Upregulated TGF-ß1 contributes to hyperglycaemia in type 2 diabetes by potentiating glucagon signalling.

AIMS/HYPOTHESIS: Glucagon-stimulated hepatic gluconeogenesis contributes to endogenous glucose production during fasting. Recent studies suggest that TGF-ß is able to promote hepatic gluconeogenesis in mice. However, the physiological relevance of serum TGF-ß levels to human glucose metabolism and the mechanism by which TGF-ß enhances gluconeogenesis remain largely unknown. As enhanced gluconeogenesis is a signature feature of type 2 diabetes, elucidating the molecular mechanisms underlying TGF-ß-promoted hepatic gluconeogenesis would allow us to better understand the process of normal glucose production and the pathophysiology of this process in type 2 diabetes. This study aimed to investigate the contribution of upregulated TGF-ß1 in human type 2 diabetes and the molecular mechanism underlying the action of TGF-ß1 in glucose metabolism. METHODS: Serum levels of TGF-ß1 were measured by ELISA in 74 control participants with normal glucose tolerance and 75 participants with type 2 diabetes. Human liver tissue was collected from participants without obesity and with or without type 2 diabetes for the measurement of TGF-ß1 and glucagon signalling. To investigate the role of Smad3, a key signalling molecule downstream of the TGF-ß1 receptor, in mediating the effect of TGF-ß1 on glucagon signalling, we generated Smad3 knockout mice. Glucose levels in Smad3 knockout mice were measured during prolonged fasting and a glucagon tolerance test. Mouse primary hepatocytes were isolated from Smad3 knockout and wild-type (WT) mice to investigate the underlying molecular mechanisms. Smad3 phosphorylation was detected by western blotting, levels of cAMP were detected by ELISA and levels of protein kinase A (PKA)/cAMP response element-binding protein (CREB) phosphorylation were detected by western blotting. The dissociation of PKA subunits was measured by immunoprecipitation. RESULTS: We observed higher levels of serum TGF-ß1 in participants without obesity and with type 2 diabetes than in healthy control participants, which was positively correlated with HbA1c and fasting blood glucose levels. In addition, hyperactivation of the CREB and Smad3 signalling pathways was observed in the liver of participants with type 2 diabetes. Treating WT mouse primary hepatocytes with TGF-ß1 greatly potentiated glucagon-stimulated PKA/CREB phosphorylation and hepatic gluconeogenesis. Mechanistically, TGF-ß1 treatment induced the binding of Smad3 to the regulatory subunit of PKA (PKA-R), which prevented the association of PKA-R with the catalytic subunit of PKA (PKA-C) and led to the potentiation of glucagon-stimulated PKA signalling and gluconeogenesis. CONCLUSIONS/INTERPRETATION: The hepatic TGF-ß1/Smad3 pathway sensitises the effect of glucagon/PKA signalling on gluconeogenesis and synergistically promotes hepatic glucose production. Reducing serum levels of TGF-ß1 and/or preventing hyperactivation of TGF-ß1 signalling could be a novel approach for alleviating hyperglycaemia in type 2 diabetes.

Receptor Interacting Protein Kinase Pathways Regulate Innate B Cell Developmental Checkpoints But Not Effector Function in Mice.

Mutations in the scaffolding domain of Receptor Interacting Protein kinases (RIP) underlie the recently described human autoimmune syndrome, CRIA, characterized by lymphadenopathy, splenomegaly, and autoantibody production. While disease mechanisms for CRIA remain undescribed, RIP kinases work together with caspase-8 to regulate cell death, which is critical for normal differentiation of many cell types. Here, we describe a key role for RIP1 in facilitating innate B cell differentiation and subsequent activation. By comparing RIP1, RIP3, and caspase-8 triple deficient and RIP3, caspase-8 double deficient mice, we identified selective contributions of RIP1 to an accumulation of murine splenic Marginal Zone (MZ) B cells and B1-b cells. We used mixed bone-marrow chimeras to determine that innate B cell commitment required B cell-intrinsic RIP1, RIP3, and caspase-8 sufficiency. RIP1 regulated MZ B cell development rather than differentiation and RIP1 mediates its innate immune effects independent of the RIP1 kinase domain. NP-KLH/alum and NP-Ficoll vaccination of mice doubly deficient in both caspase-8 and RIP3 or deficient in all three proteins (RIP3, caspase-8, and RIP1) revealed uniquely delayed T-dependent and T-independent IgG responses, abnormal splenic germinal center architecture, and reduced extrafollicular plasmablast formation compared to WT mice. Thus, RIP kinases and caspase-8 jointly orchestrate B cell fate and delayed effector function through a B cell-intrinsic mechanism.

