Abstract
The aim of this study was to investigate the interaction of Stat5 with key effector proteins Erk2 and Shc after activation by growth hormone (GH), using Chinese Hamster Ovary (CHO) cells stably expressing the wild type rabbit growth hormone receptor (GHR). In coimmunoprecipitation experiments, we show GH-induced formation of complexes consisting of Stat5a and Erk2, and Stat5a and Stat5b association with the protein adaptor Shc. In CHO cells treated with GH, a rapid association of tyrosine and serine phosphorylated Stat5a with activated Erk2 is observed. In contrast, Shc proteins interact with non-phosphorylated forms of Stat5. Using truncated and tyrosine mutants of the GHR, we identify a carboxy-terminal domain of the receptor, which is critical for serine phosphorylation of Stat5a and Stat5a/Erk2 complex formation. In addition, tyrosine residues of this region of the GHR are not required for Stat5a/Erk2 interaction but are essential for Stat5a serine phosphorylation. Moreover, we detect serine phosphorylated proteins associated with Erk2, Shc and Stat5: both Stat5 isoforms interact with a serine phosphorylated protein of 63 kDa, which is shown to be related to the serine-threonine kinase Akt-1. Our results support the importance of serine phosphorylation cascades in GH signaling and open another pathway of GH signal transduction.
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