General B1 stuff Flashcards
Statins (MOA)
- HMG CoA Reductase Inhibitors
- Structural similarity of all statins to HMG CoA substrate
-Act as reversible competitive inhibitors for the active site on HMG CoA reductase, the initial rate-limiting step in cholesterol biosynthesis;
have a higher affinity for the enzyme than natural substrate, HMG-CoA.
-Inhibition of an early and rate-limiting step in cholesterol synthesis, particularly in the hepatocyte, results in an increased need for exogenous (extracellular) cholesterol; this need is met by increased uptake of LDL particles which are rich in cholesterol
-Increase in LDL receptor gene.
o mechanism: In response to the reduced free cholesterol content within hepatocytes, membrane-bound Sterol Regulatory Element- Binding Proteins (SREBPs) are cleaved by a protease and translocated to the nucleus. The transcription factors then bind the sterol-responsive element of the LDL receptor gene, enhancing transcription and increasing the synthesis of LDL receptors (see fig 3)
-up-regulation of LDL receptor results in increased catabolism of LDL. Plasma concentration of LDL falls and less LDL is available to react with cellular elements in blood and blood vessel walls
Statins (pharmacokinetics)
- first-pass hepatic uptake of all statins, mediated primarily by the organic anion transporter OATP1B1
- limits systemic exposure to active compound
- all HMG CoA reductase inhibitors seem to preferentially effect the
liver - lovastatin (and simvastatin) administered as inactive lactone
- on uptake by liver cells, lactone prodrug is hydrolyzed to the beta-
hydroxy acid (which is the active form and principal metabolite) - atorvastatin administered in active, open-ring form
- highly bound to plasma proteins
- plasma concentrations peak at 1-4 hours;
o t1/2 of simvastatin is 12 hours o t1/2 of atorvastatin is 20 hours
all other statins have t 1/2 of 1 to 4 hours; clinical implications of this unclear - liver biotransforms all statins; about 70% of statin metabolites excreted by liver
atorvastatin, simvastatin, lovastatin metabolized by CYP3A4
Intercalated discs (transverse part)
Transmits force
- a modified Z-band (z-line)
- consists of fascia adeherens (N-cadherins) & desmosomes
Intercalated discs (lateral part)
CM-CM signaling
-gap junctions (nexi) & a few desmosomes
Excitation-contraction coupling (electrical)
Excitation (electrical)
- AP: depolarization —> T-tubules
- phase 2 of AP: L-type Cav1.2 —> Ca++ influx (highly enriched in cardiomyocytes
- ryanodine receptors in SR: —>—>—> Ca+++ (CICR….calcium induced calcium release
Monckeberg Arteriolosclerosis
- calcific deposits in media (& internal elastic lamina) of medium-sized muscular arteries (typically radial and ulnar arteries)
- > 50 years of age
- no obstruction to the blood flow
- usually not clinically significant
- unrelated to atherosclerosis
- its cause is not completely understood
Chronology of Heart Attack
1) immediate: myocyte death —> MB-CK and cTnl
2) +15 hours: inflammation
3) +2-3 days: wound healing via cardiac fibroblasts….collagen deposition (fibrosis)
4) +204 days: angiogenesis (clinical enhancement via VEGF, FGF)
5) Scar deposition (due to collagen cross-linking)
can pre-existing myocytes mobilize to fix damaged myocardium?
maybe
proliferation can be induced by…
1) inhibiting p38 MAP kinase
2) pro-proliferative agent
Can adult stem cells in the heart fix damaged heart?
- no, but maybe mobilize
- stem cells ID’s by expression of c-Kit
- treat heart with drugs/growth factors to mobilize and expand c-Kit+ cells within their niches
can c-kit+ adult stem cells be transplanted to fix injured myocardium?
- maybe….phase 1 trial SCIPIO
- some function is resotred and infarcted area is reduced, while no harm is done
can transplanted bone marrow cells fix the heart?
maybe. …
- paracrine effect…functional benefit, but little or no re-muscularization
Transplantation of CM derived from iPSCs
-best bet to re-muscularize
-
Continuous capillary
Where?
- CNS
- heart
- skeletal muscle
- lung
Item transported?
-oxygen
fenestrated capillary
where?
- endocrine glands
- GI tract
- kidneys
item transported?
