Structure-activity relationships of methoxychalcones as inducers of heme oxygenase-1

Chalcones are naturally occurring flavonoids composed of two aromatic rings connected by a three-carbon unit forming an alpha-beta unsaturated carbonyl group. They are pharmacologically relevant because of their ability to exert anticarcinogenic, antimicrobial, and anti-inflammatory activities. Recent evidence indicates that the bioactivity of hydroxy-chalcones is correlated with their intrinsic property to induce the antioxidant and cytoprotective enzyme heme oxygenase-1 (HO-1). In the present study, we assessed how the methoxy substituents positioned on the two aromatic rings affect the anti-inflammatory action of different chalcones in relation to their ability to increase heme oxygenase in RAW246.7 macrophages. Structure-activity relationships of methoxychlacones were qualitatively and quantitatively examined and correlated with inhibition of endotoxin-mediated nitrite production and cytotoxic effects. Our data indicate that (i) a progressive increase in heme oxygenase activity is obtained by sequentially increasing the number of methoxy substituents in the 3,4,5- and 3',4',5'-positions of the aromatic rings; (ii) methoxy substituents placed either in the 2,4,6-positions or alone in the 4- or 4'-position are ineffective; (iii) increased heme oxygenase activity by chalcones is lost when the alpha-beta double bond and the carbonyl group are reduced or protected; (iv) the anti-inflammatory activity and cytotoxicity profiles of the chalcones examined correlate with their potency as HO-1 inducers; and (v) chalcone-mediated HO-1 induction is reduced by thiols or inhibitors of phosphatidylinositol-3 kinase (PI3K) pathway. This study provides additional information on the structural features that methoxychalcones and natural antioxidants need to possess to be considered as therapeutic agents for maximizing HO-1 expression and activity.