Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic growth factor that has been shown to support call proliferation in murine fibroblasts engineered to stably express both chains of the human GM-CSF receptor (NIH-GMR). Because the proto-oncogene c-fos is believed to provide a link between short-term signals elicited at the membrane and long-term cellular response, we chose to study the mechanism of GM-CSF-dependent cell regulation using c-fos promoter activity as a molecular marker in both NIH-GMR transfectants and in the CD34+ cell line TF-1. The importance of c-fos and related AP-1 activity in GM-CSF signalling was suggested by a tight correlation between GM-CSF-dependent activation of the c-fos promoter and cell proliferation and by the inhibitory effect of a trans-dominant c-fos mutant on cell growth. To evaluate the contribution of the serum response factor (SRF) associated with the ternary complex factor (TCF) and of STAT proteins to c-fos promoter activation in response to GM-CSF, the SRF binding site (SRE) and/or the STAT binding site (SIE) were inactivated. In serum-free medium, both SRE and SIE are essential to c-fos promoter activation by GM-CSF in NIH-GMR transfectants and in TF-1 cells. No response to GM-CSF was observed when both sites were mutated. The nature of the STAT family member was further investigated by Wester blots and DNA retardation assays using an SIE probe. Our data indicate that GM-CSF induced DNA binding of both STAT1 and STAT3 in NIH-GMR and mainly of STAT3 in TF-1 cells. STAT5 tyrosine phosphorylation was also observed in TF-1 cells. Finally, expression of a dominant negative MAPK mutant, ERK192A, resulted in a decrease of both SRE- and SIE-dependent activation of c-fos promoter by GM-CSF, suggesting that STAT1/3 are regulated not only by tyrosine kinases, but also partially by MAPK.

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