LRG1 is an adipokine that mediates obesity-induced hepatosteatosis and insulin resistance.

Dysregulation in adipokine biosynthesis and function contributes to obesity-induced metabolic diseases. However, the identities and functions of many of the obesity-induced secretory molecules remain unknown. Here, we report the identification of leucine-rich alpha-2-glycoprotein 1 (LRG1) as an obesity-associated adipokine that exacerbates high fat diet-induced hepatosteatosis and insulin resistance. Serum levels of LRG1 were markedly elevated in obese humans and mice compared with their respective controls. LRG1 deficiency in mice greatly alleviated diet-induced hepatosteatosis, obesity, and insulin resistance. Mechanistically, LRG1 bound with high selectivity to the liver and promoted hepatosteatosis by increasing de novo lipogenesis and suppressing fatty acid ß-oxidation. LRG1 also inhibited hepatic insulin signaling by downregulating insulin receptor substrates 1 and 2. Our study identified LRG1 as a key molecule that mediates the crosstalk between adipocytes and hepatocytes in diet-induced hepatosteatosis and insulin resistance. Suppressing LRG1 expression and function may be a promising strategy for the treatment of obesity-related metabolic diseases.

The miR-182-5p/FGF21/acetylcholine axis mediates the crosstalk between adipocytes and macrophages to promote beige fat thermogenesis.

A dynamically regulated microenvironment, which is mediated by crosstalk between adipocytes and neighboring cells, is critical for adipose tissue homeostasis and function. However, information on key molecules and/or signaling pathways regulating the crosstalk remains limited. In this study, we identify adipocyte miRNA-182-5p (miR-182-5p) as a crucial antiobesity molecule that stimulated beige fat thermogenesis by promoting the crosstalk between adipocytes and macrophages. miR-182-5p was highly enriched in thermogenic adipocytes, and its expression was markedly stimulated by cold exposure in mice. In contrast, miR-182-5p expression was significantly reduced in adipose tissues of obese humans and mice. Knockout of miR-185-5p decreased cold-induced beige fat thermogenesis whereas overexpression of miR-185-5p increased beiging and thermogenesis in mice. Mechanistically, miR-182-5p promoted FGF21 expression and secretion in adipocytes by suppressing nuclear receptor subfamily 1 group D member 1 (Nr1d1) at 5'-UTR, which in turn stimulates acetylcholine synthesis and release in macrophages. Increased acetylcholine expression activated the nicotine acetylcholine receptor in adipocytes, which stimulated PKA signaling and consequent thermogenic gene expression. Our study reveals a key role of the miR-182-5p/FGF21/acetylcholine/acetylcholine receptor axis that mediates the crosstalk between adipocytes and macrophages to promote beige fat thermogenesis. Activation of the miR-182-5p-induced signaling pathway in adipose tissue may be an effective approach to ameliorate obesity and associated metabolic diseases.

Adiponectin Alleviates Diet-Induced Inflammation in the Liver by Suppressing MCP-1 Expression and Macrophage Infiltration.

Adiponectin is an adipokine that exerts insulin-sensitizing and anti-inflammatory roles in insulin target tissues including liver. While the insulin-sensitizing function of adiponectin has been extensively investigated, the precise mechanism by which adiponectin alleviates diet-induced hepatic inflammation remains elusive. Here, we report that hepatocyte-specific knockout (KO) of the adaptor protein APPL2 enhanced adiponectin sensitivity and prevented mice from developing high-fat diet-induced inflammation, insulin resistance, and glucose intolerance, although it caused fatty liver. The improved anti-inflammatory and insulin-sensitizing effects in the APPL2 hepatocyte-specific KO mice were largely reversed by knocking out adiponectin. Mechanistically, hepatocyte APPL2 deficiency enhances adiponectin signaling in the liver, which blocks TNF-α-stimulated MCP-1 expression via inhibiting the mTORC1 signaling pathway, leading to reduced macrophage infiltration and thus reduced inflammation in the liver. With results taken together, our study uncovers a mechanism underlying the anti-inflammatory role of adiponectin in the liver and reveals the hepatic APPL2-mTORC1-MCP-1 axis as a potential target for treating overnutrition-induced inflammation in the liver.

