Internalization of insulin and its receptor in the isolated rat adipose cell: Time-course and insulin concentration dependency

The time-course and insulin concentration dependency of internalization of insulin and its receptor have been examined in isolated rat adipose cells at 37°C. The internalization of insulin was assessed by examining the subcellular distribution of cell-associated [¹²⁵I]insulin among plasma membrane, and high-density (endoplasmic reticulum-enriched) and low-density (Golgi-enriched) microsomal membrane fractions prepared by differential ultracentrifugation. The distribution of receptors was measured by the steady-state exchange binding of fresh [¹²⁵I]insulin to these same membrane fractions. At 37°C, insulin binding to intact cells is accompanied initially by the rapid appearance of intact insulin in the plasma membrane fraction, and subsequently, by its rapid appearance in both the high-density and low-density microsomal membrane fractions. An apparent steady-state distribution of insulin per mg of membrane protein among these subcellular fractions is achieved within 30 min in a ratio of 1:1.54:0.80, respectively. Concomitantly, insulin binding to intact cells is associated with the rapid disappearance of approx. 30% of the insulin receptors initially present in the plasma membrane fraction and appearance of 20–30% of those lost in the low-density microsomal membrane fraction. However, the number of receptors in the high-density microsomal membrane fraction does not change. This redistribution of receptors also appears to reach a steady-state within 30 min. Both processes are insulin concentration-dependent, correlating with receptor occupancy in the intact cell, and are partially inhibited at 16°C. While the steady-state subcellular distributions of insulin and its receptor do not correlate with that of acid phosphatase, chloroquine markedly increases the levels of insulin associated with all three membrane fractions in apparent proportion to the distribution of this lysosomal marker enzyme activity, without more than marginally potentiating insulin's effects on the distribution of receptors. These results demonstrate that insulin, initially bound to the plasma membrane of the isolated rat adipose cell, is rapidly translocated by a receptor-mediated process into at least two intracellular compartments associated with the cell's high- and low-density microsomes. Furthermore, insulin simultaneously induces the translocation of its own receptor from the plasma membrane into the latter compartment. These translocations appear to represent the internalization and partial dissociation of the insulin-receptor complex through insulin-induced receptor cycling.     

Internalization of insulin receptors in the isolated rat adipose cell. Demonstration of the vectorial disposition of receptor subunits

The internalization of the insulin receptor in the isolated rat adipose cell and the spatial orientation of the alpha (Mr = 135,000) and beta (Mr = 95,000) subunits of the receptor in the plasma membrane have been examined. The receptor subunits were labeled by lactoperoxidase/Na125I iodination, a technique which side-specifically labels membrane proteins in intact cells and impermeable membrane vesicles. Internalization was induced by incubating cells for 30 min at 37 degrees C in the presence of saturating insulin. Plasma, high density microsomal (endoplasmic reticulum-enriched), and low density microsomal (Golgi-enriched) membrane fractions were prepared by differential ultracentrifugation. Receptor subunit iodination was analyzed by immunoprecipitation with anti-receptor antibodies, sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and autoradiography. When intact cells were surface-labeled and incubated in the absence of insulin, the alpha and beta receptor subunits were clearly observed in the plasma membrane fraction and their quantities in the microsomal membrane fractions paralleled plasma membrane contamination. Following receptor internalization, however, both subunits were decreased in the plasma membrane fraction by 20-30% and concomitantly and stoichiometrically increased in the high and low density microsomal membrane fractions, without alterations in either their apparent molecular size or proportion. In contrast, when the isolated particulate membrane fractions were directly iodinated, both subunits were labeled in the plasma membrane fraction whereas only the beta subunit was prominently labeled in the two microsomal membrane fractions. Iodination of the subcellular fractions following their solubilization in Triton X-100 again clearly labeled both subunits in all three membrane fractions in identical proportions. These results suggest that 1) insulin receptor internalization comprises the translocation of both major receptor subunits from the plasma membrane into at least two different intracellular membrane compartments associated, respectively, with the endoplasmic reticulum and Golgi-enriched membrane fractions, 2) this translocation occurs without receptor loss or alterations in receptor subunit structure, and 3) the alpha receptor subunit is primarily, if not exclusively, exposed on the extracellular surface of the plasma membrane while the beta receptor subunit traverses the membrane, and this vectorial disposition is inverted during internalization.     

