Glucose starvation activates AMPK via an AMP/ADP-independent mechanism that involves fructose-1,6-bisphosphate and aldolase. AMPK is a central regulator of metabolic homeostasis, and its dysfunction may result in various diseases including diabetes, obesity, and cancer. AMPK is known to be activated under stressful conditions, including glucose starvation. It has been assumed that upon glucose deprivation AMPK activation occurs in the canonical AMP/ADP-dependent manner, with reduced metabolism of glucose causing falling ATP and increasing AMP and ADP. Here, Sheng-Cai Lin and colleagues show that this is not the case, and that glucose starvation activates AMPK via a different route, in an AMP/ADP-independent manner. During glycolysis, glucose is converted to fructose-1,6-bisphosphate (FBP), which is then processed by FBP aldolases. The authors show that the absence of glucose results in a reduction of FBP-bound aldolase, which triggers LKB1 phosphorylation and activation of AMPK. This study thus uncovers FBP as the critical metabolite that signals glucose availability and FBP aldolases as the sensors that relay the information to AMPK. The major energy source for most cells is glucose, from which ATP is generated via glycolysis and/or oxidative metabolism. Glucose deprivation activates AMP-activated protein kinase (AMPK)1, but it is unclear whether this activation occurs solely via changes in AMP or ADP, the classical activators of AMPK2,3,4,5. Here, we describe an AMP/ADP-independent mechanism that triggers AMPK activation by sensing the absence of fructose-1,6-bisphosphate (FBP), with AMPK being progressively activated as extracellular glucose and intracellular FBP decrease. When unoccupied by FBP, aldolases promote the formation of a lysosomal complex containing at least v-ATPase, ragulator, axin, liver kinase B1 (LKB1) and AMPK, which has previously been shown to be required for AMPK activation6,7. Knockdown of aldolases activates AMPK even in cells with abundant glucose, whereas the catalysis-defective D34S aldolase mutant, which still binds FBP, blocks AMPK activation. Cell-free reconstitution assays show that addition of FBP disrupts the association of axin and LKB1 with v-ATPase and ragulator. Importantly, in some cell types AMP/ATP and ADP/ATP ratios remain unchanged during acute glucose starvation, and intact AMP-binding sites on AMPK are not required for AMPK activation. These results establish that aldolase, as well as being a glycolytic enzyme, is a sensor of glucose availability that regulates AMPK.

Mice lacking the p110δ catalytic subunit of phosphatidylinositol 3-kinase have reduced numbers of B1 and marginal zone B cells, reduced levels of serum immunoglobulins, respond poorly to immunization with type II thymus-independent antigen, and are defective in their primary and secondary responses to thymus-dependent antigen. p110δ^−/− B cells proliferate poorly in response to B cell receptor (BCR) or CD40 signals in vitro, fail to activate protein kinase B, and are prone to apoptosis. p110δ function is required for BCR-mediated calcium flux, activation of phosphlipaseCγ2, and Bruton9s tyrosine kinase. Thus, p110δ plays a critical role in B cell homeostasis and function.

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.

The PI3K signaling pathway regulates cell growth and movement and is heavily mutated in cancer. Class I PI3Ks synthesize the lipid messenger PI(3,4,5)P3. PI(3,4,5)P3 can be dephosphorylated by 3- or 5-phosphatases, the latter producing PI(3,4)P2. The PTEN tumor suppressor is thought to function primarily as a PI(3,4,5)P3 3-phosphatase, limiting activation of this pathway. Here we show that PTEN also functions as a PI(3,4)P2 3-phosphatase, both in vitro and in vivo. PTEN is a major PI(3,4)P2 phosphatase in Mcf10a cytosol, and loss of PTEN and INPP4B, a known PI(3,4)P2 4-phosphatase, leads to synergistic accumulation of PI(3,4)P2, which correlated with increased invadopodia in epidermal growth factor (EGF)-stimulated cells. PTEN deletion increased PI(3,4)P2 levels in a mouse model of prostate cancer, and it inversely correlated with PI(3,4)P2 levels across several EGF-stimulated prostate and breast cancer lines. These results point to a role for PI(3,4)P2 in the phenotype caused by loss-of-function mutations or deletions in PTEN.