Identification and characterization of a novel adiponectin receptor agonist adipo anti-inflammation agonist and its anti-inflammatory effects in vitro and in vivo.

BACKGROUND AND PURPOSE: Adiponectin (APN) is an adipokine secreted from adipocytes that binds to APN receptors AdipoR1 and AdipoR2 and exerts an anti-inflammatory response through mechanisms not fully understood. There is a need to develop small molecules that activate AdipoR1 and AdipoR2 and to be used to inhibit the inflammatory response in lipopolysaccharide (LPS)-induced endotoxemia and other inflammatory disorders. EXPERIMENTAL APPROACH: We designed 10 new structural analogues of an AdipoR agonist, AdipoRon (APR), and assessed their anti-inflammatory properties. Bone marrow-derived macrophages (BMMs) and peritoneal macrophages (PEMs) were isolated from mice. Levels of pro-inflammatory cytokines were measured by reverse transcription and real-time quantitative polymerase chain reaction (qRT-PCR), enzyme-linked immunosorbent assay (ELISA) and microarray in LPS-induced endotoxemia mice and diet-induced obesity (DIO) mice in which systemic inflammation prevails. Western blotting, immunohistochemistry (IHC), siRNA interference and immunoprecipitation were used to detect signalling pathways. KEY RESULTS: A novel APN receptor agonist named adipo anti-inflammation agonist (AdipoAI) strongly suppresses inflammation in DIO and endotoxemia mice, as well as in cultured macrophages. We also found that AdipoAI attenuated the association of AdipoR1 and APPL1 via myeloid differentiation marker 88 (MyD88) signalling, thus inhibiting activation of nuclear factor kappa B (NF-κB), mitogen-activated protein kinase (MAPK) and c-Maf pathways and limiting the production of pro-inflammatory cytokines in LPS-induced macrophages. CONCLUSION AND IMPLICATIONS: AdipoAI is a promising alternative therapeutic approach to APN and APR to suppress inflammation in LPS-induced endotoxemia and other inflammatory disorders via distinct signalling pathways.

APPL1 negatively regulates bone mass, possibly by controlling the fate of bone marrow mesenchymal progenitor cells.

Adiponectin is an adipokine that can exert a regulatory function on bone metabolism. However, there are many contradictions between clinical and pre-clinical studies on adiponectin. APPL1 is an adaptor protein that can interact with adiponectin receptors. In the current study, we found that knockout of the Appl1 gene in male mice was associated with higher bone volume and numbers of trabeculae than in females or controls. The trabecular thickness, cortical thickness, ratio of bone volume/trabecular volume, cross-sectional bone area, and mean polar moment of inertia increased in Appl1 KO mice compared with wild-type mice. The number of osteoblasts increased but the number of adipocytes decreased in Appl1 KO mice. Knockdown of Appl1 impaired adipogenesis in bone marrow-derived mesenchymal stem cells. Mineralization was increased by knockdown of Appl1 during osteoblast differentiation. Data from differentiation-related genes showed results consistent with the in vivo effects. In summary, this study provides further clarification of the effect of the adiponectin signaling pathway on bone metabolism.

Anti-inflammatory and anti-proliferative action of adiponectin mediated by insulin signaling cascade in human vascular smooth muscle cells.

