Xiangdong Wu

Type 2 diabetes in mice induces hepatic overexpression of sulfatase 2, a novel factor that suppresses uptake of remnant lipoproteins

Type 2 diabetes mellitus (T2DM) impairs hepatic clearance of atherogenic postprandial remnant lipoproteins. Our work and that of others have identified syndecan‐1 heparan sulfate proteoglycans (HSPGs) as remnant lipoprotein receptors. Nevertheless, defects in the T2DM liver have not been molecularly characterized, and neither has the correction that occurs upon caloric restriction. We used microarrays to compare expression of proteoglycan‐related genes in livers from control db/m mice; obese, T2DM db/db littermates fed ad libitum (AL); and db/db mice pair‐fed to match the intake of db/m mice. Surprisingly, the arrays identified only one gene whose dysregulation by T2DM would disrupt HSPG structure: the heparan sulfate glucosamine‐6‐O‐endosulfatase‐2 (Sulf2). SULF2 degrades HSPGs by removing 6‐O sulfate groups, but had no previously known role in diabetes or lipoprotein biology. Follow‐up quantitative polymerase chain reaction assays revealed a striking 11‐fold induction of Sulf2 messenger RNA in the livers of AL T2DM mice compared with controls. Immunoblots demonstrated induction of SULF2 in AL livers, with restoration toward normal in livers from pair‐fed db/db mice. Knockdown of SULF2 in cultured hepatocytes doubled HSPG‐mediated catabolism of model remnant lipoproteins. Notably, co‐immunoprecipitations revealed a persistent physical association of SULF2 with syndecan‐1. To identify mechanisms of SULF2 dysregulation in T2DM, we found that advanced glycosylation end products provoked a 10‐fold induction in SULF2 expression by cultured hepatocytes and an approximately 50% impairment in their catabolism of remnants and very low‐density lipoprotein, an effect that was entirely reversed by SULF2 knockdown. Adiponectin and insulin each suppressed SULF2 protein in cultured liver cells and in murine livers in vivo, consistent with a role in energy flux. Likewise, both hormones enhanced remnant lipoprotein catabolism in vitro. Conclusion: SULF2 is an unexpected suppressor of atherogenic lipoprotein clearance by hepatocytes and an attractive target for inhibition. (HEPATOLOGY 2010;.)

NOX4 Pathway as a Source of Selective Insulin Resistance and Responsiveness

Objective—Type 2 diabetes mellitus and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis; hyperglycemia, which causes microvascular disease; and hypertension. These features share a common, but unexplained, origin—namely, pathway-selective insulin resistance and responsiveness. Here, we undertook a comprehensive characterization of pathway-selective insulin resistance and responsiveness in liver and hepatocytes by examining 18 downstream targets of the insulin receptor, surveying the AKT, ERK, and NAD(P)H oxidase 4 pathways. Methods and Results—Injection of insulin into hyperphagic, obese type 2 diabetic db/db mice failed to inactivate hepatic protein tyrosine phosphatase gene family members, a crucial action of NAD(P)H oxidase 4 previously thought to be required for all signaling through AKT and ERK. Insulin-stimulated type 2 diabetic livers unexpectedly produced an unusual form of AKT that was phosphorylated at Thr308 (pT308), with only weak insulin-stimulated phosphorylation at Ser473. Remarkably, knockdown or inhibition of NAD(P)H oxidase 4 in cultured hepatocytes recapitulated the entire complicated pattern of pathway-selective insulin resistance and responsiveness seen in vivo—namely, monophosphorylated pT308-AKT, impaired insulin-stimulated pathways for lowering plasma lipids and glucose, but continued lipogenic pathways and robust ERK activation. Conclusion—Functional disturbance of a single molecule, NAD(P)H oxidase 4, is sufficient to induce the key harmful features of deranged insulin signaling in type 2 diabetes mellitus, obesity, and other conditions associated with hyperinsulinemia and pathway-selective insulin resistance and responsiveness.

The role of pathway-selective insulin resistance and responsiveness in diabetic dyslipoproteinemia.

Purpose of review Type 2 diabetes mellitus (T2DM) and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis, hyperglycemia, which causes microvascular disease, and hypertension. These features share a common, but unexplained, origin – namely, pathway-selective insulin resistance and responsiveness (SEIRR). Here, we review recent work on hepatic SEIRR indicating that deranged insulin signaling may have a remarkably simple molecular basis. Recent findings Comprehensive examination of a set of 18 insulin targets revealed that T2DM liver in vivo exhibits a specific defect in the ability of the NAD(P)H oxidase 4 (NOX4) to inactivate protein tyrosine phosphatase gene family members after stimulation with insulin, and that impairment of this single molecule, NOX4, in cultured hepatocytes recapitulates all features of hepatic SEIRR in vivo. These features include insulin-stimulated generation of an unusual monophosphorylated form of AKT at Thr308 (pT308-AKT) with only weak phosphorylation at Ser473, impaired insulin-stimulated pathways for lowering plasma levels of lipids and glucose, but continued lipogenic pathways and robust extracellular signal-regulated kinase activation. This new study, in combination with important prior work, provides clues to several long-standing mysteries, such as how AKT might regulate lipid-lowering and glucose-lowering pathways that become insulin-resistant but also lipogenic pathways that remain insulin-responsive, as well as a potential role for NOX4 in insulin-stimulated generation of oxysterol ligands for LXR, a key lipogenic factor. Summary These findings suggest a unified molecular explanation for fatty liver, atherogenic dyslipoproteinemia, hyperglycemia, and hence accelerated atherosclerosis and microvascular disease in T2DM, obesity, and related syndromes of positive caloric imbalance.

