Cross-talk between phorbol ester-mediated signaling and tyrosine kinase proto-oncogenes. II. Comparison of phorbol ester and sphingomyelinase-induced phosphorylation of ErbB2 and ErbB3

Megakaryocytes regulate expression of Pyk2 isoforms and caspase-mediated cleavage of actin in osteoblasts

The proliferation and differentiation of osteoblast (OB) precursors are essential for elaborating the bone-forming activity of mature OBs. However, the mechanisms regulating OB proliferation and function are largely unknown. We reported that OB proliferation is enhanced by megakaryocytes (MKs) via a process that is regulated in part by integrin signaling. The tyrosine kinase Pyk2 has been shown to regulate cell proliferation and survival in a variety of cells. Pyk2 is also activated by integrin signaling and regulates actin remodeling in bone-resorbing osteoclasts. In this study, we examined the role of Pyk2 and actin in the MK-mediated increase in OB proliferation. Calvarial OBs were cultured in the presence of MKs for various times, and Pyk2 signaling cascades in OBs were examined by Western blotting, subcellular fractionation, and microscopy. We found that MKs regulate the temporal expression of Pyk2 and its subcellular localization. We also found that MKs regulate the expression of two alternatively spliced isoforms of Pyk2 in OBs, which may regulate OB differentiation and proliferation. MKs also induced cytoskeletal reorganization in OBs, which was associated with the caspase-mediated cleavage of actin, an increase in focal adhesions, and the formation of apical membrane ruffles. Moreover, BrdU incorporation in MK-stimulated OBs was blocked by the actin-polymerizing agent, jasplakinolide. Collectively, our studies reveal that Pyk2 and actin play an important role in MK-regulated signaling cascades that control OB proliferation and may be important for therapeutic interventions aimed at increasing bone formation in metabolic diseases of the skeleton.

p38 mitogen-activated protein kinase-dependent transactivation of ErbB receptor family: a novel common mechanism for stress-induced IRS-1 serine phosphorylation and insulin resistance

OBJECTIVE

Stress stimuli such as tumor necrosis factor (TNF) have been shown to induce insulin receptor substrate (IRS)-1 serine phosphorylation and insulin resistance by transactivation of ErbB receptors. We aimed at elucidating the potential role of p38 mitogen-activated protein kinase (p38MAPK) in mediating stress-induced ErbB receptors activation.

RESEARCH DESIGN AND METHODS

p38MAPK effect on ErbBs transactivation and insulin signaling was assessed in Fao or HepG2 cells, exposed to stress stimuli, and on metabolic parameters in ob/ob and C57/BL6 mice.

RESULTS

High-fat diet-fed mice and ob/ob mice exhibited elevated hepatic p38MAPK activation associated with glucose intolerance and hyperinsulinemia. Liver expression of dominant-negative (DN)-p38MAPKα in ob/ob mice reduced fasting insulin levels and improved glucose tolerance, whereas C57/BL6 mice overexpressing wild-type p38MAPKα exhibited enhanced IRS-1 serine phosphorylation and reduced insulin-stimulated IRS-1 tyrosine phosphorylation. Fao or HepG2 cells exposed to TNF, anisomycin, or sphingomyelinase demonstrated rapid transactivation of ErbB receptors leading to PI3-kinase/Akt activation and IRS-1 serine phosphorylation. p38MAPK inhibition either by SB203580, by small interfering RNA, or by DN-p38MAPKα decreased ErbB receptors transactivation and IRS-1 serine phosphorylation and partially restored insulin-stimulated IRS-1 tyrosine phosphorylation. When cells were incubated with specific ErbB receptors antagonists or in cells lacking ErbB receptors, anisomycin- and TNF-induced IRS-1 serine phosphorylation was attenuated, despite intact p38MAPK activation. The stress-induced p38MAPK activation leading to ErbB receptors transactivation was associated with intracellular reactive oxygen species generation and was attenuated by treatment with antioxidants.

