Protein tyrosine phosphorylation induced by lysophosphatidic acid in Rat-1 fibroblasts. Evidence that phosphorylation of map kinase is mediated by the Gi-p21ras pathway

Prostaglandin F2 alpha stimulates formation of p21ras-GTP complex and mitogen-activated protein kinase in NIH-3T3 cells via Gq-protein-coupled pathway

Prostaglandin (PG) F2 alpha activated mitogen-activated protein (MAP) kinase and MAP kinase kinase in NIH-3T3 cells by a mechanism that was completely inhibited by protein kinase inhibitors, staurosporine (20 nM) or H-7 (20 microM), but was insensitive to pretreatment with islet-activating protein (100 ng/ml; 24 h) or 12-O-tetradecanoylphorbol 13-acetate (2.5 microM; 24 h). PGF2 alpha stimulation also led to a significant increase in Ras.GTP complex. Transfection of a cDNA encoding a constitutively active mutant of Gq alpha-subunit (Q209L) mimicked PGF2 alpha-induced MAP kinase activation, increase in Ras.GTP complex, and DNA synthesis in these cells, suggesting that activation of Gq mediates the PGF2 alpha-activation of Ras-MAP kinase pathway and mitogenesis in NIH-3T3 cells. These data provide a new insight into regulatory mechanisms of Ras-MAP kinase pathway through heterotrimeric G-protein-mediated pathways.

Lysophosphatidic acid signalling

Lysophosphatidic acid is an intercellular phospholipid messenger that is released from platelets (and probably other cells) and evokes multiple biological responses, ranging from induction of mitogenesis to neurite retraction, by activating a specific G protein coupled receptor. Recent studies indicate that the lysophosphatidic acid receptor acts via the small GTP-binding proteins Ras and Rho to stimulate cell proliferation and to trigger actin-based cytoskeletal events, respectively.

Guanosine 5'-3-O-(thio)triphosphate stimulates tyrosine phosphorylation of p125FAK and paxillin in permeabilized Swiss 3T3 cells. Role of p21rho

Addition of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) to streptolysin O-permeabilized Swiss 3T3 cells induced tyrosine phosphorylation of M(r) 110,000-130,000 and 70,000-80,000 bands. Specifically, GTP gamma S stimulated tyrosine phosphorylation of both focal adhesion kinase (p125FAK) and paxillin. GTP gamma S induced tyrosine phosphorylation was dose-dependent (EC50 of 2.5 microM) and reached maximum levels after 1.5 min for the M(r) 110,000-130,000 band and 2 min for the M(r) 70,000-80,000 paxillin band. Guanosine 5'-O-(2-thiodiphosphate) inhibited GTP gamma S-induced tyrosine phosphorylation with an IC50 of 100 microM. Protein kinase C did not mediate GTP gamma S-induced tyrosine phosphorylation. Varying the Ca2+ concentration from 0 to 6 microM did not increase tyrosine phosphorylation above basal levels and did not affect the ability of GTP gamma S to induce tyrosine phosphorylation. GTP gamma S was able to stimulate tyrosine phosphorylation in the presence of nanomolar concentrations of Mg2+. Furthermore, 30 microM AlF4- only weakly induced tyrosine phosphorylation in permeabilized cells. Pretreatment with the Clostridium botulinum C3 exoenzyme which inactivates p21rho, markedly reduced the ability of GTP gamma S to stimulate tyrosine phosphorylation of M(r) 110,000-130,000 and 70,000-80,000 bands including p125FAK and paxillin in permeabilized Swiss 3T3 cells. Furthermore, a peptide of p21rho (p21rho17-44) inhibited GTP gamma S-induced tyrosine phosphorylation in a dose-dependent manner (IC50 1 microM). This peptide also inhibited tyrosine phosphorylation of p125FAK and paxillin. In contrast, 20 microM p21ras17-44 peptide failed to inhibit GTP gamma S-induced tyrosine phosphorylation. Using permeabilized cells, our findings demonstrate that GTP gamma S stimulates tyrosine phosphorylation of p125FAK and paxillin and that a functional p21rho is implicated in this process.

