Faculty Bibliography

Dietary carotenoids like beta-carotene are converted within the body either to retinoid, via beta-carotene-15,15'-dioxygenase (BCO1), or to beta-apo-carotenoids, via beta-carotene-9',10'-oxygenase 2. Some beta-apo-carotenoids are potent antagonists of retinoic acid receptor (RAR)-mediated transcriptional regulation, which is required to ensure normal heart development and functions. We established liquid chromatography tandem mass spectrometery methods for measuring concentrations of 10 beta-apo-carotenoids in mouse plasma, liver, and heart and assessed how these are influenced by Bco1 deficiency and beta-carotene intake. Surprisingly, Bco1(-/-) mice had an increase in heart levels of retinol, nonesterified fatty acids, and ceramides and a decrease in heart triglycerides. These lipid changes were accompanied by elevations in levels of genes important to retinoid metabolism, specifically retinol dehydrogenase 10 and retinol-binding protein 4, as well as genes involved in lipid metabolism, including peroxisome proliferator-activated receptor-gamma, lipoprotein lipase, Cd36, stearoyl-CoA desaturase 1, and fatty acid synthase. We also obtained evidence of compromised heart function, as assessed by two-dimensional echocardiography, in Bco1(-/-) mice. However, the total absence of Bco1 did not substantially affect beta-apo-carotenoid concentrations in the heart. beta-Carotene administration to matched Bco1(-/-) and wild-type mice elevated total beta-apo-carotenal levels in the heart, liver, and plasma and total beta-apo-carotenoic acid levels in the liver. Thus, BCO1 modulates heart metabolism and function, possibly by altering levels of cofactors required for the actions of nuclear hormone receptors.

We used human cardiomyocyte-derived cells to create an in vitro model to study lipid metabolism and explored the effects of PPARgamma; ACSL1 and ATGL on fatty acid-induced ER stress. Compared to oleate, palmitate treatment resulted in less intracellular accumulation of lipid droplets and more ER stress, as measured by upregulation of CHOP, ATF6 and GRP78 gene expression and phosphorylation of eukaryotic initiation factor 2a (EIF2a). Both ACSL1 and PPARgamma adenovirus-mediated expression augmented neutral lipid accumulation and reduced palmitate-induced upregulation of ER stress markers to levels similar to those in the oleate and control treatment groups. This suggests that increased channeling of non-esterified free fatty acids (NEFA) towards storage in the form of neutral lipids in lipid droplets protects against palmitate-induced ER stress. Overexpression of ATGL in cells incubated with oleate-containing medium increased NEFA release and stimulated expression of ER stress markers. Thus, inefficient creation of lipid droplets as well greater release of stored lipids induces ER stress.

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final step in triglyceride synthesis, the conversion of diacylglycerol (DAG) to triglyceride. Dgat1(-/-) mice exhibit a number of beneficial metabolic effects including reduced obesity and improved insulin sensitivity and no known cardiac dysfunction. In contrast, failing human hearts have severely reduced DGAT1 expression associated with accumulation of DAGs and ceramides. To test whether DGAT1 loss alone affects heart function, we created cardiomyocyte-specific DGAT1 knock-out (hDgat1(-/-)) mice. hDgat1(-/-) mouse hearts had 95% increased DAG and 85% increased ceramides compared with floxed controls. 50% of these mice died by 9 months of age. The heart failure marker brain natriuretic peptide increased 5-fold in hDgat1(-/-) hearts, and fractional shortening (FS) was reduced. This was associated with increased expression of peroxisome proliferator-activated receptor alpha and cluster of differentiation 36. We crossed hDgat1(-/-) mice with previously described enterocyte-specific Dgat1 knock-out mice (hiDgat1(-/-)). This corrected the early mortality, improved FS, and reduced cardiac ceramide and DAG content. Treatment of hDgat1(-/-) mice with the glucagon-like peptide 1 receptor agonist exenatide also improved FS and reduced heart DAG and ceramide content. Increased fatty acid uptake into hDgat1(-/-) hearts was normalized by exenatide. Reduced activation of protein kinase Calpha (PKCalpha), which is increased by DAG and ceramides, paralleled the reductions in these lipids. Our mouse studies show that loss of DGAT1 reproduces the lipid abnormalities seen in severe human heart failure.

While there has been considerable focus on the role and treatment of LDL cholesterol levels, a definitive role of triglycerides in the management of cardiovascular disease has been uncertain. Notably, with increasing triglyceride levels, there is a parallel increase in cholesterol levels carried by triglyceride-rich lipoproteins, which has prompted interest in the use of non-HDL cholesterol levels as a tool guiding interventions. Recent studies have provided evidence for an independent role of triglyceride levels as a cardiovascular risk factor, and recently, an Endocrine Society guideline was published for treatment of hypertriglyceridemia. In contrast to the relative uncertainty regarding triglycerides and cardiovascular disease, a role of very high triglyceride levels as a risk factor for pancreatitis has been well known. The present paper summarizes the underlying evidence for a risk role for triglyceride levels in cardiovascular disease and pancreatitis, current treatment recommendations and areas of future research.