Decreased secretion of adiponectin through its intracellular accumulation in adipose tissue during tobacco smoke exposure

BackgroundCigarette smoking is associated with an increased risk of type 2 diabetes mellitus (T2DM). Smokers exhibit low circulating levels of total adiponectin (ADPN) and high-molecular-weight (HMW) ADPN multimers. Blood concentrations of HMW ADPN multimers closely correlate with insulin sensitivity for handling glucose. How tobacco smoke exposure lowers blood levels of ADPN, however, has not been investigated. In the current study, we examined the effects of tobacco smoke exposure in vitro and in vivo on the intracellular and extracellular distribution of ADPN and its HMW multimers, as well as potential mechanisms.FindingsWe found that exposure of cultured adipocytes to tobacco smoke extract (TSE) suppressed total ADPN secretion, and TSE administration to mice lowered their plasma ADPN concentrations. Surprisingly, TSE caused intracellular accumulation of HMW ADPN in cultured adipocytes and in the adipose tissue of wild-type mice, while preferentially decreasing HMW ADPN in culture medium and in plasma. Importantly, we found that TSE up-regulated the ADPN retention chaperone ERp44, which colocalized with ADPN in the endoplasmic reticulum. In addition, TSE down-regulated DsbA-L, a factor for ADPN secretion.ConclusionsTobacco smoke exposure traps HMW ADPN intracellularly, thereby blocking its secretion. Our results provide a novel mechanism for hypoadiponectinemia, and may help to explain the increased risk of T2DM in smokers.

An oxide transport chain essential for balanced insulin signaling

Patients with overnutrition, obesity, the atherometabolic syndrome, and type 2 diabetes mellitus exhibit imbalanced insulin action, also called pathway-selective insulin resistance. To control glycemia, they require hyperinsulinemia that then overdrives ERK and hepatic de-novo lipogenesis. We recently reported that NADPH oxidase-4 regulates balanced insulin action. Here, we show that NADPH oxidase-4 is part of a new limb of insulin signaling that we abbreviate “NSAPP” after its five major proteins. The NSAPP pathway is an oxide transport chain that begins when insulin stimulates NADPH oxidase-4 to generate . NADPH oxidase-4 hands to superoxide dismutase-3 for conversion into H2O2. The pathway ends when aquaporin-3 channels H2O2 across the membrane to inactivate PTEN. Disruption of any component of the NSAPP chain, from NADPH oxidase-4 up to PTEN, leaves PTEN persistently active, thereby producing the same deadly pattern of imbalanced insulin action seen clinically. Unraveling the molecular basis for NSAPP dysfunction in overnutrition has now become a top priority.

Suppression of Hepatic FLOT1 (Flotillin-1) by Type 2 Diabetes Mellitus Impairs the Disposal of Remnant Lipoproteins via Syndecan-1

Objective— Type 2 diabetes mellitus (T2DM) and the atherometabolic syndrome exhibit a deadly dyslipoproteinemia that arises in part from impaired hepatic disposal of C-TRLs (cholesterol- and triglyceride-rich remnant apoB [apolipoprotein B] lipoproteins). We previously identified syndecan-1 as a receptor for C-TRLs that directly mediates endocytosis via rafts, independent from coated pits. Caveolins and flotillins form rafts but facilitate distinct endocytotic pathways. We now investigated their participation in syndecan-1–mediated disposal of C-TRLs and their expression in T2DM liver. Approach and Results— In cultured liver cells and nondiabetic murine livers, we found that syndecan-1 coimmunoprecipitates with FLOT1 (flotillin-1) but not with CAV1 (caveolin-1). Binding of C-TRLs to syndecan-1 on the surface of liver cells enhanced syndecan-1/FLOT1 association. The 2 molecules then trafficked together into the lysosomes, implying limited if any recycling back to the cell surface. The interaction requires the transmembrane/cytoplasmic region of syndecan-1 and the N-terminal hydrophobic domain of FLOT1. Knockdown of FLOT1 in cultured liver cells substantially inhibited syndecan-1 endocytosis. Livers from obese, T2DM KKAy mice exhibited 60% to 70% less FLOT1 protein and mRNA than in nondiabetic KK livers. An adenoviral construct to enhance hepatic expression of wild-type FLOT1 in T2DM mice normalized plasma triglycerides, whereas a mutant FLOT1 missing its N-terminal hydrophobic domain had no effect. Moreover, the adenoviral vector for wild-type FLOT1 lowered plasma triglyceride excursions and normalized retinyl excursions in T2DM KKAy mice after a corn oil gavage, without affecting postprandial production of C-TRLs. Conclusions— FLOT1 is a novel participant in the disposal of harmful C-TRLs via syndecan-1. Low expression of FLOT1 in T2DM liver may contribute to metabolic dyslipoproteinemia.