Studies of the identity and physiological function of mesenchymal stromal cells (MSCs) have been hampered by a lack of markers that permit both prospective identification and fate mapping in vivo. We found that Leptin Receptor (LepR) is a marker that highly enriches bone marrow MSCs. Approximately 0.3% of bone marrow cells were LepR , 10% of which were CFU-Fs, accounting for 94% of bone marrow CFU-Fs. LepR cells formed bone, cartilage, and adipocytes in culture and upon transplantation in vivo. LepR cells were -GFP , -DsRed , and GFP , markers which also highly enriched CFU-Fs, but negative for -CreER and -CreER, markers which were unlikely to be found in CFU-Fs. Fate-mapping showed that LepR cells arose postnatally and gave rise to most bone and adipocytes formed in adult bone marrow, including bone regenerated after irradiation or fracture. LepR cells were quiescent, but they proliferated after injury. Therefore, LepR cells are the major source of bone and adipocytes in adult bone marrow. Zhou et al. reveal that cells positive for Leptin receptor (LepR) are the main source of mesenchymal stromal cells (MSCs), bone-forming progenitors, and adipocytes in adult mouse bone marrow.
Cancer cells have been at the centre of cell metabolism research, but the metabolism of stromal and immune cells has received less attention. Nonetheless, these cells influence the progression of malignant, inflammatory and metabolic disorders. Here we discuss the metabolic adaptations of stromal and immune cells in health and disease, and highlight how metabolism determines their differentiation and function.
Much interest is currently focused on the emerging role of tumor-stroma interactions essential for supporting tumor progression. Carcinoma-associated fibroblasts (CAFs), frequently present in the stroma of human breast carcinomas, include a large number of myofibroblasts, a hallmark of activated fibroblasts. These fibroblasts have an ability to substantially promote tumorigenesis. However, the precise cellular origins of CAFs and the molecular mechanisms by which these cells evolve into tumor-promoting myofibroblasts remain unclear. Using a coimplantation breast tumor xenograft model, we show that resident human mammary fibroblasts progressively convert into CAF myofibroblasts during the course of tumor progression. These cells increasingly acquire two autocrine signaling loops, mediated by TGF-β and SDF-1 cytokines, which both act in autostimulatory and cross-communicating fashions. These autocrine-signaling loops initiate and maintain the differentiation of fibroblasts into myofibroblasts and the concurrent tumor-promoting phenotype. Collectively, these findings indicate that the establishment of the self-sustaining TGF-β and SDF-1 autocrine signaling gives rise to tumor-promoting CAF myofibroblasts during tumor progression. This autocrine-signaling mechanism may prove to be an attractive therapeutic target to block the evolution of tumor-promoting CAFs.
BM mesenchymal stromal cells (BM-MSCs) support multiple myeloma (MM) cell growth, but little is known about the putative mechanisms by which the BM microenvironment plays an oncogenic role in this disease. Cell-cell communication is mediated by exosomes. In this study, we showed that MM BM-MSCs release exosomes that are transferred to MM cells, thereby resulting in modulation of tumor growth in vivo. Exosomal microRNA (miR) content differed between MM and normal BM-MSCs, with a lower content of the tumor suppressor miR-15a. In addition, MM BM-MSC-derived exosomes had higher levels of oncogenic proteins, cytokines, and adhesion molecules compared with exosomes from the cells of origin. Importantly, whereas MM BM-MSC-derived exosomes promoted MM tumor growth, normal BM-MSC exosomes inhibited the growth of MM cells. In summary, these in vitro and in vivo studies demonstrated that exosome transfer from BM-MSCs to clonal plasma cells represents a previously undescribed and unique mechanism that highlights the contribution of BM-MSCs to MM disease progression.
Increased numbers of S100A4⁺ cells are associated with poor prognosis in patients who have cancer. Although the metastatic capabilities of S100A4⁺ cancer cells have been examined, the functional role of S100A4⁺ stromal cells in metastasis is largely unknown. To study the contribution of S100A4⁺ stromal cells in metastasis, we used transgenic mice that express viral thymidine kinase under control of the S100A4 promoter to specifically ablate S100A4⁺ stromal cells. Depletion of S100A4⁺ stromal cells significantly reduced metastatic colonization without affecting primary tumor growth. Multiple bone marrow transplantation studies demonstrated that these effects of S100A4⁺ stromal cells are attributable to local non-bone marrow-derived S100A4⁺ cells, which are likely fibroblasts in this setting. Reduction in metastasis due to the loss of S100A4⁺ fibroblasts correlated with a concomitant decrease in the expression of several ECM molecules and growth factors, particularly Tenascin-C and VEGF-A. The functional importance of stromal Tenascin-C and S100A4⁺ fibroblast-derived VEGF-A in metastasis was established by examining Tenascin-C null mice and transgenic mice expressing Cre recombinase under control of the S100A4 promoter crossed with mice carrying VEGF-A alleles flanked by loxP sites, which exhibited a significant decrease in metastatic colonization without effects on primary tumor growth. In particular, S100A4⁺ fibroblast-derived VEGF-A plays an important role in the establishment of an angiogenic microenvironment at the metastatic site to facilitate colonization, whereas stromal Tenascin-C may provide protection from apoptosis. Our study demonstrates a crucial role for local S100A4⁺ fibroblasts in providing the permissive "soil" for metastatic colonization, a challenging step in the metastatic cascade.
