Effect of PANDER in βTC6-cell lipoapoptosis and the protective role of exendin-4

Improvement of Lipotoxicity-Induced Islet β Cellular Insulin Secretion Disorder by Osteocalcin

BACKGROUND

Osteocalcin (OCN) has been proved to be closely related with the development of type 2 diabetes mellitus (T2DM). We aimed to study if OCN could improve the disorder of islet cell caused by lipotoxicity.

METHODS

Alizarin red staining was used to investigate the mineralization. Western blotting and ELISA methods were used to measure protein expression. Immunofluorescence staining was used to investigate the protein nuclear transfer.

RESULTS

High glucose and high fat inhibited the differentiation of osteoblast precursors. Overexpression of insulin receptor (InsR^OE) significantly promoted the Runx2 and OCN expression. The increase of insulin, Gprc6a, and Glut2 by osteoblast culture medium overexpressing insulin receptor was reversed by osteocalcin neutralizing antibody. Undercarboxylated osteocalcin (ucOC) suppressed the lipotoxic islet β-cell damage caused by palmitic acid. The FOXO1 from intranuclear to extranuclear was also significantly increased after ucOC treatment compared with the group PA. Knockdown of Gprc6a or suppression of PI3K/AKT signal pathway could reverse the upregulation of GPRC6A/PI3K/AKT/FoxO1/Pdx1 caused by ucOC.

CONCLUSION

OCN could activate the FOXO1 signaling pathway to regulate GLUT2 expression and improve the insulin secretion disorder caused by lipotoxicity.

Exendin‑4 inhibits lipotoxicity‑induced oxidative stress in β‑cells by inhibiting the activation of TLR4/NF‑κB signaling pathway

The present study aimed to investigate the relationship between the protective effects of exendin‑4 (EX‑4) on lipotoxicity‑induced oxidative stress and meta‑inflammation in β‑cells and the toll‑like receptor 4 (TLR4)/NF‑κB signaling pathway. Lipotoxicity, hydrogen peroxide (H2O2)‑induced oxidative stress in β cells, obese Sprague Dawley rats and TLR4 truncation rats were utilized in the present study. The expression levels were detected by western blotting; cell apoptosis was detected by TUNEL assay; and the intracellular reactive oxygen species (ROS) levels were analyzed using a ROS assay kit. The findings of the present study showed that EX‑4 inhibited the expression of TLR4, NF‑κB p65 subunit and p47phox in a concentration‑dependent manner, and decreased the intracellular level of ROS. Additionally, silencing of TLR4 expression enhanced the protective effects of EX‑4, while overexpression of TLR4 attenuated these protective influences. Simultaneously, it was demonstrated that TLR4 was involved in the process of EX‑4 intervention to inhibit H2O2‑induced oxidative stress in islet β‑cells. Moreover, it was found that EX‑4 also inhibited TLR4‑ or NF‑κB agonist‑induced oxidative stress. These results were also confirmed in an animal model of obese rats, in which EX‑4 was able to improve the function of β‑cells, attenuate oxidative stress, and inhibit the expression levels of TLR4 and NF‑κB p65 subunit in the pancreas of the diet‑induced obese rats. Furthermore, truncation of the TLR4 gene in SD rats delayed the aforementioned damage. In summary, EX‑4 may inhibit lipotoxicity‑induced oxidative stress in β‑cells by inhibiting the activation of the TLR4/NF‑κB signaling pathway.

