Biot'himie ( 1991 ) 73, 67-70 © Soci6t6 franqaise de biochimie el biologie moldculaire / Elsevier, Paris
67
Glucose transporters: structure, function, and regulation F A s s i m a c o p o u l o s - J e a n n e t , I Cusin, R M Greco-Perotto, J Terrettaz, F R o h n e r - J e a n r e n a u d , N Zarjevski, B J e a n r e n a u d Laboratoires de Recherches Mdtaholiques. University of Geneva. 64. Av tit, ia Roseraie. 1211 Geneva 4. Switzerland (Reveived 15 October 1990: accepted 14 December 1990)
S u m m a r y - - Glucose is transported into the cell by facilitated diffusion via a family of structurally related proteins, whose expression is tissue-specific. One of these transporters, GLUT4, is expressed specifically in insulin-sensitive tissues. A possible change in the synthesis and/or in the amount of GLUT4 has therefore been studied in situations associated with an increase or a decrease in the effect of insulin on glucose transport. Chronic hyperinsulinemia in rats produces a hyper-response of white adipose tissue to insulin and resistance in skeletal muscle. The hyper-response of white adipose tissue is associated with an increase in GLUT4 mRNA and protein. In contrast, in skeletal muscle, a decrease in GLUT4 mRNA and a decrease (tibialis) or no change (diaphragm) in GLUT4 protein are measured, suggesting a divergent regulation by insulin of glucose transport and transporters in the 2 tissues. In rodents, brown adipose tissue is very sensitive to insulin. The response of this tissue to insulin is decreased in obese insulin-resistantfa/fa rats. Treatment with a 13-adrenergic agonist increases insulin-stimulated glucose transport, GLUT4 protein and mRNA. The data suggest that transporter synthesis can be modulated in vivo by insulin (muscle, white adipose tissue) or by catecholamines (brown adipose tissue).
glucose t r a n s p o r t / insulin resistance / skeletal muscle / white adipose tissue / brown adipose tissue
All mammalian cells transport glucose, which is necessary f(~r their metabolism. The transport being UjV
1 ~1~
11 Itat~,u
Ul
i 1 U.~IUII,
till,.
~ 1 11 I~. l ~ J ( ~ l
I ~ U l
f~ttUI
U!
glucose transport is glucose itself. In some types of cell, in skeletal muscle and heart and in white and brown adipose tissue, this transport is stimulated by insulin. By using different techniques about 12 yr ago, 2 groups independently showed the now classical scheme of insulin action on glucose transport. After binding to its receptor, the hormone promotes the translocation of glucose transporters from an intracellular pool to the plasma membrane [!, 2]. This translocation precedes the effect of the hormone on glucose transport by some minutes. This mechanism, initially studied in vitro in isolated white adipocytes, has now been demonstrated in rat brown adipose tissue [3], heart [4], and more recently in skeletal muscle [51. Skeletal muscle is especially difficult to study, due to its numerous compartments and particular morphology. A strict parallelism between the number of transporters and glucose transport is not always observed. For this reason, the notion of intrinsic activity of the transporters has been added to that of the number of
transporters, without knowing to which modification of the transporter or its environment this activity nnrlr~,cnnn¢~c t iu LI. t . , ~Lp t i ~ . ~ o ~ J V
]• ~. ~li .l m o rnll~ . . . . . . . .
~tndiP~ nHhlishe, d r . . . . . . . . .