- hormones
- nutrients
- ions
sinusoidal capillary
where?
- BM
- spleen
- liver
item transported?
-whole cells
Atherosclerosis: clinical syndromes
Elastic arteries
- aorta: aneurysm with rupture
- carotid arteries: occlusion causing stroke*
- iliac arteries: occlusion causing gangrene*
Large/medium sized muscular arteries
- coronary arteries: occlusion causing MI*
- popliteal arteries: occlusion causing gangrene*
- renal artery: narrowing/occlusion causing secondary hypertension*
- mesenteric arteries: narrowing/occlusion causing bowel infarction*
*possibly associated with thrombosis
Advanced/vulnerable plaque
At risk for:
1) Rupture, ulceration, erosion, and hemorrhage
- lead to thrombosis, embolism
- progressive luminal narrowing (leading to critical stenosis)
2) Atheroembolism
3) aneurysm formation
- wall weakening leading to aneurysm and rupture
Atherosclerosis pathogenesis
Hypothesis:chronicinflammatoryresponseof arterial wall to endothelial injury
• Components of process:
– Endothelial injury
– Hemodynamic disturbances – Lipid accumulation
– Inflammation
– Infection
– Smooth muscle proliferation
Chronic endothelial injury/dysfunction: ↑permeability, enhanced leukocyte adhesion – Specific cause unknown – Strongly suspected causes: • Hemodynamic disturbances: plaques occur in areas of disturbed flow patterns (at ostia and vessel branch points) • Hypercholesterolemia • Other possible contributors: – Hypertension – Cigarette smoke toxins – Homocysteine – Infectious agents
• Inflammation:
– Adhesion molecules (from endothelial cells) attract leukocytes
– Monocyte adhesion, migration & transformation to macrophages
• Initially protective response, but ultimately cause lesion
progression
– T lymphs: secretion of cytokines & fibrogenic mediators
Infection: Importance is unclear at present
– Herpes virus
– CMV
– Chlamydia pneumoniae
• Smooth muscle cells: Proliferation and migration into intima with production of matrix proteins
ApoA-I
HDL
-activates LCAT; interacts with ABC transporter
ApoB-48
Chylomicrons
-cholesterol transport/clearance
ApoB-100
VLDL, LDL
-binds to LDL receptor
ApoC-II
Chylomicrons, VLDL, HDL
-activates lipoprotein lipase
ApoC-III
Chylomicrons, VLDL, HDL
-inhibits lipoprotein lipase
ApoE
Chylomicrons, VLDL, HDL
-triggers clearance of VLDL and chylomicron remnants
NPC1L1 (Niemann-Pick C1-Like 1 protein)
- intestinal cholesterol and plant sterol absorption is mediated by this
- TARGET OF ezetimibe, a cholesterol absorption inhibitor
ABCG5/G8
-export plant sterols back into the intestinal lumen
Sitosterolemia
- autosomal recessive disorder have mutations in either ABCG5 or ABCG8
- absorb unusually large amounts of plant sterols
- fail to excrete dietary sterols into the bile
- accumulate plant sterols in the blood and tissues
- accumulation is associated with TENDON AND SUBCUTANEOUS XANTHOMAS and a markedly increased risk of PREMATURE CHD
Type III hyperlipoproteinemia
- inherited absence of a functional apoE
- inhibition of remnant clearance by the LDL receptor and LRP
- increased TG and cholesterol-rich remnant lipoproteins in the plasma
MTP
- helps transfer TG to the VLDL core
- also required for TG transfer to chylomicrons in the intestine
abetalipoproteinemia
- mutations of MTP
- vitamin deficiency
- fat in stool
- developmental delays
ACAT
Acyl-CoA:cholesterol acyltransferase
-esterifies cholesterol to form cholesteryl esters (CE)
- dietary or endogenous cholesterol in excess of need for membrane synthesis is metabolized to CE by ACAT for storage
- important physiological role in control of cellular FC pool that serves as substrate for bile acid and steroid hormone formation
PCSK9
-A serine protease that decreases the steady-state level of expression of the LDL receptor on the hepatocyte cell membrane (binds to the EGF-A domain of LDLR)
• Promotes intracellular degradation of the LDL receptor
• LDLr/PCSK9 complex gets internalized and targeted to the lysosomal compartment for degradation
– Prevents LDLR recycling to the cell surface
– Reduces LDLR population on cell surface
– Reduces clearance of LDL from the circulation, hence increased plasma LDL levels
Lp(a)
An LDL-like particle where apoB-100 is covalently bound to apolipoprotein(a) [apo(a)]
• Apo(a) proteins vary in size due to a variable number of so-called kringle IV repeats in the LPA gene (highly homologous to plasminogen).
