Studies of glucose transport system of fat cells: effects of insulin and insulin mimickers

Regulation of pyruvate dehydrogenase by insulin and polyamines within electropermeabilized fat-cells and isolated mitochondria

1. Regulation of the mammalian pyruvate dehydrogenase (PDH) complex by insulin and polyamines has been examined by using electropermeabilized rat epididymal fat-cells and isolated mitochondria. The complex could be regulated within the permeabilized cells not only by insulin, but also by certain low-M(r) species, including Ca2+ and the polyamine spermidine. 2. Both spermine and spermidine increased the level of active dephosphorylated PDH (PDHa) in isolated adipose-tissue mitochondria 2-3-fold, with half-maximal effects at 0.9 mM and 1.7 mM respectively. By contrast, PDH activity in rat heart mitochondria was essentially insensitive to the effects of these polyamines. 3. The effects on PDH activity of incubation of adipose-tissue mitochondria with spermine persisted through re-isolation and re-incubation of the mitochondria in the absence of the polyamine. 4. No evidence was found of any increase in the concentration of spermine associated with purified mitochondrial fractions prepared from insulin-treated tissue. 5. Overall, the data provide further evidence against a role for polyamines in the rapid stimulation of PDH by insulin, but suggest that polyamines may be important in mediating longer-term changes in the activity of the complex.

Role of insulin receptor phosphorylation in the insulinomimetic effects of hydrogen peroxide

The oxidant H2O2 has many insulin-like effects in rat adipocytes. To determine whether these effects could be mediated by the tyrosine kinase activity of the insulin receptor, the ability of H2O2 to stimulate receptor phosphorylation in intact adipocytes and partially purified insulin receptors has been examined. Phosphorylation of the beta subunit of the insulin receptor was increased approximately 2-fold by treatment of intact cells with 3 mM H2O2, a concentration that maximally stimulates 2-deoxyglucose uptake. Stimulation of receptor phosphorylation was rapid, reaching maximal levels within 5 min, and preceded activation of glucose transport. Phosphoamino acid analysis of insulin receptors from H2O2-treated adipocytes showed that 32P incorporation into phosphotyrosine and phosphoserine residues of the beta subunit was enhanced. Furthermore, partially purified receptors from H2O2-treated cells exhibit increased tyrosine kinase activity, as measured by phosphorylation of the peptide Glu80Tyr20. In contrast, the direct addition of H2O2 to partially purified insulin receptors did not stimulate tyrosine kinase activity or insulin receptor autophosphorylation. This was not due to breakdown of H2O2 or oxidation of ATP or the required divalent cations. To define the factors involved in H2O2's effect, we have examined receptor phosphorylation in fat cell homogenates and purified plasma membranes. Although insulin stimulated receptor phosphorylation in both of these systems, H2O2 was only effective in the cell homogenates. These data demonstrate that, under certain conditions, H2O2 stimulates insulin receptor phosphorylation and tyrosine kinase activity, suggesting that the insulin-like effects of H2O2 may be mediated by stimulation of insulin receptor phosphorylation. This does not appear to be a direct effect of H2O2 on the insulin receptor and requires nonplasma membrane cellular constituents.

Is copper effect on glucose incorporation mediated by the insulin receptor in rat adipose tissue?

The insulin receptor is not the site for the stimulatory effect of copper on glucose incorporation into total lipids by adipose tissue; prewashing of adipose tissue of rats fed a stock diet in an insulin-free medium increases the glucose incorporation into total lipids in the presence of 0.1 unit insulin from 220 above control to 430% in the nondiabetic and from 154 to 230% in the streptozotocin-diabetic rat. In contrast, glucose incorporation in the presence of CuCl2 X 2H2O is unaffected by prewashing, being the same in the prewashed as in the unwashed adipose tissue. On the other hand, mild trypsin digestion of adipocytes decreases the glucose incorporation in the presence of 28.5 mU insulin, from 201 to 126% - whereas the effect of copper on glucose incorporation is the same in the trypsin-treated or untreated adipocyte. In order to obtain maximal copper effect the adipocyte has to be preconditioned by insulin; preincubation of diabetic adipose tissue first for one hour with 0.1 unit insulin, and another hour with 100 micrograms CuCl2 X 2H2O added to the medium, results in greater glucose incorporation (230% above control) than when incubated alone with either CuCl2 X 2H2O (125%) or insulin (154%) for two hours. In addition to its previously noted effect on the in vivo insulin release, copper increased the number of the insulin receptor sites in adipocytes.

Comparison of glucose and fructose transport into adipocytes of the rat

The purpose of these studies was to define the properties of the systems that transport hexoses into adipocytes. Glucose appears to enter adipocytes on a single transport system whose maximum velocity is stimulated by insulin and which is competitively inhibited by cytochalasin B, 5-thioglucose, fructose, mannose and 3-O-methylglucose. In contrast, fructose enters adipocytes by at least two separate mechanisms, one an insulin-sensitive transporter (probably the glucose transporter) and the other a mechanism that is insensitive to insulin. The fructose concentration required for half-maximal rates of transport is at least an order of magnitude higher than that for glucose and the maximum velocity of fructose transport is more than double that for glucose.

Direct demonstration of separate receptors for growth and metabolic activities of insulin and multiplication-stimulating activity (an insulinlike growth factor) using antibodies to the insulin receptor

Insulin and such insulinlike growth factors as multiplication stimulating activity (MSA) are related polypeptides that have common biological activities. Both insulin and MSA produce acute metabolic responses (stimulation of glucose oxidation in isolated fat cells) as well as growth effects (stimulation of [(3)H]thymidine incorporation into DNA in cultured fibroblasts). In addition, most cells have separate receptors for insulin and insulinlike growth factors, and both peptides have weaker affinity for each other's specific receptors than for their own. To determine, therefore, whether these effects are mediated by receptors for insulin, insulinlike growth factors, or both, we have selectively blocked insulin receptors with a specific antagonist, namely Fab fragments derived from naturally occurring antibodies to the insulin receptor. In rat adipocytes, 10 mug/ml of antireceptor Fab inhibited insulin binding by 90%, whereas it inhibited MSA binding <5%. The anti-insulin receptor Fab is without intrinsic biological activity, but acts as a competitive inhibitor of insulin receptors. Blockade of insulin receptors with Fab fragments produced a 30-fold rightward shift in the dose response for stimulation of glucose oxidation by both insulin and MSA. The dose-response curves for stimulation of oxidation by vitamin K(5) and spermine, agents that stimulate glucose oxidation through noninsulin receptor pathways, were not affected by the blockade of insulin receptors with Fab antibody fragments. These data suggest that this acute metabolic effect of both insulin and MSA is mediated via the insulin receptor. In cultured human fibroblasts, 10 mug/ml of Fab inhibited insulin binding by 90% and MSA binding by 15%. In fibroblasts, however, blockade of the insulin receptor did not alter the dose response for stimulation of thymidine incorporation into DNA by either insulin or MSA. Furthermore, intact antireceptor antibody immunoglobulin (Ig)G, which produces multiple other insulinlike effects, and Fab fragments of antireceptor antibody did not stimulate thymidine incorporation. These data demonstrate directly that the insulin receptor mediates the metabolic effects of insulin and MSA, whereas the growth-promoting action of both peptides is mediated by the MSA receptor or other growth factors.