The role of transforming growth factor-β in PEG-rHuMGDF-induced reversible myelofibrosis in rats

Pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) injected at a suprapharmacologic dose (100 mu g/kg) daily for 5d in normal rats caused marked increases in marrow megakaryocytes and platelet counts at 6-8d followed by gradual decreases to control levels at 10-20d. Interestingly, in addition to the expected thrombopoiesis, PEG-rHuMGDF was associated with myelofibrosis with a predominance of reticulin fibres at day 10 followed by complete normalization by day 20, At 6-8d, the levels of transforming growth factor-beta (TGF-beta 1) in the extracellular fluid of the marrow the platelet poor plasma, and the platelet extract were increased 23-, 7- and 2-fold, respectively. The elevated levels of TGF-beta 1 were gradually reduced to baseline levels at 13-20d in accordance with the normalization of myelofibrosis and thrombopoiesis. An ultrastructural analysis showed that large fragments of megakaryocytes were deposited in the marrow parenchyma of PEG-rHuMGDF-treated rats at day 6. PEG-rHuMGDF administration at pharmacologic doses (1 and 10 mu g/kg) did not induce the deposition of reticulin fibres in the marrow. These findings suggest that TGF-beta 1 leaked from megakaryocytes is involved in the development of the PEG-rHuMGDF-induced myelofibrosis and that this is a reversible process related to the regulation of the excess production of platelets.