A c-Cbl yeast two hybrid screen reveals interactions with 14-3-3 isoforms and cytoskeletal components

The 14-3-3σ protein promotes HCC anoikis resistance by inhibiting EGFR degradation and thereby activating the EGFR-dependent ERK1/2 signaling pathway

Resistance to anoikis, cell death due to matrix detachment, is acquired during tumor progression. The 14-3-3σ protein is implicated in the development of chemo- and radiation resistance, indicating a poor prognosis in multiple human cancers. However, its function in anoikis resistance and metastasis in hepatocellular carcinoma (HCC) is currently unknown.

Methods: Protein expression levels of 14-3-3σ were measured in paired HCC and normal tissue samples using western blot and immunohistochemical (IHC) staining. Statistical analysis was performed to evaluate the clinical correlation between 14-3-3σ expression, clinicopathological features, and overall survival. Artificial modulation of 14-3-3σ (downregulation and overexpression) was performed to explore the role of 14-3-3σ in HCC anoikis resistance and tumor metastasis in vitro and in vivo. Association of 14-3-3σ with epidermal growth factor receptor (EGFR) was assayed by co-immunoprecipitation. Effects of ectopic 14-3-3σ expression or knockdown on EGFR signaling, ligand-induced EGFR degradation and ubiquitination were examined using immunoblotting and co-immunoprecipitation, immunofluorescence staining, and flow cytometry analysis. The levels of EGFR ubiquitination, the interaction between EGFR and 14-3-3σ, and the association of EGFR with c-Cbl after EGF stimulation, in 14-3-3σ overexpressing or knockdown cells were examined to elucidate the mechanism by which 14-3-3σ inhibits EGFR degradation. Using gain-of-function or loss-of-function strategies, we further investigated the role of the EGFR signaling pathway and its downstream target machinery in 14-3-3σ-mediated anoikis resistance of HCC cells.

Results: We demonstrated that 14-3-3σ was upregulated in HCC tissues, whereby its overexpression was correlated with aggressive clinicopathological features and a poor prognosis. In vitro and in vivo experiments indicated that 14-3-3σ promoted anoikis resistance and metastasis of HCC cells. Mechanistically, we show that 14-3-3σ can interact with EGFR and significantly inhibit EGF-induced degradation of EGFR, stabilizing the activated receptor, and therefore prolong the activation of EGFR signaling. We demonstrated that 14-3-3σ downregulated ligand-induced EGFR degradation by inhibiting EGFR-c-Cbl association and subsequent c-Cbl-mediated EGFR ubiquitination. We further verified that activation of the ERK1/2 pathway was responsible for 14-3-3σ-mediated anoikis resistance of HCC cells. Moreover, EGFR inactivation could reverse the 14-3-3σ-mediated effects on ERK1/2 phosphorylation and anoikis resistance. Expression of 14-3-3σ and EGFR were found to be positively correlated in human HCC tissues.

Conclusions: Our results indicate that 14-3-3σ plays a pivotal role in the anoikis resistance and metastasis of HCC cells, presumably by inhibiting EGFR degradation and regulating the activation of the EGFR-dependent ERK1/2 pathway. To our best knowledge, this is the first report of the role of 14-3-3σ in the anoikis resistance of HCC cells, offering new research directions for the treatment of metastatic cancer by targeting 14-3-3σ.

