ABCC6 prevents ectopic mineralization seen in pseudoxanthoma elasticum by inducing cellular nucleotide release

Abstract

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disease characterized by progressive ectopic mineralization of the skin, eyes, and arteries, for which no effective treatment exists. PXE is caused by inactivating mutations in the gene encoding ATPbinding cassette sub-family C member 6 (ABCC6), an ATP-dependent efflux transporter present mainly in the liver. Abcc6 −/− mice have been instrumental in demonstrating that PXE is a metabolic disease caused by the absence of an unknown factor in the circulation, the presence of which depends on ABCC6 in the liver. Why absence of this factor results in PXE has remained a mystery. Here we report that medium from HEK293 cells overexpressing either human or rat ABCC6 potently inhibits mineralization in vitro, whereas medium from HEK293 control cells does not. Untargeted metabolomics revealed that cells expressing ABCC6 excrete large amounts of nucleoside triphosphates, even though ABCC6 itself does not transport nucleoside triphosphates. Extracellularly, ectonucleotidases hydrolyze the excreted nucleoside triphosphates to nucleoside monophosphates and inorganic pyrophosphate (PPi), a strong inhibitor of mineralization that plays a pivotal role in several mineralization disorders similar to PXE. The in vivo relevance of our data are demonstrated in Abcc6 −/− mice, which had plasma PPi levels <40% of those found in WT mice. This study provides insight into how ABCC6 affects PXE. Our data indicate that the factor that normally prevents PXE is PPi, which is provided to the circulation in the form of nucleoside triphosphates via an asyet unidentified but ABCC6-dependent mechanism.ATP secretion | ectopic calcification | MRP6 | ENPP1

Dysregulation of gene expression in ABCC6 knockdown HepG2 cells

“…It is indeed possible that, once secreted in the extracellular space, the ABCC6 substrates may act as autocrine factors promoting the expression of anti-mineralization genes and downregulating the expression of pro-mineralization genes. In agreement with this hypothesis, it was recently reported that cells overexpressing ABCC6 secrete an unidentified substrate that in turn induces a large efflux of nucleotide triphosphates providing the extracellular pyrophosphate, a potent inhibitor of mineralization [29]. Alternatively, it can be hypothesized that ABCC6 deficiency leads to an Brought to you by | MIT Libraries Authenticated Download Date | 5/13/18 10:37 AM intracellular increase of substrates that might modulate the expression of genes involved in mineralization processes.…”

Pyrophosphate: a key inhibitor of mineralisation

“…Pseudoxanthoma elasticum is an autosomal recessive condition characterised by reduced plasma pyrophosphate levels and progressive ectopic mineralisation of the skin, eyes and arteries [91][92][93]. It is primarily caused by inactivating mutations in the ATP-binding cassette subfamily C member 6 (ABCC6) gene [94].…”

“…ABCC6 is primarily expressed in the liver and mediates ATP release from hepatocytes. Abcc6 -/-knockout animals exhibit the symptoms of pseudoxanthoma elasticum and display a 40% reduction in plasma pyrophosphate levels [91]. Studies using these animals have suggested that ATP release is impaired in cells lacking ABCC6 and that the lack of substrate for NPP1 results in lower circulating pyrophosphate levels and the development of pseudoxanthoma elasticum [66].…”

Pseudoxanthoma Elasticum: A Pediatric Case

“…Psödoksantoma elastikum (PKE), elastik fibrillerde kalsifikasyon nedeniyle, özellikle deri, gözün Bruch membranı ve kan damarlarında hasara neden olan multisistemik ve metabolik bir bağ dokusu hastalığıdır (1)(2)(3)(4). Otozomal resesif yolla kalıtılır (5).…”