Insulin-stimulated phosphatidylinositol 3-kinase. Association with a 185-kDa tyrosine-phosphorylated protein (IRS-1) and localization in a low density membrane vesicle

Insulin stimulates the appearance of anti-tyrosine(P)-immunoprecipitable phosphatidylinositol 3-kinase (PI 3-kinase) activity in adipocytes, predominantly in an intracellular membrane fraction (Kelly, K. L., Ruderman, N. B., and Chen, K. S. (1992) J. Biol. Chem. 267, 3423-3428). Neither the mechanism underlying this activation nor the precise subcellular compartment in which it occurs is known. To address these questions, studies were performed using isolated rat adipocytes and subcellular fractions of these cells. In intact cells, insulin stimulated the rapid appearance of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in 32P-labeled adipocytes without changing the labeling of phosphatidylinositol 3-phosphate, phosphatidylinositol 4-phosphate, or phosphatidylinositol 4,5-bisphosphate. This effect was accompanied by the tyrosyl phosphorylation of a 185-kDa protein, tentatively identified as IRS-1, with which PI 3-kinase became associated. The majority of the p85, the regulatory subunit of PI 3-kinase, in untreated adipocytes was present in the cytosol; however, neither the activity of PI 3-kinase nor the total amount of p85 in this fraction was modified in response to insulin. In contrast, insulin increased the association of p85 with IRS-1, the tyrosyl phosphorylation of the IRS-1 associated with p85, and the total activity of PI 3-kinase in the plasma membranes and low density membranes. After insulin treatment, similar amounts of p85 were bound to IRS-1 in the low density and plasma membrane fractions; however, tyrosyl-phosphorylated IRS-1 and PI 3-kinase activity were an order of magnitude greater in the low density membranes. The complex of tyrosyl-phosphorylated IRS-1.p85 that formed in response to insulin was localized to a very low density vesicle subpopulation that could be distinguished from vesicles containing the GLUT-4 glucose transporter and the insulin receptor. These data suggest that the activation of PI 3-kinase by insulin in the adipocyte involves the formation of a complex between IRS-1 and PI 3-kinase in a very low density membrane fraction that is not enriched in GLUT-4 or insulin receptors. They also suggest that PI 3-kinase activation correlates more closely with the extent of tyrosyl phosphorylation of the IRS-1 complexed to PI 3-kinase than it does to the amount of p85 bound to IRS-1.     

Simplified body-surface electrocardiographic maps with depolarization magnitude and direction

A new technique is presented for extracting the magnitude and direction of ventricular depolarization at the body surface from surface electrocardiographic (ECG) map data. Bipolar electrocardiograms were obtained from 36 sites on the chest surface in five normal subjects. The direction and magnitude of depolarization as seen from the chest surface were calculated for 18 body-surface areas centred between electrode positions V1 and V6. Each area was bounded by three electrodes with an electrode spacing of 5 cm. A major depolarization component could be calculated for all triangular areas, with 48% of areas having a smaller second component. The area with the greatest magnitude in each subject had a depolarization vector pointing downwards and to the left, with an average angle to the horizontal of 55 degrees. This was consistent with an average angle of 51 degrees obtained from the subjects' 12-lead electrocardiograms. There was more variability in vector angle between adjacent areas on the right-hand side. At the V5/V6 areas, close to the cardiac apex, the vector component had an upwards orientation in all subjects, opposing the overall downward component of ventricular depolarization. The technique was able to determine local depolarization directions which were in agreement with the normal cardiac vector derived from standard electrocardiography. Reversal of the vector direction close to the cardiac apex and the collision of depolarization components from different directions could be detected. This simple form of body-surface mapping can reduce the essential features of depolarization to a single map, and provide information not directly available from a 12-lead electrocardiogram.     

Giant cutaneous horn: a patient report

A large cutaneous horn was excised from the left side of the nose and cheek of a 68-year-old woman. Reconstruction was performed with a split-thickness skin graft. Histologically the lesion represented squamous cell carcinoma. The nature of malignant degeneration in cutaneous horns is discussed.     

Evolutionary shifts in three major structural features of the mitochondrial genome among iguanian lizards

A phylogenetic tree for major lineages of iguanian lizards is estimated from 1,488 aligned base positions (858 informative) of newly reported mitochondrial DNA sequences representing coding regions for eight tRNAs, ND2, and portions of ND1 and COI. Two well-supported groups are defined, the Acrodonta and the Iguanidae (sensu lato). This phylogenetic hypothesis is used to investigate evolutionary shifts in mitochondrial gene order, origin for light-strand replication, and secondary structure of tRNACys. These three characters shift together on the branch leading to acrodont lizards. Plate tectonics and the fossil record indicate that these characters changed in the Jurassic. We propose that changes to the secondary structure of tRNACys may destroy function of the origin for light-strand replication which, in turn, may facilitate shifts in gene order.