. . . . . . . .
r e c e n t l va
have shown that glucose transport by facilitated diffusion is performed via a family of proteins encoded by different genes. These proteins are specifically expressed by the tissues, one type of cell being able to express more than one type of transporter. These different transporters have different biological functions and regulations. Up to now, 5 proteins transporting glucose by facilitated diffusion have been described, their genes cloned and sequenced. These transporters are now numbered according to the order of their description (table I). It should be mentioned that GLUT I and GLUT4 coexist in insulin-sensitive tissues. GLUT4 being much more abundant than GLUT I. GLUT I seems to play a role mostly in basal glucose transport. Therefore, in the basal state. GLUT I is the main transporter present on the plasma membrane and GLUT4 is mostly intracellular. After insulin stimulation GLUT4 is translocated to the cell surface, and also GLUT I but to a much lesser extent. In the insulin-sensitive tissues, the 2 types of transporters travel between the
6S
F Assimacopoulos-Jeannet et ai
Table !. Glucose transporters. Adapted from I61: ND: not described. No ~" amino acids
T l t l t l , ~ i / ) ~ If'If'l"
K,,,fi," glucose (raM)
Tisslle
distribution
..........................................
Ubiquitous, human erythrocytes, blood/brain barrier, placenta
GLUTi
492
2-10
GLUT2
524
20-40
Liver, 8-cells of the islets of Langerhans, kidney, intestine
GLUT3
496
ND
Foetal muscle, brain, placenta kidney
GLUT4
509
2- i 0
Skeletal muscle, heart, white and brown adipose tissues
GLUT5
50 !
ND
Intestine
inside and the surface of the cells, suggesting that the cell itself confers this property of translocation to the transporter. Mueckler et ai [7] and other groups have proposed a model of transporter orientation in the plasma membrane. The general structure of the different transporters is the same and consists of 12 transmembrane domains, with one site of N-glycosylation on the loop between domain 1 and 2. The N and C terminal ends are cytoplasmic. A model has been proposed suggesting that the protein forms a channel opening alternatively to the outside and the inside of the cell. Site-directed mutagenesis experiments using GLUT i modified in the C terminal domain (deletion of 37 of the 42 amin6 acids) and expressed in CHO cells have OlIUOil
I.llKgt
Ikll~..,
iilUIkKIHIllL
13
I~.,AIJII,.,~,~aLU ,
IIII.,UIIJUIaLK~U
Ill
the membrane but functionally inactive, probably because it is blocked in an inside-directed form [8]. It should also been mentioned that several serine- threonine are present on the C-terminal part of GLUT4, and in this type only [9]. They could be potential substrates for cAMP-dependent protein kinase. James et al have shown that isoproterenol stimulates the phosphorylation of GLUT4 in white adipocytes [9]. The role of phosphorylation of the protein on its activity is still unknown. Insulin does not phosphorylate the glucose transporter. We will now describe the regulation of glucose transport and transporters in vivo in some physiological and pathophysiological states in insulin-sensitive tissues. Glucose transport being in most situations the limiting step for its metabolism, the transporter could play a role in the development of insulin-resistance in humans and/or in several animal models of insulin resistance. It has been suggested that in some of these pathologies, hyperinsulinemia precedes the appearance of insulin resistance. In order to mimic this
situation and to study its consequences on glucose transport, glucose transporter protein and its mRNA, normal rats were infused with insulin delivered continuously via a minipump. Hypoglycemia was corrected by an infusion of glucose delivered via jugular catheter. The animals were therefore normoglycemic but hyperinsulinemic. After 4 d of treatment, the animals were submitted to an euglycemic hyperinsulinemic clamp combined with the use of 2-deoxyglucose to measure the effect of insulin on glucose utilization by the individual tissues. These experiments show that after 4 d of hyperinsulinemia, white adipose tissue presented a net increase in glucose utilization (transport plus phosphorylation) (table II). Similar measurements performed on skeletal muscle showed on the contrary, a decrease in the effect of insulin on the same parameter. Hyperinsulinemia therefore produced an increased response to insulin in white adipose tissue and an insulin resistance in several skeletal muscles. Can these changes be explained by modifications in the synthesis and/or in the number of transporters? Transporter number was quantified by immunoblot using an antibody specific for GLUT4. Table II shows that in parallel with the increase in the effect of insulin on glucose transport, a 2-fold increase in the number of transporters was measured in white adipose tissue. In contrast, in skeletal muscle, either a decrease in GLUT4 (tibialis) or no change (diaphragm) was observed. These measurements were performed on homogenates and provided no information on the distribution of the transporters between the plasma membranes and the intracellular pool. GLUT4 mRNA was also measured after 4 d of hyperinsulinemia. It was found to be markedly increased in white adipose tissue and decreased in skeletal muscle. These results strongly suggest that hyperinsulinemia pen" se