Epithelial Keratins Modulate cMet Expression and Signaling and Promote InlB-Mediated Listeria monocytogenes Infection of HeLa Cells

The host cytoskeleton is a major target for bacterial pathogens during infection. In particular, pathogens usurp the actin cytoskeleton function to strongly adhere to the host cell surface, to induce plasma membrane remodeling allowing invasion and to spread from cell to cell and disseminate to the whole organism. Keratins are cytoskeletal proteins that are the major components of intermediate filaments in epithelial cells however, their role in bacterial infection has been disregarded. Here we investigate the role of the major epithelial keratins, keratins 8 and 18 (K8 and K18), in the cellular infection by Listeria monocytogenes. We found that K8 and K18 are required for successful InlB/cMet-dependent L. monocytogenes infection, but are dispensable for InlA/E-cadherin-mediated invasion. Both K8 and K18 accumulate at InlB-mediated internalization sites following actin recruitment and modulate actin dynamics at those sites. We also reveal the key role of K8 and K18 in HGF-induced signaling which occurs downstream the activation of cMet. Strikingly, we show here that K18, and at a less extent K8, controls the expression of cMet and other surface receptors such TfR and integrin β1, by promoting the stability of their corresponding transcripts. Together, our results reveal novel functions for major epithelial keratins in the modulation of actin dynamics at the bacterial entry sites and in the control of surface receptors mRNA stability and expression.

Ablation of the 14-3-3gamma Protein Results in Neuronal Migration Delay and Morphological Defects in the Developing Cerebral Cortex

14-3-3 proteins are ubiquitously-expressed and multifunctional proteins. There are seven isoforms in mammals with a high level of homology, suggesting potential functional redundancy. We previously found that two of seven isoforms, 14-3-3epsilon and 14-3-3zeta, are important for brain development, in particular, radial migration of pyramidal neurons in the developing cerebral cortex. In this work, we analyzed the function of another isoform, the protein 14-3-3gamma, with respect to neuronal migration in the developing cortex. We found that in utero 14-3-3gamma-deficiency resulted in delays in neuronal migration as well as morphological defects. Migrating neurons deficient in 14-3-3gamma displayed a thicker leading process stem, and the basal ends of neurons were not able to reach the boundary between the cortical plate and the marginal zone. Consistent with the results obtained from in utero electroporation, time-lapse live imaging of brain slices revealed that the ablation of the 14-3-3gamma proteins in pyramidal neurons slowed down their migration. In addition, the 14-3-3gamma deficient neurons showed morphological abnormalities, including increased multipolar neurons with a thicker leading processes stem during migration. These results indicate that the 14-3-3gamma proteins play an important role in radial migration by regulating the morphology of migrating neurons in the cerebral cortex. The findings underscore the pathological phenotypes of brain development associated with the disruption of different 14-3-3 proteins and will advance the preclinical data regarding disorders caused by neuronal migration defects.

Three-Dimensional Structure Prediction of the Interaction of CD34 with the SH3 Domain of Crk-L

The monomeric 115-kDa surface protein CD34, which is present on many stem cell populations, has been useful to enumerate the quality and viability of cell suspensions for engraftment. Although these studies assure the validity of CD34 as a stem cell marker, the functional role of this molecule has not been defined. CD34 has been demonstrated to regulate adhesion, differentiation, and proliferation of hematopoietic stem cells and other progenitors. The cytoplasmic domain of CD34 is known to be essential for its function. However, it is not clear how this domain's interactions with other molecules support the functional activity of CD34. Here we show that the cytoplasmic tail of CD34 is structurally similar to the carboxyl terminus of the gap junction protein Connexin 43 (Cx43). Because the activity of CD34 is mediated through its interaction with an SH3 domain of an intracellular protein, we attempted to define the SH3 binding region and amino acids involved in this interaction. We identified Glu325 to Ser334 as potential SH3 binding sites. Our results suggest that the interaction of the cytoplasmic tail of CD34 with the shallow proline-rich motif-binding groove of Crk-L is essential for the function of CD34 in stem cell development.

