Carboxy-terminal truncated STAT5 is induced by interleukin-2 and GM-CSF in human neutrophils

IL-2 and GM-CSF are potent activators of polymorphonuclear neutrophils (PMN) biologic activity. IL-2 and GM-CSF-mediated activation of STAT proteins was examined in nuclear extracts of human PMN. We found that both cytokines induced STAT5-like DNA-binding complexes that could not be supershifted using C-terminal-specific anti-STAT5 antibodies. Therefore, we performed oligoprecipitation experiments with a STAT5-biotinylated DNA probe (biotin-MGFe) and the precipitated proteins were identified by Western immunoblotting. We found that GM-CSF and IL-2 induced the DNA-binding activity of a C-terminal truncated isoform of STAT5. The truncated STAT5 form was present in the nucleus of PMN but the cytoplasmic extracts contained full-length STAT5, suggesting that PMN proteolytically process full-length STAT5 proteins. Proteolytic experiments demonstrated that PMN express a protease activity capable of producing C-terminal processed STAT5 proteins. In many settings, C-terminal truncation of the STAT5 protein leads to inhibition of STAT5 biological activity. Two known STAT5 regulated genes, encoding pim-1 and OSM proteins, failed to be induced by GM-CSF in PMN. These findings provide new insights to a mechanism by which PMN, a terminally differentiated cell, may regulate gene transcription by alternative proteolytic processing.