CONCLUSIONS

Hepatic p38MAPK is activated following various stress stimuli. This event is upstream to ErbB receptors transactivation and plays an important role in stress-induced IRS-1 serine phosphorylation and insulin resistance.

Activation of epidermal akt by diverse mouse skin tumor promoters

Akt is a serine/threonine kinase involved in a variety of cellular responses, including cell proliferation and cell survival. Recent studies from our laboratory suggest that Akt signaling may play an important role in skin tumor promotion. To explore this premise, we examined epidermal Akt activation and signaling in response to chemically diverse skin tumor promoters. Mice received single or multiple applications of 12-O-tetradecanoylphorbol-13-acetate (TPA), okadaic acid, or chrysarobin. All three tumor promoters were able to activate epidermal Akt as early as 1 h after treatment. Activation of Akt following tumor promoter treatment led to enhanced downstream signaling, including hyperphosphorylation of glycogen synthase kinase-3beta and Bad. Structure activity studies with phorbol ester analogues revealed that the magnitude of activation paralleled tumor-promoting activity. In cultured primary keratinocytes, TPA treatment also led to activation of Akt. Activation of the epidermal growth factor receptor (EGFR) seemed to underlie the ability of TPA to activate Akt as both PD153035, an inhibitor of EGFR, and GW2974, a dual-specific inhibitor of both EGFR and erbB2, were able to effectively reduce TPA-induced Akt phosphorylation as well as TPA-stimulated EGFR and erbB2 tyrosine phosphorylation in a dose-dependent manner. Furthermore, inhibition of protein kinase C (PKC) activity blocked TPA-stimulated heparin-binding EGF production and EGFR transactivation. Inhibition of PKC also led to a decreased association of Akt with the PP2A catalytic subunit, leading to increased Akt phosphorylation. However, combination of EGFR inhibitor and PKC inhibitor completely abrogated TPA-induced activation of Akt. Collectively, the current results support the hypothesis that elevated Akt activity and subsequent activation of downstream signaling pathways contribute significantly to skin tumor promotion. In addition, signaling through the EGFR via EGFR homodimers or EGFR/erbB2 heterodimers may be the primary event leading to Akt activation during tumor promotion in mouse skin.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

Two types of calcium channels regulating activation of proline-rich tyrosine kinase 2 induced by transient brain ischemia in rat hippocampus

Tyrosine phosphorylation is an important means for regulating post-ischemic signal transduction. In this article, brain ischemia was induced by four-vessel occlusion, and the effect of ischemia/reperfusion on proline-rich tyrosine kinase 2 (Pyk2) was studied. Tyrosine phosphorylation of Pyk2 in Sprague-Dawley rat hippocampus after transient (15 min) brain ischemia and reperfusion was examined by immunoprecipitation and immunoblot. Kinase activity of Pyk2 was examined by the method of (32)P-incorporation into poly(Glu-Tyr). Tyrosine phosphorylation and kinase activity of Pyk2 decreased slightly after ischemia, then increased after reperfusion and reached the peak levels (5.1 and 1.8 times the levels of the sham-operated group, respectively) at 1 h of reperfusion. Both the increases were partly inhibited by NMDA receptor antagonist ketamine and L-type voltage-gated calcium channel antagonist nifedipine administered 20 min before ischemia. The results suggested that Pyk2 was activated after transient brain ischemia and reperfusion, and it might play an important role in mediating post-ischemic signal transduction events.

L-type voltage-gated calcium channel attends regulation of tyrosine phosphorylation of NMDA receptor subunit 2A induced by transient brain ischemia

To study the mechanism of the L-type voltage-gated calcium channel (L-VGCC) involved in neuronal injury induced by brain ischemia and reperfusion (I/R), transient (15 min) brain ischemia was induced by four-vessel occlusion of Sprague-Dawley (SD) rats. Tyrosine phosphorylation of NR2A and interaction of NR2A with Src and Pyk2 in hippocampus induced by brain ischemia and reperfusion (I/R) were determined by immunoprecipitation and immunoblot(ting). Tyrosine phosphorylation of NR2A in hippocampus was enhanced after I/R. Interaction of NR2A with Src and Pyk2, tyrosine phosphorylation and kinase activity of Src and Pyk2 also increased after I/R. All the increases were partly inhibited by L-VGCC antagonist nifedipine administered to rats 20 min prior to brain ischemia. The results suggested that increase of tyrosine phosphorylation of NR2A induced by I/R had a relation to activation of L-VGCC. Src and Pyk2 interacting with NR2A might also be involved in this regulation of the tyrosine phosphorylation of NR2A induced by I/R.

