ApoC-III helical structure determines its ability to bind plasma lipoproteins and inhibit Lipoprotein Lipase-mediated triglyceride lipolysis

In humans, apolipoprotein C-III (apoC-III) plasma levels have been associated with increased risk of cardiovascular disease. This association is in part explained by the effects of apoC-III on triglyceride (TG) metabolism; apoC-III raises plasma TG by increasing very low density lipoprotein (VLDL) secretion, inhibiting lipoprotein lipase (LPL)-mediated TG lipolysis, and impairing the removal of triglyceride-rich lipoprotein (TRL) remnants from the circulation. In this study, we explored the structure-function relationship the interaction of apoC-III with plasma lipoproteins and its ultimate impact on LPL activity. The structural and functional properties of wild-type (WT) apoC-III were compared with two missense variants previously associated with lower (A23T) and higher (Q38K) plasma TG. ApoC-III in the lipid-free state is unstructured but its helix content and stability increases when bound to lipid. Lipid-bound apoC-III contains two alpha helices spanning residues amino acids 11 - 38 (helix 1) and 44 – 64 (helix 2). Investigation of the structural and functional consequences of the A23T and Q38K variants showed that these amino acid substitutions within helix 1 do not significantly alter the stability of the helical structure but affect its hydrophilic-lipophilic properties. The A23T substitution impairs lipoprotein binding capacity, reduces LPL inhibition, and ultimately leads to lower plasma TG levels. Conversely, the Q to K substitution at position 38 enhances the lipid affinity of helix 1, increases TRL binding capacity and LPL inhibition, and is associated with hypertriglyceridemia. This study indicates that structural modifications that perturb the hydrophilic/lipophilic properties of the alpha helices can modulate the hypertriglyceridemic effects of apoC-III.