The aryl hydrocarbon receptor nuclear translocator is an essential regulator of murine hematopoietic stem cell viability.

Hypoxia-inducible factors (HIFs) are master regulators of the transcriptional response to low oxygen and play essential roles in embryonic development, tissue homeostasis, and disease. Recent studies have demonstrated that hematopoietic stem cells (HSCs) within the bone marrow localize to a hypoxic niche and that HIF-1α promotes HSC adaptation to stress. Because the related factor HIF-2α is also expressed in HSCs, the combined role of HIF-1α and HIF-2α in HSC maintenance is unclear. To this end, we have conditionally deleted the HIF-α dimerization partner, the aryl hydrocarbon receptor nuclear translocator (ARNT) in the hematopoietic system to ablate activity of both HIF-1α and HIF-2α and assessed the functional consequence of ARNT deficiency on fetal liver and adult hematopoiesis. We determined that ARNT is essential for adult and fetal HSC viability and homeostasis. Importantly, conditional knockout of both Hif-1α and Hif-2α phenocopied key aspects of these HSC phenotypes, demonstrating that the impact of Arnt deletion is primarily HIF dependent. ARNT-deficient long-term HSCs underwent apoptosis, potentially because of reduced B-cell lymphoma 2 (BCL-2) and vascular endothelial growth factor A (VEGF-A) expression. Our results suggest that HIF activity may regulate HSC homeostasis through these prosurvival factors.

Context Matters: Response Heterogeneity to Collagen-Targeting Approaches in Desmoplastic Cancers.

The deposition of collagen-rich desmoplastic tissue is a well-documented feature of the solid tumor microenvironment (TME). However, efforts to target the desmoplastic extracellular matrix (ECM) en masse, or collagen molecules more specifically, have been met with mixed and sometimes paradoxical results. In this review, we posit that these discrepancies are due-at least in part-to the incredible diversity of the collagen superfamily. Specifically, whereas studies of "collagen-targeting" approaches frequently refer to "collagen" as a single molecule or relatively homogeneous molecular family, 28 individual collagens have been identified in mammalian tissues, each with a unique structure, supramolecular assembly pattern, tissue distribution, and/or function. Moreover, some collagen species have been shown to exert both pro- and anti-neoplastic effects in the desmoplastic TME, even within the same cancer type. Therefore, herein, we describe the diversity of the collagen family in normal tissues and highlight the context-specific roles of individual collagen molecules in desmoplastic tumors. We further discuss how this heterogeneity relates to the variable efficacy of "collagen-targeting" strategies in this setting and provide guidance for future directions in the field.