Regulation of STAT protein synthesis by c-Cbl

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Abstract
Many cytokines and growth factors induce transcription of immediate early response genes by activating members of the Signal Transducers and Activators of Transcription (STAT) family. Although significant progress has been made in understanding the events that lead to the activation of STAT proteins, less is known about the regulation of their expression. Here we report that murine embryonic fibroblasts derived from c-Cbl-deficient mice display significantly increased levels of STAT1 and STAT5 protein. In contrast, STAT2 and STAT3 expression, as well as the levels of the tyrosine kinases Jak1 and Tyk2, appear to be regulated independently of c-Cbl. Interestingly, the half-life of STAT1 was unaffected by the presence of c-Cbl, indicating that c-Cbl acts independently of STAT1 degradation. Further analysis revealed similar levels of STAT1 mRNA, however, a dramatically increased rate of STAT1 protein synthesis was observed in c-Cbl-deficient cells. Thus, our findings demonstrate an additional control mechanism over STAT1 function, and also provide a novel biological effect of the Cbl protein family.