Role of ATP during the initiation of microvascularization. Acceleration of an autocrine sensing mechanism facilitating chemotaxis by inorganic polyphosphate
2018
The
in vitro
tube formation assay with human umbilical vein endothelial cells (HUVEC) was applied to identify the extra- and intracellular sources of metabolic energy/ATP required for cell migration during the initial stage of microvascularization. Extracellularly, the physiological energy-rich polymer, inorganic polyphosphate [polyP], applied as biomimetic
amorphous calcium
polyP microparticles [Ca-polyP-MP], is functioning as a substrate for ATP generation most likely
via
the action of the alkaline phosphatase [ALP] and the
adenylate kinase [AK]
. T
he linear Ca-polyP-MP with a size of 40 phosphate units, close to the physiological polyP
in the
blood platelets
, were found to increase endothelial cell tube formation, as well as the intracellular ATP levels. De
pletion of extracellular ATP with apyrase
suppressed tube formation during the initial incubation period.
Inhibition experiments revealed that inhibitors (levamisole and Ap5A) of the
enzymes involved in extracellular ATP generation strongly reduced the
Ca-polyP-MP induced tube formation
. The stimulatory effect of Ca-polyP-MP was also diminished by the glycolysis inhibitor oxamate and
trifluoperazine which blocks
endocytosis, as well as by MRS2211, an antagonist of the P2Y 13 receptor. Oligomycin, an inhibitor of the mitochondrial F 0 F 1 -ATP synthase, displayed at lower concentrations no effect on tube formation. Electron microscopic data revealed that after cellular uptake, the Ca-polyP-MP accumulate close to the cell membrane. We conclude that in HUVEC
exposed to polyP,
ATP is formed extracellularly
via the coupled ALP-AK reaction, and intracellularly during glycolysis
. The results suggest
an autocrine signaling pathway of ATP with
polyP as an extracellular store of metabolic energy for endothelial cell migration.
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