FAM3B (PANDER) functions as a co-activator of FOXO1 to promote gluconeogenesis in hepatocytes

FAM3B, also known as PANcreatic DERived factor (PANDER), promotes gluconeogenesis and lipogenesis in hepatocytes. However, the underlying mechanism(s) still remains largely unclear. This study determined the mechanism of PANDER-induced FOXO1 activation in hepatocytes. In mouse livers and cultured hepatocytes, PANDER protein is located in both the cytoplasm and nucleus. Nuclear PANDER distribution was increased in the livers of obese mice. In cultured mouse and human hepatocytes, PANDER was co-localized with FOXO1 in the nucleus. PANDER directly interacted with FOXO1 in mouse and human hepatocytes. PANDER overexpression enhanced PANDER-FOXO1 interaction, and detained FOXO1 in the nucleus upon insulin stimulation in hepatocytes. With the increase in PANDER-FOXO1 interaction, PANDER overexpression upregulated the expression of gluconeogenic genes and promoted gluconeogenesis in both human and mouse hepatocytes. Luciferase reporter assays further revealed that PANDER augmented the transcriptional activity of FOXO1 on gluconeogenic genes. Moreover, PANDER overexpression also interfered the binding of AS1842856, a specific FOXO1 inhibitor, with FOXO1, and impaired its inhibitory effects on gluconeogenic gene expression and gluconeogenesis in hepatocytes. siRNA mediated-silencing of FOXO1 inhibited PANDER-promoted gluconeogenic gene expression and glucose production in hepatocytes. In conclusion, PANDER protein is abundantly present in the nucleus, where it functions as a new co-activator of FOXO1 to induce gluconeogenic gene expression in hepatocytes.

FAM3 gene family: A promising therapeutical target for NAFLD and type 2 diabetes

Non-alcoholic fatty liver disease (NAFLD) and diabetes are severe public health issues worldwide. The Family with sequence similarity 3 (FAM3) gene family consists of four members designated as FAM3A, FAM3B, FAM3C and FAM3D, respectively. Recently, there had been increasing evidence that FAM3A, FAM3B and FAM3C are important regulators of glucose and lipid metabolism. FAM3A expression is reduced in the livers of diabetic rodents and NAFLD patients. Hepatic FAM3A restoration activates ATP-P2 receptor-Akt and AMPK pathways to attenuate steatosis and hyperglycemia in obese diabetic mice. FAM3C expression is also reduced in the liver under diabetic condition. FAM3C is a new hepatokine that activates HSF1-CaM-Akt pathway and represses mTOR-SREBP1-FAS pathway to suppress hepatic gluconeogenesis and lipogenesis. In contrast, hepatic expression of FAM3B, also called PANDER, is increased under obese state. FAM3B promotes hepatic lipogenesis and gluconeogenesis by repressing Akt and AMPK activities, and activating lipogenic pathway. Under obese state, the imbalance among hepatic FAM3A, FAM3B and FAM3C signaling networks plays important roles in the pathogenesis of NAFLD and type 2 diabetes. This review briefly discussed the latest research progress on the roles and mechanisms of FAM3A, FAM3B and FAM3C in the regulation of hepatic glucose and lipid metabolism.

Pancreatic-derived factor impaired glucagon-like Peptide-1 production from GLUTag enterendorine L-cell line and intestines

PURPOSE

Pancreatic-derived factor (PANDER) is a pancreatic islet-specific cytokine that co-secretes with insulin. However, its biological function remains largely unknown. We have recently shown that the intestine might be its novel target tissue. The aim of this study was to clarify whether PANDER impacts the production of glucagon-like peptide-1 (GLP-1).

METHODS

We treated GLUTag cells from the mouse intestine L cell line with recombinant PANDER protein and hepatic overexpression of PANDER in an obese murine model.

RESULTS

In GLUTag cells, PANDER exposure led to decreased proglucagon gene mRNA expression and GLP-1 secretion without affecting cell viability or caspase-3 activation. Overexpression of PANDER in mice induced glucose intolerance and impaired glucose-stimulated GLP-1 secretion Moreover, PANDER blocked insulin-induced GLP-1 secretion by inhibiting the insulin signalling-Wnt pathway and directly inhibited the cAMP/PKA pathway.

CONCLUSIONS

Our findings indicate that intestinal L cells are responsive to PANDER, and elevated PANDER levels impair GLP-1 production in vitro and in vivo.

