A chronic signaling TGFb zebrafish reporter identifies immune response in melanoma.

Developmental signaling pathways associated with growth factors such as TGFb are commonly dysregulated in melanoma. Here we identified a human TGFb enhancer specifically activated in melanoma cells treated with TGFB1 ligand. We generated stable transgenic zebrafish with this TGFb Induced Enhancer driving green fluorescent protein (TIE:EGFP). TIE:EGFP was not expressed in normal melanocytes or early melanomas but was expressed in spatially distinct regions of advanced melanomas. Single-cell RNA-sequencing revealed that TIE:EGFP+ melanoma cells down-regulated interferon response while up-regulating a novel set of chronic TGFb target genes. ChIP-sequencing demonstrated that AP-1 factor binding is required for activation of chronic TGFb response. Overexpression of SATB2, a chromatin remodeler associated with tumor spreading, showed activation of TGFb signaling in early melanomas. Confocal imaging and flow cytometric analysis showed that macrophages localize to TIE:EGFP+ regions and preferentially phagocytose TIE:EGFP+ melanoma cells compared to TIE:EGFP- melanoma cells. This work identifies a TGFb induced immune response and demonstrates the need for the development of chronic TGFb biomarkers to predict patient response to TGFb inhibitors.

MITF regulates IDH1 and NNT and drives a transcriptional program protecting cutaneous melanoma from reactive oxygen species.

Microphthalmia-associated transcription factor (MITF) plays pivotal roles in melanocyte development, function, and melanoma pathogenesis. MITF amplification occurs in melanoma and has been associated with resistance to targeted therapies. Here, we show that MITF regulates a global antioxidant program that increases survival of melanoma cell lines by protecting the cells from reactive oxygen species (ROS)-induced damage. In addition, this redox program is correlated with MITF expression in human melanoma cell lines and patient-derived melanoma samples. Using a zebrafish melanoma model, we show that MITF decreases ROS-mediated DNA damage in vivo . Some of the MITF target genes involved, such as IDH1 and NNT , are regulated through direct MITF binding to canonical enhancer box (E-BOX) sequences proximal to their promoters. Utilizing functional experiments, we demonstrate the role of MITF and its target genes in reducing cytosolic and mitochondrial ROS. Collectively, our data identify MITF as a significant driver of the cellular antioxidant state. One Sentence Summary: MITF promote melanoma survival via increasing ROS tolerance.

NNT mediates redox-dependent pigmentation via a UVB- and MITF-independent mechanism.

Ultraviolet (UV) light and incompletely understood genetic and epigenetic variations determine skin color. Here we describe an UV- and microphthalmia-associated transcription factor (MITF)-independent mechanism of skin pigmentation. Targeting the mitochondrial redox-regulating enzyme nicotinamide nucleotide transhydrogenase (NNT) resulted in cellular redox changes that affect tyrosinase degradation. These changes regulate melanosome maturation and, consequently, eumelanin levels and pigmentation. Topical application of small-molecule inhibitors yielded skin darkening in human skin, and mice with decreased NNT function displayed increased pigmentation. Additionally, genetic modification of NNT in zebrafish alters melanocytic pigmentation. Analysis of four diverse human cohorts revealed significant associations of skin color, tanning, and sun protection use with various single-nucleotide polymorphisms within NNT. NNT levels were independent of UVB irradiation and redox modulation. Individuals with postinflammatory hyperpigmentation or lentigines displayed decreased skin NNT levels, suggesting an NNT-driven, redox-dependent pigmentation mechanism that can be targeted with NNT-modifying topical drugs for medical and cosmetic purposes.

Neural crest state activation in NRAS driven melanoma, but not in NRAS-driven melanocyte expansion.

NRAS mutations are frequently found in many deadly malignancies and are the second most common oncogene driving malignant melanoma. Here, we generate a rapid transient transgenic zebrafish model of NRASQ61R-mutant melanoma. These fish develop extensive melanocytic proliferation in approximately 4 weeks. The majority of these lesions do not engraft upon transplantation and lack overt histologic features of malignancy. Our previous work demonstrated that activation of a neural crest cell transcriptional program is a key initiating event in zebrafish BRAF/p53-driven melanomas using the fluorescent reporter crestin:EGFP. By 8-12 weeks of age, some lesions progress to malignant melanoma and have cytologic atypia, destructive tissue invasion, and express neural crest progenitor markers, including crestin:EGFP. Our studies demonstrate that NRASQ61R induces extensive melanocyte expansion, which arise during zebrafish development and lack a transformed phenotype. These early lesions are highly predisposed to reactivate a neural crest progenitor fate and form malignant melanomas.