After confirmation of the presence of adiponectin (ADPN) receptors and intra-cellular binding proteins in coronary artery smooth muscle cells (VSMC), we tested the hypotheses that, in acute insulin resistance: (i) the activation/inactivation of metabolic and mitogenic insulin signaling pathways are inversely affected by ADPN and, (ii) changes in VSMC migration/proliferation rates correlate with signal activity/inactivity. In primary cultures of VSMC exposed to high glucose and palmitate plus insulin, the expression of PI-3 kinase (Akt and m-TOR), MAP-Kinase (Erk and p-38) molecules, and inflammatory markers (TLR-4 and IkB-α) were assessed with Western blot, in the absence/presence of AdipoRon (AR). Migration and proliferation rates were measured in similar experimental conditions. There were decreases of ~ 25% (p-Akt) and 40-60% (p-mTOR) expressions with high glucose/palmitate, which reversed when AR was added were. Elevations in p-Erk and p-p38 expressions were obliterated by AR. Although, no changes were detected with high glucose and palmitate, when AR was added, a decline in inflammatory activity was substantiated by a ~ 50% decrease in TLR-4 and 40-60% increase in IkBα expression. Functional assays showed 10-20% rise in VSMC proliferation with high glucose and palmitate, but addition of AR lead to 15-25% decline. The degree of VSMC migration was reduced with AR addition by ~ 15%, ~ 35% and 55%, in VSMC exposed to 5 mM, 25 mM glucose and 25 mM + 200 µM palmitate, respectively. Changes in intracellular molecular messaging in experiments mimicking acute insulin resistance suggest that anti-inflammatory and anti-atherogenic actions of ADPN in VSMC are mediated via insulin signaling pathways.

Mitochondrial stress-activated cGAS-STING pathway inhibits thermogenic program and contributes to overnutrition-induced obesity in mice.

Obesity is a global epidemic that is caused by excessive energy intake or inefficient energy expenditure. Brown or beige fat dissipates energy as heat through non-shivering thermogenesis by their high density of mitochondria. However, how the mitochondrial stress-induced signal is coupled to the cellular thermogenic program remains elusive. Here, we show that mitochondrial DNA escape-induced activation of the cGAS-STING pathway negatively regulates thermogenesis in fat-specific DsbA-L knockout mice, a model of adipose tissue mitochondrial stress. Conversely, fat-specific overexpression of DsbA-L or knockout of STING protects mice against high-fat diet-induced obesity. Mechanistically, activation of the cGAS-STING pathway in adipocytes activated phosphodiesterase PDE3B/PDE4, leading to decreased cAMP levels and PKA signaling, thus reduced thermogenesis. Our study demonstrates that mitochondrial stress-activated cGAS-STING pathway functions as a sentinel signal that suppresses thermogenesis in adipose tissue. Targeting adipose cGAS-STING pathway may thus be a potential therapeutic strategy to counteract overnutrition-induced obesity and its associated metabolic diseases.

The tumor suppressor TMEM127 regulates insulin sensitivity in a tissue-specific manner.

Understanding the molecular components of insulin signaling is relevant to effectively manage insulin resistance. We investigated the phenotype of the TMEM127 tumor suppressor gene deficiency in vivo. Whole-body Tmem127 knockout mice have decreased adiposity and maintain insulin sensitivity, low hepatic fat deposition and peripheral glucose clearance after a high-fat diet. Liver-specific and adipose-specific Tmem127 deletion partially overlap global Tmem127 loss: liver Tmem127 promotes hepatic gluconeogenesis and inhibits peripheral glucose uptake, while adipose Tmem127 downregulates adipogenesis and hepatic glucose production. mTORC2 is activated in TMEM127-deficient hepatocytes suggesting that it interacts with TMEM127 to control insulin sensitivity. Murine hepatic Tmem127 expression is increased in insulin-resistant states and is reversed by diet or the insulin sensitizer pioglitazone. Importantly, human liver TMEM127 expression correlates with steatohepatitis and insulin resistance. Our results suggest that besides tumor suppression activities, TMEM127 is a nutrient-sensing component of glucose/lipid homeostasis and may be a target in insulin resistance.

Potential Roles of Adiponectin Isoforms in Human Obesity with Delayed Wound Healing.

