Role of the insulin receptor C-terminal acidic domain in the modulation of the receptor kinase by polybasic effectors

Regulation of heart insulin receptor tyrosine kinase activity by magnesium and spermine

Insulin action and aspects of the insulin-signaling pathway have been studied in the heart although the direct regulation of the heart's insulin receptor has not been explored. This study describes the first purification and characterization of the mammalian (rabbit, rat and bovine) heart insulin receptor. The rabbit heart IR showed maximum insulin binding of 18 microg/mg (approximately 1 mole insulin/mole (alpha2beta2) receptor) and a curvilinear Scatchard plot with a high affinity KD for insulin binding of approximately 4 nM at optimal pH (7.8) and NaCl concentration (150 mM). The insulin receptor tyrosine kinase activity was stimulated by insulin, Mg2+ (half-maximum response at approximately 5.6-10.6 nM and approximately 8.5 mM, respectively) and by the physiological polyamines, spermine and spermidine. The stimulation by Mg2+ and the polyamines occurred with and without insulin. These characteristics of the heart insulin receptor provide a mechanism for regulating the activity of the receptor's tyrosine kinase activity by the intracellular free Mg2+ concentration and the polyamines in the absence and presence of insulin.

Prostaglandylinositol cyclic phosphate (cPIP): a novel second messenger of insulin action. Comparative analysis of two kinds of "insulin mediators"

Insulin induces a broad spectrum of effects over a wide time interval. It also stimulates the phosphorylation of some cellular proteins, while decreasing the state of phosphorylation of others. These observations indicate the presence of different, but not necessarily mutually exclusive, pathways of insulin action. One well-known pathway represents a phosphorylation cascade initiated by the tyrosine kinase activity of the insulin receptor followed by involvement of different MAP-kinases. Another pathway suggests the existence of low molecular weight insulin mediators whose synthesis and/or release is initiated by insulin. Comparable analysis of two kinds of insulin mediators, namely inositolphosphoglycans and prostaglandylinositol cyclic phosphate (cPIP), has been carried out. It has been shown that the expression of a number of enzymes, such as phospholipase A(2), phospholipase C, cyclo-oxygenase and IRS-1-like enzyme, could regulate the biosynthesis of cPIP in both normal and diabetes-related conditions. Data on the activity of a key enzyme of cPIP biosynthesis termed cPIP synthase (IRS-1-like enzyme) in various monkey tissues before and twice during an euglycemic hyperinsulinemic clamp have been presented. It has been concluded that in vivo insulin increases cPIP synthase activity in both liver and subcutaneous adipose tissue of lean normal monkeys. It has been also suggested that abnormal production of cPIP could be related to several pathologies including glucocorticoid-induced insulin resistance and diabetic embryopathy. Further studies on cPIP and other types of insulin mediators are necessary to aid our understanding of insulin action.

Purification and characterization of a feline hepatic insulin receptor

OBJECTIVE

To elucidate the functional characteristics of a highly purified soluble liver insulin receptor in cats.

SAMPLE POPULATION

Frozen livers from domestic cats were obtained commercially.

PROCEDURES

The feline hepatic insulin receptor was purified from Triton X-100 solubilized plasma membranes by the use of several chromatography matrices, including affinity chromatography on an insulin-Sepharose matrix.

RESULTS

The receptor, although not homogeneous, was purified 3,000-fold. Two silver-stained protein bands were identified following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with molecular weight of 134,000 and 97,000, which are similar to insulin receptors isolated from other animals. This isolated receptor had steady-state insulin binding by 40 minutes at 24 C. Optimal insulin binding occurred at pH 7.8 and with 150 mM NaCl. Under these conditions, a curvilinear Scatchard plot was obtained with the isolated receptor. Using a 2 binding-site model, the feline insulin receptor had a high-affinity low-capacity site with a dissociation constant (KD; nM) of 3 and a low-affinity high-capacity site with a K(D) of 1,180. The receptor also had tyrosine kinase activity toward an exogenous substrate that was stimulated by insulin and protamine.

CONCLUSIONS AND CLINICAL RELEVANCE

Many of the reported characteristics of the liver insulin receptor in cats are similar to those for the receptor isolated from other animals and tissues, although some differences exist. These similarities suggest that characterization of the feline insulin receptor is important to understanding insulin resistance in cats with diabetes as well as in humans with diabetes.

Detection of protein tyrosine kinase activity using a high-capacity streptavidin-coated membrane and optimized biotinylated peptide substrates

A protein tyrosine kinase (PTK) assay system is described that uses a series of optimized biotinylated peptide substrates in conjunction with a streptavidin-coated matrix (SAM(2)) biotin capture membrane. The SAM(2) biotin capture membrane provides low backgrounds and high linear binding capacity (up to approximately 3.6 nmol of biotinylated PTK peptide/cm(2)), resulting in high signal-to-noise ratios and greater reproducibility. Capture of the phosphorylated peptide substrates onto the SAM(2) membrane is rapid and occurs independent of the amino acid sequence of the peptide, thereby overcoming difficulties commonly encountered with other methodologies. Two broad-specificity biotinylated PTK peptide substrates were identified with optimum kinetic properties, allowing members from eight distinct classes of enzymes, including transmembrane (epidermal growth factor receptor (EGFR), fibroblast growth factor receptor, insulin receptor, and platelet-derived growth factor receptor) and cytoplasmic (p43(abl), p56(lck), p60(src), and p93(fes)) PTKs, to be analyzed. A third biotinylated peptide substrate, shown to be highly selective for the EGFR, was used to illustrate the versatility of this system for both broad specificity and highly selective detection of PTK activity. The ability to accurately detect activity under optimum conditions and with crude cell extract samples, including kinetic analysis and with enzyme detection limits in the low femtomole range, supports the utility of this assay system for studying PTK enzymes.