Insulin-independent Mechanism Research Articles

Exercise signalling to glucose transport in skeletal muscle.

Contraction-induced glucose uptake in skeletal muscle is mediated by an insulin-independent mechanism that leads to translocation of the GLUT4 glucose transporter to the muscle surface membrane from an intracellular storage site. Although the signalling events that increase glucose transport in response to muscle contraction are not fully elucidated, the aim of the present review is to briefly present the current understanding of the molecular signalling mechanisms involved. Glucose uptake may be regulated by Ca(2+)-sensitive contraction-related mechanisms, possibly involving Ca(2+)/calmodulin-dependent protein kinase II and some isoforms of protein kinase C. In addition, glucose transport may be regulated by mechanisms that reflect the metabolic status of the muscle, probably involving the 5'AMP-activated protein kinase. Furthermore, the p38 mitogen-activated protein kinase may be involved in activating the GLUT4 translocated to the surface membrane. Nevertheless, the picture is incomplete, and fibre type differences also seem to be involved.

Addition of a Fenugreek Extract to Glucose Feedings Increases Post Exercise Glycogen Resynthesis

0855 The resynthesis of muscle glycogen post exercise is critical to endurance performance during subsequent exercise bouts. Previous research generally supports the concept that supplemental protein and/or select amino acids may accelerate rates of glycogen resynthesis beyond carbohydrate alone. PURPOSE: The purpose of this study was to determine the effects of ingesting a commercial supplement containing 4-hydroxyisoleucine (4 OH-Ile), isolated from fenugreek seeds, with a glucose beverage (GLU+4 OH-Ile) on rates of post-exercise muscle glycogen resynthesis in trained male cyclists. METHODS: Following an overnight fast (12 hr), subjects completed a 90-minute glycogen depletion interval ride after which a muscle biopsy was obtained from the vastus lateralis (0hr). During one trial, subjects received a 1.8 g.kg BW−1 BW oral dose of dextrose (GLU) immediately post biopsy and again 2 hours later. During another trial, subjects received the same dose of dextrose with 2.0 mg.kg−1 4-OH-Ile (GLU+4 OH-Ile) at the same feeding schedule. A second muscle biopsy was obtained from the same leg 4 hours after the initial feeding (4hr). Blood samples for insulin and glucose were obtained throughout the recovery period. Trials were completed double blind in a randomized, counterbalanced order. Statistical significance was set at p<0.05. RESULTS: Post exercise (0hr) muscle glycogen concentration was similar for both trials. Overall, there was a significant increase in glucose and insulin concentrations from 0hr throughout the majority of the 4-hour recovery period. However, there were no significant differences in glucose or insulin concentration between the two trials at any time point. Although muscle glycogen concentration significantly increased from immediately post exercise (0hr) to 4hr of recovery for both trials, the average rate of muscle glycogen resynthesis was 63% greater during the GLU+4 OH-Ile trial (10.6 ± 3.3 vs. 6.5 ± 2.6 g.kg wet wt.−1.hr−1 hr. for the GLU+4 OH-Ile and GLU trials, respectively). CONCLUSION: These data demonstrate that ingestion of the commercial 4-hydroxyisoleucine supplement along with an oral dose of dextrose stimulates skeletal muscle glucose uptake and muscle glycogen resynthesis by way of an insulin-independent mechanism. Funded by Technical Sourcing International, Inc.

P38gamma MAPK regulation of glucose transporter expression and glucose uptake in L6 myotubes and mouse skeletal muscle.

