Pitfalls and Considerations in Determining the Potency and Mutant Selectivity of Covalent Epidermal Growth Factor Receptor Inhibitors.

Enzyme inhibitors that form covalent bonds with their targets are being increasingly pursued in drug development. Assessing their biochemical activity relies on time-dependent assays, which are distinct and more complex compared with methods commonly employed for reversible-binding inhibitors. To provide general guidance to the covalent inhibitor development community, we explored methods and reported kinetic values and experimental factors in determining the biochemical activity of various covalent epidermal growth factor receptor (EGFR) inhibitors. We showcase how liquid handling and assay reagents impact kinetic parameters and potency interpretations, which are critical for structure-kinetic relationships and covalent drug design. Additionally, we include benchmark kinetic values with reference inhibitors, which are imperative, as covalent EGFR inhibitor kinetic values are infrequently consistent in the literature. This overview seeks to inform best practices for developing new covalent inhibitors and highlight appropriate steps to address gaps in knowledge presently limiting assay reliability and reproducibility.

Structural Basis for Inhibition of Mutant EGFR with Lazertinib (YH25448).

Lazertinib (YH25448) is a novel third-generation tyrosine kinase inhibitor (TKI) developed as a treatment for EGFR mutant non-small cell lung cancer. To better understand the nature of lazertinib inhibition, we determined crystal structures of lazertinib in complex with both WT and mutant EGFR and compared its binding mode to that of structurally related EGFR TKIs. We observe that lazertinib binds EGFR with a distinctive pyrazole moiety enabling hydrogen bonds and van der Waals interactions facilitated through hydrophilic amine and hydrophobic phenyl groups, respectively. Biochemical assays and cell studies confirm that lazertinib effectively targets EGFR(L858R/T790M) and to a lesser extent HER2. The molecular basis for lazertinib inhibition of EGFR reported here highlights previously unexplored binding interactions leading to improved medicinal chemistry properties compared to clinically approved osimertinib (AZD9291) and offers novel strategies for structure-guided design of tyrosine kinase inhibitors.

Safety profiling of genetically engineered Pim-1 kinase overexpression for oncogenicity risk in human c-kit+ cardiac interstitial cells.

Advancement of stem cell-based treatment will involve next-generation approaches to enhance therapeutic efficacy which is often modest, particularly in the context of myocardial regenerative therapy. Our group has previously demonstrated the beneficial effect of genetic modification of cardiac stem cells with Pim-1 kinase overexpression to rejuvenate aged cells as well as potentiate myocardial repair. Despite these encouraging findings, concerns were raised regarding potential for oncogenic risk associated with Pim-1 kinase overexpression. Testing of Pim-1 engineered c-kit+ cardiac interstitial cells (cCIC) derived from heart failure patient samples for indices of oncogenic risk was undertaken using multiple assessments including soft agar colony formation, micronucleation, gamma-Histone 2AX foci, and transcriptome profiling. Collectively, findings demonstrate comparable phenotypic and biological properties of cCIC following Pim-1 overexpression compared with using baseline control cells with no evidence for oncogenic phenotype. Using a highly selective and continuous sensor for quantitative assessment of PIM1 kinase activity revealed a sevenfold increase in Pim-1 engineered vs. control cells. Kinase activity profiling using a panel of sensors for other kinases demonstrates elevation of IKKs), AKT/SGK, CDK1-3, p38, and ERK1/2 in addition to Pim-1 consistent with heightened kinase activity correlating with Pim-1 overexpression that may contribute to Pim-1-mediated effects. Enhancement of cellular survival, proliferation, and other beneficial properties to augment stem cell-mediated repair without oncogenic risk is a feasible, logical, and safe approach to improve efficacy and overcome current limitations inherent to cellular adoptive transfer therapeutic interventions.

An algebraic model to determine substrate kinetic parameters by global nonlinear fit of progress curves.

