Recurrent mutations are frequently associated with transcription factor (TF) binding sites (TFBS) in melanoma, but the mechanism driving mutagenesis at TFBS is unclear. Here, we use a method called CPD-seq to map the distribution of UV-induced cyclobutane pyrimidine dimers (CPDs) across the human genome at single nucleotide resolution. Our results indicate that CPD lesions are elevated at active TFBS, an effect that is primarily due to E26 transformation-specific (ETS) TFs. We show that ETS TFs induce a unique signature of CPD hotspots that are highly correlated with recurrent mutations in melanomas, despite high repair activity at these sites. ETS1 protein renders its DNA binding targets extremely susceptible to UV damage in vitro, due to binding-induced perturbations in the DNA structure that favor CPD formation. These findings define a mechanism responsible for recurrent mutations in melanoma and reveal that DNA binding by ETS TFs is inherently mutagenic in UV-exposed cells.

Polycationic macromolecules and cationic peptides acting as PTDs (protein transduction domains) and CPPs (cell-penetrating peptides) represent important classes of agents used for the import and delivery of a wide range of molecular cargoes into cells. Their entry into cells is typically initiated through interaction with cell-surface HS (heparan sulfate) molecules via electrostatic interactions, followed by endocytosis of the resulting complexes. However, the endocytic mechanism employed (clathrin-mediated endocytosis, caveolar uptake or macropinocytosis), defining the migration of these peptides into cells, depends on parameters such as the nature of the cationic agent itself and complex formation with cargo, as well as the nature and distribution of proteoglycans expressed on the cell surface. Moreover, a survey of the literature suggests that endocytic pathways should not be considered as mutually exclusive, as more than one entry mechanism may be operational for a given cationic complex in a particular cell type. Specifically, the observed import may best be explained by the distribution and uptake of cell-surface HSPGs (heparan sulfate proteoglycans), such as syndecans and glypicans, which have been shown to mediate the uptake of many ligands besides cationic polymers. A brief overview of the roles of HSPGs in ligand internalization is presented, as well as mechanistic hypotheses based on the known properties of these cell-surface markers. The identification and investigation of interactions made by glycosaminoglycans and core proteins of HSPGs with PTDs and cationic polymers will be crucial in defining their uptake by cells.

Fibroblasts are polymorphic cells with pleiotropic roles in organ morphogenesis, tissue homeostasis and immune responses. In fibrotic diseases, fibroblasts synthesize abundant amounts of extracellular matrix which lead to scaring and organ failure. In contrast, the hallmark feature of fibroblasts in arthritis is matrix degradation by the release of metalloproteinases and degrading enzymes, and subsequent tissue destruction. The mechanisms driving these functionally opposing pro-fibrotic and pro-inflammatory phenotypes of fibroblasts are enigmatic. We identified the transcription factor PU.1 as an essential orchestrator of the pro-fibrotic gene expression program. The interplay between transcriptional and post-transcriptional mechanisms which normally control PU.1 expression is perturbed in various fibrotic diseases, resulting in upregulation of PU.1, induction of fibrosis-associated gene sets, and a phenotypic switch in matrix-producing pro-fibrotic fibroblasts. In contrast, pharmacological and genetic inactivation of PU.1 disrupts the fibrotic network and enables re-programming of fibrotic fibroblasts into resting fibroblasts with regression of fibrosis in different organs.

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