Functional characterization of the transcription factor early growth response 1 (Egr1) in arteriogenesis

The number of patients suffering from obstructive arterial disease
is still increasing. Stimulation of a patient’s collateralization
(arteriogenesis), though an auspicious therapeutic approach, is
still not part of current therapy regimes. Further studies on the
molecular level are needed to understand the genetic regulation in
this process. The transcription factor early growth response 1
(Egr1) was shown to partic-ipate in leukocyte recruitment and cell
proliferation in vitro. This work contributes to the acquisition of
new insights into its mode of action in vivo. Using a model of
peripheral arteriogenesis, Egr1 was found significantly upregulated
in growing col-laterals of wild-type mice (WT), both on mRNA
(2.24fold) and protein level (2.3fold). Egr1 stained positive in EC
and vSMCs of collaterals as well as in nerves. In LDI measurements
conducted over the period of 21 days evidenced a delayed perfusion
recovery after femoral artery ligation in Egr1-/- mice compared to
WT mice (day7: 0.46±0.05 in Egr1-/- vs. WT (0.73±0.04), day 14:
0.65±0.02 in Egr1-/- vs. 0.88±0.04 in WT and day 21: 0.79 ±0.03 in
Egr1-/- vs. 0.96±0.02 in WT). Under baseline conditions, Egr1-/-
showed increased levels of monocytes (521.89±52.9 cells/µl vs.
326.56±21.6 cells/µl in WT) and granulocytes (811.79±79.96 cells/µl
vs. WT 559.88±34.57 cells/µl) in the circulation but reduced levels
in adductor muscles (18.14±2.73 cells/µl vs. 51.22±4.38 cells/µl in
WT) as evidenced by FACS analyses. After femoral artery ligation,
more macrophages were detected in the perivascular space of
collateral arteries in Egr1-/- (8.10±0.99 per vessel) vs. WT
(6.12±0.45 per vessel) mice. The mRNA of leukocyte recruitment
mediators monocyte chemoattractant protein 1 (MCP-1), intercellular
adhesion molecule 1 (ICAM-1) and urokinase plasminogen activator
(uPA) were found upregulated in both groups. Whereas other Egr
family members (Egr2-4) did not show an upregulation in WT
collateral arteries, they were found significantly upregulated in
Egr1-/- mice suggesting a mechanism of counter-balancing Egr1
deficiency. A closer look at cell cycle regulators revealed that
cyclin E and cdc20 were found upregulated in WT as well as in
Egr1-/- mice. However, cyclin D1 was hardly detectable under Egr1
deficiency conferring Egr1 an unique role for cyclin D1
transcription. vSMC phenotype switch is a critical step towards
vSMC proliferation and therefore arteriogenesis. In this context,
the downregu-lation of alpha smooth muscle actin (αSM-actin) and of
the transcriptional repressor, splicing factor-1 (SF-1) has been
shown to be critical in vitro. During arteriogenesis, SF-1 has been
found downregulat-ed in collaterals of WT mice but was 1.64fold
upregulated in Egr1-/-. Similar was true for αSM-actin. Whereas in
WT mice αSM-actin is downregulated at 12h after ligation Egr1
deficient mice evidenced an upregulation of αSM-actin. The strong
upregulation of the nonselective proliferation marker ki67 in WT
mice was not detectable under Egr1 deficiency evidencing
furthermore a delay in vascular cell proliferation. Conclusion:
Compensation for deficiency of Egr1 function in leukocyte
recruitment can be mediated by other transcription factors;
however, Egr1 is indispensable for effective vascular cell cycle
progression and phenotype switch in arteriogenesis.