• There is a general inverse correlation between the size of the apo(a) isoform and the Lp(a) plasma concentration
• Apo(a) is expressed by human or non-human primate liver cells (hepatocytes), also the site of Lp(a) assembly
• The half-life of Lp(a) in the circulation is about 3 to 4 days
• Lp(a) is a risk factor for cardiovascular disease
ABCA1
ATP binding cassette transporter (ABCA1) helps release free cholesterol to apoA-I to make discoidal HDL
Membrane transporter expressed abundantly in the liver, intestine, macrophages, brain and other tissues
• Promotes efflux of cellular phospholipid and cholesterol to lipid-free apoA-I (primarily secreted from liver and intestines), but NOT to spherical HDL
• Mutations in ABCA1 result in:
– Severely reduced cholesterol efflux to apoA-I
– Markedly reduced HDL levels as poorly-lipidated nascent HDL are metabolized rapidly
apoA-I
- Primary protein component of HDL
- Synthesized in liver and intestine and is required for normal production of HDL
- Mutations in the apoA-I gene that cause HDL deficiency are variable in their clinical expression and often are associated with accelerated atherogenesis.
- Conversely, overexpression of apoA-I in transgenic mice protects against experimentally induced atherogenesis.
- ApoA-I mutations are also known to reduce the capacity of apoA-I to activate LCAT
The ABCA1-mediated transfer of phospholipid and FC to apoA-I results in the formation of nascent or discoidal HDL (pre-HDL)
• Majority of pre-HDL formation occurs at the liver and intestine (also sites of apoA-I synthesis and ABCA1 expression)
• Discoidal pre-HDL can then acquire free unesterified cholesterol from the cell membranes of tissues, such as arterial wall macrophages.
Tangier Disease
Tangier Disease results from mutations in ABCA1
Loss-of-function mutations of ABCA1 cause the defect observed in Tangier disease, a genetic disorder characterized by extremely low levels of HDL and cholesterol (CE) accumulation in the liver, spleen, tonsils, and neurons of peripheral nerves.
• Extremely rare, affecting ~ 100 people world- wide
• Near-absence of normal HDL particles since patients fail to form discoidal OR spherical HDL
• Enlarged spleens and tonsils (orange in color due to accumulation of carotenoids)
• Very low total plasma cholesterol
• ABCA1 mutations account for ~ 10% of subjects with low HDL levels
Lecithin-Cholesterol Acyl Transferase (LCAT)
Lecithin-Cholesterol Acyl Transferase (LCAT) helps form the CE core of HDL
- Secreted by liver and circulates in blood, at times, by physically associating with HDL
- After free cholesterol is acquired by the preHDL, it is esterified by LCAT
- The newly esterified and nonpolar cholesterol moves into the core of the discoidal HDL
- As the CE content increases, the HDL particle becomes spherical.
- Spherical HDL particles further enlarge by accepting more free cholesterol, which is in turn esterified by LCAT
- apoA-I activates LCAT
LCAT deficiency
-fish eye disease
Homozygotes
- corneal clouding
- nephropathy
- hemolytic anemia
- HDL deficiency
Heterozygotes
-half-normal HDL-C levels (frequently
ABCG1
ABCG1 transfers cholesterol to spherical HDL
- Expressed in spleen, thymus, lung and brain, liver and macrophages
- Promotes cholesterol efflux to HDL, and not lipid-free apoA-I
- Contributes to HDL remodeling
- Alters distribution of cholesterol on the cell membranes and allows its removal by HDL
CETP
Cholesteryl ester transfer protein (CETP) exchanges lipids between LDL and HDL
• Promotes transfer of CE from HDL to VLDL, IDL and LDL, in exchange for TG
• In humans, CETP accounts for the removal of about two-thirds of the CE associated with HDL (via LDLR-mediated endocytosis)
• Deficiency in humans is associated with increased HDL levels
– CETP deficiency in a Japanese population is associated with reduced risk for CAD
• Recent therapies have focused on inhibiting CETP to keep HDL levels high
-study: inhibiting CETP to keep HDL high….thought to target it…doubles HDL…however clinical trials had DYING patients….
