Erythropoietin Binding and Induced Differentiation of Rauscher Erythroleukemia Cell Line Red 5-1.5

Hematopoietic growth factor receptors

The molecular cloning for most of the hematopoietic growth factor receptors has been achieved over the past few years and revealed that they can by assigned to two discrete receptor families, namely the hematopoietic growth factor superfamily (HRS) and the receptor tyrosine kinase family (RTK). The members of the HRS, including granulocyte-macrophage colony-stimulating factor receptor (GM-CSF-R), interleukin 3 receptor (IL-3-R), granulocyte CSF receptor (G-CSF-R) and erythropoietin receptor (Epo-R), share a common binding domain and the absence of a tyrosine kinase domain in their cytoplasmic portion. In some cases (e.g., GM-CSF-R), the high-affinity receptor structure is obtained through the association of the low-affinity binding chain (alpha chain) with an accessory protein (beta chain). It is conceivable that this protein might also represent the common subunit shared by GM-CSF-R and by IL-3-R when they are co-expressed to form the putative GM-CSF-R/IL-3-R complex. Although tyrosine phosphorylation following ligand receptor activation seems to be a common event in the HRS, its role in the signal transduction mechanisms is unknown. Due to the structural analogies among the members of this family any new insight into one particular receptor member, such as its subunit structure and its signal transduction pathways, will be generalizable to the other family members. The subclass III of the RTK family, including the CSF-1-R and c-kit, is characterized by an additional insert into the kinase domain that recognizes and binds protein substrates. Ligand induced activation of the kinase domain and its signaling potential are mediated by receptor oligomerization which stabilizes interactions between adjacent cytoplasmic domains and leads to activation of kinase function by molecular interaction. Interestingly, the receptors included in this subclass are the products of well known cellular proto-oncogenes. A large variety of structural alteration found in receptor-derived oncogene products may lead to constitutive activation of receptor signals that, consequently, result in the subversion of the mechanisms controlling the cell growth.

Glycosylation of the murine erythropoietin receptor

Murine erythropoietin-responsive Rauscher Red 5-1.5 cells were used to determine the contribution of glycosylation to the size and function of the erythropoietin receptor. The half life of the receptors was determined to be 4 h. The number of receptors was not significantly decreased in cells treated for 48 h with inhibitors of glycosylation (tunicamycin, glucosamine or swainsonine) and their affinity was slightly enhanced in tunicamycin- or glucosamine-treated cells. Erythropoietin was cross-linked with two proteins of 104 and 86 kDa. Their molecular masses were not significantly reduced in cells treated with the glycosylation inhibitors. When immunoprecipitated cross-linked receptors were digested with endoglycosidases, the molecular masses of both proteins were only slightly modified giving values of 100 and 82 kDa. Thus we can conclude that the proteins cross-linked to erythropoietin are very weakly glycosylated.

Purification of erythropoiesis-stimulating activity (ESA) from one Actinomycetes strain

An Actinomycetes strain was found to produce a protein which stimulated the growth of murine erythroid progenitors in vitro. The protein was purified to homogeneity and was named Erythropoiesis-Stimulating Activity (ESA). ESA was an acidic glycoprotein with a molecular weight around 80,000. When mouse bone marrow cells were cultured in methylcellulose medium containing physiological concentrations of erythropoietin (0.048 units/ml), addition of ESA stimulated the growth of colony forming unit-erythroid (CFU-E) up to three-fold a in dose dependent manner. ESA was not active on the other hematopoietic progenitors.