Phosphate as a Pathogen of Arteriosclerosis and Aging

Abstract

During the evolution of skeletons, terrestrial vertebrates acquired strong bones made of calcium-phosphate. By keeping the extracellular fluid in a supersaturated condition regarding calcium and phosphate ions, they created the bone when and where they wanted simply by providing a cue for precipitation. To secure this strategy, they acquired a novel endocrine system to strictly control the extracellular phosphate concentration. In response to phosphate intake, fibroblast growth factor-23 (FGF23) is secreted from the bone and acts on the kidney through binding to its receptor Klotho to increase urinary phosphate excretion, thereby maintaining phosphate homeostasis. The FGF23-Klotho endocrine system, when disrupted in mice, results in hyperphosphatemia and vascular calcification. Besides, mice lacking Klotho or FGF23 suffer from complex aging-like phenotypes, which are alleviated by placing them on a low- phosphate diet, indicating that phosphate is primarily responsible for the accelerated aging. Phosphate acquires the ability to induce cell damage and inflammation when precipitated with calcium. In the blood, calcium-phosphate crystals are adsorbed by serum protein fetuin-A and prevented from growing into large precipitates. Consequently, nanoparticles that comprised calcium-phosphate crystals and fetuin-A, termed calciprotein particles (CPPs), are generated and dispersed as colloids. CPPs increase in the blood with an increase in serum phosphate and age. Circulating CPP levels correlate positively with vascular stiffness and chronic non-infectious inflammation, raising the possibility that CPPs may be an endogenous pro-aging factor. Terrestrial vertebrates with the bone made of calcium- phosphate may be destined to age due to calcium-phosphate in the blood.

Keywords: Aging; Calciprotein particles (CPPs); Fibroblast growth factor-23 (FGF23); Inflammation; Klotho; Phosphate; Vascular calcification.

Fig. 1.

Structure of the FGF23-FGFR-Klotho ternary complex¹⁷⁾ The extracellular domain of Klotho protein is composed of two internal repeats, termed KL1 and KL2 domains, with homology to family 1 glycosidases. KL2 domain sends out the receptor binding arm (RBA), which has an intrinsically disordered structure and interacts with the D3 domain of fibroblast growth factor receptor (FGFR). Once the RBA binds to the D3 domain of FGFR, the binding pocket for FGF23 is generated. Formation of the ternary complex composed of FGFR, FGF23, and Klotho is required for activation of the canonical FGF signaling. Modified from Refs. and .

Fig. 2.

Average life span of mammals is inversely correlated with serum phosphate levels²³⁾ 1, kl/kl mouse; 2, mouse; 3, rat; 4, hamster; 5, gerbil; 6, nutria, 7, rabbit; 8, guinea pig; 9, sheep; 10, squirrel; 11, porcupine; 12, naked mole rat; 13, flying fox; 14, bear; 15, rhinoceros; 16, elephant; 17, human; 18, human (centenarian).

Fig. 3.

Formation of calciprotein particles (CPPs) Once the concentration of calcium and phosphate ions exceeds the solubility limit in the blood, amorphous calcium–phosphate (CaPi) is precipitated and immediately adsorbed by serum protein fetuin-A to generate calciprotein monomers (CPMs). CPMs spontaneously aggregate to generate primary CPPs. Primary CPPs further undergo aggregation and phase transition of calcium–phosphate from the amorphous phase (white circle) to the crystalline phase (black circle) to generate secondary CPPs. Calcium–phosphate in CPMs and primary CPPs are in the amorphous phase, whereas secondary CPPs contain crystalline calcium–phosphate.

Fig. 4.

The FGF23–Klotho endocrine axis In response to dietary phosphate (Pi) and calcium (Ca) intake, CPPs appear in the blood and extravasate from sinusoids in the bone marrow. CPPs act on osteoblasts/osteocytes to make them produce and secrete FGF23. FGF23 circulates in the blood and binds to the FGFR–Klotho complex expressed on renal tubular cells to suppress reabsorption of phosphate and synthesis of active vitamin D. Suppression of phosphate reabsorption increases urinary phosphate excretion, thereby lowering blood phosphate levels that have transiently increased after phosphate ingestion (postprandial hyperphosphatemia) to their baseline levels. Suppression of serum active vitamin D levels attenuates calcium absorption from the intestine. The ability of FGF23 to restore serum phosphate levels and attenuate calcium absorption reduces blood CPP levels and closes the negative feedback loop.

Fig. 5.

A hypothesis on the mechanism of vascular calcification Secondary CPPs have the activity that induces phosphorylation (P) of NFκB in vascular smooth muscle cells (VSMCs), followed by osteoblastic transformation and inflammatory responses. It is assumed that secondary CPPs are endocytosed and transferred to lysosomes in which calcium–phosphate may be dissolved under the acidic environment. These lysosomes may fuse with multivesicular bodies (MVBs) and provide calcium, phosphate, and fetuin-A to vesicles within MVB, which are secreted as matrix vesicles (MVs). Calcium–phosphate crystals grow in the MVs and accumulate on the bone matrix (blue lines) secreted from the transformed VSMCs. Zinc (Zn) inhibits vascular calcification through binding to its cell surface receptor GPR39 and inducing expression of TNFα-induced protein 3 (TNFAIP3), a potent inhibitor of NFκB activation. Magnesium (Mg) inhibits vascular calcification possibly through suppressing the transition of primary CPPs to secondary CPPs.

Fig. 6.

Colloidal particles as pathogens of arteriosclerosis Two distinct forms of arteriosclerosis, atherosclerosis and vascular calcification, can be viewed collectively to be caused by mistargeting of circulating colloidal particles (lipoproteins and CPPs) containing insoluble materials (lipids and calcium–phosphate) to arterial walls. White circles, amorphous calcium–phosphate; black circles, crystalline calcium–phosphate; gray circles, fetuin-A; yellow circles, triglyceride; orange circles, cholesterol; blue circles, apoproteins.