Effects of endogenously activated transforming growth factor-beta on growth and differentiation of retinoic...

Retinoic acid (RA)-treated HL-60 cells were used as a model to study differentiation of granulocytic leukemias. RA induces these cells to mature into granulocytes and to decrease growth. Mediators of these RA effects have not been identified definitively, but transforming growth factor-beta (TGF-beta) has been implicated in regulating proliferation and differentiation of myelogenous leukemic cells. The role of TGF-beta in RA-dependent differentiation and cessation of growth was examined by adding neutralizing anti-TGF-beta IgG to RA-treated HL-60 cells, followed by assessing cell growth and markers of granulocytic differentiation over 5 days. After addition of neutralizing anti-TGF-beta IgG, growth of RA-treated HL-60 cells was maintained at control levels, but granulocytic differentiation continued. These experiments demonstrated that the antiproliferative activity of RA was TGF-beta dependent but that differentiation was not. Because most cell types secrete TGF-beta in a biologically inactive complex, a TGF-beta-dependent effect requires cells to activate the latent form of TGF-beta. Active and total TGF-beta levels were quantitated in media harvested from control and RA-treated cells using a luciferase-based bioassay for TGF-beta activity. Similar levels of total TGF-beta were observed between control and RA-treated cells. RA-treated cells produced active TGF-beta (18-24 pg/ml) after 1, 2, and 3 days of treatment, whereas negligible levels were produced by control cultures. Activation of endogenous latent TGF-beta by RA-treated cells occurred through a plasmin-independent mechanism.