GROWTH HORMONE-INDUCED SIGNAL-TRANSDUCTION DEPENDS ON AN INTACT UBIQUITIN SYSTEM

The growth hormone receptor (GHR) is a ubiquitinated cell surface prot ein, Ligand binding and receptor dimerization activate the cytosolic k inase Jak2. This event initiates signal transduction via STAT proteins . Expression of GHR in a Chinese hamster ovary (CHO) cell line, which exhibits a temperature-sensitive defect in ubiquitin conjugation (CHO- ts20), as well as in wild type cells (CHO-E36) has shown that endocyto sis of the receptor requires an intact ubiquitin conjugation system (S trous G. J., van Kerkhof, P., Govers, R., Ciechanover A., and Schwartz , A. L. (1996) EMBO J. 15, 3806-3812). We have now examined the requir ement for ubiquitin conjugation in growth factor-mediated signal trans duction. In CHO-E36 and in CHO-ts20 cells at the permissive temperatur e, STAT proteins were activated in a growth factor-dependent fashion. However, no activation of STAT proteins was observed at the nonpermiss ive temperature in CHO-ts20 cells. Neither tyrosine phosphorylation of GHR nor of Jak2 was inhibited at the nonpermissive temperature. When tyrosine phosphorylation was inhibited following treatment with stauro sporin, ubiquitination of the receptor proceeded normally, Furthermore , mutation of GHR phenylalanine-327, which prevents GHR endocytosis, i nhibited receptor ubiquitination but allowed normal JAK/STAT-mediated signal transduction. Thus, these data provide evidence that the ubiqui tin conjugation system is involved in the Jak/STAT signaling pathway, be it not at the initial stage(s) of Jak2 activity.