Expression and compartmentalization of caveolin in adipose cells: coordinate regulation with and structural segregation from GLUT4.

Native rat adipocytes and the mouse adipocyte cell line, 3T3-L1, possess transport vesicles of apparently uniform composition and size which translocate the tissue-specific glucose transporter isoform, GLUT4, from an intracellular pool to the cell surface in an insulin-sensitive fashion. Caveolin, the presumed structural protein of caveolae, has also been proposed to function in vesicular transport. Thus, we studied the expression and subcellular distribution of caveolin in adipocytes. We found that rat fat cells express the highest level of caveolin protein of any tissue studied, and caveolin is also expressed at high levels in cardiac muscle, another tissue possessing insulin responsive GLUT4 translocation. Both proteins are absent from 3T3-L1 fibroblasts and undergo a dramatic coordinate increase in expression upon differentiation of these cells into adipocytes. However, unlike GLUT4 in rat adipocytes not exposed to insulin, the majority of caveolin is present in the plasma membrane. In native rat adipocytes, intracellular GLUT4 and caveolin reside in vesicles practically indistinguishable by their size and buoyant density in sucrose gradients, and both proteins show insulin-dependent translocation to the cell surface. However, by immunoadsorption of GLUT4-containing vesicles with anti-GLUT4 antibody, we show that these vesicles have no detectable caveolin, and therefore, this protein is present in a distinct vesicle population. Thus, caveolin has no direct structural relation to the organization of the intracellular glucose transporting machinery in fat cells.

Coated vesicles participate in the receptor-mediated endocytosis of insulin.

We have purified coated vesicles from rat liver by differential ultracentrifugation. Electron micrographs of these preparations reveal only the polyhedral structures typical of coated vesicles. SDS PAGE of the coated vesicle preparation followed by Coomassie Blue staining of proteins reveals a protein composition also typical of coated vesicles. We determined that these rat liver coated vesicles possess a latent insulin binding capability. That is, little if any specific binding of 125I-insulin to coated vesicles is observed in the absence of detergent. However, coated vesicles treated with the detergent octyl glucoside exhibit a substantial specific 125I-insulin binding capacity. We visualized the insulin binding structure of coated vesicles by cross-linking 125I-insulin to detergent-solubilized coated vesicles using the bifunctional reagent disuccinimidyl suberate followed by electrophoresis and autoradiography. The receptor structure thus identified is identical to that of the high-affinity insulin receptor present in a variety of tissues. We isolated liver coated vesicles from rats which had received injections of 125I-insulin in the hepatic portal vein. We found that insulin administered in this fashion was rapidly and specifically taken up by liver coated vesicles. Taken together, these data are compatible with a functional role for coated vesicles in the receptor-mediated endocytosis of insulin.

Hormone binding alters the conformation of the insulin receptor.

Fat cells or fat cell membranes were briefly subjected to mild proteolysis under conditions where insulin receptors were either free or bound to (125)I-labeled insulin. When receptors were then affinity-labeled to visualize the effects of this treatment, it was observed that receptors that had been occupied by ligand during proteolysis exhibited greater rates of degradation than unoccupied receptors. These results demonstrate that insulin-receptor interaction induces a change in receptor structure that may be related to signal transmission.

Effect of thyroid status on insulin action in rat adipocytes and skeletal muscle.

