Hyperosmolar stress induces monocyte chemoattractant protein 1 expression in retinal pigmented epithelial arising retinal pigmented epithelial 19 cells

Purpose Diabetes is a chronic inflammatory disease that may damage the blood-retinal barrier, leading to diabetic retinopathy (DR). Blood-retinal barrier rupture may subject the retinal pigmented epithelial cells to a hyperosmolar stress (HOS), activating the transcription factor nuclear factor of activated T cells 5 (NFAT5). In addition, inflammatory cytokines, such as monocyte chemoattractant protein 1 (MCP-1/CCL2), play a crucial role in DR. The aims of our study were to determine whether HOS induces MCP-1 levels in arising retinal pigmented epithelial 19 (ARPE-19) cells and to decipher the responsible intracellular cascade involved in such stimulation. Methods ARPE-19 cells or ARPE-19 cells transfected with dominant negative NFAT5 plasmid or NFAT5 short hairpin RNA plasmids were preincubated or not for 1 h in the absence or presence of a protein kinase or transcription factor inhibitor and then incubated for 8 h with iso-osmolar or hyperosmolar medium in the absence or presence of inhibitor. NFAT5 reporter gene activity was quantified by luminescence. MCP-1 messenger RNA (mRNA) and protein levels were determined by quantitative real-time PCR and enzyme-linked immunosorbent assay, respectively. Biologically active MCP-1 was assessed by a calcium mobilization assay performed using Chinese hamster ovary cells expressing or not the MCP-1 receptor and apoaequorin. Results In response to HOS, ARPE-19 cells showed a significant increase in MCP-1 mRNA levels independent of NFAT5 activation. Moreover, the MCP-1 protein secreted by ARPE-19 in response to HOS is biologically active. The use of various inhibitors of protein kinase and transcription factors suggest that the HOS-induced increase in MCP-1 mRNA levels is dependent on a protein kinase C (PKC) and/or a MEK1/2-p38 pathway activating p53, as well as a PKC-p38-PI3K-PDK1-AKT activating hypoxia-inducible factor 1 alpha (HIF1α). Conclusion HOS increases the expression of MCP-1 mRNA and protein levels in ARPE-19 cells, and the secreted MCP-1 is biologically active. The HOS-induced increase of MCP-1 mRNA appears to be independent of NFAT5 activation. Despite the activation of NFAT5 upon HOS and the presence of NFAT5 binding sites in the MCP-1 gene promoter, activated NFAT5 may not be sufficient to induce MCP-1 gene transactivation in response to HOS in ARPE-19 cells. The intracellular cascade involved in the HOS-induced increase of MCP-1 mRNA in ARPE-19 cells may consist of a PKC-p38-PI3K-PDK1-AKT-HIF1α axis and/or a MEK1/2-p38-p53 axis.