Abstract
Thioredoxin-interacting protein (TXNIP) was originally identified in HL-60 cells as the vitamin D₃ upregulated protein 1, and is now known to be involved in diverse cellular processes, such as maintenance of glucose homeostasis, redox balance, and apoptosis. Besides the initial characterization, little is known about if and how 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] induces TXNIP expression. We therefore screened multiple cancerous cell lines of different tissue origins, and observed induction, repression, or no change in TXNIP expression in response to 1,25(OH)₂D₃. In-depth analyses on HL-60 cells revealed a rapid and transient increase in TXNIP mRNA levels by 1,25(OH)₂D₃ (3-24 h), followed by a clear reduction at later time points. Furthermore, a strong induction in protein levels was observed only after 96 h of 1,25(OH)₂D₃ treatment. Induction of TXNIP expression by 1,25(OH)₂D₃ was found to be dependent on the availability of glucose in the culture medium, as well as the presence of a functional glucose transport system, indicating an inter-dependence of 1,25(OH)₂D₃ actions and glucose-sensing mechanisms. Moreover, the inhibition of de novo protein synthesis by cycloheximide reduced TXNIP half-life in 24 h, but not in 96 h-1,25(OH)₂D₃-treated HL-60 cells, demonstrating a possible influence of 1,25(OH)₂D₃ on TXNIP stability in long-term treatment.