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)

Classical and novel GH receptor signaling pathways

In this review, I summarize historical and recent features of the classical pathways activated by growth hormone (GH) through the cell surface GH receptor (GHR). GHR is a cytokine receptor superfamily member that signals by activating the non-receptor tyrosine kinase, JAK2, and members of the Src family kinases. Activation of the GHR engages STATs, PI3K, and ERK pathways, among others, and details of these now-classical pathways are presented. Modulating elements, including the SOCS proteins, phosphatases, and regulated GHR metalloproteolysis, are discussed. In addition, a novel physical and functional interaction of GHR with IGF-1R is summarized and discussed in terms of its mechanisms, consequences, and physiological and therapeutic implications.

Growth Hormone Receptor Regulation in Cancer and Chronic Diseases

The GHR signaling pathway plays important roles in growth, metabolism, cell cycle control, immunity, homeostatic processes, and chemoresistance via both the JAK/STAT and the SRC pathways. Dysregulation of GHR signaling is associated with various diseases and chronic conditions such as acromegaly, cancer, aging, metabolic disease, fibroses, inflammation and autoimmunity. Numerous studies entailing the GHR signaling pathway have been conducted for various cancers. Diverse factors mediate the up- or down-regulation of GHR signaling through post-translational modifications. Of the numerous modifications, ubiquitination and deubiquitination are prominent events. Ubiquitination by E3 ligase attaches ubiquitins to target proteins and induces proteasomal degradation or starts the sequence of events that leads to endocytosis and lysosomal degradation. In this review, we discuss the role of first line effectors that act directly on the GHR at the cell surface including ADAM17, JAK2, SRC family member Lyn, Ubc13/CHIP, proteasome, βTrCP, CK2, STAT5b, and SOCS2. Activity of all, except JAK2, Lyn and STAT5b, counteract GHR signaling. Loss of their function increases the GH-induced signaling in favor of aging and certain chronic diseases, exemplified by increased lung cancer risk in case of a mutation in the SOCS2-GHR interaction site. Insight in their roles in GHR signaling can be applied for cancer and other therapeutic strategies.

Why Should Growth Hormone (GH) Be Considered a Promising Therapeutic Agent for Arteriogenesis? Insights from the GHAS Trial

Despite the important role that the growth hormone (GH)/IGF-I axis plays in vascular homeostasis, these kind of growth factors barely appear in articles addressing the neovascularization process. Currently, the vascular endothelium is considered as an authentic gland of internal secretion due to the wide variety of released factors and functions with local effects, including the paracrine/autocrine production of GH or IGF-I, for which the endothelium has specific receptors. In this comprehensive review, the evidence involving these proangiogenic hormones in arteriogenesis dealing with the arterial occlusion and making of them a potential therapy is described. All the elements that trigger the local and systemic production of GH/IGF-I, as well as their possible roles both in physiological and pathological conditions are analyzed. All of the evidence is combined with important data from the GHAS trial, in which GH or a placebo were administrated to patients suffering from critical limb ischemia with no option for revascularization. We postulate that GH, alone or in combination, should be considered as a promising therapeutic agent for helping in the approach of ischemic disease.

Growth hormone pathways signaling for cell proliferation and survival in hippocampal neural precursors from postnatal mice

Background

Accumulating evidence suggests that growth hormone (GH) may play a major role in the regulation of postnatal neurogenesis, thus supporting the possibility that it may be also involved in promoting brain repair after brain injury. In order to gain further insight on this possibility, in this study we have investigated the pathways signaling the effect of GH treatment on the proliferation and survival of hippocampal subgranular zone (SGZ)-derived neurospheres.

Results

Our results demonstrate that GH treatment promotes both proliferation and survival of SGZ neurospheres. By using specific chemical inhibitors we have been also able to demonstrate that GH treatment promotes the activation of both Akt-mTOR and JNK signaling pathways, while blockade of these pathways either reduces or abolishes the GH effects. In contrast, no effect of GH on the activation of the Ras-ERK pathway was observed after GH treatment, despite blockade of this signaling path also resulted in a significant reduction of GH effects. Interestingly, SGZ cells were also capable of producing GH, and blockade of endogenous GH also resulted in a decrease in the proliferation and survival of SGZ neurospheres.

Conclusions

Altogether, our findings suggest that GH treatment may promote the proliferation and survival of neural progenitors. This effect may be elicited by cooperating with locally-produced GH in order to increase the response of neural progenitors to adequate stimuli. On this view, the possibility of using GH treatment to promote neurogenesis and cell survival in some acquired neural injuries may be envisaged.

Research resource: identification of novel growth hormone-regulated phosphorylation sites by quantitative phosphoproteomics

GH and GH receptors are expressed throughout life, and GH elicits a diverse range of responses, including growth and altered metabolism. It is therefore important to understand the full spectrum of GH signaling pathways and cellular responses. We applied mass spectrometry-based phosphoproteomics combined with stable isotope labeling with amino acids in cell culture to identify proteins rapidly phosphorylated in response to GH in 3T3-F442A preadipocytes. We identified 132 phosphosites in 95 proteins that exhibited rapid (5 or 15 min) GH-dependent statistically significant increases in phosphorylation by more than or equal to 50% and 96 phosphosites in 46 proteins that were down-regulated by GH by more than or equal to 30%. Several of the GH-stimulated phosphorylation sites were known (e.g. regulatory Thr/Tyr in Erks 1 and 2, Tyr in signal transducers and activators of transcription (Stat) 5a and 5b, Ser939 in tuberous sclerosis protein (TSC) 2 or tuberin). The remaining 126 GH-stimulated sites were not previously associated with GH. Kyoto Encyclopedia of Genes and Genomes pathway analysis of GH-stimulated sites indicated enrichment in proteins associated with the insulin and mammalian target of rapamycin (mTOR) pathways, regulation of the actin cytoskeleton, and focal adhesions. Akt/protein kinase A consensus sites (RXRXXS/T) were the most commonly phosphorylated consensus sites. Immunoblotting confirmed GH-stimulated phosphorylation of all seven novel GH-dependent sites tested [regulatory sites in proline-rich Akt substrate, 40 kDA (PRAS40), regulatory associated protein of mTOR, ATP-citrate lyase, Na⁺/H⁺ exchanger-1, N-myc downstream regulated gene 1, and Shc]). The immunoblot results suggest that many, if not most, of the GH-stimulated phosphosites identified in this large-scale quantitative phosphoproteomics analysis, including sites in multiple proteins in the Akt/ mTOR complex 1 pathway, are phosphorylated in response to GH. Their identification significantly broadens our thinking of GH-regulated cell functions.

Identification of targets of c-Src tyrosine kinase by chemical complementation and phosphoproteomics

The cellular proto-oncogene c-Src is a nonreceptor tyrosine kinase involved in cell growth and cytoskeletal regulation. Despite being dysregulated in a variety of human cancers, its precise functions are not fully understood. Identification of the substrates of c-Src remains a major challenge, because there is no simple way to directly stimulate its activity. Here we combine the chemical rescue of mutant c-Src and global quantitative phosphoproteomics to obtain the first high resolution snapshot of the range of tyrosine phosphorylation events that occur in the cell immediately after specific c-Src stimulation. After enrichment by anti-phosphotyrosine antibodies, we identified 29 potential novel c-Src substrate proteins. Tyrosine phosphopeptide mapping allowed the identification of 382 nonredundant tyrosine phosphopeptides on 213 phosphoproteins. Stable isotope labeling of amino acids in cell culture-based quantitation allowed the detection of 97 nonredundant tyrosine phosphopeptides whose level of phosphorylation is increased by c-Src. A large number of previously uncharacterized c-Src putative protein targets and phosphorylation sites are presented here, a majority of which play key roles in signaling and cytoskeletal networks, particularly in cell adhesion. Integrin signaling and focal adhesion kinase signaling pathway are two of the most altered pathways upon c-Src activation through chemical rescue. In this context, our study revealed the temporal connection between c-Src activation and the GTPase Rap1, known to stimulate integrin-dependent adhesion. Chemical rescue of c-Src provided a tool to dissect the spatiotemporal mechanism of activation of the Rap1 guanine exchange factor, C3G, one of the identified potential c-Src substrates that plays a role in focal adhesion signaling. In addition to unveiling the role of c-Src in the cell and, specifically, in the Crk-C3G-Rap1 pathway, these results exemplify a strategy for obtaining a comprehensive understanding of the functions of nonreceptor tyrosine kinases with high specificity and kinetic resolution.

