Oxidative Stress Related to Plasmalemmal and Mitochondrial Phosphate Transporters in Vascular Calcification

Abstract

Inorganic phosphate (Pi) is essential for maintaining cellular function but excess of Pi leads to serious complications, including vascular calcification. Accumulating evidence suggests that oxidative stress contributes to the pathogenic progression of calcific changes. However, the molecular mechanism underlying Pi-induced reactive oxygen species (ROS) generation and its detrimental consequences remain unclear. Type III Na⁺-dependent Pi cotransporter, PiT-1/-2, play a significant role in Pi uptake of vascular smooth muscle cells. Pi influx via PiT-1/-2 increases the abundance of PiT-1/-2 and depolarization-activated Ca²⁺ entry due to its electrogenic properties, which may lead to Ca²⁺ and Pi overload and oxidative stress. At least four mitochondrial Pi transporters are suggested, among which the phosphate carrier (PiC) is known to be mainly involved in mitochondrial Pi uptake. Pi transport via PiC may induce hyperpolarization and superoxide generation, which may lead to mitochondrial dysfunction and endoplasmic reticulum stress, together with generation of cytosolic ROS. Increase in net influx of Ca²⁺ and Pi and their accumulation in the cytosol and mitochondrial matrix synergistically increases oxidative stress and osteogenic differentiation, which could be prevented by suppressing either Ca²⁺ or Pi overload. Therapeutic strategies targeting plasmalemmal and mitochondrial Pi transports can protect against Pi-induced oxidative stress and vascular calcification.

Keywords: calcium; mitochondria; oxidative stress; phosphate; transporters; vascular calcification.

Figure 1

Plasmalemmal Pi transporters. The characteristics and tissue expressions of type II and III NaPi cotransporters are illustrated. The functional abundance of type II NaPi cotransporters are regulated by hormones, such as PTH and FGF23, in order to normalize serum Pi levels upon the alterations in dietary Pi. However, high extracellular Pi directly increases the functional abundance of type III NaPi cotransporters, leading to intracellular Pi accumulation.

Figure 2

Mitochondrial Pi transporters. Four mitochondrial carriers involved in Pi transport across the inner mitochondrial membrane and their hypothetical transport mechanisms are illustrated. PiC is the main route of mitochondrial Pi uptake driven by pH gradient. DIC exchanges dicarboxylate for Pi or other dicarboxylate, but also transport adenine nucleotides and glutathione. UCP2 performs Pi transport with C₄ metabolites including malate, oxaloacetate, and aspartate. ATP-Mg/Pi transporter has EF hand domains in N-terminal leading to having a Ca²⁺ activated properties.

Figure 3

Oxidative stress related to plasmalemmal and mitochondrial Pi transporters in vascular calcification. Type III Na-Pi cotransporters, PiT-1 and PiT-2, can trigger plasma membrane depolarization due to their electrogenic properties. This depolarization couples cytosolic Ca²⁺ influx with Pi, leading to more Ca²⁺ and Pi loading in cytosol. Suppressing the entrance of either Ca²⁺ or Pi successfully attenuate oxidative stress. In addition, Pi influx activates ERK and AKT signaling further augment PiT-1/-2 abundance either by translational regulation or by surface trafficking. Mitochondrial Pi transport, mainly mediated by PiC, elicits mitochondrial hyperpolarization and superoxide generation. All these cytosolic and mitochondrial oxidative stress induces NF-B activation, osteogenic gene upregulation, ER stress and apoptosis leading to vascular calcification.

Figure 4

Oxidative stress by Ca and Pi overloads in vascular calcification. Ca and Pi overloads accelerates cytosolic and mitochondrial oxidative stress, which can induce more mitochondrial dysfunction and ER stress. Depletion of the ER Ca pool further increases ROS generation from mitochondrial and the ER, which also disturbs lysosomal Ca homeostasis and autophagy defects. All these pathologic alterations in organelles aggravate oxidative stress and disease progression including arterial medial calcification.