Adiponectin is an adipokine with anti-insulin resistance and anti-inflammatory functions. It exists in serum predominantly in three multimeric complexes: the trimer, hexamer, and high-molecular-weight forms. Although recent studies indicate that adiponectin promotes wound healing in rodents, its role in the wound healing process in humans is unknown. This study investigated the expression levels of adiponectin in adipose tissue and serum of women who experienced either normal or delayed wound healing after abdominal plastic surgery. We found that obese women with delayed healing had slightly lower total adiponectin levels in their adipose tissue compared with women with normal healing rates. Among the different isoforms of adiponectin, levels of the trimer forms were significantly reduced in adipose tissue, but not the serum, of obese women with delayed healing compared to women who healed normally. This study provides clinical evidence for a potential role of low-molecular-weight oligomers of adiponectin in the wound healing process as well as implications for an autocrine and/or paracrine mechanism of adiponectin action in adipose tissues.

DsbA-L ameliorates high glucose induced tubular damage through maintaining MAM integrity.

BACKGROUND: The mitochondrial associated endoplasmic reticulum (ER) membrane (MAM) provides a platform for communication between the mitochondria and ER, and it plays a vital role in many biological functions. Disulphide-bond A oxidoreductase-like protein (DsbA-L), expressed in the MAM, serves as an antioxidant and reduces ER stress. However, the role of DsbA-L and MAM in kidney pathobiology remains unclear. METHODS: Molecular biology techniques, transmission electron microscopy (TEM), in situ proximity ligation assays (PLAs), confocal microscopy, TUNEL staining and flow cytometry were utilized to analyse apoptosis and status of MAM in DsbA-L mutant mice. FINDINGS: We showed that MAM was significantly reduced in the kidneys of streptozotocin-induced diabetic mice, which correlated with the extent of renal injury. We also observed a correlation between the loss of MAM integrity and increased apoptosis and renal injury in diabetic nephropathy (DN). These alterations were further exacerbated in diabetic DsbA-L gene-deficient mice (DsbA-L-/-). In vitro, overexpression of DsbA-L in HK-2 cells restored MAM integrity and reduced apoptosis induced by high-glucose ambience. These beneficial effects were partially blocked by overexpression of FATE-1, a MAM uncoupling protein. Finally, the expression of DsbA-L was positively correlated with MAM integrity in the kidneys of DN patients but negatively correlated with apoptosis and renal injury. INTERPRETATION: Our results indicate that DsbA-L exerts an antiapoptotic effect by maintaining MAM integrity, which is apparently disrupted in DN. FUND: This work was supported by the National Natural Science Foundation of China (81730018), the National Key R&D Program of China (2016YFC1305501) and NIH (DK60635).

Disulfide-bond A oxidoreductase-like protein protects against ectopic fat deposition and lipid-related kidney damage in diabetic nephropathy.

Ectopic fat deposition (EFD) in the kidney has been shown to play a causal role in diabetic nephropathy; however, the mechanism underlying EFD remains elusive. By transcriptome analysis, we found decreased expression levels of disulfide-bond A oxidoreductase-like protein (DsbA-L) in the kidneys of diabetic mice (induced by high-fat diet plus Streptozotocin) compared with control mice. Increased expression of adipocyte differentiation-related protein and abnormal levels of collagen I, fibronectin, and phosphorylated 5'AMP-activated kinase (p-AMPK), adipose triglyceride lipase (p-ATGL), and HMG-CoA reductase (p-HMGCR) were also observed in diabetic mice. These alterations were accompanied by deposition of lipid droplets in the kidney, and were more pronounced in diabetic DsbA-L knockout mice. In vitro, overexpression of DsbA-L ameliorated high glucose-induced intracellular lipid droplet deposition in a human proximal tubular cell line, and DsbA-L siRNA aggravated lipid droplet deposition and reduced the levels of p-AMPK, p-ATGL, and p-HMGCR. High glucose and palmitic acid treatment enhanced the expression of interleukin-1ß and interleukin-18; these enhancements were further increased after treatment with DsbA-L siRNA but alleviated by co-treatment with an AMPK activator. In kidney biopsy tissue from patients with diabetic nephropathy, DsbA-L expression was negatively correlated with EFD and tubular damage. Collectively, these results suggest that DsbA-L has a protective role against EFD and lipid-related kidney damage in diabetic nephropathy. Activation of the AMPK pathway is a potential mechanism underlying DsbA-L action in the kidney.

Alternative splicing variant of the scaffold protein APPL1 suppresses hepatic adiponectin signaling and function.

