Ceramide is generated in response to numerous stress-inducing stimuli and has been implicated in the regulation of diverse cellular responses, including cell death, differentiation, and insulin sensitivity. Recent evidence indicates that ceramide may regulate these responses by inhibiting the stimulus-mediated activation of protein kinase B (PKB), a key determinant of cell fate and insulin action. Here we show that inhibition of this kinase involves atypical PKCζ, which physically interacts with PKB in unstimulated cells. Insulin reduces the PKB-PKCζ interaction and stimulates PKB. However, dissociation of the kinase complex and the attendant hormonal activation of PKB were prevented by ceramide. Under these circumstances, ceramide activated PKCζ, leading to phosphorylation of the PKB-PH domain on Thr³⁴. This phosphorylation inhibited phosphatidylinositol 3,4,5-trisphosphate (PIP₃) binding to PKB, thereby preventing activation of the kinase by insulin. In contrast, a PKB-PH domain with a T34A mutation retained the ability to bind PIP₃ even in the presence of a ceramide-activated PKCζ and, as such, expression of PKB T34A mutant in L6 cells was resistant to inhibition by ceramide treatment. Inhibitors of PKCζ and a kinase-dead PKCζ both antagonized the inhibitory effect of ceramide on PKB. Since PKB confers a prosurvival signal and regulates numerous pathways in response to insulin, suppressing its activation by a PKCζ-dependent process may be one mechanism by which ceramide promotes cell death and induces insulin resistance.

Despite its importance in terms of energy homeostasis, the role of AMP-activated protein kinase in adipose tissue remains controversial. Initial studies have described an anti-lipolytic role for AMP-activated protein kinase, whereas more recent studies have suggested the converse. Thus we have addressed the role of AMP-activated protein kinase in adipose tissue by modulating AMP-activated protein kinase activity in primary rodent adipocytes using pharmacological activators or by adenoviral expression of dominant negative or constitutively active forms of the kinase. We then studied the effects of AMP-activated protein kinase activity modulation on lipolytic mechanisms. Finally, we analyzed the consequences of a genetic deletion of AMP-activated protein kinase in mouse adipocytes. AMP-activated protein kinase activity in adipocytes is represented mainly by the α₁ isoform and is induced by all of the stimuli that increase cAMP in adipocytes, including fasting. When AMP-activated protein kinase activity is increased by 5-aminoimidazole-4-carboxamide-riboside, phenformin, or by the expression of a constitutively active form, isoproterenol-induced lipolysis is strongly reduced. Conversely, when AMP-activated protein kinase activity is decreased either by a dominant negative form or in AMP-activated protein kinase α₁ knock-out mice, lipolysis is increased. We present data suggesting that AMP-activated protein kinase acts on hormone-sensitive lipase by blocking its translocation to the lipid droplet. We conclude that, in mature adipocytes, AMP-activated protein kinase activation has a clear anti-lipolytic effect.

Phosphatidylinositol 3-kinase (PI 3-kinase) has been implicated in the regulation of numerous cellular processes, including the insulin-induced regulation of glycogen synthase kinase 3 (GSK-3) and glucose transport. The hormonal-induced inactivation of GSK-3 is mediated by protein kinase B (PKB), a downstream target of PI 3-kinase, whose involvement in other insulin-stimulated responses remains poorly defined at present. In this study, we investigated whether the uptake of glucose, system A amino acid transport, and cellular protein synthesis are regulated by PKBalpha in L6 skeletal muscle cells. L6 cells stably overexpressing wild-type PKBalpha (wtPKBalpha) or a constitutively active membrane-targeted PKBalpha (mPKBalpha) showed a 3- and 15-fold increase in PKB activity, respectively. Both wtPKBalpha and mPKBalpha expression led to a significant increase in the basal uptake of glucose and methyl-aminoisobutyric acid (a substrate for the system A amino acid transporter), at least to a level seen in control cells treated with insulin. The stimulation in glucose transport was facilitated, in part, by the increased translocation of GLUT4 to the plasma membrane and also through an increase in the cellular synthesis of GLUT3. In the absence of insulin, only muscle cells expressing the constitutively active PKBalpha showed a significant increase in protein synthesis and an inhibition in GSK-3. Our results indicate that constitutive activation of PKBalpha in skeletal muscle stimulates the uptake of glucose, system A amino acids, and protein synthesis and promotes the inactivation of GSK-3. These observations imply that PKBalpha may have a role in the insulin-regulated control of these processes in skeletal muscle.

Non-esterified fatty acids (NEFAs) have been implicated in the pathogenesis of skeletal muscle insulin resistance that may develop, in part, as a consequence of a direct inhibitory effect on early insulin signalling events. Here we report work investigating the mechanism by which palmitate (a saturated free fatty acid) inhibits insulin action in rat L6 myotubes. Palmitate suppressed the insulin-induced plasma membrane recruitment and phosphorylation of protein kinase B (PKB) and this was associated with a loss in insulin-stimulated glucose transport. The inhibition in PKB was not due to a loss in insulin receptor substrate (IRS)1 tyrosine phosphorylation, IRS-1/p85 (phosphoinositide 3-kinase) association or suppression in phosphatidyl 3,4,5 triphosphate synthesis, but was attributable to an elevated intracellular synthesis of ceramide (6-fold) from palmitate and a concomitant activation of protein kinase PKCζ (5-fold). Inhibitors of serine palmitoyl transferase suppressed the intracellular synthesis of ceramide from palmitate, prevented PKCζ activation, and antagonized the inhibition in PKB recruitment/phosphorylation and the loss in insulin-stimulated glucose transport elicited by the NEFA. Inhibiting the palmitate-induced activation of PKCζ with Ro 31.8220, also prevented the loss in the insulin-dependent phosphorylation of PKB caused by palmitate. These findings indicate that intracellular ceramide synthesis and PKCζ activation are important aspects of the mechanism by which palmitate desensitizes L6 muscle cells to insulin.