Faculty Bibliography

The LDL receptor related protein 1 (LRP1) partakes in metabolic and signaling events regulated in a tissue-specific manner. The function of LRP1 in airways has not been studied. We aimed to study the function of LRP1 in smoke-induced disease. We found that bronchial epithelium of patients with chronic obstructive pulmonary disease (COPD) and airway epithelium of mice exposed to smoke had increased LRP1 expression. We then knocked out LRP1 in human bronchial epithelial cells in vitro and in airway epithelial club cells in mice. In vitro, LRP1 knockdown decreased cell migration and increased TGFβ activation. Tamoxifen-inducible airway-specific LRP1 knockout mice (club Lrp1^-/-) induced after complete lung development had increased inflammation in the bronchoalveolar space and lung parenchyma at baseline. After 6 months of smoke exposure, club Lrp1^-/- mice showed a combined restrictive and obstructive phenotype, with lower compliance, inspiratory capacity, and FEV0.05/FVC than WT smoke-exposed mice. This was associated with increased values of Ashcroft fibrotic index. Proteomic analysis of room air exposed-club Lrp1^-/- mice showed significantly decreased levels of proteins involved in cytoskeleton signaling and xenobiotic detoxification, as well as decreased levels of glutathione. The proteome fingerprint created by smoke eclipsed many of the original differences, but club Lrp1^-/- mice continued to have decreased lung glutathione levels and increased protein oxidative damage and airway cell proliferation. Therefore, LRP1 deficiency leads to greater lung inflammation and damage and exacerbates smoke-induced lung disease.

Despite the success of LDL-lowering drugs in reducing cardiovascular disease (CVD), there remains a large burden of residual disease due in part to persistent dyslipidemia characterized by elevated levels of triglyceride-rich lipoproteins (TRLs) and reduced levels of HDL. This form of dyslipidemia is increasing globally as a result of the rising prevalence of obesity and metabolic syndrome. Accumulating evidence suggests that impaired hepatic clearance of cholesterol-rich TRL remnants leads to their accumulation in arteries, promoting foam cell formation and inflammation. Low levels of HDL may associate with reduced cholesterol efflux from foam cells, aggravating atherosclerosis. While fibrates and fish oils reduce TRL, they have not been uniformly successful in reducing CVD, and there is a large unmet need for new approaches to reduce remnants and CVD. Rare genetic variants that lower triglyceride levels via activation of lipolysis and associate with reduced CVD suggest new approaches to treating dyslipidemia. Apolipoprotein C3 (APOC3) and angiopoietin-like 3 (ANGPTL3) have emerged as targets for inhibition by antibody, antisense, or RNAi approaches. Inhibition of either molecule lowers TRL but respectively raises or lowers HDL levels. Large clinical trials of such agents in patients with high CVD risk and elevated levels of TRL will be required to demonstrate efficacy of these approaches.

The Abetalipoproteinemia and Related Disorders Foundation was established in 2019 to provide guidance and support for the life-long management of inherited hypocholesterolemia disorders. Our mission is "to improve the lives of individuals and families affected by abetalipoproteinemia and related disorders". This review explains the molecular mechanisms behind the monogenic hypobetalipoproteinemia disorders and details their specific pathophysiology, clinical presentation and management throughout the lifespan. In this review, we focus on abetalipoproteinemia, homozygous hypobetalipoproteinemia and chylomicron retention disease; rare genetic conditions that manifest early in life and cause severe complications without appropriate treatment. Absent to low plasma lipid levels, in particular cholesterol and triglyceride, along with malabsorption of fat and fat-soluble vitamins are characteristic features of these diseases. We summarize the genetic basis of these disorders, provide guidance in their diagnosis and suggest treatment regimens including high dose fat-soluble vitamins as therapeutics. A section on preconception counseling and other special considerations pertaining to pregnancy is included. This information may be useful for patients, caregivers, physicians and insurance agencies involved in the management and support of affected individuals.