Inhibitory GH receptor extracellular domain monoclonal antibodies: three-dimensional epitope mapping

GH receptor (GHR) mediates the anabolic and metabolic effects of GH. We previously characterized a monoclonal antibody (anti-GHR(ext-mAb)) that reacts with subdomain 2 of the rabbit GHR extracellular domain (ECD) and is a conformation-specific inhibitor of GH signaling in cells bearing rabbit or human GHR. Notably, this antibody has little effect on GH binding and also inhibits inducible metalloproteolysis of the GHR that occurs in the perimembranous ECD stem region. In the current study, we demonstrate that anti-GHR(ext-mAb) inhibits GH-dependent cellular proliferation and also inhibits hepatic GH signaling in vivo in mice that adenovirally express rabbit GHR, as assessed with our noninvasive bioluminescence hepatic signaling assay. A separate monoclonal antibody (anti-GHR(mAb 18.24)) is a sister clone of anti-GHR(ext-mAb). Here, we demonstrate that anti-GHR(mAb 18.24) also inhibits rabbit and human GHR signaling and inducible receptor proteolysis. Further, we use a random PCR-generated mutagenic expression system to map the three-dimensional epitopes in the rabbit GHR ECD for both anti-GHR(ext-mAb) and anti-GHR(mAb 18.24). We find that each of the two antibodies has similar, but nonidentical, discontinuous epitopes that include regions of subdomain 2 encompassing the dimerization interface. These results have fundamental implications for understanding the role of the dimerization interface and subdomain 2 in GHR activation and regulated GHR metalloproteolysis and may inform development of therapeutics that target GHR.

JAK2, but not Src family kinases, is required for STAT, ERK, and Akt signaling in response to growth hormone in preadipocytes and hepatoma cells

Janus kinase 2 (JAK2), a tyrosine kinase that associates with the GH receptor and is activated by GH, has been implicated as a key mediator of GH signaling. Several published reports suggest that members of the Src family of tyrosine kinases may also participate in GH signaling. We therefore investigated the extent to which JAK2 and Src family kinases mediate GH activation of signal transducers and activators of transcription (STATs) 1, 3, and 5a/b, ERKs 1 and 2, and Akt, in the highly GH-responsive cell lines 3T3-F442A preadipocytes and H4IIE hepatoma cells. GH activation of Src family kinases was not detected in either cell line. Further, blocking basal activity of Src kinases with the Src inhibitors PP1 and PP2 did not inhibit GH activation of STATs 1, 3, or 5a/b, or ERKs 1 and 2. When levels of JAK2 were depressed by short hairpin RNA in 3T3-F442A and H4IIE cells, GH-stimulated activation of STATs 1, 3, and 5a/b, ERKs 1 and 2, and Akt were significantly reduced; however, basal activity of Src family kinases was unaffected. These results were supported genetically by experiments showing that GH robustly activates JAK2, STATs 3 and 5a/b, ERKs 1 and 2, and Akt in murine embryonic fibroblasts derived from Src/Yes/ Fyn triple-knockout embryos that lack known Src kinases. These results strongly suggest that JAK2, but not Src family kinases, is critical for transducing these GH signals in 3T3-F442A and H4IIE cells.

Septins regulate actin organization and cell-cycle arrest through nuclear accumulation of NCK mediated by SOCS7

Mammalian septins are GTP-binding proteins the functions of which are not well understood. Knockdown of SEPT2, 6, and 7 causes stress fibers to disintegrate and cells to lose polarity. We now show that this phenotype is induced by nuclear accumulation of the adaptor protein NCK, as the effects can be reversed or induced by cytoplasmic or nuclear NCK, respectively. NCK is carried into the nucleus by SOCS7 (suppressor of cytokine signaling 7), which possesses nuclear import/export signals. SOCS7 interacts with septins and NCK through distinct domains. DNA damage induces actin and septin rearrangement and rapid nuclear accumulation of NCK and SOCS7. Moreover, NCK expression is essential for cell-cycle arrest. The septin-SOCS7-NCK axis intersects with the canonical DNA damage cascade downstream of ATM/ATR and is essential for p53 Ser15 phosphorylation. These data illuminate an unanticipated connection between septins, SOCS7, NCK signaling, and the DNA damage response.

Regulation of Suppressor of Cytokine Signaling 3 (SOC3) by Growth Hormone in Pro-B Cells

Suppressor of cytokine signaling 3 (SOCS3) is expressed by lymphoid cells and can modulate the sensitivity of these cells to cytokine stimulation through inhibition of Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling pathways. This study employed a mouse pro-B cell line expressing the human GH receptor (BaF/3-GHR), to elucidate the signal transduction pathways used by GH to elicit SOCS3 expression. GH treatment of these cells caused a rapid, dose-dependent increase in SOCS3 mRNA expression, which was independent of de novo protein synthesis. As expected, GH treatment increased JAK-dependent STAT5 tyrosine phosphorylation, which bound to the proximal STAT response element (pSRE) on the SOCS3 promoter. This process appeared to involve STAT5b, rather than STAT5a. In addition, GH activation of the SOCS3 promoter required a nearby activator protein (AP) 1/cAMP response element (CRE), which bound cAMP response element binding protein, c-Fos, and c-Jun. Moreover, inhibitors of p38 MAPK and c-Jun N-terminal kinase prevented GH-stimulation of SOCS3 mRNA expression in these cells, suggesting a role for these kinases in SOCS3 transcription. Importantly, GH stimulation increased binding of FOXO3a to the SOCS3 promoter at a site overlapping the AP1/CRE response element, and overexpression of FOXO3a in these cells augmented SOCS3 promoter activation. In addition, we show a direct interaction between FOXO3a and STAT5 in these cells, which may provide a link between STAT5 and the AP1 transcription factors on the SOCS3 promoter. We conclude that regulation of SOCS3 expression by GH in a pro-B cell involves not only the pSRE, but also a transcriptionally active complex involving cAMP response element binding protein/c-Fos/c-Jun and FOXO3a. This study has implications for cytokine regulation of SOCS gene expression in lymphoid cells.

The p85alpha regulatory subunit of phosphoinositide 3-kinase potentiates c-Jun N-terminal kinase-mediated insulin resistance

Insulin resistance is a defining feature of type 2 diabetes and the metabolic syndrome. While the molecular mechanisms of insulin resistance are multiple, recent evidence suggests that attenuation of insulin signaling by c-Jun N-terminal kinase (JNK) may be a central part of the pathobiology of insulin resistance. Here we demonstrate that the p85alpha regulatory subunit of phosphoinositide 3-kinase (PI3K), a key mediator of insulin's metabolic actions, is also required for the activation of JNK in states of insulin resistance, including high-fat diet-induced obesity and JNK1 overexpression. The requirement of the p85alpha regulatory subunit for JNK occurs independently of its role as a component of the PI3K heterodimer and occurs only in response to specific stimuli, namely, insulin and tunicamycin, a chemical that induces endoplasmic reticulum stress. We further show that insulin and p85 activate JNK by via cdc42 and MKK4. The activation of this cdc42/JNK pathway requires both an intact N terminus and functional SH2 domains within the C terminus of the p85alpha regulatory subunit. Thus, p85alpha plays a dual role in regulating insulin sensitivity and may mediate cross talk between the PI3K and stress kinase pathways.

