Tumor progression, including metastasis, is significantly influenced by factors in the tumor microenvironment (TME) such as mechanical force, shear stress, chemotaxis, and hypoxia. At present, most cancer studies investigate tumor metastasis by conventional cell culture methods and animal models, which are limited in data interpretation. Although patient tissue analysis, such as human patient-derived xenografts (PDX), can provide important clinical relevant information, they may not be feasible for functional studies as they are costly and time-consuming. Thus, in vitro three-dimensional (3D) models are rapidly being developed that mimic TME and allow functional investigations of metastatic mechanisms and drug responses. One of those new 3D models is tumor-on-a-chip technology that provides a powerful in vitro platform for cancer research, with the ability to mimic the complex physiological architecture and precise spatiotemporal control. Tumor-on-a-chip technology can provide integrated features including 3D scaffolding, multicellular culture, and a vasculature system to simulate dynamic flow in vivo. Here, we review a select set of recent achievements in tumor-on-a-chip approaches and present potential directions for tumor-on-a-chip systems in the future for areas including mechanical and chemical mimetic systems. We also discuss challenges and perspectives in both biological factors and engineering methods for tumor-on-a-chip progress. These approaches will allow in the future for the tumor-on-a-chip systems to test therapeutic approaches for individuals through using their cancerous cells gathered through approaches like biopsies, which then will contribute toward personalized medicine treatments for improving their outcomes.

Transcription factors control the rate of transcription of genetic information from DNA to messenger RNA, by binding specific DNA sequences in promoter regions. Transcriptional gene control is a rate-limiting process that is tightly regulated and based on transient environmental signals which are translated into long-term changes in gene transcription. Post-translational modifications (PTMs) on transcription factors by phosphorylation or acetylation have profound effects not only on sub-cellular localization but also on substrate specificity through changes in DNA binding capacity. During times of cellular stress, specific transcription factors are in place to help protect the cell from damage by initiating the transcription of antioxidant response genes. Here we discuss PTMs caused by reactive oxygen species (ROS), such as H2O2, that can expeditiously regulate the activation of transcription factors involved in the oxidative stress response. Part of this rapid regulation are proteins involved in H2O2-related reduction and oxidation (redox) reactions such as redoxins, H2O2 scavengers described to interact with transcription factors. Redoxins have highly reactive cysteines of rate constants around 6-10-1 s-1 that engage in nucleophilic substitution of a thiol-disulfide with another thiol in inter-disulfide exchange reactions. We propose here that H2O2 signal transduction induced inter-disulfide exchange reactions between redoxin cysteines and cysteine thiols of transcription factors to allow for rapid and precise on and off switching of transcription factor activity. Thus, redoxins are essential modulators of stress response pathways beyond H2O2 scavenging capacity.

Significance: It has been proposed that cancer cells are heavily dependent on their antioxidant defences for survival and growth. Peroxiredoxins are a family of abundant thiol-dependent peroxidases that break down hydrogen peroxide, and they have a central role in the maintenance and response of cells to alterations in redox homeostasis. As such, they are potential targets for disrupting tumor growth. Recent Advances: Genetic disruption of peroxiredoxin expression in mice leads to an increased incidence of neoplastic disease, consistent with a role for peroxiredoxins in protecting genomic integrity. In contrast, many human tumors display increased levels of peroxiredoxin expression, suggesting that strengthened antioxidant defences provide a survival advantage for tumor progression. Peroxiredoxin inhibitors are being developed and explored as therapeutic agents in different cancer models. Critical Issues: It is important to complement peroxiredoxin knockout and expression studies with improved understanding of the biological function of the peroxiredoxins. While current results can be interpreted within the context that peroxiredoxins scavenge hydroperoxides, some peroxiredoxin family members appear to have more complex roles in regulating the response of cells to oxidative stress through protein interactions with constituents of other signaling pathways. Future Directions: Further mechanistic information is required for understanding the role of oxidative stress in cancer, the function of peroxiredoxins in normal versus cancer cells, and for the design and testing of specific peroxiredoxin inhibitors that display selectivity to malignant cells.

Recent studies suggest that Peroxiredoxin 1 (Prdx1), in addition to its known H₂O₂-scavenging function, mediates cell signaling through redox-specific protein–protein interactions. Our data illustrate how Prdx1 specifically coordinates p38MAPK-induced signaling through regulating p38MAPKα phosphatases in an H₂O₂ dose-dependent manner. MAPK phosphatases (MKP-1 and/or MKP-5), which are known to dephosphorylate and deactivate the senescence-inducing MAPK p38α, belong to a group of redox-sensitive phosphatases (protein tyrosine phosphatases) characterized by a low pKa cysteine in their active sites. We found that Prdx1 bound to both MKP-1 and MKP-5, but dissociated from MKP-1 when the Prdx1 peroxidatic cysteine Cys52 was over-oxidized to sulfonic acid, which in turn resulted in MKP-1 oxidation-induced oligomerization and inactivity toward p38MAPKα. Conversely, over-oxidation of Prdx1-Cys52 was enhancing in the Prdx1:MKP-5 complex with increasing amounts of H₂O₂ concentrations and correlated with a protection from oxidation-induced oligomerization and inactivation of MKP-5 so that activation toward p38MAPK was maintained. Further examination of this Prdx1-specific mechanism in a model of reactive oxygen species-induced senescence of human breast epithelial cells revealed the specific activation of MKP-5, resulting in decreased p38MAPKα activity. Taken together, our data suggest that Prdx1 orchestrates redox signaling in an H₂O₂ dose-dependent manner through the oxidation status of its peroxidatic cysteine Cys52.

Precision in redox signaling is attained through posttranslational protein modifications such as oxidation of protein thiols. The peroxidase peroxiredoxin 1 (PRDX1) regulates signal transduction through changes in thiol oxidation of its cysteines. We demonstrate here that PRDX1 is a binding partner for the tumor suppressive transcription factor FOXO3 that directly regulates the FOXO3 stress response. Heightened oxidative stress evokes formation of disulfide-bound heterotrimers linking dimeric PRDX1 to monomeric FOXO3. Absence of PRDX1 enhances FOXO3 nuclear localization and transcription that are dependent on the presence of Cys31 or Cys150 within FOXO3. Notably, FOXO3-T32 phosphorylation is constitutively enhanced in these mutants, but nuclear translocation of mutant FOXO3 is restored with PI3K inhibition. Here we show that on H₂O₂ exposure, transcription of tumor suppressive miRNAs let-7b and let-7c is regulated by FOXO3 or PRDX1 expression levels and that let-7c is a novel target for FOXO3. Conjointly, inhibition of let-7 microRNAs increases let-7-phenotypes in PRDX1-deficient breast cancer cells. Altogether, these data ascertain the existence of an H₂O₂-sensitive PRDX1-FOXO3 signaling axis that fine tunes FOXO3 activity toward the transcription of gene targets in response to oxidative stress. Antioxid. Redox Signal. 28, 62–77.