CD36 mediates the uptake of long-chain fatty acids (FAs), a major energy substrate for the myocardium. Under excessive FA supply, CD36 can cause cardiac lipid accumulation and inflammation while its deletion reduces heart FA uptake and lipid content and increases glucose utilization. As a result, CD36 was proposed as a therapeutic target for obesity-associated heart disease. However, more recent reports have shown that CD36 deficiency suppresses myocardial flexibility in fuel preference between glucose and FAs, impairing tissue energy balance, while CD36 absence in tissue macrophages reduces efferocytosis and myocardial repair after injury. In line with the latter homeostatic functions, we had previously reported that CD36^-/- mice have chronic subclinical inflammation. Lipids are important for the maintenance of tissue homeostasis and there is limited information on heart lipid metabolism in CD36 deficiency. Here, we document in the hearts of unchallenged CD36^-/- mice abnormalities in the metabolism of triglycerides, plasmalogens, cardiolipins, acylcarnitines, and arachidonic acid, and the altered remodeling of these lipids in response to an overnight fast. The hearts were examined for evidence of inflammation by monitoring the presence of neutrophils and pro-inflammatory monocytes/macrophages using the respective positron emission tomography (PET) tracers, ⁶⁴Cu-AMD3100 and ⁶⁸Ga-DOTA-ECL1i. We detected significant immune cell infiltration in unchallenged CD36^-/- hearts as compared with controls and immune infiltration was also observed in hearts of mice with cardiomyocyte-specific CD36 deficiency. Together, the data show that the CD36^-/- heart is in a non-homeostatic state that could compromise its stress response. Non-invasive immune cell monitoring in humans with partial or total CD36 deficiency could help evaluate the risk of impaired heart remodeling and disease.

Loss-of-function mutations in the transcription factor CREB3L3 (CREBH) associate with severe hypertriglyceridemia in humans. CREBH is believed to lower plasma triglycerides by augmenting the activity of lipoprotein lipase (LPL). However, by using a mouse model of type 1 diabetes mellitus (T1DM), we found that greater liver expression of active CREBH normalized both elevated plasma triglycerides and cholesterol. Residual triglyceride-rich lipoprotein (TRL) remnants were enriched in apolipoprotein E (APOE) and impoverished in APOC3, an apolipoprotein composition indicative of increased hepatic clearance. The underlying mechanism was independent of LPL, as CREBH reduced both triglycerides and cholesterol in LPL-deficient mice. Instead, APOE was critical for CREBH's ability to lower circulating remnant lipoproteins because it failed to reduce TRL cholesterol in Apoe-/- mice. Importantly, individuals with CREB3L3 loss-of-function mutations exhibited increased levels of remnant lipoproteins that were deprived of APOE. Recent evidence suggests that impaired clearance of TRL remnants promotes cardiovascular disease in patients with T1DM. Consistently, we found that hepatic expression of CREBH prevented the progression of diabetes-accelerated atherosclerosis. Our results support the proposal that CREBH acts through an APOE-dependent pathway to increase hepatic clearance of remnant lipoproteins. They also implicate elevated levels of remnants in the pathogenesis of atherosclerosis in T1DM.

OBJECTIVE: CONCLUSIONS:Overall, these findings reveal a novel and unexpected function for Pcpe2 in modulating SR-BI expression and function as it relates to adipose tissue expansion and cholesterol balance in both mice and humans.

