Dual Control of LDL-cholesterol Levels by ANGPTL3 and ANGPTL8

This study demonstrates that while ANGPTL3 inactivation lowers lipids independently of LDL receptor secretion, the proteolytic cleavage of ANGPTL3 and the presence of ANGPTL8 are essential for differentially inhibiting lipases to further reduce LDL-cholesterol, suggesting that dual-targeted therapies could offer superior cardiovascular benefits.

The Gist

The Big Picture: Two Traffic Cops and a Highway

Imagine your bloodstream is a busy highway. On this highway, there are delivery trucks called VLDL (Very Low-Density Lipoproteins). These trucks carry triglycerides (fats) and cholesterol to different parts of your body.

If there are too many trucks or they move too slowly, traffic jams occur, leading to clogged arteries (heart disease). To keep traffic flowing, your body has "traffic cops" (enzymes) that break down the fat in these trucks so they can unload their cargo and disappear.

The main traffic cops are:

1. LPL (Lipoprotein Lipase): The "Fat Breaker." It clears triglycerides.
2. EL (Endothelial Lipase): The "Cholesterol Remodeler." It changes the shape of the trucks, helping clear cholesterol.

The problem is that your body has two "bad guys" (proteins) that tie up these traffic cops, stopping them from working. These bad guys are ANGPTL3 (A3) and ANGPTL8 (A8). When they are active, traffic jams happen, and cholesterol levels rise.

The Old Story vs. The New Discovery

The Old Story:
Scientists knew that if you turned off A3, the traffic cops could work again, and cholesterol levels would drop. This is why a drug called evinacumab was developed to block A3. However, the drugs weren't perfect. Sometimes they didn't lower cholesterol enough, and scientists didn't fully understand why or how A3 worked. They also didn't know if A3 needed a partner to do its job.

The New Discovery (This Paper):
This study, conducted on mice, reveals that A3 and A8 are a dynamic duo, but they work differently depending on the job.

1. The "Scissors" Effect (A3 Cutting)

Think of A3 as a long rope. In your body, a pair of molecular "scissors" (an enzyme called furin) often cuts this rope in half.

• The Full Rope (A3-FL): The whole protein.
• The Cut Rope (A3-Nter): The top half of the protein.

The researchers found that to stop the Fat Breaker (LPL) effectively, you need both the full rope and the cut rope working together, along with their partner A8. It's like a three-person tug-of-war team; if you miss one person, the rope doesn't get pulled tight enough to stop the fat.

However, to stop the Cholesterol Remodeler (EL), you don't need the rope to be cut. You just need the partner A8 to be there.

2. The Secret Weapon: A8

The study found a surprising twist: A8 is actually a major player in lowering cholesterol, not just triglycerides.

• When the researchers turned off A8 in mice, cholesterol levels dropped by about 20%.
• But here's the catch: A8 can only lower cholesterol if A3 is present to help it. A8 is like a specialized tool, but it needs A3 to hold the handle. Without A3, A8 is useless for cholesterol.

3. The "Secret Exit" Mystery

One of the most interesting findings is how the cholesterol gets cleared.
Usually, when you lower cholesterol, you think the liver is just making fewer delivery trucks. But this study proved that A3 doesn't stop the liver from making trucks. The liver keeps pumping them out at the same rate.

Instead, when A3 is blocked, the trucks are cleared out of the highway much faster by a "secret exit" that we don't fully understand yet. It's as if the traffic cops (LPL and EL) are working so efficiently that they strip the trucks of their cargo so quickly that the trucks vanish before they can cause a jam, even though the factory (the liver) is still churning them out.

The "Dual Control" Strategy

The paper concludes that to get the best results for heart health, we shouldn't just target A3. We should target both A3 and A8.

• Current Strategy: Trying to block A3 alone is like trying to stop a car by pulling the driver out, but the car still has a passenger (A8) who can still drive it a little bit.
• New Strategy: Blocking both A3 and A8 is like removing both the driver and the passenger. This ensures the traffic cops (LPL and EL) are completely free to clear the highway.