Skeletal muscle expresses at least three p38 MAPKs (alpha, beta, gamma). However, no studies have examined the potential regulation of glucose uptake by p38gamma, the isoform predominantly expressed in skeletal muscle and highly regulated by exercise. L6 myotubes were transfected with empty vector (pCAGGS), activating MKK6 (MKK6CA), or p38gamma-specific siRNA. MKK6CA-transfected cells had higher rates of basal 2-deoxy-d-[3H]glucose (2-DG) uptake (P < 0.05) but lower rates of 2,4-dinitrophenol (DNP)-stimulated glucose uptake, an uncoupler of oxidative phosphorylation that operates through an insulin-independent mechanism (P < 0.05). These effects were reversed when MKK6CA cells were cotransfected with p38gamma-specific siRNA. To determine whether the p38gamma isoform is involved in the regulation of contraction-stimulated glucose uptake in adult skeletal muscle, the tibialis anterior muscles of mice were injected with pCAGGS or wild-type p38gamma (p38gammaWT) followed by intramuscular electroporation. Basal and contraction-stimulated 2-DG uptake in vivo was determined 14 days later. Overexpression of p38gammaWT resulted in higher basal rates of glucose uptake compared with pCAGGS (P < 0.05). Muscles overexpressing p38gammaWT showed a trend for lower in situ contraction-mediated glucose uptake (P = 0.08) and significantly lower total GLUT4 levels (P < 0.05). These data suggest that p38gamma increases basal glucose uptake and decreases DNP- and contraction-stimulated glucose uptake, partially by affecting levels of glucose transporter expression in skeletal muscle. These findings are consistent with the hypothesis that activation of stress kinases such as p38 are negative regulators of stimulated glucose uptake in peripheral tissues.

Signalling to glucose transport in skeletal muscle during exercise.

Exercise-induced glucose uptake in skeletal muscle is mediated by an insulin-independent mechanism. Although the signalling events that increase glucose transport in response to muscle contraction are not fully elucidated, the aim of the present review is to briefly present the current understanding of the molecular signalling mechanisms involved. Glucose uptake may be regulated by Ca++-sensitive contraction-related mechanisms possibly involving protein kinase C, and by mechanisms that reflect the metabolic status of the muscle and may involve the AMP-activated protein kinase. Furthermore the p38 mitogen activated protein kinase may be involved. Still, the picture is incomplete and a substantial part of the exercise/contraction-induced signalling mechanism to glucose transport remains unknown.

AMP-activated protein kinase and muscle glucose uptake.

The AMP-activated protein kinase (AMPK) is an enzyme that is activated in situations where there are changes in the cellular energy status such as muscle contraction and hypoxia. AMPK can also be pharmacologically activated by the compound 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) and the antidiabetic agent metformin. Several studies support the hypothesis that AMPK plays an important role in the stimulation of muscle glucose uptake by these physiological and pharmacological stimuli. In isolated rat muscles, activation of AMPK is associated with increases in glucose uptake through an insulin-independent mechanism. Studies done in rodents have shown that the activation of AMPK by AICAR is accompanied by decreases in blood glucose concentrations, in part due to enhanced muscle glucose uptake. Similar to exercise, AICAR not only directly stimulates glucose uptake into the skeletal muscle, but also enhances insulin sensitivity. The activation of AMPK and associated increases in muscle glucose uptake are affected by factors such as glycogen content, exercise training and fibre type. The effects of AMPK on muscle glucose uptake makes this protein a promising pharmacological target for the treatment of type 2 diabetes.

A possible role for AMP-activated protein kinase in exercise-induced glucose utilization: insights from humans and transgenic animals

Exercise-induced glucose uptake in skeletal muscle is mediated by an insulin-independent mechanism, but the actual signals to glucose transport in response to muscle contraction have not been identified. The 5'-AMP-activated protein kinase (AMPK) has emerged as a putative mediator of contraction-induced glucose transport, although no conclusive evidence has been provided so far. Recent experiments in AMPK transgenic mice suggest that glucose transport induced by 5-amino-4-imidazolecarboxamide riboside (AICAR) or hypoxia is mediated by AMPK. In contrast, contraction-induced glucose transport in rodent skeletal muscle induced by electrical stimulation in vitro or in situ is not influenced or is only partially reduced by abolishing both or one of the catalytic AMPK subunits. This is compatible with exercise studies done in humans, where no tight correlation is found between AMPK activity and glucose uptake during exercise. Taken together, these results question an essential role of AMPK in exercise-induced glucose uptake and imply that one or more additional pathways are involved in mediating glucose transport in skeletal muscle during exercise.

Leucine promotes glucose uptake in skeletal muscles of rats

Soleus muscles isolated from normal rats were incubated to evaluate whether or not leucine promotes glucose uptake under insulin-free conditions, using a labeled 2-deoxyglucose uptake assay. Glucose uptake was promoted by 2 mM leucine. A metabolite of leucine, α-ketoisocaproic acid (α-KIC), also exhibited a similar stimulatory effect, although this was not as potent as leucine. Stimulation of glucose uptake by leucine was completely canceled by pre-treatment with either 10 μM LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), or 6 μM GF109203X, a specific inhibitor of protein kinase C (PKC). No significant change was observed by pre-treatment with 1 μM rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR). These results suggest that leucine stimulates glucose transport in skeletal muscle via PI3-kinase and PKC pathways independently of the mammalian target of mTOR. They also suggest that leucine stimulates glucose transport by an insulin-independent mechanism.

