Role of protein kinase C isoforms in phorbol ester-induced vascular endothelial growth factor expression in human glioblastoma cells

Aberrant expression of the potent angiogenic cytokine, vascular endothelial growth factor (VEGF), has been demonstrated to be associated with most human solid tumors. Both transcriptional and post-transcriptional mechanisms have been shown to modulate VEGF expression in a multitude of cell types. Here we report that when protein kinase C (PKC) pathways were activated in human glioblastoma U373 cells by phorbol 12-myristate 13- acetate (PMA), VEGF mRNA expression was up-regulated via a post- transcriptional mRNA stabilization mechanism. PMA treatment exhibited no increase in VEGF-specific transcriptional activation as determined by run- off transcription assays and VEGF promoter-luciferase reporter assays. However, PMA increased VEGF mRNA half-life from 0.8 to 3.6 h which was blocked by PKC inhibitors but not by protein kinase A or cyclic nucleotide- dependent protein kinase inhibitors. When U373 cells were transfected with antisense oligonucleotide sequences to the translation start sites of PKC- α, -β, -γ, -δ, -ε, or -ζ isoforms, both PKC-α and -ζ antisense oligonucleotides showed substantial inhibition of PMA-induced VEGF mRNA. In addition, overexpression of PKC-ζ resulted in a strong constitutive upregulation of VEGF mRNA expression. This study demonstrates for the first time that specific PKC isoforms regulate VEGF mRNA expression through posttranscriptional mechanisms.