Glucose transporters can produce divergent modifications in the response to insulin of white adipose tissue and skeletal muscle that could explain the coexistence of a hyperactive white adipose tissue and a resistant skeletal muscle in several animal models of obesity. These results also demonstrate that in some situations there is a good correlation between the response to insulin, the number of glucose transporters, and the level of G L U T 4 mRNA. In other situations it is postulated that other parameters exist which determine the response to insulin, which could involve either the intrinsic activity of the transporters, or the process of translocation. The experiments clearly show that hyperinsulinemia can rapidly lead to insulin resistance in skeletal muscle. Brown adipose tissue is not representative of insulin-responsive tissue in humans. In rodents, the tissue responds to insulin by a 40-100-fold increase in glucose utilization. In addition, this tissue modifies its ,-response to insulin in several physiological states (late phase of gestation, lactation, during which the tissue is insulin-resistant), in models of inherited ¢fa/fa rats) or acquired (fat rich diet, lesions of the ventromedial hypothalamus) obesities. In all these situations, a decrease in the content and/or the turnover of catecholamines has been described. The possible relationship between the decreased catecholamine content and the response to insulin of the tissue has been studied in a model of insulin-resistant obese rats, the f a / f a rat. The animals were treated with a 13-adrenergic agonist specific for brown adipose tissue. Glucose transport in response to insulin was measured in the treated and non-treated animals, as well as the amount of G L U T 4 protein and m R N A . After the treatment with the Idl-dUll~::ll~l~lt..,
l~l.~OlU~t, a l l
|llt..,ll.,a~t,,,
hi
*t ihl l~, ,
lt..,,.~[../Ull,,~,
tu
insulin of glucose transport was measured, as well as increase in the number of G L U T 4 protein per mg protein in microsomal membranes. When this parameter was corrected for the yield of microsomes and expressed by whole tissue, a 2-fold increase in the number of transporters was measured. This increase is due at least partially to an increase in the transporter synthesis since 24 h after the administration of the agonist, a 4-fold increase in G L U T 4 m R N A was measured in the tissue. An increase in the amount of G L U T 4 is probably not the only factor involved in the increased response to insulin, but can contribute to it to a large extent. In the 3 insulin-sensitive tissues studied, a hormonal regulation of the maximal response to insulin has been demonstrated. In white adipose tissue, insulin positively regulates its effect via an increase in the number of transporters due to an increase in the transcription and/or in the stability of G L U T 4 mRNA. In skeletal muscle, the global effect of the hormone is a decrease in the insulin-stimulated glucose transport;
69
!1. Effect of hyperinsulinemia on glucose transport. transporter and transporter mRNA. Adapted from i10. I! ]. Data are expressed as percent of control non treated animals.
Table
Tissue
Maximal response (%)
GLUT4 protein (%)
mRNA (%)
White adipose tissue
+ 250
+ 220
+ 350
Tibialis
-
35
-
-
40
Diaphragm
-
35
-
30
30 =
this decrease is only partially explained by a decrease in the number of transporters. In brown adipose tissue, catecholamines could modulate the maximal response to insulin by positively regulating the number of transporters. Is the response to insulin always determined by the number of transporters itself regulated at the transcriptional level in insulin-resistant humans? Most of the studies, often still in the form of abstracts [12], seem to come to a similar conclusion, ie that there is no decrease in the total number of transporters or in transporter m R N A in skeletal muscle when the syndrome is fully established. The possibility remains that the translocation process itself is responsible for the insulin resistance. Such a hypothesis remains difficult to study, since muscle fractionation requires too much material to be performed on a biopsy. Let us mention that a certain degree of polymorphism for it has been observed in the human G L U T I and GLUT4 genes [13]; no form could, however, be associated with the presence of type II diabetes. The cause of insulin-resistance in human skeletal muscle and in some animal models [ 14] rema!- to be determined; it could involve modification i~ coupling proteins between the receptor and/or the t~ansporter or in the translocation process itself.