Alternative splicing as a mechanism for regulating 14-3-3 binding: interactions between hD53 (TPD52L1) and 14-3-3 proteins

D52 (TPD52)-like proteins are coiled-coil motif-bearing proteins first identified through their expression in human breast carcinoma, which have been proposed to represent signalling intermediates and regulators of vesicle trafficking. D52-like gene transcripts are subject to alternative splicing, with sequences encoding a region termed insert 3 being affected in all three D52-like genes. We have now identified a 14-3-3 binding motif within one of two alternatively spliced exons encoding insert 3. As predicted from the distribution of 14-3-3 binding motifs in four hD52-like bait proteins tested, only a hD53 isoform encoding a 14-3-3 binding motif bound both 14-3-3beta and 14-3-3zeta preys in the yeast two-hybrid system. Since D53 proteins carrying 14-3-3 binding motifs are predicted to be widely expressed, polyclonal antisera were derived to specifically detect these isoforms. Using soluble protein extracts from breast carcinoma cell lines, pull-down assays replicated interactions between recombinant 14-3-3beta and 14-3-3zeta isoforms and exogenously expressed hD53, and co-immunoprecipitation analyses demonstrated interactions between endogenous 14-3-3 and both endogenously and exogenously-expressed hD53 protein. Co-expressed hD53 and 14-3-3 proteins were similarly demonstrated to co-localise within the cytoplasm of breast carcinoma cell lines. These results identify 14-3-3 proteins as partners for hD53, and alternative splicing as a mechanism for regulating 14-3-3 binding.

Intracellular localization and dynamics of myosin-II and myosin-IC in live Acanthamoeba by transient transfection of EGFP fusion proteins

We developed a reliable method for transient transfection of Acanthamoeba using Superfect (Qiagen) and a vector with the Acanthamoeba ubiquitin promoter and enhanced green fluorescent protein (EGFP) as the reporter gene. The transfection efficiency was 3% for profilin-I-EGFP and EGFP-myosin-II tail, and less than 0.5% for larger constructs such as full length myosin-II or myosin-IC. Profilin-I-EGFP was distributed throughout the cytoplasm as observed previously with rhodamine-labeled profilin, while EGFP alone accumulated in the nucleus. EGFP fused to full length myosin-II or to the C-terminal 256 residues of the myosin-II tail concentrated in fluorescent spots similar to thick filaments and minifilaments identified previously in fixed cells with fluorescent antibodies. Thick filaments were located in the dorsal cytoplasm and along the lateral margins of the back half of the cell. Thick filaments formed behind the leading edge and moved continuously towards the rear of the cell, where they disassembled. If phosphorylation of the myosin-II heavy chain was prevented by mutation of all three phosphorylated serines to alanine, thick filaments of unphosphorylated myosin-II accumulated around vesicles of various sizes. EGFP-myosin-IC was spread throughout the cytoplasm but concentrated transiently around contractile vacuoles and macropinocytosis cups providing that the construct included both the head and a tail with the SH3 domain.

MADM, a novel adaptor protein that mediates phosphorylation of the 14-3-3 binding site of myeloid leukemia factor 1

A yeast two-hybrid screen was conducted to identify binding partners of Mlf1, an oncoprotein recently identified in a translocation with nucleophosmin that causes acute myeloid leukemia. Two proteins isolated in this screen were 14-3-3zeta and a novel adaptor, Madm. Mlf1 contains a classic RSXSXP sequence for 14-3-3 binding and is associated with 14-3-3zeta via this phosphorylated motif. Madm co-immunoprecipitated with Mlf1 and co-localized in the cytoplasm. In addition, Madm recruited a serine kinase, which phosphorylated both Madm and Mlf1 including the RSXSXP motif. In contrast to wild-type Mlf1, the oncogenic fusion protein nucleophosmin (NPM)-MLF1 did not bind 14-3-3zeta, had altered Madm binding, and localized exclusively in the nucleus. Ectopic expression of Madm in M1 myeloid cells suppressed cytokine-induced differentiation unlike Mlf1, which promotes maturation. Because the Mlf1 binding region of Madm and its own dimerization domain overlapped, the levels of Madm and Mlf1 may affect complex formation and regulate differentiation. In summary, this study has identified two partner proteins of Mlf1 that may influence its subcellular localization and biological function.

