From the basic expression vector p/hVEGF165, containing a cDNA sequence encoding the 165-amino-acid isoform of human vascular endothelial growth factor (VEGF165), we generated an improved construct (p163/hVEGF165) by subcloning at the 5' exact end of the VEGF165 cDNA a 163-bp IRES element belonging to the 1,014-bp, 5'-untranslated region of the murine VEGF gene. This IRES structure caused enhanced synthesis and increased secretion of the mature protein both in HEK-293 and in COS-7 cells, when compared to the basic construct. Both p/hVEGF165 and p163/hVEGF165 vectors were tested for in vivo angiogenic activity on a novel hirudinean model. As expected, the p/hVEGF165 vector injected as naked DNA was able to induce angiogenesis in the non-vascularized muscular tissue of Hirudo medicinalis. This model also allowed us to monitor intracellular synthesis of VEGF165 and subsequent interstitial secretion from muscle cells. Interestingly, significantly larger muscle tissue areas underwent marked angiogenesis when the improved vector p163/hVEGF165 was injected in H. medicinalis. It thus appears that the p163/hVEGF165 construct allows enhanced expression of the human VEGF165 gene, which in turn is responsible for increased secretion of biologically active growth factor by transduced cells. Since a naked-DNA vector very similar to p/hVEGF165 was recently found to be very active in humans for treatment of heart and limb ischemia, we suggest that our improved construct might be further tested in view of potential therapeutic applications.

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