Growth factors - Pharmaceutical Biotechnology: Concepts and Aapplications - page 276

EPO is present in serum and (at very low concentrations) in urine, particularly of anaemic
individuals. This cytokine/hormone was fi rst purifi ed in 1971 from the plasma of anaemic
sheep, and small quantities of human EPO were later purifi ed (in 1977) from over 2500 l
of urine collected from anaemic patients. Large-scale purifi cation from native sources was
thus impractical. The isolation (in 1985) of the human EPO gene from a genomic DNA
library facilitated its transfection into CHO cells. This now facilitates large-scale commercial
production of the recombinant human product (rhEPO), which has found widespread medical
application.
EPO stimulates erythropoiesis by:

• increasing the number of committed cells capable of differentiating into erythrocytes;

• accelerating the rate of differentiation of such precursors;

• increasing the rate of haemoglobin synthesis in developing cells.

An overview of the best-characterized stages in the process of erythropoiesis is given in
Figure 10.4. The erythroid precursor cells, BFU-E (burst forming unit-erythroid), display EPO
receptors on their surface. The growth and differentiation of these cells into CFU-Es (where E
stands for erythroid) require the presence of not only EPO, but also IL-3 and/or GM-CSF. CFU-E
cells display the greatest density of EPO cell surface receptors. These cells, not surprisingly, also
display the greatest biological response to EPO. Progressively more mature erythrocyte precursors
display progressively less EPO receptors on their cell surfaces. Erythrocytes themselves are devoid
of EPO receptors. EPO binding to its receptor on CFU-E cells promote their differentiation into
proerythroblasts and the rate at which this differentiation occurs appears to determine the rate of
erythropoiesis. CFU-E cells also are responsive to IGF-1.
Although the major physiological role of EPO is certainly to promote red blood cell pro-
duction, EPO mRNA has also been detected in bone marrow macrophages, as well as some
multipotential haemopoietic stem cells. Although the physiological relevance is unclear, it is
possible that EPO produced by such sources may play a localized paracrine (or autocrine) role
in promoting erythroid differentiation. The level of EPO production in the kidneys (or liver)
is primarily regulated by the oxygen demand of the producer cells, relative to their oxygen
supply.
The EPO receptor is a member of the haemopoietic cytokine receptor superfamily. Its in-
tracellular domain displays no known catalytic activity, but it appears to couple directly to the
JAK2 kinase (Chapter 8) that likely promotes the early events of EPO signal transduction. Other
studies have implicated additional possible signalling mechanisms, including the involvement of
G proteins, protein kinase C and Ca 2 . The exact molecular events underlining EPO signal trans-
duction remain to be elucidated in detail.

10.2.6.1 Therapeutic applications of erythropoietin

A number of clinical circumstances have been identifi ed which are characterized by an often
profoundly depressed rate of erythropoiesis (Table 10.7). Many, if not all, such conditions could