Effects of human serum albumin complexed with free fatty acids on cell viability and insulin secretion in the hamster pancreatic β-cell line HIT-T15

Albumin is an interface between blood plasma and cell membrane, and not just a sponge

Albumin is the most abundant protein in blood plasma and acts as a carrier for many circulating molecules. Hypoalbuminaemia, mostly caused by either renal or liver disease or malnutrition, can perturb vascular homeostasis and is involved in the development of multiple diseases. Here we review four functions of albumin and the consequences of hypoalbuminaemia on vascular homeostasis. (i) Albumin is the main determinant of plasma colloid osmotic pressure. Hypoalbuminaemia was therefore thought to be the main mechanism for oedema in nephrotic syndrome (NS), however, experimental studies showed that intrarenal mechanisms rather than hypoalbuminaemia determine formation and, in particular, maintenance of oedema. (ii) Albumin functions as an interface between lysophosphatidylcholine (LPC) and circulating factors (lipoproteins and erythrocytes) and the endothelium. Consequently, hypoalbuminaemia results in higher LPC levels in lipoproteins and erythrocyte membrane, thereby increasing atherosclerotic properties of low-density lipoprotein and blood viscosity, respectively. Furthermore, albumin dose-dependently restores LPC-induced inhibition of vasodilation. (iii) Hypoalbuminaemia impacts on vascular nitric oxide (NO) signalling by directly increasing NO production in endothelial cells, leading to reduced NO sensitivity of vascular smooth muscle cells. (iv) Lastly, albumin binds free fatty acids (FFAs). FFAs can induce vascular smooth muscle cell apoptosis, uncouple endothelial NO synthase and decrease endothelium-dependent vasodilation. Unbound FFAs can increase the formation of reactive oxygen species by mitochondrial uncoupling in multiple cell types and induce hypertriglyceridemia in NS. In conclusion, albumin acts as an interface in the circulation and hypoalbuminaemia impairs multiple aspects of vascular function that may underlie the association of hypoalbuminaemia with adverse outcomes. However, hypoalbuminaemia is not a key to oedema in NS. These insights have therapeutic implications.

Effects of the Major Structured Triacylglycerols in Human Milk on Lipid Metabolism of Hepatocyte Cells in Vitro

The effect of structured triacylglycerols [1-oleoyl-2-palmitoyl-3-linoleoylglycerol (OPL), 3-dilinoleoyl-2-palmitoylglycerol (LPL), and 1,3-dioleoyl-2-palmitoylglycerol (OPO)] in human milk on the lipid metabolism was unclear. Hence, this study investigated the effects of different structured triacylglycerols and their mixtures (M) (OPL/LPL/OPO in M1, M2, and M3 were 1.5:0.5:1, 1.2:1.2:1, and 0.5:0.2:1, respectively) on lipid and expression levels of some critical proteins involved in lipid metabolism in LO2 cells. Results showed that there was more lipid accumulation in the LO2 cells exposed to 2,3-dioleoyl-1-palmitoylglycerol (POO) than OPL, LPL, and OPO (p < 0.05), and more lipid accumulation was observed in the OPL group compared to LPL and OPO groups (p < 0.05). Moreover, there was more lipid accumulation in the M3 group compared to M1 and M2 groups. The expression level of diacylglycerol acyltransferase was highest in the POO group compared to LPL, OPO, and OPL groups and was higher in the M3 group than M1 and M2 groups. The expression levels of acetyl-CoA carboxylase 1 and long-chain acyl-CoA synthetase 1 were highest in the OPL group compared to OPO and LPL groups. In comparison to OPO and LPL, OPL seemed to be more likely to increase the content of triacylglycerols and cholesterol in LO2 cells; therefore, whether this was beneficial to the growth and development of infants needs further verification.