We propose that the time course of an enzyme reaction following the Michaelis-Menten reaction mechanism can be conveniently described by a newly derived algebraic equation, which includes the Lambert Omega function. Following Northrop's ideas [Anal. Biochem.321, 457-461, 1983], the integrated rate equation contains the Michaelis constant (KM) and the specificity number (kS≡kcat/KM) as adjustable parameters, but not the turnover number kcat. A modification of the usual global-fit approach involves a combinatorial treatment of nominal substrate concentrations being treated as fixed or alternately optimized model parameters. The newly proposed method is compared with the standard approach based on the "initial linear region" of the reaction progress curves, followed by nonlinear fit of initial rates to the hyperbolic Michaelis-Menten equation. A representative set of three chelation-enhanced fluorescence EGFR kinase substrates is used for experimental illustration. In one case, both data analysis methods (linear and nonlinear) produced identical results. However, in another test case, the standard method incorrectly reported a finite (50-70 µM) KM value, whereas the more rigorous global nonlinear fit shows that the KM is immeasurably high.

Kinetic assay for characterization of spleen tyrosine kinase activity and inhibition with recombinant kinase and crude cell lysates.

Spleen tyrosine kinase (Syk) is involved in the activation of cells implicated in allergic or autoimmune diseases and certain cancers. Therefore, Syk inhibitors may prove to be effective in treating diseases where Syk activity or expression is increased or deregulated. We developed a continuous and direct (noncoupled) fluorescence intensity assay for measuring Syk activity using purified recombinant enzyme or crude lysates generated from anti-immunoglobulin M (IgM) antibody-treated RAMOS cells. The assay is based on the chelation-enhanced fluorophore 8-hydroxy-5-(N,N-dimethylsulfonamido)-2-methylquinoline (referred to as Sox), which has been incorporated into a peptide substrate selected for robust detection of Syk activity. This homogeneous assay is simple to use, provides considerably more information, and has been adapted to a 384-well, low-volume microtiter plate format that can be used for the high-throughput identification and kinetic characterization of Syk inhibitors. The assay can be performed with a wide range of adenosine triphosphate (ATP) concentrations and, therefore, can be used to analyze ATP-competitive and ATP-noncompetitive/allosteric kinase inhibitors. Measurement of Syk activity in RAMOS crude cell lysates or immunoprecipitation (IP) capture formats may serve as a physiologically more relevant enzyme source. These Sox-based continuous and homogeneous assays provide a valuable set of tools for studying Syk signaling and for defining inhibitors that may be more effective in controlling disease.

Pim-1 kinase antagonizes aspects of myocardial hypertrophy and compensation to pathological pressure overload.

Pim-1 kinase exerts potent cardioprotective effects in the myocardium downstream of AKT, but the participation of Pim-1 in cardiac hypertrophy requires investigation. Cardiac-specific expression of Pim-1 (Pim-WT) or the dominant-negative mutant of Pim-1 (Pim-DN) in transgenic mice together with adenoviral-mediated overexpression of these Pim-1 constructs was used to delineate the role of Pim-1 in hypertrophy. Transgenic overexpression of Pim-1 protects mice from pressure-overload-induced hypertrophy relative to wild-type controls as evidenced by improved hemodynamic function, decreased apoptosis, increases in antihypertrophic proteins, smaller myocyte size, and inhibition of hypertrophic signaling after challenge. Similarly, Pim-1 overexpression in neonatal rat cardiomyocyte cultures inhibits hypertrophy induced by endothelin-1. On the cellular level, hearts of Pim-WT mice show enhanced incorporation of BrdU into myocytes and a hypercellular phenotype compared to wild-type controls after hypertrophic challenge. In comparison, transgenic overexpression of Pim-DN leads to dilated cardiomyopathy characterized by increased apoptosis, fibrosis, and severely depressed cardiac function. Furthermore, overexpression of Pim-DN leads to reduced contractility as evidenced by reduced Ca(2+) transient amplitude and decreased percentage of cell shortening in isolated myocytes. These data support a pivotal role for Pim-1 in modulation of hypertrophy by impacting responses on molecular, cellular, and organ levels.

Myocardial induction of nucleostemin in response to postnatal growth and pathological challenge.

