Tyrosine-phosphorylated proteins: mediators of signal transduction from the tyrosine kinases

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

Phosphosite-Specific Antibodies: A Brief Update on Generation and Applications

Phosphate addition is a posttranslational modification of proteins, and this modification can affect the activity and other properties of intracellular proteins. Different animal species can be used to generate phosphosite-specific antibodies as either polyclonals or monoclonals, and each approach offers its own benefits and disadvantages. The validation of phosphosite-specific antibodies requires multiple techniques and tactics to demonstrate their specificity. These antibodies can be used in arrays, flow cytometry, and imaging platforms. The specificity of phosphosite-specific antibodies is vital for their use in proteomics and profiling of disease.

EGFR(s) in aging and carcinogenesis of the gastrointestinal tract

Cells of the gastrointestinal (GI) mucosa are subject to a constant process of renewal which, in normal adults, reflects a balance between the rates of cell production and cell loss. Detailed knowledge of these events is, therefore, essential for a better understanding of the normal aging processes as well as many GI diseases, particularly malignancy, that represent disorders of tissue growth. In general, many GI dysfunctions, including malignancy, increase with advancing age, and aging itself is associated with alterations in structural and functional integrity of the GI tract. Although the regulatory mechanisms for age-related increase in the incidence of GI-cancers are yet to be fully delineated, recent evidence suggests a role for epidermal growth family receptors and its family members {referred to as EGFR(s)} in the development and progression of carcinogenesis during aging. The present communication discusses the involvement of EGFR(s) in regulating events of GI cancers during advancing age and summarizes the current available therapeutics targeting these receptors. The current review also describes the effectiveness of ErbB inhibitors as well as combination therapies. Additionally, the involvement of GI stem cells in the development of the age-related rise in GI cancers is emphasized.

Overview of the generation, validation, and application of phosphosite-specific antibodies

Protein phosphorylation is a universal key posttranslational modification that affects the activity and other properties of intracellular proteins. Phosphosite-specific antibodies can be produced as polyclonals or monoclonals in different animal species, and each approach offers its own benefits and disadvantages. The validation of phosphosite-specific antibodies requires multiple techniques and tactics to demonstrate their specificity. These antibodies can be used in arrays, flow cytometry, and imaging platforms. The specificity of phosphosite-specific antibodies is key for their use in proteomics and profiling of disease.

Magnolol induces apoptosis via inhibiting the EGFR/PI3K/Akt signaling pathway in human prostate cancer cells

We observed that treatment of prostate cancer cells for 24 h with magnolol, a phenolic component extracted from the root and stem bark of the oriental herb Magnolia officinalis, induced apoptotic cell death in a dose- and time-dependent manner. A sustained inhibition of the major survival signal, Akt, occurred in magnolol-treated cells. Treatment of PC-3 cells with an apoptosis-inducing concentration of magnolol (60 microM) resulted in a rapid decrease in the level of phosphorylated Akt leading to inhibition of its kinase activity. Magnolol treatment (60 microM) also caused a decrease in Ser((136)) phosphorylation of Bad (a proapoptotic protein), which is a downstream target of Akt. Protein interaction assay revealed that Bcl-xL, an anti-apoptotic protein, was associated with Bad during treatment with magnolol. We also observed that during treatment with magnolol, translocation of Bax to the mitochondrial membrane occurred and the translocation was accompanied by cytochrome c release, and cleavage of procaspase-8, -9, -3, and poly(ADP-ribose) polymerase (PARP). Similar results were observed in human colon cancer HCT116Bax(+/-) cell line, but not HCT116Bax(-/-) cell line. Interestingly, at similar concentrations (60 microM), magnolol treatment did not affect the viability of normal human prostate epithelial cell (PrEC) line. We also observed that apoptotic cell death by magnolol was associated with significant inhibition of pEGFR, pPI3K, and pAkt. These results suggest that one of the mechanisms of the apoptotic activity of magnolol involves its effect on epidermal growth factor receptor (EGFR)-mediated signaling transduction pathways.

Development of a Cell-Based Assay for Measurement of c-Met Phosphorylation Using AlphaScreenTMTechnology and High-Content Imaging Analysis

c-Met is a receptor tyrosine kinase (RTK) with a critical role in many fundamental cellular processes, including cell proliferation and differentiation. Deregulated c-Met signaling has been implicated in both the initiation and progression of human cancers and therefore represents an attractive target for anticancer therapy. Monitoring the phosphorylation status of relevant tyrosine residues provides an important method of assessing c-Met kinase activity. This report describes a novel assay to monitor c-Met phosphorylation in cells using Amplified Luminescent Proximity Homogeneous Assay (AlphaScreen™) technology. Using AlphaScreen™, the authors were able to detect both global and site-specific phosphorylation of c-Met in transformed cell lines. Data obtained from the AlphaScreen™ assay were compared to data obtained from a high-content imaging (HCI) method developed in parallel to monitor c-Met phosphorylation at the single cell level. The AlphaScreen™ assay was miniaturized to a 384-well format with acceptable signal-to-background ratio (S/B) and Z′ statistics and was employed to measure c-Met kinase activity in situ after treatment with potent c-Met-specific kinase inhibitors. The authors discuss the utility of quantifying endogenous cellular c-Met phosphorylation in lead optimization and how the modular design of the AlphaScreen™ assay allows its adaptation to measure cellular activity of other kinases. ( Journal of Biomolecular Screening 2009:404-411)

Regulation of endothelial barrier function by reactive oxygen and nitrogen species

Excessive generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), by activated neutrophils and endothelial cells, has been implicated in the pathophysiology of endothelial barrier dysfunction. Disruption of the integrity of this barrier markedly increases permeability to fluids, solutes and inflammatory cells and is the hallmark of many disorders such as acute lung injury (ALI) and sepsis. There has been considerable progress in our understanding of the sequence of molecular and structural events that mediate the response of endothelial cells to oxidants and nitrosants. In addition, substantial experimental evidence demonstrates improvement of endothelial barrier dysfunction with antioxidant strategies. However, no significant benefits have been observed, so far, in clinical trials of antioxidants for the treatment of endothelial barrier dysfunction. This article will review the available evidence implicating ROS and RNS in endothelial barrier dysfunction, explore potential underlying mechanisms, and identify areas of further research.

Mechanisms of curcumin- and EGF-receptor related protein (ERRP)-dependent growth inhibition of colon cancer cells

Numerous dietary and pharmacological agents have been proposed as alternative strategies for treatment and prevention of colorectal cancer. Curcumin, an active ingredient of turmeric, that inhibits growth of malignant neoplasms, has a promising role in the prevention and treatment of colorectal cancer. EGF-R related protein (ERRP), a recently identified pan-erbB inhibitor, is a potential therapeutic agent for colorectal cancer. Here we examine whether curcumin together with ERRP will cause a greater inhibition of growth of colon cancer cells than either agent alone and the mechanisms of this inhibition. Human colon cancer HCT-116 or HT-29 cells were incubated with increasing doses of curcumin (up to 10 microM) or ERRP (up to 5 microg/ml), or a combination of both for 48 h. We observed that the cell growth inhibition and stimulation of apoptosis in response to the combinatorial treatment was significantly greater than that caused by either agent alone. These changes were associated with decreased activation (tyrosine phosphorylation) of EGFR, ErbB-2, ErbB-3, and/or IGF-1R. Whereas curcumin inhibited constitutive activation of both EGFR and IGF-1R, ERRP decreased activation of EGFR, ErbB-2, and ErbB-3 but had no effect on IGF-1R. Further, the combination therapy caused a greater attenuation of downstream effectors such as NF-kappaB, Akt and BAD activation, and down-regulation of procaspase-3 than that noted with either agent alone. The superior effects of the combinatorial treatment could partly be attributed to inhibition of constitutive activation of EGFRs and IGF-1R signaling pathways.

