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The human ZIP4 gene (SLC39A4) is a candidate for the genetic disorder of zinc metabolism acrodermatitis enteropathica. To understand its role in zinc homeostasis, we examined the function and expression of mouse ZIP4. This gene encodes a well conserved eight-transmembrane protein that can specifically increase the influx of zinc into transfected cells. Expression of this gene is robust in tissues involved in nutrient uptake, such as the intestines and embryonic visceral yolk sac, and is dynamically regulated by zinc. Dietary zinc deficiency causes a marked increase in the accumulation of ZIP4 mRNA in these tissues, whereas injection of zinc or increasing zinc content of the diet rapidly reduces its abundance. Zinc can also regulate the accumulation of ZIP4 protein at the apical surface of enterocytes and visceral endoderm cells. These results provide compelling evidence that ZIP4 is a zinc transporter that plays an important role in zinc homeostasis, a process that is defective in acrodermatitis enteropathica in humans.A long recognized disease of zinc metabolism is the human genetic disorder acrodermatitis enteropathica (AE) 1 (1, 2). This autosomal recessive disorder causes classic symptoms of zinc deficiency (3), such as dermatological lesions, changes in the gastric mucosa associated with digestive system problems, lack of weight gain, and immune and reproductive problems (4 -8). Remarkably, these symptoms can be ameliorated by dietary zinc supplement (4, 6, 9 -11), consistent with the finding of reduced, but not eliminated, uptake of 65 Zn by the intestine from AE patients (12, 13), and the reduced uptake and total content of zinc in AE fibroblasts (14,15). Recent genetic mapping localized the AE gene to chromosome 8q24.3 (16) and led to its identification as a member of the ZIP superfamily (17, 18). That gene was named hZIP4 (the Human Genome Organization Nomenclature Committee named this gene SLC39A4).ZIP4 was found to be expressed in enterocytes and to reside in the plasma membrane. Mutations in hZIP4 were detected in AE patients (17, 18), strongly suggesting that they cause this genetic disorder.The recently identified ZIP superfamily of metal ion uptake transporters (19,20) are found in all eukaryotes, and many of its members mediate zinc uptake. In yeast, ZRT1 encodes the high affinity zinc transporter, and ZRT2 encodes the low affinity zinc transport system. The Arabidopsis iron-regulated transporter gene (IRT1) encodes a metal transporter that has remarkable sequence similarity with the yeast ZRTs and with other Arabidopsis zinc transporters (19, 21). Thus, the acronym ZIP was adopted to reflect ZRT/IRT-related proteins. Many members of the ZIP gene superfamily have now been detected based on sequence homology with yeast and Arabidopsis ZIP genes (22, 23). The ZIP proteins typically have eight membrane-spanning domains, and spanning domain four contains fully conserved histidyl and glycyl residues in an amphipathic ␣-helix. These proteins also often have a histidine-rich intracellular loop between s...

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Synthesis of metallothionein-I (MT-I) and heme oxygenase mRNAs is rapidly and transiently induced by H2O2 in mouse hepatoma cells (Hepa) and this effect is blocked by catalase. Menadione, which generates free radicals, also induces these mRNAs. Deletion mutagenesis revealed that a region between -42 and -153 in the mouse MT-I promoter was essential for induction of a CAT reporter gene. A multimer of a 16 bp sequence (-101 to -86) that includes an antioxidant response element and overlapping adenovirus major late transcription factor binding site elevated basal expression and allowed induction by H2O2 when inserted upstream of a minimal promoter. However, deletion of this region (-100 to -89) from the intact MT-I promoter (-153) did not completely eliminate response. Multiple copies of a metal response element also permitted response to H2O2. These results suggest that induction of MT-I gene transcription by H2O2 is mediated by at least two different elements within the proximal MT-I gene promoter and suggest a previously undescribed function of the MRE. Induction of MT gene transcription by ROS and the subsequent scavenging of ROS by the MT peptide is reminiscent of the metal regulatory loop and is consistent with the hypothesized protective functions of MT.

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