Cecilia Vitali

HDL and cholesterol handling in the brain

Cholesterol is an essential component of both the peripheral nervous system and central nervous system (CNS) of mammals. Brain cholesterol is synthesized in situ by astrocytes and oligodendrocytes and is almost completely isolated from other pools of cholesterol in the body, but a small fraction can be taken up from the circulation as 27-hydroxycholesterol, or via the scavenger receptor class B type I. Glial cells synthesize native high-density lipoprotein (HDL)-like particles, which are remodelled by enzymes and lipid transfer proteins, presumably as it occurs in plasma. The major apolipoprotein constituent of HDL in the CNS is apolipoprotein E, which is produced by astrocytes and microglia. Apolipoprotein A-I, the major protein component of plasma HDL, is not synthesized in the CNS, but can enter and become a component of CNS lipoproteins. Low HDL-C levels have been shown to be associated with cognitive impairment and various neurodegenerative diseases. On the contrary, no clear association with brain disorders has been shown in genetic HDL defects, with the exception of Tangier disease. Mutations in a wide variety of lipid handling genes can result in human diseases, often with a neuronal phenotype caused by dysfunctional intracellular lipid trafficking.

Recombinant human LCAT normalizes plasma lipoprotein profile in LCAT deficiency

Lecithin:cholesterol acyltransferase (LCAT) is the enzyme responsible for cholesterol esterification in plasma. Mutations in the LCAT gene leads to two rare disorders, familial LCAT deficiency and fish-eye disease, both characterized by severe hypoalphalipoproteinemia associated with several lipoprotein abnormalities. No specific treatment is presently available for genetic LCAT deficiency. In the present study, recombinant human LCAT was expressed and tested for its ability to correct the lipoprotein profile in LCAT deficient plasma. The results show that rhLCAT efficiently reduces the amount of unesterified cholesterol (-30%) and promotes the production of plasma cholesteryl esters (+210%) in LCAT deficient plasma. rhLCAT induces a marked increase in HDL-C levels (+89%) and induces the maturation of small preβ-HDL into alpha-migrating particles. Moreover, the abnormal phospholipid-rich particles migrating in the LDL region were converted in normally sized LDL.

Role of LCAT in Atherosclerosis

Lecithin:cholesterol acyltransferase (LCAT) is the only enzyme capable of esterifying cholesterol in plasma, thus determining the maturation of high-density lipoproteins. Because it maintains an unesterified cholesterol gradient between peripheral cells and extracellular acceptors, for a long time, LCAT has been considered as a key enzyme in reverse cholesterol transport. However, despite the fact that it has been more than 50 years since the identification of LCAT, the role of this enzyme in the pathogenesis of atherosclerosis is still debated. A number of studies have been conducted in different animal models, with contradictory results. Studies in humans, in particular in the general population, in subjects at high cardiovascular risk, and in carriers of genetic LCAT deficiency in an excellent model to evaluate the correlation between the reduction of LCAT activity and atherosclerosis also gave conflicting results. This review provides a comprehensive overview of the controversial findings obtained in animals and humans, strengthening the necessity of further investigation to establish how LCAT could be regulated in a promising therapeutic strategy to reduce cardiovascular risk.

Lipoprotein X Causes Renal Disease in LCAT Deficiency

Human familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) is characterized by low HDL, accumulation of an abnormal cholesterol-rich multilamellar particle called lipoprotein-X (LpX) in plasma, and renal disease. The aim of our study was to determine if LpX is nephrotoxic and to gain insight into the pathogenesis of FLD renal disease. We administered a synthetic LpX, nearly identical to endogenous LpX in its physical, chemical and biologic characteristics, to wild-type and Lcat-/- mice. Our in vitro and in vivo studies demonstrated an apoA-I and LCAT-dependent pathway for LpX conversion to HDL-like particles, which likely mediates normal plasma clearance of LpX. Plasma clearance of exogenous LpX was markedly delayed in Lcat-/- mice, which have low HDL, but only minimal amounts of endogenous LpX and do not spontaneously develop renal disease. Chronically administered exogenous LpX deposited in all renal glomerular cellular and matrical compartments of Lcat-/- mice, and induced proteinuria and nephrotoxic gene changes, as well as all of the hallmarks of FLD renal disease as assessed by histological, TEM, and SEM analyses. Extensive in vivo EM studies revealed LpX uptake by macropinocytosis into mouse glomerular endothelial cells, podocytes, and mesangial cells and delivery to lysosomes where it was degraded. Endocytosed LpX appeared to be degraded by both human podocyte and mesangial cell lysosomal PLA2 and induced podocyte secretion of pro-inflammatory IL-6 in vitro and renal Cxl10 expression in Lcat-/- mice. In conclusion, LpX is a nephrotoxic particle that in the absence of Lcat induces all of the histological and functional hallmarks of FLD and hence may serve as a biomarker for monitoring recombinant LCAT therapy. In addition, our studies suggest that LpX-induced loss of endothelial barrier function and release of cytokines by renal glomerular cells likely plays a role in the initiation and progression of FLD nephrosis.

Off-target effects of thrombolytic drugs: apolipoprotein A-I proteolysis by alteplase and tenecteplase

The administration of thrombolytic drugs is of proven benefit in a variety of clinical conditions requiring acute revascularization, including acute myocardial infarction (AMI), ischemic stroke, pulmonary embolism, and venous thrombosis. Generated plasmin can degrade non-target proteins, including apolipoprotein A-I (apoA-I), the major protein constituent of high-density lipoproteins (HDL). Aim of the present study was to compare the extent of apoA-I proteolytic degradation in AMI patients treated with two thrombolytic drugs, alteplase and the genetically engineered t-PA variant tenecteplase. ApoA-I degradation was evaluated in sera from 38 AMI patients treated with alteplase or tenecteplase. In vitro, apoA-I degradation was tested by incubating control sera or purified HDL with alteplase or tenecteplase at different concentrations (5-100 μg/ml). Treatment with alteplase and tenecteplase results in apoA-I proteolysis; the extent of apoA-I degradation was more pronounced in alteplase-treated patients than in tenecteplase-treated patients. In vitro, the extent of apoA-I proteolysis was higher in alteplase-treated sera than in tenecteplase-treated sera, in the whole drug concentration range. No direct effect of the two thrombolytic agents on apoA-I degradation was observed. In addition to apoA-I, apoA-IV was also degraded by the two thrombolytic agents and again proteolytic degradation was higher with alteplase than tenecteplase. In conclusion, this study indicates that both alteplase and tenecteplase cause plasmin-mediated proteolysis of apoA-I, with alteplase resulting in a greater apoA-I degradation than tenecteplase, potentially causing a transient impairment of HDL atheroprotective functions.

HDL and endothelial protection: examining evidence from HDL inherited disorders

Abstract Epidemiological studies have clearly demonstrated that plasma concentrations of HDL cholesterol are strongly and inversely associated with cardiovascular risk. Besides playing a major role in reverse cholesterol transport, the process by which excess cholesterol in the arterial wall is removed by HDL and delivered to the liver for excretion, HDL have other atheroprotective functions. In particular, HDL can contribute to the maintenance of endothelial cell homeostasis by inhibiting cell adhesion molecule expression, by promoting the release of bioactive molecules such as nitric oxide and prostacyclin, and by regulating cell proliferation and migration. HDL inherited disorders represent a unique tool to understand the relationship between HDL concentration, HDL function and HDL–mediated atheroprotection.