Stem cell-specific proteins and regulatory pathways that determine self-renewal and differentiation have become of fundamental importance in understanding regenerative and reparative processes in the myocardium. One such regulatory protein, named nucleostemin, has been studied in the context of stem cells and several cancer cell lines, where expression is associated with proliferation and maintenance of a primitive cellular phenotype. We find nucleostemin is present in young myocardium and is also induced following cardiomyopathic injury. Nucleostemin expression in cardiomyocytes is induced by fibroblast growth factor-2 and accumulates in response to Pim-1 kinase activity. Cardiac stem cells also express nucleostemin that is diminished in response to commitment to a differentiated phenotype. Overexpression of nucleostemin in cultured cardiac stem cells increases proliferation while preserving telomere length, providing a mechanistic basis for potential actions of nucleostemin in promotion of cell survival and proliferation as seen in other cell types.

Activation of FOXO1 by Cdk1 in cycling cells and postmitotic neurons.

Activation of cyclin-dependent kinase 1 (Cdk1) has been linked to cell death of postmitotic neurons in brain development and disease. We found that Cdk1 phosphorylated the transcription factor FOXO1 at Ser249 in vitro and in vivo. The phosphorylation of FOXO1 at Ser249 disrupted FOXO1 binding with 14-3-3 proteins and thereby promoted the nuclear accumulation of FOXO1 and stimulated FOXO1-dependent transcription, leading to cell death in neurons. In proliferating cells, Cdk1 induced FOXO1 Ser249 phosphorylation at the G2/M phase of the cell cycle, resulting in FOXO1-dependent expression of the mitotic regulator Polo-like kinase (Plk). These findings define a conserved signaling link between Cdk1 and FOXO1 that may have a key role in diverse biological processes, including the degeneration of postmitotic neurons.

Evolution of the c-kit-positive cell response to pathological challenge in the myocardium.

Cumulative evidence indicates that myocardium responds to growth or injury by recruitment of stem and/or progenitor cells that participate in repair and regenerative processes. Unequivocal identification of this population has been hampered by lack of reagents or markers specific to the recruited population, leading to controversies regarding the nature of these cells. Use of a transgenic mouse expressing green fluorescent protein driven by the c-kit promoter allows for unambiguous identification of this cell population. Green fluorescent protein (GFP) driven by the c-kit promoter labels a fraction of the c-kit+ cells recognized by antibody labeling for c-kit protein. Expression of GFP by the c-kit promoter and accumulation of GFP-positive cells in the myocardium is relatively high at birth compared with adult and declines between postnatal weeks 1 and 2, which tracks in parallel with expression of c-kit protein and c-kit-positive cells. Acute cardiomyopathic injury by infarction prompts increased expression of both GFP protein and GFP-labeled cells in the region of infarction relative to remote myocardium. Similar increases were observed for c-kit protein and cells with a slightly earlier onset and decline relative to the GFP signal. Cells coexpressing GFP, c-kit, and cardiogenic markers were apparent at 1-2 weeks postinfarction. Cardiac-resident c-kit+ cell cultures derived from the transgenic line express GFP that is diminished in parallel with c-kit by induction of differentiation. The use of genetically engineered mice validates and extends the concept of c-kit+ cells participating in the response to myocardial injury.

Pim-1 regulates cardiomyocyte survival downstream of Akt.

The serine-threonine kinases Pim-1 and Akt regulate cellular proliferation and survival. Although Akt is known to be a crucial signaling protein in the myocardium, the role of Pim-1 has been overlooked. Pim-1 expression in the myocardium of mice decreased during postnatal development, re-emerged after acute pathological injury in mice and was increased in failing hearts of both mice and humans. Cardioprotective stimuli associated with Akt activation induced Pim-1 expression, but compensatory increases in Akt abundance and phosphorylation after pathological injury by infarction or pressure overload did not protect the myocardium in Pim-1-deficient mice. Transgenic expression of Pim-1 in the myocardium protected mice from infarction injury, and Pim-1 expression inhibited cardiomyocyte apoptosis with concomitant increases in Bcl-2 and Bcl-X(L) protein levels, as well as in Bad phosphorylation levels. Relative to nontransgenic controls, calcium dynamics were significantly enhanced in Pim-1-overexpressing transgenic hearts, associated with increased expression of SERCA2a, and were depressed in Pim-1-deficient hearts. Collectively, these data suggest that Pim-1 is a crucial facet of cardioprotection downstream of Akt.

