Cholesterol Synthesis and Transport

Cholesterol synthesis and transport describes how cells make cholesterol through the mevalonate pathway, how the supply is regulated by feedback, and how cholesterol moves between the liver and peripheral tissues aboard lipoproteins. Brown and Goldstein's receptor-mediated pathway is the central organising principle, explaining how the LDL receptor and sterol-sensing machinery keep cellular and whole-body cholesterol in balance.

Definition

Cholesterol synthesis and transport is the set of regulated pathways by which cells synthesise cholesterol from acetyl-CoA through the mevalonate pathway and by which cholesterol is distributed among tissues via lipoproteins and the LDL receptor, with feedback control maintaining cholesterol homeostasis.

Scope

The topic covers de novo cholesterol biosynthesis from acetyl-CoA via HMG-CoA reductase, transcriptional control by sterol regulatory element-binding proteins (SREBPs), receptor-mediated uptake of LDL, and reverse cholesterol transport by HDL. It is a biochemical and physiological reference topic and does not give diagnostic or therapeutic instructions.

Core questions

• How is cholesterol synthesised from acetyl-CoA and where is the pathway controlled?
• How does the LDL receptor mediate cellular cholesterol uptake?
• How do SREBPs sense and regulate sterol supply?
• How does reverse cholesterol transport return cholesterol to the liver?

Key concepts

• Mevalonate pathway
• HMG-CoA reductase
• LDL receptor
• SREBP and sterol sensing
• Reverse cholesterol transport
• Cholesterol homeostasis

Key theories

Receptor-mediated pathway for cholesterol homeostasis

Brown and Goldstein showed that cells take up LDL-borne cholesterol through the LDL receptor, and that intracellular sterol levels feed back to control receptor number and de novo synthesis, keeping cholesterol balanced.

SREBP transcriptional control of lipid synthesis

Sterol regulatory element-binding proteins act as membrane-bound transcription factors that are proteolytically activated when sterols are low, switching on the genes for cholesterol and fatty-acid synthesis.

Mechanisms

Cholesterol is built from acetyl-CoA through the mevalonate pathway, with HMG-CoA reductase as the committed, rate-limiting step. When cellular sterol content falls, SREBPs are released from the endoplasmic reticulum membrane and transported to the nucleus, where they activate genes for cholesterol synthesis and for the LDL receptor. The LDL receptor then binds circulating LDL and brings it into the cell by endocytosis, delivering cholesterol and suppressing further synthesis — the feedback loop characterised by Brown and Goldstein. Excess peripheral cholesterol is removed by HDL-mediated reverse cholesterol transport back to the liver for excretion.

Clinical relevance

Understanding these pathways explains why the LDL receptor is central to cholesterol-lowering biology and why elevated LDL cholesterol contributes to atherosclerosis. The content is provided for reference and conceptual understanding and does not constitute individualised medical advice.

History

The cholesterol biosynthetic pathway was elucidated across the mid-twentieth century, with Konrad Bloch's work on its many steps recognised by the 1964 Nobel Prize. Brown and Goldstein's discovery of the LDL receptor and the feedback pathway in the 1970s and 1980s provided the regulatory framework, and later work identified SREBPs as the transcriptional switch coordinating sterol supply.

Key figures

• Michael Brown
• Joseph Goldstein
• Jay Horton
• Konrad Bloch

Seminal works

• brown-goldstein-1986
• horton-2002

Frequently asked questions

What is the rate-limiting step of cholesterol synthesis?

HMG-CoA reductase catalyses the committed, rate-limiting step of the mevalonate pathway, and its activity is tightly regulated by cellular sterol levels.

What does the LDL receptor do?

The LDL receptor binds circulating LDL particles and brings them into cells, supplying cholesterol while providing feedback that turns down further synthesis and receptor production.