Basic fibroblast growth factor (bFGF) is a potent mitogen for human bone marrow stromal cells. Normally, large numbers of human bone marrow stromal cells are difficult to obtain. However, nanogram/ml concentrations of bFGF stimulate the growth of passaged bone marrow stromal cells both in media formulated for optimal growth of stromal cells and in a simple mixture of RPMI-1640 and 10% fetal calf serum facilitating the successive expansion of stromal cells through multiple passages. bFGF also greatly accelerates the formation of a primary stromal cell layer following inoculation of newly harvested bone marrow cells into dishes. In the presence of bFGF, the stromal cells attain high densities, lose their contact inhibition and grow in multilayered sheets. Heparin greatly potentiates the stimulatory effect of low concentrations of bFGF. The effects of bFGF are fully reversible: cells cultured in the presence of this factor for multiple passages revert to normal growth rates following trypsinization and subculture. A short (4 h) exposure of the cells to bFGF elicits profound growth stimulation. This supports the hypothesis that this factor binds to glycosaminoglycans in the cell matrix which act as a storage reservoir for this cytokine.

Plasminogen activators (PAs) and/or plasmin may be involved in hematopoietic regulation. These enzymes release biologically relevant cytokines such as basic fibroblast growth factor (bFGF) from matrix and cell surfaces. In addition, transforming growth factor beta (TGF beta) and interleukin-1 beta (IL-1 beta) are converted from inactive to active forms by plasmin. Therefore, we studied the regulation of PAs and their specific inhibitors, PA inhibitor 1 (PAI-1) and PA inhibitor 2 (PAI-2), in human bone marrow stromal fibroblasts by IL-1 beta, bFGF, and TGF beta. All three cytokines stimulated PA secretion. IL-1 beta at 10(4) U/mL increased urokinase (u-PA) levels approximately 10-fold, bFGF at 0.2 ng/mL also increased production 10-fold, but increased predominantly tissue PA (t-PA) expression. TGF beta at 0.2 ng/mL increased u-PA production up to 300-fold. PAI-1 and PAI-2 are also regulated by these cytokines. IL-1 beta decreased PAI-1 levels by 50% and stimulated PAI-2 levels sixfold. bFGF had minimal effects on PAI-1 and TGF beta increased PAI-1 levels twofold. Neither of these agents had an effect on PAI-2 levels. Thus, three cytokines relevant to bone marrow physiology regulate PA and inhibitor production by human bone marrow stromal fibroblasts. In this manner PA and plasmin generation in specific microenvironments in the bone marrow may be one of the factors orchestrating the complex series of events, which results in an efficient exquisitely regulated hematopoietic process.