The 4.1/ezrin/radixin/moesin domain of the DAL-1/Protein 4.1B tumour suppressor interacts with 14-3-3 proteins

The Protein 4.1 family contains at least two members that function as tumour suppressors, the neurofibromatosis 2 gene product merlin and the recently identified differentially expressed in adenocarcinoma of the lung (DAL-1)/Protein 4.1B molecule. DAL-1/Protein 4.1B loss is observed in a variety of tumours, including breast and lung cancers as well as meningiomas. We have previously demonstrated that DAL-1/Protein 4.1B interacts with some but not all merlin-binding proteins, raising the possibility that DAL-1/Protein 4.1B associates with additional unique proteins specific to its function as a negative growth regulator. Using yeast two-hybrid interaction cloning, we identified three 14-3-3 isoforms, beta, gamma and eta, to be DAL-1/Protein 4.1B-binding proteins. These interactions were verified by using glutathione S-transferase affinity chromatography in vitro and co-immunoprecipitation in vivo. The interaction of 14-3-3 with DAL-1/Protein 4.1B was specific, as 14-3-3 did not bind to the related Protein 4.1 family members merlin, ezrin or radixin. The DAL-1/Protein 4.1B domain that mediates 14-3-3 binding was mapped to residues Pro(244) and Leu(280) within the 4.1/ezrin/radixin/moesin domain. The identification of this novel DAL-1/Protein 4.1B-interacting protein represents the first step towards elucidating its potentially unique mechanism of action.

Mitogenesis and endocytosis: What's at the INTERSECTIoN?

Endocytosis is a regulated physiological process by which cell surface proteins are internalized along with extracellular factors such as nutrients, pathogens, peptides, toxins, etc. The process begins with the invagination of small regions of the plasma membrane which ultimately form intracellullar vesicles. These internalized vesicles may shuttle back to the plasma membrane to recycle the membrane components or they may be targeted for degradation. One role for endocytosis is in the attenuation of receptor signaling. For example, desensitization of activated membrane bound receptors such as G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs) occurs, in part, through endocytosis of the activated receptor. However, accumulating evidence suggests that endocytosis also mediates intracellular signaling. In this review, we discuss the experimental data that implicate endocytosis as a critical component in cellular signal transduction, both in the initiation of a signal as well as in the termination of a signal. Furthermore, we focus our attention on a recently described adaptor protein, intersectin (ITSN), which provides a link to both the endocytic and the mitogenic machinery of a cell. Thus, ITSN functions at a crossroad in the biochemical regulation of cell function.

Suppression of apoptosis and granulocyte colony-stimulating factor-induced differentiation by an oncogenic form of Cbl

OBJECTIVE

The retroviral oncogene v-Cbl causes pre-B cell lymphomas and myeloid leukemias in mice, and its Drosophila homologue is oncogenic, causing enhanced receptor tyrosine kinase signaling. The human Cbl gene resides at 11q23. The aim of this study is to determine the effect of oncogenic Cbl on growth-regulating responses.

MATERIALS AND METHODS

The oncogenic mutant of Cbl (CblDelta1-357) was transfected into factor-dependent 32Dcl3 myeloid cells. Consequently, cell survival and differentiation were measured. Lyn, Syk, MAP kinase, and phosphatidylinositol 3'(PI3')-kinase activities, protein phosphorylation, Bcl-2 promoter activity, ubiquitination, and levels of Bcl-2, Bax, Bad, and Bcl-x(L) were determined. In addition, the effect of v-Cbl on TF-1 cell survival upon granulocyte-macrophage colony-stimulating factor withdrawal was studied.

