Mice lacking thioredoxin-interacting protein provide evidence linking cellular redox state to appropriate response to nutritional signals.

Thioredoxin-interacting protein (Txnip) is a ubiquitous protein that binds with high affinity to thioredoxin and inhibits its ability to reduce sulfhydryl groups via NADPH oxidation. HcB-19 mice contain a nonsense mutation in Txnip that eliminates its expression. Unlike normal animals, HcB-19 mice have approximately 3-fold increase in insulin levels when fasted. The C-peptide/insulin ratio is normal, suggesting that the hyperinsulinemia is due to increased insulin secretion. Fasted HcB-19 mice are hypoglycemic, hypertriglyceridemic, and have higher than normal levels of ketone bodies. Ablation of pancreatic beta-cells with streptozotocin completely blocks the fasting-induced hypoglycemia/hypertriglyceridemia, suggesting that these abnormalities are due to excess insulin secretion. This is supported by increased hepatic mRNA levels of the insulin-inducible, lipogenic transcription factor sterol-responsive element-binding protein-1c and two of its targets, acetyl-CoA carboxylase and fatty acid synthase. During a prolonged fast, the hyperinsulinemia up-regulates lipogenesis but fails to down-regulate hepatic phosphoenolpyruvate carboxykinase mRNA expression. Hepatic ratios of reduced:oxidized glutathione, established regulators of gluconeogenic/glycolytic/lipogenic enzymes, were elevated 30% in HcB-19 mice, suggesting a loss of Txnip-enhanced sulfhydryl reduction. The altered hepatic enzymatic profiles of HcB-19 mice divert phosphoenolpyruvate to glyceroneogenesis and lipogenesis rather than gluconeogenesis. Our findings implicate Txnip-modulated sulfhydryl redox as a central regulator of insulin secretion in beta-cells and regulation of many of the branch-points of gluconeogenesis/glycolysis/lipogenesis.

Microsomal triglyceride transfer protein is essential for hepatic secretion of apoB-100 and apoB-48 but not triglyceride.

Despite a complete lack of microsomal triglyceride transfer protein (MTP), L35 rat hepatoma cells secrete triglyceride-containing lipoproteins, albeit at a rate 25% of that of parental FAO hepatoma cells, which express high levels of MTP. The inability to express MTP was associated with a complete block in the secretion of both apolipoprotein (apo)B-100 and apoB-48. Stable expression of a MTP transgene restored the secretion of both apoB-100 and apoB-48 in L35 cells, indicating that MTP is essential for the secretion of both forms of apoB. Treatment with the MTP inhibitor BMS-200150 reduced the secretion of triglyceride by 70% in FAO cells, whereas the inhibitor did not affect the secretion of triglycerides by L35 cells. Thus, in the presence of the MTP inhibitor, both cell types secreted triglycerides at similar rates. Essentially, all of the triglycerides secreted by L35 cells were associated with HDL containing apoA-IV and apoE but devoid of apoB-100 or apoB-48. These results suggest that these triglyceride-containing lipoproteins are assembled and secreted via a pathway that is independent of both apoB and MTP. Our findings support the concept that apoB and MTP co-evolved and provided a means to augment the secretion of triglyceride through the formation of lipoproteins containing large hydrophobic cores enriched with triglycerides.

Regulation of cholesterol-7alpha-hydroxylase: BAREly missing a SHP.

Cholesterol-7alpha-hydroxylase (CYP7A1) regulates the pathway through which cholesterol is converted into bile acids. The unique detergent properties of bile acids are essential for the digestion and intestinal absorption of hydrophobic nutrients. Bile acids have potent toxic properties (e.g., membrane disruption) and there are a plethora of mechanisms to limit their accumulation in blood and tissues. The discovery of farnesoid X receptor (FXR), the nuclear receptor activated specifically by bile acids, has opened new insights into these mechanisms. Bile acid activation of FXR has been shown to repress the expression of CYP7A1 via increasing the expression of small heterodimer partner (SHP), a non-DNA binding protein. The increased abundance of SHP causes it to associate with liver receptor homolog (LRH)-1, an obligate factor required for transcription of CYP7A1. Recent studies show there is an "FXR/SHP-independent" mechanism that also represses CYP7A1 expression. This "FXR/SHP-independent" pathway involves the interaction of bile acids with liver macrophages (i.e., Kupffer cells), which induces the expression, and secretion of cytokines. These inflammatory cytokines, which include tumor necrosis factor alpha and interleukin-1beta, act upon liver parenchymal cells causing a rapid repression of the CYP7A1 gene.