Basal and Inducible Expression of the Thiol-sensitive ART2.1 Ecto-ADP-ribosyltransferase in Myeloid and Lymphoid Leukocytes

ADP-ribosylation of cell surface proteins in mammalian cells is a post-translational modification by which ecto-ADP-ribosyltransferases (ARTs) transfer ADP-ribose from extracellular NAD to protein targets. The ART2 locus at murine chromosome 7 encompasses the tandem Art2a and Art2b genes that encode the distinct ART2.1 and ART2.2 proteins. Although both ecto-enzymes share 80% sequence identity, ART2.1 activity is uniquely regulated by an allosteric disulfide bond that is reducible in the presence of extracellular thiols, such as cysteine and glutathione, that accumulate in hypoxic and ischemic tissues. Previous studies have characterized the expression of ART2.1 and ART2.2 in murine T lymphocytes but not in other major classes of lymphoid and myeloid leukocytes. Here, we describe the expression of ART2.1 activity in a wide range of freshly isolated or tissue-cultured murine myeloid and lymphoid leukocytes. Spleen-derived macrophages, dendritic cells (DC), and B cells constitutively express ART2.1 as their predominant ART while spleen T cells express both ART2.1 and the thiol-independent ART2.2 isoform. Although bone-marrow-derived macrophages (BMDM) and dendritic cells (BMDC) constitutively express ART2.1 at low levels, it is markedly up-regulated when these cells are stimulated in vitro with IFNbeta or IFNgamma. ART2.1 expression and activity in splenic B cells is modestly up-regulated during incubation in vitro for 24 h, a condition that promotes B cell apoptosis. This increase in ART2.1 is attenuated by IL-4 (a B cell survival factor), but is not affected by IFNbeta/gamma, suggesting a possible induction of ART2.1 as an ancillary response to B cell apoptosis. In contrast, ART2.1 and ART2.2, which are highly expressed in freshly isolated splenic T cells, are markedly down-regulated when purified T cells are incubated in vitro for 12-24 h. Studies with the BW5147 mouse thymocyte line verified basal expression of ART2.1 and ART2.2, as in primary spleen T cells, and demonstrated that both isoforms can be up-regulated when T cells are maintained in the presence of IFNs. Comparison of the surface proteins which are ADP-ribosylated by ART2.1 in the different leukocyte subtypes indicated both shared and cell-specific proteins as ART2.1 substrates. The LFA-1 integrin, a major target for ART2.2 in T cells, is also ADP-ribosylated by the ART2.1 expressed in macrophages. Thus, ART2.1, in contrast to ART2.2, is expressed in a broad range of myeloid and lymphoid leukocytes. The thiol redox-sensitive nature of this ecto-enzyme suggests an involvement in purinergic signaling that occurs in the combined context of inflammation and hypoxia/ischemia.