Recent advances in growth hormone signaling

Growth hormone (GH) is a major regulatory factor for overall body growth as evidenced by the height extremes in people with abnormal circulating GH levels or GH receptor (GHR) disruptions. GH also affects metabolism, cardiac and immune function, mental agility and aging. Currently, GH is being used therapeutically for a variety of clinical conditions including promotion of growth in short statured children, treatment of adults with GH deficiency and HIV-associated wasting. To help reveal previous unrecognized functions of GH, better understand the known functions of GH, and avoid adverse consequences that are often associated with exogenous GH administration, careful delineation of the molecular mechanisms whereby GH induces its diverse effects is needed. GH is a peptide hormone that is secreted into the circulation by the anterior pituitary and acts upon various target tissues expressing GHR. GH binding of GHR activates the tyrosine kinase Janus kinase 2 (JAK2), thus initiating a multitude of signaling cascades that result in a variety of biological responses including cellular proliferation, differentiation and migration, prevention of apoptosis, cytoskeletal reorganization and regulation of metabolic pathways. A number of signaling proteins and pathways activated by GH have been identified, including JAKs, signal transducers and activators of transcription (Stats), the mitogen activated protein kinase (MAPK) pathway, and the phosphatidylinositol 3'-kinase (PI3K) pathway. Although these signal transduction pathways have been well characterized, the manner by which GH activates these pathways, the downstream signals induced by these pathways, and the cross-talk with other pathways are not completely understood. Recent findings have added vital information to our understanding of these downstream signals induced by GH and mechanisms that terminate GH signaling, and identified new GH signaling proteins and pathways. This review will highlight some of these findings, many of which are unexpected and some of which challenge previously held beliefs about the mechanisms of GH signaling.

Phosphorylation of JAK2 at serine 523: a negative regulator of JAK2 that is stimulated by growth hormone and epidermal growth factor

The tyrosine kinase JAK2 is a key signaling protein for at least 20 receptors in the cytokine/hematopoietin receptor superfamily and is a component of signaling for multiple receptor tyrosine kinases and several G-protein-coupled receptors. In this study, phosphopeptide affinity enrichment and mass spectrometry identified serine 523 (Ser523) in JAK2 as a site of phosphorylation. A phosphoserine 523 antibody revealed that Ser523 is rapidly but transiently phosphorylated in response to growth hormone (GH). MEK1 inhibitor UO126 suppresses GH-dependent phosphorylation of Ser523, suggesting that extracellular signal-regulated kinases (ERKs) 1 and/or 2 or another kinase downstream of MEK1 phosphorylate Ser523 in response to GH. Other ERK activators, phorbol 12-myristate 13-acetate and epidermal growth factor, also stimulate phosphorylation of Ser523. When Ser523 in JAK2 was mutated, JAK2 kinase activity as well as GH-dependent tyrosyl phosphorylation of JAK2 and Stat5 was enhanced, suggesting that phosphorylation of Ser523 inhibits JAK2 kinase activity. We hypothesize that phosphorylation of Ser523 in JAK2 by ERKs 1 and/or 2 or other as-yet-unidentified kinases acts in a negative feedback manner to dampen activation of JAK2 in response to GH and provides a mechanism by which prior exposure to environmental factors that regulate Ser523 phosphorylation might modulate the cell's response to GH.

Src transduces signaling via growth hormone (GH)-activated GH receptor (GHR) tyrosine-phosphorylating GHR and STAT5 in human leukemia cells

Most human leukemia cells are shown to express growth hormone receptor (GHR) and some of them proliferate in response to GH. We demonstrate that Src contributes to GHR-mediated signal transduction via STAT5 activation in F-36P human leukemia cells stimulated with GH. The tyrosine phosphorylation levels of GHR and STAT5 induced by GH decreased in the presence of PP2 Src kinase inhibitor. When GHR and wild-type Src were co-expressed in COS7 cells, GHR was markedly tyrosine phosphorylated as well as when Jak2 was co-expressed with GHR, but not when kinase-inactive Src co-expressed. The treatment of F-36P cells with the antisense src oligonucleotides, which selectively decreased the Src expression, reduced the rhGH-induced tyrosine phosphorylation of the STAT5 activation sites.

Influence of the crosstalk between growth hormone and insulin signalling on the modulation of insulin sensitivity

Growth hormone (GH) is an important modulator of insulin sensitivity. Multiple mechanisms appear to be involved in this modulatory effect. GH does not interact directly with the insulin receptor (IR), but conditions of GH excess are associated in general with hyperinsulinemia that induces a reduction of IR levels and impairment of its kinase activity. Several post-receptor events are shared between GH and insulin. This signaling crosstalk could be involved in the diabetogenic effects of GH. The utilization of animal models of GH excess, deficiency or resistance provided evidence that the signaling pathway leading to stimulation of the phosphatidylinositol 3-kinase (PI3K)/Akt cascade is an important site of regulation, and pointed to the liver as the major site of GH-induced insulin resistance. In skeletal muscle, GH-induced insulin resistance might involve an increase in the amount of the p85 subunit of PI3K that plays a negative role in insulin signalling. GH also reduces insulin sensitivity by enhancing events that negatively modulate insulin signaling such as stimulation of serine phosphorylation of IRS-1, which prevents its recruitment to the IR and induction of the suppressor of cytokine signalling (SOCS)-1 and SOCS-3 which modulate the signalling potential of the IRS proteins. In addition, GH has been shown to decrease the expression of the insulin-sensitizing adipo-cytokines adiponectin and visfatin. Finally, genetic manipulation of mice indicated that whereas GH plays a major role in reducing insulin sensitivity, circulating IGF-I also participates in the control of insulin sensitivity and plays an important role in the hormonal balance between GH and insulin.

Role of the cytokine-induced SH2 domain-containing protein CIS in growth hormone receptor internalization

The cytokine-inducible SH2 domain-containing protein CIS inhibits signaling from the growth hormone (GH) receptor (GHR) to STAT5b by a proteasome-dependent mechanism. Here, we used the GH-responsive rat liver cell line CWSV-1 to investigate the role of CIS and the proteasome in GH-induced GHR internalization. Cell-surface GHR localization and internalization were monitored in GH-stimulated cells by confocal immunofluorescence microscopy using an antibody directed against the GHR extracellular domain. In GH naïve cells, GHR was detected in small, randomly distributed granules on the cell surface and in the cytoplasm, with accumulation in the perinuclear area. GH treatment induced a rapid (within 5 min) internalization of GH.GHR complexes, which coincided with the onset of GHR tyrosine phosphorylation and the appearance in the cytosol of distinct granular structures containing internalized GH. GHR signaling to STAT5b continued for approximately 30-40 min, however, indicating that GHR signaling and deactivation of the GH.GHR complex both proceed from an intracellular compartment. The internalization of GH and GHR was inhibited by CIS-R107K, a dominant-negative SH2 domain mutant of CIS, and by the proteasome inhibitors MG132 and epoxomicin, which prolong GHR signaling to STAT5b. GH pulse-chase studies established that the internalized GH.GHR complexes did not recycle back to the cell surface in significant amounts under these conditions. Given the established specificity of CIS-R107K for blocking the GHR signaling inhibitory actions of CIS, but not those of other SOCS/CIS family members, these findings implicate CIS and the proteasome in the control of GHR internalization following receptor activation and suggest that CIS-dependent receptor internalization is a prerequisite for efficient termination of GHR signaling.

