Rapid effects of hormones on enzymes of lipid metabolism

Metabolic control by AMPK in white adipose tissue

White adipose tissue (WAT) plays an important role in the integration of whole-body metabolism by storing fat and mobilizing triacylglycerol when needed. The released free fatty acids can then be oxidized by other tissues to provide ATP. AMP-activated protein kinase (AMPK) is a key regulator of metabolic pathways, and can be targeted by a new generation of direct, small-molecule activators. AMPK activation in WAT inhibits insulin-stimulated lipogenesis and in some situations also inhibits insulin-stimulated glucose uptake, but AMPK-induced inhibition of β-adrenergic agonist-stimulated lipolysis might need to be re-evaluated in vivo. The lack of dramatic effects of AMPK activation on basal metabolism in WAT could be advantageous when treating type 2 diabetes with pharmacological pan-AMPK activators.

AMPK activation by SC4 inhibits noradrenaline-induced lipolysis and insulin-stimulated lipogenesis in white adipose tissue

The effects of small-molecule AMP-activated protein kinase (AMPK) activators in rat epididymal adipocytes were compared. SC4 was the most effective and submaximal doses of SC4 and 5-amino-4-imidazolecarboxamide (AICA) riboside were combined to study the effects of AMPK activation in white adipose tissue (WAT). Incubation of rat adipocytes with SC4 + AICA riboside inhibited noradrenaline-induced lipolysis and decreased hormone-sensitive lipase (HSL) Ser563 phosphorylation, without affecting HSL Ser565 phosphorylation. Preincubation of fat pads from wild-type (WT) mice with SC4 + AICA riboside inhibited insulin-stimulated lipogenesis from glucose or acetate and these effects were lost in AMPKα1 knockout (KO) mice, indicating AMPKα1 dependency. Moreover, in fat pads from acetyl-CoA carboxylase (ACC)1/2 S79A/S212A double knockin versus WT mice, the effect of SC4 + AICA riboside to inhibit insulin-stimulated lipogenesis from acetate was lost, pinpointing ACC as the main AMPK target. Treatment with SC4 + AICA riboside decreased insulin-stimulated glucose uptake, an effect that was still observed in fat pads from AMPKα1 KO versus WT mice, suggesting the effect was partly AMPKα1-independent. SC4 + AICA riboside treatment had no effect on the insulin-induced increase in palmitate esterification nor on sn-glycerol-3-phosphate-O-acyltransferase activity. Therefore in WAT, AMPK activation inhibits noradrenaline-induced lipolysis and suppresses insulin-stimulated lipogenesis primarily by inactivating ACC and by inhibiting glucose uptake.

Effects of PKB/Akt inhibitors on insulin-stimulated lipogenesis and phosphorylation state of lipogenic enzymes in white adipose tissue

We investigated acute effects of two allosteric protein kinase B (PKB) inhibitors, MK-2206 and Akti-1/2, on insulin-stimulated lipogenesis in rat epididymal adipocytes incubated with fructose as carbohydrate substrate. In parallel, the phosphorylation state of lipogenic enzymes in adipocytes and incubated epididymal fat pads was monitored by immunoblotting. Preincubation of rat epididymal adipocytes with PKB inhibitors dose-dependently inhibited the following: insulin-stimulated lipogenesis, increased PKB Ser473 phosphorylation, increased PKB activity and decreased acetyl-CoA carboxylase (ACC) Ser79 phosphorylation. In contrast, the effect of insulin to decrease the phosphorylation of pyruvate dehydrogenase (PDH) at Ser293 and Ser300 was not abolished by PKB inhibition. Insulin treatment also induced ATP-citrate lyase (ACL) Ser454 phosphorylation, but this effect was less sensitive to PKB inhibitors than ACC dephosphorylation by insulin. In incubated rat epididymal fat pads, Akti-1/2 treatment reversed insulin-induced ACC dephosphorylation, while ACL phosphorylation by insulin was maintained. ACL and ACC purified from white adipose tissue were poor substrates for PKBα in vitro. However, effects of wortmannin and torin, along with Akti-1/2 and MK-2206, on recognized PKB target phosphorylation by insulin were similar to their effects on insulin-induced ACL phosphorylation, suggesting that PKB could be the physiological kinase for ACL phosphorylation by insulin. In incubated epididymal fat pads from wild-type versus ACC1/2 S79A/S212A knockin mice, effects of insulin to increase lipogenesis from radioactive fructose or from radioactive acetate were reduced but not abolished. Together, the results support a key role for PKB in mediating insulin-stimulated lipogenesis by decreasing ACC phosphorylation, but not by decreasing PDH phosphorylation.

Substrate cycling between triglyceride and fatty acid in human adipocytes

Substrate cycles in metabolism require energy and generate heat, and they may be involved in thermogenesis. We have studied one such cycle between triglyceride and fatty acid in isolated human adipocytes using a nonisotopic technique. In the absence of added hormone, and with 5 mmol/L (90 mg/dL) glucose in the incubation medium, lipolysis and fatty acid reesterification coexisted such that 40 +/- 4% (mean +/- SEM) of the fatty acid produced was cycled back into triglyceride. In 51 individual subjects the range was from 0% to 100%. Both lipolysis and the quantity of fatty acid recycled correlated positively with cell volume (P less than .001 and P less than .005, respectively). Norepinephrine (10(-6) mol/L) alone (33 experiments) increased lipolysis 3.1-fold, and reduced the percentage of fatty acid reesterified. Cycling was similar to that in the basal state. Lipolysis was inhibited 46% by postabsorptive levels of insulin alone (18 experiments), but the proportion of fatty acid reesterified increased such that the quantity cycled back into triglyceride was similar to that observed in the basal state. In the presence of both norepinephrine and insulin (18 experiments), lipolysis was increased by 58% while 31 +/- 4% of the fatty acid released was reesterified. In consequence, the quantity of fatty acid cycled back into triglyceride increased 2.1-fold. Increasing the insulin level fivefold or the medium glucose concentration to 20 mmol/L produced no further increase in the quantity of fatty acid reesterified. A substrate cycle exists, therefore, between triglyceride and fatty acid in human adipose tissue, and its activity is modified by norepinephrine and insulin.(ABSTRACT TRUNCATED AT 250 WORDS)