Acknowledgments
This study was supported by grant 32.26405.89 from the FNSRS (CH-Beme) and by a grant-in-aid from Nestl6 SA (CH-Vevey).
References
Suzuki K, Kono T (1980) Evidence that insulin causes transiocation of glucose transport activity to the plasma membrane from an intracellular storage site. Proc Natl Acad Sci USA 77.2542-2525
70 2
3
4
5
6 7
8
F Assimacopoulos-Jeannet et ol Cu~,hman S\V. \Vardzala LJ (19807 Potential mechanisn.ts of insulin action on glucose transporter in isolated rat adiIx~xe ceil: apparent translocation of intracellular hansport systems to the plasma membrane..! Biol Chem 256, 475~---I.762 Greco-Perotto RM. Assimacopoulos-Jeannet F. Jeanrenaud B (19871 Insulin modifies the properties of glucose transporters in rat brown adipose tissue. Biochem .I 247. 6?,--68 Zaninetti D. Greco-Perotto RM. Je::nrenaud B (19881 Heart glucose transport and transporters in rat heart: regulation by insulin, workload and glucose. Di,ibetologia 31. 108-113 Hirschman MF. Goodyear LI. Wardzala LJ. Horton ES 199(17 Identification of an intraceilular pool of glucose transporter from basal and insulin-stimulated rat skeletal muscle. J BhJI Chem 265.987-99 I Pilch P (19901 Glucose transporters: what's in a name'? Emlocrim~h~g.v ! 26. 3-5 Mueckler M. Caruso C, Baldwin SA, Panico M. Blench I, Morris HR. Allard WJ. Lienhard GE. Lodish HF (1985) Sequence and structure of a human glucose transporter. Science 229.94 !-945 Oka Y. Asano T. Shibasaki Y. Lin JL Tsukuda K. Katapiri H. Akanuma Y. Takaku F (19901 C-terminal truncated glucose transporter is locked into an inward-
9
10
il
12
13 14
facing form without transport activity. Nature 345. 550-553 James D, Hiken J. Lawrence Jr JC (19891 lsoproterenol stimulates phosphorylation of the insulin regulatable glucose transporter in rat adipocytes. Proc Natl At'ad Sci USA 86. 8368-8372 Cusin I, Terretaz J, Rohner-Jeanrenaud F, Jeanrenaud B (19901 Metabolic consequences of hyperinsulinemia imposed on normal rats in glucose handling by white adipose tissue, muscles and liver. Biochem J 267, 99-103 Cusin 1, Terrettaz J, Rohner-Jeanrenaud F, Zarjevski N, Assimacopoulos-Jeannet F, Jeanrenaud B (19901 Hyperinsulinemia "ncreases the amount of GLUT4 mRNA in white adipose tissue and decreases that of muscle: a clue for increased fat depot and insulin resistance. Endocrim~h~gy ! 27, 3246-3248 Groop L, Koranyi LI, Eriksson J, Widen E (19901 Insulin resistance in NIDDM is not associated with a defect in the expression of insulin-responsive glucose transporter (GLUT4) gene in human skeletal muscle. Diabetes 39 (suppl 1), 49A Oelbaum RS, Li SR, Baroni MG, Stocks J, Alcolado JC, Galton DJ (19901 Polymorphisms of glucose transporter gene in type 2 diabetes. Diabetologia 33, A I4 Koranyi L, James D, Mueckler M, Permutt MA (19901 Glucose transporter level in spontaneously obese (db/db) insulin-resistant mice. J Clin Invest 85,962-967
67
Glucose transporters: structure, function, and regulation F A s s i m a c o p o u l o s - J e a n n e t , I Cusin, R M Greco-Perotto, J Terrettaz, F R o h n e r - J e a n r e n a u d , N Zarjevski, B J e a n r e n a u d Laboratoires de Recherches Mdtaholiques. University of Geneva. 64. Av tit, ia Roseraie. 1211 Geneva 4. Switzerland (Reveived 15 October 1990: accepted 14 December 1990)
