Dyslipidemia Flashcards
What is Cyp7a1?
7alpha-hydroxylase
RLS enzyme in bile acid production in mice
Is stimulated by the presence of cholesterol which does not occur in humans - this gives mice resistance to hypercholesteremia
In humans Cyp7a1 is regulated by LXR + oxysterols binding
Benefit of converting cholesterol to bile acids rather than excreting it as biliary cholesterol?
Once cholesterol has been converted to bile acids, it will be excreted and there is no reverse reaction
Biliary cholesterol mixes with dietary cholesterol and can be reabsorbed as a result (no distinguishing between the two sources)
Acquired vs. Genetic Dyslipidemia
Malnutrition (deficiency or excess) is the cause of acquired dyslipidemia
Genetic mutations predispose people to disease
Ex. Familial hypercholesteremia
What is familial hypercholesterolemia?
Familial Hypercholesterolemia is a good study subject because
FH caused by absence of LDLR function
Can be readily diagnosed
Use FH fibroblast cells to culture and study, as well as blood samples for lipoprotein examination (centrifuge to separate different size lipoproteins)
Data related to familial hypercholesterolemia
Normal cells have low cellular CE, UC synthesis and HMG-coA reductase activity
FH cells have high cellular CE, UC synth and HMG-CoA reductase activity
In response to lipoprotein depletion: levels in LH cells remain the same
Normal cells mobilize CE for transfer to lipoproteins, increased UC synthesis and increased HMG-coA reductase activity
AKA no cholesterol sensing
LDLR mutations in FH?
1) synthesis of LDLR
2) Transport of LDLR from ER to golgi apparatus
3) Binding of LDL
4) Clustering of LDLR in coated pits
SRE is
Sterol Response Element
Short and specific DNA sequence located in promoter region of genes which response to cholesterol levels (SREBP binds and stimulates transcription)
Ex. are on HMGR and LDLR genes
What dictates binding of SREBP to SRE?
By the concentration of unesterified cholesterol in the cell (decreased levels)
Requires proteolytic processing of SREBP precursor molecules in membrane to release SRE binding domain (multi domain protein like LDLR)
Types of SREBPs and functions in mammalian cells
SREBP-1a - strong activator of ALL SREBP genes
SREBP-1c - regulates genes involved with FA metabolism and TAG synthesis more so than cholesterol (weaker than 1a)
ex. steroyl coA desaturase
SREBP-2 - preferentially regulates cholesterol synthesis regulating genes
the SREBP genes (which encode SREBPs)
SREBP-1a and SREBP-1c are encoded by the same gene that undergoes alternative splicing
SREBP-2 encoded by a separate gene
Natural vs synthetic genes
Natural - a chunk of genetic material from an organism (not altered)
Synthetic - a designed combination of genes from different organisms
Transgenesis
When it is useful to use
Genes introduced into another genome (zygote) for permanent propagation
Useful for studying how human genes function in vivo
ex. introducing synthetic gene into mice produces over-expression of LDLR which is not sensitive to cellular cholesterol (because it is transgenic)
separation of lipoproteins using size exclusion chromatography
large particles (chylomicrons) elute faster because they have less volume to traverse
Allows separating Lps by fractions
Takeaway from over expression of LDLR in transgenic mice experiment
Over expression of LDLR gave transgenic mice resistance to hypercholesterolemia due to high constant uptake of cholesterol into cells
Cell concentrations of cholesterol don’t signal LDLR transgene regulation in transgenic mice
Even on a mouse diet blood LDL was lower than in wild type
Benefit of knockout mice
permits very specific genetic modifications to be created
Allows for direct assessment of gene function
mutations can be global, local (specific tissues) or introduced at certain stages of development
Takeaway from targeted disruption of LDLR in knockout mice
No sexual dimorphisms
Wildtype mice were resistant to hypercholesterolemia
Heterozygous had higher LDL levels, less resistant
Homozygous knockout mouse was not resistant to hypercholesterolemia on either diet
Similarities between knockout LDLR-deficient mice and hypercholesterolemia in humans
Same elevated blood cholesterol, LDL affected, hypercholesterolemia after consuming cholesterol (cannot transport LDL into the cells), premature atherosclerosis or susceptibility to it
statins can treat it by halting HMG-coA reductase
Specific SREBP knockout mice revealed
SREBP-2 is essential for life (embryonic lethal)
SREBP-1a/c: HMGR and LDLR increased slightly, FAS decreased slightly
Specific SREBP transgenic mice revealed
Increased 1a –> increase of all gene expression HMGR, FAS and LDLR
Increased 1c –> small increase in FAS
Increased 2 –> large HMGR increase, similar FAS and LDLR expression to increased 1a
