Sporadic activation of an oxidative stress-dependent NRF2-p53 signaling network in breast epithelial spheroids and premalignancies.

Breast and mammary epithelial cells experience different local environments during tissue development and tumorigenesis. Microenvironmental heterogeneity gives rise to distinct cell regulatory states whose identity and importance are just beginning to be appreciated. Cellular states diversify when clonal three-dimensional (3D) spheroids are cultured in basement membrane, and one such state is associated with stress tolerance and poor response to anticancer therapeutics. Here, we found that this state was jointly coordinated by the NRF2 and p53 pathways, which were costabilized by spontaneous oxidative stress within 3D cultures. Inhibition of NRF2 or p53 individually disrupted some of the transcripts defining the regulatory state but did not yield a notable phenotype in nontransformed breast epithelial cells. In contrast, combined perturbation prevented 3D growth in an oxidative stress-dependent manner. By integrating systems models of NRF2 and p53 signaling in a single oxidative stress network, we recapitulated these observations and made predictions about oxidative stress profiles during 3D growth. NRF2 and p53 signaling were similarly coordinated in normal breast epithelial tissue and hormone-negative ductal carcinoma in situ lesions but were uncoupled in triple-negative breast cancer (TNBC), a subtype in which p53 is usually mutated. Using the integrated model, we correlated the extent of this uncoupling in TNBC cell lines with the importance of NRF2 in the 3D growth of these cell lines and their predicted handling of oxidative stress. Our results point to an oxidative stress tolerance network that is important for single cells during glandular development and the early stages of breast cancer.

Tumor-Suppressor Inactivation of GDF11 Occurs by Precursor Sequestration in Triple-Negative Breast Cancer.

Triple-negative breast cancer (TNBC) is an aggressive and heterogeneous carcinoma in which various tumor-suppressor genes are lost by mutation, deletion, or silencing. Here we report a tumor-suppressive mode of action for growth-differentiation factor 11 (GDF11) and an unusual mechanism of its inactivation in TNBC. GDF11 promotes an epithelial, anti-invasive phenotype in 3D triple-negative cultures and intraductal xenografts by sustaining expression of E-cadherin and inhibitor of differentiation 2 (ID2). Surprisingly, clinical TNBCs retain the GDF11 locus and expression of the protein itself. GDF11 bioactivity is instead lost because of deficiencies in its convertase, proprotein convertase subtilisin/kexin type 5 (PCSK5), causing inactive GDF11 precursor to accumulate intracellularly. PCSK5 reconstitution mobilizes the latent TNBC reservoir of GDF11 in vitro and suppresses triple-negative mammary cancer metastasis to the lung of syngeneic hosts. Intracellular GDF11 retention adds to the concept of tumor-suppressor inactivation and reveals a cell-biological vulnerability for TNBCs lacking therapeutically actionable mutations.

Computational Models of Reactive Oxygen Species as Metabolic Byproducts and Signal-Transduction Modulators.

Reactive oxygen species (ROS) are widely involved in intracellular signaling and human pathologies, but their precise roles have been difficult to enumerate and integrate holistically. The context- and dose-dependent intracellular effects of ROS can lead to contradictory experimental results and confounded interpretations. For example, lower levels of ROS promote cell signaling and proliferation, whereas abundant ROS cause overwhelming damage to biomolecules and cellular apoptosis or senescence. These complexities raise the question of whether the many facets of ROS biology can be joined under a common mechanistic framework using computational modeling. Here, we take inventory of some current models for ROS production or ROS regulation of signaling pathways. Several models captured non-intuitive observations or made predictions that were later verified by experiment. There remains a need for systems-level analyses that jointly incorporate ROS production, handling, and modulation of multiple signal-transduction cascades.