INSULIN-LIKE GROWTH-FACTOR-I AUGMENTS ERYTHROPOIETIN-INDUCED PROLIFERATION THROUGH ENHANCED TYROSINE PHOSPHORYLATION OF STAT5

Insulin-like growth factor (IGF-I) is known to synergistically stimula te the proliferation of hematopoietic cells in combination with other hematopoietic growth factors. However, the precise mechanism underlyin g the cooperative effects of IGF-I is unknown. In a human interleukin- 3 or erythropoietin (EPO)-dependent cell line, F-36P, IGF-I alone fail ed to stimulate DNA synthesis but did augment the EPO dependent DNA sy nthesis of F-36P cells. The treatment of F-36P cells with a combinatio n of EPO and IGF-I (EPO/IGF-I) was found to enhance EPO-induced tyrosi ne phosphorylation of STATE, whereas IGF-I alone did not. Furthermore, c-CIS mRNA expression, one of the target molecules of STATE, was more effectively induced by EPO/IGF-I than by EPO alone. To examine the me chanisms of the EPO- and EPO/ IGF-I-induced proliferation of F-36P cel ls, we expressed dominant negative (dn) mutants of STATE and Ras in an inducible system. The EPO-induced DNA synthesis and the cooperative e ffect of EPO/IGF-I were significantly inhibited by the inducible expre ssion of dn-STAT5 or dn-Ras. In addition, the inducible expression of dn-Ras abolished the IGF-I-enhanced tyrosine phosphorylation of STATE. These results suggest that IGF-I may augment EPO-induced proliferatio n by enhancing tyrosine phosphorylation of STATE and raise the possibi lity that Ras may be involved in the augmentation of STATE tyrosyl pho sphorylation.