Assay of phosphotyrosyl protein phosphatase using synthetic peptide 1142-1153 of the insulin receptor

In vivo modulation of N-myristoyltransferase activity by orthovanadate

N-Myristoyltransferase (NMT) catalyses the transfer of myristate from myristoyl-CoA to the NH2-terminal glycine residue of several proteins and are important in signal transduction. STZ-induced diabetes (an animal model for insulin-dependent diabetes mellitus, IDDM) resulted in a 2-fold increase in rat liver NMT activity as compared with control animals. In obese Zucker (fa/fa) rats (an animal model for non-insulin dependent diabetes mellitus, NIDDM) there was a approximately 4.7-fold lower liver particulate NMT activity as compared with the control lean rat livers. Administration of sodium orthovanadate to the diabetic rats normalised liver NMT activity. These results would indicate that the rat liver particulate N-myristoyltransferase activity appears to be inversely proportional to the level of plasma insulin, implicating insulin in the control of N-myristoylation.

Skeletal muscle protein tyrosine phosphatase activity and tyrosine phosphatase 1B protein content are associated with insulin action and resistance

Particulate and cytosolic protein tyrosine phosphatase (PTPase) activity was measured in skeletal muscle from 15 insulin-sensitive subjects and 5 insulin-resistant nondiabetic subjects, as well as 18 subjects with non-insulin-dependent diabetes mellitus (NIDDM). Approximately 90% of total PTPase activity resided in the particulate fraction. In comparison with lean nondiabetic subjects, particulate PTPase activity was reduced 21% (P < 0.05) and 22% (P < 0.005) in obese nondiabetic and NIDDM subjects, respectively. PTPase1B protein levels were likewise decreased by 38% in NIDDM subjects (P < 0.05). During hyperinsulinemic glucose clamps, glucose disposal rates (GDR) increased approximately sixfold in lean control and twofold in NIDDM subjects, while particulate PTPase activity did not change. However, a strong positive correlation (r = 0.64, P < 0.001) existed between particulate PTPase activity and insulin-stimulated GDR. In five obese NIDDM subjects, weight loss of approximately 10% body wt resulted in a significant and corresponding increase in both particulate PTPase activity and insulin-stimulated GDR. These findings indicate that skeletal muscle particulate PTPase activity and PTPase1B protein content reflect in vivo insulin sensitivity and are reduced in insulin resistant states. We conclude that skeletal muscle PTPase activity is involved in the chronic, but not acute regulation of insulin action, and that the decreased enzyme activity may have a role in the insulin resistance of obesity and NIDDM.

Regulation of an hepatic low-M(r) membrane-associated protein-tyrosine phosphatase

Protein-tyrosine phosphatases (PTPases), active against autophosphorylated insulin and epidermal growth factor (EGF) receptors in rat liver, are predominantly membrane associated. Fasting of rats for 48 h decreased hepatic particulate PTPase activity by 15.0-26.9%. This reduction in particulate PTPase activity was due to a rather specific decrease in activity of > 85% of a single species of PTPase, termed PTPase I. Disappearance of PTPase I activity from the particulate fraction was not accounted for by its translocation to the cytosol. PTPase I displayed the highest activity against autophosphorylated insulin and EGF receptors, relative to activity against a 32P-labelled peptide substrate, of three PTPases resolved from the liver particulate fraction. The M(r) value of PTPase I, as determined by gel filtration on a Superose 12 column was approx. 42,000, indicating that PTPase I belongs to the low-M(r) class of PTPases. An antibody raised against PTPase 1B, the prototype of this class of PTPases, did not react with PTPase I in Western blots. The potential importance of the novel change in activity of PTPase I in the regulation of insulin-receptor signal transduction is discussed.

