Lyn for Growth Hormone Receptor

Abstract

Growth hormone (GH) signals through its receptor GHR to stimulate proliferation. Receptor-associated kinases of the Jak family are well known for their role in coupling the GHR to activation of members of the transcription factor family known as STATs. Rowlinson et al . compared the crystal structures of the extracellular portion of the GHR bound to GH or an antagonist and noticed that the region called the F-G loop was in a different position. They analyzed cells expressing various mutations in the F-G loop and found that proliferation of transfected cells in response to GH was only partially reduced when compared with cells expressing wild-type GHR. Signaling through the Jak-STAT pathway was not altered in the cells expressing the F-G mutant receptors, but activation of the mitogen-activated protein kinase known as ERK in response to GH was decreased. A Src family kinase (SFK) was phosphorylated in response to GH in cells expressing the wild-type receptor, but not those expressing the F-G mutant receptors. The SFK Lyn was coimmunoprecipitated with the wild-type GHR and with the F-G mutant receptors in the presence or absence of agonist; however, only in cells expressing the wild-type receptor was Lyn phosphorylated in response to GH and thus activated. Thus, Lyn constitutively associated with the transfected wild-type or mutant receptors, but only wild-type receptors activated the kinase in response to agonist. Activation of ERK did not require Jak signaling, but instead appeared to proceed through a Lyn to phospholipase C pathway, because pharmacological inhibition of phospholipase C or SFK blocked ERK activation by GH in cells transfected with the wild-type receptor. Modeling of receptors with mutations predicted to alter the transmembrane domain alignment and then analysis of the activation of STAT or ERK in cells transfected with these receptors suggested that transmembrane domain alignment may influence the efficiency of signaling through one pathway or the other. These studies provide insight into the mechanism by which GH-stimulated Lyn-ERK and Jak-STAT signaling may be decoupled, which may have clinical relevance in the development of therapeutics to alter one pathway specifically. S. W. Rowlinson, H. Yoshizato, J. L. Barclay, A. J. Brooks, S. N. Behncken, L. M. Kerr, K. Millard, K. Palethorpe, K. Nielsen, J. Clyde-Smith, J. F. Hancock, M. J. Waters, An agonist-induced conformational change in the growth hormone receptor determines the choice of signalling pathway. Nat. Cell Biol. 10 , 740-747 (2008). [PubMed]