Akt promotes increased cardiomyocyte cycling and expansion of the cardiac progenitor cell population.

Activation of Akt is associated with enhanced cell cycling and cellular proliferation in nonmyocytes, but this effect of nuclear Akt accumulation has not been explored in the context of the myocardium. Cardiac-specific expression of nuclear-targeted Akt (Akt/nuc) in transgenics prolongs postnatal cell cycling as evidenced by increased numbers of Ki67+ cardiomyocytes at 2 to 3 weeks after birth. Similarly, nuclear-targeting of Akt promotes expansion of the presumptive cardiac progenitor cell population as assessed by immunolabeling for c-kit in combination with myocyte-specific markers Nkx 2.5 or MEF 2C. Increases in pro-proliferative cytokines, including tumor-necrosis superfamily 8, interleukin-17e, and hepatocyte growth factor, were found in nuclear-targeted Akt myocardial samples. Concurrent signaling mediated by paracrine factors downstream of Akt/nuc expression may be responsible for phenotypic effects of nuclear-targeted Akt in the myocardium, including enhanced cell proliferation and expansion of the stem cell population.

Atrial natriuretic peptide promotes cardiomyocyte survival by cGMP-dependent nuclear accumulation of zyxin and Akt.

This study delineates a mechanism for antiapoptotic signaling initiated by atrial natriuretic peptide (ANP) stimulation leading to elevation of cGMP levels and subsequent nuclear accumulation of Akt kinase associated with zyxin, a cytoskeletal LIM-domain protein. Nuclear targeting of zyxin induces resistance to cell death coincident with nuclear accumulation of activated Akt. Nuclear translocation of zyxin triggered by cGMP also promotes nuclear Akt accumulation. Additional supportive evidence for nuclear accumulation of zyxin-enhancing cardiomyocyte survival includes the following: (a) promotion of zyxin nuclear localization by cardioprotective stimuli; (b) zyxin association with phospho-Akt473 induced by cardioprotective stimuli; and (c) recruitment of zyxin to the nucleus by activated nuclear-targeted Akt as well as recruitment of Akt by nuclear-targeted zyxin. Nuclear accumulation of zyxin requires both Akt activation and nuclear localization. Potentiation of cell survival is sensitive to stimulation intensity with high-level induction by ANP or cGMP signaling leading to apoptotic cell death rather than enhancing resistance to apoptotic stimuli. Myocardial nuclear accumulation of zyxin and Akt responds similarly in vivo following treatment of mice with ANP or cGMP. Thus, zyxin and activated Akt participate in a cGMP-dependent signaling cascade leading from ANP receptors to nuclear accumulation of both molecules. Nuclear accumulation of zyxin and activated Akt may represent a fundamental mechanism that facilitates nuclear-signal transduction and potentiates cell survival.

Phosphorylation of phosphoprotein enriched in astrocytes (PEA-15) regulates extracellular signal-regulated kinase-dependent transcription and cell proliferation.

Cell cycle progression is dependent on the nuclear localization and transcriptional effects of activated extracellular signal-regulated kinase (ERK)1 and ERK2 mitogen-activated protein (MAP) kinases (ERK1/2). Phosphoprotein enriched in astrocytes (PEA-15) binds ERK1/2 and inhibits their nuclear localization, thus blocking cell proliferation. Here, we report that phosphorylation of PEA-15 blocks its interaction with ERK1/2 in vitro and in vivo and that phosphorylation of both Ser104 and Ser116 is required for this effect. Using phosphomimetic and nonphosphorylatable mutants of PEA-15, we found that PEA-15 phosphorylation abrogates its capacity to block the nuclear localization and transcriptional activities of ERK1/2; this phosphorylation therefore enables the proliferation of cells that express high levels of PEA-15. Additionally, we report that PEA-15 phosphorylation can modulate nontranscriptional activities of ERK1/2, such as the modulation of the affinity of integrin adhesion receptors. Finally, we used a novel anti-phospho-specific PEA-15 antibody to establish that PEA-15 is phosphorylated in situ in normal mammary epithelium. These results define a novel posttranslational mechanism for controlling the subcellular localization of ERK1/2 and for specifying the output of MAP kinase signaling.

