TNFα Inhibits IGFBP-3 through Activation of p38α and Casein Kinase 2 in Human Retinal Endothelial Cells

We recently reported a reciprocal relationship between tumor necrosis factor alpha (TNF alpha) and insulin-like receptor growth factor binding protein 3 (IGFBP-3) in whole retina of normal and IGFBP-3 knockout mice. A similar relationship was also observed in cultured retinal endothelial cells (REC). We found that TNF alpha significantly reduced IGFBP-3 levels and vice-versa, IGFBP-3 can lower TNF alpha and TNF alpha receptor expression. Since IGFBP-3 is protective to the diabetic retina and TNF alpha is causative in the development of diabetic retinopathy, we wanted to better understand the cellular mechanisms by which TNF alpha can reduce IGFBP-3 levels. For these studies, primary human retinal endothelial cells (REC) were used since these cells undergo TNF alpha-mediated apoptosis under conditions of high glucose conditions and contribute to diabetic retinopathy. We first cultured REC in normal or high glucose, treated with exogenous TNF alpha, then measured changes in potential signaling pathways, with a focus on P38 mitogen-activated protein kinase alpha (P38 alpha) and casein kinase 2 (CK2) as these pathways have been linked to both TNF alpha and IGFBP-3. We found that TNF alpha significantly increased phosphorylation of P38 alpha and CK2. Furthermore, specific inhibitors of P38 alpha or CK2 blocked TNF alpha inhibition of IGFBP-3 expression, demonstrating that TNF alpha reduces IGFBP-3 through activation of P38 alpha and CK2. Since TNF alpha and IGFBP-3 are key mediators of retinal damage and protection respectively in diabetic retinopathy, increased understanding of the relationship between these two proteins will offer new therapeutic options for treatment.