Involvement of a phosphotyrosine protein phosphatase in the suppression of platelet-derived growth factor receptor autophosphorylation in ras-transformed cells

Desensitization of the PDGFbeta receptor by modulation of the cytoskeleton: the role of p21(Ras) and Rho family GTPases

Ligand-induced PDGF-type beta receptor (PDGFbeta-R) autophosphorylation is profoundly suppressed in cells transformed by activated p21(Ras). We report here that the integrity of the actin cytoskeleton is a critical regulator of PDGFbeta-R function in the presence of p21(Ras). Morphological reversion of Balb cells expressing a constitutively activated p21(Ras), with re-formation of actin stress fibers and cytoskeletal architecture, rendering them phenotypically similar to untransformed fibroblasts, allowed recovery of ligand-dependent PDGFbeta-R autophosphorylation. Conversely, disruption of the actin cytoskeleton in Balb/c-3T3 cells obliterated the normal ligand-induced phosphorylation of the PDGFbeta-R. The Rho family GTPases Rac and Rho are activated by p21(Ras) and are critical mediators of cell motility and morphology via their influence on the actin cytoskeleton. Transient expression of wild-type or constitutively active mutant forms of RhoA suppressed ligand-dependent PDGFbeta-R autophosphorylation and downstream signal transduction. These studies demonstrate the necessary role of Rho in the inhibition of PDGFbeta-R autophosphorylation in cells containing activated p21(Ras) and also demonstrate the importance of cell context and the integrity of the actin cytoskeleton in the regulation of PDGFbeta-R ligand-induced autophosphorylation.

Oncogenic Ras modulates epidermal growth factor responsiveness in endometrial carcinomas

Since the majority of endometrial carcinomas do not contain any detectable ras mutations, the precise contribution of aberrant Ras function, if any, to endometrial carcinoma development remains to be determined. Since there is considerable evidence that Ras transformation is associated with a decreased requirement for growth factors, we compared the growth response of endometrial carcinoma cells harbouring wild-type (Ishikawa cells) or mutated (HHUA cells) K-ras to epidermal growth factor (EGF). K-ras mutation did not significantly affect the level of the EGF receptor (EGFR) expressed in these carcinoma cells. EGF could stimulate the growth of Ishikawa, but not HHUA cells. Furthermore, EGF caused elevation of Ras-GTP levels in Ishikawa, but not HHUA cells. However, the introduction of mutated, but not normal, K-ras into Ishikawa cells rendered them non-responsive to EGF growth stimulation. Thus, the presence of mutated K-ras alone modulated the growth response of endometrial carcinoma cells to EGF. An inhibitor of the EGFR tyrosine kinase activity could prevent soft agar colony formation of Ishikawa cells, but not HHUA or mutant K-ras(12V)-transfected Ishikawa cells. Taken together, these results suggest that mutated K-ras causes a loss of responsiveness to EGF stimulation and that EGFR function is dispensable for the growth of mutant Ras-positive endometrial carcinoma cells.

Abnormal protein tyrosine phosphorylation in fibroblasts from hyperapobetalipoproteinemia subjects

The stimulatory effects of three normal human serum basic proteins (BP), BP I (Mr 14,000, pI 9.10), BP II (Mr 27, 500, pI 8.48), and BP III (Mr 55,000, pI 8.73) on cellular triglyceride and cholesterol formation require intact protein-tyrosine kinase phosphorylation (TKP). Here we examined whether there is an abnormality in TKP in cultured fibroblasts from 11 patients with hyperapobetalipoproteinemia (hyperapoB) that manifest two acylation-stimulatory defects, decreased stimulation of triglyceride synthesis by BP I but enhanced formation of cholesterol by BP II. Soluble and insoluble proteins in Triton X-100 extracts were isolated by immunoprecipitation with a monoclonal anti-phosphotyrosine antibody (MAPA) bound to agarose beads and by ultracentrifugation, respectively, from confluent fibroblasts after incubation for 24 h in supplemented serum-free and lipid-free medium (DMEM/F12). Western blots of insoluble proteins showed that group (Gp) II (Mr 36,000-55,000) and Gp III (Mr 14,000-35,000) from hyperapoB cells, grown in DMEM/F12 medium without BP, had significantly decreased reactivity to MAPA. No significant differences in reactivity to MAPA were detected between normal and hyperapoB cells for Gp I (Mr 97-120,000). BP II, but not BP I or BP III, reversed the decreased reactivity of Gp II and Gp III to MAPA in hyperapoB cells. Sodium vanadate, an inhibitor of phosphotyrosine phosphatases, did not reverse the deficiency in TKP or the 50% deficiency in the stimulation of mass triglyceride by BP I in hyperapoB cells. Tyrosine-phosphorylated Erk-2, a mitogen-activated protein kinase, identified as one of the proteins in Gp II, was significantly decreased in hyperapoB cells. These results provide further evidence for abnormal protein TKP in hyperapoB cells and suggest a possible link between atherosclerotic changes in hyperapoB patients and growth factors upstream from mitogen-activated protein kinase.

