Ja Young Kim-Muller

Proatherogenic Abnormalities of Lipid Metabolism in SirT1 Transgenic Mice Are Mediated through Creb Deacetylation

Dyslipidemia and atherosclerosis are associated with reduced insulin sensitivity and diabetes, but the mechanism is unclear. Gain of function of the gene encoding deacetylase SirT1 improves insulin sensitivity and could be expected to protect against lipid abnormalities. Surprisingly, when transgenic mice overexpressing SirT1 (SirBACO) are placed on atherogenic diet, they maintain better glucose homeostasis, but develop worse lipid profiles and larger atherosclerotic lesions than controls. We show that transcription factor cAMP response element binding protein (Creb) is deacetylated in SirBACO mice. We identify Lys136 is a substrate for SirT1-dependent deacetylation that affects Creb activity by preventing its cAMP-dependent phosphorylation, leading to reduced expression of glucogenic genes and promoting hepatic lipid accumulation and secretion. Expression of constitutively acetylated Creb (K136Q) in SirBACO mice mimics Creb activation and abolishes the dyslipidemic and insulin-sensitizing effects of SirT1 gain of function. We propose that SirT1-dependent Creb deacetylation regulates the balance between glucose and lipid metabolism, integrating fasting signals.

Metabolic Inflexibility Impairs Insulin Secretion and Results In MODY-like Diabetes in Triple FoxO-Deficient Mice

Pancreatic β cell failure in type 2 diabetes is associated with functional abnormalities of insulin secretion and deficits of β cell mass. Its unclear how one begets the other. We have shown that loss of β cell mass can be ascribed to impaired FoxO1 function in different models of diabetes. Here we show that ablation of the three FoxO genes (1, 3a, and 4) in mature β cells results in early-onset, maturity-onset diabetes of the young (MODY)-like diabetes, with abnormalities of the MODY networks Hnf4α, Hnf1α, and Pdx1. FoxO-deficient β cells are metabolically inflexible, i.e., they preferentially utilize lipids rather than carbohydrates as an energy source. This results in impaired ATP generation and reduced Ca(2+)-dependent insulin secretion. The present findings demonstrate a secretory defect caused by impaired FoxO activity that antedates dedifferentiation. We propose that defects in both pancreatic β cell function and mass arise through FoxO-dependent mechanisms during diabetes progression.

Aldehyde dehydrogenase 1a3 defines a subset of failing pancreatic β cells in diabetic mice

Insulin-producing β cells become dedifferentiated during diabetes progression. An impaired ability to select substrates for oxidative phosphorylation, or metabolic inflexibility, initiates progression from β-cell dysfunction to β-cell dedifferentiation. The identification of pathways involved in dedifferentiation may provide clues to its reversal. Here we isolate and functionally characterize failing β cells from various experimental models of diabetes and report a striking enrichment in the expression of aldehyde dehydrogenase 1 isoform A3 (ALDH+) as β cells become dedifferentiated. Flow-sorted ALDH+ islet cells demonstrate impaired glucose-induced insulin secretion, are depleted of Foxo1 and MafA, and include a Neurogenin3-positive subset. RNA sequencing analysis demonstrates that ALDH+ cells are characterized by: (i) impaired oxidative phosphorylation and mitochondrial complex I, IV and V; (ii) activated RICTOR; and (iii) progenitor cell markers. We propose that impaired mitochondrial function marks the progression from metabolic inflexibility to dedifferentiation in the natural history of β-cell failure.

Selective Insulin Sensitizers

Can the livers insensitivity to insulin in diabetes be overcome with another hormone? Type 2 diabetes mellitus and its complications are, with cardiovascular diseases, leading threats to public health in the 21st century. In the United States, type 2 diabetes care accounts for a third of federal health insurance (Medicare) expenditures—nearly half of it to treat associated macrovascular problems (1). The cornerstone of type 2 diabetes is insulin resistance—a decreased sensitivity of tissues and organs, such as the liver, to the metabolic effects of the hormone insulin (the other major cause is failure of the pancreas to produce insulin). Yet, except for thiazolidinediones—whose checkered safety history, troublesome side effects, and regulatory setbacks stifled widespread adoption by clinicians—treatment options for insulin resistance have generally remained unchanged since the 1940s. Although insulin signaling pathways in cells have been largely deciphered (2), the key mediators of insulin signaling are poor drug targets because they either lack a suitable ligand-binding domain, or are shared with other cellular pathways that regulate cell growth and proliferation. This realization spawned research into “alternative pathways” that control insulin resistance. On page 1621 of this issue, Kir et al. (3) find that human fibroblast growth factor 19 (FGF19) can boost certain effects of insulin on the mammalian liver, raising interest and questions about possible therapies involving this molecule.

Blunted Refeeding Response and Increased Locomotor Activity in Mice Lacking FoxO1 in Synapsin-Cre–Expressing Neurons

Successful development of antiobesity agents requires detailed knowledge of neural pathways controlling body weight, eating behavior, and peripheral metabolism. Genetic ablation of FoxO1 in selected hypothalamic neurons decreases food intake, increases energy expenditure, and improves glucose homeostasis, highlighting the role of this gene in insulin and leptin signaling. However, little is known about potential effects of FoxO1 in other neurons. To address this question, we executed a broad-based neuronal ablation of FoxO1 using Synapsin promoter–driven Cre to delete floxed Foxo1 alleles. Lineage-tracing experiments showed that NPY/AgRP and POMC neurons were minimally affected by the knockout. Nonetheless, Syn-Cre-Foxo1 knockouts demonstrated a catabolic energy homeostatic phenotype with a blunted refeeding response, increased sensitivity to leptin and amino acid signaling, and increased locomotor activity, likely attributable to increased melanocortinergic tone. We confirmed these data in mice lacking the three Foxo genes. The effects on locomotor activity could be reversed by direct delivery of constitutively active FoxO1 to the mediobasal hypothalamus, but not to the suprachiasmatic nucleus. The data reveal that the integrative function of FoxO1 extends beyond the arcuate nucleus, suggesting that central nervous system inhibition of FoxO1 function can be leveraged to promote hormone sensitivity and prevent a positive energy balance.

