Ras signaling in the activation of glucose transport by insulin

An approach involving microinjection and microanalysis has been developed to investigate signal-transduction pathways involved in the hormonal control of metabolism. We have applied this strategy to investigate the role of Ras signaling in the acute activation of glucose transport by insulin in cardiac myocytes. Glucose transport activity was assessed by measuring the initial rate of accumulation of 2-deoxyglucose 6-phosphate (dGlc6P) in individual cells after incubation in 2-deoxyglucose. Insulin increased accumulation of dGlc6P by 3- to 4-fold, consistent with its stimulatory effect on glucose transport. Accumulation of dGlc6P was increased severalfold by microinjecting the nonhydrolyzable GTP analogue, guanosine 5'-[gamma-thio]triphosphate, which activates members of the Ras superfamily of GTP-binding proteins. Injecting activated Ha-Ras protein also mimicked insulin by increasing dGlc6P; whereas, injecting a Ras protein lacking the COOH-terminal site of fatty acylation required for Ras function was without effect. Introducing the neutralizing Ras antibody Y13-259 into cells attenuated the effect of insulin. These findings implicate Ras in the acute regulation of metabolism by insulin.

Glucose transporters in single skeletal muscle fibers. Relationship to hexokinase and regulation by contractile activity

Glucose transport and phosphorylation are the first steps in the utilization of extracellular glucose by skeletal muscle. We have examined the relationships between proteins mediating these steps in single fibers of identified type dissected from rabbit skeletal muscle. The level of the glucose transporter isoform GLUT4, measured by immunoblotting, varied among fibers by a factor of 20 (slow oxidative > fast oxidative glycolytic > fast glycolytic). In fibers from the tibialis anterior muscle, GLUT4 was correlated (r2 = 0.75) with the activity of malate dehydrogenase, an enzyme representative of oxidative energy metabolism. In these fibers a strong correlation (r2 = 0.70) was also observed between GLUT4 and hexokinase activity. GLUT1 levels were barely detectable, regardless of fiber type. To investigate the possible role of muscle activity in controlling the expression of transporters, tibialis anterior muscles were activated by chronic electrical stimulation of the peroneal nerves. GLUT1 levels increased after 1 day of stimulation to a plateau that was severalfold higher than the level in non-stimulated cells. Hexokinase activity and the GLUT4 level changed in parallel: both were increased by approximately 2.5-fold after 1 day and by 14-fold after 21 days. Thus, while both GLUT1 and GLUT4 were regulated by muscle activity, only GLUT4 expression was coordinated with the expression of hexokinase.

Molecular cloning and tissue distribution of PHAS-I, an intracellular target for insulin and growth factors

Although the actions of insulin and a number of growth factors that signal via protein-tyrosine kinase receptors are believed to involve increased phosphorylation of key intracellular proteins, relatively few of the downstream phosphoproteins have been identified. In this report we describe a cDNA encoding one of the most prominent insulin-stimulated phosphoproteins in rat adipocytes. The cDNA encodes a protein, designated PHAS-I, which has 117 amino acids and a M(r) of 12,400. When translated in vitro and subjected to SDS/PAGE, PHAS-I migrates anomalously, having an apparent M(r) of 21,000. The predicted amino acid composition is interesting in that approximately 45% of the PHAS-I protein is accounted for by only four amino acids--serine, threonine, proline, and glycine. The PHAS-I gene is expressed in a variety of tissues, although the highest levels of mRNA are present in fat and skeletal muscle, two of the most insulin-responsive tissues. The nucleotide and deduced amino acid sequences of PHAS-I differ from any that have been reported, and homology screening provided no clues concerning the function of the protein. However, in view of its tissue distribution and the fact that the protein is phosphorylated in response to insulin, we speculate that PHAS-I is important in insulin action.

Glucose transport and phosphorylation in single cardiac myocytes: rate-limiting steps in glucose metabolism

