1. 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

2. 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

3. 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

4. HDL RECEPTORS
SCAVENGER RECEPTORS
CUBILIN
MEGALIN

5. 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

6. 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

7. 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

8. 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

9. 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…

10. 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

11. 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 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

12. 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

13. 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

14. 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

15. 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.