In addition to their stem/progenitor properties, mesenchymal stromal cells (MSCs) possess broad immunoregulatory properties that are being investigated for potential clinical application in treating immune-based disorders. An informed view of the scope of this clinical potential will require a clear understanding of the dynamic interplay between MSCs and the innate and adaptive immune systems. In this Review, we outline current insights into the ways in which MSCs sense and control inflammation, highlighting the central role of macrophage polarization. We also draw attention to functional differences seen between vivo and in vitro contexts and between species. Finally, we discuss progress toward clinical application of MSCs, focusing on GvHD as a case study. This Review discusses the role of MSCs in sensing and controlling inflammation, highlighting the central role of macrophage polarization, differences based on species and physiological contexts, and potential clinical applications.
A large proportion of colorectal cancers (CRCs) display mutational inactivation of the TGF-β pathway, yet, paradoxically, they are characterized by elevated TGF-β production. Here, we unveil a prometastatic program induced by TGF-β in the microenvironment that associates with a high risk of CRC relapse upon treatment. The activity of TGF-β on stromal cells increases the efficiency of organ colonization by CRC cells, whereas mice treated with a pharmacological inhibitor of TGFBR1 are resilient to metastasis formation. Secretion of IL11 by TGF-β-stimulated cancer-associated fibroblasts (CAFs) triggers GP130/STAT3 signaling in tumor cells. This crosstalk confers a survival advantage to metastatic cells. The dependency on the TGF-β stromal program for metastasis initiation could be exploited to improve the diagnosis and treatment of CRC. ► TGF-β pathway mutant cells require a stromal TGF-β program for metastasis ► CRC patients with low levels of stromal TGF-β program do not relapse ► Pharmacological blockade of TGF-β stromal signaling prevents metastasis initiation ► A TGF-β/IL-11/GP130 signaling cycle confers metastatic organ colonization capacity
Stromal communication with cancer cells can influence treatment response. We show that stromal and breast cancer (BrCa) cells utilize paracrine and juxtacrine signaling to drive chemotherapy and radiation resistance. Upon heterotypic interaction, exosomes are transferred from stromal to BrCa cells. RNA within exosomes, which are largely noncoding transcripts and transposable elements, stimulates the pattern recognition receptor RIG-I to activate STAT1-dependent antiviral signaling. In parallel, stromal cells also activate NOTCH3 on BrCa cells. The paracrine antiviral and juxtacrine NOTCH3 pathways converge as STAT1 facilitates transcriptional responses to NOTCH3 and expands therapy-resistant tumor-initiating cells. Primary human and/or mouse BrCa analysis support the role of antiviral/NOTCH3 pathways in NOTCH signaling and stroma-mediated resistance, which is abrogated by combination therapy with gamma secretase inhibitors. Thus, stromal cells orchestrate an intricate crosstalk with BrCa cells by utilizing exosomes to instigate antiviral signaling. This expands BrCa subpopulations adept at resisting therapy and reinitiating tumor growth. Stromal cells in the tumor microenvironment protect breast cancer cells and promote resistance to radio and chemotherapy through activation of a RIG-I-dependent “antiviral” program and engagement of NOTCH3 signaling.
Reprogramming of stromal cell metabolism toward glycolysis in cancerous tissue provides a source of high-energy metabolic intermediates which support proliferation, invasion and metastasis of invading tumour cells; a phenomenon known as the ‘Reverse Warburg’ effect. We have explored whether a similar interplay may fuel pathogenic processes in autoinflammatory diseases such as rheumatoid arthritis (RA). In RA, fibroblasts are themselves transformed to an aggressive, tumour-like phenotype and also reside in close proximity to energetically demanding immune cells including resident macrophages, infiltrating neutrophils and autoaggressive lymphocytes.We used nuclear magnetic resonance spectroscopy and metabolic flux analysis to identify pathological changes to metabolism in both primary human RA synovial cells, and murine cells from TNF-ΔARE and collagen-induced models of arthritis.We have shown that the fibroblast metabotype correlates with C-reactive protein as a systemic marker of inflammation, and can predict resolution of acute synovitis or progression to chronic RA prior. We have confirmed the clinical prognostic potential of metabolic profiling in inflammatory diseases and demonstrated that synovial fibroblasts increase glycolysis but not mitochondrial respiration in response to inflammatory cues such as tumour necrosis factor-α. Furthermore, analysis of monocarboxylate transporters (MCT-4) supports the likelihood that increased glycolytic products such as lactate may be shuttled between fibroblasts and myeloid cells in situ.Our findings shed light on the metabolic relationships between cells at sites of autoinflammatory pathology and support the therapeutic targeting of the glycolytic pathway beyond oncology.European Union's FP7 Health Programme FP7-HEALTH-F2–2012–305549AR UK Rheumatoid Arthritis Pathogenesis Centre of Excellence 20298
Activation of myofibroblast rich stroma is a rate-limiting step essential for cancer progression. The responsible factors are not fully understood, but TGF beta 1 is probably critical. A proportion of TGF beta 1 is associated with extracellular nano-vesicles termed exosomes, secreted by carcinoma cells, and the relative importance of soluble and vesicular TGF beta in stromal activation is presented. Prostate cancer exosomes triggered TGF beta 1-dependent fibroblast differentiation, to a distinctive myofibroblast phenotype resembling stromal cells isolated from cancerous prostate tissue; supporting angiogenesis in vitro and accelerating tumour growth in vivo. Myofibroblasts generated using soluble TGF beta 1 were not pro-angiogenic or tumour-promoting. Cleaving heparan sulphate side chains from the exosome surface had no impact on TGF beta levels yet attenuated SMAD-dependent signalling and myofibroblastic differentiation. Eliminating exosomes from the cancer cell secretome, targeting Rab27a, abolished differentiation and lead to failure in stroma-assisted tumour growth in vivo. Exosomal TGF beta 1 is therefore required for the formation of tumour-promoting stroma.