Dichloroacetate stimulates glycogen accumulation in primary hepatocytes through an insulin-independent mechanism.

Dichloroacetate (DCA), a by-product of water chlorination, causes liver cancer in B6C3F1 mice. A hallmark response observed in mice exposed to carcinogenic doses of DCA is an accumulation of hepatic glycogen content. To distinguish whether the in vivo glycogenic effect of DCA was dependent on insulin and insulin signaling proteins, experiments were conducted in isolated hepatocytes where insulin concentrations could be controlled. In hepatocytes isolated from male B6C3F1 mice, DCA increased glycogen levels in a dose-related manner, independently of insulin. The accumulation of hepatocellular glycogen induced by DCA was not the result of decreased glycogenolysis, since DCA had no effect on the rate of glucagon-stimulated glycogen breakdown. Glycogen accumulation caused by DCA treatment was not hindered by inhibitors of extracellular-regulated protein kinase kinase (Erk1/2 kinase or MEK) or p70 kDa S6 protein kinase (p70(S6K)), but was completely blocked by the phosphatidylinositol 3-kinase (PI3K) inhibitors, LY294002 and wortmannin. Similarly, insulin-stimulated glycogen deposition was not influenced by the Erk1/2 kinase inhibitor, PD098509, or the p70(S6K) inhibitor, rapamycin. Unlike DCA-stimulated glycogen deposition, PI3K-inhibition only partially blocked the glycogenic effect of insulin. DCA did not cause phosphorylation of the downstream PI3K target protein, protein kinase B (PKB/Akt). The phosphorylation of PKB/Akt did not correlate to insulin-stimulated glycogenesis either. Similar to insulin, DCA in the medium decreased IR expression in isolated hepatocytes. The results indicate DCA increases hepatocellular glycogen accumulation through a PI3K-dependent mechanism that does not involve PKB/Akt and is, at least in part, different from the classical insulin-stimulated glycogenesis pathway. Somewhat surprisingly, insulin-stimulated glycogenesis also appears not to involve PKB/Akt in isolated murine hepatocytes.

AMP-activated protein kinase (AMPK) is activated in muscle of subjects with type 2 diabetes during exercise.

Insulin-stimulated GLUT4 translocation is impaired in people with type 2 diabetes. In contrast, exercise results in a normal increase in GLUT4 translocation and glucose uptake in these patients. Several groups have recently hypothesized that exercise increases glucose uptake via an insulin-independent mechanism mediated by the activation of AMP-activated protein kinase (AMPK). If this hypothesis is correct, people with type 2 diabetes should have normal AMPK activation in response to exercise. Seven subjects with type 2 diabetes and eight matched control subjects exercised on a cycle ergometer for 45 min at 70% of maximum workload. Biopsies of vastus lateralis muscle were taken before exercise, after 20 and 45 min of exercise, and at 30 min postexercise. Blood glucose concentrations decreased from 7.6 to 4.77 mmol/l with 45 min of exercise in the diabetic group and did not change in the control group. Exercise significantly increased AMPK alpha2 activity 2.7-fold over basal at 20 min in both groups and remained elevated throughout the protocol, but there was no effect of exercise on AMPK alpha1 activity. Subjects with type 2 diabetes had similar protein expression of AMPK alpha1, alpha2, and beta1 in muscle compared with control subjects. AMPK alpha2 was shown to represent approximately two-thirds of the total alpha mRNA in the muscle from both groups. In conclusion, people with type 2 diabetes have normal exercise-induced AMPK alpha2 activity and normal expression of the alpha1, alpha2 and beta1 isoforms. Pharmacological activation of AMPK may be an attractive target for the treatment of type 2 diabetes.

