Regulation of Cholesterol Biosynthesis Flashcards
2 Regulations of cholesterol through de novo synthesis
- Feeding studies have shown that endogenous synthesis of cholesterol was reduced by an increase in diet cholesterol.
- HMG-CoA reductase was identified as an obvious target for regulation.
Rate limiting enzyme of cholesterol synthesis
-HMG-CoA reductase can be regulated at the transcriptional and post-transcriptional levels:
a) Transcriptional control: long term regulation
b) Post-transcriptional control: short (phosphorylation) and long (degradation) term regulations
Where is HMG-CoA Reductase found
Hmg-coa reductase is in the membrane facing the outside of the er lumen
Short-term regulation of HMG-CoA reductase (inhibition) Post Transcript
1) In human, HMG-CoA reductase can be phosphorylated at Serine 872 (Ser872) by AMP-activated protein kinase (AMPK).
-Phosphorylation of Ser872 completely abolish HMG-CoA reductase activity.
2) Remember that AMPK can also phosphorylate ACC (acetyl-CoA Carboxylase) leading to an inhibition of the enzyme activity and consequently, inhibits the synthesis of fatty acids (FAs).
3) The AMPK system act as an energy sensor.
-Remember that we need 36 ATP and 26 NADPH to produce one molecule of cholesterol
EXAM QUESTION: Please describe a mechanism that can shut down the production of fatty acid and cholesterol at the same time
AMPK does that because it acts as an energy sensor and it is too costly to produce cholesterol and fatty acids (so don’t produce these molecules when were starving, AMPK lets us know if the energy is right for us to make these high energy molecules)
Short-term regulation of HMG-CoA reductase (activation) Post Transcipt
1) The phosphorylated HMG-CoA reductase (Ser872) can be dephosphorylated by Protein phosphatase 2A (PP2A).
-Dephosphorylation of Ser872 reactivates HMG-CoA reductase activity.
2) Remember that PP2A can also dephosphorylate ACC (acetyl-CoA Carboxylase) leading to an activation of the enzyme activity and consequently, promotes the synthesis of fatty acids (FAs).
3) PP2A activity can be regulated by insulin and glucagon/epinephrine.
-Insulin: lots of glucose = lots of ATP = phosphorylation = cholesterol synthesis takes place
-Glucagon/Epinephrine: opposite effect of insulin = inhibits cholesterol synthesis
Long-term regulation of HMG-CoA reductase (Post-transcript)
1) HMG-CoA reductase can be regulated by controlled proteolysis.
2) N-terminal transmembrane domain of HMG-CoA reductase acts as a “cholesterol sensor”
-When sterol accumulates in in the ER membrane, HMG-CoA reductase is degraded rapidly (T1/2<1hr, from 12h).
-Membrane sterol binds to HMG-CoA reductase.
-This binding leads to the recruitment of Insigs proteins which are associated to a ubiquitination complex.
-Insigs: Insulin-Induced Genes
3) HMG-CoA reductase is ubiquitinated, released from the ER membrane and degraded by the proteosome.
Transcriptional regulation of gene expression
Long-term transcriptional regulation of HMG-CoA reductase process
-The promoter region of the HMG-CoA reductase gene contains a cis element termed SRE (sterol-regulatory element or sterol-responsive element)
-Binding of the trans element SREBP (SRE binding protein) to SRE activates transcription of HMG-CoA reductase
-Availability of SREBP is governed by cholesterol concentrations within the cells
Regulation of HMG-CoA reductase gene expression by levels of cholesterol.
What is SREBP
SREBP is a transcription factor that belongs to the basic helix-loop-helix-leucine-zipper DNA binding protein family (bHLH)
Mammalian SREBP is encoded by
- SREBP-1
- SREBP-2
SREBP-2 is
-SREBP-2 is responsible for regulating expression of genes involved in cholesterol metabolism
-SREBP-2 expression is ubiquitious.
Sterol regulatory element binding protein (SREBP) structure
basic helix-loop-helix-leucine-zipper DNA binding protein family (bHLH) (dimer)
Explain the schematic representation of SREBP-2 protein
NOTE: TM = transmembrane domain
SREBP-2 is found where
ER
Fate of SREBP-2: low cholesterol levels (Step 1)
INSIG gets degraded when cholesterol concentration is low
Fate of SREBP-2: low cholesterol levels (Step 2)
SCAP escorts SREBP to Golgi when cholesterol level is low
Fate of SREBP-2: low cholesterol levels (Step 2)
SCAP escorts SREBP to Golgi when cholesterol level is low
Fate of SREBP-2: low cholesterol levels
Cleavage of SREBP-2 by endopeptidases SP1 and SP2 located in golgi apparatus
Fate of SREBP-2: High cholesterol levels
NOTE: Insig binds tightly to SREBP and SCAP
SREBP-2 is a
SREBP-2 is a master switch, because it regulates every enzyme involved in cholesterol metabolism
SREBP-2 increases
LDL receptors = more free LDL in cell picked up to make cholesterol
LDL receptor expression is increased by
NOTE: Low cholesterol in liver = activate SREBP = activated LDL receptor and PCSK9 expression = increase cholesterol content in liver
-This will work in peripheral tissue as well because LDL and SREBP is found in all tissues
LDLR 4 characteristics
1) LDLR is a type-I membrane protein
2) A type-I or type-II membrane protein has one transmembrane (TM) domain
T3) he N-terminus of type-I protein is oriented facing the outside of plasma membrane
4) The N-terminus of type-II protein is facing the inside of plasma membrane
The ligand for LDLR is [blank] and is located [blank]
-The ligand for LDLR is the LDL-bound apoB100.
-The ligand-binding domain is located in the extracellular portion of LDLR
Explain endocytosis of LDL
1) Binding of LDL to the LDLR results in endocytosis (Endocytosis of LDL is mediated by LDLR)
2) LDLs are degraded into lipids and amino acids
3) LDLRs are recycled back to the plasma membrane
SREBP-2 also increases the expression of
PCSK9
What is the role of PCSK9
Will Prevent LDLR from being recycled, will send to lysosome to degrade it
SUMMARY: PCSK9 makes sure that the LDL process does not go overboard (makes sure it only happens for a fixed amount of time and don’t produce too much cholesterol)
The Cholesterol 7a-hydroxylase catalyzes the
rate-limiting step in the synthesis of bile acid
2 Characteristics of cholesterol 7a-hydroxylase catalyzes the rate-limiting step in the synthesis of bile acid
1) The expression of cholesterol 7a-hydroxylase in the liver is upregulated when levels of cholesterol are high
-Through the action of the LXR (liver X receptor) nuclear receptor
2) The expression of cholesterol 7a-hydroxylase in the liver is downregulated when levels of cholesterol are low.
-This inhibition in gene transcription could be attributed to the action of SREBP1. (not 100% certain)
Explain the Storage of cholesterol ester under high cholesterol conditions
1) When concentrations of cholesterol are high, SREBP2 is tightly bound to the ER membrane.
-No expression of SREBP2 target genes, mainly HMGCR and LDLR.
2) However, an increased level of cholesterol in the ER membrane, which is a good indicator of high concentration of cholesterol in the cell, leads to an increased in Acyl-CoA cholesterol Acyl Transferase (ACAT) activity.
-This leads to storage of cholesterol in the form of cholesterol esters (CE).
