Considerable data support the idea that Foxo1 drives the liver transcriptional program during fasting and is inhibited by Akt after feeding. Mice with hepatic deletion of Akt1 and Akt2 were glucose intolerant, insulin resistant, and defective in the transcriptional response to feeding in liver. These defects were normalized upon concomitant liver–specific deletion of Foxo1. Surprisingly, in the absence of both Akt and Foxo1, mice adapted appropriately to both the fasted and fed state, and insulin suppressed hepatic glucose production normally. Gene expression analysis revealed that deletion of Akt in liver led to constitutive activation of Foxo1–dependent gene expression, but once again concomitant ablation of Foxo1 restored postprandial regulation, preventing its inhibition of the metabolic response to nutrient intake. These results are inconsistent with the canonical model of hepatic metabolism in which Akt is an obligate intermediate for insulin’s actions. Rather they demonstrate that a major role of hepatic Akt is to restrain Foxo1 activity, and in the absence of Foxo1, Akt is largely dispensable for hepatic metabolic regulation in vivo.

Type 2 diabetes mellitus, a disease with significant effects on the health and economy of Western societies, involves disturbances in both lipid and carbohydrate metabolism. In the insulin-resistant or diabetic state, the liver is unresponsive to the actions of insulin with regard to the suppression of glucose output but continues to produce large amounts of lipid, the latter mimicking the fed, insulin-replete condition. The disordered distribution of lipids contributes to the cardiovascular disease that is the greatest cause of mortality of type 2 diabetes mellitus. Yet the precise signal transduction pathways by which insulin regulates hepatic lipid synthesis and degradation remain largely unknown. Here we describe a mechanism by which insulin, through the intermediary protein kinase Akt2/protein kinase B (PKB)-beta, elicits the phosphorylation and inhibition of the transcriptional coactivator peroxisome proliferator-activated receptor-coactivator 1alpha (PGC-1alpha), a global regulator of hepatic metabolism during fasting. Phosphorylation prevents the recruitment of PGC-1alpha to the cognate promoters, impairing its ability to promote gluconeogenesis and fatty acid oxidation. These results define a mechanism by which insulin controls lipid catabolism in the liver and suggest a novel site for therapy in type 2 diabetes mellitus.

Summary Insulin-resistant syndromes such as type II diabetes mellitus (T2DM) involve disrupted temporal coordination of hepatic metabolism such that synthesis and secretion of lipid and glucose are inappropriately engaged concurrently. Here we test the hypothesis that a combination of direct and indirect actions of insulin on liver can lead to the metabolic phenotype exhibited in T2DM without a defect in proximal hepatic insulin signaling. First, we show that the insulin-dependent inhibition of Foxo1 and activation of mTorc1 by Akt is both necessary and sufficient for the induction of lipogenesis and the lipogenic gene program. In marked contrast, insulin, acting in vivo independent of hepatocyte insulin signaling can suppress glucose production by reducing serum free fatty acids. These studies support the hypothesis that under conditions of obesity and diabetes, intact hepatic insulin signaling can maintain lipogenesis while excess circulating FFAs become a dominant positive regulator of HGP.

A hallmark of the pathology of Alzheimer's disease is the accumulation of the microtubule-associated protein tau into fibrillar aggregates. Recent studies suggest that they accumulate because cytosolic chaperones fail to clear abnormally phosphorylated tau, preserving a pool of toxic tau intermediates within the neuron. We describe a mechanism for tau clearance involving a major cellular kinase, Akt. During stress, Akt is ubiquitinated and degraded by the tau ubiquitin ligase CHIP, and this largely depends on the Hsp90 complex. Akt also prevents CHIP-induced tau ubiquitination and its subsequent degradation, either by regulating the Hsp90/CHIP complex directly or by competing as a client protein with tau for binding. Akt levels tightly regulate the expression of CHIP, such that, as Akt levels are suppressed, CHIP levels also decrease, suggesting a potential stress response feedback mechanism between ligase and kinase activity. We also show that Akt and the microtubule affinity-regulating kinase 2 (PAR1/MARK2), a known tau kinase, interact directly. Akt enhances the activity of PAR1 to promote tau hyperphosphorylation at S262/S356, a tau species that is not recognized by the CHIP/Hsp90 complex. Moreover, Akt1 knockout mice have reduced levels of tau phosphorylated at PAR1/MARK2 consensus sites. Hence, Akt serves as a major regulator of tau biology by manipulating both tau kinases and protein quality control, providing a link to several common pathways that have demonstrated dysfunction in Alzheimer's disease.Alzheimer's disease ͉ cell signaling ͉ chaperone ͉ MARK2/PAR1