Region-specific role for Pten in maintenance of epithelial phenotype and integrity.

Previous studies have demonstrated resistance to naphthalene-induced injury in proximal airways of mice with lung epithelial-specific deletion of the tumor-suppressor gene Pten, attributed to increased proliferation of airway progenitors. We tested effects of Pten loss following bleomycin injury, a model typically used to study distal lung epithelial injury, in conditional PtenSFTPC-cre knockout mice. Pten-deficient airway epithelium exhibited marked hyperplasia, particularly in small bronchioles and at bronchoalveolar duct junctions, with reduced E-cadherin and ß-catenin expression between cells toward the luminal aspect of the hyperplastic epithelium. Bronchiolar epithelial and alveolar epithelial type II (AT2) cells in PtenSFTPC-cre mice showed decreased expression of epithelial markers and increased expression of mesenchymal markers, suggesting at least partial epithelial-mesenchymal transition at baseline. Surprisingly, and in contrast to previous studies, mutant mice were exquisitely sensitive to bleomycin, manifesting rapid weight loss, respiratory distress, increased early mortality (by day 5), and reduced dynamic lung compliance. This was accompanied by sloughing of the hyperplastic airway epithelium with occlusion of small bronchioles by cellular debris, without evidence of increased parenchymal lung injury. Increased airway epithelial cell apoptosis due to loss of antioxidant defenses, reflected by decreased expression of superoxide dismutase 3, in combination with deficient intercellular adhesion, likely predisposed to airway sloughing in knockout mice. These findings demonstrate an important role for Pten in maintenance of airway epithelial phenotype integrity and indicate that responses to Pten deletion in respiratory epithelium following acute lung injury are highly context-dependent and region-specific.

p53 Regulates Progenitor Cell Quiescence and Differentiation in the Airway.

Mechanisms that regulate progenitor cell quiescence and differentiation in slowly replacing tissues are not fully understood. Here, we demonstrate that the tumor suppressor p53 regulates both proliferation and differentiation of progenitors in the airway epithelium. p53 loss decreased ciliated cell differentiation and increased the self-renewal and proliferative capacity of club progenitors, increasing epithelial cell density. p53-deficient progenitors generated a pseudostratified epithelium containing basal-like cells in vitro and putative bronchioalveolar stem cells in vivo. Conversely, an additional copy of p53 increased quiescence and ciliated cell differentiation, highlighting the importance of tight regulation of p53 levels. Using single-cell RNA sequencing, we found that loss of p53 altered the molecular phenotype of progenitors and differentially modulated cell-cycle regulatory genes. Together, these findings reveal that p53 is an essential regulator of progenitor cell behavior, which shapes our understanding of stem cell quiescence during homeostasis and in cancer development.

Distal airway epithelial progenitor cells are radiosensitive to High-LET radiation.

Exposure to high-linear energy transfer (LET) radiation occurs in a variety of situations, including charged particle radiotherapy, radiological accidents, and space travel. However, the extent of normal tissue injury in the lungs following high-LET radiation exposure is unknown. Here we show that exposure to high-LET radiation led to a prolonged loss of in vitro colony forming ability by airway epithelial progenitor cells. Furthermore, exposure to high-LET radiation induced clonal expansion of a subset of progenitor cells in the distal airway epithelium. Clonal expansion following high-LET radiation exposure was correlated with elevated progenitor cell apoptosis, persistent γ-H2AX foci, and defects in mitotic progression of distal airway progenitors. We discovered that the effects of high-LET radiation exposure on progenitor cells occur in a p53-dependent manner. These data show that high-LET radiation depletes the distal airway progenitor pool by inducing cell death and loss of progenitor function, leading to clonal expansion. Importantly, high-LET radiation induces greater long-term damage to normal lung tissue than the relative equivalent dose of low-LET γ-rays, which has implications in therapeutic development and risk assessment.