Novel Strategy for Validating the Existence and Mechanism of the “Gut–Liver Axis” in Vivo by a Hypoxia-Sensitive NIR Fluorescent Probe

Liver fibrosis is a major stage in the development of liver disease, and it is important to investigate its pathogenesis for early intervention or even reversal. Recent studies have found that intestinal disease can aggravate liver fibrosis through the role of the "gut–liver axis". Hypoxia is considered to be a typical characteristic of many diseases including ulcerative colitis and liver fibrosis. However, there is no fluorescent probe for hypoxia detection to investigate the "gut–liver axis". Herein we design and synthesize a turn-on fluorescent probe termed Cy-AP, which displays high sensitivity and selectivity to hypoxia given by sodium dithionite (Na2S2O4) in vitro with near-infrared (NIR) emission (725 nm). The possible response mechanism of Cy-AP toward hypoxia is given and proved though HPLC, MS, and theory calculation. Moreover, on the basis of low cell cytotoxicity, the probe is used in visualizing hypoxia in four cell lines (HepG2, HCT116, HeLa, and MCF-7 cells) by fluorescence imaging, flow cytometry, and 3D imaging. Furthermore, due to its NIR emission, Cy-AP can monitor the hypoxia condition in vivo such as in liver fibrosis mice and ulcerative colitis mice models. In particular, the probe can validate the existence and mechanism of the "gut–liver axis" in vivo by monitoring hypoxia. To the best of our knowledge, this is the first work to give a strategy for studying the "gut–liver axis" by a NIR hypoxia-sensitive fluorescent probe, which will provide some powerful support for delaying the progression of liver fibrosis and thus promoting the treatment of liver disease.