CHARACTERIZATION OF GROWTH HORMONE-INDUCED TYROSINE-PHOSPHORYLATED PROTEINS IN MOUSE CELLS THAT EXPRESS GH RECEPTORS

Following the growth hormone (GH) and GH receptor (R) interaction, the receptor and Janus tyrosine kinase 2 (JAK2) become tyrosine phosphory lated along with other intracellular proteins. Previously, we reported that GH induces tyrosine phosphorylation of intracellular proteins wi th molecular masses of approx 95 kDa (pp95) in mouse 3T3-F442A preadip ocytes and in mouse L-cells that express recombinant GHRs. We have stu died this GH-induced phosphorylation event in greater detail. Three pr oteins with apparent molecular masses of 93, 95, and 96 kDa showed inc reased tyrosine phosphorylation in a time-dependent manner following G H treatment of cells that express GH receptors. GH-induced tyrosine ph osphorylation of these proteins is independent of activation of protei n kinase C (PKC). Cell fractionation studies revealed that the majorit y of tyrosine-phosphorylated pp95/96 is located in the cytoplasm. pp95 and pp96 have pIs of approx 6.2. Immunoprecipitation and Western blot analyses revealed that pp93 and pp95/96 are not immunologically relat ed with Stat1, Stat3, Stat4, JAK2, and GHR. Thus, pp93 and pp95/96 may be important GH signal transducers independent of PKC activation and different from the characterized members in the JAK-STAT pathway.