Why This Matters for You

1. Better Drugs: Current drugs that target A3 are good, but they might not be strong enough for everyone. This research suggests that drugs designed to block both A3 and A8 (or disrupt their partnership) could lower cholesterol even more effectively.
2. Safety: Some older drugs caused liver problems. By understanding exactly how these proteins work, scientists can design better drugs that lower cholesterol without hurting the liver.
3. The "Cut" Matters: The study shows that the way A3 is chopped up in your body matters. Future therapies might need to ensure they stop all forms of A3 (both the full version and the cut version) to get the maximum benefit.

In a Nutshell

Think of your blood cholesterol like a crowded dance floor. A3 and A8 are the bouncers who stop the dancers (lipases) from clearing the floor.

• To clear the floor of fat, you need to fire the bouncers and make sure the rope they are holding is cut in a specific way.
• To clear the floor of cholesterol, you just need to fire the bouncers and their partner A8.
• The best way to keep the dance floor empty is to fire both bouncers and their partner. This "dual control" approach could lead to the next generation of heart-healthy medicines.

Technical Summary

1. Problem Statement

Angiopoietin-like protein 3 (ANGPTL3 or A3) is a validated therapeutic target for lowering plasma lipids and reducing atherosclerotic cardiovascular disease (ASCVD) risk. However, current pharmacological approaches (monoclonal antibodies, antisense oligonucleotides, siRNA) have failed to achieve the magnitude of lipid lowering seen in individuals with complete genetic loss-of-function of A3. Key gaps in understanding include:

• Mechanism of Action: The specific contribution of different circulating forms of A3 (full-length A3-FL, N-terminal fragment A3-Nter, and C-terminal fragment A3-Cter) to lipid metabolism is unclear.
• Role of A8: The paralog ANGPTL8 (A8) is known to potentiate A3-mediated inhibition of Lipoprotein Lipase (LPL), but its specific role in inhibiting Endothelial Lipase (EL) and its necessity for cholesterol lowering are poorly defined.
• Therapeutic Efficacy: It is unknown whether the failure of current therapies to lower LDL-cholesterol (LDL-C) sufficiently is due to incomplete suppression of A3 or a failure to target the A3/A8 complex effectively.

2. Methodology

The authors employed a multi-faceted approach combining in vivo genetic models, ex vivo liver perfusion, and in vitro structural biology:

• Genetic Mouse Models:
  • Generated Ldlr-/- mice (to accentuate lipid phenotypes) expressing specific A3 isoforms:
    • A3-FL only: Mice with furin site mutations (R221A, R224A) preventing cleavage.
    • A3-Nter only: Mice with a premature stop codon (residues 1-224).
    • A8-deficient: CRISPR-Cas9 generated Ldlr-/-;A8-/- mice.
    • Combinations: Crosses to create mice expressing various combinations of A3-FL, A3-Nter, and A8.
  • Used AAV-mediated overexpression to reconstitute A8 or introduce mutagenized A8 variants.
• Ex Vivo Liver Perfusion:
  • Performed liver perfusions on wildtype and A3-/- mice (with and without Ldlr-/- background) to directly measure the secretion rates of VLDL-ApoB and VLDL-triglycerides (TG), isolating secretion from clearance mechanisms.
• Biochemical Assays:
  • Size-Exclusion Chromatography (SEC) & Native Gels: To characterize the size and composition (A3-FL, A3-Nter, A8) of circulating protein complexes.
  • Lipase Activity Assays: Measured post-heparin LPL and Endothelial Lipase (EL/PLA1) activities in plasma fractions.
  • In Vitro Inhibition: Recombinant A3 and A3/A8 complexes were tested against purified EL activity.
• Structural Modeling:
  • Used AlphaFold to model the interaction interfaces between EL and A3 homotrimers (A3₃) versus A3/A8 heterotrimers (A3₂A8).
  • Validated models via site-directed mutagenesis of A8 (introducing A3-like residues) and testing functional outcomes in mice.

