EXPRESSION OF THE LACTOTRANSFERRIN RECEPTOR DURING THE DIFFERENTIATION PROCESS OF THE MEGAKARYOCYTE DAMI CELL-LINE

In order to determine whether the human lactotransferrin receptor rece ntly described on platelets was also present on hematopoietic precurso rs, we investigated its presence and characteristics on the megakaryoc ytic Dami cell line. The reversible binding of human razino)methyl]thi o}acetyl)aminofluorescein-labeled lactotransferrin showed that such a receptor was only present on the subpopulation of the largest cells. T he increase in numbers of large cells during culture was paralleled by a concurrent increase in lactotransferrin receptor positive cells. Sc atchard analysis of the binding of [I-125]-labeled lactotransferrin sh owed that a single affinity class of binding site was present (K-d = 4 46 +/- 40 nM) and that there were 52 +/- 3 x 10(5) sites per cell. The mouse monoclonal antibody DP5B3G10, specific for the human lactotrans ferrin receptor, allowed its characterization as a 105 kDa protein on Western blots. The same monoclonal antibody was used to separate the s mall and large cell subpopulations of Dami cells by panning. Separate culture of the small cells showed that the receptor appeared prior to and independent from endomitosis. In contrast, GPIb was expressed only by large megakaryocytes. The use of conditioned medium from cultures of whole Dami cell populations indicated that a soluble factor is invo lved in differentiation, but not in the appearance of the lactotransfe rrin receptor.