Molecular Mechanisms of Apoptosis Induction and Its Regulation by Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death contribute significantly to the pathogenesis of type 2 diabetes. One of the main factors responsible for β-cell dysfunction and subsequent cell death is chronic exposure to increased concentrations of FAs (fatty acids). The effect of FAs seems to depend particularly on the degree of their saturation. Saturated FAs induce apoptosis in pancreatic β-cells, whereas unsaturated FAs are well tolerated and are even capable of inhibiting the pro-apoptotic effect of saturated FAs. Molecular mechanisms of apoptosis induction by saturated FAs in β-cells are not completely elucidated. Saturated FAs induce ER stress, which in turn leads to activation of all ER stress pathways. When ER stress is severe or prolonged, apoptosis is induced. The main mediator seems to be the CHOP transcription factor. Via regulation of expression/activity of pro- and anti-apoptotic Bcl-2 family members, and potentially also through the increase in ROS production, CHOP switches on the mitochondrial pathway of apoptosis induction. ER stress signalling also possibly leads to autophagy signalling, which may activate caspase-8. Saturated FAs activate or inhibit various signalling pathways, i.e., p38 MAPK signalling, ERK signalling, ceramide signalling, Akt signalling and PKCδ signalling. This may lead to the activation of the mitochondrial pathway of apoptosis, as well. Particularly, the inhibition of the pro-survival Akt signalling seems to play an important role. This inhibition may be mediated by multiple pathways (e.g., ER stress signalling, PKCδ and ceramide) and could also consequence in autophagy signalling. Experimental evidence indicates the involvement of certain miRNAs in mechanisms of FA-induced β-cell apoptosis, as well. In the rather rare situations when unsaturated FAs are also shown to be pro-apoptotic, the mechanisms mediating this effect in β-cells seem to be the same as for saturated FAs. To conclude, FA-induced apoptosis rather appears to be preceded by complex cross talks of multiple signalling pathways. Some of these pathways may be regulated by decreased membrane fluidity due to saturated FA incorporation. Few data are available concerning molecular mechanisms mediating the protective effect of unsaturated FAs on the effect of saturated FAs. It seems that the main possible mechanism represents a rather inhibitory intervention into saturated FA-induced pro-apoptotic signalling than activation of some pro-survival signalling pathway(s) or metabolic interference in β-cells. This inhibitory intervention may be due to an increase of membrane fluidity.

TAK-875 Mitigates β-Cell Lipotoxicity-Induced Metaflammation Damage through Inhibiting the TLR4-NF-κB Pathway

Metabolic inflammatory damage, characterized by Toll-like receptor 4 (TLR4) signaling activation, is a major mechanism underlying lipotoxicity-induced β-cell damage. The present study is aimed at determining whether G protein-coupled receptor 4 (GPR40) agonist can improve β-cell lipotoxicity-induced damage by inhibiting the TLR4-NF-κB pathway. Lipotoxicity, inflammation-damaged β-cells, obese SD, and TLR4^KO rat models were used in the study. In vitro, TAK-875 inhibited the lipotoxicity- and LPS-induced β-cell apoptosis in a concentration-dependent manner, improved the insulin secretion, and inhibited the expression of TLR4 and NF-κB subunit P65. Besides, silencing of TLR4 expression enhanced the protective effects of TAK-875, while TLR4 overexpression attenuated this protective effect. Activation of TLR4 or NF-κB attenuated the antagonism of TAK-875 on PA-induced damage. Moreover, the above process of TAK-875 was partially independent of GPR40 expression. TAK-875 reduced the body weight and inflammatory factors, rebalanced the number and distribution of α or β-cells, inhibited the apoptosis of islet cells, and inhibited the expression of TLR4 and NF-κB subunit P65 in obese rats. Further knockout of the rat TLR4 gene delayed the damage induced by the high-fat diet and synergy with the action of TAK-875. These data suggest that GPR40 agonists antagonized the lipotoxicity β-cell damage by inhibiting the TLR4-NF-κB pathway.

The Transcription Factor p8 Regulates Autophagy in Response to Palmitic Acid Stress via a Mammalian Target of Rapamycin (mTOR)-independent Signaling Pathway

Autophagy is an evolutionarily conserved degradative process that allows cells to maintain homoeostasis in numerous physiological situations. This process also functions as an essential protective response to endoplasmic reticulum (ER) stress, which promotes the removal and degradation of unfolded proteins. However, little is known regarding the mechanism by which autophagy is initiated and regulated in response to ER stress. In this study, different types of autophagy were identified in human gastric cancer MKN45 cells in response to the stress induced by nutrient starvation or lipotoxicity in which the regulation of these pathways is mammalian target of rapamycin (mTOR)-dependent or -independent, respectively. Interestingly, we found that p8, a stress-inducible transcription factor, was enhanced in MKN45 cells treated with palmitic acid to induce lipotoxicity. Furthermore, an increase in autophagy was observed in MKN45 cells stably overexpressing p8 using a lentivirus system, and autophagy induced by palmitic acid was blocked by p8 RNAi compared with the control. Western blotting analyses showed that autophagy was regulated by p8 or mTOR in response to the protein kinase-like endoplasmic reticulum kinase/activating transcription factor 6-mediated ER stress of lipotoxicity or the parkin-mediated mitochondrial stress of nutrient starvation, respectively. Furthermore, our results indicated that autophagy induced by palmitic acid is mTOR-independent, but this autophagy pathway was regulated by p8 via p53- and PKCα-mediated signaling in MKN45 cells. Our findings provide insights into the role of p8 in regulating autophagy induced by the lipotoxic effects of excess fat accumulation in cells.

