EFFECTS OF TRANSFORMING GROWTH-FACTOR-BETA ON MEGAKARYOCYTIC CELL-FUSION AND ENDOMITOSIS

Transforming growth factor beta (TGF-beta) is a cytokine secreted by megakaryocytes and platelets which inhibits the proliferation of megakaryocyte progenitors in vitro. We have studied the effects of TGF-beta on more mature megakaryocytes, using cell lines CMK, DAMI, and CHRF as well as isolated human marrow megakaryocytes as models. Proliferation of these megakaryocytic cell lines was inhibited by TGF-beta-1 at concentrations of 10-100 ng/ml. Although these cell lines secreted small amounts of TGF-beta (up to 100 pg/ml), it was predominantly in an inactive form. Upon induction with 12-phorbol 13-myristate acetate (PMA), they secreted greater amounts of TGF-beta (500-1250 pg/ml), most of which was still inactive in a bioassay. Addition of exogenous active TGF-beta-1 had no effect on ploidy of unstimulated megakaryocytic cell lines or on [H-3]thymidine incorporation of isolated human marrow megakaryocytes, Following PMA induction, exogenous TGF-beta-1 had a significant inhibitory effect on ploidy in DAMI cells but not CMK or CHRF cells, suggesting that the phenomenon is restricted to DAMI cells. Because certain cell lines may fuse in vitro, and phorbol esters can promote this phenomenon, we investigated the possibility that fusion was contributing to the increase in ploidy in megakaryocytic cells. Unstimulated megakaryocytic cells did not show spontaneous fusion; when PMA was added to the cultures, fusion was markedly increased, particularly in DAMI cells. These results demonstrate that in vitro fusion accounts for a proportion of the apparent increase in DNA content of megakaryocytic cell lines upon induction with PMA. The inhibitory effects of TGF-beta in megakaryocytopoiesis appear most clear on progenitors of this lineage rather than in their mature, committed progeny.