ATHEROSCLEROSIS THE HDL RECEPTORS A REVIEW AND Good morning fellow students, Dr Stewart and Mr Mennella. Today I will be presenting a talk on… As we have learned in this class, atherosclerosis is HDL is HDL receptor importance A REVIEW

High Density Lipoprotein (HDL) acts as a cholesterol scavenger probably obtains cholesterol from cell surface membranes using lecithin:cholesterol acyltransferase (LCAT) transfers some cholesterol esters (CE) to VLDL and LDL using cholesterol ester transfer protein (CETP) transport of cholesterol and CE from tissues to liver and steriodogenic cells excrete cholesterol from body in form of bile acids use as building blocks for steroids Very low density lipoprotein Low density lipoprotein After this slide put the pic of liver cycle and lipoproteins

IMPORTANCE Cardiovascular disease is the #1 killer in the US 40% of all deaths in the US 1.5 million new cases every year chronic illness The Framingham Heart Study shows that a 1% reduction in an individual's total serum cholesterol level translates into an approximate 2% reduction in heart disease risk

HDL RECEPTORS SCAVENGER RECEPTORS CUBILIN MEGALIN

SCAVENGER RECEPTORS There are many classes of these receptors Scavenger Receptor Class B Type 1 (SR-B1) Scavenger Receptor Class B Type 2 (SR-B2) Cluster Designation (CD) 36 Focus : SR-B1 and CD 36 Type 1 and 2 differ only in their C-terminal cytoplasmic tails. Its postulated that this difference may be important to the functioning of the scavenger receptor 1 In recent studies 3 of these receptors have been tested for HDL binding affinities

SR-B1 identified as a receptor for LDLs greatest expression in hepatic and steriodogenic cells in liver, ovary, lung and adrenal glands FUNCTIONS decreases plasma levels of HDL and non-HDL cholesterol mediates HDL CE selective uptake anchors HDL molecules to plasma membranes without internalizing or degrading them

STRUCTURE OF SR-B1 2 transmembrane domains 2 cytoplasmic domains consists of amino and carboxyl terminals C-terminal may facilitate uptake of CE into cells N-terminal responsible for proper targeting of receptor to plasma membrane an extracellular domain contains a cysteine rich region 9 putative sites for N linked glycosylation binding site for HDL CE greatest efficiency for uptake and binding a non-aqueous channel between the HDL and the plasma membrane In a most recent study, the following has been discovered… C-terminal facilitates selective uptake of cholesteryl ester into cells ?? C-terminal found to be dispensable for targeting to plasma m. and any activity Channel: facilitate the transport of lipids through membrane ie free cholesterol

This diagram shows the similarity between SR-B1 and CD36 SR-B1 and CD36 are similar in size They show considerable amino acid sequence identity Very similar in predicted secondary structure HDL binds with high affinity to both of them Studies show that its the extracellular domain of SR-B1 that mediates the high efficiency selective uptake of CE from receptor bound HDL particles In the study, chimeric receptors obtained by swapping various domains between these receptors, were used to determine which part of the protein was actually involved

Bi-directional free cholesterol flux HOT OF THE PRESS! Extracellular domain HDL CE uptake Bi-directional free cholesterol flux alteration of membrane cholesterol mass and distribution Cytoplasmic domain : N and C-terminals C-terminal tail is dispensable for activity as well as for targeting to the plasma membrane it does not confer an enhanced HDL CE selective uptake activity N-terminal has no new activity roles A new study that was published in Biochemistry last week shows the following…

CUBILIN Intestinal receptor for the endocytosis of intrinsic factor-vitamin B12 high expression in kidney, yolk sac, placenta and hepatic cells binds to HDL and apolipoprotein A-1 (apo A-1) endocytosis of 127I-HDL inhibited by IgG antibodies against apo A-1 and cubilin deficiency results in loss of apo A-1 in urine uptake of apo A-1 and HDL from kidney tubules not a receptor for LDL or its derivatives

STRUCTURE OF CUBILIN a short N terminal sequence this region is involved in membrane association has a amphipathic helix pattern which is a potential site for hydrophobic interactions 8 epidermal growth factor repeats 27 CUB (Complement components Clr/Cls, Uegf, and bone morphogenic protein-1) domain cluster ligand binding sites: domains 5-8 bind intrinsic factor-vitamin B12 domains 13-14 is a receptor-associated protein binding site As you can see from your handouts, cubilin has… The 27 CUB is where binding occurs for cubilin It has different sites for the vitamin B12 and for HDL apo A-1

Cubilin does not have apparent transmembrane and cytoplasmic domains MEGALIN does not bind to HDL, delipidated HDL or apo A-1 co-purifies with cubilin exhibits a coincident pattern of mRNA expression in mouse tissues including kidney, ileum, placenta and yolk sac – same as cubilin suppression of megalin activity results in reduced cell surface expression of cubilin megalin antibodies inhibit HDL uptake may play a role in the intracellular trafficking of cubilin Because cubilin did not appear to have transmembrane and cytoplasmic domains, questions arose as to how it could perform its function It seemed to be attached to the top surface of the cell membranes Thus, the possibility of a co-receptor was explored and megalin was discovered Studies show that megalin does not bind to hdl, etc And that in purification procedures, it comes out with cubilin Its expressed in the same tissues as cubilin When antibodies are used to suppress its activity and expression, cubilin expression on cell surface membranes is decreased. In addition, megalin antibodies have been shown to inhibit HDL uptake There is the possibility that megalin may be responsible for the intracellular trafficking of cubilin

ATHEROSCLEROSIS the higher the plasma concentrations of HDL the lower the risk mechanism is unknown strong correlation between atherosclerotic lesions and VLDL and LDL levels combination of SR-B1 overexpression and a low fat diet demonstrates strong anti-atherogenic potential overexpression of hepatic SR-B1 reduces advanced atherosclerotic lesions decreases HDL cholesterol levels moderately reduces non HDL cholesterol levels

FUTURE WORKS Structure of Megalin Details of its interaction with cubilin Determining the extent to which megalin is involved in HDL uptake Crystalline structure of SR-B1 Details of the non-aqueous channel and its function Mechanistic details of relation between atherosclerosis and HDL plasma concentrations

ACKNOWLEDGEMENTS Dr Caro-Beth Stewart, Ph.D. University at Albany Department of Biological Sciences Mr. Tom Mennella Eireann Collins To end, I‘d like to thank Dr Stewart and Mr. Mennella for their help with this project. For the opportunity to learn and the experience of giving a talk Eireann: For all her hard work and constructive criticisms and help in putting this presentation together.