Adiponectin is an adipocyte-derived hormone with antidiabetic activities that include increasing the sensitivity of cells to insulin. Adaptor protein containing pleckstrin homology domain, phosphotyrosine-binding domain, and leucine zipper motif (APPL1) stimulates adiponectin signaling and promotes adiponectin's insulin-sensitizing effects by binding to two adiponectin receptors, AdipoR1 and AdipoR2, and the insulin receptor. In this study, we report an alternative splicing variant of APPL1 (APPL1sv) that is highly expressed in mouse liver, pancreas, and spleen tissues. The expression levels of APPL1sv in liver tissues were enhanced in a mouse model of obesity and diabetic dyslipidemia (i.e. db/db mice) and reduced in calorie-restricted mice compared with ad libitum-fed mice. APPL1sv overexpression or suppression inhibited or enhanced, respectively, adiponectin-stimulated phosphorylation of AMP protein kinase (AMPK) in mouse hepatocytes. We also found that APPL1sv binds to AdipoR1 and AdipoR2 under basal conditions and that adiponectin treatment reduces this binding. Overexpression of APPL1sv blocked adiponectin-induced interactions of APPL1 with the adiponectin receptors. Moreover, adenovirus-mediated and short hairpin RNA-based suppression of APPL1sv greatly reduced high fat diet-induced insulin resistance and hepatic glucose production in mice. Our study identifies a key suppressor of hepatic adiponectin signaling and insulin sensitivity, a finding that may shed light on identifying effective therapeutic targets for treating insulin resistance and type 2 diabetes.

DsbA-L prevents obesity-induced inflammation and insulin resistance by suppressing the mtDNA release-activated cGAS-cGAMP-STING pathway.

Chronic inflammation in adipose tissue plays a key role in obesity-induced insulin resistance. However, the mechanisms underlying obesity-induced inflammation remain elusive. Here we show that obesity promotes mtDNA release into the cytosol, where it triggers inflammatory responses by activating the DNA-sensing cGAS-cGAMP-STING pathway. Fat-specific knockout of disulfide-bond A oxidoreductase-like protein (DsbA-L), a chaperone-like protein originally identified in the mitochondrial matrix, impaired mitochondrial function and promoted mtDNA release, leading to activation of the cGAS-cGAMP-STING pathway and inflammatory responses. Conversely, fat-specific overexpression of DsbA-L protected mice against high-fat diet-induced activation of the cGAS-cGAMP-STING pathway and inflammation. Taken together, we identify DsbA-L as a key molecule that maintains mitochondrial integrity. DsbA-L deficiency promotes inflammation and insulin resistance by activating the cGAS-cGAMP-STING pathway. Our study also reveals that, in addition to its well-characterized roles in innate immune surveillance, the cGAS-cGAMP-STING pathway plays an important role in mediating obesity-induced metabolic dysfunction.

Hepatic DsbA-L protects mice from diet-induced hepatosteatosis and insulin resistance.

Hepatic insulin resistance and hepatosteatosis in diet-induced obesity are associated with various metabolic diseases, yet the underlying mechanisms remain to be fully elucidated. Here we show that the expression levels of the disulfide-bond A oxidoreductase-like protein (DsbA-L) are significantly reduced in the liver of obese mice and humans. Liver-specific knockout or adenovirus-mediated overexpression of DsbA-L exacerbates or alleviates, respectively, high-fat diet-induced mitochondrial dysfunction, hepatosteatosis, and insulin resistance in mice. Mechanistically, we found that DsbA-L is localized in mitochondria and that its deficiency is associated with impairment of maximum respiratory capacity, elevated cellular oxidative stress, and increased JNK activity. Our results identify DsbA-L as a critical regulator of mitochondrial function, and its down-regulation in the liver may contribute to obesity-induced hepatosteatosis and whole body insulin resistance.-Chen, H., Bai, J., Dong, F., Fang, H., Zhang, Y., Meng, W., Liu, B., Luo, Y., Liu, M., Bai, Y., Abdul-Ghani, M. A., Li, R., Wu, J., Zeng, R., Zhou, Z., Dong, L. Q., Liu, F. Hepatic DsbA-L protects mice from diet-induced hepatosteatosis and insulin resistance.