Effect of tyrosine kinase inhibitors on luteinizing hormone-releasing hormone (LHRH)-induced gonadotropin release from the anterior pituitary

A range of selective tyrosine kinase inhibitors, piceatannol, methyl-2,5-dihydroxycinnamate (MDC), genistein, psi-tectorigenin and lavendustin A, all reduced luteinizing hormone-releasing hormone (LHRH)-induced luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release from pro-oestrous rat hemipituitaries incubated in vitro. In general, both 'initial' release and the augmented release resulting from LHRH self-priming, were reduced in parallel in a concentration-dependent fashion. The effects of piceatannol were independent of the steroidal status of the pituitary tissue. Both piceatannol and MDC greatly reduced LH release by ionomycin and a protein kinase C (PKC) activator, phorbol 12,13-dibutyrate (PDBu), suggesting that the tyrosine kinase(s)-dependent step is in the later stages of the stimulus-secretion pathway activated by the LHRH receptor. These data were supported by immunoblots for phosphotyrosine showing that in the gonadotrope-derived alpha T3-1 cell line, treatment with LHRH caused piceatannol-sensitive increases in specific tyrosine phosphorylation of several proteins (major bands at 65-75 and 120-130 kDa). Treatment of cells with PDBu mimicked the tyrosine phosphorylations evoked by LHRH whereas the PKC inhibitor, GF109203X, partially reduced both LHRH- and PDBu-induced tyrosine phosphorylations. Direct effects of MDC and piceatannol on PKC were assessed in an in vitro PKC assay; piceatannol, but not MDC, inhibited PKC activity but at considerably higher concentrations than required for inhibition of LHRH-induced gonadotropin secretion. These data support a role for tyrosine kinase activation in LHRH-induced secretion.

Insulin stimulates the tyrosine dephosphorylation of pp125 focal adhesion kinase

The phosphorylation state of pp125 focal adhesion kinase in response to insulin was examined in parental and transfected Rat-1 fibroblasts expressing both wild-type (HIRc cells) and mutant human insulin receptor cDNAs lacking the C-terminal twin tyrosine phosphorylation sites (YF2 cells) or a deletion mutant lacking the distal 43 amino acids of the beta-subunit (delta CT cells). In HIRc cells insulin stimulated the tyrosine dephosphorylation of pp125fak, whereas IGF-I did not. In contrast, the tyrosine phosphorylation state of pp125fak was unchanged in the parental Rat-1 fibroblasts and the YF2 or delta CT mutant cell lines in response to insulin. Analysis of the supernatants revealed that pp125fak was only one component of the major M(r), 120-130-kDa phosphotyrosine band seen in HIRc cells. We conclude that: 1) in contrast to other growth factors, insulin stimulates the dephosphorylation of pp125fak; 2) the presence of the insulin receptor C-terminal tyrosines 1328 and 1334 is required for the insulin-stimulated tyrosine dephosphorylation of pp125fak, suggesting a possible SH2 domain-dependent interaction; 3) insulin may modulate integrin-mediated signaling through pp125fak by altering the phosphorylation state of pp125fak.

Regulation of Ras-mediated signalling: more than one way to skin a cat

A powerful combination of genetics and biochemistry has provided details of how Ras-directed signalling interacts with and is regulated by other cellular signalling pathways. This might ultimately lead to the control of deregulated signalling by oncogenic Ras. Recently, progress has been made in understanding the regulation of Ras-mediated activation of the Raf-1-ERK2 kinase cascade through crosstalk with protein kinase C and cyclic-AMP-dependent protein kinase.

Angiotensin II induces phosphatidic acid formation in neonatal rat cardiac fibroblasts: evaluation of the roles of phospholipases C and D

