Ninjurin1 deficiency aggravates colitis development by promoting M1 macrophage polarization and inducing microbial imbalance.

Disruption of colonic homeostasis caused by aberrant M1/M2 macrophage polarization and dysbiosis contributes to inflammatory bowel disease (IBD) pathogenesis. However, the molecular factors mediating colonic homeostasis are not well characterized. Here, we found that Ninjurin1 (Ninj1) limits colon inflammation by regulating macrophage polarization and microbiota composition under homeostatic conditions and during colitis development. Ninj1 deletion in mice induced hypersusceptibility to colitis, with increased prevalence of colitogenic Prevotellaceae strains and decreased immunoregulatory Lachnospiraceae strains. Upon co-housing (CoH) with WT mice, Ninj1-/- mice showed increased Lachnospiraceae and decreased Prevotellaceae abundance, with subsequent improvement of colitis. Under homeostatic conditions, M1 macrophage frequency was higher in the Ninj1-/- mouse colons than wild-type (WT) mouse colons, which may contribute to increased basal colonic inflammation and microbial imbalance. Following colitis induction, Ninj1 expression was increased in macrophages; meanwhile Ninj1-/- mice showed severe colitis development and impaired recovery, associated with decreased M2 macrophages and escalated microbial imbalance. In vitro, Ninj1 knockdown in mouse and human macrophages activated M1 polarization and restricted M2 polarization. Finally, the transfer of WT macrophages ameliorated severe colitis in Ninj1-/- mice. These findings suggest that Ninj1 mediates colonic homeostasis by modulating M1/M2 macrophage balance and preventing extensive dysbiosis, with implications for IBD prevention and therapy.

Ninjurin1 positively regulates osteoclast development by enhancing the survival of prefusion osteoclasts.

Osteoclasts (OCs) are bone-resorbing cells that originate from hematopoietic stem cells and develop through the fusion of mononuclear myeloid precursors. Dysregulation of OC development causes bone disorders such as osteopetrosis, osteoporosis, and rheumatoid arthritis. Although the molecular mechanisms underlying osteoclastogenesis have been well established, the means by which OCs maintain their survival during OC development remain unknown. We found that Ninjurin1 (Ninj1) expression is dynamically regulated during osteoclastogenesis and that Ninj1-/- mice exhibit increased trabecular bone volume owing to impaired OC development. Ninj1 deficiency did not alter OC differentiation, transmigration, fusion, or actin ring formation but increased Caspase-9-dependent intrinsic apoptosis in prefusion OCs (preOCs). Overexpression of Ninj1 enhanced the survival of mouse macrophage/preOC RAW264.7 cells in osteoclastogenic culture, suggesting that Ninj1 is important for the survival of preOCs. Finally, analysis of publicly available microarray data sets revealed a potent correlation between high NINJ1 expression and destructive bone disorders in humans. Our data indicate that Ninj1 plays an important role in bone homeostasis by enhancing the survival of preOCs.

A-kinase anchoring protein 12 is downregulated in human hepatocellular carcinoma and its deficiency in mice aggravates thioacetamide-induced liver injury.

AKAP12 belongs to A-kinase anchoring protein (AKAP) family of scaffold proteins and is known as a tumor suppressor in several human cancer types. Its role as a tumor suppressor in hepatocellular carcinoma (HCC) was proposed due to its downregulation and epigenetic modification in human HCC; however, the effect of its deficiency on liver injuries, such as liver fibrosis and cancer has been poorly studied. By analyzing tumor and non-tumor tissues of 15 patients with HCC, it was confirmed that AKAP12 expression was downregulated in human HCC as compared with adjacent non-tumor tissues. Immunohistochemical staining of mouse liver tissue for AKAP12 revealed that its sinusoidal expression was diminished in capillarized endothelium after 8 weeks of thioacetamide (TAA) administration. AKAP12 deficiency resulted in the promotion of ductular response of biliary epithelial cells, whereas overall fibrosis and myofibroblast activation were comparable between genotypes after short-term TAA treatment. The mRNA expressions of some fibrosis-related genes such as those encoding epithelial cell adhesion molecule, collagen type 1 α1 and elastin were upregulated in liver tissues of AKAP12-knockout mice. Long-term administration of TAA for 26 weeks led to the development of liver tumors; the incidence of tumor development was higher in AKAP12-deficient mice than in wild-type littermates. Together, these results suggest that AKAP12 functions as a tumor suppressor in liver cancer and is associated with the regulation of hepatic non-parenchymal cells.

