Phorbol Ester 12-O-tetradecanoylphorbol-13-acetate down-regulates expression of the c-kit proto-oncogene product

Modulation of expression of the c-kit proto-oncogene product, the receptor for the recently indentified stem cell factor, was studied on 12-O-tetradecanoylphorbol-13-acetate (TPA)-stimulated cultures of CD34⁺ normal bone marrow progenitor cells, blast cells from patients with primary acute myelogenous leukemia, cells from the leukemia cell lines HEL and MO7E, as well as cultured HMC-1 mast cells. Expression of c-kit was assessed on both RNA and protein level employing standard Northern blotting, reverse transcription, and polymerase chain reaction-based Southern blotting, as well as cell surface labeling with anti-c-kit mAb YB5.B8. Treatment of virtually all cell types with nontoxic concentrations of TPA (10^-9 M) for at least 48 h was associated with down-regulation of synthesis of c-kit transcripts and stem cell factor-receptor surface expression. Studies on the mechanism of action of TPA utilizing the HEL erythroleukemia line showed that TPA was primarily acting by accelerating the turnover of c-kit RNA most likely through induction of a destabilizing protein. The effect of TPA on c-kit expression levels was independent of TPA-mediated induction of differentiation since other compounds including IFN-γ, vitamin D3, retinoic acid, arabinofuranosylcytosine, butyric acid, and camptothecin, which also effectively induced differentiation of HEL cells, failed to alter levels of c-kit expression.