Luminescent insulin-Au(III) conjugate retains insulin biological properties in human microglia

The aims of this study were to establish if insulin conjugated with auric gold Au(III) can be visualized in living cells expressing insulin receptors, and if key steps in insulin signaling are undisturbed. Au(III) is coordinated with three tyrosines of insulin and is strongly stabilized by coordination with these tyrosines into a metal-ion coordinated insulin template. We combined theoretical and experimental approaches, including computational analyses, immunocytochemistry and functional assays to analyze the cellular effects of this construct. Results in this study show that the insulin-Au(III) conjugate does not impair several steps in insulin signaling and downstream metabolic functions in lysosomes and lipid droplets. Namely, the signaling kinases protein kinase B (Akt1) and mitogen-activated protein kinases (ERK1/2) are phosphorylated to a comparable extent as with unmodified insulin. Immunocytochemistry also shows a comparable decrease in nuclear transcription factor EB (TFEB), an essential regulator of lysosomal biogenesis. Fluorescence co-labelling of lysosomes with LysoTracker and lipid droplets with BODIPY 493/503 shows normal organellar morphology and slightly enhanced lipid droplet content. The luminescent properties of insulin-Au(III) could serve to combine biochemical and imaging studies in human cells beyond microglia.