Abstract
Cytokines that signal through Class II receptors form a distinct family that includes the interferons and interleukin 10 (IL-10). Recent identification of several IL-10 homologs has defined a cytokine subfamily that includes AK155, IL-19, IL-20, IL-22, and IL-24. Within this subfamily, IL-19, IL-20, and IL-24 exhibit substantial sharing of receptor complexes; all three are capable of signaling through IL-20RA/IL-20RB, and IL-20 and IL-24 both can also use IL-22R/IL-20RB. However, the biological effects of these three cytokines appear quite distinct: immune activity with IL-19, skin biology with IL-20, and tumor apoptosis with IL-24. To more fully elucidate their interactions with the receptor complexes, we have performed a series of in vitro assays. Reporter, proliferation, and direct STAT activation assays using cell lines expressing transfected receptors revealed differences between the receptor complexes. IL-19 and IL-24 also exhibited growth inhibition on a cell line endogenously expressing all three receptor subunits, an effect that was seen at cytokine levels two orders of magnitude above those required for STAT activation or proliferation. These results demonstrate that, although this subclass exhibits receptor complex redundancy, there are differences in ligand/receptor interactions and in signal transduction that may lead to specificity and a distinct biology for each cytokine.
Highlights
Cytokines that signal through Class II receptors form a distinct family that includes the interferons and interleukin 10 (IL-10)
IL-19 and IL-24 exhibited growth inhibition on a cell line endogenously expressing all three receptor subunits, an effect that was seen at cytokine levels two orders of magnitude above those required for STAT activation or proliferation
We show that functional differences exist in the IL-20 subfamily in which IL-20, IL-19, and IL-24 exhibit sharing of receptor complexes
Summary
RT-PCR Analysis on Human Tissues—RT-PCR was performed on a human Rapid-Scan gene expression panel (Origene Technologies, Inc.) using primers 5Ј-ccccagacacggtctacagcat-3Ј and 5Ј-gggtcaggccgaagaactcatat-3Ј to amplify a 440-bp fragment of human IL22R. A human alpha actin probe was used as a positive control for the tissue samples, which corresponds to nt 603–1328 of sequence NM_001613. All tissues were tested with positive control probes and confirmed to be suitable for in situ hybridization analysis. Cells were cultured at 5000 cells/well with 60 pM human IL-20, IL-19, or IL-24 in 96-well flat-bottomed plates at 37 °C. Cells were plated in culture media at 5000 cells/well in 96-well flat-bottomed tissue culture-treated plates (Corning Costar) and allowed to adhere for 24 h in a 37 °C, 5% CO2 incubator. IL-20RA- and IL-20RB-soluble receptors were expressed as homodimeric IgG fusion proteins in BHK570 cells. The receptor subunits were co-expressed in BHK570 cells, and the IgG fusions were isolated from the culture media by protein-A chromatography. The heterodimers were purified away from the homodimers by immobilized metal chromatography utilizing an imidazole step gradient elution
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