Internalization and activation of the rat liver insulin receptor kinase in vivo

The preparation of clearly delineated plasmalemma (PM) and endosomal subcellular fractions from rat liver has allowed us to compare insulin receptor (IR) kinase activity at the cell surface and in hepatic endosomes (ENs) as a function of dose and time after injected insulin. Tyrosine kinase activity in PM and ENs was measured, after solubilization and partial purification by wheat germ agglutinin chromatography (lectin-purified), using poly(Glu:Tyr) as substrate. Following the injection of a subsaturating dose of insulin (1.5 micrograms/100 g body weight), lectin-purified receptor showed peak activation at 30 s in PM and at 2 min in ENs. As observed previously (Khan, M. N., Savoie, S., Bergeron, J. J. M., and Posner, B. I. (1986) J. Biol. Chem. 261, 8462-8472) autophosphorylation activity was also augmented following insulin injection. In a pattern virtually identical to that of exogenous kinase activity, autophosphorylation attained peak activity at 30 s in PM and at 2 min in ENs. The time course of IR autophosphorylation in intact membranes was very similar to that observed for lectin purified receptors and was seen with an injected insulin dose as low as 150 ng/100 g body weight. Phosphatase treatment of the solubilized endosomal receptor abolished its enhanced activity. Hence, insulin treatment led to in vivo receptor phosphorylation which was reflected in the enhancement of both tyrosine kinase and autophosphorylation activities. Significant differences in the phosphorylation activities of PM and ENs were observed. Phosphoamino acid analyses revealed that the activated IR of intact PM was autophosphorylated in vitro, at both serine (55%) and tyrosine (45%) residues; whereas the activated IR of intact ENs was phosphorylated in vitro exclusively on tyrosine autophosphorylation specific activity for the activated IR of ENs was 3- to 4-fold that of the IR of PM. This was observed for the lectin purified IRs as well as for IRs of intact cell fractions. The reduced level of IR autophosphorylation in PM was not due to occlusion of tyrosine acceptor sites by prior in vivo phosphorylation. The rapidity with which activated IR accumulates in ENs as well as the sensitivity of endosomal IR kinase to activation by injected insulin are consistent with the endosomal apparatus serving a physiologically significant site for the regulation of transmembrane signaling.     

Internalization of the neonatal brain insulin receptor

Using 10-15 day neonatal rabbit brain cells, we studied the internalization (n = 6) and intracellular degradation (n = 8) of specifically bound 125I-insulin. In addition we investigated the association between the internalization of the specifically bound 125I-insulin and the metabolic effects of insulin such as glucose (n = 13) and amino-acid (leucine) uptake (n = 6). Phenylarsine oxide (10 microM), an agent that inhibits the internalization of the insulin receptor (n = 6) decreased the specifically bound 125I-insulin in the intact and trypsin-resistant (inside) part of the brain cells by 50% (p less than 0.05). On the other hand chloroquine (100 microM), a lysosomotropic agent that interferes with the intracellular degradation of the insulin receptor (n = 8) increased two-fold the 125I-insulin specifically bound to the intact and trypsin resistant part of the cells (p less than 0.05). Both these agents did not alter the time-dependent basal glucose uptake by the brain cells. Glucose alone regulated its own uptake (n = 4) whereas 1 X 10(-6) M insulin did not augment the glucose uptake (n = 11+13) above basal. Similarly leucine regulated the leucine uptake (n = 4) but insulin did not alter this basal uptake by the brain cells (n = 6). In summary we observed no associated glucose or leucine uptake along with the presence of internalization and intracellular degradation of specifically bound 125I-insulin in the brain cells.     

