Authors
Fa-Xing Yu, Shuang-Ru Goh, Ru-Ping Dai and Yan Luo

Abstract
Eukaryotic cells sense extracellular glucose concentrations via diverse mechanisms in order to regulate the expression of genes involved in metabolic control. One such example is the tight correlation between the expression of thioredoxin-interacting protein (Txnip) and extracellular glucose levels. In this report, we show that the transcription of the Txnip gene is induced by adenosine-containing molecules, of which an intact adenosine moiety is necessary and sufficient. Txnip promoter contains a carbohydrate response element (ChoRE), which mediates the induction of Txnip expression by these molecules in a glucose-dependent manner. Max-like protein X (MLX) and MondoA are transcription factors previously shown to stimulate glucose-dependent Txnip expression, and are shown here to convey stimulatory signals from extracellular adenosine-containing molecules to the Txnip promoter. These molecules may exert their stimulatory functions by amplifying glucose signaling to facilitate the Txnip production, which in turn inhibits cellular thioredoxin activity and glucose transport. This revelation may form the basis for developing pharmacological interventions or modulations of certain cellular processes in which glucose plays a fundamental role.

Figure Legend
(A) Molecules containing an intact adenosine group induce Txnip expression. The mRNA levels of Txnip are scored by quantitative Real-Time PCR. (B) The induction of Txnip expression by adenosine-containing molecules is mediated by carbohydrate response element (ChoRE) and its associated transcription factors MondoA and Max-like protein X (MLX). The promoter activity of Txnip gene diminishes and is not induced by NAD+ in the presence of siRNAs against MondoA or MLX; however, the effect of siRNAs on a ChoRE-mutant Txnip promoter is negligent. (C) The cellular thioredoxin activity is repressed by NAD+. (D) Glucose up-take is inhibited by NAD+. In (B) to (D), NAD+ represents the adenosine-containing molecules. (E) A negative feed-back loop model for the regulation of the cellular glucose transport by glucose and adenosine-containing molecules. Glucose induces Txnip expression, and the accumulation of Txnip in cells feeds back to inhibit glucose transport; in the presence of adenosine-containing molecules, the Txnip accumulation is facilitated, which modulates the kinetics of this negative feed-back loop.