Altered AKAP12 expression in portal fibroblasts and liver sinusoids mediates transition from hepatic fibrogenesis to fibrosis resolution.

Liver fibrosis can be reversed by removing its causative injuries; however, the molecular mechanisms mediating the resolution of liver fibrogenesis are poorly understood. We investigate the role of a scaffold protein, A-Kinase Anchoring Protein 12 (AKAP12), during liver fibrosis onset, and resolution. Biliary fibrogenesis and fibrosis resolution was induced in wild-type (WT) or AKAP12-deficient C57BL/6 mice through different feeding regimens with 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-containing chow. AKAP12 expression in portal fibroblasts (PFs) and liver sinusoidal endothelial cells (LSECs) gradually decreased as fibrosis progressed but was restored after cessation of the fibrotic challenge. Histological analysis of human liver specimens with varying degrees of fibrosis of different etiologies revealed that AKAP12 expression diminishes in hepatic fibrosis from its early stages onward. AKAP12 KO mice displayed reduced fibrosis resolution in a DDC-induced biliary fibrosis model, which was accompanied by impaired normalization of myofibroblasts and capillarized sinusoids. RNA sequencing of the liver transcriptome revealed that genes related to ECM accumulation and vascular remodeling were mostly elevated in AKAP12 KO samples. Gene ontology (GO) and bioinformatic pathway analyses identified that the differentially expressed genes were significantly enriched in GO categories and pathways, such as the adenosine 3',5'-cyclic monophosphate (cAMP) pathway. Knockdown of the AKAP12 gene in cultured primary PFs revealed that AKAP12 inhibited PF activation in association with the adenosine 3',5'-cyclic monophosphate (cAMP) pathway. Moreover, AKAP12 knockdown in LSECs led to enhanced angiogenesis, endothelin-1 expression and alterations in laminin composition. Collectively, this study demonstrates that AKAP12-mediated regulation of PFs and LSECs has a central role in resolving hepatic fibrosis.

SAMHD1 acetylation enhances its deoxynucleotide triphosphohydrolase activity and promotes cancer cell proliferation.

SAM domain and HD domain containing protein 1 (SAMHD1) is a deoxynucleotide triphosphohydrolase (dNTPase) that inhibits retroviruses by depleting intracellular deoxynucleotide triphosphates (dNTPs) in non-cycling myeloid cells. Although SAMHD1 is expressed ubiquitously throughout the human body, the molecular mechanisms regulating its enzymatic activity and function in non-immune cells are relatively unexplored. Here, we demonstrate that the dNTPase activity of SAMHD1 is regulated by acetylation, which promotes cell cycle progression in cancer cells. SAMHD1 is acetylated at residue lysine 405 (K405) in vitro and in vivo by an acetylatransferase, arrest defective protein 1 (ARD1). Acetylated SAMHD1 wildtype proteins have enhanced dNTPase activity in vitro, whereas non-acetylated arginine substituted mutants (K405R) do not. K405R mutant expressing cancer cells have reduced G1/S transition and slower proliferation compared to wildtype. SAMHD1 acetylation levels are strongest during the G1 phase, indicating a role during G1 phase. Collectively, these findings suggest that SAMHD1 acetylation enhances its dNTPase activity and promotes cancer cell proliferation. Therefore, SAMHD1 acetylation may be a potent therapeutic target for cancer treatment.

ARD1-mediated aurora kinase A acetylation promotes cell proliferation and migration.

Aurora kinase A (AuA) is a prerequisite for centrosome maturation, separation, and mitotic spindle assembly, thus, it is essential for cell cycle regulation. Overexpression of AuA is implicated in poor prognosis of many types of cancer. However, the regulatory mechanisms underlying the functions of AuA are still not fully understood. Here, we report that AuA colocalizes with arrest defective protein 1 (ARD1) acetyltransferase during cell division and cell migration. Additionally, AuA is acetylated by ARD1 at lysine residues at positions 75 and 125. The double mutations at K75/K125 abolished the kinase activity of AuA. Moreover, the double mutant AuA exhibited diminished ability to promote cell proliferation and cell migration. Mechanistic studies revealed that AuA acetylation at K75/K125 promoted cell proliferation via activation of cyclin E/CDK2 and cyclin B1. In addition, AuA acetylation stimulated cell migration by activating the p38/AKT/MMP-2 pathway. Our findings indicate that ARD1-mediated acetylation of AuA enhances cell proliferation and migration, and probably contributes to cancer development.

