Functional Expression of c-kit by Acute Myelogenous Leukemia Blasts Is Enhanced by Tumor Necrosis Factor-α Through Posttranscriptional mRNA Stabilization by a Labile Protein

Abstract

The c-kit proto-oncogene encodes a transmembrane glycoprotein identical to the receptor for the recently cloned stem cell factor (SCF). The present study examines constitutive synthesis of transcripts in primary acute myelogenous leukemia (AML) blasts and the effects of recombinant human tumor necrosis factor (TNF)-alpha on c-kit mRNA expression in these cells. The c-kit transcripts were detectable at low levels in 10 of 10 different AML samples investigated. TNF treatment of AML cells was associated with enhanced c-kit mRNA expression in all specimens. Nuclear run-on transcription assays indicated that the c-kit gene was transcriptionally active in all leukemias examined and the rate of transcription was unaffected by exposure to TNF, suggesting posttranscriptional control mechanisms of c-kit mRNA accumulation. In the absence of TNF, the half-life of c-kit transcripts was 2 to 3 hours, while in TNF-treated AML cells, c-kit half-life was found to be 5 to 9 hours. Inhibition of protein synthesis reduced TNF-induced c-kit mRNA expression by Northern blot analysis, but did not affect the rate of c-kit gene transcription. In the presence of inhibition of protein synthesis, the half-life of c-kit transcripts in TNF-induced leukemia cells decreased to 2 to 4 hours. These findings indicate that levels of c-kit mRNA are controlled by a labile protein that is involved in TNF-mediated stabilization of c-kit transcripts. The effects of TNF-alpha also extended to the protein level in that TNF-alpha treatment of primary AMLs was associated with enhanced surface expression of the SCF receptor by some of these cells. While exogenous SCF induced clonogenic growth of all primary AML samples investigated, TNF-alpha failed to stimulate leukemic cells to proliferate. However, the combination of SCF and TNF-alpha resulted in synergistic growth stimulation in seven of nine different AML specimens investigated. The finding of transmodulation of the SCF receptor through posttranscriptional modifications might further contribute to our understanding of the synergistic interplay of TNF-alpha and SCF.