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 hum an solid tumors. Both transcriptional and post-transcriptional mechanisms h ave been shown to modulate VEGF expression in a multitude of cell types. He re we report that when protein kinase C (PKC) pathways were activated in hu man glioblastoma U373 cells by phorbol 12-myristate 13-acetate (PMA), VEGF mRNA expression was up-regulated via a post-transcriptional mRNA stabilizat ion mechanism. PMA treatment exhibited no increase in VEGF-specific transcr iptional 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 prote in 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-alpha, -beta, -gamma, -delta, -epsilon, or -zeta isoforms, both PKC-alpha and -zeta antisense oligonucleotides showed substantial inhibition of PMA-induced VEGF mRNA. In addition, overexpressio n of PKC-zeta resulted in a strong constitutive up-regulation of VEGF mRNA expression. This study demonstrates for the first time that specific PKC is oforms regulate VEGF mRNA expression through post-transcriptional mechanism s.