RESULTS

32Dcl3 and TF-1 cells expressing v-Cbl showed resistance to apoptosis upon growth factor withdrawal, and 32Dcl3 cells completely failed to respond to granulocyte colony-stimulating factor's induction of differentiation. Basal activities of Lyn, Syk, and PI3'-kinase were elevated in the v-Cbl line. There was neither enhanced tyrosine phosphorylation of cellular protein content, Cbl, or Jak2, nor serine phosphorylation of MAP kinase or Akt. After factor withdrawal, the level of Bcl-2 was greater in v-Cbl cells than in control cells.

CONCLUSIONS

Neither increased Bcl-2 promoter activity nor decreased ubiquitination of Bcl-2 could account for increased Bcl-2 levels. v-Cbl-expressing 32Dcl3 cells were resistant to differentiation. v-Cbl suppresses apoptosis and differentiation, possibly through enhancement of Lyn, Syk, and PI3'-kinase activities and Bcl-2.

A Drosophila analogue of v-Cbl is a dominant-negative oncoprotein in vivo

Cells rely on the ability to receive and interpret external signals to regulate growth, differentiation, and death. Positive transduction of these signals to the cytoplasm and nucleus has been extensively characterized, and genetic studies in Drosophila have made major contributions to the understanding of these pathways. Less well understood, but equally important, are the mechanisms underlying signal down-regulation. Here we report biochemical and genetic characterization of the Drosophila homologue of c-Cbl, a negative regulator of signal transduction with ubiquitin-protein ligase activity. A new isoform of D-Cbl, D-CblL, has been identified that contains SH3-binding and UBA domains previously reported to be absent. Genetic analysis demonstrates that Dv-cbl, analogous to the mammalian v-cbl oncogene, is a dominant negative mutation able to enhance signalling from the Drosophila Egfr and cooperate with activating mutations in the sevenless pathway to produce melanotic tumours. In addition, our data show genetic and biochemical links between D-Cbl and proteins involved in endocytosis and ubiquitination, suggesting that v-Cbl may exert its oncogenic effect by enhancing receptor signalling as a consequence of suppressing receptor endocytosis.

Regulation of the enzymatic and motor activities of myosin I

Myosins I were the first unconventional myosins to be purified and they remain the best characterized. They have been implicated in various motile processes, including organelle translocation, ion channel gating and cytoskeletal reorganization but their exact cellular functions are still unclear. All members of the myosin I family, from yeast to man, have three structural domains: a catalytic head domain that binds ATP and actin; a tail domain believed to be involved in targeting the myosins to specific subcellular locations and a junction or neck domain that connects them and interacts with light chains. In this review we discuss how each of these three domains contributes to the regulation of myosin I enzymatic activity, motor activity and subcellular localization.

The endocytic protein intersectin is a major binding partner for the Ras exchange factor mSos1 in rat brain

We recently identified intersectin, a protein containing two EH and five SH3 domains, as a component of the endocytic machinery. The N-terminal SH3 domain (SH3A), unlike other SH3 domains from intersectin or various endocytic proteins, specifically inhibits intermediate events leading to the formation of clathrin-coated pits. We have now identified a brain-enriched, 170 kDa protein (p170) that interacts specifically with SH3A. Screening of combinatorial peptides reveals the optimal ligand for SH3A as Pp(V/I)PPR, and the 170 kDa mammalian son-of-sevenless (mSos1) protein, a guanine-nucleotide exchange factor for Ras, con- tains two copies of the matching sequence, PPVPPR. Immunodepletion studies confirm that p170 is mSos1. Intersectin and mSos1 are co-enriched in nerve terminals and are co-immunoprecipitated from brain extracts. SH3A competes with the SH3 domains of Grb2 in binding to mSos1, and the intersectin-mSos1 complex can be separated from Grb2 by sucrose gradient centrifugation. Overexpression of the SH3 domains of intersectin blocks epidermal growth factor-mediated Ras activation. These results suggest that intersectin functions in cell signaling in addition to its role in endocytosis and may link these cellular processes.

