Hif-1α regulates differentiation of limb bud mesenchyme and joint development

Recent evidence suggests that low oxygen tension (hypoxia) may control fetal development and differentiation. A crucial mediator of the adaptive response of cells to hypoxia is the transcription factor Hif-1α. In this study, we provide evidence that mesenchymal condensations that give origin to endochondral bones are hypoxic during fetal development, and we demonstrate that Hif-1α is expressed and transcriptionally active in limb bud mesenchyme and in mesenchymal condensations. To investigate the role of Hif-1α in mesenchymal condensations and in early chondrogenesis, we conditionally inactivated Hif-1α in limb bud mesenchyme using a Prx1 promoter-driven Cre transgenic mouse. Conditional knockout of Hif-1α in limb bud mesenchyme does not impair mesenchyme condensation, but alters the formation of the cartilaginous primordia. Late hypertrophic differentiation is also affected as a result of the delay in early chondrogenesis. In addition, mutant mice show a striking impairment of joint development. Our study demonstrates a crucial, and previously unrecognized, role of Hif-1α in early chondrogenesis and joint formation

HIF signaling in osteoblast-lineage cells promotes systemic breast cancer growth and metastasis in mice

Bone metastasis involves dynamic interplay between tumor cells and the local stromal environment. In bones, local hypoxia and activation of the hypoxia-inducible factor (HIF)-1α in osteoblasts are essential to maintain skeletal homeostasis. However, the role of osteoblast-specific HIF signaling in cancer metastasis is unknown. Here, we show that osteoprogenitor cells (OPCs) are located in hypoxic niches in the bone marrow and that activation of HIF signaling in these cells increases bone mass and favors breast cancer metastasis to bone locally. Remarkably, HIF signaling in osteoblast-lineage cells also promotes breast cancer growth and dissemination remotely, in the lungs and in other tissues distant from bones. Mechanistically, we found that activation of HIF signaling in OPCs increases blood levels of the chemokine C-X-C motif ligand 12 (CXCL12), which leads to a systemic increase of breast cancer cell proliferation and dissemination through direct activation of the CXCR4 receptor. Hence, our data reveal a previously unrecognized role of the hypoxic osteogenic niche in promoting tumorigenesis beyond the local bone microenvironment. They also support the concept that the skeleton is an important regulator of the systemic tumor environment

Calpain-6 controls the fate of sarcoma stem cells by promoting autophagy and preventing senescence

International audienceSarcomas are still unsolved therapeutic challenges. Cancer stem cells are believed to contribute to sarcoma development, but lack of specific markers prevents their characterization and targeting. Here, we show that calpain-6 expression is associated with cancer stem cell features. In mouse models of bone sarcoma, calpain-6-expressing cells have unique tumor-initiating and metastatic capacities. Calpain-6 levels are especially high in tumors that have been successfully propagated in mouse to establish patient-derived xenografts. We found that calpain-6 levels are increased by hypoxia in vitro and calpain-6 is detected within hypoxic areas in tumors. Furthermore, calpain-6 expression depends on the stem cell transcription network that involves Oct4, Nanog, and Sox2 and is activated by hypoxia. Calpain-6 knockdown blocks tumor development in mouse and induces depletion of the cancer stem cell population. Data from transcriptomic analyses reveal that calpain-6 expression in sarcomas inversely correlates with senescence markers. Calpain-6 knockdown suppresses hypoxia-dependent prevention of senescence entry and also promotion of autophagic flux. Together, our results demonstrate that calpain-6 identifies sarcoma cells with stem-like properties and is a mediator of hypoxia to prevent senescence, promote autophagy, and maintain the tumor-initiating cell population. These findings open what we believe is a novel therapeutic avenue for targeting sarcoma stem cells

Hif-1alpha regulates differentiation of limb bud mesenchyme and joint development.