Obesity is associated with infiltration of macrophages into adipose tissue (AT), contributing to insulin resistance and diabetes. However, relatively little is known regarding the origin of AT macrophages (ATMs). We discovered that murine models of obesity have prominent monocytosis and neutrophilia, associated with proliferation and expansion of bone marrow (BM) myeloid progenitors. AT transplantation conferred myeloid progenitor proliferation in lean recipients, while weight loss in both mice and humans (via gastric bypass) was associated with a reversal of monocytosis and neutrophilia. Adipose S100A8/A9 induced ATM TLR4/MyD88 and NLRP3 inflammasome-dependent IL-1beta production. IL-1beta interacted with the IL-1 receptor on BM myeloid progenitors to stimulate the production of monocytes and neutrophils. These studies uncover a positive feedback loop between ATMs and BM myeloid progenitors and suggest that inhibition of TLR4 ligands or the NLRP3-IL-1beta signaling axis could reduce AT inflammation and insulin resistance in obesity.

Triglyceride-rich lipoproteins (TRLs) undergo lipolysis by lipoprotein lipase (LPL), an enzyme that is transported to the capillary lumen by an endothelial cell protein, GPIHBP1. For LPL-mediated lipolysis to occur, TRLs must bind to the lumen of capillaries. This process is often assumed to involve heparan sulfate proteoglycans (HSPGs), but we suspected that TRL margination might instead require GPIHBP1. Indeed, TRLs marginate along the heart capillaries of wild-type but not Gpihbp1-/- mice, as judged by fluorescence microscopy, quantitative assays with infrared-dye-labeled lipoproteins, and EM tomography. Both cell-culture and in vivo studies showed that TRL margination depends on LPL bound to GPIHBP1. Notably, the expression of LPL by endothelial cells in Gpihbp1-/- mice did not restore defective TRL margination, implying that the binding of LPL to HSPGs is ineffective in promoting TRL margination. Our studies show that GPIHBP1-bound LPL is the main determinant of TRL margination.

The rodent heart accumulates TGs and lipid droplets during fasting. The sources of heart lipids could be either FFAs liberated from adipose tissue or FAs from lipoprotein-associated TGs via the action of lipoprotein lipase (LpL). Because circulating levels of FFAs increase during fasting, it has been assumed that albumin transported FFAs are the source of lipids within heart lipid droplets. We studied mice with three genetic mutations: peroxisomal proliferator-activated receptor alpha deficiency, cluster of differentiation 36 (CD36) deficiency, and heart-specific LpL deletion. All three genetically altered groups of mice had defective accumulation of lipid droplet TGs. Moreover, hearts from mice treated with poloxamer 407, an inhibitor of lipoprotein TG lipolysis, also failed to accumulate TGs, despite increased uptake of FFAs. TG storage did not impair maximal cardiac function as measured by stress echocardiography. Thus, LpL hydrolysis of circulating lipoproteins is required for the accumulation of lipids in the heart of fasting mice.

In vitro studies have demonstrated a critical role for high-mobility group box 1 (HMGB1) in autophagy and the autophagic clearance of dysfunctional mitochondria, resulting in severe mitochondrial fragmentation and profound disturbances of mitochondrial respiration in HMGB1-deficient cells. Here, we investigated the effects of HMGB1 deficiency on autophagy and mitochondrial function in vivo, using conditional Hmgb1 ablation in the liver and heart. Unexpectedly, deletion of Hmgb1 in hepatocytes or cardiomyocytes, two cell types with abundant mitochondria, did not alter mitochondrial structure or function, organ function, or long-term survival. Moreover, hepatic autophagy and mitophagy occurred normally in the absence of Hmgb1, and absence of Hmgb1 did not significantly affect baseline and glucocorticoid-induced hepatic gene expression. Collectively, our findings suggest that HMGB1 is dispensable for autophagy, mitochondrial quality control, the regulation of gene expression, and organ function in the adult organism.

Sustained increases in glucose flux via the aldose reductase (AR) pathway have been linked to diabetic vascular complications. Previous studies revealed that glucose flux via AR mediates endothelial dysfunction and leads to lesional hemorrhage in diabetic human AR (hAR) expressing mice in an apoE(-/-) background. Our studies revealed sustained activation of Egr-1 with subsequent induction of its downstream target genes tissue factor (TF) and vascular cell adhesion molecule-1 (VCAM-1) in diabetic apoE(-/-)hAR mice aortas and in high glucose-treated primary murine aortic endothelial cells expressing hAR. Furthermore, we observed that flux via AR impaired NAD(+) homeostasis and reduced activity of NAD(+)-dependent deacetylase Sirt-1 leading to acetylation and prolonged expression of Egr-1 in hyperglycemic conditions. In conclusion, our data demonstrate a novel mechanism by which glucose flux via AR triggers activation, acetylation, and prolonged expression of Egr-1 leading to proinflammatory and prothrombotic responses in diabetic atherosclerosis.