Insulin production in the retina drives autocrine signalling and metabolism reprogramming of the ARPE-19, a retinal pigment epithelium cellular model

The retinal pigment epithelium (RPE) plays a pivotal role in retinal homeostasis and energy metabolism. A recent study demonstrates that RPE cells release insulin in response to photoreceptor outer segment (POS) phagocytosis and starvation conditions. However, the downstream signalling pathway of this local insulin production has not yet been identified. Therefore, using the ARPE-19 cell line as an in vitro model of human RPE, we have investigated insulin signalling in basal conditions and after rod OS phagocytosis. Our data show that ARPE-19 cells express key pancreatic β-cell markers, including the transcription factor Pancreatic and Duodenal Homeobox-1 (PDX-1), which translocates to the nucleus in response to phagocytosis, and prohormone convertase 1/3 (PC1/3). In addition, ARPE-19 cells synthesize and secrete insulin already in basal conditions, increasing their release after phagocytosis. The RPE-secreted insulin acts in an autocrine manner, activating the canonical insulin signalling pathway and leading to increased phosphorylation of insulin receptor (IR), insulin receptor substrate-1 (IRS-1), and AKT. An upregulation of the insulin-responsive glucose transporter GLUT4 and increased glucose uptake was also observed, fueling the ARPE-19 cells' oxidative energy metabolism, incrementing the oxidative phosphorylation activity, probably to sustain the high energy demand associated with phagocytosis. At the same time, a decrease in lactate release has been observed. These features may have important implications for understanding retinal energy metabolism and developing novel therapeutic strategies for retinal neurodegenerative diseases.