Lipases constitute an important class of water-soluble enzymes that catalyze the hydrolysis of hydrophobic triacylglycerol (TAG). Their enzymatic activity is typically measured using multistep procedures involving isolation and quantification of the hydrolyzed products. We report here a new fluorescence method to measure lipase activity in real time that does not require the separation of substrates from products. We developed this method using adipose triglyceride lipase (ATGL) and lipoprotein lipase (LpL) as model lipases. We first incubated a source of ATGL or LpL with substrate vesicles containing nitrobenzoxadiazole (NBD)-labeled TAG, then measured increases in NBD fluorescence, and calculated enzyme activities. Incorporation of NBD-TAG into phosphatidylcholine (PC) vesicles resulted in some hydrolysis; however, incorporation of phosphatidylinositol into these NBD-TAG/PC vesicles and increasing the ratio of NBD-TAG to PC greatly enhanced substrate hydrolysis. This assay was also useful in measuring the activity of pancreatic lipase and hormone-sensitive lipase. Next, we tested several small-molecule lipase inhibitors and found that orlistat inhibits all lipases, indicating that it is a pan-lipase inhibitor. In short, we describe a simple, rapid, fluorescence-based triacylglycerol hydrolysis assay to assess four major TAG hydrolases: intracellular ATGL and hormone-sensitive lipase, LpL localized at the extracellular endothelium, and pancreatic lipase present in the intestinal lumen. The major advantages of this method are its speed, simplicity, and elimination of product isolation. This assay is potentially applicable to a wide range of lipases, is amenable to high-throughput screening to discover novel modulators of triacylglycerol hydrolases, and can be used for diagnostic purposes.

Despite the decades-old knowledge that diabetes mellitus is a major risk factor for cardiovascular disease, the reasons for this association are only partially understood. While this association is true for both type 1 and type 2 diabetes, different pathophysiological processes may be responsible. Lipids and other risk factors are indeed important, whereas the role of glucose is less clear. This lack of clarity stems from clinical trials that do not unambiguously show that intensive glycemic control reduces cardiovascular events. Animal models have provided mechanisms that link diabetes to increased atherosclerosis, and evidence consistent with the importance of factors beyond hyperglycemia has emerged. We review clinical, pathological, and animal studies exploring the pathogenesis of atherosclerosis in humans living with diabetes and in mouse models of diabetes. An increased effort to identify risk factors beyond glucose is now needed to prevent the increased cardiovascular disease risk associated with diabetes.

Lipids released from circulating lipoproteins by intravascular action of lipoprotein lipase (LpL) reach parenchymal cells in tissues with a non-fenestrated endothelium by transfer through or around endothelial cells. The actions of LpL are controlled at multiple sites, its synthesis and release by myocytes and adipocytes, its transit and association with the endothelial cell luminal surface, and finally its activation and inhibition by a number of proteins and by its product non-esterified fatty acids. Multiple pathways mediate endothelial transit of lipids into muscle and adipose tissues. These include movement of fatty acids via the endothelial cell fatty acid transporter CD36 and movement of whole or partially LpL-hydrolyzed lipoproteins via other apical endothelial cell receptors such as SR-B1and Alk1. Lipids also likely change the barrier function of the endothelium and operation of the paracellular pathway around endothelial cells. This review summarizes in vitro and in vivo support for the key role of endothelial cells in delivery of lipids and highlights incompletely understood processes that are the focus of active investigation.

Although tissue uptake of fatty acids from chylomicrons is primarily via lipoprotein lipase (LpL) hydrolysis of triglycerides (TGs), studies of patients with genetic LpL deficiency suggest additional pathways deliver dietary lipids to tissues. Despite an intact endothelial cell (EC) barrier, hyperchylomicronemic patients accumulate chylomicron-derived lipids within skin macrophages, leading to the clinical finding eruptive xanthomas. We explored whether an LpL-independent pathway exists for transfer of circulating lipids across the EC barrier. We found that LpL-deficient mice had a marked increase in aortic EC lipid droplets before and after a fat gavage. Cultured ECs internalized chylomicrons, which were hydrolyzed within lysosomes. The products of this hydrolysis fueled lipid droplet biogenesis in ECs and triggered lipid accumulation in cocultured macrophages. EC chylomicron uptake was inhibited by competition with HDL and knockdown of the scavenger receptor-BI (SR-BI). In vivo, SR-BI knockdown reduced TG accumulation in aortic ECs and skin macrophages of LpL-deficient mice. Thus, ECs internalize chylomicrons, metabolize them in lysosomes, and either store or release their lipids. This latter process may allow accumulation of TGs within skin macrophages and illustrates a pathway that might be responsible for creation of eruptive xanthomas.