Fine-tuning vascular fate during endothelial-mesenchymal transition

In the heart and other organs, endothelial-mesenchymal transition (EndMT) has emerged as an important developmental process that involves coordinated migration, differentiation, and proliferation of the endothelium. In multiple disease states including cancer angiogenesis and cardiovascular disease, the processes that regulate EndMT are recapitulated, albeit in an uncoordinated and dysregulated manner. Members of the transforming growth factor beta (TGFβ) superfamily are well known to impart cellular plasticity during EndMT by the timely activation (or repression) of transcription factors and miRNAs in addition to epigenetic regulation of gene expression. On the other hand, fibroblast growth factors (FGFs) are reported to augment or oppose TGFβ-driven EndMT in specific contexts. Here, we have synthesized the currently understood roles of TGFβ and FGF signalling during EndMT and have provided a new, comprehensive paradigm that delineates how an autocrine and paracrine TGFβ/FGF axis coordinates endothelial cell specification and plasticity. We also provide new guidelines and nomenclature that considers factors such as endothelial cell heterogeneity to better define EndMT across different vascular beds. This perspective should therefore help to clarify why TGFβ and FGF can both cooperate with or oppose one another during the complex process of EndMT in both health and disease. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Endothelial Cell Phenotype, a Major Determinant of Venous Thrombo-Inflammation

Reduced blood flow velocity in the vein triggers inflammation and is associated with the release into the extracellular space of alarmins or damage-associated molecular patterns (DAMPs). These molecules include extracellular nucleic acids, extracellular purinergic nucleotides (ATP, ADP), cytokines and extracellular HMGB1. They are recognized as a danger signal by immune cells, platelets and endothelial cells. Hence, endothelial cells are capable of sensing environmental cues through a wide variety of receptors expressed at the plasma membrane. The endothelium is then responding by expressing pro-coagulant proteins, including tissue factor, and inflammatory molecules such as cytokines and chemokines involved in the recruitment and activation of platelets and leukocytes. This ultimately leads to thrombosis, which is an active pro-inflammatory process, tightly regulated, that needs to be properly resolved to avoid further vascular damages. These mechanisms are often dysregulated, which promote fibrinolysis defects, activation of the immune system and irreversible vascular damages further contributing to thrombotic and inflammatory processes. The concept of thrombo-inflammation is now widely used to describe the complex interactions between the coagulation and inflammation in various cardiovascular diseases. In endothelial cells, activating signals converge to multiple intracellular pathways leading to phenotypical changes turning them into inflammatory-like cells. Accumulating evidence suggest that endothelial to mesenchymal transition (EndMT) may be a major mechanism of endothelial dysfunction induced during inflammation and thrombosis. EndMT is a biological process where endothelial cells lose their endothelial characteristics and acquire mesenchymal markers and functions. Endothelial dysfunction might play a central role in orchestrating and amplifying thrombo-inflammation thought induction of EndMT processes. Mechanisms regulating endothelial dysfunction have been only partially uncovered in the context of thrombotic diseases. In the present review, we focus on the importance of the endothelial phenotype and discuss how endothelial plasticity may regulate the interplay between thrombosis and inflammation. We discuss how the endothelial cells are sensing and responding to environmental cues and contribute to thrombo-inflammation with a particular focus on venous thromboembolism (VTE). A better understanding of the precise mechanisms involved and the specific role of endothelial cells is needed to characterize VTE incidence and address the risk of recurrent VTE and its sequelae.

