A patient takes a statin, which inhibits HMG-CoA reductase. Serum LDL cholesterol falls significantly. What is the primary mechanism by which statins lower blood LDL levels?
AStatins directly block LDL particles from binding their receptors in the bloodstream
BInhibiting HMG-CoA reductase lowers intracellular cholesterol, activating SREBP, which upregulates LDL receptor expression, pulling more LDL from the blood
CStatins stimulate bile acid synthesis, converting more cholesterol to bile acids that are then excreted
DStatins block ACAT, preventing cholesterol esterification and forcing cells to export excess cholesterol as LDL
The mechanism is indirect and counterintuitive: statins inhibit synthesis, which lowers intracellular cholesterol, which activates the SCAP-SREBP pathway, which upregulates both HMG-CoA reductase and LDL receptor expression. The LDL receptor upregulation is the key effect — more receptors on cell surfaces capture more LDL from the blood. Statins lower serum LDL not by blocking it directly, but by making cells more hungry for it through the feedback regulatory system.
Question 2 Multiple Choice
Under normal cellular conditions, SCAP is bound to Insig in the ER and the SREBP pathway is inactive. What change triggers SREBP activation and movement to the Golgi?
AHigh intracellular cholesterol causes SCAP to release Insig and escort SREBP to the Golgi
BLow intracellular cholesterol causes SCAP to release Insig, allowing the SCAP-SREBP complex to travel to the Golgi
CSREBP is cleaved directly in the ER by Site-1 protease when cholesterol falls below a threshold
DLDL receptor activation signals back to the nucleus to release SREBP from its precursor form
The system is a negative feedback loop: low cholesterol releases the brake. When cholesterol in the ER membrane drops, cholesterol dissociates from SCAP's sterol-sensing domain, SCAP releases Insig, and the SCAP-SREBP complex travels to the Golgi. There, Site-1 and Site-2 proteases cleave SREBP, releasing its active transcription factor fragment. High cholesterol does the opposite — it locks SCAP-SREBP in the ER by promoting the SCAP-Insig interaction.
Question 3 True / False
When intracellular cholesterol is low, SREBP activation upregulates both HMG-CoA reductase (increasing synthesis) and LDL receptor expression (increasing uptake).
TTrue
FFalse
Answer: True
SREBP is a transcription factor that drives expression of multiple genes in the cholesterol homeostasis network, including HMG-CoA reductase (the rate-limiting synthesis enzyme) and the LDL receptor (which imports cholesterol from the blood). This dual upregulation makes the response to low cholesterol more powerful: the cell simultaneously increases its own production and its ability to scavenge cholesterol from circulation.
Question 4 True / False
Statins lower blood cholesterol primarily by blocking intestinal absorption of dietary cholesterol.
TTrue
FFalse
Answer: False
Statins inhibit HMG-CoA reductase, the rate-limiting enzyme in the mevalonate pathway of cholesterol *synthesis* — not absorption. Drugs that block intestinal absorption (such as ezetimibe) work by a different mechanism. Statins reduce intracellular cholesterol in liver cells, which activates SREBP, which upregulates LDL receptors, which then clear LDL from the blood. The serum LDL reduction comes primarily from increased receptor-mediated uptake, not from blocking absorption.
Question 5 Short Answer
Explain how the SCAP-SREBP-Insig system functions as a feedback sensor for cholesterol homeostasis.
Think about your answer, then reveal below.
Model answer: SCAP has a sterol-sensing domain that directly binds cholesterol. When cholesterol is abundant, cholesterol-bound SCAP is retained in the ER by interaction with Insig, keeping SREBP inactive. When cholesterol is scarce, SCAP releases Insig, escorts SREBP to the Golgi, where proteases cleave it and release its active transcription factor fragment. This fragment enters the nucleus and upregulates cholesterol synthesis and LDL receptor genes. High cholesterol thus suppresses the pathway that makes more cholesterol — a classic negative feedback loop.
The elegance of this system is that cholesterol itself is the signal: it acts directly on SCAP's sterol-sensing domain, with no intermediary second messengers needed. This allows the cell to respond immediately to changes in membrane cholesterol levels. The system is also graded — partial cholesterol depletion produces partial SREBP activation — allowing fine-tuned homeostatic control rather than an all-or-nothing switch.