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A light‐switchable transgene system could be a powerful optogenetic tool for the precise manipulation of spatiotemporal gene expression in multicellular organisms. We have developed the LightOn system, which consists of a single chimeric protein (GAVPO) that can homodimerize and bind to promoters upon exposure to blue light, activating transcription of a target gene. This article describes protocols for precise control of gene expression in mammalian cells and mice using the LightOn system. These protocols can be carried out in an ordinary laboratory, as both liposome‐mediated transfection and hydrodynamic tail vein injection are routine methods that can easily transfer the LightOn system to mammalian cells and mouse liver, respectively. The illumination equipment can also be easily obtained. The LightOn system can provide a robust, convenient means to control the expression of a gene of interest, with unprecedented temporal and spatial accuracy in manipulating an extremely broad range of biological processes. Curr. Protoc. Chem. Biol. 5:111‐129 © 2013 by John Wiley & Sons, Inc.

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Background Hepatocellular carcinoma (HCC) is a leading cause of death in humans. Clinical investigations have reported that insulin-like growth factor 2 (IGF2) mRNA binding protein 1 (IGF2BP1) is overexpressed in HCC, promoting oncogene activation, and is associated with cancer progression and poor prognosis. Previous research from our laboratory found that coptisine (COP) could inhibit the proliferation of HCC cells, but the exact mechanism was unknown. Methods The activity of COP against HCC was evaluated by the experiments with HCC cells and HuH7 xenografted mice. Based on transcriptome analysis, cellular thermal shift assay (CETSA), and mass spectrometry analysis, the key direct target of COP in inhibiting the proliferation of HCC were discovered. In vitro and in vivo assays were performed to study the biological function of IGF2BP1 in HCC. Screening of E3 ligases and deubiquitin enzyme interacting with IGF2BP1 by IP-mass spectrometry and website prediction. The molecular mechanism of COP causes protein ubiquitination and degradation of IGF2BP1 was demonstrated by CO-IP, immunofluorescence and protein stability assays. Result The results disclosed that COP could inhibit the growth of liver cancer cells and tumors and IGF2BP1 was identified as the direct target of COP. IGF2BP1 overexpression and silencing in vivo and in vitro confirmed that IGF2BP1 could significantly influence the growth of liver cancer cells and tumors. Mechanistic studies revealed that COP could either increase the interaction between E3 ligase Pre-mRNA Processing Factor 19 (PRPF19) and IGF2BP1 or decrease the deubiquitin function of the deubiquitin enzyme Ubiquitin Specific Peptidase 10 (USP10), resulting in increased ubiquitin degradation of IGF2BP1. Conclusions These findings suggest that COP has an anti-HCC activity by reducing IGF2BP1 expression. It opens up new avenues for research into modern pharmacology and the molecular mechanisms of traditional Chinese medicine against tumors.