To reduce the side effects of chemotherapy, nontoxic prodrugs activated by the tumor microenvironment are urgently required for use in cancer treatment. In this work, we developed prodrug 4 for tumor-targeting treatment and imaging of the anticancer drug release in vivo. Taking advantage of the high glutathione (GSH) concentration in cancer cells, the disulfide bond in prodrug 4 was cleaved, resulting in the release of an active anticancer drug and a near-infrared (NIR) fluorescence dye turn-on. Furthermore, contrast to the free anticancer drug, the prodrug exhibited higher cytotoxicity to hepatoma cells than that to normal HL-7702 cells. Thus, prodrug 4 is a promising platform for specific tumor-activatable drug delivery system, because of its favorable features of in situ and in vivo monitoring of the drug release and therapeutic efficacy.

Rationale: Selenium has been shown to have chemotherapeutic effects against cancer. However, the anti-cancer mechanism of selenium is not fully understood, and the role of hydrogen selenide (H 2 Se), which is a common metabolite of dietary selenium compounds, has not been elucidated due to the lack of detection methods. In this study, we revealed a new anti-cancer mechanism of selenite with the help of a H 2 Se fluorescent probe. Methods: HepG2 cells were cultured under a simulated tumor hypoxic microenvironment. The H 2 Se and H 2 O 2 levels were detected by fluorescent probes in living cells and in mice. Autophagic and apoptotic proteins were detected by Western blotting. The redox of HMGB1 protein were analyzed by non-reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis. Results: After pharmacological doses of Na 2 SeO 3 treatment of HepG2 cells under hypoxic conditions, high levels of H 2 Se were produced before cell death. The H 2 Se accumulation resulted in reductive stress instead of oxidative stress, which was induced by Na 2 SeO 3 treatment under normoxic conditions. Furthermore, H 2 Se targeted the HMGB1 protein and induced cell autophagy. H 2 Se could interrupt the disulfide bond in HMGB1 and promote its secretion. The reduced HMGB1 outside the cells stimulated cell autophagy by inhibiting the Akt/mTOR axis. Here, autophagy played a dual role, i.e., mild autophagy inhibited apoptosis, while excessive autophagy led to autophagy-associated cell death. Conclusions: These results show that H 2 Se plays a key role during HepG2 cell death induced by selenite. Our findings reveal a new anti-cancer mechanism of selenite and provide a new research area for selenium studies.

The caspase cascade is an ensemble of very important signaling molecules that plays a critical role in cell apoptosis. Real-time monitoring of the upstream and downstream activation relationships of the caspases in the signal pathway is of great significance for understanding the regulatory mechanisms of these signaling molecules in the development of various diseases. Herein, a multicolor fluorescent nanoprobe, GNP-Se-Casp, has been developed based on Au−Se bonding for real-time in situ monitoring caspase-(casp-) 3, 8, and 9 during cell apoptosis. In the real-time fluorescence imaging of apoptotic HeLa cells induced by staurosporine using GNP-Se-Casp, the fluorescence signals corresponding to casp-8 and casp-9 sequentially turn on, followed by the appearance of the fluorescence of casp-3, which visualizes the upstream and downstream relationships of casp-3, -8, and -9. Thus, GNP-Se-Casp is an effective tool for real-time in situ monitoring of caspase cascade activation in the apoptosis process of tumor cells. This design strategy is easily adaptable to in situ detection of other signal molecules, especially those with upstream and downstream activation relationships.

The discovery that hypobromous acid (HOBr) can regulate the activity of collagen IV has attracted great attention. However, HOBr as an important reactive small molecule has hardly ever been studied using a detection method suitable for organisms. Herein, a high-quantum-yield mitochondria-targeting near-infrared (NIR) fluorescent probe for HOBr, RhSN-mito, was designed. RhSN-mito was easily obtained by the Suzuki cross-coupling reaction. The test results show that RhSN-mito can rapidly respond to HOBr with ultrasensitivity and high selectivity. The achievement of ultrasensitivity lies in the high signal-to-noise ratio and the highest fluorescence quantum yield of the reaction product (Φ = 0.68) in the near-infrared region, as far as we know. RhSN-mito is successfully applied to image native HOBr in mitochondria of HepG2 cells and zebrafish. Thus, RhSN-mito is a powerful tool for detecting native HOBr in vivo and is expected to provide a method to further study the physiological and pathological functions related to HOBr.

Hydrogen selenide (HSe) is an important metabolite of dietary Se compounds and has been implicated in various pathological and physiological processes. The development of highly sensitive and selective methods for the sensing of HSe is therefore very important. Herein, we developed a fluorescent probe (hemicyanine (Hcy)-HSe) for detecting HSe based on a new HSe-specific receptor unit, 1,2-dithiane-4,5-diol. Hcy-HSe showed high selectivity toward HSe over thiols (RSH), hydrogen sulfide (HS), and selenocysteine (Sec) and was further exploited for the fluorescence imaging of HSe both in living cells and in vivo. Furthermore, with the aid of Hcy-HSe, we demonstrated that HSe can be generated and gradually accumulated in HepG2 cells under hypoxic conditions and in the solid tumor after treatment with NaSeO.