Proliferation of normal cells is regulated through the complex interaction of each cell with neighboring homologous and heterologous cells and extracellular tissue components forming its microenvironment. In addition to the cell-cell and cell-extracellular matrix contact, the proliferation of cells is regulated by a variety of soluble growth factors acting as endocrine, paracrine, or autocrine stimulators of cell division (Sporn and Roberts 1988). These growth factors, defined as polypeptides that stimulate cell proliferation through binding to specific high affinity cell membrane receptors (Goustin et al. 1986), have pleiotropic effects which are often diametrically opposite from one another in different cell systems. Thus, a growth factor that is highly mitogenic for one cell type may act as an inhibitor of proliferation of other cells and vice versa. The transforming growth factor beta (TGF-β) stimulates the growth of nonneoplastic fibroblasts in soft agar, while inhibiting the growth of many normal and neoplastic cells (Sporn and Roberts 1985). On the other hand a polypeptide that induces differentiation and halts the proliferation of leukemia cells promotes the growth of undifferentiated embryonic cells (Williams et al. 1988).