A phorbol ester-binding inhibitory factor from human placenta. Partial purification, characterization and biological effects.

A factor which inhibits the binding of 3H-phorbol-12,13-dibutyrate (PDBu) on different types of cells has been partially purified from human placenta. This factor, phorbol ester binding inhibitory factor (PEBIF), is sensitive to pepsin, but resistant to trypsin, heat and acid (pH 3) treatments and can be precipitated by 80% ethanol with no loss of activity. Inhibition occurs both at 37 degrees C and 4 degrees C and is rapid and reversible. Inhibition on human epithelial cells (FL), on mouse erythroleukaemia cells (FELC clone 19-10) and on rat liver cells (IAR clone 6-1 and clone 20) is non-competitive, whereas on IAR clone 6 and clone 6-7 it is competitive. Differentiation of FELC induced by hexamethylene bisacetamide (HMBA) can be inhibited by 12-O-tetradecanoyl phorbol-13-acetate (TPA) only if TPA-sensitive cells are used, and no inhibition was observed with TPA-resistant cells. PEBIF can also inhibit the differentiation induced by HMBA of TPA-sensitive cells and has no effect on the differentiation of TPA-resistant cells. The extent of inhibition of PDBu binding by PEBIF was similar in these two clones. Like TPA, PEBIF can increase 2-deoxyglucose uptake in mouse fibroblasts (BALB/3T3 cells). Thus, TPA and PEBIF share two biological responses; however, PEBIF failed to mimic other TPA effects, such as induction of Epstein-Barr virus from Raji cells, inhibition of intercellular communication and induction of differentiation of human promyelocytic leukaemia cells (HL-60).