Inhibition of granulocyte colony-stimulating factor-mediated myeloid maturation by low level expression of the differentiation-defective class IV granulocyte colony-stimulating factor receptor isoform

In acute myeloid leukemia (AML), granulocyte colony-stimulating factor rece ptor (G-CSFR) proliferative and maturational signaling pathways are uncoupl ed. Seven human G-CSFR mRNA isoforms exist, named class I through class VII , The 183-amino acid cytosolic domain of the class I isoform provides all s ignaling activities. The class IV isoform is "differentiation defective" be cause the carboxyterminal 87 amino acids are replaced with 34 amino acids o f novel sequence. In more than 50% of AML samples, the class IV/class I G-C SFR mRNA ratio is aberrantly elevated compared to normal CD34(+) bone marro w cells, We hypothesized that the increased relative expression of class IV G-CSFR in AML uncouples proliferative and maturational G-CSFR signaling pa thways. To test this, we transfected the G-CSF-responsive murine cell line 32Dcl3 with class IV G-CSFR cDNA, After 10 days of G-CSF stimulation, clone s expressing class IV G-CSFR had greater percentages of myeloblasts and pro myelocytes than control (53% +/- 13% versus 3% +/- 2%), Differential counts over time demonstrated delayed G-CSF-driven maturation in 5 class IV-expre ssing clones, with 2 clones demonstrating a subpopulation that completely f ailed to differentiate, Heterologous class IV expression did not affect G-C SF-dependent proliferation. Class IV/ murine G-CSFR mRNA ratios after 24 ho urs of G-CSF stimulation for 3 of the 5 clones (range, 0.090 to 0.245; mean , 0.152 +/- 0.055) are within the range of class IV/class mRNA ratios seen in patients with AML, This indicates that aberrantly increased relative cla ss IV G-CSFR expression seen in AML can uncouple G-CSFR proliferative end m aturational signaling pathways, (Blood, 2000; 95:3335-3340) (C) 2000 by The American Society of Hematology.