Isolated adipocytes and soleus muscles prepared from mature rats, rendered hypothyroid by a low iodine diet and propylthiouracil, markedly resisted the ability of insulin to increase glucose utilization. In adipocytes, the sum of basal d-(1-(14)C)-glucose conversion to CO(2), glyceride-glycerol, and fatty acid was unaltered by hypothyroidism, although conversion to fatty acid was decreased. The response of each of these metabolic pathways to insulin at all concentrations tested was greatly diminished in hypothyroid rat adipocytes. 3-O-Methylglucose transport rates in the presence of insulin were not significantly different in adipocytes from hypothyroid as compared with euthyroid rats, although basal transport rates were significantly higher in the hypothyroid state. Lipolysis and cyclic AMP accumulation in adipocytes from hypothyroid rats in response to theophylline were markedly diminished compared with euthyroid controls, but insulin was about as effective in inhibiting lipolysis in these cells as in those derived from euthyroid animals. The binding of (125)I-insulin to adipocytes at several hormone concentrations was also shown to be unaffected by hypothyroidism. In soleus muscle, basal glucose conversion to H(2)O and glycogen was unaltered in the hypothyroid state, whereas insulin action on these pathways was markedly inhibited. The decrease in muscle insulin responsiveness was less marked than that observed in adipocytes. Uptake of either 2-deoxyglucose or l-arabinose in the presence or absence of insulin was similar in soleus muscles derived from euthryoid vs. hypothyroid rats. Similarly, insulin action on the conversion of soleus muscle glycogen synthase D to the I form in the absence of glucose was unaltered by hypothyroidism. We conclude that (a) hypothyroidism in mature rats leads to a marked decrease in the responsiveness of glucose metabolism in adipocytes and skeletal muscle to insulin; (b) no detectable impairment of the membrane insulin effector systems that mediate the regulation of adipocyte hexose transport and glycogen synthase is caused by hypothyroidism in this animal model; and (c) the cellular defect that leads to apparent insulin resistance of adipocyte and soleus muscle glucose utilization resides at the level of one or more intracellular enzymes involved in glucose catabolism.

Structural and functional characterization of the human T lymphocyte receptor for insulin-like growth factor I in vitro.

Growth factor receptors for T lymphocytes, such as interleukin 2 and insulin, are present on activated but not resting T lymphocytes. We sought to determine if insulin-like growth factor I (IGF-I) could act as a growth factor for human T cells and to characterize its receptor on resting and activated cells. Recombinant IGF-I induced two separate functions. It was chemotactic for and increased incorporation of tritiated thymidine into both unactivated (resting) and mitogen-activated T cells. High-affinity 125I-IGF-I binding to human T cells was saturable with an apparent Kd of 1.2 +/- .6 X 10(-10) M for binding to activated T cells and 1.2 +/- .9 X 10(-10) for unactivated T cells. The calculated binding for activated cells was 330 +/- 90 and for resting cells 45 +/- 9 high-affinity receptor sites per cell. Affinity cross-linking of 125I-IGF-I to resting or activated T cells revealed a radioligand-receptor complex of 360,000 mol wt when analyzed by SDS-PAGE without reduction and complexes of 270,000 and 135,000 mol wt upon reduction; prior incubation with excess unlabeled IGF-I prevented formation of the 125I-IGF-I receptor complex. Our data suggest that both resting and activated T lymphocytes bear functional IGF-I receptors similar to those found in other tissues. These receptors may mediate T cell growth and chemotaxis.

Binding of beta-amyloid to the p75 neurotrophin receptor induces apoptosis. A possible mechanism for Alzheimer's disease.

Alzheimer's disease is a neurodegenerative disorder characterized by the extracellular deposition in the brain of aggregated beta-amyloid peptide, presumed to play a pathogenic role, and by preferential loss of neurons that express the 75-kD neurotrophin receptor (p75NTR). Using rat cortical neurons and NIH-3T3 cell line engineered to stably express p75NTR, we find that the beta-amyloid peptide specifically binds the p75NTR. Furthermore, 3T3 cells expressing p75NTR, but not wild-type control cells lacking the receptor, undergo apoptosis in the presence of aggregated beta-amyloid. Normal neural crest-derived melanocytes that express physiologic levels of p75NTR undergo apoptosis in the presence of aggregated beta-amyloid, but not in the presence of control peptide synthesized in reverse. These data imply that neuronal death in Alzheimer's disease is mediated, at least in part, by the interaction of beta-amyloid with p75NTR, and suggest new targets for therapeutic intervention.

Stimulation of C2C12 myoblast growth by basic fibroblast growth factor and insulin-like growth factor 1 can occur via mitogen-activated protein kinase-dependent and -independent pathways.

