Modulation of thrombospondin receptor expression during HL-60 cell differentiation

Thrombospondin (TSP), a multifunctional homotrimeric glycoprotein of approximately 450,000 M(r), is a component of the extracellular matrix that mediates the adhesive interactions of several different cell types including hematopoietic progenitor cells. We have used the promyelocytic leukemia HL-60 cell line to examine TSP receptor expression during differentiation of leukocytes along either the monocyte/macrophage or the polymorphonuclear leukocyte (PMN) pathway. 125I-labeled TSP binding to undifferentiated or differentiated HL-60 cells was time-dependent reaching saturation by 45 min. Undifferentiated HL-60 cells expressed a single class of heparin-inhibitable TSP receptors. Treating HL-60 cells with PMA induced their differentiation to macrophage-like cells and resulted in a concomitant 10-fold increase in TSP receptor expression. As with undifferentiated cells, a single class of heparin-inhibitable receptors was observed. Treating HL-60 cells with DMSO induced their differentiation to PMN-like cells and resulted in a fivefold increase in TSP receptor expression. However, in this case two classes of binding sites were apparent on PMN-like cells, only 40% of which were heparin inhibitable. This is reminiscent of TSP binding to normal peripheral blood PMN (S.J. Suchard, L.A. Boxer, and V.M. Dixit. 1991. J. Immunol. 147:651). In parallel studies, we also examined TSP synthesis during HL-60 cell differentiation. Undifferentiated HL-60 cells synthesized and secreted TSP as assessed by immunoprecipitation. TSP synthesis increased about fourfold when cells were differentiated toward PMN-like cells. In contrast, TSP was not detected in macrophage-like cells. RNase protection assays showed that TSP transcript levels paralleled TSP protein expression during differentiation. These findings suggest that expression of both TSP and TSP receptors are differentially regulated during blood cell maturation.