Retracted: Aspirin inhibits serine phosphorylation of insulin receptor substrate 1 in growth hormone treated animals

In this study, we demonstrate that pretreatment with aspirin inhibits GH-induced insulin resistance. GH was observed to lead to serine phosphorylation of IRS-1, a phenomenon which was reversed by aspirin in liver, muscle and WAT in parallel with a reduction in JNK activity. In addition, our data show an impairment of insulin activation in the IR/IRS/PI(3)kinase pathway and a reduction in IRS-1 protein levels in rats treated with GH, which was also reversed in the animals pretreated with aspirin. Overall, these results provide new insights into the mechanism of GH-induced insulin resistance.

C3G-mediated suppression of oncogene-induced focus formation in fibroblasts involves inhibition of ERK activation, cyclin A expression and alterations of anchorage-independent growth

We showed previously that exogenous overexpression of C3G, a guanine nucleotide releasing factor (GEF) for Rap1 and R-Ras proteins, blocks the focus-forming activity of cotransfected, activated, sis, ras and v-raf oncogenes in NIH 3T3 cells. In this report, we show that C3G also interferes with dbl and R-Ras focus-forming activity and demonstrate that the transformation suppressor ability of C3G maps to its Crk-binding region (SH3-b domain). Using full-length C3G and C3GDeltaCat mutant, lacking catalytic domain, we showed here that overexpression of cotransfected C3G or C3GDeltaCat inhibited oncogenic Hraslys12-mediated phosphorylation of ERK, without altering Ras and Raf-1 kinase activation. We also showed that, overexpressed C3G and C3GdeltaCat inhibited the viability of oncogenic Ras-induced colonies in soft agar, indicating that C3G interferes with the anchorage-independent growth of Ras-transformed cells in a Rap1-independent manner. Consistent with both observations, overexpression of exogenous C3G and C3GDeltaCat also caused downregulation of Ras-induced cyclin A expression. Altogether, our results indicate that C3G interferes with at least two separate aspects of oncogenic transformation - cell cycle progression and loss of contact inhibition - and that these inhibitory effects probably account for its transformation suppressor activity.

Transactivation of the insulin-like growth factor-I receptor by angiotensin II mediates downstream signaling from the angiotensin II type 1 receptor to phosphatidylinositol 3-kinase

Angiotensin II (AngII) activates phosphatidylinositol 3-kinase (PI3-kinase), a known effector of receptor tyrosine kinases. Treatment of smooth muscle cells with AngII has also been shown to promote phosphorylation of various tyrosine kinase receptors. We therefore investigated the relationship between AngII and IGF-I receptor activation in smooth muscle cells with a phosphorylation-specific antibody. Our experiments showed that IGF-I receptor phosphorylation was maximally stimulated within 10 min by AngII. Inclusion of an IGF-I-neutralizing antibody in the culture media did not prevent IGF-I receptor phosphorylation after AngII treatment, which argues that a paracrine/autocrine loop is not required. Furthermore, this process was blocked by losartan and 1-(1,1-dimethylethyl)-1-(4-methylphenyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine (PP-1), indicating stimulation of IGF-I receptor phosphorylation occurs via AngII type 1 receptor-dependent activation of Src kinase. The functional significance of IGF-I receptor transactivation was examined with selective inhibitors of the IGF-I receptor kinase (AG1024, AG538). When AngII-treated cells were incubated with AG1024 or AG538, phosphorylation of the regulatory p85 subunit of PI3-kinase was blocked. Furthermore, phosphorylation of the downstream factor p70(S6K) did not occur. In contrast, AG1024 did not prevent MAPK or Src kinase activation by AngII. AG1024 also did not inhibit AngII-dependent cell migration, although this process was blocked by inhibitors of the epidermal growth factor and platelet-derived growth factor receptors. Transactivation of the IGF-I receptor is therefore a critical mediator of PI3-kinase activation by AngII but is not required for stimulation of the MAPK cascade.

Cleavage of p130Cas in anoikis

p130Cas is a multifunctional signaling adaptor protein. It integrates and relays signals generated from a variety of extracellular stimuli and regulates a number of cellular activities including cell death. In this study, we analyzed the regulation and function of p130Cas in anoikis, a type of apoptosis caused by disruption of cell-matrix interactions. We found that p130Cas was specifically cleaved during anoikis in anoikis-sensitive epithelial cells, but not in anoikis-resistant tumor cells. There is a close correlation between p130Cas cleavage and anoikis. Furthermore, we found that the cleavage of p130Cas, as well as another focal adhesion component FAK, is different from that of caspase substrate PARP and spectrin. Although caspases and calpain were found to be involved in the cleavage of p130Cas, there appear to be other unidentified proteases that are mainly responsible for the cleavage of p130Cas, particularly at the early stage of anoikis. Overexpression of the p130Cas cleavage product induced apoptosis. Taken together, these data suggest that there are novel proteases involved in the cleavage of p130Cas during anoikis, which may be functionally involved in the onset of anoikis. p130Cas may have a dual role in the regulation of anoikis. On one hand, it mediates a survival signal from cell-matrix interactions when cells are attached to the extracellular matrix. On the other hand, it participates in executing cell death when cell-matrix interactions are disrupted. These observations provide new insights into the understanding of the function of p130Cas and the molecular mechanism of anoikis.

Clustering-induced tyrosine phosphorylation of nephrin by Src family kinases

BACKGROUND

Nephrin is a recently discovered protein of the immunoglobulin (Ig) superfamily. In the kidney, it is located at the slit diaphragm, which forms the decisive size-selective filter of glomerular ultrafiltration barrier and locates between the interdigitating foot processes of podocytes. Nephrin is mutated in congenital nephrosis of the Finnish type (NPHS1) and has been demonstrated to be an essential component of the slit diaphragm. Based on its domain structure, nephrin is likely to be a cell-cell or cell-matrix adhesion protein that may have a signaling function. In this study, we hypothesized that the clustering of nephrin with antibodies on cell surface mimics the situation where the interaction between nephrin and its extracellular ligand(s) is altered.

METHODS

Nephrin was clustered on the surface of stably transfected HEK293 cells by a monoclonal antinephrin antibody and polyclonal secondary antibody. Clusters were visualized by immunofluorescence microscopy. Changes in protein phosphorylation were studied employing immunoprecipitations and Western blot analysis. A specific inhibitor and cotransfection experiments were used to investigate role of Src family kinases in nephrin phosphorylation.

RESULTS

Clustering of nephrin induced its own tyrosine phosphorylation. This phosphorylation was inhibited by PP2, an inhibitor of Src family kinases. Several members of Src family kinases were able to induce nephrin phosphorylation when cotransfected to HEK293 cells with nephrin. Moreover, the Src family kinase Fyn was consistently found to be coimmunoprecipitated with nephrin. Interestingly, clustering of nephrin induced also tyrosine phosphorylation of a 46 kD protein that was as well found to be coimmunoprecipitated with nephrin.

CONCLUSION

Nephrin is a signaling protein phosphorylated by Src family kinases.