Troglitazone inhibits expression of the phosphoenolpyruvate carboxykinase gene by an insulin-independent mechanism

Troglitazone is an oral insulin-sensitizing drug used to treat patients with type 2 diabetes. A major feature of this hyperglycemic state is the presence of increased rates of hepatic gluconeogenesis, which troglitazone is able to ameliorate. In this study, we examined the molecular basis for this property of troglitazone by exploring the effects of this compound on the expression of the two genes encoding the major regulatory enzymes of gluconeogenesis, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) in primary cultures of rat hepatocytes. Insulin is able to inhibit expression of both of these genes, which was verified in our model system. Troglitazone significantly reduced mRNA levels of PEPCK and G6Pase in rat hepatocytes isolated from normal and Zucker–diabetic rats, but to a lesser extent than that observed with insulin. Interestingly, troglitazone was unable to reduce cAMP-induced levels of PEPCK mRNA, suggesting that the molecular mechanism whereby troglitazone exerted its effects on gene expression differed from that of insulin. This was further supported by the observation that troglitazone was able to reduce PEPCK mRNA levels in the presence of the insulin signaling pathway inhibitors wortmannin, rapamycin, and PD98059. These results indicate that troglitazone can regulate the expression of specific genes in an insulin-independent manner, and that genes encoding gluconeogenic enzymes are targets for the inhibitory effects of this drug.

13C NMR study of the effects of leptin treatment on kinetics of hepatic intermediary metabolism.

The recent discovery of leptin receptors in peripheral tissue raises questions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular mechanisms are responsible for leptin's effects on carbohydrate and lipid metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive 13C NMR methodology was used to follow the kinetics of intermediary metabolism by monitoring flux of 13C-labeled substrate through several multistep pathways. In perfused liver from either ob/ob or lean mice, we found that acute treatment with leptin in vitro modulates pathways controlling carbohydrate flux into 13C-labeled glycogen, thereby rapidly enhancing synthesis by an insulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from 13C-labeled acetyl-CoA by the de novo synthesis route. Chronic leptin treatment in vivo induced homeostatic changes that resulted in a tripling of the rate of glycogen synthesis via the gluconeogenic pathway from [2-13C]pyruvate in ob/ob mouse liver perfused in the absence of the hormone. Consistent with the 13C NMR results, leptin treatment of the ob/ob mouse in vivo resulted in significantly increased hepatic glycogen synthase activity. Chronic treatment with leptin in vivo exerted the opposite effect of acute treatment in vitro and markedly decreased hepatic de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the 13C NMR findings, activities of hepatic acetyl-CoA carboxylase and fatty acid synthase were significantly reduced by chronic treatment of the ob/ob mouse with leptin. Our data represent a demonstration of direct effects of leptin in the regulation of metabolism in the intact functioning liver.

Interaction between BTBR and C57BL/6J genomes produces an insulin resistance syndrome in (BTBR x C57BL/6J) F1 mice.

Insulin resistance is a common syndrome that often precedes the development of noninsulin-dependent diabetes mellitus (NIDDM). Both diet and genetic factors are associated with insulin resistance. BTBR and C57BL/6J (B6) mice have normal insulin responsiveness and normal fasting plasma insulin levels. However, a cross between these two strains yielded male offspring with severe insulin resistance. Surprisingly, on a basal diet (6.5% fat), the insulin resistance was not associated with fasting hyperinsulinemia. However, a 15% fat diet produced significant hyperinsulinemia in the male mice (twofold at 10 weeks; P < .05). At 10 weeks of age, visceral fat contributed approximately 4.3% of the total body weight in the males versus 1.8% in females. In the males, levels of plasma triacylglycerol and total cholesterol increased 40% and 30%, respectively, compared to females. Plasma free fatty acid concentrations were unchanged. Oral glucose tolerance tests revealed significant levels of hyperglycemia and hyperinsulinemia 15 to 90 minutes after oral glucose administration in the male mice. This was particularly dramatic in males on a 15% fat diet. Glucose transport was examined in skeletal muscles in (BTBR x B6)F1 mice. In the nonhyperinsulinemic animals (females), insulin stimulated 2-deoxyglucose transport 3.5-fold in the soleus and 2.8-fold in the extensor digitorum longus muscles. By contrast, glucose transport was not stimulated in the hyperinsulinemic male mice. Hypoxia stimulates glucose transport through an insulin-independent mechanism. This is known to involve the translocation of GLUT4 from an intracellular pool to the plasma membrane. In the insulin-resistant male mice, hypoxia induced glucose transport as effectively as it did in the insulin-responsive mice. Thus, defective glucose transport in the (BTBR x B6)F1 mice is specific for insulin-stimulated glucose transport. This is similar to what has been observed in muscles taken from obese NIDDM patients. These animals represent an excellent genetic model for studying insulin resistance and investigating the transition from insulin resistance in the absence of hyperinsulinemia to insulin resistance with hyperinsulinemia.

