Differentiation with elaidate tends to impair insulin-dependent glucose uptake and GLUT4 translocation in 3T3-L1 adipocytes

Peripheral Aβ acts as a negative modulator of insulin secretion

The cerebral accumulation of amyloid β (Aβ) is a hallmark of Alzheimer’s disease (AD). While type 2 diabetes mellitus is known to be a risk factor for AD, the underlying mechanisms remain unclear. In the present study, we demonstrate that plasma Aβ is produced from glucose- and insulin-susceptible peripheral tissues, such as the pancreas, adipose tissues, skeletal muscles, and liver, to inhibit insulin secretion from islet β-cells. Our findings suggest a physiological role of peripheral Aβ in glucose and insulin metabolism and a possible mechanism linking diabetes to AD. In addition, although plasma Aβ levels are currently used as a diagnostic biomarker of AD, our data suggest they should be used with caution.

Type 2 diabetes mellitus is known to be a risk factor for Alzheimer’s disease (AD), but the underlying mechanisms remain unclear. In AD, the cerebral accumulation of amyloid β (Aβ) triggers a pathological cascade leading to neurodegeneration. Plasma Aβ levels are thought to reflect the brain amyloid pathology and currently used as a diagnostic biomarker of AD. However, amyloid precursor protein and Aβ-generating enzymes, β- and γ-secretases, are widely expressed in various peripheral tissues. Previous reports have shown that glucose and insulin loading cause a transient increase of plasma Aβ in mice and humans. These findings led us to speculate that plasma Aβ is produced from glucose- and insulin-susceptible peripheral tissues to play a role in glucose and insulin metabolism. To test this hypothesis, we investigated the effects of glucose and insulin on Aβ secretion and the effect of Aβ on insulin secretion in vivo, ex vivo, and in vitro. Aβ was found to be secreted from β-cells of the pancreas along with insulin upon glucose stimulation. Upon insulin stimulation, Aβ was secreted from cells of insulin-targeted organs, such as adipose tissues, skeletal muscles, and the liver, along with their organokines. Furthermore, Aβ inhibited the glucose-triggered insulin secretion from β-cells, slowing down glucose clearance from the blood. These results suggest that peripheral Aβ acts as a negative modulator of insulin secretion. Our findings provide a possible mechanism linking diabetes to AD and call attention to how plasma Aβ levels are used in AD diagnosis.

Exposure to Stearate Activates the IRE1α/XBP-1 Pathway in 3T3-L1 Adipocytes

In the endoplasmic reticulum (ER), accumulation of abnormal proteins with malformed higher-order structures activates signaling pathways (inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 (XBP-1) pathway, protein kinase RNA-activated-like endoplasmic reticulum kinase (PERK)/CCAAT/enhancer binding protein-homologous protein (CHOP) pathway and activating transcription factor 6α (ATF6α) pathway) that result in a cellular response suppressing the production of abnormal proteins or inducing apoptosis. These responses are collectively known as the unfolded protein response (UPR). Recently, it has been suggested that the UPR induced by saturated fatty acids in hepatocytes and pancreatic β cells is involved in the development of metabolic diseases such as diabetes. The effect of palmitate, a saturated fatty acid, on the UPR has also been investigated in adipocytes, which are associated with the development of metabolic disorders, but the results were inconclusive. Therefore, as the major saturated fatty acids present in the daily diet are palmitate and stearate, we examined the effects of these saturated fatty acids on UPR in adipocytes. Here, we show that saturated fatty acids caused limited activation of the UPR in adipocytes. Exposure to stearate for several hours elevated the ratio of spliced XBP-1 mRNA, and this effect was stronger than that of palmitate. Moreover, the phosphorylation level of IRE1α, upstream of XBP-1 and expression levels of its downstream targets such as DNAJB9 and Pdia6 were elevated in 3T3-L1 adipocytes exposed to stearate. On the other hand, stearate did not affect the phosphorylation of PERK, its activation of CHOP, or the cleavage of ATF6α. Thus, in adipocytes, exposure to stearate activates the UPR via the IRE1α/XBP-1 pathway, but not the PERK/CHOP and ATF6α pathway.

The Effect of Trans Fatty Acids on Human Health: Regulation and Consumption Patterns

Health effects of trans fatty acids (TFAs) on human organisms can vary according to their type, structure, composition, and origin. Even though the adverse health effects of industrial TFAs (iTFAs) have been widely discussed, the health effects of natural TFAs (nTFAs) are still questionable. Hence, it is important to review the literature and provide an overall picture on the health effects of different TFAs coming from industrial and ruminant sources, underlining those types that have adverse health effects as well as suggesting methods for reducing their harmful effects. Multiple databases (PubMed, Medline, Cochrane Library, etc.) were searched with the key words "trans fatty acid sources", "ruminant", "industrial", "conjugated trans linoleic acid", "human", "coronary heart disease", "cancer", etc. Reference lists of the studies were scanned discussing the health effects of iTFAs and nTFAs. The review of the literature showed that iTFAs are found to be more harmful than ruminant-produced nTFAs. Although several beneficial effects (such as reduced risk of diabetes) for nTFAs have been observed, they should be used with caution. Since during labeling it is usually not mentioned whether the TFAs contained in food are of industrial or natural origin, the general suggestion is to reduce their consumption.

Elaidate, a trans fatty acid, suppresses insulin signaling for glucose uptake in a manner distinct from that of stearate

The dietary intake of elaidate (elaidic acid), a trans-fatty acid, is associated with the development of various diseases. Since elaidate is a C18 unsaturated fatty acid with a steric structure similar to that of a C18 saturated fatty acid (stearate), we previously revealed that insulin-dependent glucose uptake was impaired in adipocytes exposed to elaidate prior to and during differentiation similar to stearate. However, it is still unknown whether the mechanism of impairment of insulin-dependent glucose uptake due to elaidate is similar to that of stearate. Here, we indicate that persistent exposure to elaidate has particular effects on insulin signaling and GLUT4 dynamics. Insulin-induced accumulation of Akt at the plasma membrane (PM) and elevations of phosphorylated Akt and AS160 levels in whole cells were suppressed in adipocytes persistently exposed to 50 μM elaidate. Interestingly, persistent exposure to the same concentration of stearate has no effect on the phosphorylated Akt and AS160 levels. When cells were exposed to these fatty acids, elaidate suppressed insulin-induced fusion, but not translocation, of GLUT4 storage vesicles in the PM, whereas stearate did not suppress the fusion and translocation of GLUT4 storage, indicating that elaidate has suppressive effects on the accumulation of Akt and fusion of GLUT4 storage vesicles and that both elaidate and stearate vary in the mechanisms by which they impair insulin-dependent glucose uptake.