b-Thujaplicin (BT), also known as hinokitiol, is a tropolone-related compound found in the heart wood in several cupressaceous plants such as the Hinoki cypress, Chamaecyprais obtuse, and the western red cedar, Thuja Plicata DON.1,2) BT has a wide variety of biological functions, including antibacterial and antifungal activity, 3) acting as an iron chelator for Salmonella typhimurium, 4) anti-tumor activity, 5) and induction of differentiation. 6) Thus far, however, the molecular mechanisms by which BT exerts such a broad spectrum of biological effects have not been elucidated.Metallothioneins (MT) are ubiquitously distributed, low molecular weight proteins with a high cysteine content and high binding capacity for metals such as zinc and cadmium.
7)MT function in zinc homeostasis 7) and detoxification of heavy metals. 8,9) Several experiments have also shown that MT acts as a reactive oxygen species (ROS) scavenger, 10,11) and MT induction has protective effects against oxidative stresses such as anticancer drugs 12) and ultraviolet (UV).
13)MT gene expression is induced not only by heavy metals but also by a wide variety of stress-inducing agents such as ultraviolet, 14) X-ray, 15) hypoxia, 16) hydrogen peroxide, 17) and a number of cytokines including interferon-a and b, TNF-a, and interleukin 1 and 6. [18][19][20][21][22] Recently, we found that BT induces cellular metallothionein in mouse keratinocytes in vitro and in vivo, resulting in reduction of apoptotic cell formation caused by UV-B irradiation.
23)The molecular mechanism underlying MT gene regulation has been extensively investigated (reviewed in ref. 24). In the proximal promoters of MT genes, multiple copies of metal response elements (MREs) reside, and these cis-elements are essential for MT gene induction by zinc and cadmium. MREs were shown to mediate the transcriptional response of MT genes to oxidative stress in mouse hepatoma cells.25) A metal-responsive transcription factor, termed MTF-1, which binds to MREs and activates MT gene transcription, has been cloned from mouse and human. 26,27) Recently, the involvement of protein kinase-C (PKC) has been demonstrated in zinc-and cadmium-induced MT gene expression, as determined by Northern analyses using a specific PKC inhibitor, chelerythrin. 28) However, the precise regulation of MT gene expression by other inducing agents has yet to be elucidated.In the present study, we report the first evidence for BT as a potent transactivator for the human MT-IIA gene. Moreover, inhibition experiments revealed a possible involvement of protein kinase C and reactive oxygen species in the transduction mechanism.
MATERIALS AND METHODSChemicals BT was a kind gift from Takasago (Tokyo, Japan). Cycloheximide, H7, chelerythrin, HA1004, and Nacetylcysteine (NAC) were all purchased from Sigma (St. Louis, MO, U.S.A.).Cell Culture The spontaneously transformed human epidermal keratinocyte cell line, HaCaT, (kindly provided by Dr. Husenig) was cultured in Dulbecco's modified minimal essential medium (DMEM) supplemented...