Hepatic ATF6 Increases Fatty Acid Oxidation to Attenuate Hepatic Steatosis in Mice Through Peroxisome Proliferator-Activated Receptor α.

The endoplasmic reticulum quality control protein activating transcription factor 6 (ATF6) has emerged as a novel metabolic regulator. Here, we show that adenovirus-mediated overexpression of the dominant-negative form of ATF6 (dnATF6) increases susceptibility to develop hepatic steatosis in diet-induced insulin-resistant mice and fasted mice. Overexpression of dnATF6 or small interfering RNA-mediated knockdown of ATF6 decreases the transcriptional activity of peroxisome proliferator-activated receptor α (PPARα)/retinoid X receptor complex, and inhibits oxygen consumption rates in hepatocytes, possibly through inhibition of the binding of PPARα to the promoter of its target gene. Intriguingly, ATF6 physically interacts with PPARα, enhances the transcriptional activity of PPARα, and triggers activation of PPARα downstream targets, such as CPT1α and MCAD, in hepatocytes. Furthermore, hepatic overexpression of the active form of ATF6 promotes hepatic fatty acid oxidation and protects against hepatic steatosis in diet-induced insulin-resistant mice. These data delineate the mechanism by which ATF6 controls the activity of PPARα and hepatic mitochondria fatty acid oxidation. Therefore, strategies to activate ATF6 could be used as an alternative avenue to improve liver function and treat hepatic steatosis in obesity.

Adiponectin signaling and function in insulin target tissues.

Obesity-linked type 2 diabetes is one of the paramount causes of morbidity and mortality worldwide, posing a major threat on human health, productivity, and quality of life. Despite great progress made towards a better understanding of the molecular basis of diabetes, the available clinical counter-measures against insulin resistance, a defect that is central to obesity-linked type 2 diabetes, remain inadequate. Adiponectin, an abundant adipocyte-secreted factor with a wide-range of biological activities, improves insulin sensitivity in major insulin target tissues, modulates inflammatory responses, and plays a crucial role in the regulation of energy metabolism. However, adiponectin as a promising therapeutic approach has not been thoroughly explored in the context of pharmacological intervention, and extensive efforts are being devoted to gain mechanistic understanding of adiponectin signaling and its regulation, and reveal therapeutic targets. Here, we discuss tissue- and cell-specific functions of adiponectin, with an emphasis on the regulation of adiponectin signaling pathways, and the potential crosstalk between the adiponectin and other signaling pathways involved in metabolic regulation. Understanding better just why and how adiponectin and its downstream effector molecules work will be essential, together with empirical trials, to guide us to therapies that target the root cause(s) of type 2 diabetes and insulin resistance.

Appl1 and Appl2 are Expendable for Mouse Development But Are Essential for HGF-Induced Akt Activation and Migration in Mouse Embryonic Fibroblasts.

Although Appl1 and Appl2 have been implicated in multiple cellular activities, we and others have found that Appl1 is dispensable for mouse embryonic development, suggesting that Appl2 can substitute for Appl1 during development. To address this possibility, we generated conditionally targeted Appl2 mice. We found that ubiquitous Appl2 knockout (Appl2-/-) mice, much like Appl1-/- mice, are viable and grow normally to adulthood. Intriguingly, when Appl1-/- mice were crossed with Appl2-/- mice, we found that homozygous Appl1;Appl2 double knockout (DKO) animals are also viable and grossly normal with regard to reproductive potential and postnatal growth. Appl2-null and DKO mice were found to exhibit altered red blood cell physiology, with erythrocytes from these mice generally being larger and having a more irregular shape than erythrocytes from wild type mice. Although Appl1/2 proteins have been previously shown to have a very strong interaction with phosphatidylinositol-3 kinase (Pi3k) in thymic T cells, Pi3k-Akt signaling and cellular differentiation was unaltered in thymocytes from Appl1;Appl2 (DKO) mice. However, Appl1/2-null mouse embryonic fibroblasts exhibited defects in HGF-induced Akt activation, migration, and invasion. Taken together, these data suggest that Appl1 and Appl2 are required for robust HGF cell signaling but are dispensable for embryonic development and reproduction.