S u m m a r y - - Glucose is transported into the cell by facilitated diffusion via a family of structurally related proteins, whose expression is tissue-specific. One of these transporters, GLUT4, is expressed specifically in insulin-sensitive tissues. A possible change in the synthesis and/or in the amount of GLUT4 has therefore been studied in situations associated with an increase or a decrease in the effect of insulin on glucose transport. Chronic hyperinsulinemia in rats produces a hyper-response of white adipose tissue to insulin and resistance in skeletal muscle. The hyper-response of white adipose tissue is associated with an increase in GLUT4 mRNA and protein. In contrast, in skeletal muscle, a decrease in GLUT4 mRNA and a decrease (tibialis) or no change (diaphragm) in GLUT4 protein are measured, suggesting a divergent regulation by insulin of glucose transport and transporters in the 2 tissues. In rodents, brown adipose tissue is very sensitive to insulin. The response of this tissue to insulin is decreased in obese insulin-resistantfa/fa rats. Treatment with a 13-adrenergic agonist increases insulin-stimulated glucose transport, GLUT4 protein and mRNA. The data suggest that transporter synthesis can be modulated in vivo by insulin (muscle, white adipose tissue) or by catecholamines (brown adipose tissue).
glucose t r a n s p o r t / insulin resistance / skeletal muscle / white adipose tissue / brown adipose tissue
All mammalian cells transport glucose, which is necessary f(~r their metabolism. The transport being UjV
1 ~1~
11 Itat~,u
Ul
i 1 U.~IUII,
till,.
~ 1 11 I~. l ~ J ( ~ l
I ~ U l
f~ttUI
U!
glucose transport is glucose itself. In some types of cell, in skeletal muscle and heart and in white and brown adipose tissue, this transport is stimulated by insulin. By using different techniques about 12 yr ago, 2 groups independently showed the now classical scheme of insulin action on glucose transport. After binding to its receptor, the hormone promotes the translocation of glucose transporters from an intracellular pool to the plasma membrane [!, 2]. This translocation precedes the effect of the hormone on glucose transport by some minutes. This mechanism, initially studied in vitro in isolated white adipocytes, has now been demonstrated in rat brown adipose tissue [3], heart [4], and more recently in skeletal muscle [51. Skeletal muscle is especially difficult to study, due to its numerous compartments and particular morphology. A strict parallelism between the number of transporters and glucose transport is not always observed. For this reason, the notion of intrinsic activity of the transporters has been added to that of the number of
transporters, without knowing to which modification of the transporter or its environment this activity nnrlr~,cnnn¢~c t iu LI. t . , ~Lp t i ~ . ~ o ~ J V
]• ~. ~li .l m o rnll~ . . . . . . . .
~tndiP~ nHhlishe, d r . . . . . . . . .
. . . . . . . .
