Insulin activates the kinase activity of the Raf-1 proto-oncogene by increasing its serine phosphorylation

Insulin was found to stimulate the serine/threonine kinase activity of the proto-oncogene product Raf-1. This stimulation was observed in HeLa, NIH 3T3, and Chinese hamster ovary cells, all overexpressing the human insulin receptor. In the HeLa cells, 100 pM insulin gave a significant increase in Raf-1 kinase activity, and 100 nM insulin caused a maximal 2-5-fold increase in activity. The increase in activity was detected after 2 min of insulin treatment and peaked after 5 min. In addition to stimulating Raf-1 kinase activity, insulin caused a shift in the electrophoretic mobility of the Raf-1 protein and an increase in the amount of serine phosphorylation of Raf-1. Moreover, a serine/threonine-specific phosphatase, phosphatase 1, but not two tyrosine-specific phosphatases, was found to deactivate the insulin-activated Raf-1 kinase activity. These findings indicate that insulin activates the serine/threonine kinase activity of the Raf-1 proto-oncogene by increasing its content of phosphoserine.     

Identification of p130(cas) as a substrate for the cytosolic protein tyrosine phosphatase PTP-PEST.

PTP-PEST is a ubiquitously expressed, cytosolic, mammalian protein tyrosine phosphatase (PTP) which exhibits high specific activity in vitro. We have investigated the substrate specificity of PTP-PEST by a novel substrate-trapping approach in combination with in vitro dephosphorylation experiments. We initially identified a prominent 130-kDa tyrosine-phosphorylated protein in pervanadate-treated HeLa cell lysates which was preferentially dephosphorylated by PTP-PEST in vitro. In order to identify this potential substrate, mutant (substrate-trapping) forms of PTP-PEST were generated which lack catalytic activity but retain the ability to bind substrates. These mutant proteins associated in stable complexes exclusively with the same 130-kDa protein, which was identified as p130(cas) by immunoblotting. This exclusive association was observed in lysates from several cell lines and in transfected COS cells, but was not observed with other members of the PTP family, strongly suggesting that p130(cas) represents a major physiologically relevant substrate for PTP-PEST. Our studies suggest potential roles for PTP-PEST in regulation of p130(cas) function. These functions include mitogen- and cell adhesion-induced signalling events and probable roles in transformation by various oncogenes. These results provide the first demonstration of a PTP having an inherently restricted substrate specificity in vitro and in vivo. The methods used to identify p130(cas) as a specific substrate for PTP-PEST are potentially applicable to any PTP and should therefore prove useful in determining the physiological substrates of other members of the PTP family.     

Effect of microinjection of a low-Mr human placenta protein tyrosine phosphatase on induction of meiotic cell division in Xenopus oocytes.

Homogeneous preparations of a protein phosphatase that is specific for phosphotyrosyl residues (protein tyrosine phosphatase [PTPase] 1B) were isolated from human placenta and microinjected into Xenopus oocytes. This resulted in an increase in activity of up to 10-fold over control levels, as measured in homogenates with use of an artificial substrate (reduced carboxamidomethylated and maleylated lysozyme). Microinjected PTPase was stable for at least 18 h. It is distributed within the oocyte in a manner similar to the endogenous activity and is suggestive of an interaction with cellular structures or molecules located predominantly in the animal hemisphere. The phosphatase markedly retarded (by up to 5 h) maturation induced by insulin. This, in conjunction with the demonstration that PTPase 1B abolished insulin stimulation of an S6 peptide (RRLSSLRA) kinase concomitant with a decrease in the phosphorylation of tyrosyl residues in a protein with the same apparent Mr as the beta subunit of the insulin and insulinlike growth factor 1 receptors (M. F. Cicirelli, N. K. Tonks, C. D. Diltz, E. H. Fischer, and E. G. Krebs, submitted for publication), provides further support for an essential role of protein tyrosine phosphorylation in insulin action. Furthermore, maturation was significantly retarded even when the PTPase was injected 2 to 4 h after exposure of the cells to insulin. PTPase 1B also retarded maturation induced by progesterone and maturation-promoting factor, which presumably do not act through the insulin receptor. These data point to a second site of action of the PTPase in the pathway of meiotic cell division, downstream of the insulin receptor and following the appearance of active maturation-promoting factor.     