Therapeutic potential of EGFR-related protein, a universal EGFR family antagonist

Interference with the activation of growth factor receptors, specifically epidermal growth factor receptor (EGFR) and/or other member(s) of its family (human epidermal growth factor [HER]-2, -3 and -4) represents a promising strategy for development of novel and selective anticancer therapies. Indeed, a number of inhibitors that target either EGFR or HER-2, but not both, have been developed for treatment of epithelial cancers. However, since most solid tumors express different EGFRs, identification of inhibitor(s) targeting multiple EGFR family members may provide a therapeutic benefit to a broader patient population. To this end, the author proposes that EGFR-related protein (ERRP), a recently isolated negative regulator of EGFR that possesses a substantial homology to the extracellular ligand-binding domain of EGFR and its family members, is a pan-ErbB inhibitor that targets multiple members of the EGFR family. This review discusses the significance of EbB [corrected] family of receptors in epithelial cancers, and describes isolation, characterization and the mechanisms of action of ERRP as well as its potential application as a therapeutic agent for a wide variety of epithelial cancers.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Cell cycle- and apoptosis-regulatory protein-1 is involved in apoptosis signaling by epidermal growth factor receptor

CARP-1, a novel apoptosis inducer, regulates apoptosis signaling by diverse agents, including adriamycin and growth factors. Epidermal growth factor receptor (EGFR)-related protein (ERRP), a pan-ErbB inhibitor, inhibits EGFR and stimulates apoptosis. Treatments of cells with ERRP or Iressa (an EGFR tyrosine kinase inhibitor) results in elevated CARP-1 levels, whereas antisense-dependent depletion of CARP-1 causes inhibition of apoptosis by ERRP. CARP-1 is a tyrosine-phosphorylated protein, and ERRP treatments cause elevated tyrosine phosphorylation of CARP-1. CARP-1 contains multiple, nonoverlapping apoptosis-inducing subdomains; one such subdomain is present within amino acids 1-198. Wild-type or CARP-1-(1-198) proteins that have substitution of tyrosine 192 to phenylalanine abrogate apoptosis by ERRP. In addition, apoptosis mediated by wild type or CARP-1-(1-198), and not CARP-1-(1-198(Y192F)), results in activation of caspase-9 and increased phosphorylation of p38 MAPK. However, the expression of dominant-negative forms of p38 MAPK activators MKK3 or MKK6 proteins inhibits apoptosis induced by both the full-length and truncated (amino acids 1-198) proteins. Together, data demonstrate that tyrosine 192 of CARP-1 is a target of apoptosis signaling, and CARP-1, in turn, promotes apoptosis by activating p38 MAPK and caspase-9.

Is methylglyoxal a causative factor for hypertension development?This paper is one of a selection of papers published in this Special Issue, entitled Young Investigator's Forum

Hypertension is a life-threatening disease that is associated with increased cardiovascular risks. Causes and mechanisms for hypertension development remain poorly understood. Methylglyoxal (MG), a highly reactive molecule, is a metabolite of sugar. Increased circulation and tissue levels of MG have been documented not only in diabetes but also in hypertension. Many recent studies also link MG-induced vascular damage to the pathogenic process of hypertension. As such, an etiological role of MG in hypertension development is proposed.

Epidermal growth factor receptor (EGFR)-related protein inhibits multiple members of the EGFR family in colon and breast cancer cells

Inactivation of epidermal growth factor receptor (EGFR) family members represents a promising strategy for the development of selective therapies against epithelial cancers. Current anti-EGFR therapies, such as cetuximab (Erbitux), gefitinib (Iressa), or trastuzumab (Herceptin), target EGFR or HER-2 but not both. Because solid tumors express different EGFRs, identification of inhibitor(s), targeting multiple EGFR family members may provide a therapeutic benefit to a broader patient population. We have identified a natural inhibitor of EGFRs called EGFR-related protein (ERRP), a 53 to 55 kDa protein that is present in most, if not all, normal human epithelial cells. The growth of colon (HCT-116, Caco2, and HT-29) and breast (MDA-MB-468 and SKBR-3) cancer cells expressing varying levels of EGFR, HER-2, and/or HER-4 was inhibited by recombinant ERRP in a dose-dependent manner. In contrast, ERRP caused no inhibition of growth of normal mouse fibroblast cell lines (NIH-3T3, NIH-3T3/P67), and the growth of nontransformed rat small intestinal IEC-6 cells expressing relatively low levels of EGFRs was inhibited only at high doses of ERRP. Transforming growth factor-alpha or heparin-binding epidermal growth factor-induced activation of EGFR and HER-2 was inhibited by ERRP in colon and breast cancer cells expressing high levels of EGFR or HER-2. In contrast, cetuximab inhibited the growth- and ligand-induced activation of EGFR in cell lines expressing high levels of EGFR, whereas trastuzumab was effective only in HER-2-overexpressing cells. ERRP and trastuzumab, but not cetuximab, attenuated heregulin-alpha-induced activation of colon and breast cancer cells that expressed high levels of HER-2. Furthermore, ERRP, but not cetuximab or trastuzumab, significantly induced apoptosis of colon and breast cancer cells. None of these agents induced apoptosis of either NIH-3T3 mouse fibroblast or normal rat small intestinal IEC cells. Our results suggest that ERRP is an effective pan-erbB inhibitor and, thus, may be a potential therapeutic agent for a wide variety of epithelial cancers expressing different levels and subclasses of EGFRs.

Epidermal growth factor receptor–related peptide inhibits growth of PC-3 prostate cancer cells

Interference with the activation of growth factor receptors, specifically epidermal growth factor receptor (EGFR), represents a promising strategy for the development of novel and selective anticancer therapies. We reported that EGFR-related peptide (ERRP), a recently isolated negative regulator of EGFR, could be a potential therapeutic agent for colorectal cancer. To determine whether ERRP could potentially be a therapeutic agent for prostate carcinoma, we examined the effect of recombinant ERRP on the growth of the prostate cancer cell line PC-3 in vitro. Events of the EGFR signal transduction pathways were also examined. ERRP caused a marked inhibition of cell growth in a dose- and time-dependent manner and also induced apoptosis. The latter was evidenced by increased number of apoptotic cells, activation of caspase-3, and cleavage of poly(ADP-ribose)polymerase. The transforming growth factor-α–induced stimulation of cell growth and activation of EGFR was also inhibited by ERRP. These changes were accompanied by a concomitant attenuation of activation of Akt and mitogen-activated protein kinases as well as basal and transforming growth factor-α–induced activation of nuclear factor-κB. Inhibition of EGFR activation by ERRP could be partly attributed to increased sequestration of EGFR ligands. In summary, our data show that ERRP inhibits the growth of prostate cancer cells by attenuating EGFR signaling processes. ERRP could potentially be an effective therapeutic agent for prostate cancer.

Tyrosine kinase inhibitors suppress alpha1-adrenoceptor mediated contraction in human radial, internal mammary arteries and saphenous vein

The aim of this study is to determine whether the tyrosine kinase plays a role in the contractile response of human radial artery (RA), internal mammary artery (IMA) and saphenous vein (SV) to alpha(1)-adrenoceptor (AR) stimulation. The tyrosine kinase inhibitors, genistein and tyrphostin, significantly inhibited alpha(1)-AR mediated contractile response in a dose-dependent and non-competitive manner. Genistein at 10 microM inhibited 39%, 54% and 72% of PE-induced maximum contraction, and tyrphostin at 50 microM inhibited 41%, 68% and 39% of the contraction in the human RA, IMA and SV respectively. These results suggest that tyrosine kinases participate in regulation of signal transduction that is associated with alpha(1)-AR mediated contractile response in human blood vessels.

