytoplasmic protein tyrosine kinase critically involved in the cellular control of motility. Using a mutational strategy, we have found that the formation of the Nck-2-FAK complex is mediated by interactions involving multiple SH2 and SH3 domains of Nck-2. The Nck-2 SH2 domain-mediated interaction with FAK is dependent on phosphorylation of Tyr397, a site that is involved in the regulation of cell motility. A fraction of Nck-2 co-localizes with FAK at cell periphery in spreading cells. Furthermore, overexpression of Nck-2 modestly decreased cell motility, whereas overexpression of a mutant form of Nck-2 containing the SH2 domain but lacking the SH3 domains significantly promoted cell motility. These results identify a novel interaction between Nck-2 and FAK and suggest a role of Nck-2 in the modulation of cell motility.

Modulation of protein translation by Nck-1

In mammals, Nck represented by two genes, is a 47-kDa SH2/SH3 domain-containing protein lacking intrinsic enzymatic function. Here, we reported that the first and the third SH3 domains of Nck-1 interact with the C-terminal region of the beta subunit of the eukaryotic initiation factor 2 (eIF2 beta). Binding of eIF2 beta was specific to the SH3 domains of Nck-1, and in vivo, the interaction Nck/eIF2 beta was demonstrated by reciprocal coimmunoprecipitations. In addition, Nck was detected in a molecular complex with eIF2 beta in an enriched ribosomal fraction, whereas no other SH2/SH3 domain-containing adapters were found. Cell fractionation studies demonstrated that the presence of Nck in purified ribosomal fractions was enhanced after insulin stimulation, suggesting that growth factors dynamically regulate translocation of Nck to ribosomes. In HEK293 cells, we observed that transient overexpression of Nck-1 significantly enhanced Cap-dependent and -independent protein translation. This effect of Nck-1 required the integrity of its first and third SH3 domains originally found to interact with eIF2 beta. Finally, in vitro, Nck-1 also increased protein translation, revealing a direct role for Nck-1 in this process. Our study demonstrates that in addition to mediate receptor tyrosine kinase signaling, Nck-1 modulates protein translation potentially through its direct interaction with an intrinsic component of the protein translation machinery.