Hierarchical Role of Fetuin-A and Acidic Serum Proteins in the Formation and Stabilization of Calcium Phosphate Particles

Abstract

The serum protein fetuin-A is a potent systemic inhibitor of soft tissue calcification. Fetuin-A is highly effective in the formation and stabilization of protein-mineral colloids, referred to as calciprotein particles (CPPs). These particles ripen in vitro in a two-step process, indicated by a morphological conversion from spheres to larger prolate ellipsoids. Using a combined light scattering and electron microscopic imaging approach we determined that the second-stage particles resulted from a highly anisotropic outgrowth of the first-stage particles. Electron microscopy of ascites fluid from a patient with calcifying peritonitis revealed particles reminiscent of secondary CPPs. Thus, CPPs form in the body and undergo the two-step ripening at least in pathological conditions. Unlike in vitro generated CPPs, ascites-derived CPPs contained little fetuin-A but large amounts of albumin. This prompted us to study the role of fetuin-A combined with other serum proteins in CPP formation. Fetuin-A was indispensable for primary CPP formation. Albumin and acidic proteins in general greatly enhanced the fetuin-A triggered formation of secondary CPPs and, thus, substituted substantial amounts of fetuin-A without loss of inhibition of calcium phosphate precipitation. Thus, direct mineral deposition from solute in the body is unlikely even at low fetuin-A serum levels as long as sufficient bulk acidic protein is available. Collectively fetuin-A and other acidic bulk plasma proteins may be considered as mineral chaperones mediating the stabilization, safe transport, and clearance in the body of calcium and phosphate as colloidal complexes, thus, preventing ectopic calcification.

Nanoparticle-Based Test Measures Overall Propensity for Calcification in Serum

“…When trapped in body compartments, primary CPPs can undergo transition to secondary CPPs, which was shown in a case of progressive calcifying peritonitis 7,23 and cases of amputated limbs of diabetic patients. 24 A weakness of our method is that, as a serum test, it does not by nature take into account the established contribution of cells, including vascular smooth muscle cells 25 and calcifying myeloid cells, 26 in promoting vascular calcifications in vivo.…”

“…4,5 We and others have identified the serum protein fetuin-A as a major systemic inhibitor of calcification. [6][7][8][9] Together with additional blood components, fetuin-A prevents the supersaturated calcium and phosphate from precipitating by forming soluble colloidal protein-mineral nanoparticles. Calcification takes place when this humoral line of defense, inherent in small molecules (particularly pyrophosphate and magnesium) and proteins (particularly fetuin-A and albumin), is overwhelmed.…”

Mineralization by Inhibitor Exclusion

“…More recent studies have shown that a fetuin-mineral complex is formed in the course of the fetuin-mediated inhibition of apatite growth and precipitation in serum containing increased calcium and phosphate (24,25). Purified fetuin also potently inhibits the growth of apatite crystals from supersaturated solutions of calcium phosphate (12,24).…”

Metabolismo mineral óseo en pacientes con enfermedad renal crónica: revisión sobre su fisiopatología y morbimortalidad

“…Es miembro de la superfamilia de inhibidores de la cistein proteasa. Es un transportador de factores de crecimiento, que se une al factor de crecimiento transformante-(TGF-b) y a la proteína morfogenética (44,45) . Disminuye la activación de macrófagos y la liberación de citokinas proinflamatorias.…”

“…No afecta la mineralización ósea normal. Adicionalmente, inhibe la apoptosis de las células musculares lisas de los vasos sanguíneos (45) . En modelos animales de ERC los ratones CKD-2 mostraron incremento en la producción de inhibidores de Wnt (wingless/integration) como Dickkopf-1 (Dkk1) y esclerostina y disminución de Klotho soluble (46) .…”