Src-CrkII-C3G-dependent activation of Rap1 switches growth hormone-stimulated p44/42 MAP kinase and JNK/SAPK activities

We demonstrate here that growth hormone (GH) stimulates the activation of Rap1 and Rap2 in NIH-3T3 cells. Full activation of Rap1 and Rap2 by GH necessitated the combined activity of both JAK2 and c-Src kinases, although c-Src was predominantly required. GH-stimulated Rap1 and Rap2 activity was also demonstrated to be CrkII-C3G-dependent. GH stimulated the tyrosine phosphorylation of C3G, which again required the combined activity of JAK2 and c-Src. C3G tyrosine residue 504 was required for GH-stimulated Rap activation. Activated Rap1 inhibited GH-stimulated activation of RalA and subsequent GH-stimulated p44/42 MAP kinase activity and Elk-1-mediated transcription. In addition, we demonstrated that C3G-Rap1 mediated CrkII enhancement of GH-stimulated JNK/SAPK activity. We have therefore identified a linear JAK2-independent pathway switching GH-stimulated p44/42 MAP kinase and JNK/SAPK activities.

Anchorage-independent phosphorylation of p130(Cas) protects lung adenocarcinoma cells from anoikis

The regulation and function of the signaling adaptor protein p130(Cas) in tumor cell anchorage-independent survival, or anoikis resistance, were investigated in human lung adenocarcinoma cells. The tyrosine phosphorylation and function of p130(Cas) during cell detachment were analyzed in tumor cells and compared with that of normal epithelial cells. Cell detachment trigged rapid dephosphorylation of p130(Cas) in the nontumorigenic and anoikis-sensitive normal epithelial cells, but had no effect on the tyrosine phosphorylation of p130(Cas) in the anoikis-resistant lung adenocarcinoma cells. Further analysis revealed that the total tyrosine kinase activities associated with p130(Cas) in the lung tumor cells are anchorage-independent and are significantly higher than that in the normal cells, in which the p130(Cas)-associated tyrosine kinase activities are anchorage-dependent. Analysis of two known p130(Cas)-associated tyrosine kinases FAK and Src indicated that the regulation of tyrosine phosphorylation of FAK and Src are altered in the tumor cells. Inhibition of Src specifically abolished phosphorylation of p130(Cas) and induced anoikis. Furthermore, overexpression of dominant-negative forms of p130(Cas) also induced apoptosis. Taken together, these data suggest that p130(Cas) mediates a cell survival signal from cell-matrix interaction. Alterations in tumor cells that lead to constitutive phosphorylation of p130(Cas) can prevent cells from anoikis, hence contribute to tumor cell anchorage independence and metastasis.

Angiotensin II signaling pathways mediated by tyrosine kinases

Angiotensin II (AngII) plays a critical role in control of cardiovascular and renal homeostasis. In addition to its physiological action as a vasoconstrictor, growing evidence supports the notion that AngII contributes to cardiovascular diseases such as hypertension, atherosclerosis, and heart failure. The physiological and pathological actions of AngII in adults are mediated largely via the AngII type 1 receptor (AT1R), a heterotrimeric G-protein-coupled receptor (GPCR). Besides coupling with heterotrimeric G proteins to activate phospholipase C-beta (PLC-beta), AT1R also activates receptor tyrosine kinases (PDGF-R, EGF-R and IGF-R) and non-receptor tyrosine kinases (Src, Fyn, Yes, proline-rich tyrosine kinase 2 (Pyk2), focal adhesion kinase (FAK) and JAK2). These tyrosine kinases play critical roles in AngII-stimulated cell signal events.

YXXL motifs in SH2-Bbeta are phosphorylated by JAK2, JAK1, and platelet-derived growth factor receptor and are required for membrane ruffling

SH2-Bbeta binds to the activated form of JAK2 and various receptor tyrosine kinases. It is a potent stimulator of JAK2, is required for growth hormone (GH)-induced membrane ruffling, and increases mitogenesis stimulated by platelet-derived growth factor (PDGF) and insulin-like growth factor I. Its domain structure suggests that SH2-Bbeta may act as an adapter protein to recruit downstream signaling proteins to kinase.SH2-Bbeta complexes. SH2-Bbeta is tyrosyl-phosphorylated in response to GH and interferon-gamma, stimulators of JAK2, as well as in response to PDGF and nerve growth factor. To begin to elucidate the role of tyrosyl phosphorylation in the function of SH2-Bbeta, we used phosphopeptide mapping, mutagenesis, and a phosphotyrosine-specific antibody to identify Tyr-439 and Tyr-494 in SH2-Bbeta as targets of JAK2 both in vitro and in intact cells. SH2-Bbeta lacking Tyr-439 and Tyr-494 inhibits GH-induced membrane ruffling but still activates JAK2. We provide evidence that JAK1, like JAK2, phosphorylates Tyr-439 and Tyr-494 in SH2-Bbeta and that PDGF receptor phosphorylates SH2-Bbeta on Tyr-439. Therefore, phosphorylated Tyr-439 and/or Tyr-494 in SH2-Bbeta may provide a binding site for one or more proteins linking cytokine receptor.JAK2 complexes and/or receptor tyrosine kinases to the actin cytoskeleton.

A role for Grb2-associated binder-1 in growth hormone signaling

GH signaling begins with activation of the GH receptor (GHR)-associated cytoplasmic tyrosine kinase, Janus kinase-2. GH-induced Janus kinase-2 activation leads to engagement of several signaling pathways, including the extracellular signal-regulated kinase (ERK), mitogen-activated protein kinase, phosphoinositol 3-kinase, and signal transducer and activator of transcription-5 (STAT5) pathways. Previous work suggests that ERK activation in response to GH may be modulated by several proteins acting as docking molecules, including the epidermal growth factor receptor (EGFR) and insulin receptor substrate-1. In this study we investigate potential roles for the pleckstrin homology (PH) domain-containing insulin receptor substrate-like protein, Grb-2-associated binder-1 (Gab1), in GH signaling. We find in 3T3-F442A preadipocytes that GH promotes tyrosine phosphorylation of Gab1 and its association with SHP2, an Src homology 2-containing cytoplasmic tyrosine phosphatase. The Grb2 adapter protein, in contrast, is specifically coimmunoprecipitated with Gab1, even in the absence of GH exposure. Using a COS-7 cell transient reconstitution system, we observed that GH-induced Gab1 tyrosine phosphorylation is dependent on the Gab1 PH domain, whereas GH-induced coimmunoprecipitation of SHP2 requires tyrosine 627 of Gab1, as previously reported for EGF-induced Gab1-SHP2 association. Deletion of the Gab1 PH domain significantly attenuates GH-induced ERK activation and trans-activation of a c-fos enhancer-driven reporter construct compared with wild-type Gab1 in this system. In contrast, GH-induced STAT5 tyrosine phosphorylation and STAT5-dependent trans-activation are similar in cells expressing wild-type or PH domain-deleted Gab1. Notably, neither the ERK nor the STAT5 GH-dependent signaling outcome is affected by expression of the Gab1 mutant with tyrosine 627 changed to phenylalanine. Finally, we observed GH-dependent translocation of a wild-type, but not a PH domain-deleted, Gab1-green fluorescent protein chimera from the cytoplasm to the plasma membrane. Our results suggest selective involvement of Gab1 in GH-induced ERK activation and implicate the Gab1 PH domain as critical in this involvement.

