No abstract

Selenium (Se) is an essential micronutrient element, and the biological significance of Se is predominantly dependent on its incorporation as selenocysteine (Sec), the genetically encoded 21st amino acid in protein synthesis, into the active site of selenoproteins, which have broad functions, ranging from redox regulation and anti-inflammation to the production of active thyroid hormones. Compared to its counterpart Cys, there are only limited probes for selective recognition of Sec, and such selectivity is strictly restricted at low pH conditions. We reported herein the design, synthesis, and biological evaluations of a series of potential Sec probes based on the mechanism of nucleophilic aromatic substitution. After the initial screening, the structural determinants for selective recognition of Sec were recapitulated. The follow-up studies identified that probe 19 (Sel-green) responds to Sec and other selenols with more than 100-fold increase of emission in neutral aqueous solution (pH 7.4), while there is no significant interference from the biological thiols, amines, or alcohols. Sel-green was successfully applied to quantify the Sec content in the selenoenzyme thioredoxin reductase and image endogenous Sec in live HepG2 cells. With the aid of Sel-green, we further demonstrated that the cytotoxicity of different selenocompounds is correlated to their ability metabolizing to selenols in cells. To the best of our knowledge, Sel-green is the first selenol probe that works under physiological conditions. The elucidation of the structure-activity relationship for selective recognition of selenols paves the way for further design of novel probes to better understand the pivotal role of Sec as well as selenoproteins in vivo.

quinone oxidoreductase, thioredoxin, and thioredoxin reductase. A mechanistic study indicates that the α,β-unsaturated ketone structure in Xn and activation of the transcription factor Nrf2 are key determinants for the cytoprotection of Xn. Targeting the Nrf2 by Xn discloses a previously unrecognized mechanism underlying the biological action of Xn. Our results demonstrate that Xn is a novel small-molecule activator of Nrf2 in neuronal cells and suggest that Xn might be a potential candidate for the prevention of neurodegenerative disorders.

The selenoprotein thioredoxin reductase (TrxR) plays a pivotal role in regulating cellular redox homeostasis and has attracted increasing attention as a promising anticancer drug target. We report here that 2-(4-aminophenyl)-1,3,2-dithiarsinane (PAO-PDT, 4), a potent and highly selective small molecule inhibitor of TrxR, stoichiometrically binds to the C-terminal selenocysteine/cysteine pair in the enzyme in vitro and induces oxidative stress-mediated apoptosis in HL-60 cells. The molecular action of 4 in cells involves inhibition of TrxR, elevation of reactive oxygen species, depletion of cellular thiols, and activation of caspase-3. Knockdown of TrxR sensitizes the cells to 4 treatment, whereas overexpression of the functional enzyme alleviates the cytotoxicity, providing physiological relevance for targeting TrxR by 4 in cells. The simplicity of the structure and the presence of an easily manipulated amine group will facilitate the further development of 4 as a potential cancer chemotherapeutic agent.

The selenoprotein thioredoxin reductases (TrxRs) are attractive targets for anticancer drugs development. Xanthohumol (Xn), a naturally occurring polyphenol chalcone from hops, has received increasing attention because of its multiple pharmacological activities. We synthesized Xn and its 43 analogues and discovered that compound 13n displayed the highest cytotoxicity toward HeLa cells (IC50 = 1.4 μM). Structure-activity relationship study indicates that the prenyl group is not necessary for cytotoxicity, and introducing electron-withdrawing group, especially on the meta-position, is favored. In addition, methylation of the phenoxyl groups generally improves the potency. Mechanistic study revealed that 13n selectively inhibits TrxR and induces reactive oxygen species and apoptosis in HeLa cells. Cells overexpressing TrxR are resistant to 13n insult, while knockdown of TrxR sensitizes cells to 13n treatment, highlighting the physiological significance of targeting TrxR by 13n. The clarification of the structural determinants for the potency would guide the design of novel potent molecules for future development.