The hyperresponsiveness of cells expressing truncated erythropoietin receptors is contingent on insulin-like growth factor-1 in fetal calf serum.

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70(S6K). Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.