Study on the mechanism of epidermal growth factor-induced proliferation of hepatoma cells

AIM: Many growth factors, such as epidermal growth factor (EGF), are associated with the carcinogenesis. EGF plays its role in the proliferation of hepatoma cells through binding with EGF receptor (EGFR) and a series of signal transduction. But the postreceptor pathway is still not clear. In the present experiment, we studied the effect of tyrosine kinase, protein kinase C, Na⁺/H⁺ exchange, calmodulin and voltage-dependent Ca²⁺ channel on EGF-induced hepatoma cell proliferation.

METHODS: Hepatoma cell line SMMC7721 was cultured in RPMI1640 serum-free medium. In order to study the effect of thyrosine kinase, protein kinase C, Na⁺/H⁺ exchange, calmodulin and voltage-dependent Ca²⁺ channel on human heptoma cell proliferation induced by epidermal growth factor (EGF), DNA synthesis rate of hepatoma cells was measured by the method of ³H-TdR incorporation.

RESULTS: EGF (10^-9 M) stimulated the proliferation of heptoma cells significantly (³H-TdR incorporation was 1880 ± 281 cpm/well, P < 0.05), and this effect was significantly inhibited by tyrosine kinase inhibitor genistein (³H-TdR incorporation was 808 ± 209 cpm/well, P < 0.001). Calmodulin inhibitor W-7, protein kinase C inhibitor H-7 and Na⁺/H⁺ exchange inhibitor amiloride individually had significant inhibiting effect on EGF-induced proliferation of hepatoma cells (³H-TdR incorporation was 978 ± 87.3 cpm/well, 1241 ± 147 cpm/well, 1380 ± 189 cpm/well, respectivly, P < 0.001, P < 0.01, P < 0.05), but they all had no effect on the basal level proliferation of cultured hepatoma cells (³H-TdR incorporation was 1284 ± 260 cpm/well, 1179 ± 150 cpm/well, 1392 ± 152 cpm/well, respectivly, ³H-TdR incorporation of the control was 1353 ± 175 cpm/well, P > 0.05). Voltage-dependent Ca²⁺ channel inhibitor verapamil had no inhibition on EGF-induced proliferation of hepatoma cells (³H-TdR incorporation was 1637 ± 133 cpm/well, P > 0.05), it also had no effect on the basal level proliferation of cultured hepatoma cells (³H-TdR incorporation was 1196 ± 12 cpm/well, P > 0.05).

CONCLUSION: Our data suggest that tyrosine kinase, Ca²⁺-calmodulin-dependent pathway, protein kinase C and Na⁺/H⁺ exchange play a critical role in EGF-induced proliferation of hepatoma cells and that the effect of EGF is independent of voltage-dependent Ca²⁺ channel.

Protein kinase C induces actin reorganization via a Src- and Rho-dependent pathway

We have investigated the mechanism of PKC-induced actin reorganization in A7r5 vascular smooth muscle cells. PKC activation by 12-O-tetradecanoylphorbol-13-acetate induces the disassembly of actin stress fibers concomitant with the appearance of membrane ruffles. PKC also induces rapid tyrosine phosphorylation in these cells. As we could show, utilizing the Src-specific inhibitor PP2 and a kinase-deficient c-Src mutant, actin reorganization is dependent on PKC-induced Src activation. Subsequently, the activity of the small G-protein RhoA is decreased, whereas Rac and Cdc42 activities remain unchanged. Disassembly of actin stress fibers could also be observed using the Rho kinase-specific inhibitor Y-27632, indicating that the decrease in RhoA activity on its own is responsible for actin reorganization. In addition, we show that tyrosine phosphorylation of p190RhoGAP is increased upon 12-O-tetradecanoylphorbol-13-acetate stimulation, directly linking Src activation to a decrease in RhoA activity. Our data provide substantial evidence for a model elucidating the molecular mechanisms of PKC-induced actin rearrangements.

