CSF-1 is the well-known ligand for CSF-1R, which plays a vital role in monocyte-macrophage generation, survival, and function. IL-34 is a newly discovered cytokine that also signals through CSF-1R. Although there are limited data for downstream signaling and pathway activation for CSF-1, none are published, to date, for expression profiles of IL-34. The objective of this study was to characterize and compare the signaling pathways downstream of the CSF-1R receptor, based on these two ligands. This was accomplished through transcriptional profiling and pathway analysis of CD14(+) human monocytes differentiated with each ligand. Additionally, cells were treated with a CSF-1R inhibitor GW2580 to establish that observations associated with each ligand were CSF-1R mediated. Gene expression profiles were generated for each condition using Agilent 4x44K Whole Human Genome Microarrays. Overall profiles generated by each cytokine were similar (~75% of genes) with a dampened effect noted on some pathways (~25% of genes) with IL-34. One key difference observed, between the two cytokines was in the repression of CCR2 message. A similar divergence in protein level was established by FACS analysis. The differential effect on CCR2 expression has major implications for monocyte/macrophage biology including homeostasis and function. Further study of IL-34 effects on monocyte/macrophage biology will shed light on the specific role each ligand plays and the context in which these roles are important. To our knowledge, this study is the first to illustrate downstream transcriptional profiles and pathways of IL-34 in comparison with CSF-1 and identify notable differences in CCR2 expression.
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