A selective small molecule c-MET Inhibitor, PHA665752, cooperates with rapamycin.

PURPOSE: c-MET is believed to be an attractive receptor target for molecular therapeutic inhibition. TPR-MET, a constitutively active oncogenic variant of MET, serves as excellent model for testing c-MET inhibitors. Here, we characterized a small molecule c-MET inhibitor, PHA665752, and tested its cooperation with the mammalian target of rapamycin inhibitor as potential targeted therapy. EXPERIMENTAL DESIGN: The effect of PHA665752 treatment was determined on cell growth, motility and migration, apoptosis, and cell-cycle arrest of TPR-MET-transformed cells. Moreover, the effect of PHA665752 on the phosphorylation on MET, as well as its downstream effectors, p-AKT and p-S6K, was also determined. Finally, growth of TPR-MET-transformed cells was tested in the presence of PHA665752 and rapamycin. H441 non-small cell lung cancer (NSCLC) cells (with activated c-Met) were also tested against both PHA665752 and rapamycin. RESULTS: PHA665752 specifically inhibited cell growth in BaF3. TPR-MET cells (IC(50) < 0.06 micromol/L), induced apoptosis and cell cycle arrest. Constitutive cell motility and migration of the BaF3. TPR-MET cells was also inhibited. PHA665752 inhibited specific phosphorylation of TPR-MET as well as phosphorylation of downstream targets of the mammalian target of rapamycin pathway. When combined with PHA665752, rapamycin showed cooperative inhibition to reduce growth of BaF3. TPR-MET- and c-MET-expressing H441 NSCLC cells. CONCLUSIONS: PHA665752 is a potent small molecule-selective c-MET inhibitor and is highly active against TPR-MET-transformed cells both biologically and biochemically. PHA665752 is also active against H441 NSCLC cells. The c-MET inhibitor can cooperate with rapamycin in therapeutic inhibition of NSCLC, and in vivo studies of this combination against c-MET expressing cancers would be merited.

Functional expression and mutations of c-Met and its therapeutic inhibition with SU11274 and small interfering RNA in non-small cell lung cancer.

Non-small cell lung cancer (NSCLC) is a difficult disease to treat. The c-Met receptor is an attractive potential target for novel therapeutic inhibition in human cancers. We provide strong evidence that c-Met is overexpressed, activated, and sometimes mutated in NSCLC cell lines and tumor tissues. Expression of c-Met was found in all (100%) of the NSCLC tumor tissues examined (n = 23) and most (89%) of the cell lines (n = 9). Sixty-one percent of tumor tissues strongly expressed total c-Met, especially adenocarcinoma (67%). Specific expression of phospho-Met (p-Met) [Y1003] and [Y1230/1234/1235] was seen by immunohistochemistry. p-Met expression was preferentially observed at the NSCLC tumor invasive fronts. c-Met alterations were identified within the semaphorin domain (E168D, L299F, S323G, and N375S) and the juxtamembrane domain (R988C, R988C + T1010I, S1058P, and alternative splice product skipping entire juxtamembrane domain) of a NSCLC cell line and adenocarcinoma tissues. We validated c-Met as potential therapeutic target using small interfering RNA down-regulation of the receptor expression by 50% to 60% in NSCLC cells. This led to inhibition of p-Met and phospho-AKT and up to 57.1 +/- 7.2% cell viability inhibition at 72 hours. The selective small molecule inhibitor of c-Met SU11274 inhibited cell viability in c-Met-expressing NSCLC cells. SU11274 also abrogated hepatocyte growth factor-induced phosphorylation of c-Met and its downstream signaling. Here, we provide first direct evidence by small interfering RNA targeting and small molecule inhibitor that c-Met is important in NSCLC biology and biochemistry. These results indicate that c-Met inhibition will be an important therapeutic strategy against NSCLC to improve its clinical outcome.

Beta3 tyrosine phosphorylation and alphavbeta3-mediated adhesion are required for Vav1 association and Rho activation in leukocytes.