r e c e n t l va
have shown that glucose transport by facilitated diffusion is performed via a family of proteins encoded by different genes. These proteins are specifically expressed by the tissues, one type of cell being able to express more than one type of transporter. These different transporters have different biological functions and regulations. Up to now, 5 proteins transporting glucose by facilitated diffusion have been described, their genes cloned and sequenced. These transporters are now numbered according to the order of their description (table I). It should be mentioned that GLUT I and GLUT4 coexist in insulin-sensitive tissues. GLUT4 being much more abundant than GLUT I. GLUT I seems to play a role mostly in basal glucose transport. Therefore, in the basal state. GLUT I is the main transporter present on the plasma membrane and GLUT4 is mostly intracellular. After insulin stimulation GLUT4 is translocated to the cell surface, and also GLUT I but to a much lesser extent. In the insulin-sensitive tissues, the 2 types of transporters travel between the
6S
F Assimacopoulos-Jeannet et ai
Table !. Glucose transporters. Adapted from I61: ND: not described. No ~" amino acids
T l t l t l , ~ i / ) ~ If'If'l"
K,,,fi," glucose (raM)
Tisslle
distribution
..........................................
Ubiquitous, human erythrocytes, blood/brain barrier, placenta
GLUTi
492
2-10
GLUT2
524
20-40
Liver, 8-cells of the islets of Langerhans, kidney, intestine
GLUT3
496
ND
Foetal muscle, brain, placenta kidney
GLUT4
509
2- i 0
Skeletal muscle, heart, white and brown adipose tissues
GLUT5
50 !
ND
Intestine
inside and the surface of the cells, suggesting that the cell itself confers this property of translocation to the transporter. Mueckler et ai [7] and other groups have proposed a model of transporter orientation in the plasma membrane. The general structure of the different transporters is the same and consists of 12 transmembrane domains, with one site of N-glycosylation on the loop between domain 1 and 2. The N and C terminal ends are cytoplasmic. A model has been proposed suggesting that the protein forms a channel opening alternatively to the outside and the inside of the cell. Site-directed mutagenesis experiments using GLUT i modified in the C terminal domain (deletion of 37 of the 42 amin6 acids) and expressed in CHO cells have OlIUOil
I.llKgt
Ikll~..,
iilUIkKIHIllL
13
I~.,AIJII,.,~,~aLU ,
IIII.,UIIJUIaLK~U
Ill
the membrane but functionally inactive, probably because it is blocked in an inside-directed form [8]. It should also been mentioned that several serine- threonine are present on the C-terminal part of GLUT4, and in this type only [9]. They could be potential substrates for cAMP-dependent protein kinase. James et al have shown that isoproterenol stimulates the phosphorylation of GLUT4 in white adipocytes [9]. The role of phosphorylation of the protein on its activity is still unknown. Insulin does not phosphorylate the glucose transporter. We will now describe the regulation of glucose transport and transporters in vivo in some physiological and pathophysiological states in insulin-sensitive tissues. Glucose transport being in most situations the limiting step for its metabolism, the transporter could play a role in the development of insulin-resistance in humans and/or in several animal models of insulin resistance. It has been suggested that in some of these pathologies, hyperinsulinemia precedes the appearance of insulin resistance. In order to mimic this
situation and to study its consequences on glucose transport, glucose transporter protein and its mRNA, normal rats were infused with insulin delivered continuously via a minipump. Hypoglycemia was corrected by an infusion of glucose delivered via jugular catheter. The animals were therefore normoglycemic but hyperinsulinemic. After 4 d of treatment, the animals were submitted to an euglycemic hyperinsulinemic clamp combined with the use of 2-deoxyglucose to measure the effect of insulin on glucose utilization by the individual tissues. These experiments show that after 4 d of hyperinsulinemia, white adipose tissue presented a net increase in glucose utilization (transport plus phosphorylation) (table II). Similar measurements performed on skeletal muscle showed on the contrary, a decrease in the effect of insulin on the same parameter. Hyperinsulinemia therefore produced an increased response to insulin in white adipose tissue and an insulin resistance in several skeletal muscles. Can these changes be explained by modifications in the synthesis and/or in the number of transporters? Transporter number was quantified by immunoblot using an antibody specific for GLUT4. Table II shows that in parallel with the increase in the effect of insulin on glucose transport, a 2-fold increase in the number of transporters was measured in white adipose tissue. In contrast, in skeletal muscle, either a decrease in GLUT4 (tibialis) or no change (diaphragm) was observed. These measurements were performed on homogenates and provided no information on the distribution of the transporters between the plasma membranes and the intracellular pool. GLUT4 mRNA was also measured after 4 d of hyperinsulinemia. It was found to be markedly increased in white adipose tissue and decreased in skeletal muscle. These results strongly suggest that hyperinsulinemia pen" se
