Biology Reference
In-Depth Information
EPOR [61]. EPO has been shown to activate the common
-chain of the IL-3
and GM-CSF receptors through EPO-specific signaling pathways [62, 63].
These cross-talk phenomena need further investigation, particularly with
regard to the biology of EPOR in non-hematopoietic tissues.
β
Regulation of EPOR expression
EPOR
gene expression can be regulated at the transcriptional and post-tran-
scriptional levels and by post-translational modification [64-66]. At the level
of transcription, EPOR mRNA expression is controlled by
cis
acting elements
located upstream of the
EPOR
gene that mediate positive and negative regula-
tory effects. The transcription start site of the EPOR gene is located 141 bp
upstream of the initiation codon (Fig. 2). The minimal EPOR promoter region
(nt-76 to +33 of the transcription start site) does not contain a TATA or CAAT
box, but contains binding sites for the ubiquitous transcription factor Sp1 at
nt-20 and GATA does include transcription factors at nt-45. A positive regula-
tory domain from nt+1 to +78 contains potential binding sites (CANNTG) for
proteins of the helix-loop-helix family [67]. A negative regulatory domain has
been identified upstream of the minimal EPOR promoter region from nt-1050
to -450 which carries Alu repetitive elements [67, 68]. A second negative reg-
ulatory domain exists downstream of the transcription start site (nt+79 to
+135), where a potential binding site for Spl-like proteins exists (nt+85). Both
negative regulatory domains have an independent and non-cumulative effect on
the expression of the human
EPOR
gene [67]; however, erythroid-specific
expression of
EPOR
gene can be achieved if sequences are available which are
further downstream localized of the transcription start site (nt+1 to +135).
Since minimal promoters of several erythroid genes contain at least one
GATA binding site in close association with a CACCC or Sp1 motif, an essen-
tial role in regulating EPOR on hematopoietic cells is suggested [69]. The min-
imal EPO promoter can be transactivated in non-erythroid cells by the co-
transfection with GATA-1 [70, 71]. Maouche et al. concluded from reporter
gene assays in different cell lines that the fragment of
EPOR
gene containing
the GATA and Sp1 sites is not erythroid specific [67]. Mice with targeted
EPOR
-/-
deletion which were transgenic for two different GATA-1 minigene
cassettes with hematopoietic regulatory domains (GATA-1-HRD) were res-
cued from the lethal defect of EPOR deficiency [22]; however, further studies
are necessary to determine the specific regulatory processes of EPOR expres-
sion in non-hematopoietic tissues [67].
EPOR expression also underlies other regulatory processes. In the human
EPO-dependent leukemia cell line UT-7, treatment with EPO or GM-CSF
results in a transient decrease of EPOR mRNA expression. The down-regula-
tion of EPOR is preceded by a transient down-regulation of GATA-1 mRNA.
The expression of both EPOR and GATA-1 concomitantly decreases at the
G0/G1 phase and increases at the S and G2/M phases of the cell cycle.
