The Prolactin Receptor and Severely Truncated Erythropoietin Receptors Support Differentiation of Erythroid Progenitors

Abstract

The EpoR 1 belongs to a large family of cytokine receptors, many of which are required for the proliferation and differentiation of hematopoietic as well as other cell types (4 -7). Throughout life, eight different hematopoietic lineages arise from pluripotent stem cells in the bone marrow (8, 9). The exact role of growth factors in this process is not clear and has been described broadly by two alternative hypotheses. The stochastic hypothesis suggests that commitment of a progenitor to a particular lineage is a stochastic event, subsequent to which cell differentiation proceeds along a predetermined program; growth factors are merely required to ensure the survival and proliferation of committed progenitors (10 -14). The contrasting inductive, or instructive, hypothesis attributes to growth factors a direct role in cell differentiation, predicting that growth factor receptors transduce signals that uniquely specify the differentiation outcome of a progenitor (15)(16)(17). A number of hybrid hypotheses have also been proposed, where, for example, committed progenitors arise stochastically, but their subsequent differentiation and acquisition of the mature phenotype are uniquely induced by lineage-specific growth factors (18).Although Epo is essential for the production of red blood cells, it is not thought to play a role in the generation of committed erythroid progenitors from pluripotent progenitors: expression of recombinant EpoR does not bias the lineage commitment of pluripotent hematopoietic progenitors (19,20), and normal numbers of committed erythroid BFU-e and CFU-e progenitors are found in the fetal livers of EpoR Ϫ/Ϫ mutant mice (21). However, there is a unique requirement for EpoR activation during the subsequent proliferation and terminal differentiation of committed erythroid progenitors: the EpoR Ϫ/Ϫ CFU-e and BFU-e progenitors fail to give rise to mature erythrocytes unless EpoR is expressed by retroviral infection (21); and in vitro, other growth factors only partially substitute for . It is not known whether EpoR activation is essential at this stage of erythroid differentiation because of an EpoR-unique instructive signal or whether it is simply required for functions that are not unique to EpoR, such as its proliferative and anti-apoptotic effects.Some evidence for the capability of EpoR to promote the erythroid phenotype comes from the ability of Epo to induce surface expression of glycophorin (25) and transcription of the ␤-globin gene (26, 27) in pre-B Ba/F3 cells expressing a transfected EpoR. However, the uncertain lineage commitment of many cell lines and their incomplete differentiation response makes them less suitable for the study of signaling in differentiation. EpoR-mediated signaling for proliferation can be studied in a number of interleukin-3-dependent cell lines, where heterologous expression of EpoR allows Epo to support cell proliferation (1, 2). Only the membrane proximal ϳ120 amino acids is essential for this function (1-3). Similarly truncated mutants of other cytokine r...

Integrative Signaling by Minimal Erythropoietin Receptor Forms and c-Kit

“…1, 2, and 6, FDCER-HY343F cells failed to grow or survive in the presence of Epo yet efficiently supported c-Myc transcript induction. Notably, this result is consistent with the reported inactivity of cytoplasmic Tyr(P)-deficient chimeric receptor forms as assayed for CFU-e forming activity in transduced fetal cells (12), with the limited activity of a Tyr(P)-deficient Epo receptor form as assayed in fetal liver cells from Epo receptor Ϫ/Ϫ mice (11), and with the loss in CFU-e…”

“…Despite the complexity of this signaling network, studies of tyrosine-mutated and -truncated Epo receptors in cell lines (9,10), murine fetal liver (11,12), and adult murine marrow and spleen (13) have established that signals necessary for Epo receptor-dependent erythroid development are supported by receptor forms retaining only a membrane-proximal box domain for JAK2 binding plus a single phosphotyrosine Tyr(P)-343 STAT5 binding site. Additionally, in JAK2 Ϫ/Ϫ mice (14) (as in Epo receptor Ϫ/Ϫ mice (Ref.…”

Cytokine receptors and hematopoietic differentiation

“…Transduction of primary erythroid cells with a retrovirus encoding the M-CSFR enabled generation of erythroid colonies in response to M-CSF, and experiments with multipotential progenitors showed the same trend (McArthur et al, 1994;Pharr et al, 1994). The prolactin receptor, growth hormone receptor and c-MPL were all shown to be able to replace the requirement for EPOR in primary erythroid cell differentiation in vitro (Socolovsky et al, 1997;Goldsmith et al, 1998). Strong experimental support for the stochastic model came from the demonstration that ectopic expression of the prosurvival factor bcl-2 in certain hematopoietic cell lines resulted in hematopoietic differentiation in the absence of cytokines (Fairbairn et al, 1993).…”

The origin of hematopoietic cell type diversity

“…Similarly, mice lacking one or more than one of the colony-stimulating factors or their receptors develop normally, albeit with some deficiency in myeloid cell function. More intriguing is the fact that erythroid colony formation in vitro is observed in EpoR-deficient cells that are stimulated via other types of receptors, either the prolactin receptor (Socolovsky et al, 1997), or c-Mpl (Kieran et al, 1996), suggesting that erythroid cell differentiation is intrinsically determined and that Epo is required only to sustain cell survival. Finally, to verify whether c-Mpl or the G-CSF receptor has a deterministic effect on lineage outcome in megakaryocyte or granulocyte developments, the c-mpl gene was replaced by homologous recombination with a chimeric construct encoding the extracellular domain of c-Mpl and the intracellular domain of G-CSFR (Stoffel et al, 1999).…”