Abstract
We report that human galectin-1 (dGal-1), a small dimeric beta-galactoside-binding protein, induces phosphatidylserine (PS) exposure, measured by Annexin V staining, on human promyelocytic HL-60 cells, T leukemic MOLT-4 cells, and fMet-Leu-Phe-activated, but not resting, human neutrophils. This effect of dGal-1 on HL-60 and MOLT-4 cells is enhanced by pretreatment of the cells with neuraminidase, but treatment of resting neutrophils with neuraminidase does not enhance their sensitivity to dGal-1. Although the induction of staining with Annexin V is often associated with apoptosis, the dGal-1-treated HL-60 cells, MOLT-4 cells, and activated neutrophils do not undergo apoptosis, and there is no detectable DNA fragmentation. HL-60 and MOLT-4 cells treated with dGal-1 continue to grow normally. By contrast, camptothecin-treated HL-60 cells, etoposide-treated MOLT-4 cells, and anti-Fas-treated neutrophils exhibit extensive DNA fragmentation and/or cell death. Lactose inhibits the dGal-1-induced effects, indicating that dGal-1-induced signaling requires binding to cell surface beta-galactosides. The dimeric form of Gal-1 is required for signaling, because a monomeric mutant form of Gal-1, termed mGal-1, binds to cells but does not cause these effects. Importantly, dGal-1, but not mGal-1, treatment of HL-60 cells and activated human neutrophils significantly promotes their phagocytosis by activated mouse macrophages. These dGal-1-induced effects are distinguishable from apoptosis, but like apoptotic agents, prepare cells for phagocytic removal. Such effects of dGal-1 may contribute to leukocyte homeostasis.
Highlights
We report that human galectin-1, a small dimeric -galactoside-binding protein, induces phosphatidylserine (PS) exposure, measured by Annexin V staining, on human promyelocytic HL-60 cells, T leukemic MOLT-4 cells, and fMet-Leu-Phe-activated, but not resting, human neutrophils
Binding of dGal-1 and monomeric form of galectin-1 (mGal-1) to HL-60 Cells, MOLT-4 Cells, Resting Human Neutrophils, and Activated Human Neutrophils—The human recombinant galectin-1 preparation was purified by affinity chromatography on lactosyl-Sepharose, as described under “Experimental Procedures.”
DGal-1enhanced exposure of Annexin V binding sites on HL-60 cells, MOLT-4 cells and activated, but not resting, human neutrophils is not associated with apoptosis
Summary
FasL, Fas ligand; dGal-1, dimeric human ture, circulating neutrophils in vitro [5,6,7], mice deficient in FasL (gld) or Fas (lpr) have essentially normal numbers of circulating mature granulocytes [8]. Phagocytosis is required for resolution of the inflammatory process and leukocyte homeostasis in vivo (14 –16) These results suggest that factors not yet defined may regulate leukocyte turnover in tissues. Such observations led us to explore whether the basement membrane and extracellular matrix might harbor other proteins capable of binding to leukocytes and inducing their apoptosis or phagocytic recognition. DGal-1 is a widely expressed dimeric protein (subunit ϳ14.6 kDa), which is a member of the galectin family of lectins [17,18,19] It is secreted by many cell types, including human endothelial cells [20, 21], and is found in the basement membrane and extracellular matrices around capillary walls [22, 23]. These results suggest that dGal-1 could induce surface changes that play a role in leukocyte turnover in tissues independently of apoptosis
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