ATOX1

ATOX1 Full Name

ATX1 antioxidant protein 1 homolog (yeast)

ATOX1 Introduction

ATOX1 (antioxidant 1 copper chaperone, also known as HAH1) is a small copper-binding protein that functions as a copper chaperone, delivering copper ions to copper-dependent enzymes and transporters within the secretory pathway. Copper is an essential trace element required as a catalytic cofactor for numerous enzymes involved in mitochondrial respiration, neurotransmitter synthesis, iron metabolism, antioxidant defense, and connective tissue formation. However, free copper ions are highly toxic due to their ability to generate reactive oxygen species (ROS) through Fenton-type reactions. ATOX1 is part of a highly conserved system that ensures safe and specific delivery of copper to its target proteins while preventing copper toxicity. ATOX1 binds cytosolic copper and transports it to the copper-transporting ATPases ATP7A (Menkes protein) and ATP7B (Wilson protein) in the trans-Golgi network, which then incorporate copper into secreted cuproenzymes including tyrosinase (melanin synthesis), lysyl oxidase (collagen and elastin crosslinking), ceruloplasmin (iron homeostasis), and dopamine beta-hydroxylase (neurotransmitter synthesis). Beyond its canonical role in copper homeostasis, ATOX1 also functions as a transcription factor and antioxidant protein, linking copper metabolism to gene expression and oxidative stress responses.

Figure 1. Schematic structure of ATOX1.

Copper Chaperone Function and Cellular Copper Homeostasis
ATOX1 plays an essential role in cellular copper homeostasis by specifically delivering copper to the copper-transporting ATPases ATP7A and ATP7B. The copper delivery mechanism proceeds through several well-characterized steps. First, cytosolic copper is imported by the copper transporter CTR1 (SLC31A1) and bound by glutathione or other low-molecular-weight ligands. ATOX1 then acquires copper from these ligands through a metal-transfer reaction, forming a stable copper-ATOX1 complex. The copper-loaded ATOX1 then diffuses to the trans-Golgi network, where it interacts with the N-terminal metal-binding domains (MBDs) of ATP7A or ATP7B through electrostatic and hydrophobic interactions. Each ATPase contains six MBDs, and ATOX1 transfers copper to these domains through a ligand-exchange reaction involving the conserved cysteine residues in both proteins. Upon copper binding, ATP7A and ATP7B undergo conformational changes that activate their ATPase activity, enabling them to pump copper into the Golgi lumen. Within the Golgi, copper is incorporated into secreted cuproenzymes. When cellular copper levels are elevated, ATP7A and ATP7B traffic from the Golgi to the plasma membrane (ATP7A) or to the bile canalicular membrane (ATP7B) to export excess copper from the cell. This dynamic localization is regulated by copper levels and ensures copper homeostasis. ATOX1 also participates in the copper-dependent degradation of ATP7B, providing feedback regulation. Genetic deletion of Atox1 in mice leads to copper accumulation in the kidneys and brain, reduced activity of copper-dependent enzymes, and defects in placental copper transfer, highlighting the essential role of ATOX1 in copper distribution.

Nuclear Function as a Transcription Factor and Therapeutic Implications
Beyond its cytoplasmic role as a copper chaperone, ATOX1 also translocates to the nucleus, where it functions as a copper-dependent transcription factor. Nuclear translocation is mediated by the C-terminal nuclear localization signal (KKGK) and is enhanced by elevated copper levels. Once in the nucleus, ATOX1 binds to specific DNA sequences in the promoter regions of target genes, particularly the antioxidant response element (ARE) and the metal response element (MRE). ATOX1 directly activates the transcription of cyclin D1 (CCND1), promoting cell cycle progression, as well as superoxide dismutase 1 (SOD1), an important antioxidant enzyme. ATOX1 also regulates the expression of genes involved in copper uptake (CTR1), copper export (ATP7A, ATP7B), and the oxidative stress response (HO-1, NQO1). This nuclear function links copper availability directly to transcriptional programs that control cell proliferation, antioxidant defense, and copper homeostasis itself. Therapeutically, ATOX1 has emerged as a potential target for anticancer therapy. Small molecule inhibitors that block ATOX1 copper binding or its interaction with ATP7A/ATP7B have been developed, including trientine and tetrathiomolybdate, which are already used clinically for Wilson disease. These agents are being repurposed for cancer therapy, with clinical trials ongoing in breast cancer, ovarian cancer, and glioblastoma. Additionally, ATOX1 may serve as a predictive biomarker for response to copper chelation therapy, with high ATOX1 expression potentially indicating greater sensitivity. In neurodegenerative diseases, strategies to modulate ATOX1 function are more complex, as both copper deficiency and excess can be harmful. Restoring ATOX1 function in conditions of copper deficiency or inhibiting ATOX1-mediated copper delivery in conditions of copper excess represent potential therapeutic approaches that require further investigation.

Alternate Names for ATOX1

ATOX1; ATX1 antioxidant protein 1 homolog (yeast); ATX1 (antioxidant protein 1, yeast) homolog 1; copper transport protein ATOX1; HAH1; metal transport protein ATX1; ATX1; MGC138453; MGC138455;