Involvement of protein tyrosine phosphorylation and reduction of cellular sulfhydryl groups in cell death induced by 1' -acetoxychavicol acetate in Ehrlich ascites tumor cells

Elucidation of the mechanisms underlying potential anticancer drugs continues and unraveling these mechanisms would not only provide a conceptual framework for drug design but also promote use of natural products for chemotherapy. To further evaluate the efficacy of the anticancer activity of 1'-acetoxychavicol acetate (ACA), this study investigates the underlying mechanisms by which ACA induces death of Ehrlich ascites tumor cells. ACA treatment induced loss of cell viability, and Western blotting analysis revealed that the compound stimulated tyrosine phosphorylation of several proteins with 27 and 70 kDa proteins being regulated in both dose- and time-dependent manner prior to loss of viability. Protein tyrosine kinase inhibitor herbimycin A moderately protected cells from ACA-induced toxicity. In addition, cellular glutathione and protein sulfydryl groups were also significantly reduced both dose- and time-dependently during evidence of cell death. Replenishing thiol levels by antioxidant, N-acetylcysteine (NAC), an excellent supplier of glutathione and precursor of glutathione, substantially recovered the viability loss, but the recovery being time-dependent, as late addition of NAC (at least 30 min after ACA addition to cultures) was, however, ineffective. Addition of NAC to ACA treated cultures also abolished tyrosine phosphorylation of the 27 kDa protein. These results, at least partly, identify cellular sulfhydryl groups and protein tyrosine phosphorylation as targets of ACA cytotoxicity in tumor cells.

Mitogen-activated protein kinase plays an important role in hemolysate-induced contraction in rabbit basilar artery

OBJECT

Mitogen-activated protein kinase (MAPK) is an important signaling factor in the vascular proliferation and contraction, the two features of cerebral vasospasm following subarachnoid hemorrhage. We studied the possible involvement of MAPK in hemolysate-induced signal transduction and contraction in rabbit basilar artery.

METHODS

Isometric tension was used to record the contractile response of rabbit basilar artery to hemolysate. Western blots using antibodies for MAPK were conducted. 1) Hemolysate produced a concentration-dependent contraction of rabbit basilar artery. Pre-incubation of arteries with MAPK kinase inhibitor PD-98059 markedly reduced the contraction induced by hemolysate. PD-98059 also relaxed, in a concentration-dependent fashion, the sustained contraction induced by hemolysate (10%). 2) Hemolysate produced a time-dependent elevation of MAPK immunoreactivity in Western blot in rabbit basilar artery. MAPK was enhanced 3 min after hemolysate exposure and the effect reached maximum at 5 min. The immunoreactivity of MAPK decayed slowly with time, but the level of MAPK was still higher than the basal level even at two hours after exposure to hemolysate. 3) Pre-incubation of arteries with MAPK kinase inhibitor PD-98059 abolished the effect of hemolysate on MAPK immunoreactivity.

CONCLUSION

Hemolysate produced contraction of rabbit basilar artery possibly by activation of MAPK. MAPK inhibitors may be useful in the treatment of cerebral vasospasm.

Caveolin-1 and a 29-kDa caveolin-associated protein are phosphorylated on tyrosine in cells expressing a temperature-sensitive v-Abl kinase

Caveolin-1 was originally identified as a tyrosine-phosphorylated protein in v-Src-transformed cells and it was suggested that phosphorylation of this protein could mediate transformation by the tyrosine kinase class of oncogenes (J. R. Glenney, 1989, J. Biol. Chem. 264, 20163--20166). We found that caveolin-1 is also phosphorylated on tyrosine in v-Abl-transformed cells. In fact, caveolin-1 and a caveolin-associated protein of 29 kDa are among the strongest phosphotyrosine signals detected in the Abl-expressing cells. In addition, v-Abl shows a preferential phosphorylation of caveolin-1 and the 29-kDa caveolin-associated protein over other proteins in the caveolin-enriched Triton-resistant cell fraction. These data indicate that caveolin-1 and the 29-kDa caveolin-associated protein may be preferred substrates of the Abl kinase. Caveolin-1 is phosphorylated at tyrosine 14 in v-Abl-expressing cells as has been observed previously in v-Src-expressing cells. However, using a temperature-sensitive allele of v-Abl (ts120 v-Abl) we provide evidence that caveolin-1 phosphorylation is not sufficient to mediate the loss of caveolin expression or loss of cell adhesion induced by v-Abl.

Changes in cytoskeletal organization in polyoma middle T antigen-transformed fibroblasts: involvement of protein phosphatase 2A and src tyrosine kinases

The major transforming activity of polyomavirus, middle T antigen, targets several cellular regulatory effectors including protein phosphatase 2A and src tyrosine kinases. Although transformed cells exhibit profound morphological changes, little is known about how middle T antigen-induced changes in the cellular regulatory environment specifically affect the cytoskeleton. We have investigated these changes in 10T(1/2) mouse fibroblasts transformed with polyoma middle T antigen. Immunofluorescence microscopy revealed that expression of middle T antigen (Pym T cells) depleted the stable (acetylated) microtubule array and increased the sensitivity of dynamic (tyrosinated) microtubules to nocodazole-induced disassembly. These effects were associated with a modest but statistically significant (P</=0.05) increase in recovery of protein phosphatase 2A activity with microtubules. Middle T antigen expression also depleted the normal cellular complement of actin stress fibers and focal adhesions, in parallel with changes in the distribution of src tyrosine kinases. Herbimycin A promoted recovery of paxillin and phosphotyrosine into nascent focal adhesion sites, in addition to restoring normal src tyrosine kinase distribution. However, herbimycin A did not restore actin stress fibers or parental-type microtubules to Pym T cells. We suggest that regulation of the microtubule array by middle T antigen may occur through direct effects including redistribution of protein phosphatase 2A as well as indirect effects such as altered interactions with actin-based stress fibers.

c-Src association with and phosphorylation of p58gag, a membrane- and microfilament-associated retroviral Gag-like protein in a xenotransplantable rat mammary tumor

The retroviral Gag-like protein p58gag expressed in a highly metastatic ascites rat mammary adenocarcinoma has been implicated in cell surface changes contributing to xenotransplantability. p58gag is present in the cells in a plasma membrane- and microfilament-associated signal transduction particle containing Src and is phosphorylated on tyrosine. Overlay analyses and affinity chromatography with glutathione S-transferase (GST) fusion proteins of Src homology-3 (SH3) domains showed direct binding of the Src but not the Crk SH3 domain to p58gag. This association was confirmed by co-immunoprecipitation of partially purified p58gag from ascites cell lysates with platelet Src. Further, a GST-p58gag fusion protein bound full length c-Src from either platelets or c-Src-expressing insect cells. The GST-p58gag fusion protein, but not GST, was phosphorylated by platelet or insect cell-expressed c-Src, but not by a kinase negative c-Src variant. The binding of GST-p58gag to c-Src was almost completely abolished by a 50-fold excess of the GST-SH3 domain of Src, and a parallel decrease in tyrosine phosphorylation of p58gag was observed. These results demonstrate that p58gag is tyrosine-phosphorylated as a consequence of its specific association with c-Src via its SH3 domain. These observations suggest a mechanism by which Gag proteins may contribute to retroviral maturation or pathogenesis through binding and relocalization of SH3 domain-containing proteins such as Src-like tyrosine kinases to sites of association of microfilaments with the plasma membrane.

Mitogen-activated protein kinase mediation of hemolysate-induced contraction in rabbit basilar artery

OBJECT

Mitogen-activated protein kinase (MAPK) is an important signaling factor in vascular proliferation and contraction, which are the two features of cerebral vasospasm that follow subarachnoid hemorrhage. The authors studied the possible involvement of MAPK in hemolysate-induced signal transduction and contraction in rabbit basilar artery (BA).