Hsp70 acetylation prevents caspase-dependent/independent apoptosis and autophagic cell death in cancer cells.

Cancer cells are continuously challenged by adverse environmental factors including hypoxia, metabolite restriction, and immune reactions, and must adopt diverse strategies to survive. Heat shock protein (Hsp) 70 plays a central role in protection against stress-induced cell death by maintaining protein homeostasis and interfering with the process of programmed cell death. Recent findings have suggested that Hsp70 acetylation is a key regulatory modification required for its chaperone activity, but its relevance in the process of programmed cell death and the underlying mechanisms involved are not well understood. In this study, we sought to investigate mechanisms mediated by Hsp70 acetylation in relation to apoptotic and autophagic programmed cell death. Upon stress-induced apoptosis, Hsp70 acetylation inhibits apoptotic cell death, mediated by Hsp70 association with apoptotic protease-activating factor (Apaf)-1 and apoptosis-inducing factor (AIF), key modulators of caspase-dependent and -independent apoptotic pathways, respectively. Hsp70 acetylation also attenuated autophagic cell death associated with upregulation of autophagy-related genes and autophagosome induction. Collectively, these results suggest that the acetylation of Hsp70 plays key regulatory roles in cell death pathways as well as in its function as a chaperone, together enabling cellular protection in response to stress.

Structural environment built by AKAP12+ colon mesenchymal cells drives M2 macrophages during inflammation recovery.

Macrophages exhibit phenotypic plasticity, as they have the ability to switch their functional phenotypes during inflammation and recovery. Simultaneously, the mechanical environment actively changes. However, how these dynamic alterations affect the macrophage phenotype is unknown. Here, we observed that the extracellular matrix (ECM) constructed by AKAP12+ colon mesenchymal cells (CMCs) generated M2 macrophages by regulating their shape during recovery. Notably, rounded macrophages were present in the linear and loose ECM of inflamed colons and polarized to the M1 phenotype. In contrast, ramified macrophages emerged in the contracted ECM of recovering colons and mainly expressed M2 macrophage markers. These contracted structures were not observed in the inflamed colons of AKAP12 knockout (KO) mice. Consequently, the proportion of M2 macrophages in inflamed colons was lower in AKAP12 KO mice than in WT mice. In addition, clinical symptoms and histological damage were more severe in AKAP12 KO mice than in WT mice. In experimentally remodeled collagen gels, WT CMCs drove the formation of a more compacted structure than AKAP12 KO CMCs, which promoted the polarization of macrophages toward an M2 phenotype. These results demonstrated that tissue contraction during recovery provides macrophages with the physical cues that drive M2 polarization.

AMP-activated protein kinase activator, HL156A reduces thioacetamide-induced liver fibrosis in mice and inhibits the activation of cultured hepatic stellate cells and macrophages.

Cirrhosis, the end-stage of hepatic fibrosis, is not only life-threatening by itself, but also a causative factor of liver cancer. Despite efforts to develop treatment for liver fibrosis, there are no approved agents as anti-fibrotic drugs to date. In the present study, we aimed to investigate the anti-fibrotic effect of the AMP-activated protein kinase (AMPK) activator, HL156A. A mouse model of thioacetamide (TAA)-induced liver fibrosis was used to examine the effect of HL156A in vivo. Mice received either TAA alone or a combination of TAA and HL156A intraperitoneally for a total duration of 6 weeks. Including HL156A during exposure to TAA significantly reduced extracellular matrix (ECM) deposition and production of the hepatic transforming growth factor-ß1 (TGF-ß1). Immunohistochemical analysis revealed that the activation of hepatic stellate cells and the capillarization of liver sinusoids were also diminished significantly by HL156A co-treatment. The anti-fibrotic effect of HL156A was further studied in vitro by using a rat hepatic stellate cell line, HSC-T6 cells. The induction of α-smooth muscle actin (α-SMA) by TGF-ß1 treatment was reversed by HL156A, which was likely via the activation of AMPK. Moreover, HL156A showed anti-inflammatory effects on macrophages. Treatment with HL156A diminished LPS-induced activation of both Raw264.7 macrophage cells and primary cultured mouse macrophages. Taken together, these results imply that the AMPK activator HL156A inhibits hepatic fibrosis via multiple mechanisms and could be a potentially effective agent for fibrosis treatment.

Ninjurin1 regulates lipopolysaccharide-induced inflammation through direct binding.

