Characterization of sites of serine phosphorylation in human placental insulin receptor copurified with insulin-stimulated serine kinase activity by two-dimensional thin-layer peptide mapping

Brazilin inhibits activities of protein kinase C and insulin receptor serine kinase in rat liver

Hypoglycemic action of brazilin was found to be based on the improvement of peripheral glucose utility, and this action might be correlated with the insulin action pathway. In the present study we investigated the effect of brazilin on the insulin receptor autophosphorylation, protein kinase C (PKC), protein phosphatase and insulin receptor serine kinase in order to confirm whether the hypoglycemic mechanism is concerned with insulin action pathway. Brazilin was found to inhibit PKC and insulin receptor serine kinase, which are involved in the regulation of insulin signal pathway. But any significant effect was not shown on insulin receptor tyrosine kinase activity, autophosphorylation and phosphatase activity. These findings suggest that brazilin might enhance insulin receptor function by decreasing serine phosphorylation, which might mediate hypoglycemic effect of brazilin.

Purification and characterization of an insulin-stimulated insulin receptor serine kinase

In cells, insulin stimulates autophosphorylation of the insulin receptor on tyrosine and its phosphorylation on serine and threonine by poorly characterized kinases. Here we describe methods for the purification of an insulin-stimulated insulin receptor serine kinase from human placenta and rat liver by sequential chromatography of solubilized membranes on wheat germ agglutinin-agarose, Mono Q, phenyl-Superose, and Superose 12. On silver-stained SDS-polyacrylamide gels, the resulting kinase was homogeneous (human) or near-homogeneous (rat) and had an apparent M(r) of 40000. The apparent M(r) determined by gel filtration was also 40000, suggesting that the kinase exists as a monomer. The kinase could be reconstituted back to the insulin receptor stripped of the kinase to yield a high stoichiometry of serine phosphorylation of the insulin receptor in the presence of insulin (0.75 +/- 0.15 mol/mol of beta-subunit, mean +/- SEM, n = 3). The activity of the reconstituted kinase toward the insulin receptor was insulin-regulated, being stimulated > 5-fold by insulin. Insulin increased the catalytic activity of the reconstituted kinase. The purified kinase specifically phosphorylated serine 1078 of the insulin receptor, a major site of insulin-stimulated serine phosphorylation in vivo, showing that the purified kinase phosphorylated a physiologically relevant site on the insulin receptor. Phosphorylation of serine 1078 of the insulin receptor to high stoichiometry by the kinase did not affect insulin-stimulated exogenous protein tyrosine kinase activity of the insulin receptor. Similarly, insulin receptor phosphorylated with or without the purified kinase exhibited the same levels of tyrosine autophosphorylation and of the tyrosine kinase-activating tris-phosphorylated kinase domain species. Properties of the kinase distinguished it from kinases known to act on the insulin receptor and other kinases that are insulin-stimulated, indicating that the kinase is a novel entity. The serine kinase underwent autophosphorylation on serine and immunoprecipitated with the insulin receptor. The availability of the purified kinase should facilitate cloning of the kinase, determination of the mechanism of activation of the kinase, and study of the wider potential role of the kinase in insulin signalling, and the ability to be able to phosphorylate serine 1078 to high stoichiometry should facilitate further studies into the function of this serine phosphorylation site.

Studies on an insulin-stimulated insulin receptor serine kinase activity: separation of the kinase activity from the insulin receptor and its reconstitution back to the insulin receptor

In cells insulin stimulates autophosphorylation of the insulin receptor on tyrosine and its phosphorylation on serine and threonine by poorly characterized kinases. Recently we have achieved co-purification of the insulin receptor with insulin-stimulated insulin receptor serine kinase activity. We now show that the co-purified serine kinase activity can be removed by NaCl washing and reconstituted by adding back the NaCl eluate. Reconstitution enabled higher serine phosphorylation than achieved with the co-purified preparation. Myelin basic protein was discovered to be a potent substrate for insulin-stimulated serine phosphorylation by the co-purified preparation, with the activity responsible having similar properties to the serine kinase activity towards the receptor. Myelin basic protein was also phosphorylated on serine by the NaCl eluate. Myelin basic protein phosphorylated by the co-purified preparation or the NaCl eluate gave the same set of phosphoserine peptides. The major myelin basic protein serine kinase activity in the NaCl eluate co-purified exactly on Mono Q with the activity that restored insulin-stimulated insulin receptor serine phosphorylation. These results provide strong evidence for the true separation of the serine kinase from the insulin receptor and the distinctiveness of the serine kinase activity from the insulin receptor tyrosine kinase and mitogen-activated protein kinases. The procedures developed for the isolation of the serine kinase and the establishment of an effective in vitro substrate should allow purification of the kinase. The protocols also provide flexible systems for identifying the functions of the insulin-stimulated serine phosphorylations and the respective kinase(s).

Insulin receptor mutation at tyrosines 1162 and 1163 alters both receptor serine phosphorylation and desensitization

Chinese hamster ovary (CHO) cells expressing human insulin receptor (hIR) of the wild-type (CHO R) or hIR mutated at tyrosines 1162 and 1163 (CHO Y2) were compared for agonist-induced receptor phosphorylation of serine/threonine residues and receptor desensitization. Relative to CHO R cells, CHO Y2 cells exhibited a marked decrease in their response to insulin and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) for hIR phosphorylation on serine residues. Moreover, the tyr1162,1163 mutant hIR could not be normally phosphorylated by purified protein kinase C (PKC) in vitro. Finally, in contrast to CHO R cells, CHO Y2 cells were refractory to PMA-induced IR desensitization for subsequent activation by insulin of exogenous tyrosine kinase and glycogen synthesis. These results strongly suggest that the replacement of tyrosines 1162 and 1163 by phenylalanine residues changes the IR beta-subunit conformation and thus impedes phosphorylation of the IR at crucial serine residues and prevents PMA-induced desensitization. This supports the hypothesis that IR serine phosphorylation and desensitization are related.

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.

Insulin-stimulated serine/threonine phosphorylation of the insulin receptor: paucity of threonine 1348 phosphorylation in vitro indicates the involvement of more than one serine/threonine kinase in vivo

Immunoaffinity-purified insulin receptors were used to analyse and compare the serine/threonine sites phosphorylated on the insulin receptor in vitro (isolated receptor) with the insulin-stimulated phosphorylation in vivo (intact cells in culture). In vivo, insulin-stimulation resulted in the appearance of three phosphoserine-containing phosphopeptides and a distinct phosphothreonine peptide (threonine 1348). In vitro, similar phosphoserine peptides were observed but the phosphothreonine peptide was absent. These results indicate that multiple serine sites are phosphorylated in vivo and in vitro and that an additional protein kinase mediates insulin-stimulated insulin receptor threonine phosphorylation in vivo.

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.