Peroxiredoxin (Prx), a family of ubiquitous thiol peroxidases, functions as a redox signaling regulator that controls cellular H2O2 in mammalian cells and has recently received attention for being overexpressed in various cancer types. In this study, we show that Prx type II (PrxII) is rather silenced in gastric cancer cells. PrxII expression is severely downregulated in 9 out of the 28 gastric cancer cell lines. Strikingly, PrxII expression is completely lost in three cell lines, MKN28, MKN74 and SNU484. Loss of PrxII expression is due to DNA methyltransferase 1-dependent methylation at the promoter region of the PrxII gene. Restoration of PrxII expression using a retroviral system markedly reduces the colony-forming ability and migratory activity of both MKN28 and SNU484 cells by inhibiting Src kinase. Mechanistically, PrxII peroxidase activity is essential for regulating gastric cancer cell migration. Bioinformatics analysis from The Cancer Genome Atlas stomach cancer data (STAD) revealed significantly low PrxII expression in gastric cancer patients and a negative correlation between PrxII expression and methylation levels. More importantly, low PrxII expression also strongly correlates with poor survival in cancer patients. Thus our study suggests that PrxII may be the first thiol peroxidase that simultaneously regulates both survival and metastasis in gastric cancer cells with high clinical relevance.

There are abundant peroxiredoxin (Prx) enzymes, but an increase of cellular H2O2 level always happens in apoptotic cells. Here, we show that cellular H2O2 switches different apoptosis pathways depending on which type of Prx enzyme is absent. TNF-α-induced H2O2 burst preferentially activates the DNA damage-dependent apoptosis pathway in the absence of PrxI. By contrast, the same H2O2 burst stimulates the RIPK1-dependent apoptosis pathway in the absence of PrxII by inducing the destruction of cIAP1 in caveolar membrane. Specifically, H2O2 induces the oxidation of Cys308 residue in the cIAP1-BIR3 domain, which induces the dimerization-dependent E3 ligase activation. Thus, the reduction in cIAP level by the absence of PrxII triggers cell-autonomous apoptosis in cancer cells and tumors. Such differential functions of PrxI and PrxII are mediated by interaction with H2AX and cIAP1, respectively. Collectively, this study reveals the distinct switch roles of 2-Cys Prx isoforms in apoptosis signaling.

Desensitization and acute tolerance are terms used to describe the attenuation of receptor responsiveness by prolonged or intermittent exposure to an agonist. Unlike desensitization of G protein-coupled receptors (GPCRs), which is commonly explained by steric hindrance caused by the β-arrestins that are translocated to the activated receptors, molecular mechanisms involved in the acute tolerance of GPCRs remain unclear. Our studies with several GPCRs and related mutants showed that the acute tolerance of GPCRs could occur independently of agonist-induced β-arrestin translocation. A series of co-immunoprecipitation experiments revealed a correlation between receptor tolerance and interactions among receptors, β-arrestin2, and Gβγ. Gβγ displayed a stable interaction with receptors and β-arrestin2 in cells expressing GPCRs that were prone to undergo tolerance compared to the GPCRs that were resistant to acute tolerance. Strengthening the interaction between Gβγ and β-arrestin rendered the GPCRs to acquire the tendency of acute tolerance. Overall, stable interaction between the receptor and Gβγ complex is required for the formation of a complex with β-arrestin, and determines the potential of a particular GPCR to undergo acute tolerance. Rather than turning off the signal, β-arrestins seem to contribute on continuous signaling when they are in the context of complex with receptor and Gβγ.

PICK1, a PDZ domain-containing protein, is known to increase the reuptake activities of dopamine transporters by increasing their expressions on the cell surface. Here, we report a direct and functional interaction between PICK1 and dopamine D₃ receptors (D₃R), which act as autoreceptors to negatively regulate dopaminergic neurons. PICK1 colocalized with both dopamine D₂ receptor (D₂R) and D₃R in clusters but exerted different functional influences on them. The cell surface expression, agonist affinity, endocytosis, and signaling of D₂R were unaffected by the coexpression of PICK1. On the other hand, the surface expression and tolerance of D₃R were inhibited by the coexpression of PICK1. These findings show that PICK1 exerts multiple effects on D₃R functions.