METHODS

Isometric tension was used to record the contractile response of rabbit BA to hemolysate, and Western blots were obtained using antibodies for MAPK. The following results are reported. 1) Hemolysate produced a concentration-dependent contraction of rabbit BA; however, preincubation of arteries with the MAPK kinase (MEK) inhibitor PD-98059 markedly reduced this contraction. The administration of PD-98059 also relaxed, in a concentration-dependent fashion, the sustained contraction induced by 10% hemolysate. 2) The Janus tyrosine kinase 2 inhibitor AG-490, preincubated with arterial rings, reduced the contractile response to hemolysate but failed to relax the sustained contraction induced by this agent. The Src-tyrosine kinase inhibitor damnacanthal and the phosphatidylinositol 3-kinase inhibitor wortmannin failed to reduce hemolysate-induced contraction. 3) Hemolysate produced a time-dependent elevation of MAPK immunoreactivity as seen on Western blots of rabbit BA. The MAPK was enhanced 1 minute after hemolysate exposure and the effect reached maximum levels at 5 minutes. The immunoreactivity of MAPK decayed slowly over time, but the level of this kinase was still higher than the basal level, even at 2 hours after exposure to hemolysate. Preincubation of arteries with the MEK inhibitor PD-98059 abolished the effect of hemolysate on MAPK immunoreactivity.

CONCLUSIONS

Hemolysate produced contraction of rabbit BA, possibly by activation of MAPK, and therefore MAPK inhibitors may be useful in the treatment of cerebral vasospasm.

Role of protein tyrosine phosphorylation in erythrocyte lysate-induced intracellular free calcium concentration elevation in cerebral smooth-muscle cells

OBJECT

Tyrosine kinases play an important role in the regulation of systemic vascular smooth-muscle tone. The authors studied the involvement of protein tyrosine kinase activity in erythrocyte lysate-mediated signal transduction in cerebral smooth-muscle cells.

METHODS

Tyrosine kinase phosphorylation and intracellular free Ca++ ([Ca++]i) were measured in rat aortic and basilar artery smooth-muscle cells by using Western blot and fura 2-acetoxymethyl ester microfluorimetry. Erythrocyte lysate enhanced tyrosine phosphorylation in cultured rat aortic and basilar smooth-muscle cells and induced a rapid transient and a prolonged plateau phase of [Ca++]i response in rat basilar smooth-muscle cells. The tyrosine kinase inhibitors genistein and tyrphostin A51 (administered at concentrations of 30 or 100 microM) attenuated both phases of erythrocyte lysate-induced [Ca++]i elevation. Erythrocyte lysate was separated into low- (<10 kD, which contains adenine nucleotides) and high- (>10 kD, which contains hemoglobin) molecular-weight fractions; these fractions were tested separately in these cells. The low-molecular-weight fraction produced a similar [Ca++]i response to that of erythrocyte lysate and the high-molecular-weight fraction produced a small response. The [Ca++]i responses from both fractions were inhibited by tyrosine kinase inhibitors.

CONCLUSIONS

To the authors' knowledge, this is the first report to show that tyrosine phosphorylation may be involved in erythrocyte lysate-induced signal transduction and [Ca++]i responses in cerebral smooth-muscle cells.

Role of tyrosine phosphorylation in excitation-contraction coupling in vascular smooth muscle

Increasingly it is recognized that tyrosine phosphorylation plays an important part in the regulation of function in differentiated contractile vascular smooth muscle. Tyrosine kinases and phosphatases are present in large amounts in vascular smooth muscle and have been reported to influence a number of processes crucial to contraction, including ion channel gating, calcium homeostasis and sensitization of the contractile process to [Ca2+]i. This review summarizes current understanding regarding the role of tyrosine phosphorylation in excitation-contraction coupling in blood vessels.

Altered regulation of L-type channels by protein kinase C and protein tyrosine kinases as a pathophysiologic effect in retinal degeneration

The effect of protein tyrosine kinases (PTK) on L-type calcium channels in cultured retinal pigmented epithelium (RPE) from rats with retinal dystrophy was investigated. Barium currents through Bay K 8644 (10(-6) M) sensitive L-type channels were measured using the patch-clamp technique. The current density of L-type currents is twice as high and the inactivation time constants are much slower than in cells from nondystrophic control rats. Application of the PTK blockers genistein, lavendustin A, and herbimycin A (all 5 x 10(-6) M) led to an increase of L-type currents. Intracellular application of pp60c-src (30 U/ml) via the patch pipette led to a transient decrease of L-type currents. The protein kinase A (PKA) and PKG blocker H9 (10(-6) M) showed no effect on L-type currents. However, the protein kinase C blocker chelerythrine (10(-5) M) reduced these currents. Up-regulation of PKC by 10(-6) M 4beta-phorbol-12 myristate-13 acetate (PMA) led to a decrease of L-type currents. Additional application of genistein led to a further decrease of these currents. However, intracellular application of pp60(c-src) in PMA-treated cells led to a transient increase of L-type currents. Investigating the calcium response to bFGF application showed that RPE cells from RCS rats used different pathways than control RPE cells to increase cytosolic free calcium. This different pathway does not involve the activation of L-type channels. The present study with RPE cells from rats with retinal dystrophy shows a changed integration of PTK and PKC in channel regulation. Considering the altered response to bFGF in RCS-RPE cells, this disturbed regulation of L-type channels by tyrosine kinases is involved in the etiology of retinal degeneration in RCS rats.

Increased tyrosine phosphorylation causes redistribution of adherens junction and tight junction proteins and perturbs paracellular barrier function in MDCK epithelia

Polarized monolayers of strain II Madin-Darby canine kidney cells (MDCK II) were treated with vanadate/H2O2, known inhibitors of protein tyrosine phosphatase activity. Vanadate/H2O2 treatment resulted in a rapid increase in paracellular permeability as revealed by decreased transepithelial resistance and increased permeability to inulin. These alterations in epithelial barrier function coincided with increased phosphotyrosine immunofluorescence in the vicinity of intercellular junctions and with redistribution of F-actin, the adherens junction protein E-cadherin and the tight junction protein ZO-1. The effects of vanadate/H2O2 on intercellular junction permeability and protein distribution were completely blocked by the specific protein tyrosine kinase (PTK) inhibitor tyrphostin 25 and partially inhibited by the alternative PTK inhibitor genistein. The relative potency of these two inhibitors in blocking the effects of vanadate/H2O2 on intercellular junctions correlated with their abilities to inhibit tyrosine phosphorylation. The potent ser/thr protein kinase inhibitor staurosporine had only a small influence on the vanadate/H2O2-induced increase in paracellular permeability and did not affect the observed redistribution of intercellular junction proteins or phosphotyrosine immunofluorescence. The relative potencies of these distinct protein kinase inhibitors in reversing the effects of vanadate/H2O2 indicate that these effects are directly related to tyrosine phosphorylation. In conclusion, our data provide evidence that enhanced tyrosine phosphorylation of intercellular junction proteins in MDCK epithelia increases paracellular permeability and can also induce prominent reorganization of the junctional complex.

Syk Activation Is Required for Spreading and H2O2 Release in Adherent Human Neutrophils

Chemoattractant-stimulated polymorphonuclear leukocytes (PMNs) that are adherent to extracellular matrix proteins exhibit a massive, sustained respiratory burst that requires cell spreading. However, the signaling pathways culminating in PMN spreading are not well characterized. Studies showing that protein tyrosine phosphorylation increases with PMN spreading suggest that phosphorylation is critical for this process. In the present study, we observed increased tyrosine phosphorylation of both focal adhesion kinase and Syk in FMLP-activated PMNs that had been plated onto fibrinogen; an increase in Syk activity, but not focal adhesion kinase activity, was apparent. The time course of Syk phosphorylation correlated with the initiation of cell spreading and H2O2 release. Pretreatment of PMNs with piceatannol, a Syk-selective inhibitor, blocked Syk activity, cell spreading, and H2O2 release, indicating that Syk activity was required for the activation of adherent PMNs. Paxillin is a cytoskeletally associated protein that is also tyrosine phosphorylated during PMN spreading and H2O2 release. Paxillin phosphorylation is kinetically slower than Syk phosphorylation and is inhibited with piceatannol, suggesting that paxillin is a substrate for Syk. An analysis of Syk immunoprecipitates indicated that Syk and paxillin associate during PMN spreading. This interaction is not mediated by the src kinases Lyn and Fgr, since neither kinase coprecipitated with Syk. Syk from FMLP-activated, adherent PMNs phosphorylated paxillin-glutathione S-transferase, suggesting that paxillin is a substrate for Syk in vivo. These results indicate that PMN spreading and H2O2 release require a Syk-dependent signaling pathway leading to paxillin phosphorylation.

