Imbalanced gp130-Dependent Signaling in Macrophages Alters Macrophage Colony-Stimulating Factor Responsiveness via Regulation of c-fms Expression

Abstract

The mechanisms by which interleukin-6 (IL-6) family cytokines, which utilize the common receptor signaling subunit gp130, influence monocyte/macrophage development remain unclear. Here we have utilized macrophages devoid of either gp130-dependent STAT1/3 (gp130^{ΔSTAT/ΔSTAT}) or extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinase (gp130^Y757F/Y757F) activation to assess the individual contribution of each pathway to macrophage formation. While the inhibition by IL-6 of macrophage colony-stimulating factor (M-CSF)-induced colony formation observed in gp130^wt/wt mice was abolished in gp130^{ΔSTAT/ΔSTAT} mice, inhibition of macrophage colony formation was enhanced in gp130^Y757F/Y757F mice. In gp130^{ΔSTAT/ΔSTAT} bone marrow-derived macrophages (BMMs), both IL-6- and M-CSF-induced ERK1/2 tyrosine phosphorylation was enhanced. By contrast, tyrosine phosphorylation of ERK1/2 in response to M-CSF was reduced in gp130^Y757F/Y757F BMMs, and the pattern of ERK1/2 activation in gp130 mutant BMMs correlated with their opposing responsiveness to M-CSF-induced proliferation. When compared to the level of expression in gp130^wt/wt BMMs, c-fms expression was elevated in gp130^{ΔSTAT/ΔSTAT} BMMs but reduced in gp130^Y757F/Y757F BMMs. Finally, an ERK1/2 inhibitor suppressed M-CSF-induced BMM proliferation, and this result corresponded to a reduction in c-fms expression. Collectively, these results provide a functional and causal correlation between gp130-dependent ERK MAP kinase signaling and c-fms gene activation, a finding that provides a potential mechanism underlying the inhibition of M-CSF-dependent macrophage development by IL-6 family cytokines in mice.

We are grateful to Graham Lieschke and Ken Harder for valuable discussions and critical reading of the manuscript, and we thank Nicholas Wilson and Tony Burgess for critical reading of the manuscript. We also thank Elsbeth Richardson and Maureen Nerrie for technical assistance.

This work was supported in part by a research grant from the National Health and Medical Research Council of Australia.