Endothelial nitric oxide synthase gene transfer inhibits platelet-derived growth factor-BB stimulated focal adhesion kinase and paxillin phosphorylation in vascular smooth muscle cells

Stimulation of smooth muscle (VSM) cells of guinea pig coronary artery by platelet-derived growth factor (PDGF)-BB retards paxillin mobility (mobility shift) in SDS-PAGE in a time-dependent manner. This mobility shift may be due to tyrosine phosphorylation of paxillin. eNOS gene transfer by replication deficient recombinant adenovirus vector AD5/RSVeNOS in VSM cells inhibited PDGF-BB-stimulated mobility shift and tyrosine phosphorylation of paxillin. Concomitantly, tyrosine phosphorylation of focal adhesion kinase (FAK) was also inhibited. The inhibition of paxillin and FAK tyrosine phosphorylation did not affect stress fiber and focal adhesion formation. Considering the importance of FAK and paxillin in cell migration and proliferation, these results suggest that the FAK-paxillin pathway is a target for NO action to inhibit VSM cell migration and proliferation.

Reversion of Ras- and phosphatidylcholine-hydrolyzing phospholipase C-mediated transformation of NIH 3T3 cells by a dominant interfering mutant of protein kinase C lambda is accompanied by the loss of constitutive nuclear mitogen-activated protein kinase/extracellular signal-regulated kinase activity

The transformed phenotype of v-Ras- or Bacillus cereus phosphatidylcholine-hydrolyzing phospholipase C (PC-PLC)-expressing NIH 3T3 cells is reverted by expressing a kinase-defective mutant of protein kinase C lambda (lambdaPKC). We report here that extracellular signal-regulated kinase (ERK)-1 and -2 are constitutively activated in v-Ras- and PC-PLC-transformed cells in the absence of added growth factors. Interestingly, the activated ERKs were exclusively localized to the cell nucleus. Consistently, the transactivating potential of the C-terminal domain of Elk-1, which is activated upon ERK-mediated phosphorylation, was strongly induced in serum-starved cells expressing v-Ras or PC-PLC. Reversion of v-Ras- or PC-PLC-induced transformation by expression of dominant negative lambdaPKC abolished the nuclear ERK activation suggesting lambdaPKC as a novel, direct or indirect, activator of mitogen-activated protein kinase/ERK kinase in response to activated Ras or elevated levels of phosphatidylcholine-derived diacylglycerol. Transient transfection experiments confirmed that lambdaPKC acts downstream of Ras but upstream of mitogen-activated protein kinase/ERK kinase. We found both the v-Ras- and PC-PLC-transformed cells to be insensitive to stimulation with platelet-derived growth factor (PDGF). No detectable receptor level, autophosphorylation, or superinduction of DNA synthesis could be observed in response to treatment with PDGF. Reversion of the transformed cell lines by expression of dominant negative lambdaPKC restored the receptor level and the ability to respond to PDGF in terms of receptor autophosphorylation, ERK activation, and induction of DNA synthesis.

Enrichment of yeast protein tyrosine kinase activity by substrate affinity chromatography

The direct biochemical analysis of protein tyrosine kinases from yeast has been difficult due to their very low activity in crude cell lysates. Here we present a procedure for the enrichment and partial purification of protein tyrosine kinases from Saccharomyces cerevisiae based on single-step substrate affinity chromatography using a synthetic random co-polymer of glutamic acid and tyrosine. Fractionation of cell lysates on a poly-glutamic acid:tyrosine (4:1)-Sepharose affinity column resulted in a 4000-fold increase in tyrosine kinase activity. Active fractions contain at least six potential protein kinases as judged by in situ phosphorylation assay and Western blot analysis using anti-phosphotyrosine. We propose that this protocol may also be useful for the initial identification and purification of tyrosine kinases from other organisms exhibiting low levels of this enzymatic activity in cell lysates.