Identification of a JAK2-independent pathway regulating growth hormone (GH)-stimulated p44/42 mitogen-activated protein kinase activity. GH activation of Ral and phospholipase D is Src-dependent

We have demonstrated here that growth hormone (GH) stimulates the formation of the active GTP-bound form of both RalA and RalB in NIH-3T3 cells. Full activation of RalA and RalB by GH required the combined activity of c-Src and JAK2, both kinases activated by GH independent of the other. Activation of RalA and RalB by growth hormone did not require the activity of JAK2 per se. Ras was also activated by GH and was required for the GH-stimulated formation of GTP-bound RalA and RalB. Activation of RalA by GH subsequently resulted in increased phospholipase D activity and the formation of its metabolite, phosphatidic acid. GH-stimulated RalA-phospholipase D-dependent formation of phosphatidic acid was required for activation of p44/42 MAPK and subsequent Elk-1-mediated transcription stimulated by GH. Thus we report the identification of a JAK2-independent pathway regulating GH-stimulated p44/42 MAPK activity.

c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription

We have previously demonstrated that cellular stimulation with GH results in the formation of a multiprotein signaling complex. One component of this multiprotein signaling complex is the adapter molecule c-Cbl. Here we have examined the role of c-Cbl in the mechanism of GH signal transduction. Forced expression of c-Cbl in NIH3T3 cells did not alter GH-stimulated Janus kinase 2 tyrosine phosphorylation nor GH-stimulated p44/42 MAPK activation and consequent Elk-1- mediated transcription. c-Cbl overexpression did, however, result in enhanced and prolonged GH-stimulated activation of phosphatidylinositol 3-kinase. Forced expression of c-Cbl did not affect GH-stimulated STAT5 tyrosine phosphorylation, nuclear translocation, nor binding to DNA but markedly abrogated GH-stimulated STAT5-mediated transactivation. c-Cbl overexpression resulted in increased ubiquitination and proteosomal degradation of STAT5 and increased degradation of GH-stimulated tyrosine phosphorylated STAT5. Cellular pretreatment with the proteosomal inhibitor MG132 reversed the effect of c-Cbl overexpression with prolonged duration of GH-stimulated STAT5 tyrosine phosphorylation and restoration of STAT5-mediated transcription. Thus, c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription by direction of STAT5 for proteosomal degradation.

UCS15A, a novel small molecule, SH3 domain-mediated protein-protein interaction blocking drug

Protein-protein interactions play critical regulatory roles in mediating signal transduction. Previous studies have identified an unconventional, small-molecule, Src signal transduction inhibitor, UCS15A. UCS15A differed from conventional Src-inhibitors in that it did not alter the levels or the tyrosine kinase activity of Src. Our studies suggested that UCS15A exerted its Src-inhibitory effects by a novel mechanism that involved the disruption of protein-protein interactions mediated by Src. In the present study we have examined the ability of UCS15A to disrupt the interaction of Src-SH3 with Sam68, both in vivo and in vitro. This ability of UCS15A was not restricted to Src-SH3 mediated protein-protein interactions, since the drug was capable of disrupting the in vivo interactions of Sam68 with other SH3 domain containing proteins such as Grb2 and PLCgamma. In addition, UCS15A was capable of disrupting other typical SH3-mediated protein-protein interactions such as Grb2-Sos1, cortactin-ZO1, as well as atypical SH3-mediated protein-protein interactions such as Grb2-Gab1. However, UCS15A was unable to disrupt the non-SH3-mediated protein-protein interactions of beta-catenin, with E-cadherin and alpha-catenin. In addition, UCS15A had no effect on the SH2-mediated interaction between Grb2 and activated Epidermal Growth Factor receptor. Thus, the ability of UCS15A, to disrupt protein-protein interactions appeared to be restricted to SH3-mediated protein-protein interactions. In this regard, UCS15A represents the first example of a non-peptide, small molecule agent capable of disrupting SH3-mediated protein-protein interactions. In vitro analyses suggested that UCS15A did not bind to the SH3 domain itself but rather may interact directly with the target proline-rich domains.

Growth hormone-induced alterations in the insulin-signaling system

Growth hormone (GH) counteracts insulin action on lipid and glucose metabolism. However, the sequence of molecular events leading to these changes is poorly understood. Insulin action is initiated by binding of the hormone to its cell surface receptor (IR). This event activates the intrinsic tyrosine kinase activity residing in the beta-subunit of the IR and leads to autophosphorylation of the cytoplasmic portion of the beta-subunit and further activation of its tyrosine kinase towards several intermediate proteins, including the family of IR substrates (IRS) and the Shc proteins. When tyrosine phosphorylated, these cellular substrates connect the IR with several downstream signaling molecules. One of them is the enzyme phosphatidylinositol (PI) 3-kinase. The insulin antagonistic action of GH is not a consequence of a direct interaction with the IR. Instead, long-term exposure to GH is, in general, associated with hyperinsulinemia, which leads to a reduction of IR levels and an impairment of its tyrosine kinase activity. The signals of GH and insulin may converge at post-receptor levels. The signaling pathway leading to activation of PI 3-kinase appears to be an important site of convergence between the signals of these two hormones and seems to be mediated principally by IRS-1. Rodent models of chronic GH excess have been useful tools to investigate the mechanism by which GH induces insulin resistance. Decreased IR, IRS-1, and IRS-2 tyrosyl phosphorylation in response to insulin was found in skeletal muscle, whereas a chronic activation of the IRS-PI 3-kinase pathway was found in liver. The induction of the expression of proteins that inhibit IR signaling such as suppressors of cytokine signaling (SOCS)-1 and -6 may also be involved in this alteration. Interestingly, the modulation of insulin signaling and action observed in states of GH excess, deficiency, or resistance seems to be relevant to the changes in longevity associated with those states.

Crk family adaptors-signalling complex formation and biological roles

Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. Selective formation of multi-protein complexes by the Crk and Crk-like (CRKL) proteins depends on specific motifs recognized by their SH2 and SH3 domains. In the case of the first SH3 domains [SH3(1)] a P-x-x-P-x-K motif is crucial for highly selective binding, while the SH2 domains prefer motifs which conform to the consensus pY-x-x-P. Crk family proteins are involved in the relocalization and activation of several different effector proteins which include guanine nucleotide releasing proteins like C3G, protein kinases of the Abl- and GCK-families and small GTPases like Rap1 and Rac. Crk-type proteins have been found not only in vertebrates but also in flies and nematodes. Major insight into the function of Crk within organisms came from the genetic model organism C. elegans, where the Crk-homologue CED-2 regulates cell engulfment and phagocytosis. Other biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration and immune cell responses. Crk family adaptors also appear to play a role in mediating the action of human oncogenes like the leukaemia-inducing Bcr-Abl protein. This review summarizes some key findings and highlights recent insights and open questions.

Signal transduction via the growth hormone receptor

Rapid progress has been made recently in the definition of growth hormone (GH) receptor signal transduction pathways. It is now apparent that many cytokines, including GH, share identical or similar signalling components to exert their cellular effects. This review provides a brief discourse on the signal transduction pathways, which have been demonstrated to be utilized by GH. The identification of such pathways provides a basis for understanding the pleiotropic actions of GH. The mechanisms by which the specific cellular effects of GH are achieved remain to be elucidated.

A direct interaction between JNK1 and CrkII is critical for Rac1-induced JNK activation

CrkII, a cellular homolog of v-crk, belongs to a family of adaptor proteins that play a central role in signal transduction cascades. We demonstrate that CrkII interacts directly with c-Jun N-terminal kinase 1 (JNK1). A proline-rich sequence of JNK1 is critical for the interaction of the kinase with the N-terminal Src homology 3 (SH3) domain of CrkII. JNK1 is localized with CrkII in membrane ruffles of Crk-overexpressing cells in a Rac1-dependent manner. A JNK1 mutant (K340A) that fails to interact with CrkII is defective in Rac/epidermal growth factor-induced activation, but remains responsive to UVC irradiation. Furthermore, CrkII recruits JNK1 to a p130Cas multiprotein complex where it may be activated through a hematopoietic progenitor kinase 1- and mitogen-activated protein kinase kinase 4-dependent pathway. Together, the results presented here argue for a new mechanism of regulation of the JNK pathway through the CrkII-p130Cas adaptor complex.

