INTERLEUKIN-3-INDUCED ACTIVATION OF THE JAK STAT PATHWAY IS PROLONGEDBY PROTEASOME INHIBITORS/

One facet of cytokine receptor signaling involves the activation of si gnal transducers and activators of transcription (STATs). STATs are ra pidly activated via tyrosine phosphorylation by Janus kinase (JAK) fam ily members and subsequently inactivated within a short period. We inv estigated the effect of proteasome inhibition on interleukin-3 (IL-3) activation of the JAK/STAT pathway following stimulation of Ba/F-3 cel ls, Treatment of Ba/F-3 cells with the proteasome inhibitor, N-acetyl- L-leucinyl-L-leucinyl-norleucin (LLnL), led to stable tyrosine phospho rylation of the IL-3 receptor, beta common (pc), and STATE following s timulation. The effects of LLnL were not restricted to the JAK/STAT pa thway, as Shc and mitogen-activated protein kinase (MAPK) phosphorylat ion were also prolonged in LLnL-treated cells. Further investigation s howed these stable phosphorylation events were the result of prolonged activation of JAK2 and JAK1. These observations were confirmed using pharmacologic inhibitors. In the presence of LLnL, stable phosphorylat ion of STATE and pc was abrogated if the tyrosine kinase inhibitor, st aurosporine, was added. The effect of staurosporine on STATE phosphory lation could be overcome if the phosphatase inhibitor, vanadate, was a lso added, suggesting phosphorylated STATE could be stabilized by phos phatase, but not by proteasome inhibition per se. These observations a re consistent with the hypothesis that proteasome mediated protein deg radation can modulate the activity of the JAK/STAT pathway by regulati ng the deactivation of JAK. (C) 1998 by The American Society of Hemato logy.