Lipopolysaccharide-induced apoptosis of macrophages determines the up-regulation of concentrative nucleoside transporters Cnt1 and Cnt2 through tumornecrosis factor-alpha-dependent and -independent mechanisms

In murine bone marrow macrophages, lipopolysaccharide (LPS) induces apoptos is through the autocrine production of tumor necrosis factor-alpha (TNF-alp ha), as demonstrated by the fact that macrophages from TNF-alpha receptor I knock-out mice did not undergo early apoptosis. In these conditions LPS up -regulated the two concentrative high affinity nucleoside transporters here shown to be expressed in murine bone marrow macrophages, concentrative nuc leoside transporter (CNT) 1 and 2, in a rapid manner that is nevertheless c onsistent with the de novo synthesis of carrier proteins. This effect was n ot dependent on the presence of macrophage colony-stimulating factor, altho ugh LPS blocked the macrophage colony-stimulating factor-mediated up-regula tion of the equilibrative nucleoside transport system es. TNF-alpha mimicke d the regulatory response of nucleoside transporters triggered by LPS, but macrophages isolated from TNF-alpha receptor I knock-out mice similarly up- regulated nucleoside transport after LPS treatment. Although NO is produced by macrophages after LPS treatment, NO is not involved in these regulatory responses because LPS up-regulated CNT1 and CNT2 transport activity and ex pression in macrophages from inducible nitric oxide synthase and cationic a mino acid transporter (CAT) 2 knock-out mice, both of which lack inducible nitric oxide synthesis. These data indicate that the early proapoptotic res ponses of macrophages, involving the up-regulation of CNT transporters, fol low redundant regulatory pathways in which TNF-alpha- dependent- and -indep endent mechanisms are involved. These observations also support a role for CNT transporters in determining extracellular nucleoside availability and m odulating macrophage apoptosis.