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FLT3/FLK-2 (STK-1) Ligand Does Not Stimulate Human Megakaryopoiesis In Vitro

^a Department of Pathology and Laboratory Medicine, and
^b Internal Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA

Key Words. Hematopoiesis • Megakaryocytopoiesis • FLT3/FLK-2 (STK-1) receptor • FLT3/FLK-2 (STK-1) ligand • Growth factors • Antisense oligodeoxynucleotides

Dr. Alan M. Gewirtz, 513B-Stellar-Chance Laboratories, University of Pennsylvania School of Medicine, 422 Curie Blvd., Philadelphia, PA 19104, USA. e-mail: gewirtz{at}a1.mscf.upenn.edu

It has not yet been determined if the FLT3/FLK-2 or STK-1 Ligand (STK-1 L)/FLT3/FLK-2 or STK-1 receptor (STK-1R) axis has the ability to regulate human megakaryopoiesis in vitro. To address this question, we exposed normal human CD34⁺ marrow mononuclear cells to recombinant human STK-1L alone, or in combination with other growth factors. Colony-forming unit-megakaryocytic/thrombocytes (CFU-Meg) and BFU-E-derived colonies were then enumerated, and effects on colony size and maturation noted. As assessed by these parameters, STK-1L had no demonstrable effect on megakaryocyte colony formation. Similarly, suppressing STK-1R expression with oligodeoxynucleotides also had no influence on CFU-Meg-derived colony formation. To begin to derive a physiologic explanation for these findings, we examined freshly isolated normal human megakaryocytes for the presence of STK-1L and STK-1R mRNA. In contrast to a growing number of growth factors and growth factor receptors which appear to be expressed by megakaryocytes, normal mature human megakaryocytes express neither STK-1R or STK-1L mRNA. Accordingly, our results led us to hypothesize that if STK-1/ STK-1L have any effects on megakaryocyte development in vitro, they are likely subtle and of uncertain physiologic significance.

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