Phosphatidic acid has been proposed to contribute to the mitogenic actions of various growth factors. In 32P-labeled neonatal rat cardiac fibroblasts, 100 nM [Sar1]angiotensin II was shown to rapidly induce formation of 32P-phosphatidic acid. Levels peaked at 5 min (1.5-fold above control), but were partially sustained over 2 h. Phospholipase D contributed in part to phosphatidic acid formation, as 32P- or 3H-phosphatidylethanol was produced when cells labeled with [32P]H3PO4 or 1-O-[1,2- 3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine were stimulated in the presence of 1% ethanol. [Sar1]angiotensin II-induced phospholipase D activity was transient and mainly mediated through protein kinase C (PKC), since PKC downregulation reduced phosphatidylethanol formation by 68%. Residual activity may have been due to increased intracellular Ca2+, as ionomycin also activated phospholipase D in PKC-depleted cells. Phospholipase D did not fully account for [Sar1]angiotensin II-induced phosphatidic acid: 1) compared to PMA, a potent activator of phospholipase D, [Sar1]angiotensin II produced more phosphatidic acid relative to phosphatidylethanol, and 2) PKC downregulation did not affect [Sar1]angiotensin II-induced phosphatidic acid formation. The diacylglycerol kinase inhibitor R59949 depressed [Sar1]angiotensin II-induced phosphatidic acid formation by only 21%, indicating that activation of a phospholipase C and diacylglycerol kinase also can not account for the bulk of phosphatidic acid. Thus, additional pathways not involving phospholipases C and D, such as de novo synthesis, may contribute to [Sar1]angiotensin II-induced phosphatidic acid in these cells. Finally, as previously shown for [Sar1]angiotensin II, phosphatidic acid stimulated mitogen activated protein (MAP) kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine phosphorylation is involved in reorganization of the actin cytoskeleton in response to serum or LPA stimulation

Tyrosine phosphorylation is known to regulate the formation of focal adhesions in cells adhering to extracellular matrix (ECM). We have investigated the possible involvement of tyrosine phosphorylation and the focal adhesion kinase (FAK) in the cytoskeletal changes induced by serum or lysophosphatidic acid (LPA) in quiescent Swiss 3T3 fibroblasts. As shown previously by others, quiescent cells stimulated with serum or LPA reveal a rapid reappearance of focal adhesions and stress fibers. Here we show that this is accompanied by an increase in phosphotyrosine in focal adhesions and specifically an increase in the tyrosine phosphorylation of FAK. The LPA-stimulated reappearance of focal adhesions and stress fibers is blocked by inhibitors of phospholipase C but not by pertussis toxin (PTX), indicating that this LPA signaling pathway is mediated by phospholipase C activation and does not involve PTX-sensitive G proteins. In the absence of serum or LPA, these cytoskeletal effects and the tyrosine phosphorylation of FAK can be mimicked by sodium orthovanadate in conjunction with hydrogen peroxide, agents that inhibit protein tyrosine phosphatases and thereby elevate levels of phosphotyrosine. Two tyrosine kinase inhibitors, erbstatin and genistein block both the serum-induced tyrosine phosphorylation of FAK and the assembly of focal adhesions and stress fibers. Two other tyrosine kinase inhibitors, tyrphostins 47 and 25, previously shown to inhibit FAK, failed to prevent FAK phosphorylation or the reassembly of focal adhesions and stress fibers in response to serum. However, these inhibitors did prevent FAK phosphorylation and cytoskeletal assembly in response to lysophosphatidic acid (LPA), one component of serum previously shown to stimulate assembly of focal adhesions and stress fibers. Our findings suggest that the response to serum is complex and that although FAK phosphorylation is important, other tyrosine kinases may also be involved.

Deciphering the MAP kinase pathway

MAP kinases (MAPK) are serine/threonine kinases which are activated by a dual phosphorylation on threonine and tyrosine residues. Their specific upstream activators, called MAP kinase kinases (MAPKK), constitute a new family of dual-specific threonine/tyrosine kinases, which in turn are activated by upstream MAP kinase kinase kinases (MAPKKK). These three kinase families are successively stimulated in a cascade of activation described in various species such as mammals, frog, fly, worm or yeast. In mammals, the MAP kinase module lies on the signaling pathway triggered by numerous agonists such as growth factors, hormones, lymphokines, tumor promoters, stress factors, etc. Targets of MAP kinase have been characterized in all subcellular compartments. In yeast, genetic epistasis helped to characterize the presence of several MAP kinase modules in the same system. By complementation tests, the relationships existing between phylogenetically distant members of each kinase family have been described. The roles of the MAP kinase cascade have been analyzed by engineering various mutations in the kinases of the module. The MAP kinase cascade has thus been implicated in higher eukaryotes in cell growth, cell fate and differentiation, and in low eukaryotes, in conjugation, osmotic stress, cell wall construct and mitosis.