SIGNAL-TRANSDUCTION BY THE RECEPTORS FOR THROMBOPOIETIN (C-MPL) AND INTERLEUKIN-3 IN HEMATOPOIETIC AND NONHEMATOPOIETIC CELLS

Antisense oligonucleotide to the translation initiation sequence of hu man c-mpl reduced the proliferation of human CD34(+) bone marrow cells in response to interleukin-3 (IL-3) alone or to the combination of IL -3 and thrombopoietin (TPO). To investigate the molecular basis for th ese cytokine interactions, we analyzed the relationship between the re ceptor subunits for IL-3 and TPO and determined whether both receptors activate identical signal transduction pathways. The function of the receptor subunits was characterized in transiently transfected hepatom a cells and fibroblasts by the activation of gene expression via speci fic regulatory elements and by the stimulation of DNA-binding activity of STAT proteins. Although c-mpl and IL-3 receptor (IL-3R) reconstitu ted a qualitatively comparable gene regulatory response, there was no detectable functional interaction between their respective receptor su bunits, By comparing the receptor action in different cell lines, we o bserved that in human hepatoma cells the signaling of c-mpl was 100-fo ld less sensitive to TPO than in rat hepatoma cells. However, IL-3R si gnaling was comparable between the two cell types, suggesting that c-m pl and IL-3R do not use identical signal transducing mechanisms. The c ytoplasmic domains necessary for c-mpl signaling were determined by te sting deletion mutants. The membrane-proximal box 1 sequence motif was critical for gene regulation and for STAT protein activation that see med to involve the Janus kinase 2 (JAK2), Because IL-3R was less depen dent on JAK2 than c-mpl, different levels of JAK2 expression may accou nt, in part, for the quantitative difference in IL-3 and TPO response among various cell lines. (C) 1995 by The American Society of Hematolo gy.