Mitochondria play a critical role in cell survival and death. Mitochondrial recovery during inflammatory processes such as sepsis is associated with cell survival. Recovery of cellular respiration, mitochondrial biogenesis and function requires coordinated expression of transcription factors encoded by nuclear and mitochondrial genes, including mitochondrial transcription factor A (T-fam) and cytochrome c oxidase (COX, complex IV). LPS elicits strong host defenses in mammals with pronounced inflammatory responses but also triggers activation of survival pathways such as AKT pathway. AKT/PKB is a serine/threonine protein kinase playing an important role in cell survival, protein synthesis, and controlled inflammation in response to TLRs. Hence, we investigated the role of LPS mediated AKT activation in mitochondrial bioenergetics and function in cultured murine macrophages (B6-MCL) and bone marrow derived macrophages. We show that LPS challenge led to increased expression of T-fam and COX subunit I and IV in a time dependent manner through early phosphorylation of the PI3kinase/AKT pathway. PI3K/AKT pathway inhibitors abrogated LPS mediated T-fam and COX induction. Lack of induction was associated with decreased ATP production, increased proinflammatory cytokines (TNF-α), nitric oxide production and cell death. The TLR4 mediated AKT activation and mitochondrial biogenesis required activation of adaptor protein MyD88 and Toll-IL-1R-containing adaptor inducing IFN-β (TRIF). Importantly, using a genetic approach, we show that the AKT1 isoform is pivotal in regulating mitochondrial biogenesis in response to TLR4 agonist.

AKT activity has been reported in the epidermis associated with keratinocyte survival and differentiation. We show in developing skin that Akt activity associates first with post-proliferative, para-basal keratinocytes and later with terminally differentiated keratinocytes that are forming the fetal stratum corneum. In adult epidermis the dominant Akt activity is in these highly differentiated granular keratinocytes, involved in stratum corneum assembly. Stratum corneum is crucial for protective barrier activity, and its formation involves complex and poorly understood processes such as nuclear dissolution, keratin filament aggregation, and assembly of a multiprotein cell cornified envelope. A key protein in these processes is filaggrin. We show that one target of Akt in granular keratinocytes is HspB1 (heat shock protein 27). Loss of epidermal HspB1 caused hyperkeratinization and misprocessing of filaggrin. Akt-mediated HspB1 phosphorylation promotes a transient interaction with filaggrin and intracellular redistribution of HspB1. This is the first demonstration of a specific interaction between HspB1 and a stratum corneum protein and indicates that HspB1 has chaperone activity during stratum corneum formation. This work demonstrates a new role for Akt in epidermis.The epidermis is the primary environmental barrier, protecting from infection, allergens, and damage from UV radiation. The major constituent of this epidermal barrier is the terminally differentiated, anuclear keratinocyte. This structure is bounded by a cornified envelope, an elaborate, cross-linked protein structure covalently bound to hydrophobic lipid externally and aggregated keratin internally (1). Formation of this structure is poorly understood.The complexity of keratinocyte terminal differentiation and the importance of precisely timed and compartmentalized processing is illustrated by the maturation of the stratum corneum protein filaggrin. Filaggrin is synthesized as a high molecular mass precursor comprising multiple subunits that are sequentially processed and modified in temporally and spatially regulated steps by diverse proteases and enzymes to produce mature filaggrin subunits. Mature filaggrin is thought to be important in the aggregation and collapse of the keratin network leading to flattening of the keratinocyte and the destruction of the nucleus in granular layer (terminally differentiating) keratinocytes (2). Filaggrin is also incorporated into the cornified envelope (2). Premature or aberrant filaggrin processing can be catastrophic, leading to disruption of cornified envelope integrity and skin barrier function (3-5) and is far more damaging than reduced filaggrin levels that, in contrast, lead only to mild skin defects (6).Possible regulators of protein processing and trafficking during terminal differentiation are heat shock proteins (Hsps). 2Hsps have diverse roles as cellular chaperones, anti-apoptotic factors, stress-protective proteins, and cytoskeletal stabilizers (7,8). Human HspB1 (Hsp27) (9 -11) and the mouse HspB1...

Insulin signaling and nutrient levels coordinate the metabolic response to feeding in the liver. Insulin signals in hepatocytes to activate Akt, which inhibits Foxo1 suppressing hepatic glucose production (HGP) and allowing the transition to the postprandial state. Here we provide genetic evidence that insulin regulates HGP by both direct and indirect hepatic mechanisms. Liver-specific ablation of the IR (L-Insulin Receptor KO) induces glucose intolerance, insulin resistance and prevents the appropriate transcriptional response to feeding. Liver-specific deletion of Foxo1 (L-IRFoxo1DKO) rescues glucose tolerance and allows for normal suppression of HGP and gluconeogenic gene expression in response to insulin despite lack of autonomous liver insulin signaling. These data indicate that, in the absence of Foxo1, insulin signals via an intermediary extra-hepatic tissue to regulate liver glucose production. Importantly, a hepatic mechanism distinct from the IR-Akt-Foxo1 axis exists to regulate glucose production.

Akt is encoded by a gene family for which each isoform serves distinct but overlapping functions. Based on the phenotypes of the germ line gene disruptions, Akt1 has been associated with control of growth, whereas Akt2 has been linked to metabolic regulation. Here we show that Akt1 serves an unexpected role in the regulation of energy metabolism, as mice deficient for Akt1 exhibit protection from diet-induced obesity and its associated insulin resistance. Although skeletal muscle contributes most of the resting and exercising energy expenditure, muscle-specific deletion of Akt1 does not recapitulate the phenotype, indicating that the role of Akt1 in skeletal muscle is cell nonautonomous. These data indicate a previously unknown function of Akt1 in energy metabolism and provide a novel target for treatment of obesity.