GH Signaling Pathways Involving STAT Transcription Factors

GH stimulation. This led to the discovery that an intracellular signaling protein and transcription factor, termed STAT5b, is present in its nuclear, tyrosine phosphorylated form at a substantially higher level in male rat liver than in female rat liver 3, STATs are latent cytoplasmic transcription factors that are activated by tyrosine phosphorylation induced by a variety of cytokines and growth factors, and were first discovered as signal mediators that carry transcription signals into the nucleus in the interferon signaling pathway .  

In hypophysectomized rat liver, where there is no endogenous GH signaling, there is little or no tyrosine phosphorylated STAT5b protein in the nucleus essentially all of the STATSb protein is found in the cytosolic fraction, where it resides in a 

STATSb is not present in the nucleus at all times in male rat liver. Rather, STATSb is repeatedly activated in concert with the onset of each 

For example, if the liver is excised from a rat killed at the peak of a plasma GH pulse, then STATSb is tyrosine phosphorylated and localized to the nucleus, whereas if the liver is excised from a rat killed at a time point between successive plasma GH pulses, STATSb is inactive and cytoplasmic. Moreover, in female rats there is generally a much lower level of active, nuclear STATSb protein ( 5-10% that of the peak male liver level). This close temporal linkage between plasma GH pattern and the state of liver STATSb activation has been confirmed in intact male rats killed at times shown to be specifically associated with spontaneous peaks or troughs of the plasma GH rhythm. The key difference between male and female rat liver is that STATSb is repeatedly, and efficiently, activated by plasma GH pulses in the case of the 

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