Ninjurin1 is a transmembrane protein involved in macrophage migration and adhesion during inflammation. It was recently reported that repression of Ninjurin1 attenuated the lipopolysaccharide (LPS)-induced inflammatory response in macrophages; however, the precise mechanism by which Ninjurin1 modulates LPS-induced inflammation remains poorly understood. In the present study, we found that the interaction between Ninjurin1 and LPS contributed to the LPS-induced inflammatory response. Notably, pull-down assays using lysates from HEK293T cells transfected with human or mouse Ninjurin1 and biotinylated LPS (LPS-biotin) showed that LPS directly bound Ninjurin1. Subsequently, LPS binding assays with various truncated forms of Ninjurin1 protein revealed that amino acids (aa) 81-100 of Ninjurin1 were required for LPS binding. In addition, knockdown experiments using Ninj1 siRNA resulted in decreased nitric oxide (NO) and tumor necrosis factor-α (TNFα) secretion upon LPS treatment in Raw264.7 cells. Collectively, our results suggest that Ninjurin1 regulates the LPS-induced inflammatory response through its direct binding to LPS, thus, identifying Ninjurin1 as a putative target for the treatment of inflammatory diseases, such as sepsis and inflammation-associated carcinogenesis.

Neuropilin 1 balances ß8 integrin-activated TGFß signaling to control sprouting angiogenesis in the brain.

Angiogenesis in the developing central nervous system (CNS) is regulated by neuroepithelial cells, although the genes and pathways that couple these cells to blood vessels remain largely uncharacterized. Here, we have used biochemical, cell biological and molecular genetic approaches to demonstrate that ß8 integrin (Itgb8) and neuropilin 1 (Nrp1) cooperatively promote CNS angiogenesis by mediating adhesion and signaling events between neuroepithelial cells and vascular endothelial cells. ß8 integrin in the neuroepithelium promotes the activation of extracellular matrix (ECM)-bound latent transforming growth factor ß (TGFß) ligands and stimulates TGFß receptor signaling in endothelial cells. Nrp1 in endothelial cells suppresses TGFß activation and signaling by forming intercellular protein complexes with ß8 integrin. Cell type-specific ablation of ß8 integrin, Nrp1, or canonical TGFß receptors results in pathological angiogenesis caused by defective neuroepithelial cell-endothelial cell adhesion and imbalances in canonical TGFß signaling. Collectively, these data identify a paracrine signaling pathway that links the neuroepithelium to blood vessels and precisely balances TGFß signaling during cerebral angiogenesis.

Ninjurin1 enhances the basal motility and transendothelial migration of immune cells by inducing protrusive membrane dynamics.

Ninjurin1 is involved in the pathogenesis of experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis, by mediating leukocyte extravasation, a process that depends on homotypic binding. However, the precise regulatory mechanisms of Ninjurin1 during inflammation are largely undefined. We therefore examined the pro-migratory function of Ninjurin1 and its regulatory mechanisms in macrophages. Interestingly, Ninjurin1-deficient bone marrow-derived macrophages exhibited reduced membrane protrusion formation and dynamics, resulting in the impairment of cell motility. Furthermore, exogenous Ninjurin1 was distributed at the membrane of filopodial structures in Raw264.7 macrophage cells. In Raw264.7 cells, RNA interference of Ninjurin1 reduced the number of filopodial projections, whereas overexpression of Ninjurin1 facilitated their formation and thus promoted cell motility. Ninjurin1-induced filopodial protrusion formation required the activation of Rac1. In Raw264.7 cells penetrating an MBEC4 endothelial cell monolayer, Ninjurin1 was localized to the membrane of protrusions and promoted their formation, suggesting that Ninjurin1-induced protrusive activity contributed to transendothelial migration. Taking these data together, we conclude that Ninjurin1 enhances macrophage motility and consequent extravasation of immune cells through the regulation of protrusive membrane dynamics. We expect these findings to provide insight into the understanding of immune responses mediated by Ninjurin1.

Ninjurin1 deficiency attenuates susceptibility of experimental autoimmune encephalomyelitis in mice.

