Receptors for platelet-derived growth factor on microvascular endothelial cells

PI3 K/Akt/mTOR-mediated translational control regulates proliferation and differentiation of lineage-restricted RoSH stem cell lines

BACKGROUND

We have previously derived highly similar lineage-restricted stem cell lines, RoSH and E-RoSH cell lines from mouse embryos and CD9hi SSEA-1- differentiated mouse embryonic stem cells, respectively. These cell lines are not pluripotent and differentiate readily into endothelial cells in vitro and in vivo.

RESULTS

We investigated the signaling pathway that maintains proliferation of these cells in an undifferentiated state, and demonstrate that PI3 K/Akt/mTOR, but not Raf/MEK/Erk, signaling in these cells was active during proliferation and was downregulated during endothelial differentiation. Inhibition of PI3 K/Akt/mTOR signaling, but not Raf/MEK/Erk, reduced proliferation and induced expression of endothelial specific proteins. During differentiation or inhibition of PI3 K/Akt/mTOR signaling, cyclinD2 transcript abundance in ribosome-enriched RNA but not in total RNA was reduced with a corresponding reduction in protein level. In contrast, transcript abundance of endothelial-specific genes e.g. Kdr, Tek and Pdgfralpha in ribosome-enriched RNA fraction was not reduced and their protein levels were increased. Together these observations suggested that translational control mediated by PI3K/Akt/mTOR signaling was critical in regulating proliferation and endothelial differentiation of lineage-restricted RoSH-like stem cell lines.

CONCLUSION

This study highlights translation regulation as a critical regulatory mechanism during proliferation and differentiation in stem cells.

Activation of the platelet-derived growth factor-receptor enhances survival of murine bone endothelial cells

The activation of the microvascular endothelial cell platelet-derived growth factor (PDGF) receptor (PDGF-R) by PDGF has been implicated in neoplastic angiogenesis. Here, we established cultures of murine bone microvascular endothelial cells and examined their response to stimulation with PDGF BB ligand and to blockade of PDGF-R signaling with the tyrosine kinase inhibitor STI571 (Gleevec). The addition of STI571 to cultures of bone endothelial cells blocked PDGF BB-induced phosphorylation in a dose-dependent manner and completely abrogated the activation of downstream targets Akt and ERK1/2. Coadministration of STI571 and Taxol also induced the activation of procaspase-3 and significant apoptosis. These data suggest that phosphorylation of PDGF-R stimulates survival pathways in bone endothelial cells and that by selectively inhibiting PDGF-R signaling with STI571, the cells are rendered sensitive to Taxol treatment. The therapeutic combination of STI571 and Taxol may be a powerful tool for targeting tumor-associated endothelial cells in the skeletal compartment.

The role of PDGF-BB on the development of the collateral circulation after acute arterial occlusion

The survival of tissues in the presence of arterial occlusion is critically dependent on the development of collateral blood vessels. Identification of the biochemical mediators and their mechanism of action is fundamental to an understanding of the evolution of the collateral circulation. The ability of PDGF-BB to promote this was evaluated in an animal model of hind limb ischaemia. In order to obtain significant quantities of this mitogen for use in our animal model, human recombinant PDGF-BB was expressed in a Chinese Hamster Ovary (CHO) cell line. The transfected CHO cells produced 544 micrograms/l of PDGF-BB in serum free medium (SFM). The 30 kDa form of PDGF-BB was purified to homogeneity as judged by silver staining and amino-acid sequencing. Purified PDGF-BB was shown to be bioactive by a cell proliferation assay. The exogenous administration of PDGF-BB enhanced the recovery of blood flow after acute arterial occlusion. The results suggest that PDGF-BB may have therapeutic value in promoting collateral development following arterial occlusion.