Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) and Inflammatory Stimuli Up-Regulate Secretion of the Soluble GM-CSF Receptor in Human Monocytes: Evidence for Ectodomain Shedding of the Cell Surface GM-CSF Receptor {alpha} Subunit

Soluble GM-CSF receptor [alpha] subunit (sGMR[alpha]) is a soluble isoform of the GMR[alpha] that is believed to arise exclusively through alternative splicing of the GMR[alpha] gene product. The sGMR[alpha] mRNA is expressed in a variety of tissues, but it is not clear which cells are capable of secreting the protein. We show here that normal human monocytes, but not lymphocytes, constitutively secrete sGMR[alpha]. Stimulation of monocytes with GM-CSF, LPS, PMA, or A23187 rapidly up-regulates the secretion of sGMR[alpha] in a dose-dependent manner, demonstrating that secretion is also regulated. To determine whether sGMR[alpha] arose exclusively through alternative splicing of the GMR[alpha] gene product, or whether it could also be generated through ectodomain shedding of GMR[alpha], we engineered a murine pro-B cell line (Ba/F3) to express exclusively the cDNA for cell surface GMR[alpha] (Ba/F3.GMR[alpha]). The Ba/F3.GMR[alpha] cell line, but not the parental Ba/F3 cell line, constitutively shed a sGMR[alpha]-like protein that bound specifically to GM-CSF, was equivalent in size to recombinant alternatively spliced sGMR[alpha] (60 kDa), and was recognized specifically by a mAb raised against the ectodomain of GMR[alpha]. Furthermore, a broad-spectrum metalloprotease inhibitor (BB94) reduced constitutive and PMA-, A23187-, and LPS-induced secretion of sGMR[alpha] by monocytes, suggesting that shedding of GMR[alpha] by monocytes may be mediated in part through the activity of metalloproteases. Taken together, these observations demonstrate that sGMR[alpha] is constitutively secreted by monocytes, that GM-CSF and inflammatory mediators up-regulate sGMR[alpha] secretion, and that sGMR[alpha] arises not only through alternative splicing but also through ectodomain shedding of cell surface GMR[alpha].