It is now well-recognized that the mitogen-activated protein (MAP) kinase cascade facilitates signaling from an activated tyrosine kinase receptor to the nucleus. In fact, an increasing number of extracellular effectors have been reported to activate the MAP kinase cascade, with a significant number of cellular responses attributed to this activation. We set out to explore how two extracellular effectors, basic fibroblast growth factor (bFGF) and insulin-like growth factor 1 (IGF-1), which have both been reported to activate MAP kinase, generate quite distinct cellular responses in C2C12 myoblasts. We demonstrate here that bFGF, which is both a potent mitogen and inhibitor of myogenic differentiation, is a strong MAP kinase agonist. By contrast, IGF-1, which is equally mitogenic for C2C12 cells but ultimately enhances the differentiated phenotype, is a weak activator of the MAP kinase cascade. We further demonstrate that IGF-1 is a potent activator of both insulin receptor substrate IRS-1 tyrosyl phosphorylation and association of IRS-1 with activated phosphatidylinositol 3-kinase (PI 3-kinase). Finally, use of the specific MAP kinase kinase inhibitor, PD098059, and wortmannin, a PI 3-kinase inhibitor, suggests the existence of an IGF-1-induced, MAP kinase-independent signaling event which contributes to the mitogenic response of this factor, whereas bFGF-induced mitogenesis appears to strongly correlate with activation of the MAP kinase cascade.

The formation of an insulin-responsive vesicular cargo compartment is an early event in 3T3-L1 adipocyte differentiation.

Differentiating 3T3-L1 cells exhibit a dramatic increase in the rate of insulin-stimulated glucose transport during their conversion from proliferating fibroblasts to nonproliferating adipocytes. On day 3 of 3T3-L1 cell differentiation, basal glucose transport and cell surface transferrin binding are markedly diminished. This occurs concomitant with the formation of a distinct insulin-responsive vesicular pool of intracellular glucose transporter 1 (GLUT1) and transferrin receptors as assessed by sucrose velocity gradients. The intracellular distribution of the insulin-responsive aminopeptidase is first readily detectable on day 3, and its gradient profile and response to insulin at this time are identical to that of GLUT1. With further time of differentiation, GLUT4 is expressed and targeted to the same insulin-responsive vesicles as the other three proteins. Our data are consistent with the notion that a distinct insulin-sensitive vesicular cargo compartment forms early during fat call differentiation and its formation precedes GLUT4 expression. The development of this compartment may result from the differentiation-dependent inhibition of constitutive GLUT1 and transferrin receptor trafficking such that there is a large increase in, or the new formation of, a population of postendosomal, insulin-responsive vesicles.

Characterization and solubilization of the cytochalasin B binding component from human placental microsomes.

Human placental microsomes exhibit uptake of D-[3H]glucose which is sensitive to inhibition by cytochalasin B (apparent Ki = 0.78 microM). Characterization of [3H]cytochalasin B binding to these membranes reveals a glucose-sensitive site, inhibited by D-glucose with an ED50 = 40 mM. The glucose-sensitive cytochalasin B binding site is found to have a Kd = 0.15 microM by analysis according to Scatchard. Solubilization with octylglucoside extracts 60-70% of the glucose-sensitive binding component. Equilibrium dialysis binding of [3H]cytochalasin B to the soluble protein displays a pattern of inhibition by D-glucose similar to that observed for intact membranes, and the measurement of an ED50 = 37.5 mM D-glucose confirms the presence of the cytochalasin B binding component, putatively assigned as the glucose transporter. Further evidence is attained by photoaffinity labelling; ultraviolet-sensitive [3H]cytochalasin B incorporation into soluble protein (Mr range 42000-68000) is prevented by the presence of D-glucose. An identical photolabelling pattern is observed for incorporation of [3H]cytochalasin B into intact membrane protein, confirming the usefulness of this approach as a means of identifying the presence of the glucose transport protein under several conditions.

Insulin stimulates the tyrosine phosphorylation of a 165 kDa protein that is associated with microsomal membranes of rat adipocytes.

The beta-subunit of the insulin receptor possesses an insulin-stimulatable protein tyrosine kinase activity. It has been widely postulated that this activity may mediate the transduction of the insulin signal by phosphorylation of cellular substrates involved in the mechanism of insulin action. We have identified, by immunoblotting with antiphosphotyrosine antibodies, a 165 kDa protein in rat adipocytes that is rapidly phosphorylated in response to insulin. Phosphorylation of this protein (pp165) occurs within 5-10 s of exposure to 10 nM insulin, suggesting that it may be a direct substrate for the insulin receptor. This protein was recovered in an intracellular membrane that fractionates with the low-density microsomes. Using discontinuous sucrose density-gradient centrifugation, pp165-containing vesicles were separated from other vesicles of the low-density microsomes including the glucose transporter-containing vesicles, indicating that pp165 is probably not a regulatory component of the vesicles that translocate glucose transporters in response to insulin. However, pp165 may be involved in conveying receptor activation at the cell surface to an intracellular site of insulin action.