Involvement of a subset of tyrosine kinases and phosphatases in regulation of the beta-lactoglobulin gene promoter by prolactin

This study used pharmacological intervention to provide support for a role of kinases and phosphatases in prolactin transactivation of a milk protein gene. It was based on transient cotransfection using a rabbit prolactin receptor expression plasmid and a beta-lactoglobulin promoter/CAT reporter construct. In cotransfected CHO cells, herbimycin A and tyrphostin, two tyrosine kinase inhibitors, were able to decrease the CAT response by over 50%, along with tyrosine phosphorylation of cellular proteins, whereas genistein and lavendustine were without effect on lactoglobulin transactivation. Orthovanadate, an inactivator of tyrosine phosphatases, was able to substitute for prolactin in inducing the CAT response. Staurosporine, a non-specific kinase inhibitor, was able, when used at low concentrations (10 nM), to augment the prolactin response strikingly. Threonine/serine kinases do not appear to be involved early in beta-lactoglobulin promoter transactivation, since four C-kinase inhibitors and okadaic acid a threonine/serine phosphatase inhibitor, were without substantive effect. We conclude that specific tyrosine kinases are responsible for most of the signal transduction from the prolactin receptor to the beta-lactoglobulin gene promoter.

EJ-Ras inhibits phospholipase C gamma 1 but not actin polymerization induced by platelet-derived growth factor-BB via phosphatidylinositol 3-kinase

Transformation of fibroblast-like cells (NIH 3T3) by a constitutively activated GTP-bound isoform of p21ras (EJ-Ras) produces morphogenic changes characterized by decreased attachment to the substratum, with retraction and rounding of the cell body. Transformed fibroblasts lose their "stressed" conformation and adopt a "relaxed" morphology. The specific molecular mechanisms responsible for these changes remain uncharacterized. We found that EJ-Ras transformation of NIH 3T3 cells decreased the cellular content of polymerized actin, particularly at the expense of actin stress fibers, but induced the accumulation of actin filaments in peripheral ruffling membranes. Polymerization of actin could be induced in EJ-Ras-transformed cells by exposure to platelet-derived growth factor (PDGF)-BB to an extent similar to that observed in wild-type NIH 3T3 cells. In EJ-Ras cells, actin polymerization was independent of phospholipase C gamma 1 (PLC gamma 1) activity, because inositol tris-phosphate (IP3) production observed in control NIH 3T3 cells in response to PDGF-BB was absent. Although PDGF-BB did stimulate tyrosine phosphorylation of PLC gamma 1, the phospholipase was strongly inhibited by an inhibitory factor present in the cytoplasm of EJ-Ras-transformed cells. In addition, cytoplasmic extracts of EJ-Ras, but not of control cells, inhibited phosphatidylinositol 4,5-diphosphate (PIP2) hydrolysis catalyzed by a recombinant PLC gamma 1 in vitro. Although PIP2 hydrolysis could not contribute to the reorganization of the actin cytoskeleton induced by PDGF-BB in EJ-Ras-transformed cells, phosphatidylinositol 3-kinase (PI3-K) was necessary for actin polymerization. Wortmannin, a specific PI3-K inhibitor, not only blocked actin polymerization in both control and EJ-Ras-transformed cells but actually led to rapid actin depolymerization when these cells were exposed to PDGF-BB. Thus, in EJ-Ras-transformed cells, cell morphogenic changes in response to PDGF-BB rely importantly on PI3-K and can occur in the complete absence of IP3 production despite tyrosine phosphorylation of PLC gamma 1.

Inhibition of cellular response to platelet-derived growth factor by low M(r) phosphotyrosine protein phosphatase overexpression

The role of low M(r) phosphotyrosine protein phosphatase (PTPase) in the control of cell proliferation was studied. A synthetic gene coding for PTPase was transfected and expressed in NIH/3T3 fibroblasts. The effects of the enzyme were particularly evident when cells were stimulated by platelet-derived growth factor (PDGF). The mitogenic response to PDGF was decreased and the inhibition reached 90%. This effect was more pronounced with respect to fetal calf serum stimulation. Hormone-dependent autophosphorylation of the PDGF receptor was significantly reduced. These results demonstrate that low M(r) PTPase, a cytosolic enzyme, not only affects cellular response to PDGF but also reduces the membrane receptor autophosphorylation.

Protein tyrosine phosphatases: characterization of extracellular and intracellular domains

Protein tyrosine phosphatases (PTPs) play an important role in the regulation of cell growth and differentiation. With over 30 PTPs identified, the specific functions of these enzymes are now being addressed. The identification of extracellular domain receptor-like PTP interactions and the characterization of intracellular PTP 'targeting' domains represent recent efforts in this pursuit.