S6 KINASE P90(RSK) IN GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR-STIMULATED PROLIFERATIVE AND MATURE HEMATOPOIETIC-CELLS

The ribosomal S6 kinase p90(rsk) was studied in mature and proliferati ng hemopoietic cells in response to the human cytokine granulocyte-mac rophage colony-stimulating factor (GM-CSF). In neutrophils, GM-CSF ind uced time-dependent electrophoretic mobility shifts in immunoreactive p90(rsk) Although these shifts suggested changes in the phosphorylatio n status of the molecule, a kinase assay with whole cell lysates detec ted minimal (1.5-fold) increments in enzymatic activity. Only immunopr ecipitation followed by immune complex kinase assay or in-gel kinase a ssay performed against the RSK substrate RRLSSLRA evidenced an increas e in p90(rsk) activity (SR-fold). p90(rsk) was also detected in the GM CSF-dependent erythroleukemia cell line TF-1. Normally cultured, cytok ine-supplemented cells did not respond to further GM-CSF stimulation. However, the activity of p90(rsk) in cytokine-starved cells increased dramatically in response to short term CRI-CSF challenge. This effect was readily observable in total cell lysates (6.6-fold increase over c ontrols) and was paralleled by changes in mitogen-activated protein ki nase activity (a substrate of p90(rsk)). Thus, p90(rsk) is present in mature hemopoietic cells, but the extent of the enzymatic response to GM-CSF is significantly lower than that seen in proliferative cells.