Screening a plant extract library for inhibitors of cholecystokinin receptor CCK1 pathways.

This study describes the screening of a plant extract library for inhibitors of signal transduction pathways mediated by the cholecystokinin receptor, CCK1. CCK1 receptors are coupled to Galpha(q/11)-proteins, localized mainly in the gastrointestinal tract, and implicated in the regulation of various digestive functions. A primary screen was performed using a cell-based assay that used the beta-lactamase gene reporter controlled by the transcriptional activator NFAT. The assay was validated with the CCK1 receptor antagonist, lorglumide, and automated by the use of a liquid-handling robot MultiProbe II. Off-target hits were triaged by counterscreening against gene reporter cells activated by a combination of thapsigargin and phorbol ester. Purification of active compounds was guided by the beta-lactamase gene reporter and Ca2+ mobilization assays. Pure compounds were characterized by Ca2+ mobilization, radioligand binding, inositol-1 phosphate formation, and Eu-GTP binding assays. The selectivity of inhibition was tested against a panel of Galpha(q/11), Galpha(s), and Galpha(i/0)-coupled receptors. These studies led to the identification of a novel Galpha(q/11)-selective inhibitor.

1-(2,4-dihydroxyphenyl)-3-(2,4-dimethoxy-3-methylphenyl)propane, a novel tyrosinase inhibitor with strong depigmenting effects.

A series of diarylpropane compounds was isolated by screening a plant extract library for inhibitors of mushroom tyrosinase. The most potent compound, 1-(2,4-dihydroxyphenyl)-3-(2,4-dimethoxy-3-methylphenyl)propane (UP302: CAS# 869743-37-3), was found in the medicinal plant Dianella ensifolia. Synthetic and plant-derived versions of UP302 inhibited mushroom tyrosinase with similar potencies. UP302 inhibited mushroom tyrosinase with K(i)=0.3 microM, in a competitive and reversible fashion. UP302 was 22 times more potent than Kojic acid in inhibiting murine tyrosinase, with IC(50) values of 12 and 273 microM respectively. Experiments on mouse melanoma cells B16-F1 and on human primary melanocytes demonstrated that UP302 inhibits melanin formation with IC(50) values of 15 and 8 microM respectively. Long-term treatment of cultured melanocytes with up to 62 microM of UP302 revealed no detectable cytotoxicity. In a reconstructed skin model (MelanoDerm) topical application of 0.1% UP302 resulted in significant skin lightening and decrease of melanin production without effects on cell viability, melanocyte morphology or overall tissue histology. In conclusion, UP302 is a novel tyrosinase inhibitor that suppresses melanin production in both cultured melanocytes and reconstructed skin with high potency and without adverse side effects.