When β-cells fail: lessons from dedifferentiation.

Diabetes is caused by a combination of impaired responsiveness to insulin and reduced production of insulin by the pancreas. Until recently, the decline of insulin production had been ascribed to β‐cell death. But recent research has shown that β‐cells do not die in diabetes, but undergo a silencing process, termed “dedifferentiation.” The main implication of this discovery is that β‐cells can be revived by appropriate treatments. We have shown that mitochondrial abnormalities are a key step in the progression of β‐cell dysfunction towards dedifferentiation. In normal β‐cells, mitochondria generate energy required to sustain insulin production and its finely timed release in response to the bodys nutritional status. A normal β‐cell can adapt its mitochondrial fuel source based on substrate availability, a concept known as “metabolic flexibility.” This capability is the first casualty in the progress of β‐cell failure. β‐Cells lose the ability to select the right fuel for mitochondrial energy production. Mitochondria become overloaded, and accumulate by‐products derived from incomplete fuel utilization. Energy production stalls, and insulin production drops, setting the stage for dedifferentiation. The ultimate goal of these investigations is to explore novel treatment paradigms that will benefit people with diabetes.

FoxO1 Deacetylation Decreases Fatty Acid Oxidation in β-Cells and Sustains Insulin Secretion in Diabetes.

Pancreatic β-cell dysfunction contributes to onset and progression of type 2 diabetes. In this state β-cells become metabolically inflexible, losing the ability to select between carbohydrates and lipids as substrates for mitochondrial oxidation. These changes lead to β-cell dedifferentiation. We have proposed that FoxO proteins are activated through deacetylation-dependent nuclear translocation to forestall the progression of these abnormalities. However, how deacetylated FoxO exert their actions remains unclear. To address this question, we analyzed islet function in mice homozygous for knock-in alleles encoding deacetylated FoxO1 (6KR). Islets expressing 6KR mutant FoxO1 have enhanced insulin secretion in vivo and ex vivo and decreased fatty acid oxidation ex vivo. Remarkably, the gene expression signature associated with FoxO1 deacetylation differs from wild type by only ∼2% of the >4000 genes regulated in response to re-feeding. But this narrow swath includes key genes required for β-cell identity, lipid metabolism, and mitochondrial fatty acid and solute transport. The data support the notion that deacetylated FoxO1 protects β-cell function by limiting mitochondrial lipid utilization and raise the possibility that inhibition of fatty acid oxidation in β-cells is beneficial to diabetes treatment.

Altered Plasma Profile of Antioxidant Proteins as an Early Correlate of Pancreatic β Cell Dysfunction

Insulin resistance and β cell dysfunction contribute to the pathogenesis of type 2 diabetes. Unlike insulin resistance, β cell dysfunction remains difficult to predict and monitor, because of the inaccessibility of the endocrine pancreas, the integrated relationship with insulin sensitivity, and the paracrine effects of incretins. The goal of our study was to survey the plasma response to a metabolic challenge in order to identify factors predictive of β cell dysfunction. To this end, we combined (i) the power of unbiased iTRAQ (isobaric tag for relative and absolute quantification) mass spectrometry with (ii) direct sampling of the portal vein following an intravenous glucose/arginine challenge (IVGATT) in (iii) mice with a genetic β cell defect. By so doing, we excluded the effects of peripheral insulin sensitivity as well as those of incretins on β cells, and focused on the first phase of insulin secretion to capture the early pathophysiology of β cell dysfunction. We compared plasma protein profiles with ex vivo islet secretome and transcriptome analyses. We detected changes to 418 plasma proteins in vivo, and detected changes to 262 proteins ex vivo. The impairment of insulin secretion was associated with greater overall changes in the plasma response to IVGATT, possibly reflecting metabolic instability. Reduced levels of proteins regulating redox state and neuronal stress markers, as well as increased levels of coagulation factors, antedated the loss of insulin secretion in diabetic mice. These results suggest that a reduced complement of antioxidants in response to a mixed secretagogue challenge is an early correlate of future β cell failure.

Pair Feeding, but Not Insulin, Phloridzin, or Rosiglitazone Treatment, Curtails Markers of β-Cell Dedifferentiation in db/db Mice

β-Cell failure is a hallmark of type 2 diabetes. Among several cellular biological mechanisms of cellular dysfunction, we and others have recently proposed that dedifferentiation of β-cells can explain the slowly progressive onset and partial reversibility of β-cell failure. Accordingly, we provided evidence of such processes in humans and experimental animal models of insulin-resistant diabetes. In this study, we asked whether β-cell dedifferentiation can be prevented with diet or pharmacological treatment of diabetes. db/db mice, a widely used model of insulin-resistant diabetes and obesity, were either pair fed or treated with the Sglt inhibitor phloridzin, the insulin-sensitizer rosiglitazone, or insulin. All treatments were equally efficacious in reducing plasma glucose levels. Pair feeding and phloridzin also resulted in significant weight loss. However, pair feeding among the four treatments resulted in a reduction of β-cell dedifferentiation, as assessed by Foxo1 and Aldh1a3 immunohistochemistry. The effect of diet to partly restore β-cell function is consistent with data in human diabetes and provides another potential mechanism by which lifestyle changes act as an effective intervention against diabetes progression.