Localization and significance of pp55, a gastric mucosal membrane protein with tyrosine kinase activity

In Fischer 344 rats, induction of gastric mucosal proliferative activity, whether the result of aging or injury or occurring after administration of epidermal growth factor, gastrin, or bombesin, is associated with a rise in tyrosine kinase activity and tyrosine phosphorylation of several mucosal proteins, including a protein with a molecular mass of 53-55 kDa. We hypothesized that this phosphotyrosine membrane protein (referred to as pp55) may play a role in regulating gastric mucosal cell proliferation and differentiation. Purification and subsequent immunoprecipitation studies now show that pp55 is a tyrosine kinase. In addition, the enzyme activity in the gastric mucosa is found to be fourfold higher in aged rats than in young rats. Incubation of gastric mucosal membranes with transforming growth factor-alpha (2 x 10(-8) M) stimulates tyrosine kinase activity of pp55. Immunolocalization studies reveal that pp55 immunoreactivity is predominantly present in mucous cells that are located just above the proliferative zone and spasmolytic peptide-immunoreactive mucous neck cells. Tyrosine kinase activity as well as expression of pp55 are also greatly increased in the gastric mucosa after hypertonic saline-induced injury, a condition that results in stimulation of surface mucosal cell proliferation and differentiation. Our current data suggest that pp55 is a tyrosine kinase, likely localized to pre-surface cells. The presence of pp55 in pre-surface mucous cells and the expression and tyrosine kinase activity of this protein, which can be stimulated during mucosal cell proliferation and differentiation, strongly suggest a role for pp55 in differentiation of gastric surface mucous cells.

The cysteine of the cytoplasmic tail of glucose-dependent insulinotropic peptide receptor mediates its chronic desensitization and down-regulation

The glucose-dependent insulinotropic peptide receptor (GIP-R) is a member of G-protein-coupled, seven transmembrane-spanning receptors. Recent studies have shown that elevated serum GIP level in diabetic patients may induce chronic desensitization of the GIP-R, and that this mechanism could contribute to impaired insulin secretion. The cellular basis of down-regulation and chronic desensitization of GIP-R is unclear. To explore the role of the carboxyl terminus of the GIP-R in mediating these processes, five truncated GIP-Rs (T395, T399, T420, T431, T455) were created to delete consecutive serines from the carboxyl end. All mutants except T395 exhibit an identical ligand-binding affinity to the WT receptor. The T395 mutant, which had the entire carboxyl tail removed, does not bind to ligand. Down-regulation and desensitization was assessed by measuring the receptor number and the ability of agonist-induced cAMP or [Ca2+] generation after pre-exposure to 10(-7) M GIP for 24 h. The wild-type (WT) and T421, T431, T455 mutant GIP-Rs are maximally down-regulated by GIP preincubation, whereas T399 mutant does not, indicating that the sequence between amino acids 399 and 420 is critical for this process. Mutation analysis of this area by alanine scanning mutagenesis reveals two critical residues: serine 406 and cysteine 411. Replacement of serine 406 with arginine (S406R) or alanine (S406A) partly attenuates agonist-induced down-regulation and desensitization. In contrast, mutation of the cysteine 411 to glycine (C411G) or alanine (C411A) markedly attenuates both processes. Mutant SCRG, in which both serine 406 and cysteine 411 are mutated, behaves similar to C411G or C4111A. The data suggest that chronic desensitization and down-regulation of the GIP-R may be mediated by similar mechanisms, and that the cysteine in the carboxyl terminus plays an essential role in regulating both processes.

Cell growth inhibition by a novel vitamin K is associated with induction of protein tyrosine phosphorylation

We have shown that a synthetic vitamin K analog, 2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone or compound 5 (Cpd 5), potently inhibits cell growth and suggested that the analog exerts its effects mainly via sulfhydryl arylation rather than redox cycling. Since protein-tyrosine phosphatases (PTPases), which have pivotal roles in many cellular functions, have a critical cysteine in their active site, we have proposed PTPases as likely targets for Cpd 5. To test this hypothesis, we examined the effects of Cpd 5 on protein tyrosine phosphorylation of cellular proteins and on the activity of PTPases. We found that Cpd 5 rapidly induced protein tyrosine phosphorylation in a human hepatocellular carcinoma cell line (Hep3B) at growth inhibitory doses, and the effect was blocked by thiols but not by non-thiol antioxidants or tyrosine kinase inhibitors. Cpd 5 inhibited PTPase activity, which was also significantly antagonized by reduced glutathione. Furthermore, the well studied PTPase inhibitor orthovanadate also induced protein tyrosine phosphorylation and growth inhibition in Hep3B cells. These results suggest that inhibition of cellular PTPases by sulfhydryl arylation and subsequent perturbation of protein tyrosine phosphorylation may be involved in the mechanisms of Cpd 5-induced cell growth inhibition.

Inhibition of Ca2+ current in neonatal and adult rat ventricular myocytes by the tyrosine kinase inhibitor, genistein

Yokoshiki et al. (Yokoshiki, H., Sumii, K., Sperelakis, N., 1996. Inhibition of L-type calcium current in rat ventricular cells by the tyrosine kinase inhibitor, genistein and its inactive analog, daidzein. J. Mol. Cell. Cardiol. 28, 807-814) reported that genistein and daidzein inhibited L-type Ca2+ current (I(Ca)(L)) in young rat ventricular cells. Therefore, we investigated the developmental differences in the effect of genistein, an inhibitor of tyrosine kinases, on I(Ca)(L) in freshly-isolated neonatal (3-7 days) and adult (2-5 months) rat ventricular myocytes using whole-cell voltage clamp and single-channel recordings (cell-attached configuration). For whole-cell voltage clamp, I(Ca)(L) was measured as the peak inward current at a test potential of +10 mV by applying a 300 ms pulse from a holding potential of -40 mV. To isolate I(Ca(L), the pipette solution was Cs+-rich and the bath solution was Na+-, K+-free. Ca2+ (1.8 mM) was used as charge carrier. Bath application of 100 microM genistein (sufficient for maximal effect) decreased the basal I(Ca)(L) by 43.3% (n = 27) in neonatal cells and by 30.6% (n = 14) in adult cells (P < 0.05). In the current/voltage relationships, the potential of peak I(Ca)(L) was shifted to the right by genistein by 8.6 mV in neonatal and by 9.3 mV in adult cells. Genistein produced a shift of the steady-state inactivation curve (to the left) in neonatal cells (from -16.0 +/- 3.9 mV to -26.1 +/- 4.2 mV; P < 0.05) and in adult cells (-15.9 +/- 3.2 mV to -22.9 +/- 3.3 mV; P < 0.05); the slope factor was not affected. For single-channel recordings in cell-attached patches, Ca2+ currents were evoked by applying a 150 ms pulse from a holding potential of -40 mV to a test potential of 0 mV. The pipette solution contained 110 mM Ba2+ (as charge carrier), and the bath solution contained 150 mM K+ (to bring resting potential to near zero). Genistein (50 microM) decreased the open probability of the channels from 2.8% to 0.75% (P < 0.05) in absence of Bay K 8644, and from 24% to 7.9% (P < 0.05) in presence of Bay K 8644; the mean open time and the slope conductance of the currents were not affected. In conclusion, (1) genistein inhibits the basal I(Ca)(L) in rat ventricular cells and (2) the inhibition of I(Ca)(L) by genistein is greater in immature cells than in adult cells.

