α4β1 integrin and erythropoietin mediate temporally distinct steps in erythropoiesis: integrins in red cell development

Abstract

Erythropoietin (Epo) is essential for the terminal proliferation and differentiation of erythroid progenitor cells. Fibronectin is an important part of the erythroid niche, but its precise role in erythropoiesis is unknown. By culturing fetal liver erythroid progenitors, we show that fibronectin and Epo regulate erythroid proliferation in temporally distinct steps: an early Epo-dependent phase is followed by a fibronectin-dependent phase. In each phase, Epo and fibronectin promote expansion by preventing apoptosis partly through bcl-xL. We show that α4, α5, and β1 are the principal integrins expressed on erythroid progenitors; their down-regulation during erythropoiesis parallels the loss of cell adhesion to fibronectin. Culturing erythroid progenitors on recombinant fibronectin fragments revealed that only substrates that engage α4β1-integrin support normal proliferation. Collectively, these data suggest a two-phase model for growth factor and extracellular matrix regulation of erythropoiesis, with an early Epo-dependent, integrin-independent phase followed by an Epo-independent, α4β1-integrin–dependent phase.

Down-regulation of Myc Is Essential for Terminal Erythroid Maturation

“…The initial stages of terminal erythroid maturation are highly dependent on erythropoietin (1), whose roles in activation of erythroid specific genes, terminal proliferation, and protection against apoptosis are well understood (2). This is followed by an erythropoietin-independent, fibronectin-dependent phase where survival and proliferation of the erythroblasts require signaling by ␣4␤1 integrins (3). Upon erythropoietin stimulation, CFU-E progenitors undergo 4 -5 cell divisions accompanied by a decrease in cell size, increase in hemoglobin content, and nuclear condensation followed by withdrawal from the cell cycle (4).…”

VLA-5-mediated Adhesion to Fibronectin Accelerates Hemin-stimulated Erythroid Differentiation of K562 Cells through Induction of VLA-4 Expression

“…The ECM in the hematopoietic microenvironment is composed of various macromolecules, such as fibronectin (FN), collagens, laminins, and proteoglycans. Among them, FN is one of the most important parts of the microenvironment niche (7)(8)(9)(10)(11). Also, in erythropoiesis, the importance of the adhesion of erythroid progenitors to FN via the FN receptors VLA-4 and VLA-5 has been reported (11)(12)(13)(14)(15)(16).…”

“…Among them, FN is one of the most important parts of the microenvironment niche (7)(8)(9)(10)(11). Also, in erythropoiesis, the importance of the adhesion of erythroid progenitors to FN via the FN receptors VLA-4 and VLA-5 has been reported (11)(12)(13)(14)(15)(16). However, the substantial role of these FN receptors and their functional assignment in erythroid differentiation are not yet fully understood.…”

Cell-derived Bone marrow-mimetics induce erythroid lineage commitment of hematopoietic stem and progenitor cells ex-vivo

“…Vice versa, ECM cultured AT-and SN-cells showed reduced myeloid colony formation regarding CFU-G, CFU-M and CFU-GM (Figure 1f). It has been demonstrated that erythropoiesis requires ITGb1 mediated FN adhesion [10] and ITGb3 expression is a marker for erythroid commitment [7]. To test whether erythroid lineage commitment is associated with ITGb3 expression in our ex-vivo ECM culture, we separated CD34 + ITGb3 + and CD34 + ITGb3 -cells.…”

“…Following this line, ECM proteins exhibit multiple binding sites for colony-stimulating factors (CSFs), which can trigger myeloid lineage commitment [8] [9]. Additionally, ECM proteins like fibronectin (FN) were found to be essential for erythropoiesis [10] and pro-erythroid growth factors like IL-3 and BMP4 are incorporated in ECM scaffolds [5]. Therefore, we asked whether BM-mimetic ECM scaffolds might trigger erythropoiesis when culturing HSPC.…”