TNAP dephosphorylates phosphocholine and phosphoethanolamine and participates in triglyceride transport from the liver to the bloodstream

Tissue-nonspecific alkaline phosphatase (TNAP) is primarily known for its role in skeletal mineralization, through the hydrolysis of inorganic pyrophosphate (PP_i). Here we demonstrate that TNAP-knockout mice exhibit liver steatosis and reduced serum triglyceride levels, mirroring the effects of choline deficiency, which impairs phosphatidylcholine synthesis, an essential component of VLDL. In fasting WT mice, TNAP inhibition via SBI-425 administration decreases choline levels in blood and liver. Incubating mouse or human serum with SBI-425 inhibits the dephosphorylation of phosphocholine and phosphoethanolamine, an alternative substrate for hepatic phosphatidylcholine synthesis. In hepatocytes, TNAP inhibition impedes proliferation when the medium is supplemented with phosphocholine instead of choline. Recombinant TNAP hydrolyzes phosphocholine and phosphoethanolamine with similar efficiency than PP_i. X-ray diffraction and cryo-EM identified the residues in TNAP's active site interacting with phosphocholine, PP_i and the TNAP inhibitor. In summary, TNAP is the phosphatase enabling cellular choline uptake during fasting, participating in hepatic lipid metabolism.