Microanalytic methods were used to investigate the regulation of glucose metabolism by insulin in single myocytes isolated from adult rat ventricles. Cultured myocytes were incubated with or without insulin and, with either glucose or 2-deoxyglucose (2-DG), rinsed, and freeze-dried. Individual cells were weighed and levels of 2-DG-6-phosphate (2-DG-6-P) or glucose and glucose 6-phosphate (G-6-P) were determined after enzymatic amplification. In cells incubated with 2-DG, insulin increased the level of 2-DG-6-P by as much as 30-fold, indicative of dramatic activation of glucose transport. In cells incubated with glucose, insulin increased the levels of G-6-P by approximately threefold. Increasing extracellular glucose without insulin also increased G-6-P; however, intracellular glucose concentrations were not increased, indicating that glucose transport is rate limiting in nonstimulated myocytes. In contrast, intracellular glucose concentrations were increased by over an order of magnitude by insulin, reaching 60% of the extracellular glucose concentration. Measurements of glucose and G-6-P in the same insulin-treated cells revealed that accumulation of G-6-P reached a plateau when extracellular glucose was increased > 2 mM. At this point the estimated intracellular glucose concentration was 300 microM, or approximately 10 times the Michaelis constant of hexokinase for glucose. These results indicate that in the presence of insulin and physiological concentrations of glucose, hexokinase is saturated with glucose. Consequently, the rate-limiting step for insulin-stimulated glucose utilization is glucose phosphorylation rather than glucose transport.

Increasing cAMP attenuates activation of mitogen-activated protein kinase

Activation of the mitogen-activated protein kinase (MAP kinase) isoforms ERK1 and ERK2 was investigated in rat adipocytes. Kinase activities were measured by using myelin basic protein as substrate after the isoforms were resolved by Mono Q chromatography or by immunoprecipitation with specific antibodies. Insulin increased the activity of both isoforms by 3- to 4-fold. The beta-adrenergic agonist isoproterenol was without effect in the absence of insulin but markedly reduced the increases in ERK1 and ERK2 activities produced by the hormone. MAP kinase activation was also attenuated by forskolin and glucagon, which increase intracellular cAMP, and by dibutyryl-cAMP, 8-bromo-cAMP, and 8-(4-chlorophenylthio)-cAMP. Thus, increasing cAMP is associated with decreased activation of MAP kinase by insulin. Forskolin also inhibited activation of MAP kinase by several agents (epidermal growth factor, phorbol 12-myristate 13-acetate, and okadaic acid) that act independently of insulin receptors. Moreover, forskolin did not inhibit insulin-stimulated tyrosine phosphorylation of the insulin receptor substrate IRS-1. Therefore, the inhibitory effect on MAP kinase did not result from compromised functioning of the insulin receptor. The inhibitory effect was not confined to adipocytes, as forskolin and dibutyryl-cAMP inhibited the increase in MAP kinase activity by phorbol 12-myristate 13-acetate in wild-type CHO cells. In contrast, these agents did not inhibit MAP kinase activity in mutant CHO cells (line 10248) that express a cAMP-dependent protein kinase resistant to activation by cAMP. Our results suggest that activation of cAMP-dependent protein kinase represents a general counter-regulatory mechanism for opposing MAP kinase activation.

Biochemical analysis of a mutant Tetrahymena lacking outer dynein arms

Tetrahymena thermophila mutants homozygous for the oad mutation become nonmotile when grown at the restrictive temperature, and axonemes isolated from nonmotile mutants lack approximately 90% of their outer dynein arms. Electrophoretic analyses of axonemes isolated from nonmotile mutants (oad axonemes) indicate they contain significantly fewer of the 22 S dynein heavy chains that axonemes isolated from wild-type cells (wild-type axonemes) contain. The 22 S dynein heavy chains that remain in axonemes isolated from nonmotile, oad mutants are assembled into 22 S dynein particles that exhibit wild-type levels of ATPase activity. Two-dimensional gel electrophoresis of oad axonemes show that they are deficient in no proteins other than those proteins thought to be components of 22 S dynein. This report is the first formal proof that outer dynein arms in Tetrahymena cilia are composed of 22 S dynein.

The therapeutic and biological effects of total lymph node irradiation and autologous bone marrow infusion on malignant lymphoma patients

Forty patients with malignant lymphoma were treated by 60Co total lymphoid irradiation (TLI): 21 cases received 6 Gy and 19 received 8 Gy. Ten also received autologous bone marrow infusion (ABMI). Acute radiation damage with digestive tract reaction and hemopoietic and immunological depression was observed. Bone marrow was depressed. WBC and platelets decreased rapidly. Lymphocytes showed quantitative and qualitative changes even at the early stage. All these symptoms subsided within 40 days. TLI accompanied by irradiation of the tumor site could result in effective control. The 1-, 3-, and 5-year survival rates of malignant lymphoma patients were 30/40 (75%), 14/24 (58%) and 4/12 (33%), respectively, while in those with Hodgkin's disease alone, the 1- and 3-year survival rates were 10/13 (76%) and 5/7 (71.4%), respectively. ABMI hastened hemopoietic reconstitution, which recovered relatively quickly after TLI.

Reflection Zernike phase contrast microscope

A novel configuration for a reflection Zernike phase contrast microscope is given that has a height sensitivity of 0.5 A.