Quantitative proteomic profiling reveals hepatic lipogenesis and liver X receptor activation in the PANDER transgenic model

PANcreatic-DERived factor (PANDER) is a member of a superfamily of FAM3 proteins modulating glycemic levels by metabolic regulation of the liver and pancreas. The precise PANDER-induced hepatic signaling mechanism is still being elucidated and has been very complex due to the pleiotropic nature of this novel hormone. Our PANDER transgenic (PANTG) mouse displays a selective hepatic insulin resistant (SHIR) phenotype whereby insulin signaling is blunted yet lipogenesis is increased, a phenomena observed in type 2 diabetes. To examine the complex PANDER-induced mechanism of SHIR, we utilized quantitative mass spectrometry-based proteomic analysis using Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC) to reveal the global hepatic proteome differences within the PANTG under the metabolic states of fasting, fed and insulin-stimulated conditions. Proteomic analysis identified lipid metabolism as one of the top cellular functions differentially altered in all metabolic states. Differentially expressed proteins within the PANTG having a lipid metabolic role included ACC, ACLY, CD36, CYP7A1, FASN and SCD1. Central to the differentially expressed proteins involved in lipid metabolism was the predicted activation of the liver X receptor (LXR) pathway. Western analysis validated the increased hepatic expression of LXRα along with LXR-directed targets such as FASN and CYP7A1 within the PANTG liver. Furthermore, recombinant PANDER was capable of inducing LXR promoter activity in-vitro as determined by luciferase reporter assays. Taken together, PANDER strongly impacts hepatic lipid metabolism across metabolic states and may induce a SHIR phenotype via the LXR pathway.

Exendin-4 attenuates endoplasmic reticulum stress through a SIRT1-dependent mechanism

Accumulation of excess hepatic lipids contributes to insulin resistance and liver disease associated with endoplasmic reticulum (ER) stress. Exendin-4 is an agonist of the glucagon-like peptide 1 receptor and plays a role in improving insulin resistance and liver disease by increasing silent mating type information regulation 2 homolog (SIRT) 1. However, the effects and mechanism of action of exendin-4 on responses to palmitic acid (PA)-induced ER stress in hepatocytes have not been clearly defined. We investigated whether exendin-4 attenuates PA-induced ER stress via SIRT1 in HepG2 cells. PA treatment induced increased expression of PRKR-like endoplasmic reticulum kinase, inositol-requiring kinase 1α (IRE1α), activating transcription factor 6 (ATF6), and C/EBP homologous protein (CHOP) mRNA. Exendin-4 decreased the expression of P-IRE1α, ATF6, X-box binding protein-1 and CHOP, and increased the expression of SERCA2b. A significant decrease in the hepatic expression of PUMA, BAX, cytochrome c, and cleaved caspase-3 were observed in hepatocytes treated with exendin-4. The TUNEL assay consistently showed that exendin-4 reversed hepatocyte apoptosis induced by treatment with PA. Inhibition of SIRT1 by nicotinamide and siRNA significantly increased the expression of ER stress marker genes in cells treated with both PA and exendin-4. In conclusion, increased SIRT1 by exendin-4 attenuates PA-induced ER stress and mitochondrial dysfunction in hepatocytes.

PANDER transgenic mice display fasting hyperglycemia and hepatic insulin resistance

