Solution-based studies on the contact between the complement receptor 3 ligand-binding domain and simvastatin

Simvastatin is a primary cholesterol-lowering medication, but it has also been reported to possess anti-inflammatory properties. Notably, the CD18 integrins are targets for simvastatin antagonism of ligand binding, which may affect leukocyte adhesion and diapedesis. Lymphocyte-associated antigen (LFA)-1 is inhibited through an allosteric mechanism by binding the lactone form of simvastatin (simvastatin-lac) to a hydrophobic pocket in the major ligand binding domain, the alpha chain I domain. By contrast, crystallographic evidence showed that complement receptor 3 (CR3) is inhibited by simvastatin in its carboxylate form (simvastatin-carbox) chelated by an Mg²⁺ ion in the α_MI metal ion-dependent adhesion site (MIDAS). We now report that the affinity (K_D) of simvastatin-carbox for the α_MI is ∼650 μM, which is significantly weaker than the 50 %-inhibitory concentration of simvastatin-lac at 50 μM. The simvastatin-carbox was incapable of inhibiting CR3 binding to iC3b, nor did it exert any neuroprotective or anti-inflammatory effects in the middle cerebral artery occlusion animal model of stroke, unlike what has been reported for simvastatin-lac. From available structural data on the CR3 ligand binding domain in complex with C3d, we suggest that simvastatin-lac makes a critical ternary complex with the ligand binding domain and its ligand before engaging, in its carboxylate form, the MIDAS. In this way, both the LFA-1 and CR3 are antagonized by simvastatin-lac but through fundamentally different mechanisms.