PKC-dependent activation of FAK and src induces tyrosine phosphorylation of Cas and formation of Cas-Crk complexes

SH-SY5Y neuroblastoma cells are a well-characterized model for studying the induction of neuronal differentiation. TPA treatment of these cells induces cytoskeletal rearrangements that ultimately result in neurite extension. However, the signaling pathways that precede these changes are poorly understood. Other investigators have shown that TPA treatment of SH-SY5Y cells results in increased tyrosine phosphorylation of cytoskeletal-associated proteins, including the adapter protein Cas. In this report, we examine the events upstream and downstream of Cas phosphorylation. We show that TPA treatment induces the PKC-dependent association of tyrosine-phosphorylated Cas with Crk. The activity of two protein tyrosine kinases, Src and FAK, was shown to be necessary and sufficient for TPA-induced Cas phosphorylation. We propose that the PKC-dependent phosphorylation of Cas by Src and FAK promotes the establishment of Cas-Crk complexes and that these interactions may play an important role in regulating the actin cytoskeleton during neuronal differentiation.

UCS15A, a non-kinase inhibitor of Src signal transduction

Src tyrosine kinase plays key roles in signal transduction following growth factor stimulation and integrin-mediated cell-substrate adhesion. Since src-signal transduction defects are implicated in a multitude of human diseases, we have sought to develop new ways to identify small molecule inhibitors using a yeast-based, activated-src over-expression system. In the present study, we describe the identification of a unique src-signal transduction inhibitor, UCS15A. UCS15A was found to inhibit the src specific tyrosine phosphorylation of numerous proteins in v-src-transformed cells. Two of these phosphoproteins were identified as bona-fide src substrates, cortactin and Sam68. UCS15A differed from conventional src-inhibitors in that it did not inhibit the tyrosine kinase activity of src. In addition, UCS15A appeared to differ from src-destabilizing agents such as herbimycin and radicicol that destabilize src by interfering with Hsp90. Our studies suggest that UCS15A exerted its src-inhibitory effects by a novel mechanism that involved disruption of protein-protein interactions mediated by src. One of the biological consequences of src-inhibition by UCS15A was its ability to inhibit the bone resorption activity of osteoclasts in vitro. These data suggest that UCS15A may inhibit the bone resorption activity of osteoclasts, not by inhibiting src tyrosine kinase activity, but by disrupting the interaction of proteins associated with src, thereby modulating downstream events in the src signal transduction pathway.

Cell adhesion and focal adhesion kinase regulate insulin receptor substrate-1 expression

Integrins are transmembrane receptors involved in interactions between cells and extracellular matrix proteins. Here we show that cell adhesion regulates insulin receptor substrate-1 (IRS-1) mRNA synthesis. When fibroblasts are held in suspension, lower levels of IRS-1 mRNA, but not of IRS-2 mRNA, are detected, and this effect is due to the negative regulation of IRS-1 transcription rather than to decreased mRNA stability. Upon fibronectin- or vitronectin-mediated integrin stimulation, the level of IRS-1 mRNA was restored within 4 h. The focal adhesion kinase (FAK) is known to be activated upon integrin stimulation, and we found that IRS-1 was not expressed in FAK(-)(/-) cells. Stable re-expression of epitope-tagged FAK in FAK(-)(/-) fibroblasts (DA2 cells) restored normal levels of IRS-1 expression, confirming that IRS-1 mRNA expression is regulated by FAK. It is known that integrins activate the JNK pathway. However, in adherent FAK(-)(/-) cells, we failed to detect activation of JNK, whereas JNK was stimulated in DA2 cells. This confirms the role of FAK in integrin-induced JNK stimulation. FAK-independent stimulation of JNK with anisomycin treatment both in FAK(-)(/-) cells and in suspended FAK(+/+) cells confirmed that IRS-1 mRNA transcription can be partially regulated by JNK. We suggest that integrins can modulate insulin and insulin-like growth factor-1 signaling pathways by regulating the levels of IRS-1 in cells and that FAK-mediated signaling to JNK is one pathway involved in this process.

Angiotensin II mediated signal transduction. Important role of tyrosine kinases

It has been 100 years since the discovery of renin by Bergman and Tigerstedt. Since then, numerous studies have advanced our understanding of the renin-angiotensin system. A remarkable aspect was the discovery that angiotensin II (AngII) is the central product of the renin-angiotensin system and that this octapeptide induces multiple physiological responses in different cell types. In addition to its well known vasoconstrictive effects, growing evidence supports the notion that AngII may play a central role not only in hypertension, but also in cardiovascular and renal diseases. Binding of AngII to the seven-transmembrane angiotensin II type 1 receptor is responsible for nearly all of the physiological actions of AngII. Recent studies underscore the new concept that activation of intracellular second messengers by AngII requires tyrosine phosphorylation. An increasing number of tyrosine kinases have been shown to be activated by AngII, including the Src kinase family, the focal adhesion kinase family, the Janus kinases and receptor tyrosine kinases. These actions of AngII contribute to the pathophysiology of cardiac hypertrophy and remodeling, vascular thickening, heart failure and atherosclerosis. In this review, we discuss the important role of tyrosine kinases in AngII-mediated signal transduction. Understanding the importance of tyrosine phosphorylation in AngII-stimulated signaling events may contribute to new therapies for cardiovascular and renal diseases.

The mechanism of effect of growth hormone on preadipocyte and adipocyte function

Growth hormone (GH) is not only the major regulator of postnatal somatic growth but also exerts profound effects on body composition through a combination of anabolic, lipolytic and antinatriuretic actions. GH enhancement of the lipolytic activity of adipose tissue in combination with a reduction of triglyceride accumulation via inhibition of lipoprotein lipase activity appears to be the major mechanism by which GH results in a reduction of the total fat mass. Recently, much progress has been made in understanding the molecular mechanism by which GH affects cellular function. This review provides a brief discourse and summary of the mechanism of effects of GH on preadipocyte/adipocyte function. It is intended to provide a functional understanding of the mechanism of action of GH as it relates to adipogenesis and adipocyte function.

c-Src signaling induced by the adapters Sin and Cas is mediated by Rap1 GTPase

Oncogenic Src proteins have been extensively studied to gain insight into the signaling mechanisms of Src. To better understand signaling through wild-type Src, we used an approach that involves activation of Src signaling through the binding of physiologic ligands to the Src SH3 domain. To this end, we used full-length and truncated versions of the multiadapter molecules Cas and Sin to activate c-Src, and we examined the intracellular pathways that mediate Src signaling under these conditions. We show that although all proteins bind to and are phosphorylated by c-Src, quantitative differences exist in the ability of the different ligands to activate c-Src signaling. In addition, we show that Sin- and Cas-induced Src signaling, as assayed by transcriptional activation, is exclusively mediated through a pathway that involves the adapter Crk and the GTP-binding protein Rap1. These data are in contrast to previous observations showing Ras to mediate signaling downstream of transforming Src alleles. In our system, we found that signaling through the oncogenic SrcY527 mutant is indeed mediated by Ras. In addition, we found that Rap1 also mediates oncogenic Src signaling. Our results show for the first time that Rap1 mediates c-Src kinase signaling and reveal mechanistic differences in the signaling properties of wild-type and transforming Src proteins.

