Stretch-induced retinal vascular endothelial growth factor expression is mediated by phosphatidylinositol 3-kinase and protein kinase C (PKC)-ζ but not by stretch-induced ERK1/2, Akt, Ras, or classical/novel PKC pathways

Abstract

Stretch-induced expression of vascular endothelial growth factor (VEGF) is thought to be important in mediating the exacerbation of diabetic retinopathy by systemic hypertension. However, the mechanisms underlying stretch-induced VEGF expression are not fully understood. We present novel findings demonstrating that stretch-induced VEGF expression in retinal capillary pericytes is mediated by phosphatidylinositol (PI) 3-kinase and protein kinase C (PKC)-ζ but is not mediated by ERK1/2, classical/novel isoforms of PKC, Akt, or Ras despite their activation by stretch. Cardiac profile cyclic stretch at 60 cpm increased VEGF mRNA expression in a time- and magnitude-dependent manner without altering mRNA stability. Stretch increased ERK1/2 phosphorylation, PI 3-kinase activity, Akt phosphorylation, and PKC-ζ activity. Signaling pathways were explored using inhibitors of PKC, MEK1/2, and PI 3-kinase; adenovirus-mediated overexpression of ERK, PKC-α, PKC-δ, PKC-ζ, and Akt; and dominant negative (DN) mutants of ERK, PKC-ζ, Ras, PI 3-kinase and Akt. Although stretch activated ERK1/2 through a Ras- and PKC classical/novel isoformdependent pathway, these pathways were not responsible for stretch-induced VEGF expression. Overexpression of DN ERK and Ras had no effect on VEGF expression in these cells. In contrast, DN PI 3-kinase as well as pharmacologic inhibitors of PI 3-kinase blocked stretch-induced VEGF expression. Although stretch-induced PI 3-kinase activation increased both Akt phosphorylation and activity of PKC-ζ, VEGF expression was dependent on PKC-ζ but not Akt. In addition, PKC-ζ did not mediate stretch-induced ERK1/2 activation. These results suggest that stretch-induced expression of VEGF involves a novel mechanism dependent upon PI 3-kinase-mediated activation of PKC-ζ that is independent of stretch-induced activation of ERK1/2, classical/novel PKC isoforms, Ras, or Akt. This mechanism may play a role in the well documented association of concomitant hypertension with clinical exacerbation of neovascularization and vascular permeability.