Regulation of lipogenesis in adipocytes. Independent effects of thyroid hormones, cyclic AMP and insulin on the uptake of deoxy-D-glucose

The effect of propylthiouracyl-induced low thyroid function on secretion response and action of insulin in sheep

The effect of propylthiouracyl (PTU)-induced low thyroid function on insulin responsiveness to glucose and glucose responsiveness to insulin in sheep was studied by performing hyperglycemic and euglycemic clamp experiments. All sheep were maintained at a level of 125% daily metabolizable energy intake and were housed in an environmental room that was maintained at 20 degrees C with a 16-hr lighting period. In the first study, eight female Suffolk sheep were divided equally into two groups and were subjected to oral PTU treatments of 4 mg/kg body weight (BW) per day for 7 d (low PTU) and 8 mg/kg BW per day for 14 d (high PTU). A hyperglycemic clamp experiment was conducted in each group on both control and PTU treatment periods. Plasma concentrations of triiodothyronine and thyroxine decreased (P < 0.05) in high PTU-treated sheep compared with those of low PTU-treated and control sheep. Both PTU treatments did not significantly influence basal insulin and glucose levels. Results of the hyperglycemic clamp experiment indicated that the mean plasma insulin increment and the ratio of mean plasma insulin increment to glucose infusion rate were significantly higher (P < 0.01) in high PTU-treated sheep than in low PTU-treated and control sheep. In the second study, the PTU treatment (8 mg/kg BW per day) was applied for 17 d in four male Suffolk sheep. A euglycemic clamp experiment with two insulin infusion rates (1.0 and 10.0 mU/kg per minutes) for two sequential periods of 2 hr each and thyroid hormone responses to intrajugular injection of thyrotropin-releasing hormone (1 microgram/kg BW) were performed in each sheep on both control and PTU treatment periods. In the euglycemic clamp experiment, the glucose infusion rate and the ratio of glucose infusion rate to mean plasma insulin increment were significantly reduced (P < 0.05) during the PTU treatment period for 10.0 mU/kg BW per minute of insulin infusion rate. The response areas of plasma thyroxine and triiodothyronine to thyrotropin-releasing hormone injection were blunted (P < 0.01) in PTU-treated sheep compared with those of control sheep. The high PTU treatment induced low thyroid function, enhanced insulin secretion response, and impaired insulin action in sheep.

Effects of hypothyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin in the soleus muscle of the rat

1. The effects of hypothyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin were investigated in the isolated, incubated soleus muscle of the rat. 2. Hypothyroidism, which was induced by administration of propylthiouracil to the rats, decreased fasting plasma levels of free fatty acids and increased plasma levels of glucose but did not significantly change plasma levels of insulin. 3. The sensitivity of the rates of glycogen synthesis to insulin was increased at physiological, but decreased at supraphysiological, concentrations of insulin. 4. The rates of glycolysis in the hypothyroid muscles were decreased at all insulin concentrations studied and the EC50 for insulin was increased more than 8-fold; the latter indicates decreased sensitivity of this process to insulin. However, at physiological concentrations of insulin, the rates of glucose phosphorylation in the soleus muscles of hypothyroid rats were not different from controls. This suggests that hypothyroidism affects glucose metabolism in muscle not by affecting glucose transport but by decreasing the rate of glucose 6-phosphate conversion to lactate and increasing the rate of conversion of glucose 6-phosphate to glycogen. 5. The rates of glucose oxidation were decreased in the hypothyroid muscles at all insulin concentrations.

Decreased insulin receptor binding in hyperthyroidism

The binding of 125I-insulin to insulin receptors on circulating mononuclear leukocytes was studied in ten patients with hyperthyroidism and 20 euthyroid normal volunteers. The hyperthyroid patients demonstrated significantly elevated glucose levels following an oral glucose load, despite normal insulin secretion. The infusion of insulin resulted in a delayed hypoglycaemic effect in the hyperthyroid patients; however, the inhibition of the endogenous insulin secretion as indicated by suppression of C-peptide levels was not different from euthyroid control subjects. Insulin binding to monocytes was significantly decreased in the hyperthyroid patients. Scatchard analysis of binding data indicates that a decrease of receptor number rather than receptor affinity seems to be the cause of the lowered insulin binding in hyperthyroid patients with diffuse toxic goitre. The findings of decreased insulin receptor number, mild degree of glucose intolerance despite normal insulin secretion and the delayed hypoglycaemic effect following insulin infusion suggest that peripheral insulin resistance could be involved in the highly complex pathophysiology of glucose intolerance in hyperthyroidism.

