Copper transporters and copper chaperones: roles in cardiovascular physiology and disease

Abstract

Copper (Cu) is an essential micronutrient but excess Cu is potentially toxic. Its important propensity to cycle between two oxidation states accounts for its frequent presence as a cofactor in many physiological processes through Cu-containing enzymes, including mitochondrial energy production (via cytochrome c-oxidase), protection against oxidative stress (via superoxide dismutase), and extracellular matrix stability (via lysyl oxidase). Since free Cu is potentially toxic, the bioavailability of intracellular Cu is tightly controlled by Cu transporters and Cu chaperones. Recent evidence reveals that these Cu transport systems play an essential role in the physiological responses of cardiovascular cells, including cell growth, migration, angiogenesis and wound repair. In response to growth factors, cytokines, and hypoxia, their expression, subcellular localization, and function are tightly regulated. Cu transport systems and their regulators have also been linked to various cardiovascular pathophysiologies such as hypertension, inflammation, atherosclerosis, diabetes, cardiac hypertrophy, and cardiomyopathy. A greater appreciation of the central importance of Cu transporters and Cu chaperones in cell signaling and gene expression in cardiovascular biology offers the possibility of identifying new therapeutic targets for cardiovascular disease.

Keywords: cardiovascular diseases; copper chaperones; copper homeostasis; copper transporters; vascular physiology.

Fig. 1.

Copper trafficking pathways in mammalian cells. Once transported by Cu uptake transporter CTR1, soluble cytosolic Cu carrier proteins termed “Cu chaperones” may obtain the Cu directly from the uptake transporter or from GSH and are required for trafficking Cu to specific Cu-containing enzymes through direct protein-protein interactions. Three main copper chaperone pathways have been characterized thus far: 1) various Cu chaperones (cytochrome c-oxidases Cox1, Cox2, Cox11, and Cox17 and synthesis of cytochrome c-oxidases Sco1 and Sco2) and unknown Cu ligands (L), which deliver Cu to cytochrome c-oxidase in the mitochondria; 2) Cu chaperone for SOD1 (CCS), which delivers copper to SOD1 in the cytosol and mitochondrial intermembrane space (IMS); 3) antioxidant-1 (Atox1), which delivers copper to the secretory copper enzymes such as extracellular superoxide dismutase (ecSOD, SOD3) and lysyl oxidase (LOX) via the copper transporter ATP7A (Menkes disease copper ATPase, MNK) or ATP7B (Wilson disease copper ATPase, WND) in the trans-Golgi network. In addition to its chaperone function, Atox1 also function as a Cu-dependent transcription factor for Atox1-responsive genes such as ecSOD and cyclin D1. Cytosolic concentrations of free Cu are typically maintained at exquisitely low levels (10⁻¹⁸ M) by metal scavenging systems, including metallothionein (MT).

Fig. 2.

Role of Cu transporters and Cu chaperones in platelet-derived growth factor (PDGF)-induced vascular smooth muscle cell (VSMC) migration and vascular remodeling. PDGF promotes interaction of ATP7A with antioxidant-1 (Atox1) and translocation from the trans-Golgi network (TGN) to the lipid rafts at the leading edge. This stimulates lamellipodia formation via recruiting Rac1, which in turn promotes directional VSMC migration. This is associated with a decrease in the cellular copper level and secretory copper enzyme prolysyl oxidase (Pro-LOX) at the lipid rafts, which is processed following secretion and activated by proteolysis to a mature lysyl oxidase (LOX) and a propeptide (LOX-PP), which may promote extracellular matrix (ECM) remodeling. Secreted copper may also contribute to PDGF-induced cell migration. Atox1 also regulates inflammatory cell recruitment. Thus, Cu transporters and Cu chaperones may play an important role in vascular remodeling by regulating inflammatory cell recruitment, VSMC migration, and ECM remodeling. CTR1, copper transporter 1; PDGFR, PDGF receptor.

Fig. 3.

Protective role of the insulin-Akt2/ATP7A/superoxide dismutase 3 (SOD3) pathway against diabetes mellitus (DM)-induced endothelial dysfunction. Decreased ATP7A expression in diabetic vessels with either hypoinsulinemia (type1 DM) or selective impairment of insulin/Akt signaling (type 2 DM) contributes to decreased SOD3 activity, resulting in increased O₂^·− production and endothelial dysfunction. Mechanistically, insulin increases Akt2 binding to ATP7A to induce ATP7A phosphorylation, which may increase ATP7A protein expression via preventing proteasomal degradation as well as ATP7A translocation to the plasma membrane, which contributes to full activation of SOD3 in vascular smooth muscle cells (VSMCs) and preserves endothelial function. ECs, endothelial cells; TGN, trans-Golgi network.

Fig. 4.

Role of antioxidant-1 (Atox1) in inflammatory neovascularization in response to ischemia and wound injury, which is dependent on arteriogenesis/angiogenesis and inflammation. In response to proinflammatory cytokine TNF-α, Atox1 functions as a Cu-dependent transcription factor for NADPH oxidase organizer, p47^phox, to increase the reactive oxygen species (ROS)/NF-κB/VCAM-1/ICAM-1 axis in endothelial cells. This in turn promotes recruitment of inflammatory cells that secrete TNF-α and VEGF. In response to VEGF, Atox1 functions as a Cu chaperone for ATP7A, to increase lysyl oxidase (LOX) activity involved in angiogenesis. This process represents a novel positive feedback loop whereby the Cu chaperone protein Atox1 promotes inflammatory neovascularization. CTR1, copper transporter 1; TNFR, TNF receptor.