Glucose transporters can produce divergent modifications in the response to insulin of white adipose tissue and skeletal muscle that could explain the coexistence of a hyperactive white adipose tissue and a resistant skeletal muscle in several animal models of obesity. These results also demonstrate that in some situations there is a good correlation between the response to insulin, the number of glucose transporters, and the level of G L U T 4 mRNA. In other situations it is postulated that other parameters exist which determine the response to insulin, which could involve either the intrinsic activity of the transporters, or the process of translocation. The experiments clearly show that hyperinsulinemia can rapidly lead to insulin resistance in skeletal muscle. Brown adipose tissue is not representative of insulin-responsive tissue in humans. In rodents, the tissue responds to insulin by a 40-100-fold increase in glucose utilization. In addition, this tissue modifies its ,-response to insulin in several physiological states (late phase of gestation, lactation, during which the tissue is insulin-resistant), in models of inherited ¢fa/fa rats) or acquired (fat rich diet, lesions of the ventromedial hypothalamus) obesities. In all these situations, a decrease in the content and/or the turnover of catecholamines has been described. The possible relationship between the decreased catecholamine content and the response to insulin of the tissue has been studied in a model of insulin-resistant obese rats, the f a / f a rat. The animals were treated with a 13-adrenergic agonist specific for brown adipose tissue. Glucose transport in response to insulin was measured in the treated and non-treated animals, as well as the amount of G L U T 4 protein and m R N A . After the treatment with the Idl-dUll~::ll~l~lt..,
l~l.~OlU~t, a l l
|llt..,ll.,a~t,,,
hi
*t ihl l~, ,
lt..,,.~[../Ull,,~,
tu
insulin of glucose transport was measured, as well as increase in the number of G L U T 4 protein per mg protein in microsomal membranes. When this parameter was corrected for the yield of microsomes and expressed by whole tissue, a 2-fold increase in the number of transporters was measured. This increase is due at least partially to an increase in the transporter synthesis since 24 h after the administration of the agonist, a 4-fold increase in G L U T 4 m R N A was measured in the tissue. An increase in the amount of G L U T 4 is probably not the only factor involved in the increased response to insulin, but can contribute to it to a large extent. In the 3 insulin-sensitive tissues studied, a hormonal regulation of the maximal response to insulin has been demonstrated. In white adipose tissue, insulin positively regulates its effect via an increase in the number of transporters due to an increase in the transcription and/or in the stability of G L U T 4 mRNA. In skeletal muscle, the global effect of the hormone is a decrease in the insulin-stimulated glucose transport;
69
!1. Effect of hyperinsulinemia on glucose transport. transporter and transporter mRNA. Adapted from i10. I! ]. Data are expressed as percent of control non treated animals.
Table
Tissue
Maximal response (%)
GLUT4 protein (%)
mRNA (%)
White adipose tissue
+ 250
+ 220
+ 350
Tibialis
-
35
-
-
40
Diaphragm
-
35
-
30
30 =
this decrease is only partially explained by a decrease in the number of transporters. In brown adipose tissue, catecholamines could modulate the maximal response to insulin by positively regulating the number of transporters. Is the response to insulin always determined by the number of transporters itself regulated at the transcriptional level in insulin-resistant humans? Most of the studies, often still in the form of abstracts [12], seem to come to a similar conclusion, ie that there is no decrease in the total number of transporters or in transporter m R N A in skeletal muscle when the syndrome is fully established. The possibility remains that the translocation process itself is responsible for the insulin resistance. Such a hypothesis remains difficult to study, since muscle fractionation requires too much material to be performed on a biopsy. Let us mention that a certain degree of polymorphism for it has been observed in the human G L U T I and GLUT4 genes [13]; no form could, however, be associated with the presence of type II diabetes. The cause of insulin-resistance in human skeletal muscle and in some animal models [ 14] rema!- to be determined; it could involve modification i~ coupling proteins between the receptor and/or the t~ansporter or in the translocation process itself.