Internalization of the human insulin receptor. The insulin-independent pathway.

Internalization of the human insulin receptor requires the activation by insulin of the intrinsic kinase of the receptor. However, even in the absence of kinase activation, insulin receptors slowly enter the cells. In the present study, we addressed the question of this insulin-independent pathway of internalization. To that end, we traced insulin receptor internalization with a monoclonal antibody (mAb 83-14) directed against the alpha-subunit of the human insulin receptor. Internalization of this antibody was followed in Chinese hamster ovary (CHO) cells transfected with either normal (CHO.HIRC2) or kinase-deficient (CHO.A1018) human insulin receptors. The internalization rate of 125I-mAb 83-14 was comparable in CHO cells expressing kinase-active or kinase-inactive receptors and was similar to that observed for 125I-insulin in CHO.A1018 cells. Moreover, in CHO.HIRC2 cells, the internalization of 125I-mAb 83-14 was identical with that of its 125I-Fab fragments. Thus, mAb 83-14 represents an appropriate tool to study the constitutive internalization of the insulin receptor. Internalization of insulin receptors tagged with 125I-mAb 83-14 was unaffected by cytochalasin B, which excluded a macropinocytotic process. By contrast, internalization was sensitive to hypertonia, which abrogates clathrin-coated pits-mediated endocytosis. The implication of clathrin-coated pits in this internalization process was directly demonstrated by quantitative electron microscopic autoradiography, which showed that 125I-mAb 83-14 present on the nonvillous domain of the cell surface preferentially associate with clathrin-coated pits at all time points.     

Determinations of adipose cell size and number in suspensions of isolated rat and human adipose cells

The osmic acid fixation-Coulter electronic counter method described for determining adipose cell size and number in intact adipose tissue fragments has been modified for use with suspensions of isolated rat and human adipose cells. Mean cell sizes in tissue fragments and isolated cell suspensions prepared from the same tissue are virtually identical in rats of various weights. No statistically significant difference in mean adipose cell size between tissue and isolated cell suspension was observed in human adipose tissue although the variability was much greater than in rat tissue. The distribution of cell sizes among replicate samples is more uniform in the isolated cell preparations, possibly reflecting the considerably larger quantities of tissue used in preparing isolated cells than in determining cell size and number directly from tissue fragments. An example of the utility of the modified method during routine metabolic studies with isolated rat epididymal adipose cells is described; isolated cells of increasing size can be obtained from rats of increasing body weight, or from the separated distal and proximal portions of the fat pads of rats of the same weight.     

Biosynthesis of the insulin receptor in rat adipose cells. Intracellular processing of the Mr-190 000 pro-receptor.

We investigated the biosynthesis of the insulin receptor in primary cultures of isolated rat adipose cells. Cells were pulse-chase-labelled with [3H]mannose, and at intervals samples were homogenized. Three subcellular membrane fractions were prepared by differential centrifugation: high-density microsomal (endoplasmic-reticulum-enriched), low-density microsomal (Golgi-enriched), and plasma membranes. After detergent solubilization, the insulin receptors were immunoprecipitated with anti-receptor antibodies and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and autoradiography. After a 30 min pulse-label [3H]mannose first appeared in a band of Mr 190 000. More than 80% of the Mr-190 000 component was recovered in the microsomal fractions. Its intensity reached a maximum at 1 h in the high-density microsomal fraction and at 2 h in the low-density microsomal fraction, and thereafter declined rapidly (t 1/2 approx. 3 h) in both fractions. In the plasma-membrane fraction, the radioactivity in the major receptor subunits, of Mr 135 000 (alpha) and 95 000 (beta), rose steadily during the chase and reached a maximum at 6 h. The Mr-190 000 precursor could also be detected in the high-density microsomal fraction by affinity cross-linking to 125I-insulin. In the presence of monensin, a cationic ionophore that interferes with intracellular transport within the Golgi complex, the processing of the Mr-190 000 precursor into the alpha and beta subunits was completely inhibited. Our results suggest that the Mr-190 000 pro-receptor originates in the endoplasmic reticulum and is subsequently transferred to the Golgi complex. Maturation of the pro-receptor does not seem to be necessary for the expression of the insulin-binding site. Processing of the precursor into the mature receptor subunits appears to occur during the transfer of the pro-receptor from the Golgi complex to the plasma membrane.     