Integrin alpha(v)beta(3)-mediated adhesion of hematopoietic cells to vitronectin results in activation of the Rho GTPases. Mutation of beta(3) tyrosine residue 747, previously shown to disrupt cell adhesion, results in sustained activation of Cdc42 and diminished Rac and Rho activity. We investigated the role of the hematopoietically restricted guanine nucleotide exchange factor Vav1 in alpha(v)beta(3)-mediated adhesion. We find that Vav1, a guanine nucleotide exchange factor for Rac and Rho, associates with alpha(v)beta(3) upon cell adhesion to vitronectin and that this association requires beta(3) tyrosine phosphorylation. Expression of exogenous Vav1 demonstrates that Y160F, but not wild type or the Vav1Y174F mutant, inhibits Rac and Rho activation during alpha(v)beta(3)-mediated cell adhesion to vitronectin. Cells expressing Vav1Y160F exhibit a sustained Cdc42 activation similar to nonphosphorylatable beta(3) mutants. In addition, cytoskeletal reorganization and cell adhesion are severely suppressed in Vav1Y160F-transfected cells, and Vav1Y160F fails to associate with beta(3) integrins. Furthermore, Vav1 itself is selectively phosphorylated upon tyrosine 160 after alpha(v)beta(3)-mediated adhesion, and the association between Vav1 and beta(3) occurs in specific response to adhesion to substrate. These studies describe a phosphorylation-dependent association between beta(3) integrin and Vav1 which is essential for cell progression to a Rho-dominant phenotype during cell adhesion.

Cardiomyocyte apoptosis triggered by RAFTK/pyk2 via Src kinase is antagonized by paxillin.

Altered cellular adhesion and apoptotic signaling in cardiac remodeling requires coordinated regulation of multiple constituent proteins that comprise cytoskeletal focal adhesions. One such protein activated by cardiac remodeling is related adhesion focal tyrosine kinase (RAFTK, also known as pyk2). Adenoviral-mediated expression of RAFTK in neonatal rat cardiomyocytes involves concurrent increases in phosphorylation of Src, c-Jun N-terminal kinase, and p38 leading to characteristic apoptotic changes including cleavage of poly(ADP-ribose) polymerase, caspase-3 activation, and increased DNA laddering. DNA laddering was decreased by mutation of the Tyr(402) Src-binding site in RAFTK, suggesting a central role for Src activity in apoptotic cell death that was confirmed by adenoviral-mediated Src expression. Multiple apoptotic signaling cascades are recruited by RAFTK as demonstrated by prevention of apoptosis using caspase-3 inhibitor IV (caspase-3 specific inhibitor), PP2 (Src-specific kinase inhibitor), or Csk (cellular negative regulator for Src), as well as dominant negative constructs for p38beta or MKP-1. These RAFTK-mediated phenotypic characteristics are prevented by concurrent expression of wild-type or a phosphorylation-deficient paxillin mutated at Tyr(31) and Tyr(118). Wild-type or mutant paxillin protein accumulation in the cytoplasm has no overt effect upon cell structure, but paxillin accumulation prevents losses of myofibril organization as well as focal adhesion kinase, vinculin, and paxillin protein levels mediated by RAFTK. Apoptotic signaling cascade inhibition by paxillin indicates interruption of signaling proximal to but downstream of RAFTK activity. Chronic RAFTK activation in cardiac remodeling may represent a maladaptive reactive response that can be modulated by paxillin, opening up novel possibilities for inhibition of cardiomyocyte apoptosis and structural degeneration in heart failure.

p21-activated kinase regulates endothelial permeability through modulation of contractility.

