In 2008, Seelinger et al. conducted a literature review to clarify luteolin's antioxidant, anti-inflammatory, and antiallergic activities. They found that luteolin is a natural antioxidant that exhibits less pro-oxidant potential than the most studied flavonoid, quercetin, and that it has a better safety profile than quercetin.
Luteolin also has been shown to possess superlative radical scavenging and cytoprotective qualities, particularly in complex biological systems, where it can interact with other antioxidants. Luteolin acts as an anti-inflammatory agent by activating antioxidative enzymes, inhibiting the nuclear factor kappa B pathway, and suppressing proinflammatory compounds.
The authors concluded that more quantitative research is necessary to determine the potential therapeutic benefits of this potent antioxidant (Planta Med. 2008;74:1667–77).
Ultraviolet Protection
In 2005, Morquio et al. mixed extracts of plants known to contain potent antioxidants, specifically Achyrocline satureioides and Epilobium parviflorum, with a cosmetic base, and applied the compounds to the back skin of rabbits. Subsequently, they exposed the skin to ultraviolet irradiation for 1 hour, and intracutaneously injected the irradiated areas with sodium salicylate.
The researchers then evaluated hydroxyl radical production by measuring 2,3-dihydroxybenzoic acid (2,3-DHBA) synthesis resulting from the hydroxylation of sodium salicylate. The production of 2,3-DHBA was found to be significantly increased by the UV irradiation, but was markedly diminished in association with the application of the Achyrocline satureioides cosmetic formulation. The authors attributed this antioxidant effect to the presence of high concentrations of flavonoid aglycones, including luteolin (Phytother. Res. 2005;19:486–90).
In 2007, Sim et al. studied the structure-activity relationship of several flavonoid compounds and their antioxidant and inhibitory effects against MMP activity in vitro and in human dermal fibroblasts induced by ultraviolet A light. The compounds examined included luteolin, myricetin, quercetin, kaempferol, apigenin, and chrysin. Luteolin, with the highest number of OH groups in the B ring, was shown to have the most potent antioxidant efficacy as ascertained using the DPPH method and the xanthine/xanthine oxidase system. The authors also noted that in association with the relative antioxidant strength of the flavonoids, the compounds dose-dependently suppressed collagenase activity and MMP expression. They concluded that flavonoids with a higher number of hydroxyl groups may be the most effective at preventing UV-induced cutaneous aging (Arch. Pharm. Res. 2007;30:290–8).
Prostaglandin Inhibitor
In a 2008 study, Papaliodis et al. investigated the effect of flavonoids on niacin-induced flush in a rat model, and sought to determine whether prostaglandin D2 (PGD2) and 5-hydroxytryptamine (5-HT) were involved.
The researchers recorded three skin temperature measurements from each ear for each time point immediately before intraperitoneal injection with either niacin or a flavonoid (quercetin or luteolin). They then measured temperature every 10 minutes for 1 hour. Ear temperature was increased by niacin to a maximum of 1.9 plus or minus 0.2 °C at 45 minutes.
Quercetin and luteolin administered intraperitoneally 45 minutes before niacin blocked the niacin effect by 96% and 88%, respectively, while aspirin inhibited the niacin effect by 30%. Plasma PGD2 and 5-HT were increased twofold by niacin, while luteolin suppressed plasma PGD2 and 5-HT by 100% and 67%, respectively, and aspirin lowered only PGD2 (by 86%).
The investigators concluded that the increased skin temperature in rats caused by niacin is linked to increases in PGD2 and 5-HT, and that luteolin may be the most suitable inhibitor of niacin-induced flush because it suppresses both mediators (Br. J. Pharmacol. 2008;153:1382–7).
Currently, luteolin is included as a minor ingredient in some nutritional and herbal supplements.
Conclusion
Much more research is necessary to ascertain whether the bioactive properties of luteolin can be readily harnessed for application in dermatologic and other medical conditions. Currently, the preponderance of evidence suggests that this flavonoid is at least as promising as its fellow flavonoid quercetin.