Membrane-Associated and Soluble Granulocyte/Macrophage-Colony-Stimulating Factor Receptor α subunits are Independently Regulated in HL-60 Cells

The effects of granulocyte/macrophagecolony-stimulating factor (GM-CSF) are mediated by interaction with its composite receptor (GMR), which consists of a unique α subunit (GMRα) and a β subunit (GMRβ) that is common to the receptors for GM-CSF, interleukin 3, and interleukin 5. GMRβ is required for high-affinity binding, cell proliferation, and protein phosphorylation but has no intrinsic GM-CSF-binding activity. GMRα in isolation binds to GM-CSF with low affinity and can signal for increased glucose uptake. In addition to the membrane-bound receptor (mGMRα), there is a naturally occurring soluble isoform (sGMRα) that is released free into the pericellular milieu. Analysis of genomic sequences reveals that the soluble GMRα isoform comes about by alternative mRNA splicing. To examine GMRα expression, we developed a quantitative reverse transcription-polymerase chain reaction assay based on serial dilutions of in vitro transcribed GMRα RNA. This assay provides a strict log-log measure of GMRα RNA expression, distinguishes transcripts related to the soluble and membrane-associated isoforms, and quantitatively detects 0.1 fg of GMRα-related mRNA. There was little or no GMRα expression in two human lymphoid cell lines and in the erythroblastic leukemia cell line K562, but all myeloid cell lines tested expressed both the membrane-associated and soluble isoforms of GMRα. Baseline level of expression of both isoforms varied >20-fold among the myeloid cell lines studied. Differentiation of HL-60 cells to neutrophils with dimethyl sulfoxide led to a 2-fold downregulation of sGMRα and a 20-fold up-regulation of mGMRα. These differentiation-induced transcriptional changes were unrelated to changes in mRNA stability. These findings indicate that sGMRα is differentially expressed from mGMRα in human hematopoietic cells and that programmed downregulation of sGMRα may be important in myeloid maturation.