Resin immobilized synthetic peptides used to characterize phosphorylation and antigenic properties of insulin receptor autophosphorylation domains

To develop a common strategy in peptide design for kinase assay, antibody production and affinity purification, we investigated phosphorylation and antigenic properties of peptides immobilized on an aminated polyacrylic resin (Expansin) corresponding to autophosphorylation domains of the insulin receptor tyrosine kinase. Immobilized peptides (1143-1155) and peptide (1314-1330), designated p1151 and p1322, respectively, were good substrates for the insulin receptor with Km of 0.74 and 0.78 mM. By contrast, peptide (952-963), designated p960, was poorly phosphorylated. p1151 showed distinctive behaviour as a substrate, displaying a higher basal phosphorylation, a leftward shift of the insulin dose-response curve (ED50 = 0.7 ng mL-1 insulin compared to 20 ng mL-1 for other substrates) and an inhibition by 90% of receptor autophosphorylation (ID50 = 0.5 mM). Similar substrate behaviour was observed with another tyrosine kinase, the pp60c-src. Antibodies against P1151 and p1322 have comparable reactivity in ELISA, but the antibody against p960 was poor. While purified immunoglobulins (IgG) against both p1151 and p1322 were inhibitors of receptor autophosphorylation and kinase, in immunoprecipitation the IgG against p1151 mainly interacted with the phosphorylated receptor and that against p1322 with non-phosphorylated forms. Functional mapping of the receptor with oligoclonal 1322-antibody revealed inhibition of phosphate transfer to exogenous substrate poly(Glu,Tyr) (4:1) but not towards immobilized p1151. These data provide further support for the distinctive features of endogenous phosphorylation domain 1151. We conclude that immobilized peptides on polyacrylic resin offer a major new potential for use in kinase assays, immunization, immunoabsorbent techniques and purification of well defined oligoclonal antibodies.

Kinase inhibition by a phosphorylated peptide corresponding to the major insulin receptor autophosphorylation domain

We studied the inhibitory effect of non-phosphorylated and triphosphorylated synthetic peptides, corresponding to amino acids 1143-1155 of the insulin proreceptor (domain 1151) on autophosphorylation and kinase of the insulin receptor. Tyrosine-phosphorylated peptides were synthesized using the N-(9-fluorenylmethoxycarbonyl)-O-dibenzylphosphono-L- tyrosine. The triphosphorylated peptide (1151-P3) and the non-phosphorylated peptide (1151-NP), respectively, inhibited insulin receptor autophosphorylation by 65% and 70%, in a dose-dependent and additive manner. When the receptor was pre-phosphorylated for 1 min with [gamma-32P]ATP, 1151-P3 decreased autophosphorylation to 60% of maximum, whereas 1151-NP had no further effect. In both non-activated and preactivated receptors, 1151-P3 inhibition of receptor autophosphorylation was prevented by adding 2 mM vanadate. Kinase activity towards exogenous substrate poly(Glu4, Tyr) was dose-dependently inhibited by both analogues. This effect was independent of the state of receptor phosphorylation or the addition of vanadate. Since 1151-P3 inhibited the exogenous kinase without altering receptor endogenous autophosphorylation after the addition of vanadate, we investigated 1151-NP and 1151-P3 competition for the phosphorylation of a resin-immobilized 1151 peptide. While 1151-NP (at 2 mM) was highly competitive, inhibiting phosphate incorporation by 70%, 1151-P3 caused a four-fold increase in the phosphorylation of 1151-NP--resin. The receptor underwent conformational changes during autophosphorylation and an antibody directed against a peptide corresponding to amino acids 1314-1330 of the proreceptor (1322Ab) was previously shown to immunoprecipitate specifically the non-phosphorylated receptor forms. Nevertheless, the 1322Ab immunoprecipitated a fully autophosphorylated receptor in the presence of 1151-NP, but not of 1151-P3, thus suggesting a conformational change induced by the non-phosphorylated peptide. In conclusion, kinase inhibition was still observed after the addition of phosphate groups to three 1151-peptide tyrosines, but the peptide effect on receptor autophosphorylation, phosphorylation of homologous 1151-NP--resin and conformational changes induced in the receptor was altered dramatically. These data may provide a basis for further understanding the role of tyrosine phosphorylation in insulin receptor kinase activation or regulation.

