Autotaxin (ATX) or ecto-nucleotide pyrophosphatase/phosphodiesterase-2 (ENPP2) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA), a mitogen and chemo-attractant for many cell types. ATX-LPA signaling has roles in various pathologies including tumour progression and inflammation. However, the molecular basis of substrate recognition and catalysis, and the mechanism of interaction with target cells, has been elusive. Here we present the crystal structure of ATX, alone and in complex with a small-molecule inhibitor. We identify a hydrophobic lipid-binding pocket and map key residues required for catalysis and selection between nucleotide and phospholipid substrates. We show that ATX interacts with cell-surface integrins via its N-terminal somatomedin-B-like domains, using an atypical mechanism. Our results define determinants of substrate discrimination by the ENPP family, suggest how ATX promotes localized LPA signaling, and enable new approaches to target ATX with small-molecule therapeutics.

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1,25-Dihydroxyvitamin D3 [1,25(0H)2D3] stimulates bone resorption in man and other vertebrates, in part, by increasing the number of osteoclasts, the principal resorbing cells of bone. Because osteoclasts are very likely derived from a member(s) of the mononuclear phagocyte family, we determined if 1,25(OH)2D3 promotes maturation of these cells by studying its effects on the human promyelocytic leukemia cell line HL-60. Of the vitamin D3 metabolites tested, only 1,25(OH)2D3, at 10-10 to 10-7 M, induces the differentiation of HL60 into mono-and multinucleated macrophage-like cells. Phenotypic change is evident within 24 hr and reaches a plateau between 72 and 96 hr of incubation. The changes are metabolite-specific and include (i) adherence to substrate, (ii) acquisition of the morphological features of mature monocytes, (iii) a 4-to 6-fold enhancement in lysozyme synthesis and secretion, (iv) increase in the fraction of a-naphthyl acetate esterase-positive cells from approximately 2% to 100% of the population, and (v) the acquisition of several monocyte-associated cell surface antigens. More importantly, treated HL-60 cells acquire the capacity to bind and degrade bone matrix, two of the essential, functional characteristics of osteoclasts and related boneresorbing cells. These results, considered together with the reported action of 1,25(OH)2D3 on nontransformed mononuclear cells, are consistent with the view that vitamin D3 enhances bone resorption and osteoclastogenesis in vivo by promoting the differentiation of precursor cells.Of the several circulating factors known to affect bone resorption, one of the most potent is 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. This compound, when administered in picomolar to nanomolar concentrations, markedly stimulates resorptive activity (1) and promotes a readily measurable increase in the number of osteoclasts, the principal resorbing cells of bone (2). It is generally assumed that the appearance of increased numbers of osteoclasts is responsible for enhanced bone resorption. However, the mechanism(s) by which 1,25(OH)2D3 alters the size of the osteoclast population, and therefore resorptive activity, is presently unknown.Osteoclasts originate by fusion of circulating mononuclear precursor cells and almost certainly represent one of the endstage cells of mononuclear phagocyte differentiation (3, 4). Like osteoclasts, other mature, nonproliferative members of this family-e.g., monocytes and macrophages (Mos)-possess the capacity to attach to and degrade bone matrix (5, 6), and therefore they serve as useful models with which to study the mechanisms of bone resorption in tissue culture. In a previous study, we showed that Mos isolated from vitamin D-deficient animals exhibit, in vitro, the same bone resorptive dysfunction characteristically observed in the intact, calciferol-deprived animal (7). This dysfunction is not corrected by the direct addition of 1,25(OH)2D3 to M4 cultures, but administration of the metabolite to vitamin D-depleted animals for several days ...

Development of natural killer (NK) cells is thought to depend on interactions between NK progenitors