Acknowledgments
This study was supported by grant 32.26405.89 from the FNSRS (CH-Beme) and by a grant-in-aid from Nestl6 SA (CH-Vevey).
References
Suzuki K, Kono T (1980) Evidence that insulin causes transiocation of glucose transport activity to the plasma membrane from an intracellular storage site. Proc Natl Acad Sci USA 77.2542-2525
70 2
3
4
5
6 7
8
F Assimacopoulos-Jeannet et ol Cu~,hman S\V. \Vardzala LJ (19807 Potential mechanisn.ts of insulin action on glucose transporter in isolated rat adiIx~xe ceil: apparent translocation of intracellular hansport systems to the plasma membrane..! Biol Chem 256, 475~---I.762 Greco-Perotto RM. Assimacopoulos-Jeannet F. Jeanrenaud B (19871 Insulin modifies the properties of glucose transporters in rat brown adipose tissue. Biochem .I 247. 6?,--68 Zaninetti D. Greco-Perotto RM. Je::nrenaud B (19881 Heart glucose transport and transporters in rat heart: regulation by insulin, workload and glucose. Di,ibetologia 31. 108-113 Hirschman MF. Goodyear LI. Wardzala LJ. Horton ES 199(17 Identification of an intraceilular pool of glucose transporter from basal and insulin-stimulated rat skeletal muscle. J BhJI Chem 265.987-99 I Pilch P (19901 Glucose transporters: what's in a name'? Emlocrim~h~g.v ! 26. 3-5 Mueckler M. Caruso C, Baldwin SA, Panico M. Blench I, Morris HR. Allard WJ. Lienhard GE. Lodish HF (1985) Sequence and structure of a human glucose transporter. Science 229.94 !-945 Oka Y. Asano T. Shibasaki Y. Lin JL Tsukuda K. Katapiri H. Akanuma Y. Takaku F (19901 C-terminal truncated glucose transporter is locked into an inward-
9
10
il
12
13 14
facing form without transport activity. Nature 345. 550-553 James D, Hiken J. Lawrence Jr JC (19891 lsoproterenol stimulates phosphorylation of the insulin regulatable glucose transporter in rat adipocytes. Proc Natl At'ad Sci USA 86. 8368-8372 Cusin I, Terretaz J, Rohner-Jeanrenaud F, Jeanrenaud B (19901 Metabolic consequences of hyperinsulinemia imposed on normal rats in glucose handling by white adipose tissue, muscles and liver. Biochem J 267, 99-103 Cusin 1, Terrettaz J, Rohner-Jeanrenaud F, Zarjevski N, Assimacopoulos-Jeannet F, Jeanrenaud B (19901 Hyperinsulinemia "ncreases the amount of GLUT4 mRNA in white adipose tissue and decreases that of muscle: a clue for increased fat depot and insulin resistance. Endocrim~h~gy ! 27, 3246-3248 Groop L, Koranyi LI, Eriksson J, Widen E (19901 Insulin resistance in NIDDM is not associated with a defect in the expression of insulin-responsive glucose transporter (GLUT4) gene in human skeletal muscle. Diabetes 39 (suppl 1), 49A Oelbaum RS, Li SR, Baroni MG, Stocks J, Alcolado JC, Galton DJ (19901 Polymorphisms of glucose transporter gene in type 2 diabetes. Diabetologia 33, A I4 Koranyi L, James D, Mueckler M, Permutt MA (19901 Glucose transporter level in spontaneously obese (db/db) insulin-resistant mice. J Clin Invest 85,962-967