Two nonradioactive assays for phosphotyrosine phosphatases with activity toward the insulin receptor

Two highly sensitive, nonradiolabeled assays for protein phosphotyrosine phosphatase (PTPase) have been developed. The first assay is based on the use of chemically synthesised phosphotyrosine-containing peptides that can be separated from the dephosphorylated peptide products by HPLC. In this assay, partially purified placental PTPase 1B dephosphorylated three dodecaphosphopeptides (corresponding to insulin receptor autophosphorylation sites at positions PY1146, PY1150, and PY1151) with approximately equal affinity (Km 1.3-2.5 microM), indicating that PTPase 1B shows no distinct preference for the site of dephosphorylation in these peptides. The second assay employs either a phosphopeptide or an autophosphorylated tyrosine kinase domain immobolized on microtiter plate wells. After reaction with PTPase, the remaining unconverted phosphosubstrate is detected in an ELISA using anti-phosphotyrosine antibodies. The latter assay was used to monitor PTPase activity during purification procedures and for characterizing PTPases. Modulation of PTPase activity by orthovanadate, heparin, Zn2+, and EDTA gave similar results in both assays. The immobilized autophosphorylated IR tyrosine kinase domain was a poor substrate for bovine liver alkaline phosphatase and seminal fluid acid phosphatase. The second assay also offers the potential for comparing PTPase activity toward several autophosphorylated tyrosine kinase domains, including those of the insulin, epidermal growth factor, and platelet-derived growth factor receptors.

Dephosphorylation of autophosphorylated insulin and epidermal-growth-factor receptors by two major subtypes of protein-tyrosine-phosphatase from human placenta

The identity of protein-tyrosine-phosphatases (PTPases) active against autophosphorylated insulin receptor was probed by using an insulin-receptor-related peptide phosphorylated on tyrosine (peptide 1142-1153). Two major peaks of PTPase activity were resolved from the particulate (Triton X-100-soluble) fraction of human placenta by chromatography on DEAE-cellulose. The two peaks were purified 1300-2300-fold; other peaks of PTPase activity (greater than 15%) were not detected. Properties of the PTPases indicated that they corresponded to subtypes 1A and 1B. Both subtypes appeared capable of catalysing dephosphorylation of all autophosphorylation sites in three domains of the insulin receptor, with no appreciable difference in the pattern of dephosphorylation detected by two-dimensional tryptic-peptide mapping. The tyrosine-1150 domain of the insulin receptor in triply phosphorylated form was found to be highly sensitive to the action of both PTPases, and was dephosphorylated at least 4 times faster than the doubly and singly phosphorylated forms of the tyrosine-1150 domain or phosphorylation sites in other domains by either PTPase. This is significant, as the level of the triphosphotyrosine-1150 species has been shown to correlate well with the capacity of the insulin-receptor tyrosine kinase to phosphorylate other proteins. Both subtypes also dephosphorylated autophosphorylated epidermal-growth-factor (EGF) receptor by greater than 95%. Placental particulate (and cytosolic) PTPase activity against either receptor distributed approximately 2:1 between subtypes 1A and 1B as assayed in the presence of EDTA. In summary, PTPases within two major subtypes have been identified as phosphotyrosyl-insulin and -EGF-receptor phosphatases in vitro. The PTPases identified exhibit high affinities for substrates and high activities in cells, which is commensurate with the PTPases being important in vivo in controlling or reversing autophosphorylation-induced regulatory or signalling events.

Site-specific dephosphorylation and deactivation of the human insulin receptor tyrosine kinase by particulate and soluble phosphotyrosyl protein phosphatases

