Modified low density lipoproteins differentially bind and activate the C1 complex of complement

Several studies suggest that complement plays an important role in atherogenesis. To further investigate this question, we have studied the ability of native and modified forms of low density lipoprotein (LDL) to bind and activate C I, the complex that triggers the classical pathway of complement. For this purpose, LDL was both obtained commercially and purified according to an established procedure, and oxidized (oxLDL) and enzymatically modified (E-LDL) derivatives were generated from each preparation. Whereas the unmodified LDL and oxLDL samples did not activate C I in the presence of excess C1 inhibitor, the E-LDL derivatives obtained by sequential treatment of LDL with a protease and then with cholesterol esterase triggered efficient C I activation under these conditions, with activation levels similar to 60% upon incubation with I mu M E-LDL for 90 min at 37 degrees C. In agreement with these findings, as shown by surface plasmon resonance spectroscopy (SPR), the C1q recognition subunit of C I showed no interaction with unmodified LDL but bound to both E-LDL samples with high affinity (K-D = 58-75 nM). More unexpectedly, although they did not trigger direct C I activation, both oxLDL samples were also efficiently recognized by C1q. Whereas the oxLDL derivative of commercial LDL activated C1 to a significant extent (similar to 30%) in the presence of C-reactive protein (CRP), much lower activation levels (< 10%) were obtained using the oxLDL derivative of purified LDL. As measured by SPR, CRP bound equally well to the oxLDL and E-LDL derivatives obtained from purified LDL. These data provide the first experimental evidence that E-LDL triggers efficient C I activation under conditions close to the physiological situation, suggesting that activation of the classical complement pathway by this derivative may be a crucial factor in the pathogenesis of atherosclerosis. In contrast, it appears unlikely that oxLDL significantly activates C1 directly or in a CRP-dependent manner in vivo. (c) 2006 Elsevier Ltd. All rights reserved.