HL
Hepatic lipase (HL)
• Hydrolyzes TG and PL to generate smaller, spherical HDL particles that recirculate and acquire additional free cholesterol from tissues
• Both androgens and estrogens affect HL gene expression, but with opposite effects.
– Androgens increase HL activity, which accounts for the lower HDL-C values observed in men than in women.
– Estrogens reduce HL activity, but their impact on HDL-C levels in women is substantially less than that of androgens on HDL-C levels in men.
• HL appears to have a pivotal role in regulating HDL-C levels, as HL activity is increased in many patients with low HDL-C levels.
SR-BI
Scavenger receptor BI (SR-BI) is the HDL receptor
• Expressed in liver, ovaries, testes, adrenal glands
• Overexpression of SR-BI in mice:
– HDL-CE uptake, HDL-cholesterol
– biliary cholesterol excretion
• Disruption of SR-BI in mice:
– plasma HDL-cholesterol levels
• Transgenic and knock-out mouse models confirm an athero-protective role for SR-BI
• Recently identified mutations in human SR-BI have confirmed the importance of SR-BI in maintaining plasma cholesterol levels and support a protective role of SR-BI against atherosclerosis
• The CE core is transferred to cells via SR-BI by “selective uptake”
– Only lipid is transferred to cells
– Entire HDL particle is NOT internalized
• The lipid-depleted particle can circulate back to peripheral tissues to pick up more cholesterol
• ApoA-IandApoA-IIareremovedfromplasma synchronously and that a portion of the degradation occurs in liver and in kidney.
Regulation of Cholesterol Synthesis and Transport (4)
1) Covalent modification of HMG-CoA reductase
2) Transcriptional regulation of HMG-CoA gene
3) Activation of ACAT, which increases esterification for storage
4) Transcriptional regulation of the LDL receptor
Regulation of Cholesterol Metabolism (3)
1) AMP-dependent protein kinase
- when AMP rises, kinase
phosphorylates HMG-CoA reductase –> decreased activity , decreased cholesterol synthesis
2) Glucagon, epinephrine - cascades lead to phosphorylation, decrease activity
3) Insulin
- cascades lead to dephosphorylation, increased activity
Covalent modification provides short-term regulation.
Longer-term Regulation of HMG-CoA Reductase through Transcriptional Control
• Sterol regulatory element-binding proteins (SREBPs)
– When cellular sterol levels are high, SREBP is in ER membrane with other proteins
– When cellular sterol levels decrease, complex is cleaved, moves to the nucleus
– Activates transcription of HMG-CoA reductase and LDL receptor as well as other genes
The metabolic syndrome criteria
• Criteria – requires > 3 of the 5
- Waist > 40” in male or > 35” in female
- Fasting blood sugar > 100
- BP > 135/85 or on treatment
- Triglycerides > 150
- HDL
Why do we measure and treat lipids when it’s the lipoproteins that actually cross the endothelium?
• Since 1960, medicine has studied cholesterol and triglycerides, not lipoproteins
• Huge epidemiological data and pathology show that serum level of cholesterol is causally linked to atherosclerosis
• Numerous outcome studies that show the benefit of treatment to lower cholesterol have been based on lipid values
• No outcome studies on lipoproteins
-“proxy”
Lipid panel (calculations)
• Total cholesterol and HDL cholesterol and triglycerides are measured values
• LDL and non-HDL are calculated
• LDL is calculated by the Friedewald equation
• LDL-C= TC – (HDL-C + TG/5)
• LDL-C= TC – (HDL-C + VLDL-C) (can’t measure this)
• ____= 215 – (45 + 150/5)
• Calculation is invalid when trigs > 400
• Non-HDL = TC – HDL
= sum of all potentially atherogenic cholesterol
will probably become leading marker of treatment
= cholesterol in all VLDL + VLDL remnants + LDL
Lipid panel (risks)
- LDL-C > 100 risk for ASCVD
* HDL 1000 risk for pancreatitis
