A neutralizing APOA5 monoclonal antibody reduces amounts of lipoprotein lipase in capillaries and triggers hypertriglyceridemia

Apolipoprotein AV (APOA5) regulates intravascular triglyceride metabolism by binding to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppressing its ability to unfold the native conformation of lipoprotein lipase (LPL). LPL unfolding results in loss of catalytic activity and the detachment of LPL from the surface of cells. An <i>APOA5</i> truncation mutation (identified in two patients with hypertriglyceridemia) had suggested that the last 35 amino acids of APOA5 are important for its function. We reasoned that a monoclonal antibody (mAb) against carboxyl-terminal sequences in APOA5 could clarify functionally important amino acid residues in APOA5 and assist in elucidating the mechanism by which APOA5 regulates plasma triglyceride metabolism. Because carboxyl-terminal APOA5 sequences are evolutionarily conserved, we began by screening a human Fab bacteriophage library for binders of carboxyl-terminal APOA5 sequences. We identified one such binder and used phage DNA sequences to build a chimeric IgG₁ mAb (IBA707) against APOA5. The binding of IBA707 to APOA5 was abolished by nonconservative amino acid substitutions in conserved sequences (residues L337-I348) within a C-terminal α-helix in APOA5. The same substitutions disrupted APOA5's ability to bind and inhibit ANGPTL3/8 activity. IBA707-mediated blockade of APOA5 function reduced intracapillary LPL levels and triggered elevated plasma levels of triglycerides and ANGPTL3/8 in both fasted and refed mice. IBA707 was cleared rapidly from the plasma in <i>Apoa5</i>^+/+ mice but slowly in <i>Apoa5</i>^-/- mice. Our studies identified functionally important amino acids in APOA5 and revealed that APOA5 controls plasma triglyceride metabolism in part by modulating plasma levels of ANGPTL3/8.