Adipose tissue glucose transporters in NIDDM. Decreased levels of muscle/fat isoform.

We investigated the mechanism of peripheral insulin resistance in the adipose tissue of obese and non-insulin-dependent diabetes mellitus (NIDDM) patients at the level of the glucose-transport effector system. Freshly isolated adipocytes from obese nondiabetic and obese NIDDM subjects had decreased insulin sensitivity and responsiveness for glucose-transport stimulation compared with control subjects, with more pronounced changes associated with obese NIDDM patients. The relative abundance of muscle/fat glucose-transporter isoform in the three groups of subjects was determined by Western-blot analysis of detergent-soluble adipose tissue extracts with monoclonal antibody 1F8. Obesity per se had no effect on adipose tissue muscle/fat glucose-transporter isoform (3150 +/- 660 vs. 4495 +/- 410 counts/min [cpm]/mg protein). Furthermore, decreased levels of muscle/fat isoform in adipose tissue of NIDDM patients were also reflected in isolated adipocytes. Our results demonstrate that insulin resistance in isolated adipocytes of NIDDM patients could at least partly be due to a significant depletion of adipose tissue muscle/fat glucose-transporter isoform.

Expression of the glucose transporter isoform GLUT 4 is insufficient to confer insulin-regulatable hexose uptake to cultured muscle cells.

Glucose transporter isoform expression was studied in the skeletal muscle-like cell line, C2C12. Northern and Western blot analysis showed that the insulin-responsive muscle/fat glucose transporter isoform, GLUT 4, was expressed in these cells at very low levels, whereas the erythrocyte isoform, GLUT 1, was expressed at readily detectable levels. Insulin did not stimulate glucose transport in this cultured muscle cell line. The C2C12 cells were then transfected separately with either GLUT 1 or GLUT 4, and stable cell lines expressing high levels of mRNA and protein were isolated. GLUT 1-transfected cells exhibited a 3-fold increase in the amount of the GLUT 1 transporter protein which was accompanied by a 2- to 3-fold increase in the glucose uptake rate. However, despite at least a 10-fold increase in GLUT 4 mRNA and protein detected after GLUT 4 cDNA transfection, the glucose uptake of these cells was unchanged and remained insulin-insensitive. By laser confocal immunofluorescence imaging, it was established that the transfected GLUT 4 protein was localized almost entirely in cytoplasmic compartments. In contrast, the GLUT 1 isoform was detected both at the plasma membrane as well as in intracellular compartments. These results suggest that acute insulin stimulation of glucose transport is not solely dependent on the presence of the insulin receptor and the GLUT 4 protein, and that the presence of some additional protein(s) must be required.

Regulation of glucose-transporter function.

Glucose transport in mammals is mediated by a multigene family whose expression can be highly tissue specific. All cells express at least one transporter isoform in a constitutive fashion, because a certain level of glucose uptake is an absolute necessity, regardless of influences by various regulatory factors. The level of the constitutive transporter, usually the erythroid glucose-transporter isoform, can be regulated by environmental factors, e.g., nutrition and transformation. Certain cells express unique transporter isoforms, the quantitatively most important of which is the muscle-adipocyte glucose-transporter isoform that functions in response to insulin to clear most of the blood glucose after a meal. The available data suggest that the major insulin target tissues are uniquely able to produce this transporter isoform, sequester it in a unique organelle, and bring it to the cell surface in response to insulin. This insulin response is dramatically different from that seen in various fibroblastic cells, quantitatively and qualitatively, and suggests the expression in adipose tissue and muscle of a multigene program that defines the insulin-stimulated glucose transport of relevance to organismal glucose homeostasis.

Stimulation of collagen formation by insulin and insulin-like growth factor I in cultures of human lung fibroblasts.

