Beta-adrenergic receptor stimulation modulates iNOS protein levels through p38 and ERK1/2 signaling in human retinal endothelial cells

Diabetic retinopathy is the leading cause of blindness in working-age adults. Recently, data has suggested that some of the pathological changes that occur in the diabetic retina may be due to increased expression of inflammatory markers. We have previously reported that human retinal endothelial cells cultured in high glucose show increased protein levels of iNOS, which were reduced when cells were pre-treated with a beta-1-adrenergic receptor agonist, xamoterol. The cellular signaling involved in this response is not known. The hypothesis of this study was that modulation of PKA activity through beta-adrenergic receptor stimulation can alter members of the mitogen-activated protein kinase (MAP kinase) family to regulate iNOS protein levels. To address this hypothesis, human retinal endothelial cells were grown in high glucose medium, treated with xamoterol, and immunoblotting and ELISA analyses were done to evaluate cellular signaling of PKA, p38 MAP kinase, and p42/p44 MAP kinase. The results indicate that treatment with xamoterol significantly reduced PKA levels in the retinal endothelial cells cultured in high glucose. This reduction in PKA was followed by decreased ratios of phosphorylated p42/p44 and p38 MAP kinases. Blockade of p42/p44 or p38 signaling could restore iNOS protein levels to those prior to xamoterol stimulation, suggesting that beta-adrenergic receptor regulates iNOS protein levels through regulation of PKA and MAP kinase signaling. These results offer new avenues for therapeutic intervention for retinal diseases that involve altered inflammatory marker expression.