Biochemical and Biophysical Research Communications, Vol.443, No.1, 339-343, 2014
Shikonin shortens the circadian period: Possible involvement of Top2 inhibition
The naphthoquinone pigment, shikonin, is a natural product derived from Lithospermum erythrorhizon and an active component of a Chinese traditional herbal therapeutic. We identified shikonin as a candidate for shortening the circadian period using real-time reporter gene assays based on NIH3T3-derived stable reporter cells. Period length that became shortened in cells incubated with shikonin or etoposide reverted to that of control cells after continued incubation without these compounds. These findings indicated that shikonin and etoposide shorten the circadian period reversibly and through similar mechanisms. Topoisomerase II (Top2)-specific decatenation assays confirmed that shikonin, liker etoposide, is a Top2 inhibitor. Shikonin was incorporated into the nucleus and Top2 was located in the Small promoter, suggesting the relationship between Bmall transcription and Top2 inhibition. Top2a siRNA also shortened period length, suggesting that Top2 is involved in this process. Promoter assays showed that Top2a siRNA, etoposide and shikonin reduce Bmall promoter activity. These findings indicated that Top2 is involved in Bmall transcription and influences the circadian period, and that shikonin is a novel contributor to the control of period length in mammalian cells. (C) 2013 Elsevier Inc. All rights reserved.