Promoter Hypermethylation in Tumor Suppressing Genes p16 and FHIT and Their Relationship with Estrogen Receptor and Progesterone Receptor Status in Breast Cancer Patients from Northern India

BACKGROUND: Aberrant DNA methylation has been recognized in human breast carcinogenesis as a common molecular alteration associated with the loss of expression of a number of key regulatory genes. The present study was undertaken to determine whether methylation and expression of p16 and FHIT genes would correlate with the estrogen receptor (ER) and progesterone receptor (PR) status. METHODS: Methylation-specific polymerase chain reaction, messenger RNA (mRNA) expression analysis, immunohistochemistry, and Western blot analysis were performed to study the methylation of p16 and FHIT genes in 351 pairs of malignant/normal breast tissues. We examined the expression of ER and PR in those specimens by immunohistochemistry. Mutations of p16 and FHIT genes in tumors were detected by direct sequencing. RESULTS: The frequency of hypermethylation was 31.9% and 36.8% in p16 and FHIT genes, respectively, and showed significant harmony in concordant hypermethylation (P < .0001). In postmenopausal patients, methylation frequency in both genes is significantly higher in poorly and moderately differentiated tumors. Loss of protein expression of p16 and FHIT in 77 and 74 tumors, respectively, is associated with their methylation status in premenopausal women. CONCLUSION: We did not find any significant differences in tumor-related gene methylation patterns relevant to both ER and PR status of breast tumors.     

Regulation of insulin signaling through reversible oxidation of the protein-tyrosine phosphatases TC45 and PTP1B

Many studies have illustrated that the production of reactive oxygen species (ROS) is important for optimal tyrosine phosphorylation and signaling in response to diverse stimuli. Protein-tyrosine phosphatases (PTPs), which are important regulators of signal transduction, are exquisitely sensitive to inhibition after generation of ROS, and reversible oxidation is becoming recognized as a general physiological mechanism for regulation of PTP function. Thus, production of ROS facilitates a tyrosine phosphorylation-dependent cellular signaling response by transiently inactivating those PTPs that normally suppress the signal. In this study, we have explored the importance of reversible PTP oxidation in the signaling response to insulin. Using a modified ingel PTP assay, we show that stimulation of cells with insulin resulted in the rapid and transient oxidation and inhibition of two distinct PTPs, which we have identified as PTP1B and TC45, the 45-kDa spliced variant of the T cell protein-tyrosine phosphatase. We investigated further the role of TC45 as a regulator of insulin signaling by combining RNA interference and the use of substrate-trapping mutants. We have shown that TC45 is an inhibitor of insulin signaling, recognizing the beta-subunit of the insulin receptor as a substrate. The data also suggest that this strategy, using ligand-induced oxidation to tag specific PTPs and using interference RNA and substrate-trapping mutants to illustrate their role as regulators of particular signal transduction pathways, may be applied broadly across the PTP family to explore function.     

Hepatocyte growth factor receptor tyrosine kinase met is a substrate of the receptor protein-tyrosine phosphatase DEP-1

The receptor protein-tyrosine phosphatase (PTP) DEP-1 (CD148/PTP-eta) has been implicated in the regulation of cell growth, differentiation, and transformation, and most recently has been identified as a potential tumor suppressor gene mutated in colon, lung, and breast cancers. We have generated constructs comprising the cytoplasmic segment of DEP-1 fused to the maltose-binding protein to identify potential substrates and thereby suggest a physiological function for DEP-1. We have shown that the substrate-trapping mutant form of DEP-1 interacted with a small subset of tyrosine-phosphorylated proteins from lysates of the human breast tumor cell lines MDA-MB-231, T-47D, and T-47D/Met and have identified the hepatocyte growth factor/scatter factor receptor Met, the adapter protein Gab1, and the junctional component p120 catenin as potential substrates. Following ligand stimulation, phosphorylation of specific tyrosyl residues in Met induces mitogenic, motogenic, and morphogenic responses. When co-expressed in 293 cells, the full-length substrate-trapping mutant form of DEP-1 formed a stable complex with the chimeric receptor colony stimulating factor 1 (CSF)-Met and wild type DEP-1 dephosphorylated CSF-Met. Furthermore, we observed that DEP-1 preferentially dephosphorylated a Gab1 binding site (Tyr(1349)) and a COOH-terminal tyrosine implicated in morphogenesis (Tyr(1365)), whereas tyrosine residues in the activation loop of Met (Tyr(1230), Tyr(1234), and Tyr(1235)) were not preferred targets of the PTP. The ability of DEP-1 preferentially to dephosphorylate particular tyrosine residues that are required for Met-induced signaling suggests that DEP-1 may function in controlling the specificity of signals induced by this PTK, rather than as a simple "off-switch" to counteract PTK activity.