Separate intracellular pathways for insulin receptor recycling and insulin degradation in isolated rat adipocytes

To gain insight into the sequence of events that follow endocytotic uptake of insulin receptor complexes, we have examined the interrelationship between the degradative pathway of the insulin ligand and the recycling pathway of the insulin receptor. Tris(hydroxymethyl)aminomethane and other nonamphoteric amines were found to selectively impair insulin receptor recycling while leaving the insulin-degradative pathway intact. In contrast, low concentrations of the lysosomotropic agent chloroquine markedly inhibited intracellular insulin degradation but had little or no affect on the recycling of internalized receptors. Thus, we conclude: (1) that insulin dissociates from its receptor after endocytotic uptake and both receptor and ligand follow a separate intracellular pathway; and (2) that receptor recycling and insulin degradation can be selectively inhibited by Tris and chloroquine, respectively, highlighting the potential usefulness of these agents as intracellular probes in the study of receptor-ligand metabolism.     

Counter-regulation of insulin-stimulated glucose transport by catecholamines in the isolated rat adipose cell

The interaction between catecholamines and insulin in regulating glucose transport in isolated rat adipose cells has been evaluated. In the absence of insulin, 1 microM isoproterenol stimulates 3-O-methylglucose transport approximately 2-fold. However, isoproterenol in combination with adenosine deaminase inhibits glucose transport activity approximately 60%. N6-Phenylisopropyladenosine, a nonmetabolizable adenosine analogue, substantially reverses this inhibitory effect and actually stimulates glucose transport activity approximately 2-fold in the absence of isoproterenol. Dibutyryl cAMP inhibits glucose transport activity approximately 75% regardless of adenosine deaminase. While none of these agents significantly influences the basal concentration of plasma membrane glucose transporters, as assessed by specific D-glucose-inhibitable cytochalasin B binding, isoproterenol or dibutyryl cAMP in combination with adenosine deaminase reduces that in the low density microsomes 19 and 58%, respectively. In the presence of insulin, both isoproterenol and adenosine deaminase alone inhibit glucose transport activity approximately 25%. However, only the latter is accompanied by a corresponding decrease in the insulin-stimulated concentration of plasma membrane glucose transporters. Together, isoproterenol and adenosine deaminase inhibit insulin-stimulated glucose transport activity approximately 75%, even in the presence of 5 mM glucose to maintain cellular ATP levels. A similar inhibition is observed with dibutyryl cAMP. However, these agents decrease the insulin-stimulated concentration of plasma membrane glucose transporters only approximately 45%. Nevertheless, all of these inhibitory effects occur through decreases in the transport Vmax. In addition, N6-phenylisopropyladenosine partially reverses the inhibitory effects induced by the presence of adenosine deaminase. These results suggest that catecholamines counter-regulate basal and insulin-stimulated glucose transport in rat adipose cells through a cAMP-mediated mechanism, but only in part by modulating the translocation of glucose transporters.     

Lactate release from isolated rat adipocytes: influence of cell size, glucose concentration, insulin and epinephrine

Release of lactate was studied during in vitro incubations with isolated fat cells. Lactate release increased (approximately 3-fold) with increasing medium glucose concentration (from 3 to 12 mM) in both large and small fat cells. Large fat cells from older, fatter rats, however, released 3-4 times more lactate per cell than small fat cells from young rats when incubated with 3, 6 or 12 mM glucose. Insulin and epinephrine produced a marked stimulation of lactate release in small fat cells, but these hormones had no effect in large fat cells. Lactate accounted for only 10-15% of the glucose metabolized by small fat cells under all incubation conditions but was nearly 40% of glucose utilized by large fat cells at glucose concentrations greater than 6 mM. In conclusion, lactate is a major metabolite of glucose in adipocytes, particularly in the large fat cells. Adipose tissue may therefore be a major site of lactate production, particularly in states of altered glucose metabolism (i.e., hyperglycemia) and obesity.     

