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.