Tissue-nonspecific
alkaline phosphatase (TNAP) is necessary for skeletal mineralization by its ability to hydrolyze the mineralization inhibitor inorganic
pyrophosphate (PPi), which is mainly generated from extracellular
ATP by
ectonucleotide pyrophosphatase phosphodiesterase 1 (NPP1). Since children with TNAP deficiency develop bone metaphyseal auto-
inflammations in addition to
rickets, we hypothesized that TNAP also exerts anti-inflammatory effects relying on the hydrolysis of pro-inflammatory
adenosine nucleotides into the anti-inflammatory
adenosine. We explored this hypothesis in bone metaphyses of 7-day-old Alpl+/- mice (encoding TNAP), in mineralizing hypertrophic chondrocytes and osteoblasts, and non-mineralizing mesenchymal stem cells (MSCs) and neutrophils, which express TNAP and are present, or can be recruited in the metaphysis. Bone metaphyses of 7-day-old Alpl+/- mice had significantly increased levels of Il-1β and
Il-6 and decreased levels of the anti-inflammatory
Il-10 cytokine as compared with Alpl+/+ mice. In bone metaphyses, murine hypertrophic chondrocytes and osteoblasts, Alpl
mRNA levels were much higher than those of the
adenosine nucleotidases Npp1, Cd39 and Cd73. In hypertrophic chondrocytes, inhibition of TNAP with 25 μM of MLS-0038949 decreased the hydrolysis of
AMP and
ATP. However, TNAP inhibition did not significantly modulate
ATP- and
adenosine-associated effects in these cells. We observed that part of TNAP
proteins in hypertrophic chondrocytes was sent from the cell membrane to matrix vesicles, which may explain why TNAP participated in the hydrolysis of
ATP but did not significantly modulate its autocrine pro-inflammatory effects. In MSCs, TNAP did not participate in
ATP hydrolysis nor in secretion of inflammatory mediators. In contrast, in neutrophils, TNAP inhibition with MLS-0038949 significantly exacerbated
ATP-associated activation and secretion of IL-1β, and extended cell survival. Collectively, these results demonstrate that TNAP is a
nucleotidase in both hypertrophic chondrocytes and neutrophils, and that this
nucleotidase function is associated with autocrine effects on
inflammation only in neutrophils.