Transactivation of ErbB2 and ErbB3 by tumor necrosis factor-alpha and anisomycin leads to impaired insulin signaling through serine/threonine phosphorylation of IRS proteins

The cellular pathways involved in the impairment of insulin signaling by cellular stress, triggered by the inflammatory cytokine tumor necrosis factor-alpha (TNF) or by translational inhibitors like cycloheximide and anisomycin were studied. Similar to TNF, cycloheximide and anisomycin stimulated serine phosphorylation of IRS-1 and IRS-2, reduced their ability to interact with the insulin receptor, inhibited the insulin-induced tyrosine phosphorylation of IRS proteins, and diminished their association with phosphatidylinositol 3-kinase (PI3K). These defects were partially reversed by wortmannin and LY294002, indicating that a PI3K-regulated step is critical for the impairment of insulin signaling by cellular stress. Induction of cellular stress resulted in complex formation between PI3K and ErbB2/ErbB3 and enhanced PI3K activity, implicating ErbB proteins as downstream effectors of stress-induced insulin resistance. Indeed, stimulation of ErbB2/ErbB3 by NDFbeta1, the ErbB3 ligand, inhibited IRS protein tyrosine phosphorylation and recruitment of downstream effectors. Specific inhibitors of the ErbB2 tyrosine kinase abrogated the activation of ErbB2/ErbB3 and in parallel prevented the reduction in IRS protein functions. Taken together, our results suggest a novel mechanism by which cellular stress induces cross-talk between two different signaling pathways. Stress-dependent transactivation of ErbB2/ErbB3 receptors triggers a PI3K cascade that induces serine phosphorylation of IRS proteins culminating in insulin resistance.

Protein kinase C-mediated tyrosine phosphorylation of paxillin and focal adhesion kinase requires cytoskeletal integrity and is uncoupled to mitogen-activated protein kinase activation in human hepatoma cells

Treatment of cultured human hepatoma HepG2 cells with the protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), results in an increase in tyrosine phosphorylation of several proteins, including the focal adhesion kinase (FAK) and paxillin using anti-phosphotyrosine Western blotting and immunoprecipitation. However, when cells are in suspension or in the presence of cytochalasin D which disrupts the intracellular network of actin microfilaments, TPA loses its ability to stimulate tyrosine phosphorylation of FAK and paxillin but it still activates mitogen-activated protein kinase (MAPK) and induces PKC translocation from cytosol to the membrane in HepG2 cells. On the other hand, PD98059, a specific inhibitor of mitogen-activated protein kinase kinase, blocks TPA-induced MAPK activation but has no effect on TPA-induced tyrosine phosphorylation. Our findings suggest that TPA-induced tyrosine phosphorylation of FAK and paxillin in human hepatoma cells is PKC dependent and requires the integrity of the cell cytoskeleton but is uncoupled to the signal transduction pathway of PKC leading to the translocation of PKC and MAPK activation.

Activation via multiple signaling pathways induces down-regulation of platelet-activating factor receptors on human B lymphocytes