Direct effect of triiodothyronine on glucose utilization in adipocytes from obese and nonobese Zucker rats

The direct effects of L-T3 (triiodothyronine) on glucose utilization, 2-deoxyglucose uptake and O2 consumption were evaluated in vitro in isolated adipocytes from 6-week-old obese and nonobese Zucker rats. Adipocytes treated for 30 min with L-T3 had a 20-25% decrease in glucose conversion to CO2, fatty acids and glyceride-glycerol and uptake of 2-deoxyglucose. Incubation of adipocytes with either D-T3 or rT3 (reverse T3) did not decrease utilization of glucose. T3 also decreased the insulin-stimulated fatty acid synthesis from glucose in both obese and nonobese adipocytes. Adipocyte O2 consumption was decreased by T3 in media that contained glucose but not in media that contained either acetate or no exogenous substrate. These data indicate that T3 has a direct, immediate and specific action on adipocyte glucose utilization and may act to modulate adipose lipogenesis and lipolysis.

Effects of thyroid hormones on adipose tissue development in Sherman and Zucker rats

It has been reported that mild hyperthyroidism in the young Sherman rat induces adipose tissue hyperplasia, concomitant with cell size reduction, and that hypothyroidism induces opposite effects. The present experiments were designed to study the evolution of cellularity in retroperitoneal and epididymal adipose tissue during a long term thyroxine (T4) treatment or in T4-treated rats, after the treatment had been stopped. In both cases, hyperplasia was transient with the observation that the adipocyte number observed in 3-mo-old control rats was reached earlier in T4-treated rats. In hypothyroid rats, hypoplasia was also transient, because once the treatment was stopped, the cell number overtook that of controls. Zucker rats were also treated with T4, because hypoplasia has been observed in young obese (fa/fa) rats and these rats are reported to be hypothyroid. T4 treatment increased their adipocyte number up to the level of nonobese (Fa/fa) untreated rats, while hypertrophy, although reduced, was persistent. In Sherman and Zucker rats, adipose tissue lipoprotein lipase activity was decreased by T4 treatment in parallel with and perhaps because of adipocyte size reduction. We suggest that hyperplasia induced by thyroid hormones results from a precocious differentiation of preadipocytes and does not necessarily imply an increased preadipocytes multiplication.

Cyclic AMP, cyclic GMP and glucose utilization by diabetic rat fat cells

Fat cells from epididymal adipose tissue from normal and streptozotocin-diabetic rats were studied to determine glucose utilization and cyclic nucleotide levels. Diabetic rat fat cells present a higher cAMP content (P less than 0.05) compared with controls. Addition of insulin decreases within 10-min incubation the cAMP content in both normal and diabetic cells (P less than 0.05). However, the value obtained in the latter remains by 25% higher than that of normal cells not exposed to insulin. No changes in cGMP were detected. Pretreatment of the diabetic animals during two days with propranolol (1 mg kg body wt-1 day-1) induces the decrease to normal levels of the fat cell cAMP content. However, it persists the impairment on glucose utilization observed in fat cells from diabetic animals. It seems that the increase in the intracellular amount of cAMP found in fat cells from diabetic rats is not involved, at least directly, to the impaired glucose utilization found in the diabetic state. Furthermore, through an unknown mechanism, pretreatment with propranolol can induce a drop in fat tissue cAMP toward normal values without normalizing glucose utilization.

Metabolic aspects of the calorigenic effect of thyroid hormone in mammals

The increased BMR in hyperthyroidism may be accounted for by the use of chemical energy for metabolic processes and work. Major contributors are the heart work and futile cycling of FFA into triglyceride in adipose tissue, whereas gluconeogenesis in liver and the maintenance of sodium and potassium concentration gradients across the plasma membranes are unlikely to play any significant role. Information on the use of energy for protein turnover and urea production is lacking. The rate of oxygen uptake is not increased in the brain, spleen and testis. The main metabolic fuel seems to be free fatty acid. The mechanism which enables the hyperthyroid tissue to maintain normal concentrations of ATP, ADP and inorganic phosphate, (Pi) despite an increased turnover of energy-rich phosphates, is not fully elucidated. However, ultimately the increased rate of oxygen uptake in hyperthyroidism seems to rely upon an increased capacity for the transport of cytosolic ADP and Pi into mitochondria. The transport capacity is increased by an increased area of the mitochondrial membrane per g tissue and by a change in the kinetics of translocation of substrates for oxidative phosphorylation. Other transport processes across the mitochondrial membrane are also changed by hyperthyroidism, e.g. long chain fatty acid transport via carnitine acyl transferase, and oxaloacetate transport via substrate shuttles.

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.