Ninjurin1 is a homotypic adhesion molecule that contributes to leukocyte trafficking in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. However, in vivo gene deficiency animal studies have not yet been done. Here, we constructed Ninjurin1 knock-out (KO) mice and investigated the role of Ninjurin1 on leukocyte trafficking under inflammation conditions such as EAE and endotoxin-induced uveitis. Ninjurin1 KO mice attenuated EAE susceptibility by reducing leukocyte recruitment into the injury regions of the spinal cord and showed less adhesion of leukocytes on inflamed retinal vessels in endotoxin-induced uveitis mice. Moreover, the administration of a custom-made antibody (Ab26-37) targeting the Ninjurin1 binding domain ameliorated the EAE symptoms, showing the contribution of its adhesion activity to leukocyte trafficking. In addition, we addressed the transendothelial migration (TEM) activity of bone marrow-derived macrophages and Raw264.7 cells according to the expression level of Ninjurin1. TEM activity was decreased in Ninjurin1 KO bone marrow-derived macrophages and siNinj1 Raw264.7 cells. Consistent with this, GFP-tagged mNinj1-overexpressing Raw264.7 cells increased their TEM activity. Taken together, we have clarified the contribution of Ninjurin1 to leukocyte trafficking in vivo and delineated its direct functions to TEM, emphasizing Ninjurin1 as a beneficial therapeutic target against inflammatory diseases such as multiple sclerosis.

αvß8 integrin interacts with RhoGDI1 to regulate Rac1 and Cdc42 activation and drive glioblastoma cell invasion.

The malignant brain cancer glioblastoma multiforme (GBM) displays invasive growth behaviors that are regulated by extracellular cues within the neural microenvironment. The adhesion and signaling pathways that drive GBM cell invasion remain largely uncharacterized. Here we use human GBM cell lines, primary patient samples, and preclinical mouse models to demonstrate that integrin αvß8 is a major driver of GBM cell invasion. ß8 integrin is overexpressed in many human GBM cells, with higher integrin expression correlating with increased invasion and diminished patient survival. Silencing ß8 integrin in human GBM cells leads to impaired tumor cell invasion due to hyperactivation of the Rho GTPases Rac1 and Cdc42. ß8 integrin coimmunoprecipitates with Rho-GDP dissociation inhibitor 1 (RhoGDI1), an intracellular signaling effector that sequesters Rho GTPases in their inactive GDP-bound states. Silencing RhoGDI1 expression or uncoupling αvß8 integrin-RhoGDI1 protein interactions blocks GBM cell invasion due to Rho GTPase hyperactivation. These data reveal for the first time that αvß8 integrin, via interactions with RhoGDI1, regulates activation of Rho proteins to promote GBM cell invasiveness. Hence targeting the αvß8 integrin-RhoGDI1 signaling axis might be an effective strategy for blocking GBM cell invasion.

Endothelial expression of TGFß type II receptor is required to maintain vascular integrity during postnatal development of the central nervous system.

TGFß signalling in endothelial cells is important for angiogenesis in early embryonic development, but little is known about its role in early postnatal life. To address this we used a tamoxifen inducible Cre-LoxP strategy in neonatal mice to deplete the TypeII TGFß receptor (Tgfbr2) specifically in endothelial cells. This resulted in multiple micro-haemorrhages, and glomeruloid-like vascular tufts throughout the cerebral cortices and hypothalamus of the brain as well as in retinal tissues. A detailed examination of the retinal defects in these mutants revealed that endothelial adherens and tight junctions were in place, pericytes were recruited and there was no failure of vascular smooth muscle differentiation. However, the deeper retinal plexus failed to form in these mutants and the angiogenic sprouts stalled in their progress towards the inner nuclear layer. Instead the leading endothelial cells formed glomerular tufts with associated smooth muscle cells. This evidence suggests that TGFß signalling is not required for vessel maturation, but is essential for the organised migration of endothelial cells as they begin to enter the deeper layers of the retina. Thus, TGFß signalling is essential in vascular endothelial cells for maintaining vascular integrity at the angiogenic front as it migrates into developing neural tissues in early postnatal life.

Clusterin protects H9c2 cardiomyocytes from oxidative stress-induced apoptosis via Akt/GSK-3ß signaling pathway.

Clusterin is a secretory glycoprotein, which is highly up-regulated in a variety of normal and injury tissues undergoing apoptosis including infarct region of the myocardium. Here, we report that clusterin protects H9c2 cardiomyocytes from H2O2-induced apoptosis by triggering the activation of Akt and GSK-3ß. Treatment with H2O2 induces apoptosis of H9c2 cells by promoting caspase cleavage and cytochrome c release from mitochondria. However, co-treatment with clusterin reverses the induction of apoptotic signaling by H2O2, thereby recovers cell viability. The protective effect of clusterin on H2O2-induced apoptosis is impaired by PI3K inhibitor LY294002, which effectively suppresses clusterin-induced activation of Akt and GSK-3ß. In addition, the protective effect of clusterin is independent on its receptor megalin, because inhibition of megalin has no effect on clusterin-mediated Akt/GSK-3ß phosphoylation and H9c2 cell viability. Collectively, these results suggest that clusterin has a role protecting cardiomyocytes from oxidative stress and the Akt/GSK-3ß signaling mediates anti-apoptotic effect of clusterin.