We examined the effects of insulin and insulin-like growth factor I (IGF-I) on the production of collagen by cultures of human embryonic lung fibroblasts. Insulin at 20 ng/ml increased collagen accumulation by 58% and total protein formation by 18%. At 2 micrograms/ml, insulin increased collagen production by 2- to 3-fold and total protein production by 2-fold. The mRNA levels for alpha 1(I) and alpha 1(III) collagen chains were elevated by insulin compared with untreated control values. IGF-I at 10 ng/ml increased collagen production 2-fold. IGF-I at 100 ng/ml maximally increased collagen production 3-fold. A specific antibody to the IGF-I receptor (alpha IR-3) caused a concentration-related decline in insulin-induced collagen formation. The addition of antibody at 1 micrograms/ml, resulted in 80% inhibition of insulin-induced collagen accumulation. Higher levels of antibody were required to inhibit IGF-I mediated collagen formation. The presence of antibody (alpha IR-3) also blocked fibroblast proliferation stimulated by epidermal growth factor plus insulin. These data show that insulin-induced collagen formation is mediated primarily through an interaction with the IGF-I receptor. The modulation of extracellular matrix production by insulin may influence the repair of tissue injury and the development of the accelerated atherosclerosis that accompanies the diabetic state in humans.

Alteration in the expression of GLUT-1 and GLUT-4 protein and messenger RNA levels in denervated rat muscles.

Denervation induces insulin resistance of the glucose transport process in skeletal muscle. To determine whether this is due to alterations in the expression of muscle glucose transporters (GLUT) in different fiber types, we evaluated the amount of GLUT-1 and GLUT-4 protein and messenger RNA (mRNA) in extensor digitorum longus (EDL) and soleus at 1, 2, and 3 days after sciatotomy. Denervation elevated the basal rate of 2-[1,2-3H]deoxy-D-glucose (2-DOG) uptake in the EDL and decreased the insulin-stimulated DOG uptake in both muscles. Denervation after 1 day did not modify the GLUT-1 or the GLUT-4 protein level in either muscle. However, it increased GLUT-1 mRNA by 66% and decreased GLUT-4 mRNA by 70% in the EDL, but not in the soleus (P < 0.05). After 2 days of denervation, by which time GLUT-1 mRNA was increased 2-fold and GLUT-4 mRNA was reduced by 70%, we observed a 2-fold increase in GLUT-1 protein (P < 0.01) in the EDL and a 40-45% decrease in GLUT-4 protein in both muscles (P < 0.01). These results indicate that modifications in the expression of GLUT-1 and GLUT-4 protein cannot explain the insulin resistance of the glucose transport process in the EDL or soleus 1 day after denervation. After 2 days of denervation, however, alterations in GLUT-1 and GLUT-4 protein levels may contribute to the change in basal and insulin-stimulated DOG uptake in both the EDL and the soleus muscles.

Vanadate treatment of streptozotocin diabetic rats restores expression of the insulin-responsive glucose transporter in skeletal muscle.

Streptozotocin-treated rats were diabetic, as assessed by blood glucose and plasma insulin values, while vanadate treatment restored blood glucose values to normal. Immunoblot analysis using a monoclonal antibody to the insulin-responsive glucose transporter demonstrated a 70% decline in transporter expression in skeletal muscle of diabetic rats. Subsequent treatment of diabetic animals with vanadate resulted in renewed expression of the transporter to 87% of control levels. Northern blot analysis of total skeletal muscle RNA from diabetic animals revealed a 55% decline in the steady state level of muscle glucose transporter mRNA, while vanadate treatment led to a 187% increase in transporter mRNA over normal levels. These results support the conclusion that vanadate acts to relieve diabetic hyperglycemia by inducing expression of the insulin-responsive glucose transporter at the pretranslational level.

Insulin-responsive human adipocytes express two glucose transporter isoforms and target them to different vesicles.