PANcreatic-DERived factor (PANDER, FAM3B) is a novel protein that is highly expressed within the endocrine pancreas and to a lesser degree in other tissues. Under glucose stimulation, PANDER is co-secreted with insulin from the β-cell. Despite prior creation and characterization of acute hepatic PANDER animal models, the physiologic function remains to be elucidated from pancreas-secreted PANDER. To determine this, in this study, a transgenic mouse exclusively overexpressing PANDER from the endocrine pancreas was generated. PANDER was selectively expressed by the pancreatic-duodenal homeobox-1 (PDX1) promoter. The PANDER transgenic (PANTG) mice were metabolically and proteomically characterized to evaluate effects on glucose homeostasis, insulin sensitivity, and lipid metabolism. Fasting glucose, insulin and C-peptide levels were elevated in the PANTG compared with matched WT mice. Younger PANTG mice also displayed glucose intolerance in the absence of peripheral insulin sensitivity. Hyperinsulinemic-euglycemic clamp studies revealed that hepatic glucose production and insulin resistance were significantly increased in the PANTG with no difference in either glucose infusion rate or rate of disappearance. Fasting glucagon, corticosterones, resistin and leptin levels were also similar between PANTG and WT. Stable isotope labeling of amino acids in cell culture revealed increased gluconeogenic and lipogenic proteomic profiles within the liver of the PANTG with phosphoenol-pyruvate carboxykinase demonstrating a 3.5-fold increase in expression. This was matched with increased hepatic triglyceride content and decreased p-AMPK and p-acetyl coenzyme A carboxylase-1 signaling in the PANTG. Overall, our findings support a role of pancreatic β-cell-secreted PANDER in the regulation of hepatic insulin and lipogenenic signaling with subsequent impact on overall glycemia.

The effect of FFAR1 on pioglitazone-mediated attenuation of palmitic acid-induced oxidative stress and apoptosis in βTC6 cells

OBJECTIVE

We sought to determine whether free fatty acid receptor 1 (FFAR1), a receptor for free fatty acids on the β-cell membrane, can mediate the pioglitazone (PIO)-attenuating effect on lipoapoptosis in β cells and its relationship to oxidative stress.

METHODS

The glucose-sensitive mouse beta pancreatic cell line βTC6 was used to investigate the effect of FFAR1 on PIO-attenuating palmitic acid (PA)-induced oxidative stress and apoptosis.

RESULTS

(1) PIO reduced PA-induced lipoapoptosis in β cells and upregulated the expression of FFAR1 at the mRNA and protein levels in a dose- and time-dependent manner. Silencing of FFAR1 expression was shown to weaken the protective effect of PIO on PA-induced lipoapoptosis in βTC6 cells; while lentiviral-mediated overexpression of FFAR1 was shown to enhance the protective effect of PIO against lipoapoptosis in β cells. (2) Downregulation of FFAR1 expression reduced the attenuating effect of PIO on the expression of NAPDH oxidase subunit p47(phox), Bax, cleaved caspase 3, and the production of reactive oxygen specific (ROS) induced by lipotoxicity, thereby preventing the upregulation of the expression of bcl-2. Inducing the overexpression of FFAR1 enhanced the anti-oxidative stress effect of PIO. Similarly, these effects of FFAR1 on PIO were reproduced under conditions of oxidative stress and apoptosis in βTC6 cells that were induced by H2O2. (3) PIO was found to increase the expression of PLCγ, ERK1/2, and PPARγ in lipotoxic β cells. Silencing FFAR1 expression reduced the PIO-mediated increases in the expression of above proteins; while inducing FFAR1 overexpression showed the opposite effect. Use of an inhibitor of PLCγ, ERK1/2, PPARγ was shown to restrict the protective effect of PIO on oxidative stress and lipoapoptosis of β cells.

CONCLUSIONS

FFAR1 can mediate PIO suppression of β-cell lipoapoptosis through anti-oxidative stress, which may be related to the activation of the PLCγ-ERK1/2-PPARγ pathway.

