Studies in cell lines have shown that Jak2 is the primary tyrosine kinase involved in signal transduction by the growth hormone receptor (GHR). In addition, growth hormone (GH) stimulates tyrosine phosphorylation of Jak1 and Jak3 in certain cell lines, while the effect on Tyk2 has not been analysed. We have investigated the expression of Jak proteins in human liver and analysed their interactions with the GHR. Using Western blot analysis and immunohistochemistry, we demonstrate that Jak1, Jak2, Jak3 and Tyk2 are present in human liver. Immunoprecipitation by an antibody against the GHR (Mab 263) followed by immunoblotting with specific antibodies against Jak proteins showed that Jak1, Jak2 and Tyk2 were associated with the GHR in this tissue. We conclude that the GHR associates with Jak1, Jak2 and Tyk2 in human liver. Although experiments in vitro indicate that Jak2 mediates GH signalling, our results open the possibility that other Jak proteins may influence GHR signalling in human liver.
Copyright 1999 Harcourt Publishers Ltd.