Inhibition of FGF-1 receptor tyrosine kinase activity by PD 161570, a new protein-tyrosine kinase inhibitor

Through direct synthetic efforts we discovered a small molecule which is a 40 nanomolar inhibitor of the human FGF-1 receptor tyrosine kinase. 1-Tert-butyl-3-[6-(2,6-dichloro-phenyl)-2-(4-diethylamino-butylamino)-py rido[2,3-d]pyrimidin-7-yl]-urea (PD 161570) had about 5- and 100-fold greater selectivity toward the FGF-1 receptor (IC50 = 40 nM) compared with the PDGFbeta receptor (IC50 = 262 nM) or EGF receptor (IC50 = 3.7 microM) tyrosine kinases, respectively. In addition, PD 161570 suppressed constitutive phosphorylation of the FGF-1 receptor in both human ovarian carcinoma cells (A121(p)) and Sf9 insect cells overexpressing the human FGF-1 receptor and blocked the growth of A121(p) cells in culture. The results demonstrate a novel synthetic inhibitor with nanomolar potency and specificity towards the FGF-1 receptor tyrosine kinase.

Expression of Ca2+ channels from rat brain with model phenylketonuria in Xenopus oocytes

Ca2+ channels expressed in Xenopus oocytes using mRNA purified from the brain of the rats subjected to chronic treatment with l-phenylalanine in order to model conditions typical for the congenital disease called phenylketonuria (PKU) were studied using double microelectrode technique. The amplitude of Ca2+ channel currents (IBa, 40 mM Ba2+ as a charge carrier) directed in the oocytes by mRNA from the brain of the animals with model PKU was significantly smaller compared to the control animals (145+/-23 nA vs. 270+/-38 nA, p<0.025) while the voltage-dependence of both currents was similar and typical for that of high voltage-activated (HVA) Ca2+ channels. No evidence for the expression of low voltage-activated Ca2+ channels were found. The decrease of the overall HVA Ba2+ current under model PKU occurred primarily at the expense of the decaying, omega-conotoxin-sensitive component which accounted for about 64% of the total current amplitude in control, and apparently was associated with the activity of the expressed N-type Ca2+ channels. omega-Aga-IVA-sensitive, P/Q component of IBa that contributed not more than 10% to the total current in control showed no change under PKU conditions. In addition to the decreased amplitude, Ba2+ current from model PKU animals showed accelerated run-down during prolonged recording (50%/h compared to 15%/h in control). Our data suggest that hyperphenylalaninemic conditions affect the expression of preferentially N-type Ca2+ channels via the reduction of their specific mRNA content as well as influence the type and manner of channels regulation. The underexpression of N-type Ca2+ channels is consistent with the decrease in the overall number of synaptic contacts during PKU and may be one of the factors contributing to the severe damage of the brain function.

Tyrosine kinase and MAPK inhibition of TNF-alpha- and EGF-stimulated IEC-6 cell growth

The role of TNF-alpha in modulating intestinal crypt cell growth was examined, in comparison with EGF. Both significantly increased IEC-6 cell proliferation. Neither EGF nor TNF-alpha overcame the inhibitory effect on growth exerted by the tyrosine kinase inhibitor genistein. Immunoblots with phosphotyrosine antibodies showed increased tyrosine phosphorylation of IEC-6 cell proteins in response to EGF and TNF-alpha stimulation. TNF-alpha increased ERK1 and ERK2 MAPK phosphorylation. A MAPK assay confirmed the increased activity upon TNF-alpha stimulation. Selective inhibition of MAPK activation by PD98059 resulted in a dose dependent inhibition of TNF-alpha or EGF-induced IEC-6 cell growth. These findings suggest a role for TNF-alpha in the regulation of intestinal epithelial cell growth and that the mitogenic effect of TNF-alpha requires protein tyrosine phosphorylation and MAPK activation.

Biochemical and antiproliferative properties of 4-[ar(alk)ylamino]pyridopyrimidines, a new chemical class of potent and specific epidermal growth factor receptor tyrosine kinase inhibitor

The tyrosine kinase inhibitors PD 69896, 153717, and 158780, which belong to the chemical class 4-[ar(alk)ylamino]pyridopyrimidines, have been characterized with respect to enzymology, target specificity, and antiproliferative effects in tumor cells. These compounds were competitive inhibitors with respect to ATP against purified epidermal growth factor (EGF) receptor tyrosine kinase and inhibited EGF receptor autophosphorylation in A431 human epidermoid carcinoma with IC50 values of 2085, 110, and 13 nM, respectively. Onset of inhibition was immediate once cells were exposed to these compounds, whereas recovery of receptor autophosphorylation activity after the cells were washed free of the compound was dependent on inhibitory potency. Thus, full activity returned immediately after removal of PD 69896 but required 8 hr after exposure to PD 158780. PD 158780 was highly specific for the EGF receptor in Swiss 3T3 fibroblasts, inhibiting EGF-dependent receptor autophosphorylation and thymidine incorporation at low nanomolar concentrations while requiring micromolar levels for platelet-derived growth factor- and basic fibroblast growth factor-dependent processes. PD 158780 inhibited heregulin-stimulated phosphorylation in the SK-BR-3 and MDA-MB-453 breast carcinomas with IC50 values of 49 and 52 nM, respectively, suggesting that the compound was active against other members of the EGF receptor family. The antiproliferative effects of this series of compounds against A431 cells correlated precisely with the inhibitory potency against EGF receptor autophosphorylation. PD 158780 reduced clone formation in soft agar of fibroblasts transformed by EGF, EGF receptor, or the neu oncogene but not ras or raf, further demonstrating its high degree of specificity. Finally, this compound was active against clone formation in several breast tumors having different expression patterns of the erbB family, indicating an anticancer utility in tumors expressing these receptors.

Mechanism of inhibition of protein-tyrosine phosphatases by vanadate and pervanadate

Vanadate and pervanadate (the complexes of vanadate with hydrogen peroxide) are two commonly used general protein-tyrosine phosphatase (PTP) inhibitors. These compounds also have insulin-mimetic properties, an observation that has generated a great deal of interest and study. Since a careful kinetic study of the two inhibitors has been lacking, we sought to analyze their mechanisms of inhibition. Our results show that vanadate is a competitive inhibitor for the protein-tyrosine phosphatase PTP1B, with a Ki of 0.38+/-0.02 microM. EDTA, which is known to chelate vanadate, causes an immediate and complete reversal of the inhibition due to vanadate when added to an enzyme assay. Pervanadate, by contrast, inhibits by irreversibly oxidizing the catalytic cysteine of PTP1B, as determined by mass spectrometry. Reducing agents such as dithiothreitol that are used in PTP assays to keep the catalytic cysteine reduced and active were found to convert pervanadate rapidly to vanadate. Under certain conditions, slow time-dependent inactivation by vanadate was observed; since catalase blocked this inactivation, it was ascribed to in situ generation of hydrogen peroxide and subsequent formation of pervanadate. Implications for the use of these compounds as inhibitors and rationalization for some of their in vivo effects are considered.

Modulation of Cl- secretion in rat distal colon by genistein, a protein tyrosine kinase inhibitor

The protein tyrosine kinase inhibitor, genistein, caused an increase of short-circuit current (Isc) across the rat distal colon in forskolin-pretreated tissues, suggesting a synergistic interaction of the drug with cAMP-dependent secretion. In the absence of forskolin, genistein had a dual effect on Isc, it increased Isc in tissues with a low baseline, but decreased Isc in tissues with a high baseline Isc. The secretory effect of genistein was dependent on the presence of Cl- and was blocked by inhibitors of Cl- secretion like bumetanide, an inhibitor of the Na(+)-K(+)-Cl- cotransporter, or 5-nitro-2-(3- phenylpropylamino)-benzoate (NPPB), a Cl- channel blocker. Unidirectional flux measurements revealed that genistein inhibited Na+ and Cl- absorption and induced net Cl- secretion. The protein tyrosine phosphatase inhibitor vanadate suppressed the secretory effect of genistein. In contrast, genistein caused an inhibition of carbachol-induced, i.e. Ca(2+)-mediated secretion. Whole-cell patch-clamp experiments confirmed the synergistic effect of genistein on cAMP-induced Cl- currents. In the presence of forskolin, genistein caused a depolarization concomitant with an increase in membrane inward current. In addition, genistein caused an inhibition of a basal K+ conductance and inhibited the Ca(2+)-dependent K+ conductance stimulated by carbachol. These results suggest a complex role of the protein tyrosine kinase pathway in the control of colonic Cl- secretion, an antagonistic action on the cAMP pathway and a synergistic action on the Ca2+ pathway as revealed by the opposing effects of genistein. The physiological importance of this regulation remains to be clarified.