Acan125 binding to the SH3 domain of acanthamoeba myosin-IC

The domain organization of Acanthamoeba myosin-I, an oligomodular motor protein, includes a potentially important protein interaction module that is mostly uncharacterized. The Src homology 3, SH3, domain of myosin-I binds Acan125, a protein containing at least two consensus ligand binding domains: C-terminal SH3 binding motifs (PXXP) and N-terminal leucine-rich repeats. We report the first affinities determined for an SH3 domain of any myosin, namely, K(d) = 7 microM for a 21-residue synthetic peptide based on the PXXP domain sequence and K(d) = 0.15 microM for the PXXP domain included in the C-terminus of Acan125. These values are consistent with affinities reported for peptides and proteins that associate with SH3. By deletional analysis we show that only the PXXP domain is required for Acan125 to interact with the SH3 domain of Acanthamoeba myosin-IC (AmyoC(SH3)). The synthetic peptide described above at a concentration near the K(d) for SH3 binding blocked the interaction between native AmyoC and Acan125, mapping the interaction to the PXXP domain of Acan125 and the SH3 domain of myosin-I. These results are consistent with prototypical SH3 binding and suggest that a PXXP module is both necessary and sufficient to interact with an SH3 module of myosin-I.

The adapter protein Crkl links Cbl to C3G after integrin ligation and enhances cell migration

Crkl, an SH2-SH3-SH3 adapter protein, is one of the major tyrosine phosphoproteins detected in cells from patients with chronic myelogenous leukemia. Crkl binds to BCR/ABL through its N-terminal SH3 domain and is known to interact with several signaling proteins that have been implicated in integrin signaling, including Cbl, Cas, Hef-1, and paxillin. We have previously shown that overexpression of Crkl enhances adhesion to extracellular matrix proteins through beta(1) integrins. In this study, the effects of Crkl on spontaneous and chemokine-directed migration of the hematopoietic cell line Ba/F3 were examined. Full-length, SH2-, and SH3(N)-domain deletion mutants of Crkl were expressed transiently as fusion proteins with green fluorescent protein. Successfully transfected cells were isolated by fluorescence-activated cell sorting. The ability of these cells to migrate across a fibronectin-coated membrane, either spontaneously or in response to the chemokine stromal-derived factor-1alpha, was determined. Cells expressing green fluorescent protein alone were not distinguishable from untransfected or mock transfected Ba/F3 cells. However, Ba/F3 cells overexpressing full-length Crkl were found to have an increase in spontaneous migration of 2.8 +/- 0.6-fold in seven independent assays. The enhancement of migration required both the SH2 domain and the N-terminal SH3 domain. Migration in response to stromal-derived factor-1alpha was not significantly enhanced by overexpression of Crkl. Overexpression of Crkii also augmented spontaneous migration but to a lesser degree than did Crkl. Because the SH2 domain was required for enhanced migration, we looked for changes in phosphotyrosine containing proteins coprecipitating with Crkl, but not Crkl DeltaSH2, after integrin cross-linking. Full-length Crkl, but not CrklDeltaSH2, coprecipitated with a single major tyrosine phosphoprotein with an M(r) of approximately 120 kDa, identified as Cbl. The major Crkl SH3-binding protein in these cells was found to be the guanine nucleotide exchange factor, C3G. Interestingly, overexpression of C3G also enhanced migration, suggesting that a Cbl-Crkl-C3G complex may be involved in migration signaling in Ba/F3 cells. These data suggest that Crkl is involved in signaling pathways that regulate migration, possibly through a complex with Cbl and C3G.

