Lecture 13: Cholesterol Biosynthesis Regulation

How will we regulate de novo synthesis?

Reseach found...
- Endogenous synthesis of cholesterol reduced by increase in diet cholesterol

HMG-CoA Reductase = obvious regulation target (is RLS of cholesterol synthesis); can be regulated @ transcriptional and post-transcriptional levels
- Transcriptional: LONG-TERM
- Post-transcriptional: Short (phosphorylation) and long (degradation) term regulations
RECALL: this enzyme catalyzes HMG-CoA -> mevalonate
step 1)Phosphorylate at Serine by AMPK
- AMPK also phosphorylates ACC and inhib's Enzyme activity and inhib's synthesis of FAs
- AMPK = energy sensor (need 36 ATP and 16 NADPH per cholesterol)
step 2)Can dephosphorylate by PP2A (insulin-dependent)

Long-term: Controlled proteolysis
- N-terminal domain of the reductase = "cholesterol sensor"
- Sterol up, HMG-CoA reductase degraded rapidly

INSIG: Insulin-induced genes involved w/ ubiquitination complex
- HMG-CoA ubiquitinated/released from ER membrane/degraded by proteosome

Long-term: Promoter region for reductase gene has cis element SRE (sterol-responsive element)
- SREBP (SRE-binding protein) binds trans and activates HMG-CoA reductase transcription
- SREBP availability depends on cholesterol concentration: HIGH cholesterol = NO SREBP binding
- SREBP belongs to bHLH family
- Mammalian SREBP encoded by genes SREBP-1 and SREBP-2; regulates expression involved in chole metab

SREBP-2 chills in ER membrane, gets into the nucleus by

LOW CHOLESTEROL? INSIG degraded and liberate SCAP
- SCAP brings SREBP to Golgi cuz SCAP's the one that detects cholesterol

HIGH CHOLESTEROL? INSIG expressed, SCAP conformation altered, so it doesn't help escort SREBP

SREBP = ER protein and exits ER in low cholesterol situation
- Escort = SCAP which has the cholesterol sensing domain
SREBP cleaved by endopeptidases in Golgi, n-terminal SRE binding domain released

Lots of expression in liver, stim'd by SREBP
- Forms complex w/ SCAP to stop SCAP/SREBP exit when high cholesterol

Genes regulated by SREBP-2 in low cholesterol conditions

LDL Receptor Expression
Controlled by SREBP-2
LDL-1 = type-1 or type-2 membrane protein, one transmembrane domain
N faces outside plasma membrane in type I
N faces inside in type II

Ligand = ApoB100 associated w/ LDL

LDL to LDLR starts endocytosis; the receptors are recycled back to the plasma membrane

SREBP-2 controls PCSK9 (secreted protein that binds to extracellular part of LDL receptors
- With PCSK9, endocytosis still happens but instead @ end LDLR gets degraded instead of recycled


Familial hypercholesterolemia: autosomal co-dominant disorder with increased [LDL] and Atherosclerosis
- Xanthoma on eyelids and on skin (basically deposits of cholesterol-rich materials)
- CAUSE: LOF mutation in LDLR gene and this is frequent (1/500)
1) LDL-null: can't synthesize protein
2) ER/Golgi trafficking: LDLR doesn't get taken to plasma membrane from ER
3) LDLR Can't bind to ligand ApoB100
4) Internalization: can't internalize LDLR/LDL complex
5) Recycling: LDLR/LDL can't dissociate again within sorting endosome

1) dietary intake: lower intake
2) Endogenous rate of synthesis (HMG-CoA reductase): inhibit this
3) Cholesterol rate of use in cells: Inhibit PCSK9, increase cholesterol uptake by liver

STATIN = reduces de novo synthesis 
- more SREBP-2, produce more LDLR, clear extracellular cholesterol from blood, lower overall
- Can't remove all LDL, reach a saturation point; PCSK9 is working, LDLR receptors degrading 

GOF PCSK9: decreases LDLR recycling, increases circulating LDL
LOF PCSK9: no effect on LDLR recycling, decrease in circulating LDL