Synthesis, biological activity and mechanism of atypical carotenoid antilipoperoxidants

Lipid peroxidation has long been correlated to and proposed to be a causative factor in a myriad of human diseases. However, identifying a causative mechanistic role of lipid peroxidation has proven challenging and the role of oxidized lipids in disease pathogenesis remains enigmatic. We posited that a potent small molecule inhibitor of lipid peroxidation could be a useful probe to understand the role of lipid peroxidation in human disease. Carotenoids are an extensively studied class of polyene containing small molecules with antilipoperoxidant activity. Yet no significant progress has been made towards utilizing carotenoids either as clinically proven treatments or to definitively probe the role of lipid peroxidation in disease pathogenesis. We therefore sought to identify an exceptionally potent small molecule inhibitor of bilayer lipid peroxidation to advance our understanding of the specific role(s) this process plays in disease pathogenesis. Additionally, such a molecule could serve as a starting point for the development of small molecule replacements for missing or dysfunctional antilipoperoxidant proteins. We questioned whether environmental pressures might have driven the evolution of exceptional natural product antilipoperoxidants in microorganisms that thrive in environments of extreme oxidative stress. We specifically sought to characterize the antilipoperoxidant activity of three highly atypical carotenoids, peridinin, synechoxanthin, and chlamydaxanthin. We synthesized each of these natural products utilizing an efficient and modular building block based approach. This strategy allowed for initial activity assays as well synthesis of isotopically labeled carotenoids and scale-up for animal experiments. We found that the atypical carotenoid peridinin is an exceptionally potent inhibitor of non-enzymatic bilayer lipid peroxidation in both chemically defined liposomes and primary human endothelial cells. Mechanistic investigation showed, as compared to astaxanthin, peridinin does not act through inhibition of lipid lateral diffusion nor does it have a higher inherent rate of radical quenching activity. SSNMR experiments with a synthesized isotopologue showed that the increased potency of peridinin is due to its increased propensity to embed within the lipid bilayer. That is, in contrast to the primarily extramembranous localization of the widely used but much less potent antilipoperoxidant astaxanthin, peridinin is completely embedded within and physically spans the hydrophobic core of POPC membranes, maximizing its concentration at the site of the targeted lipid peroxidation reactions. In a final set of experiments, we utilized peridinin to probe the role of lipid peroxidation in atherosclerosis and asthma. In primary human endothelial cells, we found that small molecule-mediated inhibition of bilayer lipid peroxidation blocks stress-induced monocyte-endothelial cell adhesion, a key step in atherogenesis. In contrast, in two different mouse models of acute asthma, our preliminary results suggest that lipid peroxidation does not drive the pathogenesis of asthma. We did not observe a consistent increase in oxidized lipids in these models, nor did peridinin have any measurable effect on the asthmatic phenotype. Further studies are required to understand the biodistribution of peridinin and evaluate lipid peroxidation in clinically relevant chronic mouse models of asthma. In conclusion, we have identified peridinin as a potent and membrane embedded inhibitor of lipid peroxidation with the potential to further probe the role of lipid peroxidation in a multitude of human diseases