A general strategy to enhance the potency of chimeric transcriptional activators

Efforts to increase the potency of transcriptional activators are generally unsuccessful because poor expression of activators in mammalian cells limits their delivery to target promoters. Here we report that the effectiveness of chimeric activators can be dramatically improved by expressing them as noncovalent tetrameric bundles. Bundled activation domains are much more effective at activating a reporter gene than simple monomeric activators, presumably because, at similar expression levels, up to 4 times as many the activation domains are delivered to the target promoter. These bundled activation domains are also more effective than proteins in which activation domains are tandemly reiterated in the same polypeptide chain, because such proteins are very poorly expressed and therefore not delivered effectively. These observations suggest that there is a threshold number of activation domains that must be bound to a promoter for activation, above which promoter activity is simply a function of the number of activators bound. We show that bundling can be exploited practically to enhance the sensitivity of mammalian two-hybrid assays, enabling detection of weak interactions or those between poorly expressed proteins. Bundling also dramatically improves the performance of a small-molecule-regulated gene expression system when the expression level of regulatory protein is limiting, a situation that may be encountered in gene therapy applications.

cDNA cloning, expression pattern, and chromosomal localization of Mlf1, murine homologue of a gene involved in myelodysplasia and acute myeloid leukemia

The NPM-MLF1 fusion protein is expressed in blasts from patients with myelodysplasia/acute myeloid leukemia (MDS/AML) containing the t(3;5) chromosomal rearrangement. Nucleophosmin (NPM), a previously characterized nucleolar phosphoprotein, contributes to two other fusion proteins found in lympho-hematopoietic malignancies, anaplastic large cell lymphoma (NPM-ALK) and acute promyelocytic leukemia (NPM-RARalpha). By contrast, the function of the carboxy-terminal fusion partner, myelodysplasia/myeloid leukemia factor 1 (MLF1), is unknown. To aid in understanding normal MLF1 function, we isolated the murine cDNA, determined the chromosomal localization of Mlf1, and defined its tissue expression by in situ hybridization. Mlf1 was highly similar to its human homologue (86% and 84% identical nucleotide and amino acid sequence, respectively) and mapped to the central region of chromosome 3, within a segment lacking known mouse mutations. Mlf1 tissue distribution was restricted during both development and postnatal life, with high levels present only in skeletal, cardiac, and selected smooth muscle, gonadal tissues, and rare epithelial tissues including the nasal mucosa and the ependyma/choroid plexus in the brain. Mlf1 transcripts were undetectable in the lympho-hematopoietic organs of both the embryonic and adult mouse, suggesting that NPM-MLF1 contributes to the genesis of MDS/AML in part by enforcing the ectopic overexpression of MLF1 within hematopoietic tissues.

Growth hormone stimulates the formation of a multiprotein signaling complex involving p130(Cas) and CrkII. Resultant activation of c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK)

We have demonstrated previously that growth hormone (GH) activates focal adhesion kinase (FAK), and this activation results in the tyrosine phosphorylation of two FAK substrates, namely paxillin and tensin. We now show here in Chinese hamster ovary cells stably transfected with rat GH receptor cDNA that human (h)GH induces the formation of a large multiprotein signaling complex centered around another FAK-associated protein, p130(Cas) and the adaptor protein CrkII. hGH stimulates the tyrosine phosphorylation of both p130(Cas) and CrkII, their association, and the association of multiple other tyrosine-phosphorylated proteins to the complex. Both the c-Src and c-Fyn tyrosine kinases are tyrosine phosphorylated and activated by cellular hGH stimulation and form part of the multiprotein signaling complex as does tensin, paxillin, IRS-1, the p85 subunit of phosphatidylinositol 3-kinase, C3G, SHC, Grb-2, and Sos-1. c-Cbl and Nck are also tyrosine-phosphorylated by cellular stimulation with hGH and associate with the p130(Cas)-CrkII complex. c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) is activated in response to hGH in accordance with the formation of the abovementioned signaling complex, and hGH stimulated JNK/SAPK activity is increased in CrkII overexpressing NIH3T3 cells compared with vector transfected NIH3T3 cells. The formation of such a large multiprotein signaling complex by GH, with the resultant activation of multiple downstream effector molecules, may be central to many of the pleiotropic effects of GH.