Letter to the editor: Demonstration of tsp receptor both on the cell surface and in the cytoplasm of the megakaryoblastic leukemia cell line (MEG‐01)

Abstract

Thrombospondin (TSP), is an adhesive 450-kD multifunctional glycoprotein composed of three identical disulphide-linked chains [l]. TSP has a broad tissue distribution, is synthesized by endothelial cells, fibroblasts, smooth muscle cells, alveolar pneumocytes, monocytes, and neuroglial cells, and is known to bind heparin, fibrinogen, fibronectin, histidine-rich glycoprotein, collagen, and plasminogen.OKM5 monoclonal antibody (CD36) is reactive with platelets, endothelial cells, and monocytes [2]. It identifies an 88-kD glycoprotein and has recently been found to inhibit the specific binding of TSP to C32 melanoma cells and HT1080 fibrosarcoma cells and to react with the cellular receptor for TSP [ 3 ] . The expression of OKM5 antigen (TSP receptor) on the cell surface of megakaryoblastic leukemia has been reported previously [4]. We describe here the existence of TSP receptor on the cell surface and in the cytoplasm of the megakaryoblastic cell line MEG-01 [5]. Analysis of the culture cells (MEG-01) by means of indirect immunofluorescence technique is described. Monoclonal antibodies (MAbs) were obtained through the courtesy of Drs. A.J. McMichael, H. Shiku, and G.Goldstein, and Ortho Diagnostic Systems K.K. and the Japan Scientific Instrument Company. Cell surface antigens were analyzed by a fluorescent-activated cell sorter (FACS IV; Becton Dickinson, Mountain View, CA) using a panel of MAbs as previously described [6]. Immunophenotypically , the differentiation stage for the MEG-01 cell line was considered to belong the megakaryoblast, which was demonstrated by both the presence of the megakaryocyte-platelet-related cell surface antigens GP IIb/IIIa (J15, P2, HPL2, and HPLS), Plt-1, OKM6, and Ia (HLA-DR), and the lack of GP Ib (5F1 and AN51) [6]. The antigen expression detected by OKM5 MAb was also clearly found on the surface of this cells. In addition, OKM5 antigen increased while culturing the MEG-01 cells with phorbol diesters and was also found in the cytoplasm by means of the indirect cytoplasmic immunofluorescence test [ 7 ] . (Fig. 1). On the basis of these findings, we conclude that TSP receptor certainly exists both on the cell surface and in the cytoplasm of the megakaryoblastic cell line (MEG-01). REFERENCESI . Lawler J, Denck LH, Connolly JE, Chen JH, Chao FC: The stnicture of human platelet thrombospondin. J Biol Chem 260:3762, 19x5. 2. Talk MA, Rao PE, Westberg E, Allegan N, Makowski M, Mittler RS, Goldstein G: Patterns of antigenic expression on human monocytcs as defined by monoclonal antibodies. Cell lmmunol 78:83, 1983. 3. Asch AS, Barnwell J , Silverstein RL, Nachman RL: Isolation of the thrombospondin membrane receptor. J Clin lnvest 79: 1054, 1987. 4. Imamura N, lnada T, Mtasiwa DM, Kuramoto, A: Demonstration of TSP receptor on the cell surface of acute megakaryoblastic leukemia Am J