Platelet-activating factor receptor (PAFR) has been identified in B cell lines and primary human B cells, but the regulation of PAFR during B cell activation has not been completely elucidated. In the present study, we have investigated the effects of B cell activation on PAFR binding parameters, PAFR mRNA and PAF-triggered intracellular calcium mobilization. The human B lymphoid cell line LA350 was shown to exhibit high levels of PAFR (48,550 +/- 4,310 sites/cell) as determined by radio-ligand binding assay with PAFR antagonist [3H]WEB2086. Treatment with phorbol 12,13-dibutyrate caused a biphasic reduction of PAFR binding. The early phase was inhibited by the protein kinase C inhibitor bisindolylmaleimide I (BIM), whereas the late phase was not blocked by BIM, protein tyrosine kinase inhibitor genistein, or the mitogen-activated protein kinase/extracellular signal-related kinase inhibitor PD98059. However, staurosporine, a broad-spectrum protein kinase inhibitor, completely inhibited the late phase down-regulation. Ionomycin also decreased [3H]WEB2086 binding sites, whereas the combination of PDB and ionomycin induced a greater reduction than either agent alone. Cross-linking of B cell receptor by anti-IgM Ab also induced down-regulation of PAFR, which was abolished by genistein or PD98059, but not by BIM or staurosporine. The decrease in surface PAFR number was closely paralleled by the reduction in PAFR mRNA both in LA350 cells and human tonsillar B cells, and was associated with decreased response to PAF indicated by decreased intracellular calcium mobilization. These data show that multiple signaling pathways are involved in down-regulating PAFR expression during B cell activation and development.

RAFTK/Pyk2-mediated cellular signalling

Intracellular signal transduction following extracellular ligation by a wide variety of surface molecules involves the activation and tyrosine phosphorylation of protein tyrosine kinases (PTKs). Tyrosine phosphorylation, controlled by the coordinated actions of protein tyrosine phosphatases (PTPs) and tyrosine kinases, is a critical regulatory mechanism for various physiological processes, including cell growth, differentiation, metabolism, cell cycle regulation and cytoskeleton function. The focal adhesion PTK family consists of the focal adhesion kinase (FAK) and the RAFTK/Pyk2 kinase (also known as CAK-beta and CADTK). RAFTK/Pyk2 can be activated by a variety of extracellular signals that elevate intracellular calcium concentration, and by stress signals. RAFTK/Pyk2 is expressed mainly in the central nervous system and in cells derived from hematopoietic lineages, while FAK is widely expressed in various tissues and links transmembrane integrin receptors to intracellular pathways. This review describes the role of RAFTK/Pyk2 in various signalling cascades and details the differential signalling by FAK and RAFTK/Pyk2.

Cross-talk between phorbol ester-mediated signaling and tyrosine kinase proto-oncogenes. I. Activation of protein kinase C stimulates tyrosine phosphorylation and activation of ErbB2 and ErbB3

The tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), acutely stimulates the tyrosine phosphorylation of proteins of approximately 190, 120, and 70 kDa in the well differentiated Fao rat hepatoma cell line. This phosphorylation is dependent on protein kinase C (PKC) and is abolished by down-regulation of PKC or pretreatment with a PKC inhibitor. Purification of the 190-kDa tyrosine-phosphorylated protein revealed that it consists of both ErbB2 and ErbB3. Following PMA-induced tyrosine phosphorylation, ErbB2 and ErbB3 were able to associate with the SH2 domains of several signaling proteins including the p85alpha subunit of phosphatidylinositol 3-kinase, Syp, and Grb2. The 120-kDa protein phosphorylated in response to PMA consists of at least two proteins: focal adhesion kinase that exhibits a minor increase in tyrosine phosphorylation following treatment with PMA, and a major 120-kDa tyrosine-phosphorylated species in PMA-stimulated Fao cells which as yet is unidentified. Similarly, the 70-kDa tyrosine-phosphorylated protein also appears to represent more than one protein, including paxillin and a second protein of similar mobility which appears to be the major tyrosine phosphorylation in response to PMA. Both ErbB2 and paxillin also exhibit reduced migration on SDS-polyacrylamide gel electrophoresis following PMA treatment, suggesting that they are also phosphorylated on serine/threonine residues. The mobility shift of both of these proteins is abolished by treatment with inhibitors of PKC or mitogen-activated protein kinase/extracellular signal-related kinase kinase. These results suggest a novel mechanism of cross-talk between the serine/threonine kinase PKC and tyrosine phosphorylation pathways. The activation of ErbB2 and ErbB3 that is initiated by PMA may contribute to the tumor promoting activity of these compounds.