Mechanisms of palmitate-induced cell death in human osteoblasts

Lipotoxicity is an overload of lipids in non-adipose tissues that affects function and induces cell death. Lipotoxicity has been demonstrated in bone cells in vitro using osteoblasts and adipocytes in coculture. In this condition, lipotoxicity was induced by high levels of saturated fatty acids (mostly palmitate) secreted by cultured adipocytes acting in a paracrine manner. In the present study, we aimed to identify the underlying mechanisms of lipotoxicity in human osteoblasts. Palmitate induced autophagy in cultured osteoblasts, which was preceded by the activation of autophagosomes that surround palmitate droplets. Palmitate also induced apoptosis though the activation of the Fas/Jun kinase (JNK) apoptotic pathway. In addition, osteoblasts could be protected from lipotoxicity by inhibiting autophagy with the phosphoinositide kinase inhibitor 3-methyladenine or by inhibiting apoptosis with the JNK inhibitor SP600125. In summary, we have identified two major molecular mechanisms of lipotoxicity in osteoblasts and in doing so we have identified a new potential therapeutic approach to prevent osteoblast dysfunction and death, which are common features of age-related bone loss and osteoporosis.

Family with sequence similarity 3 gene family and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) comprises a disease spectrum ranging from simple steatosis (fatty liver) and nonalcoholic steatohepatitis to fibrosis and cirrhosis. NAFLD has become the leading cause of chronic liver diseases as well as liver-related morbidity and mortality worldwide. NAFLD is also associated with increased risk of cardiovascular diseases, hyperlipidemia, and type 2 diabetes. Insulin resistance in adipose tissues and the liver plays crucial roles in the progression of NAFLD. The family with sequence similarity 3 (FAM3) gene family is a cytokine-like gene family with four members designated FAM3A, FAM3B, FAM3C, and FAM3D, respectively. Increasing evidence suggests that the FAM3 gene family members are involved in the pathogenesis of NAFLD. In particular, FAM3B, also called pancreatic-derived factor, is an important regulator of glucose and lipid metabolism. In obesity status, increased expression and secretion of FAM3B in pancreatic islets and liver may induce lipid accumulation in the liver via the induction of hepatic insulin resistance and lipogenesis. FAM3A and FAM3D may also participate in the regulation of lipid and energy metabolism. In this brief review, we discussed the latest findings regarding the role of FAM3 gene family members, in particular FAM3B, in the pathogenesis of NAFLD.

Exendin-4 protects endothelial cells from lipoapoptosis by PKA, PI3K, eNOS, p38 MAPK, and JNK pathways

Experimental studies have indicated that endothelial cells play an important role in maintaining vascular homeostasis. We previously reported that human coronary artery endothelial cells (HCAECs) express the glucagon-like peptide 1 (GLP1) receptor and that the stable GLP1 mimetic exendin-4 is able to activate the receptor, leading to increased cell proliferation. Here, we have studied the effect of exendin-4 and native GLP1 (7-36) on lipoapoptosis and its underlying mechanisms in HCAECs. Apoptosis was assessed by DNA fragmentation and caspase-3 activation, after incubating cells with palmitate. Nitric oxide (NO) and reactive oxidative species (ROS) were analyzed. GLP1 receptor activation, PKA-, PI3K/Akt-, eNOS-, p38 MAPK-, and JNK-dependent pathways, and genetic silencing of transfection of eNOS were also studied. Palmitate-induced apoptosis stimulated cells to release NO and ROS, concomitant with upregulation of eNOS, which required activation of p38 MAPK and JNK. Exendin-4 restored the imbalance between NO and ROS production in which ROS production decreased and NO production was further augmented. Incubation with exendin-4 and GLP1 (7-36) protected HCAECs against lipoapoptosis, an effect that was blocked by PKA, PI3K/Akt, eNOS, p38 MAPK, and JNK inhibitors. Genetic silencing of eNOS also abolished the anti-apoptotic effect afforded by exendin-4. Our results support the notion that GLP1 receptor agonists restore eNOS-induced ROS production due to lipotoxicity and that such agonists protect against lipoapoptosis through PKA-PI3K/Akt-eNOS-p38 MAPK-JNK-dependent pathways via a GLP1 receptor-dependent mechanism.