Polyunsaturated fatty acids inhibit hepatic stearoyl-CoA desaturase-1 gene in diabetic mice.

Insulin and dietary fructose independently induce stearoyl-CoA desaturase 1 (SCD1) gene expression in diabetic mouse liver. In the present study, we again used diabetic mice and supplemented a high fructose diet with polyunsaturated fatty acids (PUFA) to determine the selective repression of SCD1 gene expression by dietary PUFA, as previously shown in normal mice. We saw dramatic repression of SCD1 mRNA expression, with trilinolenin at 3% and triarachidonin at 1% supplementation. We also observed significant repression of insulin-induced SCD1 mRNA upon supplementation of the noninducing starch diet with PUFA. In conclusion, our data demonstrate that PUFA negatively regulate hepatic gene expression through an insulin-independent mechanism.

Effect of soleus unweighting on stimulation of insulin-independent glucose transport activity.

Unweighting of the rat soleus by tail-cast suspension results in increased insulin action on stimulation of glucose transport, which can be explained, at least in part, by increased insulin binding and enhanced glucose transporter protein levels. Glucose transport is also activated by an insulin-independent mechanism stimulated by in vitro muscle contractions or hypoxia. Therefore, the purpose of this study was to determine if soleus unweighting leads to an enhanced response of the insulin-independent pathway for stimulation of glucose transport. The hindlimbs of juvenile male Wistar rats were suspended by a tail-cast system for 3 or 6 days. Glucose transport activity in isolated soleus strips (approximately 18 mg) was then assessed by using 2-deoxy-[1,2-3H]glucose (2-DG) uptake. Insulin (2 mU/ml) had a progressively enhanced effect on 2-DG uptake after 3 and 6 days of unweighting (+44 and +72% vs. control, respectively; both P < 0.001). At these same times, there was no difference between groups for activation of 2-DG uptake by maximally effective treatments with contractions (10 tetanuses), hypoxia (60 min), or caffeine (5 mM). These results indicate that the enhanced capacity for stimulation of glucose transport after soleus unweighting is restricted to the insulin pathway, with no apparent enhancement of the insulin-independent pathway.

Cellular mechanism of metformin action involves glucose transporter translocation from an intracellular pool to the plasma membrane in L6 muscle cells.

The effects of the oral hypoglycemic drug metformin on glucose and amino acid transporter activity and subcellular localization of GLUT1 and GLUT4 glucose transporters were tested in cultured L6 myotubes. In muscle cells preexposed to maximal doses of metformin (2 mM, for 16 h), 2-deoxyglucose uptake was stimulated by over 2-fold from 5.9 +/- 0.3 to 13.3 +/- 0.5 pmol/min.mg protein. Uptake of the nonmetabolizable amino acid analog methylaminoisobutyrate was unaffected by treatment with the drug under identical conditions. Extracellular calcium was required to preserve the full response to the biguanide. Exposure of muscle cells to insulin in the presence of metformin resulted in further activation of 2-deoxyglucose transport. The latter effect was additive to the maximum effect of metformin, suggesting that the biguanide stimulates hexose uptake into muscle cells by an insulin-independent mechanism. Glucose transporter number quantified by performing studies of D-glucose-protectable binding of cytochalasin-B in plasma membranes (PM) and internal membranes (IM) prepared from L6 myotubes revealed that a 16-h treatment with 800 microM metformin significantly elevated glucose transporter number in the PM (by 47%), with an equivalent decrement in glucose transporter number (47%) in the IM. Western blot analysis using antisera reactive with the GLUT1 and GLUT4 isoforms of glucose transporters showed that metformin caused a reduction in GLUT1 content in the IM fraction and a concomitant increase in the PM. Unlike insulin, metformin treatment had no effect on the subcellular distribution of GLUT4. We propose that the molecular basis of metformin action in skeletal muscle involves the subcellular redistribution of GLUT1 proteins from an intracellular compartment to the plasma membrane. Such a recruitment process may form an integral part of the mechanism by which the drug stimulates glucose uptake (and utilization) in skeletal muscle and facilitates lowering of blood glucose in the management of type II diabetes.

