INDUCTION OF THROMBOSPONDIN-1 BY ALL-TRANS-RETINOIC ACID MODULATES GROWTH AND DIFFERENTIATION OF HL-60 MYELOID-LEUKEMIA CELLS

Thrombospondin-1 (TSP), a multifunctional extracellular matrix protein , modulates human hematopoietic stem cell adherence and thus may play a role in blood cell proliferation and/or differentiation. The express ion of TSP was studied in the human myeloid leukemia cell line, HL-60, upon differentiation into monocytes by phorbol-13-monoacetate (PMA) o r into granulocytes by all-trans retinoic acid (RA). HL-60 cells cultu red under serum-free conditions constitutively secreted low amounts of TSP into the cultured medium, approximately 13 ng/10(6) cells/24 h. P MA used at 4 x 10(-8) M did not significantly modulate TSP secretion o ver a 24 h period. In contrast, RA at 10(-7) M induced a 5- to 10-fold increase in TSP secreted by HL-60 cells during their differentiation into granulocytes over a 5 day period. The role of secreted TSP in RA- dependent cessation of growth and differentiation was examined using b locking anti-TSP antibodies. In the presence of the polyclonal anti-TS P antibody R5 (25 mu g/ml), growth of RA-treated HL-60 cells was maint ained at control levels for up to 3 days and a concomitant delay in gr anulocytic differentiation was observed. Moreover, the addition of sol uble TSP (0.5-5 mu g/ml) to untreated HL-60 cells decreased their grow th and promoted their differentiation in a dose-dependent manner. Usin g a neutralizing antibody to transforming growth factor beta (TGF-beta ) or purified TGF-beta 1 we further demonstrated that the effects of T SP were not mediated through activation of latent TGF-beta. These stud ies indicate that TSP decreases the proliferation and promotes the dif ferentiation of HL-60 cells.