Deciphering Tumor Niches: Lessons From Solid and Hematological Malignancies

Knowledge about the hematopoietic niche has evolved considerably in recent years, in particular through in vitro analyzes, mouse models and the use of xenografts. Its complexity in the human bone marrow, in particular in a context of hematological malignancy, is more difficult to decipher by these strategies and could benefit from the knowledge acquired on the niches of solid tumors. Indeed, some common features can be suspected, since the bone marrow is a frequent site of solid tumor metastases. Recent research on solid tumors has provided very interesting information on the interactions between tumoral cells and their microenvironment, composed notably of mesenchymal, endothelial and immune cells. This review thus focuses on recent discoveries on tumor niches that could help in understanding hematopoietic niches, with special attention to 4 particular points: i) the heterogeneity of carcinoma/cancer-associated fibroblasts (CAFs) and mesenchymal stem/stromal cells (MSCs), ii) niche cytokines and chemokines, iii) the energy/oxidative metabolism and communication, especially mitochondrial transfer, and iv) the vascular niche through angiogenesis and endothelial plasticity. This review highlights actors and/or pathways of the microenvironment broadly involved in cancer processes. This opens avenues for innovative therapeutic opportunities targeting not only cancer stem cells but also their regulatory tumor niche(s), in order to improve current antitumor therapies.

Endothelial plasticity governs the site-specific leukocyte recruitment in hepatocellular cancer

The correct programming of the endothelial cell phenotype is crucial for efficient leukocyte recruitment to tumor tissue. It has been previously described that T cells infiltrated hepatocellular cancer (HCC) tissue mainly in peritumoral, stromal and tumor border areas. In the current study, phenotype features of tumor endothelial cells and their potential impact on leukocyte recruitment were analyzed in murine tissue of HCC. In the murine model, proinflammatory stimulation with IL-1β induced leukocyte recruitment in the blood vessels of peripheral tumor areas and in nonmalignant liver tissue, but not in deeper tumor blood vessels. Furthermore, peripheral tumor endothelium, but not deeper tumor blood vessels exhibited a "normalized" hepatic sinusoidal endothelial cell (HSEC)-like phenotype with regard to the expression of adhesion molecules and liver sinusoidal endothelial markers. When tumor endothelial cells were isolated and incubated in vitro, their phenotype rapidly changed and became almost identical to normal hepatic endothelial cells. Interestingly, cytokine production in HCC was strongly dysregulated as compared to normal liver, with IL-1RN exhibiting the most prominent elevation. Experiments with isolated hepatic endothelial cells showed that IL-1RN effectively antagonized the activating action of IL-1β on the expression of adhesion molecules and T cell attachment. These novel insights indicate that tumor endothelium of HCC represents a plastic system that is susceptible to microenvironmental changes. The peritumoral and tumor border areas have distinct endothelial cell phenotype, which promotes leukocyte recruitment to HCC tissue.

Longitudinal single-cell profiles of lung regeneration after viral infection reveal persistent injury-associated cell states

Functional regeneration of the lung's gas exchange surface following injury requires the coordination of a complex series of cell behaviors within the alveolar niche. Using single-cell transcriptomics combined with lineage tracing of proliferating progenitors, we examined mouse lung regeneration after influenza injury, demonstrating an asynchronously phased response across different cellular compartments. This longitudinal atlas of injury responses has produced a catalog of transient and persistent transcriptional alterations in cells as they transit across axes of differentiation. These cell states include an injury-induced capillary endothelial cell (iCAP) that arises after injury, persists indefinitely, and shares hallmarks with developing lung endothelium and endothelial aberrations found in degenerative human lung diseases. This dataset provides a foundational resource to understand the complexity of cellular and molecular responses to injury and correlations to responses found in human development and disease.

Feeding cancer's sweet tooth: specialized tumour vasculature shuttles glucose in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal neoplasm characterized by a 'fortress' of thick collagen fibres, abundant myofibroblasts, and paradoxically reduced vascularization compared to normal pancreas. Despite these features, PDAC shows no reduction in the uptake of glucose that fuels tumour cell survival. In new work published in The Journal of Pathology, Saiyin and colleagues have identified a novel adaptation of PDAC tumour endothelium; namely, 'hairy-like' basal microvilli that increase the total vascular surface area and correlate with regions of highest glucose uptake. Since basal microvilli are not present on normal pancreatic blood vessels, their presence may add diagnostic value and blocking their function is a potential new treatment strategy for PDAC. This novel finding of basal microvilli on PDAC endothelium is a striking example of how phenotypic plasticity in tumour blood vessels contributes to tumour growth and progression, independent of conventional modes of angiogenesis.