3. Bedo G, Santisteban P, Aranda A. Retinoic acid regulates growth hormone gene expression. Na- ture 1989;339:231-4

4. Regulation of growth hormone gene expression by retinoic acid. Nutr Rev 1989;47:374-5

5. Vasios GW, Gold JD, Petkovich M, Chambon P, Gudas W. A retinoic acid-responsive element is present in the 5’-flanking region of the laminin B, gene. Proc Natl Acad Sci USA. 1989;86:9099-103

6. De The H, Marchio A, Tiollais P, Dejean A. Differ- ential expression and ligand regulation of the reti-

noic acid receptor a and p genes. Embo J

7. De The H, Vivanco-Ruiz MDM, Tiollais P, Stunnen- berg H, Dejean A. Identification of a retinoic acid responsive element in the retinoic acid receptor beta gene. Nature 1990;343:177-80

8. Brand N, Petkovich M, Krust A, et al. Identification of a second human retinoic acid receptor. Nature

9. Thaller C, Eichele G. The identification and spatial distribution of retinoids in the developing chick limb bud. Nature 1987;327:625-8

1989;8:429-33

1 988 ;332 :850- 3

HOW GLUCOSE GETS INTO CELLS

Glucose entry into cells is mediated by specific carrier proteins called glucose trans- porters. Five types of glucose transporters have been identified. One is found only in tissues requiring insulin for glucose uptake: heart, skeletal muscle, and adipose tissue. This glucose transporter is rapidly recycled between microsomal storage sites and plasma membrane by an insulin-dependent process.

Of the many cellular effects of insulin, per- haps the best known is its role in stimulat- ing the uptake of glucose by certain tis- sues.’ During the last 30 years, it has been well established* that the entry of glucose into virtually all cells in mammalian tissue occurs via a carrier-mediated, facilitated- diffusion mechanism.

Paradoxically, insulin plays a role in glu- cose transport in some but not all tissues. That is, insulin rapidly increases the entry of glucose into cells of so-called “insulin- sensitive” tissues, specifically heart, skele- tal muscle, and adipose cells, but it does not regulate entry of glucose into cells of other tissues including liver, kidney, brain, and the serosal side of intestinal mucosal cells. The obvious questions are how does insulin stimulate glucose entry into insulin- sensitive tissue and why is insulin not needed for glucose entry into those tissues that are not insulin-sensitive? Answers to these questions have emerged only in re- cent years.

The molecule that carries for glucose

across the plasma membrane, the glucose transporter, has been identified as a pro- tein containing -500 amino acids3 To fa- cilitate glucose entry into the cell, the glu- cose transporter protein stretches across the plasma membrane, where it is thought to provide a “port” for glucose entry. Five distinct types of glucose transporters, which differ in structure and tissue distri- bution, have been identified. Cells from all insulin-sensitive tissues contain the same type of glucose transporter, a type not found in any non-insulin-sensitive tissue. In contrast, tissues that do not require insulin for glucose entry contain variable amounts of the other four types.

For tissues that do not require insulin for glucose entry, it is generally thought that the glucose-transporter proteins reside in the plasma membrane. In contrast, in insu- lin-sensitive tissues, the glucose trans- porter can be recycled between a storage site within the cell and the plasma mem- brane by a remarkably rapid, insulin-depen- dent process. For example, when adipo-

NUTR/l/ON REVIEWSIVOL 48, NO SISEPTEMBER 1990 357

cytes were exposed to insulin, the number of glucose transporters in the plasma membrane quickly increased to reach half- maximal within 3 min and subsequently disappeared from the cell surface just as rapidly when the cells were treated with an antiserum to i n ~ u l i n . ~

Recently Hirshman et al.5 provided con- vincing evidence that microsomes are the intracellular storage site of the glucose transporters in skeletal muscle. Rat hind limbs were perfused with and without insu- lin and glucose transporters in the plasma membrane, and microsomal fractions were measured by two different methods. The first method involved the use of a mold me- tabolite, cytochalasin B, which is competi- tive with glucose for binding by glucose transporter proteins.2 Binding by cytocha- lasin B can be used to determine the quan- tity of glucose transporters in either plasma membrane preparations or subcellular fractions. The other method involved de- tection of the glucose transporter protein by a Western blot technique with a mono- clonal antibody after the proteins were separated by sodium dodecyl sulfate-poly- acrylamide gel electrophoresis and electro- phoretic transfer to nitrocellulose. Both methods showed that when the hind limb was perfused with insulin, there was an in- creased amount of glucose transporter protein in the plasma membrane, with a concomitant decrease in transporter pro- tein associated with a microsomal fraction rich in Golgi-associated membranes. Thus, microsomes appear to be the intracellular reservoir of glucose transporters in skeletal muscle as they are in adipose tissue4 and heart muscle.6

It is important to note that insulin is not the only agent that regulates the number of glucose transporters found on the muscle cell surface. When isolated myocytes were

treated with phenylephrine, an analogue of epinephrine, glucose transport into the cells was rapidly increased, apparently be- cause of surfacing glucose transporters.’ This effect of epinephrine may account in large measure for the fact that vigorous ex- ercise by insulin-dependent diabetics can lead to hypoglycemia. In addition, contrac- tile activity as well as hypoxia, which can occur in response to vigorous exercise, are also reported to increase glucose entry into muscle cells, again attributable to the sur- facing of glucose receptors6

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Levine R. The Banting Memorial Lecture: Con- cerning the mechanism of insulin action. Diabetes

Simpson IA, Cushman SW. Hormonal regulation of mammalian glucose transport. Annu Rev Biochem

Gould GW, Gel1 GI. Facilitative glucose trans- porters: an expanding family. Trends in Biochem Sci 1990;15:18-23 Karnieli E, Zarnowski MJ, Hissin PJ, Simpson IA, Salans LB, Cushman SW. Insulin-stimulated trans- location of glucose transport in the isolated rat adipose tissue cell. J Biol Chem 1981 ;256:4772-7 Hirshman MF, Goodyear LJ, Wardzala LJ, Horton ED, Horton ES. Identification of an intracellular pool of glucose transporters from basal and insu- lin-stimulated rat skeletal muscle. J Biol Chem

Watanabe T, Smith MM, Robinson FW, Kono T. In- sulin action on glucose transport in cardiac mus- cle. J Biol Chem 1984;259:13117-22 Farese RV, Rosic N, Standaert M, et al. Further evi- dence implicating diacylglycerol generation and protein kinase G activation in agonist-induced in- creases in glucose uptake. Diabetes 1986;35:

ldstrom JP, Rennie MJ, Schersten T, Bylund-Fel- lenius AC. Membrane transport in relation to net uptake of glucose in the perfused rat hind limb: stirnulatory effect of insulin, hypoxia and contrac- tile activity. Biochem J 1986;233:131-7

1961 ;10:421-31

1986;55:1059-89

1990;265:987-91

951 -7

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