Evaluating dietary compounds in pancreatic cancer modeling systems

With the establishment of outstanding rodent models of pancreatic neoplasia and cancer, there are now systems available for evaluating the role diet, dietary supplements, and/or therapeutic compounds (which can be delivered in the diet) play in disease suppression. Several outstanding reports, which demonstrate clear inhibition or regression of pancreatic tumors following dietary manipulations, represent a noticeable advancement in the field by allowing for the contribution of diet and natural and synthetic compounds to be identified. The real goal is to provide support for translational components that will provide true chemoprevention to individuals at higher risk for developing pancreatic cancer. In addition, administration of molecules with proven efficacy in an in vivo system will screen likely candidates for future clinical trials. Despite this growing enthusiasm, it is important to note that the mere one-to-one translation of findings in rodent models to clinical outcomes is highly unlikely. Thus, careful consideration must be made to correlate findings in rodents with those in human cells with full disclosure of the subtle but often critical differences between animal models and humans. Additional concern should also be placed on the approaches employed to establish dietary components with real potential in the clinic. This chapter is focused on procedures that provide a systematic design for evaluating dietary compounds in cell culture and animal models to highlight which ones might have the greatest potential in people. The general format for this text is a stepwise use of fairly well-known approaches covered briefly but annotated with certain considerations for dietary studies. These methods include administration of a compound or a diet, measuring the cellular and molecular effects (histology, proliferation, apoptosis, RNA and protein expression, and signaling pathways), measuring the level of certain metabolites, and assessing the stability of active compounds. Though this chapter is divided into in vitro and in vivo sections, it is not an implication as to the order of experiments but an endorsement for utilizing human cells to complement work in a rodent modeling system. The notion that cell culture can provide the basis for further in vivo work is an attractive starting point, though the lack of a response in a single cell type should not necessarily prevent diet studies in rodents. The advantage of cell culture over animal models is the human origin of these cells and the ease and directness of manipulating a single cell type (particularly when exploring mechanism of action in that cell). Of course, the full effect of a diet, diet supplement, or therapeutic can only be wholly appreciated in an intact living organism with similar anatomical and physiological relevance. Thus, both approaches are considered in this chapter as each can provide unique strengths to determining the effectiveness of various dietary compounds or supplements on pancreatic neoplasia and cancer.

GPR40: a therapeutic target for mediating insulin secretion (review)

G-protein-coupled receptor 40 (GPR40), known as free fatty acid receptor 1, is mainly expressed in pancreatic β-cells and activated by medium- and long-chain fatty acids. Increasing evidence indicates that the activation of GPR40 in cells causes insulin secretion, and GPR40 has become an attractive therapeutic target for type 2 diabetes. Recently, certain novel GPR40 agonists have been identified that regulate glucose-stimulated insulin secretion, leading to the development of new drugs for the treatment of type 2 diabetes. In this review, we focus on progress in the physiological role of GPR40 and potential drugs targeting GPR40 over the past decade.

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

Chronic exposure to high concentrations of saturated fatty acids, such as palmitic acid (PA), leads to apoptosis of pancreatic β-cells through the activation of the c-Jun N-terminal kinase (JNK) signaling pathway. This study of β-cell lipoapoptosis was designed to investigate the roles of pancreatic-derived factor (PANDER), a pro-apoptosis cytokine-like peptide, and exendin-4, a long-acting agonist of the hormone glucagon-like peptide-1 (GLP-1) receptor and anti-apoptosis factor. The glucose-sensitive mouse β-pancreatic cell line, βTC6, was used to investigate the mechanisms of PA-induced apoptosis. Twenty-four hours of PA exposure led to increased PANDER expression in a dose- and time-dependent manner, and significantly increased phosphorylation of JNK. Treatment with the JNK-specific inhibitor SP600125 reduced the PA-induced PANDER expression. After the 24h of PA exposure, cells also underwent marked apoptosis and showed increased activation of the apoptosis protease, caspase-3. The small interfering (si)RNA-mediated silencing of PANDER gene expression significantly reduced both of these effects. When PA-treated βTC6 cells were exposed to exogenous exendin-4, JNK activation was inhibited, PANDER expression was decreased, and the numbers of apoptotic cells were reduced. Collectively, these results demonstrated that the JNK-mediated signaling mechanism of PA-induced β-cell apoptosis involves up-regulated expression of PANDER and activation of caspase-3. Exendin-4 may protect against lipoapoptosis by interfering with the JNK-PANDER pathway.