LDL receptor + PCSK9 axis

Category: transport

Overview

LDL receptor (LDL-R) clears circulating LDL particles by hepatic endocytosis. After internalization, LDL-R normally recycles back to the plasma membrane for hundreds of cycles. PCSK9 binds LDL-R + diverts it to lysosomal degradation, reducing surface receptor density and raising plasma LDL. Statins lower intracellular cholesterol → SREBP-2 activation → upregulates BOTH LDL-R AND PCSK9 (negating part of the LDL-R upregulation). PCSK9 inhibitors (mAb: alirocumab + evolocumab; siRNA: inclisiran with q6-monthly dosing) block PCSK9 → preserve LDL-R recycling → LDL reductions of 50-60% on top of maximal statin. Familial hypercholesterolemia: most often LDL-R loss-of-function mutations (autosomal codominant); gain-of-function PCSK9 mutations produce phenocopy.

Organ Systems

• cardiovascular
• digestive

Pathway Steps

1. circulating-ldl → hepatic-internalization — via LDL-R binding + clathrin-mediated endocytosis. Hepatic LDL receptors bind circulating LDL and internalize it, clearing cholesterol from blood — the main route lowering plasma LDL. Statins work largely by upregulating these receptors; LDLR loss-of-function mutations cause familial hypercholesterolemia with very high LDL and premature atherosclerosis.
2. ldl-receptor → recycling-or-degradation — via PCSK9 binding determines lysosomal degradation vs surface recycling. After internalization the LDL receptor normally recycles to the surface — unless PCSK9 binds it and routes it to lysosomal degradation. Blocking PCSK9 (evolocumab, alirocumab; or inclisiran siRNA) spares the receptors, dramatically lowering LDL and cutting cardiovascular events (FOURIER, ODYSSEY).

Known Modulators

• atorvastatin (activator) — SREBP-2 activation → LDL-R upregulation (statin class mechanism)
• rosuvastatin (activator) — SREBP-2 → LDL-R upregulation
• ezetimibe (inhibitor) — NPC1L1 (intestinal cholesterol absorption). reduces dietary + biliary cholesterol uptake; LDL ~20% lowering as monotherapy; IMPROVE-IT showed CV-outcome benefit added to simvastatin
• cholestyramine (inhibitor) — bile acid sequestrant (resin). binds bile acids in gut → enterohepatic interruption → hepatic LDL-R upregulation; constipation + drug-binding DDIs (separate dosing)
• colesevelam (inhibitor) — bile acid sequestrant (resin). newer resin; better-tolerated than cholestyramine; modest glycemic benefit in T2DM
• fenofibrate (activator) — PPAR-α. fibrate; ↑LPL → triglyceride lowering; ↑HDL; mild LDL effect; less myopathy with statins vs gemfibrozil (no CYP2C8 inhibition)
• gemfibrozil (activator) — PPAR-α. fibrate; CYP2C8 strong inhibitor — catastrophic cerivastatin rhabdo combo (Wave 2a v1.1) + repaglinide DDI 8×
• icosapent ethyl (activator) — EPA elevation → TG ↓ + anti-inflammatory. purified EPA ethyl ester; REDUCE-IT showed 25% CV-event reduction added to statin in high-TG patients
• bempedoic acid (inhibitor) — ATP-citrate lyase (ACLY). prodrug activated only in liver (not muscle) → no statin-myopathy mechanism; CLEAR Outcomes 2023 showed CV-outcome benefit
• evolocumab (inhibitor) — PCSK9 (extracellular). PCSK9 mAb; blocks PCSK9-mediated LDL-R degradation → ↑LDL-R recycling; FOURIER showed CV-outcome benefit; q2week SC
• alirocumab (inhibitor) — PCSK9 (extracellular). PCSK9 mAb; ODYSSEY CV outcomes; q2week SC; comparable to evolocumab
• inclisiran (inhibitor) — PCSK9 mRNA (siRNA). siRNA targeting PCSK9 transcript in hepatocytes; q6month SC dosing schedule — durable LDL lowering with minimal injections
• clofibrate (activator) — PPAR-α. first-generation fibrate; CHD signal led to phase-out; plasma-binding-displacement DDI with warfarin authored Wave 2b v1.0

References

• PMID 28304224
• PMID 30403574
• PMID 19299327