Intravenous glycerol infusions: effect on free fatty acid metabolism

The plasma-free fatty acid response to intravenous glycerol infused at 250 and 500 mumol/min was determined in five normal volunteers in the postabsorptive state. There was a drop in free fatty acid concentration in all five subjects (one-way ANOVA, p less than 0.01) after the glycerol infusion with no change in insulin concentration compared to the post-absorptive state. These results suggest that intravenous glycerol infusions decrease free fatty acid concentrations in the post-absorptive state by an insulin-independent mechanism. When pharmacologic nonisotopic glycerol infusions are used to determined lipolytic rate, simultaneous measurement of free fatty acid concentrations should be interpreted with caution.

Hepatic Ketogenesis and Peripheral Ketone Body Utilization in the Ruminant

Hepatic and alimentary ketogenesis occur at similar rates in fed, nonpregnant, nonlactating goats, sheep and dairy cows. Alimentary ketogenesis begins to diminish within 24 h after fasting but compensatory increases in hepatic ketogenesis maintain total splanchnic release and, therefore, no change in circulating concentrations of ketone bodies is observed. By the third day of fast the gut is utilizing acetoacetate and β-hydroxybutyrate and alimentary ketogenesis has ceased. Hepatic ketogenesis of both ketone bodies accelerates rapidly due to portal-drained visceral and hindquarter lipolysis and subsequent hepatic fatty acid uptake and total circulating concentrations are doubled. During pregnancy and lactation in sheep and cows alimentary ketogenesis is maintained as long as digestible organic matter intake is constant. Hepatic and total splanchnic release of β-hydroxybutyrate increases in late gestation and early lactation. Again, this is due to increased portal-drained visceral and hindquarter free fatty acid release and hepatic free fatty acid uptake. Hindquarter uptake of both ketones during late gestation is similar to the ratio observed in nonpregnant fed sheep but the percentage of utilization decreases, perhaps reflecting partitioning to uteroplacental tissues. Hindquarter uptake of both ketone bodies in sheep increases in early lactation due to increased circulating concentrations because extraction ratios are similar to those of fed animals. Ketosis during pregnancy in sheep and lactation in cows may be prevented by β-hydroxybutyrate stimulation of pancreatic insulin production. However, an insulin-independent intrahepatic mechanism apparently occurs in sheep. Such a mechanism may allow for the regulation of hepatic ketogenesis without increases in pancreatic insulin production and could be most important in lactating dairy cows, because low insulin concentrations are positively correlated to milk production. It has yet to be determined whether the insulin-independent mechanism is present in high producing dairy cows.

Inhibition of Liver Lipogenesis by Dietary Polyunsaturated Fat in Severely Diabetic Rats

Diabetic and nondiabetic rats were used to ascertain if dietary polyunsaturated fats inhibited hepatic acetyl-CoA carboxylase and fatty acid synthetase in insulin-insufficient rats as had been previously shown for normal rats. Male rats were rendered diabetic (400–600 mg glucose/100 mL blood) with streptozotocin and were fed a high fructose fat-free diet. Safflower oil or palmitate (or tallow) was added to the basal fructose diet at a level to supply 12, 24 or 36% additional digestible energy. Compared with normal rats, diabetic rats had significantly lower hepatic fatty acid biosynthesis, but the proportion of acetyl-CoA carboxylase expressing catalytic activity as determined by the avidin-inactivation technique was unaffected by diabetes. Diabetes did not lower the maximal activities of carboxylase and fatty acid synthetase. Moreover, the activities of these enzymes greatly exceeded the rate of fatty acid synthesis. At all levels of fat supplementation, the high linoleate safflower oil consistently resulted in a 50% lower rate of fatty acid biosynthesis than did comparable levels of tallow or palmitate. Safflower oil was also a more effective suppressor of the activities of acetyl-CoA carboxylase and fatty acid synthetase than the saturated fats. Our data suggest that the greater inhibition of hepatic fatty acid biosynthesis by polyenoic fatty acids is an insulin-independent mechanism.