Abstract

Copper (Cu) is an essential micronutrient for cell growth but is toxic when in excess. Copper transporter (Ctr1) plays an important role in regulating adequate Cu levels in mammalian cells. We previously showed that expression of the human high-affinity Cu transporter (hCtr1) was transcriptionally upregulated under Cu-depleted conditions and downregulated under replete conditions; moreover, elevated hCtr1 levels feedback to suppress hCtr1 expression (Song IS, et al. Mol Pharmocol 74:705-713, 2008). Expression of hCtr1 under Cu-stressed conditions is regulated by Sp1. In this study, we made the following important observations: (i) Sp1 expression is downregulated under Cu-replete conditions, but upregulated under Cu-depleted conditions. These up- and down-regulations of Sp1 in turn regulate hCtr1 expression to control Cu homeostasis. (ii) Cu-regulated Sp1 expression involved Sp1 binding to its own promoter as demonstrated by the chromatin immunoprecipitation assay; therefore, Sp1 is also transcriptionally self-regulated via hCtr1/Cu intermediation. (iii) Both zinc finger and glutamine-rich transactivation domains of Sp1 are involved in the Sp1-mediated hCtr1 and Sp1 regulation by Cu stresses. (iv) While Sp3 expression is also regulated by Cu availability, Sp3 does not regulate hCtr1 homeostasis. Collectively, our results demonstrated that mammalian cells use Sp1 oscillation in response to Cu availability to regulate Cu homeostasis through hCtr1 expression in a tripartite interregulatory relationship. These findings have important implications in mammalian Cu physiology regulation and in cancer chemotherapy using platinum drugs, because hCtr1 is also a transporter for these antitumor agents.