The GM-CSF Receptor Utilises Members of the Canonical Wnt-Signaling Pathway during Macrophage Differentiation

Over the past several years we have used a factor-dependent murine bi-potential cell line, FDB1, in combination with activated mutants of the GM-CSF receptor common beta subunit (hβc) to dissect receptor pathways and gene expression changes that can contribute to myeloid cell growth and differentiation. Of particular interest are the mechanisms by which signalling from the GM-CSF receptor can promote simultaneous granulocyte and macrophage differentiation. In this context we have characterised differentiation of FDB1 cells in response to an activated hβc mutant (FIΔ). For this mutant, which confers growth factor-independent granulocyte-macrophage differentiation in the FDB1 cell line, the presence of a single intracellular tyrosine residue (Y577) is a requirement for granulocyte differentiation. In the absence of this tyrosine residue FDB1 cells expressing FIΔ undergo uni-lineage macrophage differentiation (Brown et al., 2004). Comparative transcriptional profiling of these FDB1 populations has allowed us to identify gene expression changes associated with receptor-induced granulocyte or macrophage differentiation. An examination of the genes selectively associated with macrophage differentiation revealed a potential role for the transcription factor TCF4/TCF7L2, which is a central mediator of the canonical Wnt signalling pathway through its role as the DNA binding co-factor for β-catenin. Over-expression of TCF4 in FDB1 cells could initiate growth arrest and differentiation, under growth-promoting conditions. In FDB1 cells expressing the FIΔ mutant forced TCF4 expression resulted in an increase in the proportion of cells differentiating to macrophages. Further examination of signalling in the FDB1 FIΔ populations demonstrated that stabilisation of β-catenin is associated with the switch to macrophage differentiation. GM-CSF signalling in FDB1 cells also induces transient β-catenin stabilisation during bi-lineage GM differentiation. In FDB1 cells stabilisation of β-catenin and myeloid differentiation is also induced using the GSK3β inhibitor BIO (6-bromoindirubin-3′-oxime). The results using this unique model of receptor-induced GM differentiation are consistent with a role for the β-catenin pathway in regulating the macrophage lineage.