CrkII participation in the cellular effects of growth hormone and insulin-like growth factor-1. Phosphatidylinositol-3 kinase dependent and independent effects

We have examined the role of CrkII in the cellular response to both human growth hormone (hGH) and human insulin-like growth factor-1 (hIGF-1). We have demonstrated that overexpression of the adaptor molecule enhances both basal phosphatidylinositol 3-kinase (PI 3-kinase) activity and also dramatically enhances the ability of both hormones to stimulate PI 3-kinase activity in the cell. Many of the effects of CrkII overexpression on hGH- and hIGF-1-stimulated cellular function can then be attributed to CrkII enhancement of PI 3-kinase stimulation by these hormones. Thus, CrkII-enhanced PI 3-kinase activity is used to enhance actin filament reorganization in response to both hGH and hIGF-1, to enhance stress activated protein kinase (SAPK) activity in response to hGH, and to diminish STAT5-mediated transcription in response to hGH. It is apparent, however, that CrkII also regulates cellular function independent of its ability to stimulate PI 3-kinase activity. This is evidenced by the ability of CrkII, in a PI 3-kinase-independent manner, to diminish the activation of p44/42 mitogen-activated protein kinase in response to both hGH and hIGF-1 and to inhibit the activation of SAPK by hIGF-1. Therefore, despite the common use of CrkII to activate PI 3-kinase, CrkII also allows hGH or hIGF-1 to selectively switch the activation of SAPK. Thus, common utilization of CrkII by hGH and hIGF-1 allows the execution of common cellular effects of these hormones, concomitant with the retention of hormonal specificity.

The role of STAT proteins in growth hormone signaling

Growth hormone (GH) has long been known to be the body's primary regulator of body growth and a regulator of metabolism, yet the mechanisms by which GH regulates the transcription of specific genes required for these processes are just now being delineated. GH binding to its receptor recruits and activates the receptor-associated JAK2 that in turn phosphorylates tyrosines within itself and the GH receptor. These tyrosines form binding sites for a number of signaling proteins, including members of the family of signal transducers and activators of transcription (STAT). Among the known signaling molecules for GH, STAT proteins play a particularly prominent role in the regulation of gene transcription. This paper will review what is currently understood about which STAT proteins are regulated by GH, how they are regulated by GH, the GH-dependent genes they regulate, and discuss current theories about how GH-activated STAT signaling is regulated. Particular attention will be given to the novel role that STAT5 plays in sexually dimorphic gene expression in the liver as determined by the secretory pattern of GH and the role of STAT5 in body growth. Oncogene (2000).

SH2-B is required for growth hormone-induced actin reorganization

The Src homology-2 (SH2) domain-containing protein SH2-Bbeta is a substrate of the growth hormone (GH) receptor-associated tyrosine kinase JAK2. Here we tested whether SH2-Bbeta is involved in GH regulation of the actin cytoskeleton. Based on cell fractionation and confocal microscopy, we find SH2-Bbeta present at the plasma membrane and in the cytosol. SH2-Bbeta colocalized with filamentous actin in GH and platelet-derived growth factor (PDGF)-induced membrane ruffles. To test if SH2-Bbeta is required for actin reorganization, we transiently overexpressed wild-type or mutant SH2-Bbeta in 3T3-F442A cells and assayed for GH- and PDGF-induced membrane ruffling and fluid phase pinocytosis. Overexpression of wild-type SH2-Bbeta enhanced ruffling and pinocytosis produced by submaximal GH but not submaximal PDGF. Point mutant SH2-Bbeta (R555E) and truncation mutant DeltaC555, both lacking a functional SH2 domain, inhibited membrane ruffling and pinocytosis induced by GH and PDGF. Mutant DeltaN504, which possesses a functional SH2 domain and enhances JAK2 kinase activity in overexpression systems, also inhibited GH-stimulated membrane ruffling. DeltaN504 failed to inhibit GH-induced nuclear localization of Stat5B, indicating JAK2 is active in these cells. Taken together, these results show that SH2-Bbeta is required for GH-induced actin reorganization by a mechanism discrete from the action of SH2-Bbeta as a stimulator of JAK2 kinase activity.

Growth hormone, insulin-like growth factor I and the CNS: localization, function and mechanism of action

The growth hormone (GH) receptor and binding protein are synthesized in the CNS and are regulated differentially to their hepatic counterparts. GH is also synthesized in the CNS and is regulated differentially to its hypophyseal counterpart. Insulin-like growth factor I (IGF-I) is synthesized in the CNS and in the early postnatal period is regulated by peripherally secreted GH. Both GH and IGF-I alter the size and morphology of the CNS during development and affect differentiated cell function in the CNS, with consequent modulation of cognitive function. Differential utilization of the same signal transduction molecules indicates that GH and IGF-I possess distinct overlapping roles in CNS function.

Autocrine human growth hormone enhancement of human mammary carcinoma cell spreading is Jak2 dependent

We investigated the role of autocrine production of human (h) GH in the attachment and spreading of mammary carcinoma cells in vitro. We used a previously described model system for the study of the autocrine/paracrine role of GH in which the hGH gene (MCF-hGH) or a translation-deficient hGH gene (MCF-MUT) was stably transfected into MCF-7 cells. No differences in attachment to a collagen matrix between MCF-hGH and MCF-MUT cells were observed in either serum-free medium (SFM) or medium containing exogenous hGH, 5% serum, or 10% serum. In contrast, MCF-hGH cells spread more rapidly on a collagen matrix than did MCF-MUT cells. Exogenous hGH and 10% serum interacted with autocrine production of hGH in an additive manner to increase cell spreading. MCF-hGH cells formed filipodia and stress fibers earlier than MCF-MUT cells during the process of cell spreading and possessed marked differences in morphology after spreading. MCF-MUT cells displayed uniform and symmetrical formation of stress fibers, whereas MCF-hGH cells displayed irregular and elongated stress fiber formation. The level of cytoplasmic phosphotyrosine was increased in MCF-hGH compared with MCF-MUT cells during spreading and displayed colocalization with Janus kinase 2 (JAK2). Basal JAK2 tyrosine phosphorylation was increased, and it increased further on spreading in MCF-hGH cells compared with MCF-MUT cells. Transient transfection of JAK2 complementary DNA resulted in interaction with autocrine hGH to increase the rate of cell spreading in MCF-hGH cells compared with MCF-MUT cells. Treatment with a selective JAK2 tyrosine kinase inhibitor (AG 490) reduced the rate of MCF-hGH cell spreading to the rate of MCF-MUT cell spreading. Thus, we conclude that autocrine production of hGH enhances the rate of mammary carcinoma cell spreading in a JAK2-dependent manner.

Adaptor proteins CRK and CRKL associate with the serine/threonine protein kinase GCKR promoting GCKR and SAPK activation

STE20-related kinases play significant regulatory roles in a range of cellular responses to environmental stimuli. GCKR (also referred to as KHS1) is a serine/threonine protein kinase that has an STE20-like protein kinase domain and that stimulates the stress-activated protein kinase (SAPK, also referred to as Jun kinase or JNK) pathway. GCKR has a large C-terminal regulatory domain that provides sites for interactions with other proteins. Adaptor proteins mediate the interactions between signaling molecules. In this study we showed that the adaptor proteins Crk and CrkL associated with GCKR. When Crk-I, Crk-II, or CrkL was transiently expressed in HEK 293T cells along with GCKR, each coimmunoprecipitated with GCKR. Furthermore, in the Bcr-Abl transformed cell line, K562 endogenous GCKR and CrkL coimmunoprecipitated, indicating a constitutive association. Detection of the CrkL-GCKR interaction required the SH3 domains of CrkL and 2 regions in GCKR—1 between amino acids 387 and 395 that contains a consensus SH3 binding motif and the other between amino acids 599 and 696. Crk or CrkL overexpression increased GCKR catalytic activity. A dominant negative form of Ras abolished Crk- or CrkL-induced GCKR activation, suggesting a dependence on Ras activation for their activation of GCKR. Finally, we showed impairment of the known ability of CrkL to activate the SAPK pathway by a catalytically inactive form of GCKR or by a GCKR antisense construct. Thus, GCKR associates with other proteins through interactions mediated by SH2/SH3 adaptor proteins, which can lead to GCKR and SAPK activation.