Induction of fibroblast growth factor 21 does not require activation of the hepatic X-box binding protein 1 in mice

Mechanisms of hepatic fatty acid oxidation and ketogenesis during fasting

Fasting is part of many weight management and health-boosting regimens. Fasting causes substantial metabolic adaptations in the liver that include the stimulation of fatty acid oxidation and ketogenesis. The induction of fatty acid oxidation and ketogenesis during fasting is mainly driven by interrelated changes in plasma levels of various hormones and an increase in plasma nonesterified fatty acid (NEFA) levels and is mediated transcriptionally by the peroxisome proliferator-activated receptor (PPAR)α, supported by CREB3L3 (cyclic AMP-responsive element-binding protein 3 like 3). Compared with men, women exhibit higher ketone levels during fasting, likely due to higher NEFA availability, suggesting that the metabolic response to fasting shows sexual dimorphism. Here, we synthesize the current molecular knowledge on the impact of fasting on hepatic fatty acid oxidation and ketogenesis.

Systems levels analysis of lipid metabolism in oxygen-induced retinopathy

Hyperoxia induces glutamine-fueled anaplerosis in the Muller cells, endothelial cells, and retinal explants. Anaplerosis takes away glutamine from the biosynthetic pathway to the energy-producing TCA cycle. This process depletes biosynthetic precursors from newly proliferating endothelial cells. The induction of anaplerosis in the hyperoxic retina is a compensatory response, either to decreased glycolysis or decreased flux from glycolysis to the TCA cycle. We hypothesized that by providing substrates that feed into TCA, we could reverse or prevent glutamine-fueled anaplerosis, thereby abating the glutamine wastage for energy generation. Using an oxygen-induced retinopathy (OIR) mouse model, we first compared the difference in fatty acid metabolism between OIR-resistant BALB/cByJ and OIR susceptible C57BL/6J strains to understand if these strains exhibit metabolic difference that protects BALB/cByJ from the hyperoxic conditions and prevents their vasculature in oxygen-induced retinopathy model. Based on our findings from the metabolic comparison between two mouse strains, we hypothesized that the medium-chain fatty acid, octanoate, can feed into the TCA and serve as an alternative energy source in response to hyperoxia. Our systems levels analysis of OIR model shows that the medium chain fatty acid can serve as an alternative source to feed TCA. We here, for the first time, demonstrate that the retina can use medium-chain fatty acid octanoate to replenish TCA in normoxic and at a higher rate in hyperoxic conditions.

Single well, single-common primer pair, dual probe, duplex qPCR assay for the quantification of mRNA splicing variants

Quantifying the ratio of alternatively spliced mRNA variants of genes with known alternative splicing variants is highly relevant for many applications. Herein, we describe the validation of a quantitative PCR design for the simplified quantification of known mRNA splice variants. The assay uses a single-common primer pair, dual probe design for the determination of splicing variants in a single well configuration. We used murine XBP-1 splicing variants, XBP-1S and XBP-1U, to validate and demonstrate the performance characteristics of this approach. Using synthetic XBP-1S and XBP-1U cDNA as well as cDNA synthesized from mouse beta-cell line MIN6, we established the performance parameters and dynamic range of the assay. Reliable quantification of both variants at varying concentration gradients was shown. No cross detection of XBP-1U by the XBP-1S probe was detected and only marginal XBP-1S cross detection by the XBP-1U probe was detected at high concentration gradients that are unlikely to be relevant. We demonstrated that the assay accurately detected changes of XBP-1 splice variants in mouse liver subjected to pharmacologically induced ER stress without the need for normalization to a reference gene.

XBP1 inhibits mesangial cell apoptosis in response to oxidative stress via the PTEN/AKT pathway in diabetic nephropathy

Diabetic nephropathy (DN) is a complication of diabetes mellitus (DM) that frequently results in renal disease, and is characterized by a variety of symptoms, including albuminuria. It has been shown that apoptosis of glomerular mesangial cells (MCs) can aggravate albuminuria and contribute to the development of diabetic glomerulosclerosis. Hence, determination of the mechanisms leading to MC apoptosis may help us gain insights into the pathogenesis of DN. As our understanding of the role of high glucose (HG) in MC apoptosis remains elusive, we explored the interplay between X-box binding protein 1 (XBP1) and MC apoptosis in this study. XBP1 was observed to be downregulated both in vivo and in vitro. Treatment of XBP1-overexpressing cells with HG resulted in a decrease of reactive oxygen species (ROS) and a suppression of cell apoptosis, concomitant with decreases in cleaved caspase-3 and Bax. Subsequent analyses demonstrated that XBP1 overexpression inhibited the expression of phosphatase and tensin homolog deleted on chromosome ten (PTEN) and enhanced the activation of AKT in MCs exposed to HG. In addition, XBP1-induced injuries in MC were reversed by overexpression of PTEN, and XBP1 inhibited apoptosis, which was mediated by the activated PTEN/AKT signaling pathway. Thus, our data indicate that XBP1 can activate the PTEN/AKT signaling pathway, thereby alleviating oxidative stress caused by HG or MC apoptosis. These findings suggest that XBP1 may have potential in the development of treatment methods for DN.

The role of X-box binding protein 1 in the hepatic response to refeeding in mice

Refeeding mice after a prolonged fast is a potent stimulus of hepatic lipogenesis, but is also associated with induction of the hepatic unfolded protein response (UPR). The X-box binding protein 1 (Xbp1), a key regulator of the adaptive UPR, transcriptionally activates hepatic lipogenesis genes. We therefore determined whether hepatic Xbp1 mediates the hepatic lipogenic response to refeeding. Mice bearing a hepatocyte-specific deletion of Xbp1 and littermate controls were fasted overnight, followed by refeeding for up to 6 h. Among control mice, refeeding induced hepatic expression of activated Xbp1 and, as expected, induced hepatic expression of genes controlling de novo lipogenesis of fatty acids. Unexpectedly, deletion of hepatic Xbp1 allowed for normal induction of hepatic lipogenesis genes, yet impaired translation of SREBP1c and its targets in response to refeeding. Impaired protein translation was associated with enhanced postprandial activation of the global translational arrest protein, eukaryotic initiation factor 2α, among mice lacking hepatic Xbp1 Deletion of hepatic Xbp1 prevented postprandial induction of genes regulating protein folding and processing and shifted the pattern of postprandial UPR activation to favor proapoptotic signals. We conclude that activation of hepatic Xbp1 in the postprandial states serves the dual roles of restoring postprandial hepatic lipogenesis and proteostasis.