By means of its antiangiogenic activity, thrombospondin-1 (TSP-1) exerts indirect antitumoral action on solid tumors. Here, we investigated potential antitumor action in an in vitro cell model for promyelocytic leukemia (NB4-LR1), resistant to retinoid maturation. Purified soluble TSP-1 added to cultures induced a strong dosedependent growth inhibition and a slowly developing maturation-independent cell death. Recombinant fragments of TSP-1 allowed mapping of these activities to its type 3 repeat/C-terminal domain, features that are distinct from those of TSP-1 action on solid tumors, previously ascribed to the type 1 repeat domain. Cell death in leukemia was characterized as a caspase-independent mechanism, without DNA fragmentation, but phosphatidylserine externalization followed by membrane permeabilization. Mitochondria membrane depolarization was inherent to TSP-1 action but did not produce release of death-promoting proteins (eg, noncaspase apoptosis regulators, apoptosisinduced factor [ IntroductionCell interaction with extracellular matrix proteins is essential to regulate proliferation, differentiation, and cell death. 1-3 Thrombospondin-1 (TSP-1) belongs to a family of multidomain and multifunctional glycoproteins that regulate cell proliferation, migration, and differentiation and is important for embryonic development, morphogenesis, inflammation, tumor metastasis, and multiple other processes. 4-6 TSP-1 is a secreted component of extracellular matrix that supports attachment and motility of a variety of normal and neoplastic cell types. 5,7,8 Because it also functions as soluble protein that antagonizes the proadhesive activities of other matrix proteins, TSP-1 has been defined as a matricellular protein modulating cell-matrix interactions. 9,10 It can affect matrix structure through its ability to interact with matrix components and to activate transforming growth factor-␤1 (TGF-␤1). 11 TSP-1 is also a potent physiological inhibitor of angiogenesis by impairing endothelial cell growth and migration and by acting on endothelial cell cycle progression and apoptosis; thus, down-expression of TSP-1 is important for tumor progression. [12][13][14][15] Many of the TSP-1 functions have been located within specific domains of the molecule that bind to various membrane receptors, including integrins of the ␤1 and ␤3 family, CD47/integrinassociated protein (CD47/IAP), CD36, and proteoglycans. 5,8 Therefore, cell responses to TSP-1 probably reflect the integration of multiple membrane signals depending on the cell type and its repertoire in membrane receptors.TSP-1 function in tumor cell differentiation and apoptosis remains poorly explored. TSP-1 has been described to be positively involved in retinoic acid-induced maturation of neuroblastoma cells and the acute myeloblastic leukemic cell line 17 In the HL-60 cell line, it was shown to induce a caspase-dependent cell death mediated by CD36. 18 It is also able to trigger a caspase-independent cell death that is accompanied by selective mitochondrial changes ...

In a previous study, we have demonstrated that the platelet adhesive glycoprotein thrombospondin-1 (TSP-1) interacts specifically with the cytoskeletal protein alpha-actinin in a solid-phase binding assay. Stored in the alpha-granules of platelets, TSP-1 is secreted during cell activation and binds to the plasma membrane promoting the platelet macroaggregate formation. However, the molecular mechanism by which TSP-1 reaches and binds to the platelet surface is to date unelucidated. alpha-Actinin is an actin-binding and actinin-cross-linking protein that is present in most cells and may act as a link between the bundles of F-actin and the plasma membrane. In this study, we have investigated a possible interaction of alpha-actinin with TSP-1 in platelets by examining their respective subcellular location during the platelet activation process. By indirect immunofluorescence. alpha-actinin was found to display a granular staining in resting platelets similar to that of TSP-1. Performing postembedding immunogold labeling for electron microscopy, we detected the presence of alpha-actinin throughout the cytoplasm, but the strongest gold staining was found in organelles identified as alpha-granules on the basis of their ultrastructure and TSP-1 content. With the use of double immunogold labeling on platelets at different stages of activation by thrombin, both alpha-actinin and TSP-1 were seen redistributing from the alpha-granules to the platelet surface via the open canalicular system (OCS). At the same time, the cytoplasmic alpha-actinin concentrated toward the plasma membrane, but no colocalization with the F-actin bundles was evidenced. Finally, preembedding immunogold labeling and immunoprecipitation of 125I-surface-labeled, thrombin-activated platelets further demonstrated that alpha-actinin was expressed on the plasma membrane in the absence of any detectable expression of actin and that it could from molecular complexes with TSP-1 on activated platelets. These results suggest that alpha-actinin found to be present on the platelet surface together with TSP-1 originates in the alpha-granules by fusion of the alpha-granules with the plasma membrane during platelet exocytosis.