Collagen and collagenase gene expression in three-dimensional collagen lattices are differentially regulated by alpha 1 beta 1 and alpha 2 beta 1 integrins

The reorganization of extracellular matrix (ECM) is an important function in many biological and pathophysiological processes. Culture of fibroblasts in a three-dimensional collagenous environment represents a suitable system to study the underlying mechanisms resulting from cell-ECM interaction, which leads to reprogramming of fibroblast biosynthetic capacity. The aim of this study was to identify receptors that transduce ECM signals into cellular events, resulting in reprogramming of connective tissue metabolism. Our data demonstrate that in human skin fibroblasts alpha 1 beta 1 and alpha 2 beta 1 integrins are the major receptors responsible for regulating ECM remodeling: alpha 1 beta 1 mediates the signals inducing downregulation of collagen gene expression, whereas the alpha 2 beta 1 integrin mediates induction of collagenase (MMP-1). Applying mAb directed against different integrin subunits resulted in triggering the heterodimeric receptors and enhancing the normal biochemical response to receptor ligation. Different signal transduction inhibitors were tested for their influence on gel contraction, expression of alpha 1(I) collagen and MMP-1 in fibroblasts within collagen gels. Ortho-vanadate and herbimycin A displayed no significant effect on any of these three processes. In contrast, genistein reduced lattice contraction, and completely inhibited induction of MMP-1, whereas type I collagen down-regulation was unaltered. Calphostin C inhibited only lattice contraction. Taken together, these data indicate a role of tyrosine-specific protein kinases in mediating gel contraction and induction of MMP-1, as well as an involvement of protein kinase C in the contraction process. The data presented here indicate that different signaling pathways exist leading to the three events discussed here, and that these pathways do not per se depend upon each other.

Inhibitors of protein tyrosine kinases

The description in the past year of several novel protein tyrosine kinase inhibitors, which exhibit dramatic improvements in potency and specificity over earlier agents, will be considered a major turning point in the field. These compounds appear to have the necessary pharmacological properties to finally allow clarification of whether suppression of specific tyrosine kinases is of therapeutic benefit in certain disease states.

Blood platelet heterogeneity: a functional hierarchy in the platelet population

The platelet population in man and rat can be divided into two classes of about equal size based on the presence/absence of a p-nitrophenylphosphatase, which probably is a phosphotyrosine phosphatase (PTPase). Phosphorylation of tyrosines on several platelet proteins is implicated in platelet activation, and I carried out in vitro and in vivo experiments on rats to determine whether PTPase positive and negative platelets differed in their reaction time. I used adhesion to collagen in vitro and in vivo (longitudinal slits in aorta and vena portae) and platelet aggregates in clots formed in vivo. I present evidence that PTPase negative platelets react the fastest, most conspicuously seen in the arterial bleeding under high flow conditions, where the first platelets to respond and adhere are predominantly PTPase negative.

Non-receptor cytosolic protein tyrosine kinases from various rat tissues

Adipocytic-cytosolic non-receptor protein tyrosine kinase (CytPTK) when activated can substitute for the insulin receptor tyrosine kinase (InsRTK), in manifesting several insulin effects in insulin-receptor independent fashion. Our aims here were to utilize PolyGlu4Tyr, a good experimental exogenous substrate for protein tyrosine kinases (PTKs) in general, for studying qualitative and quantitative parameters of CytPTKs extracted from different tissue cytosols. At the same time, we would search for a unique specific marker specifically characterizing CytPTKs. High speed supernatants of spleen, thymus, smooth muscle, lung and kidney were found to be rich in CytPTK activities. Their specific activities being 6- to 13-fold that of liver or adipose cytosols. Brain, testis and adrenal cytosols were an intermediate source of CytPTK activity, whereas CytPTK activity of heart and skeletal muscle was low. It was also evaluated that the capacity of the cytosol to phosphorylate PolyGlu4Tyr is 15-50% that of the non-stimulated Triton X-100 extractable plasma membrane PTKs. Fractionation of the cytosols on superose 12 column revealed several CytPTKs within the same tissue, their peaks ranging between 30 and 450 kDa. Immunoblotting analysis showed Fyn and Lyn were present in most tissue cytosols. Upon immunoprecipitation, however, with anti-Fyn or anti-Lyn, negligible amounts (< 2%) of the total cellular CytPTK were precipitated. Thus, these general markers of CytPTKs comprise only a minor proportion of the total intracellular PolyGlu4Tyr phosphorylating capacity. To see whether a specific marker for CytPTK could be detected, we also examined the requirement of CytPTKs for divalent ions, their preferred phosphate donor and their sensitivity to inhibition by known PTK inhibitors. We found that the order of reactivity with divalent cations was Co2+ > Mn2+ > Mg2+, while Zn2+ and Ca2+ did not support CytPTK activity. The best phosphate donor was ATP (ED50 = 5 microM), but other nucleoside 3-phosphates could substitute for ATP at high concentrations. With respect to these parameters, no basic difference exists between cytosolic and plasma-membrane PTKs. The PTK inhibitors, genestein and quercetin, inhibited both cytosolic and membranal PTKs at micromolar concentrations. In contrast, staurosporine was a potent inhibitor of CytPTKs (IC50 5-20 nM) and a poor inhibitor of membranal PTKs (IC50 10-40 microM). One of the conclusions we can draw from this study is that tissue cytosols contain PolyGlu4Tyr phosphorylating capacity in quantities greater than previously assumed and that the low level of phosphotyrosine found in cells is not the result of limited intracellular levels of CytPTKs.

Inhibition of formation of filopodia after axotomy by inhibitors of protein tyrosine kinases

The activity of motile protrusions of the growth cone--filopodia, veils, and lamellipodia--is essential for directed growth of a neuronal process. The regulation of the formation of these protrusions is not well understood. Numerous filopodia and veils or lamellipodia form within minutes of transection of an Aplysia axon in culture, as the initial components of growth cones of regenerating neurites. Axotomy, therefore, provides a robust and reliable protocol for analyzing the formation of these protrusions. We evaluated the involvement of protein phosphorylation in the regulation of protrusive activity. Of the inhibitors of protein kinases assayed, only the inhibitors of protein tyrosine kinases--genistein, lavendustin A, herbimycin A, and erbstatin analogue--suppressed the formation of protrusions, as assessed by high magnification video microscopy. These drugs did not work by preventing resealing of the axon, as evident from visual inspection and by the unimpaired effectiveness of genistein or lavendustin in preventing formation of filopodia when applied after resealing. Inhibition of protein tyrosine kinases not only prevented the formation of actin-based protrusions, but also caused deterioration of the actin network underlying the protrusive area of preexisting growth cones. Consistent with an involvement of protein tyrosine phosphorylation in the generation of protrusive structures, immunocytochemistry revealed that aggregates of phosphotyrosine appeared at the margins of the axon, from which protrusions emerge shortly after axotomy. These results suggest a role for protein tyrosine phosphorylation in the formation and maintenance of actin-based protrusive structures.

