Microparticles as biomarkers of early changes leading to cardiovascular disease in chronic kidney disease

Hyperphosphataemia in patients with advanced chronic kidney disease (CKD) is thought to be an important contributor to cardiovascular risk, in part because of endothelial cell (EC) dysfunction induced by inorganic phosphate (Pi). Such patients also have an elevated circulating concentration of pro-coagulant endothelial microparticles (MPs), leading to a pro-thrombotic state, which may contribute to acute occlusive events. It is hypothesised that hyperphosphataemia leads to MP formation from ECs via an elevation of intracellular Pi, which directly inhibits phosphoprotein phosphatases, triggering a global increase in phosphorylation and cytoskeletal changes. Using cultured human endothelial cells (EAhy926), incubation with elevated extracellular Pi (2.5mM) led to a rise in intracellular Pi concentration within 90min. This was mediated by PiT-1/slc20a1 Pi transporters; and led to global accumulation of Tyr- and Ser-Thr phosphorylated proteins, a marked increase in cellular Tropomyosin-3, plasma membrane blebbing and release of 0.1 – 1 micron diameter MPs. The effect of Pi was independent of oxidative stress or apoptosis. Similarly, global inhibition of phosphoprotein phosphatases with orthovandate or fluoride also yielded a global protein phosphorylation response and rapid release of MPs. The Pi-induced MPs expressed VE-cadherin and superficial phosphatidylserine, and in a thrombin generation assay were significantly more pro-coagulant than particles derived from cells incubated in medium with a physiological level of Pi (1mM). These data demonstrate a mechanism of Pi-induced cellular stress and signalling which may be widely applicable in mammalian cells; and in ECs provides a novel pathological link between hyperphosphataemia, generation of MPs and thrombotic risk.