Endothelial cells lining the vasculature have close cell-cell associations that maintain separation of the blood fluid compartment from surrounding tissues. Permeability is regulated by a variety of growth factors and cytokines and plays a role in numerous physiological and pathological processes. We examined a potential role for the p21-activated kinase (PAK) in the regulation of vascular permeability. In both bovine aortic and human umbilical vein endothelial cells, PAK is phosphorylated on Ser141 during the activation downstream of Rac, and the phosphorylated subfraction translocates to endothelial cell-cell junctions in response to serum, VEGF, bFGF, TNFalpha, histamine, and thrombin. Blocking PAK activation or translocation prevents the increase in permeability across the cell monolayer in response to these factors. Permeability correlates with myosin phosphorylation, formation of actin stress fibers, and the appearance of paracellular pores. Inhibition of myosin phosphorylation blocks the increase in permeability. These data suggest that PAK is a central regulator of endothelial permeability induced by multiple growth factors and cytokines via an effect on cell contractility. PAK may therefore be a suitable drug target for the treatment of pathological conditions where vascular leak is a contributing factor, such as ischemia and inflammation.

Nerve growth factor pretreatment attenuates oxygen and glucose deprivation-induced c-Jun amino-terminal kinase 1 and stress-activated kinases p38alpha and p38beta activation and confers neuroprotection in the pheochromocytoma PC12 Model.

Neurotrophins such as nerve growth factor (NGF) are considered putative neuroprotective compounds in the central nervous system. To investigate the cellular and molecular neuroprotective mechanisms of NGF under ischemia, we used a unique oxygen and glucose deprivation (OGD) device. In this system we used pheochromocytoma PC12 cells to elucidate NGF neuroprotective effect. PC12 cells were exposed to OGD, followed by addition of glucose and oxygen (OGD reperfusion). Neuronal cell death induced in this model was measured by the release of lactate dehydrogenase (LDH), activation of caspase-3 and mitogen-activated protein kinases (MAPKs), measured with specific anti-phospho-antibodies. Pretreatment of the cultures with 50 ng/mL NGF, 18 h prior to OGD insult, conferred 30% neuroprotection. However, treatment of the cultures with NGF concomitantly with the OGD insult did not result in neuroprotection. Time-course experiments showed marked activation of extracellular signal-regulated protein kinase, c-Jun N-terminal kinase (JNK), and p38 MAPK isoforms during the OGD phase but not during OGD reperfusion. Pretreatment of the cultures with 50 ng/mL NGF, 18 h prior to OGD insult, resulted in 50% attenuation of OGD-induced activation of JNK1, and 20% and 50% attenuation of OGD-induced activation of p38alpha and beta, respectively. These findings support the notion that NGF confers neuroprotection from OGD insult, a phenomenon coincidentally related to differential inhibition of MAPK stress kinase isoforms, and provide the PC12 model as an in vitro OGD system to investigate molecular mechanisms of neurotoxicity and neuroprotection.

Beta 3 integrin phosphorylation is essential for Arp3 organization into leukocyte alpha V beta 3-vitronectin adhesion contacts.

Integrins play a pivotal role in self-regulated hematopoietic adhesion and migration. Leukocyte alpha(V)beta(3) integrin-mediated adhesion to vitronectin requires protein kinase C activation and phosphorylation on tyrosine 747 of the beta(3) cytoplasmic tail. We have previously shown that beta(3) phosphorylation is required for Rho activation. In this study, an antibody specific to phosphorylated beta(3) tyrosine 747 was used to localize phosphorylated alpha(V)beta(3) in vitronectin adhesive structures. Early adhesion contacts containing phosphorylated beta(3) preceded actin stress fiber formation. beta(3) phosphorylation decreased progressively throughout the course of adhesion coincident with the appearance of actin stress fibers. Time-dependent increases in colocalization of beta(3) with tyrosine 402 phosphorylated Pyk2 in similar adhesive structures was observed, providing evidence for downstream signaling complex formation. Surprisingly, Arp3 organized into similar adhesion contacts in cells expressing wild-type beta(3) but not in those expressing a nonphosphorylatable mutant of beta(3), suggesting that beta(3) phosphorylation is required for sequestration of Arp3 to adhesion complexes. Suppression of actin stress fiber formation by an inhibitor to Rho kinase disrupted Arp3 organization while prolonging beta(3) phosphorylation throughout the adhesion time course. These data confirm a requirement for beta(3) phosphorylation in alpha(V)beta(3)-mediated adhesion to vitronectin and suggest that beta(3) phosphorylation permits signaling complex assembly at the adhesion site necessary for actin stress fiber formation in leukocytes.