HS1 interacts with Lyn and is critical for erythropoietin-induced differentiation of erythroid cells

Erythroid cells terminally differentiate in response to erythropoietin bind ing its cognate receptor. Previously we have shown that the tyrosine kinase Lyn associates with the erythropoietin receptor and is essential for hemog lobin synthesis in three erythroleukemic cell lines. To understand Lyn sign aling events in erythroid cells, the yeast two-hybrid system was used to an alyze interactions with other proteins. Here we show that the hemopoietic-s pecific protein HS1 interacted directly with the SH3 domain of Lyn, via its proline-rich region. A truncated HS1, bearing the Lye-binding domain, was introduced into J2E erythroleukemic cells to determine the impact upon resp onsiveness to erythropoietin. Truncated HS1 had a striking effect on the ph enotype of the J2E line-the cells were smaller, more basophilic than the pa rental proerythoblastoid cells and had fewer surface erythropoietin recepto rs. Moreover, basal and erythropoietin-induced proliferation and differenti ation were markedly suppressed. The inability of cells containing the trunc ated HS1 to differentiate maybe a consequence of markedly reduced levels of Lyn and GATA-1. In addition, erythropoietin stimulation of these cells res ulted in rapid, endosome-mediated degradation of endogenous HS1. The trunca ted HS1 also suppressed the development of erythroid colonies from fetal li ver cells. These data show that disrupting HS1 has profoundly influenced th e ability of erythroid cells to terminally differentiate.