3. Key Contributions & Results

A. A3 Inactivation Does Not Reduce Hepatic VLDL Secretion

• Finding: Liver perfusion studies demonstrated that A3 inactivation does not alter the rate of hepatic secretion of VLDL-ApoB or VLDL-TG, nor does it change the lipid-to-protein ratio of secreted particles.
• Implication: The lipid-lowering effect of A3 deficiency is driven entirely by enhanced peripheral clearance (lipolysis) and/or hepatic uptake of existing particles, not by reduced production.

B. Distinct Requirements for LPL vs. EL Inhibition

The study revealed that the inhibition of the two major intravascular lipases (LPL and EL) relies on different molecular configurations of A3 and A8:

1. LPL Inhibition (Triglyceride Metabolism):

   • Requirement: Maximal LPL inhibition requires the co-expression of A3-FL, A3-Nter, and A8.
   • Evidence: Mice expressing only A3-FL or only A3-Nter showed intermediate TG levels. Only mice expressing both isoforms (mimicking wildtype cleavage) achieved full LPL inhibition comparable to wildtype.
   • Complex Formation: Circulating A3 exists in multiple complexes. A specific ~242 kDa complex containing A3-FL, A3-Nter, and A8 was identified as the potent LPL inhibitor.

2. EL Inhibition (Cholesterol Metabolism):

   • Requirement: Optimal EL inhibition requires A8, but does not require A3 cleavage (i.e., A3-FL alone is sufficient if A8 is present).
   • Evidence: A3-FL-only mice inhibited EL as effectively as wildtype mice. However, in A8-/- mice, EL activity increased significantly, leading to higher LDL-C levels.
   • Mechanism: A8 enhances the potency of A3-mediated EL inhibition. In vitro assays showed the A3/A8 complex inhibits EL ~20% more effectively than A3 alone.

C. A8 Inactivation Lowers LDL-C via an A3/EL-Dependent Pathway

• Finding: Inactivation of A8 in Ldlr-/- mice lowered plasma LDL-C by ~20%.
• Dependency: This effect was abolished in mice lacking A3 (Ldlr-/-;A3-/-;A8-/-) or EL (Ldlr-/-;A8-/-;Lipg-/-).
• Conclusion: A8 contributes to cholesterol lowering by facilitating A3-mediated inhibition of EL. Without A8, A3 cannot effectively inhibit EL, leading to increased LDL-C.

D. Structural Basis for Enhanced Inhibition

• AlphaFold Modeling: The A3₂A8 heterotrimer bound to EL showed a high-confidence interface where the N-terminal helix of A8 sits adjacent to the EL active site, engaging catalytic residues.
• Contrast: In the A3 homotrimer (A3₃), the corresponding helix was displaced laterally, resulting in weaker contact probabilities.
• Validation: Mutating A8 to resemble A3 (H40N, A42L, Q47H) abolished its ability to elevate cholesterol and TG, confirming that specific A8 residues are critical for the interaction with EL and LPL.

4. Significance and Clinical Implications

• Refined Therapeutic Strategy: The study challenges the notion that simply lowering total A3 levels is sufficient. It suggests that dual inhibition of A3 and A8 (or disrupting the A3/A8 complex) is a superior strategy.
  • Current A3-targeted therapies (e.g., evinacumab) may fail to fully disrupt the A3/A8 complex or achieve the necessary suppression of the specific active species.
  • A combination therapy targeting both proteins could achieve the robust LDL-C and TG lowering seen in genetic deficiency.
• Mechanistic Insight: The paper clarifies that A3 cleavage is a regulatory mechanism specifically tuned for LPL inhibition (TG metabolism), while A8 is the critical cofactor for EL inhibition (LDL-C metabolism).
• New Pathway Identification: The findings reinforce that A3/A8 lowers LDL-C independently of the LDL receptor (LDLR), LRP1, or ApoE, pointing to a distinct hepatic clearance pathway for ApoB-containing lipoproteins that is accelerated when EL is inhibited.
• Future Directions: The identification of the A3/A8-EL interface provides a structural blueprint for designing small molecules or antibodies that specifically disrupt this interaction to treat dyslipidemia and ASCVD with potentially greater efficacy and safety than current monotherapies.