We have characterized the insulin-dependent increase in glucose transport in human adipocytes using subcellular fractionation and antibodies specific for the two isoforms of the glucose transporter that are expressed in these cells. Plasma membranes isolated from untreated human fat cells contain the erythroid/GLUT1 isoform of the glucose transporter almost exclusively whereas the muscle-fat/GLUT4 transporter isoform is most abundant in intracellular microsomal membranes in resting cells. After exposure of adipocytes to insulin, the muscle-fat isoform is dramatically increased in the plasma membrane whereas the erythroid isoform barely changes in response to insulin. Thus, the total insulin-mediated increase in plasma membrane glucose transporters, confirmed by affinity labeling of both transporter isoforms, must be due to the increase in the muscle-fat/GLUT4 transporter. The two isoforms exist in different vesicle populations as shown by immunoadsorption of the muscle fat isoform-containing vesicles which are essentially devoid of the erythroid transporter. These data indicate that the insulin-mediated increases in glucose transport in human fat cells is a result of the translocation of vesicles uniquely containing the muscle-fat glucose transporter isoform.

Insulin-like growth factor I binding and receptor kinase in red and white muscle.

IGF-I receptors were partially purified from red and white skeletal muscle by lectin-affinity chromatography and the resultant fraction was depleted of insulin receptors by insulin affinity chromatography. Equilibrium binding of 125I-IGF-I to receptor preparations from red and white muscle yielded identical Scatchard plots. The integrity of the IGF-I receptor preparation in the two fiber types was identical as determined by affinity cross-linking. The tyrosine kinase activity of the receptor from red muscle was 2-3-fold more active towards exogenous substrates in both the basal and ligand-activated states as compared to white muscle. These data show that there is IGF-I-dependent kinase activity intrinsic to IGF-I receptors from skeletal muscle, and suggest that identical cellular factors may regulate the kinase activity of insulin and IGF-I receptors in a parallel manner in vivo.

Role of PPAR gamma in regulating adipocyte differentiation and insulin-responsive glucose uptake.

Adipocyte differentiation is regulated by at least two families of transcription factors, CCAAT/enhancer binding proteins (C/EBPs) and peroxisome proliferator-activated receptors (PPARs). Induction of PPAR gamma gene transcription during the differentiation of preadipocytes into adipocytes in vitro occurs following an initial phase of cell proliferation and requires a direct involvement of C/EBP beta, C/EBP delta, and glucocorticoids. Ectopic expression of PPAR gamma in non-adipogenic, Swiss 3T3 fibroblasts promotes their conversion into adipocytes as indicated by the accumulation of lipid droplets and the induction of C/EBP alpha, aP2, insulin-responsive aminopeptidase (IRAP), and glucose transporter 4 (GLUT4) expression. These PPAR gamma-expressing Swiss cells also exhibit a high level of insulin-responsive glucose uptake that is comparable to that expressed in 3T3-L1 adipocytes. In contrast, PPAR gamma-expressing NIH-3T3 fibroblasts, despite similar lipid accumulation, adipocyte morphology, and aP2 expression, do not synthesize C/EBP alpha and fail to acquire insulin sensitivity. In Swiss 3T3 cells ectopically expressing PPAR gamma, the development of insulin-responsive glucose uptake correlates with C/EBP alpha expression. Furthermore, ectopic expression of C/EBP alpha in NIH-3T3 cells induces PPAR gamma expression and adipogenesis, but also restores insulin-sensitive glucose transport. These results suggest that although PPAR gamma is sufficient to trigger the adipogenic program, C/EBP alpha is required for establishment of insulin-sensitive glucose transport in adipocytes.

Molecular mechanisms involved in GLUT4 translocation in muscle during insulin and contraction stimulation.

Studies in mammalian cells have established the existence of numerous intracellular signaling cascades that are critical intermediates in the regulation of various biological functions. Over the past few years considerable research has shown that many of these signaling proteins are expressed in skeletal muscle. However, the detailed mechanisms involved in the regulation of glucose transporter (GLUT4) translocation from intracellular compartments to the cell surface membrane in response to insulin and contractions in skeletal muscle are not well understood. In the present essay we report three different approaches to unravel the GLUT4 translocation mechanism: 1. specific pertubation of the insulin and/or contraction signaling pathways; 2. characterization of the protein composition of GLUT4-containing vesicles with the expectation that knowledge of the constituent proteins of the vesicles may help in understanding their trafficking; 3. degree of co-immunolocalization of the GLUT4 glucose transporters with other membrane marker proteins assessed by immunofluorescense and electron microscopy.