Internalization and recycling of insulin receptors in hepatoma cells. Absence of regulation by receptor occupancy.

Insulin receptors of Fao hepatoma cells were labelled with a 125I-labelled photoreactive insulin analogue or by surface iodination catalysed by lactoperoxidase. Cells were then incubated at 37 degrees C, and the cellular localization of the labelled receptors was assessed by limited exposure of intact cells to trypsin. The results show that: (1) photolabelled insulin-receptor complexes are internalized and recycled in Fao hepatoma cells; (2) the dynamics of photolabelled insulin receptors (internalization and recycling) is similar before and after down-regulation; (3) the unoccupied receptors labelled by surface iodination are internalized and recycled similarly to covalent insulin-receptor complexes; (4) insulin does not induce internalization of surface-iodinated insulin receptors. We conclude that internalization and recycling of insulin receptors are independent of receptor occupancy by insulin in Fao hepatoma cells.     

Internalization efficiency of the transferrin receptor.

Quantitative ultrastructural and biochemical methods have allowed us to obtain a coherent set of data on the internalization efficiency of the transferrin receptor (TfR). In confluent cell cultures we find that (1) the initial internalization rate of transferrin is approximately 10% per minute, and (2) around 10% of cell-surface TfRs are present in coated pits. From these data a lifetime of coated pits of ca. 1 min is derived. Furthermore, we show that coated pits constitute 1.1-1.4% of the plasma membrane area in confluent cell cultures. Thus, the TfR is concentrated six- to ninefold in coated pits compared to resident plasma membrane proteins. Moreover, we show that the concentration of TfRs in coated pits is cell density dependent, since only around 5% of the receptors are present in coated pits in low-density cultures. Correspondingly, the internalization of TfRs in high-density cell cultures is roughly twice as efficient as that in low-density cell cultures. The reduced TfR internalization efficiency at low cell density is accounted for by a concomitant decrease to 0.55% in the relative surface area occupied by coated pits.     

Caveolin-1 interacts with the insulin receptor and can differentially modulate insulin signaling in transfected Cos-7 cells and rat adipose cells

Caveolae may function as microdomains for signaling that help to determine specific biological actions mediated by the insulin receptor (IR). Caveolin-1, a major component of caveolae, contains a scaffolding domain (SD) that binds to a caveolin-1 binding motif in the kinase domain of the IR in vitro. To investigate the potential role of caveolin-1 in insulin signaling we overexpressed wild-type (Cav-WT) or mutant (Cav-Mut; F92A/V94A in SD) caveolin-1 in either Cos-7 cells cotransfected with IR or rat adipose cells (low and high levels of endogenous caveolin-1, respectively). Cav-WT coimmunoprecipitated with the IR to a much greater extent than Cav-Mut, suggesting that the SD is important for interactions between caveolin-1 and the IR in intact cells. We also constructed several IR mutants with a disrupted caveolin-1 binding motif and found that these mutants were poorly expressed and did not undergo autophosphorylation. Interestingly, overexpression of Cav-WT in Cos-7 cells significantly enhanced insulin-stimulated phosphorylation of Elk-1 (a mitogen-activated protein kinase-dependent pathway) while overexpression of Cav-Mut was without effect. In contrast, in adipose cells, overexpression of either Cav-WT or Cav-Mut did not affect insulin-stimulated phosphorylation of a cotransfected ERK2 (but did significantly inhibit basal phosphorylation of ERK2). Furthermore, we also observed a small inhibition of insulin-stimulated translocation of GLUT4 when either Cav-WT or Cav-Mut was overexpressed in adipose cells. Thus, interaction of caveolin-1 with IRs may differentially modulate insulin signaling to enhance insulin action in Cos-7 cells but inhibit insulin's effects in adipose cells.