Meteorin promotes the formation of GFAP-positive glia via activation of the Jak-STAT3 pathway.

Meteorin is an orphan ligand which has been previously reported to control neuritogenesis and angiogenesis, as well as gliogenesis. However, the precise function of this factor in CNS development and the underlying molecular mechanisms are poorly understood. Here, we demonstrate that meteorin is involved in GFAP-positive glial differentiation through activation of the Jak-STAT3 pathway, by using neurosphere and retinal explant culture systems. During embryonic brain development, meteorin is highly expressed in neural stem and radial glia cells of the ventricular zone and immature neurons outside the ventricular zone but its expression disappears spontaneously as development proceeds except in GFAP-positive astrocytes. In cultured neurospheres, meteorin activates STAT3, and in turn increases the transcriptional activity of GFAP by enhancing the binding of STAT3 to the promoter. By meteorin stimulation, differentiating neurospheres show increased numbers of GFAP-positive cells, but the effect is abrogated by a blockade of the Jak-STAT3 pathway using either a Jak inhibitor or STAT3 siRNA. Furthermore, we expand our findings to the retina, and show that meteorin increases GFAP expression in Müller glia. Together, our results suggest that meteorin promotes GFAP-positive glia formation by mediating the Jak-STAT3 signaling pathway during both cortical stem cell differentiation and retinal glia development.

Meteorin regulates angiogenesis at the gliovascular interface.

Brain microvasculature requires a coordinated interaction between endothelial cells and astrocytes at the gliovascular interface. However, the role of the factors involved in that interaction and expressed by these cells is poorly understood. In this study, we demonstrate that Meteorin is highly expressed in astrocytes of the brain and retina during the late embryonic and postnatal stages of mouse development. Most notably, Meteorin is localized to the astrocyte endfeet that surround the blood vessels. To investigate the role of Meteorin in perivascular astrocytes, we depleted endogenous levels of Meteorin in cultured astrocytes using siRNA, and found that Meteorin attenuates angiogenic activity indirectly via astrocyte-derived thrombospondin-1/-2 (TSP-1/-2). Exogenous treatment of astrocytes with Meteorin protein also promotes astrocyte expression and secretion of TSP-1/-2. The conditioned media from the Meteorin-treated astrocytes attenuated angiogenic activity of microvascular endothelial cells. This activity was reversed by inhibiting the binding of TSP-1/-2 to its receptor. Furthermore, we found that TSP-1/-2 was co-localized with Meteorin in the developing brain. Therefore, our data strongly suggests that Meteorin is expressed and secreted by perivascular astrocytes and the secreted protein upregulates TSP-1/-2 to attenuate angiogenesis in the surrounding endothelial cells and to promote vascular maturation.

Structural basis for the selective inhibition of JNK1 by the scaffolding protein JIP1 and SP600125.

The c-jun N-terminal kinase (JNK) signaling pathway is regulated by JNK-interacting protein-1 (JIP1), which is a scaffolding protein assembling the components of the JNK cascade. Overexpression of JIP1 deactivates the JNK pathway selectively by cytoplasmic retention of JNK and thereby inhibits gene expression mediated by JNK, which occurs in the nucleus. Here, we report the crystal structure of human JNK1 complexed with pepJIP1, the peptide fragment of JIP1, revealing its selectivity for JNK1 over other MAPKs and the allosteric inhibition mechanism. The van der Waals contacts by the three residues (Pro157, Leu160, and Leu162) of pepJIP1 and the hydrogen bonding between Glu329 of JNK1 and Arg156 of pepJIP1 are critical for the selective binding. Binding of the peptide also induces a hinge motion between the N- and C-terminal domains of JNK1 and distorts the ATP-binding cleft, reducing the affinity of the kinase for ATP. In addition, we also determined the ternary complex structure of pepJIP1-bound JNK1 complexed with SP600125, an ATP-competitive inhibitor of JNK, providing the basis for the JNK specificity of the compound.