Tyrosine kinase-mediated signal transduction pathways and the actions of polypeptide growth factors and G-protein-coupled agonists in smooth muscle

This synopsis focuses on the role that tyrosine kinase pathways may play in the acute regulation of smooth muscle contractility by receptor-kinase-activating growth factors, such as epidermal growth factor-urogastrone (EGF-URO) and by G-protein-coupled agonists, such as angiotensin-II. Growth factor-activated response paradigms that modulate smooth muscle contractility are summarized and the parallels between the actions of G-protein-coupled agonists and growth factors in these response systems are pointed out. A possible dynamic interplay between tyrosine kinase and tyrosine phosphatase activities to modulate tissue tension is also hypothesized. Finally, a model is proposed, wherein an intermediary tyrosine kinase pathway is suggested as a point of convergence for the regulation of smooth muscle contractility by agonists as diverse as EGF-URO and angiotensin-II.

Protein tyrosine phosphorylation in human platelets induced by interaction between glycoprotein Ib and von Willebrand factor

The interaction between von Willebrand factor (vWF) and glycoprotein Ib (GPIb) induced by ristocetin or botrocetin resulted in associated platelet aggregation, protein tyrosine phosphorylation (PTP) of a 64 kDa protein, as detected by a monoclonal antibody against phosphotyrosine (PY-20), and intracellular Ca2+ elevation that is largely dependent upon Ca2+ influx in human platelets. It is of interest that 75-80, 97 and 125 kDa proteins which are strongly tyrosine-phosphorylated in platelet activation induced by thrombin and other agonists were not detected. Neither vWF nor a coaggregating agent (ristocetin or botrocetin) alone induced aggregation, [Ca2+]i elevation or the 64 kDa PTP. NMC-4, an antibody which inhibits both ristocetin- or botrocetin-induced vWF binding to GPIb, abolished the appearance of the 64 kDa PTP as well as other responses, suggesting that it is specifically induced by the GPIb-vWF interaction. Aspirin, or ONO-3708, a competitive inhibitor of thromboxane A2, did not modify the 64 kDa PTP, while [Ca2+]i elevation was moderately suppressed. Depletion of extracellular Ca2+ or RGD peptides suppressed neither the 64 kDa PTP nor aggregation. H-7, a protein kinase C inhibitor, did not inhibit the 64 kDa PTP, while staurosporine, a potent protein kinase inhibitor, inhibited the 64 kDa PTP and Ca2+ influx, but not aggregation, in a dose-dependent manner. It is suggested that the 64 kDa PTP is associated with platelet aggregation induced by the interaction between GPIb and vWF.

Calcitonin acutely increases tyrosyl-phosphorylation of proteins in human osteosarcoma (SaOS-2) cells

In order to test the hypothesis that salmon calcitonin has direct effects to modulate tyrosyl-protein phosphorylation in human osteosarcoma cells, SaOS-2 cells (with very high steady-state levels of skeletal alkaline phosphatase) were exposed to calcitonin, in duplicate serum-free cultures, at concentrations ranging from 10(-13) to 10(-9) mol/liter, for 0-60 minutes at 37 degrees C. Phospho-tyrosyl proteins were identified by autoradiography of Western blots after incubation with 125I-labeled antiphosphotyrosine antibodies (or with unlabeled antibodies and 125I-labeled protein A) and quantitated by laser densitometry. The results of these studies revealed (1) time-dependent effects of salmon calcitonin (sCt) (at 3 x 10(-12) mol/liter) to increase the level of tyrosylphosphorylation of at least six proteins, with apparent molecular weights of 20, 25, 27, 41, 48, and 135 kD (P < 0.05 for each); and (2) dose-dependent effects of sCt (during 15 minutes of exposure) to increase the level of tyrosyl-phosphorylation of at least 10 proteins with apparent molecular weights of 19, 20, 27, 35, 41, 102, 135, 195, 220, and 244 kD (P < 0.05 for each). A supplementary study of calcitonin effects on tyrosyl-protein phosphorylation in a subpopulation of SaOS-2 cells with very low steady-state levels of skeletal alkaline activity revealed similar responses--time and dose-dependent increases in the tyrosyl-phosphorylation of at least seven proteins with apparent molecular weights of 44, 48, 57, 62, 101, 244, and 280 kD (P < 0.05 for each).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tyrosine kinase inhibitors on the contractility of rat mesenteric resistance arteries

1. A pharmacological characterization of tyrosine kinase inhibitors (TKI) belonging to two distinct groups (competitors at the ATP-binding site and the substrate-binding site, respectively) was performed, based on their effects on the contractility of rat mesenteric arteries. 2. Both the ATP-site competitors (genistein and its inactive analogue, daidzein) and the substrate-site competitors (tyrphostins A-23, A-47 and the inactive analogue, A-1) reversibly inhibited noradrenaline (NA, (10 microM)) and KCl (125 mM) induced contractions, concentration-dependently. Genistein was slightly but significantly more potent than daidzein; the tyrphostins were all less potent than genistein, and there were no significant differences between the individual potencies. The tyrosine kinase substrate-site inhibitor bis-tyrphostin had no inhibitory effect. 3. Genistein, daidzein, A-23 and A-47 each suppressed the contraction induced by Ca2+ (1 microM) in alpha-toxin permeabilized arteries. A-1 and bis-tyrphostin had little or no effect on contraction of the permeabilized arteries. 4. Genistein was significantly more potent than daidzein with respect to inhibition of the contraction induced by 200 nM Ca2+ in the presence of NA (100 microM) and GTP (3 microM). The effect of A-23, A-47, A-1 and bis-tyrphostin was similar in permeabilized arteries activated with Ca2+ (200 nM) + NA (100 microM) + GTP (3 microM) and permeabilized arteries activated with 1 microM Ca2+. 5. Genistein (30 microM) reduced the fura-2 measured intracellular calcium activity ([Ca2+]j) in arteries stimulated with NA but had no effect on [Ca2+]i in arteries stimulated with KCl (125 mM).6. The potent effect of the TKIs in this study is consistent with a role for tyrosine kinases in the mechanisms which regulate both cytoplasmic Ca2+ levels and the effect of Ca2+ on the contractile apparatus in smooth muscle cells in resistance arteries. However, the results must be interpreted cautiously because the enzyme inhibitors may have a poor specificity in intact tissues and because the presumed inactive analogues had potent effects.

Chronic desensitization and down-regulation of the gastrin-releasing peptide receptor are mediated by a protein kinase C-dependent mechanism

The cellular basis of down-regulation and desensitization in phospholipase C-linked receptors is unclear. Recent studies with some receptors suggest that elements in the carboxyl terminus of the receptor are important in mediating these processes. Three mutant gastrin-releasing peptide receptors (GRP-R) were studied: one whose last 37 carboxyl-terminal amino acids were eliminated (construct MGT346); one that replaced all of the carboxyl-terminal Ser and Thr eliminated in MGT346 with Ala, Asn, or Gly (construct JF1); and one that selectively replaced the Ser and Thr of the protein kinase C consensus sequence (PKC-CS) located within the same region with alanine (construct TS360AA). Desensitization was assessed by measuring the ability to activate phospholipase C and increase cellular [3H]inositol phosphates, or increase [Ca2+]i, after pre-exposure to 3 nM bombesin for 24 h. Wild-type GRP-R was maximally desensitized and down-regulated after a 24-h exposure to 3 nM bombesin, and removal of the PKC-CS alone markedly attenuated each process. Elimination of additional serines and threonines by truncation (MGT346) or replacement (JF1) did not decrease down-regulation or desensitization further. To confirm the necessity of second messenger activation in mediating down-regulation, we further investigated two additional mutant GRP-R that bound agonist with high affinity but fail to activate phospholipase C (constructs R139G and A263E). Neither construct underwent significant down-regulation. Removal of all GRP-R carboxyl-terminal Ser or Thr, either by MGT346 or JF1, reduced internalization by > 80%, whereas elimination of the PKC-CS in TS360AA only attenuated internalization by 21 +/- 2%. These data suggest that activation of the distal carboxyl-terminal PKC-CS is essential for chronic desensitization and down-regulation of the GRP-R, and provide no evidence for involvement of second messenger-independent processes. In contrast, internalization is equally regulated by both second messenger-dependent and independent processes.