Insulin receptor tyrosine kinase activation, induced by insulin-stimulated autophosphorylation, was measured using a synthetic peptide containing residues 1142-1153 of the insulin receptor and shown to be reversed by both particulate and soluble phosphotyrosyl protein phosphatases from rat liver. Deactivation of the tyrosine kinase was highly sensitive to phosphatase action and was correlated best with disappearance of insulin receptors triphosphorylated in the tyrosine-1150 domain. Dephosphorylation of the di- and mono-phosphorylated forms of the tyrosine-1150 domain generated during dephosphorylation or of phosphorylation sites in the C-terminal or putative juxta-membrane domains occurred 3- greater than 10-fold more slowly than deactivation of the tyrosine kinase, and these phosphorylated species did not appear to appreciably (less than 20%) contribute to tyrosine kinase activation. These results indicate that the transition from the triply to the doubly phosphorylated form of the tyrosine-1150 domain acts as an important switch for deactivation of the insulin receptor tyrosine kinase during dephosphorylation. The exquisite sensitivity of this dephosphorylation/deactivation event to phosphotyrosyl protein phosphatase action, combined with the high affinities of this phosphatases for substrates and the high activities of the phosphatases in cells, suggests that the tyrosine kinase activity expressed by insulin-stimulated insulin receptors is likely to be stringently regulated.

Hepatic protein phosphotyrosine phosphatase. Dephosphorylation of insulin and epidermal growth factor receptors in normal and alloxan diabetic rats

Polypeptide hormone signal transmission by receptor tyrosine kinases requires the rapid reversal of tyrosine phosphorylation by protein phosphotyrosine phosphatases (PPTPases). We studied hepatic PPTPases in the rat with emphasis on acute and chronic regulation by insulin. PPTPase activity with artificial substrates ([32P]Tyr-reduced, carboxyamidomethylated, and maleylated lysozyme and [32P]Tyr-poly[glutamic acid:tyrosine] 4:1) was present in distinct membrane, cytoskeletal, and cytosolic fractions. These PPTPase activities were unaffected by alloxan diabetes. Acute administration of insulin to normal animals also did not change PPTPase activity in liver plasma membranes or endosomal membranes. Although alloxan diabetes did not affect PPTPase activity measured with artificial substrates or with epidermal growth factor receptors, a decrease in insulin receptor dephosphorylation was noted. Dephosphorylation of hepatic receptors from normal and diabetic rats by membrane PPTPase from control rats was similar. These results indicate that alloxan diabetes does not lead to a generalized effect on hepatic PPTPase activity, although a substrate-specific decrease in activity with the insulin receptor may occur.

Dephosphorylation of insulin-receptor autophosphorylation sites by particulate and soluble phosphotyrosyl-protein phosphatases

Insulin stimulates autophosphorylation of the insulin receptor on multiple tyrosines in three domains: tyrosines 1316 and 1322 in the C-terminal tail, 1146, 1150 and 1151 in the tyrosine-1150 domain, and possibly 953, 960 or 972 in the juxtamembrane domain. In the present work the sequence of dephosphorylation of the various autophosphorylation sites by particulate and cytosolic preparations of phosphotyrosyl-protein phosphatase from rat liver was studied with autophosphorylated human placental insulin receptor as substrate. Both phosphatase preparations elicited a broadly similar pattern of dephosphorylation. The tyrosine-1150 domain in triphosphorylated form was found to be exquisitely sensitive to dephosphorylation, and was dephosphorylated 3-10-fold faster than the di- and monophosphorylated forms of the tyrosine-1150 domain or phosphorylation sites in other domains. The major route for dephosphorylation of the triphosphorylated tyrosine-1150 domain involved dephosphorylation of one of the phosphotyrosyl pair, 1150/1151, followed by phosphotyrosyl 1146 to generate a species monophosphorylated mainly (greater than 80%) at tyrosine 1150 or 1151. Insulin receptors monophosphorylated in the tyrosine-1150 domain disappeared slowly, and overall the other domains were completely dephosphorylated faster than the tyrosine-1150 domain. Dephosphorylation of the diphosphorylated C-terminal domain yielded insulin receptor in which the domain was singly phosphorylated at tyrosine 1322. Triphosphorylation of the insulin receptor in the tyrosine-1150 domain appears important in activating the receptor tyrosine kinase to phosphorylate other proteins. The extreme sensitivity of the triphosphorylated form of the tyrosine-1150 domain to dephosphorylation may thus be important in terminating or regulating insulin-receptor tyrosine kinase action and insulin signalling.