Thrombospondin-1 (TSP-1) and TSP-1-derived heparin-binding peptides induce promyelocytic leukemia cell differentiation and apoptosis.

Thrombospondin-1 (TSP-1) is a multifunctional adhesive glycoprotein that is synthesized by several cell types and modulates cell growth and differentiation. In this study, we showed that the amount of TSP-1 secreted by two human leukemia cell lines, HL-60 and NB4, increased markedly during differentiation of these cells by all-trans retinoic acid (ATRA) (10(-7) M), reaching about 100 ng/10(6) cells after 3 days. Addition of purified TSP-1 alone (10(-9)-5 x 10(-8) M) to HL-60 or NB4 cell cultures dose-dependently inhibited cell growth and differentiation. Differently to ATRA, TSP-1-induced differentiation of HL-60 and NB4 cells occurred independently of Bcl-2 regulation, as shown by immunofluorescence and Western immunoblotting. At day 5, TSP-1 also induced promyelocytic leukemia cell apoptosis. The percentage of apoptotic cells in NB4 cultures was higher with TSP-1 (5 x 10(-8) M) than with ATRA (10(-7) M) (46+/-3% versus 19+/-7%, p<0.001), whereas similar levels of apoptosis (37+/-7% and 38+/-6%) were reached with both agents in HL-60 cultures. Studies performed with synthetic peptides derived from the TSP-1 sequence indicated that two heparin-binding peptides, Hep-I and GGWSHW, located within the NH2-terminal and type 1 repeats respectively, were strong inducers of apoptosis of HL-60 and NB4 cells, suggesting that cell surface heparan sulfate molecules might be involved in the apoptotic effect of TSP-1 on promyelocytic cells.