Recent evidence suggests that low oxygen tension (hypoxia) may control fetal development and differentiation. A crucial mediator of the adaptive response of cells to hypoxia is the transcription factor Hif-1alpha. In this study, we provide evidence that mesenchymal condensations that give origin to endochondral bones are hypoxic during fetal development, and we demonstrate that Hif-1alpha is expressed and transcriptionally active in limb bud mesenchyme and in mesenchymal condensations. To investigate the role of Hif-1alpha in mesenchymal condensations and in early chondrogenesis, we conditionally inactivated Hif-1alpha in limb bud mesenchyme using a Prx1 promoter-driven Cre transgenic mouse. Conditional knockout of Hif-1alpha in limb bud mesenchyme does not impair mesenchyme condensation, but alters the formation of the cartilaginous primordia. Late hypertrophic differentiation is also affected as a result of the delay in early chondrogenesis. In addition, mutant mice show a striking impairment of joint development. Our study demonstrates a crucial, and previously unrecognized, role of Hif-1alpha in early chondrogenesis and joint formation

Interaction of HIF1α and β-catenin inhibits matrix metalloproteinase 13 expression and prevents cartilage damage in mice

International audienceLow oxygen tension (hypoxia) regulates chondrocyte differentiation and metabolism. Hypoxia-inducible factor 1α (HIF1α) is a crucial hypoxic factor for chondrocyte growth and survival during development. The major metalloproteinase matrix metalloproteinase 13 (MMP13) is also associated with chondrocyte hypertrophy in adult articular cartilage, the lack of which protects from cartilage degradation and osteoarthritis (OA) in mice. MMP13 is up-regulated by the Wnt/β-catenin signaling, a pathway involved in chondrocyte catabo-lism and OA. We studied the role of HIF1α in regulating Wnt signaling in cartilage and OA. We used mice with conditional knockout of Hif1α (ΔHif1α chon) with joint instability. Specific loss of HIF1α exacerbated MMP13 expression and cartilage destruction. Analysis of Wnt signal-ing in hypoxic chondrocytes showed that HIF1α lowered transcription factor 4 (TCF4)–β-catenin transcriptional activity and inhibited MMP13 expression. Indeed, HIF1α interacting with β-catenin displaced TCF4 from MMP13 regulatory sequences. Finally, ΔHif1α chon mice with OA that were injected intraarticularly with PKF118-310, an inhibitor of TCF4–β-catenin interaction, showed less cartilage degradation and reduced MMP13 expression in cartilage. Therefore, HIF1α–β-catenin interaction is a negative regulator of Wnt signaling and MMP13 transcription, thus reducing catabolism in OA. Our study contributes to the understanding of the role of HIF1α in OA and highlights the HIF1α–β-catenin interaction, thus providing new insights into the impact of hypoxia in articular cartilage

PiT1/Slc20a1 is required for endoplasmic reticulum homeostasis, chondrocyte survival and skeletal development: PiT1 regulates ER stress in chondrocytes

International audienceDuring skeletal mineralization, the sodium-phosphate co-transporter PiT1Slc20a1 is assumed to meet the phosphate requirements of bone-forming cells, although evidence is missing. Here, we used a conditional gene deletion approach to determine the role of PiT1 in growth plate chondrocytes. We show that PiT1 ablation shortly after birth generates a rapid and massive cell death in the center of the growth plate, together with an uncompensated ER stress, characterized by morphological changes and increased Chop, Atf4 and Bip expression. PiT1 expression in chondrocytes was not found at the cell membrane but co-localized with the ER marker ERp46, and was up-regulated by the unfolded protein response cascade. In addition, we identified the protein disulfide isomerase (Pdi) ER chaperone as a PiT1 binding partner and showed that PiT1 ablation impaired Pdi reductase activity. The ER stress induced by PiT1 deficiency in chondrocytes was associated with intracellular retention of aggrecan and Vegf-A, which was rescued by over-expressing a phosphate transport-deficient mutant of PiT1. Our data thus reveal a novel, Pi-transport independent function of PiT1, as a critical modulator of ER homeostasis and chondrocyte survival during endochondral ossification. This article is protected by copyright. All rights reserved