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Succinate beyond metabolism in blood
reported to express GPR91 on their cell surface,38 suggest- ing a potential role for succinate signaling in hematopoietic stem cells (HSC). At present, the lack of commercially available mouse GPR91-specific antibody makes it difficult to further explore the significance of succinate-GPR91 sig- naling pathway in the biology of HSC and progenitor cells. In this regard, generation of reporter mouse lines will be of great value in the hematopoietic field.
In vitro, succinate stimulates the proliferation capacity of erythroid and megakaryocyte progenitors, and cell prolifer- ation is significantly hampered in cells transfected with small interfering ribonucleic acid targeting GPR91.38 However, when megakaryocytes and erythroblasts are dif- ferentiated in vitro from human cord blood CD34+ cells, megakaryocytes show 7.2-fold higher GPR91 transcript expression as detected by cDNA microarrays.39 Of note, differentiation of CD34+ progenitors into megakaryocytes is performed in the presence of both thrombopoietin and IL-1b.39 As previously discussed, GPR91 signaling seems to be required for correct mast cell differentiation.35 GPR91- deficient mice show hypomorphic mast cells that display increased responses, and this can be recapitulated in vitro using WT BM cells differentiated in the absence of succi- nate.35
Following chemotherapy, succinate treatment in vivo stimulates multilineage blood cell recovery, including red blood cells, platelets and neutrophils.38 In addition, inducible deletion of the mitochondrial SDHD in mouse models leads to remarkable hematopoietic defects, includ- ing depletion of BM progenitors and differentiated cells, and apoptosis in the HSC compartment defined as LSK, i.e. lineage-negative, c-kit-positive, and sca-1-positive.40 However, the consequences of this strategy on succinate levels in LSK cells and their link to cell functional changes has not been studied. Furthermore, an inducible system was used to conditionally delete SDHD controlled in a temporal manner, using Cre mediated recombination that activates upon estrogen analog tamoxifen administration. However, this genetic approach does not allow the cell subset where the deletion occurs to be delimited, either within or outside the hematopoietic system. Given the key role of regulatory cells within the healthy HSC niche, that tightly controls the function, fate and numbers of HSC in the BM,41 it may well be that these effects are partially mediated by the stem cell niche. Future studies will be needed to validate this exciting hypothesis.
Interestingly, succinate is increased 24-fold in BM stro- mal cells derived from type 2 diabetes mellitus mice com- pared to normoglycemic mice.42 In this study, adherent cells were used after flushing out BM cells and culture of the adherent fraction for one week in vitro. Thus, levels of metabolites in this study may differ from those found in primary BM stromal cells in vivo. Furthermore, these adher- ent cells may consist of heterogeneous populations of stro- mal cells. Nevertheless, the Authors reveal an interesting mechanism that may contribute to bone dysregulation in metabolic disorders. In vitro, extracellular succinate binds to GPR91 on osteoclastic lineage cells and stimulates osteo- clast differentiation; a process that may contribute to the bone resorption seen in vivo.42 Succinate may then have an indirect effect on hematopoiesis in diabetes, as osteoclasts function as negative regulators of HSC43 whereas osteoblasts support lymphoid progenitors.44,45 Future stud- ies will be required to test these ideas.
cycle regulation in HSC,46 so identification of the potential link to succinate is of high relevance. Actually, HSC reside in hypoxic BM niches, which maintain their long-term self- renewal by mechanisms like limiting their production of ROS.47 This is performed through adaptation of their metabolism to maintain a high glycolysis rate by HIF-1α activation. These conditions change during the processes of proliferation and differentiation, with activated cells depending more on oxidative phosphorylation to meet the energy requirements.47 Besides this, HSC niches are closely related to the vasculature in the BM and have been defined as perivascular, with mainly endothelial cells and mes- enchymal stromal cells secreting factors that promote HSC maintenance.48 In this regard, GPR91 has been suggested to link capillary function to metabolic needs in other tissues, like retina, where GPR91 is essential to establish a neovas- cular network in response to injury through production of numerous angiogenic factors including vascular endothelial growth factor by retinal ganglion neurons.49 Considering that SDH mutations, HIF-1α accumulation and elevated levels of circulating succinate relate to pathology, particu- larly in hematologic malignancies,6,7,16-19 characterization of succinate signaling network, both cell- and non-cell- autonomous, is pivotal to understand the link between metabolic alterations that affect HSC function and hema- tologic malignancies.
Conclusions
Succinate is an essential intermediate of the TCA cycle at the intersection among metabolism, gene expression and intercellular communication. Initially characterized as an inflammatory signal, recent data show succinate-mediated anti-inflammatory mechanisms depending on the cellular subsets. The great plasticity of succinate biology is exempli- fied by cell-type specific responses of GPR91 machinery and on/off dynamics that may contribute to seemingly con- troversial results. In this scenario, it is interesting to hypoth- esize that succinate may play dual roles both as early driver of inflammation, through cell-autonomous processes, and as late terminator by intercellular communication with reg- ulatory cells. Future studies are required to demonstrate this idea. Furthermore, we are only starting to understand how succinylation may influence protein activation and epige- netic landscape, and thereby gene expression and cell func- tion. Studies on both succinylation regulation and impact will be highly valuable for a complete picture of succinate biology in blood cells. Finally, early hints on hematopoiesis seem to involve succinate in the survival and differentiation of blood stem cells, through mechanisms that remain unclear. Future research should carefully explore the prom- ising therapeutic value of succinate targeting in inflamma- tory diseases, including hematologic malignancies.50
Acknowledgments
Our work is supported by a joint meeting grant of the Northern Norway Regional Health Authority, the University Hospital of Northern Norway (UNN) and UiT The Arctic University of Norway (UiT) (2014/5668), Young Research Talent grants from the Research Council of Norway, (Stem Cell Program, 247596; FRIPRO Program, 250901), and grants from the Norwegian Cancer Society (6765150), the Northern Norway Regional Health Authority (HNF1338-17), and the Aakre-Stiftelsen Foundation (2016/9050) to LA.
Furthermore, HIF-1α contributes importantly to cell-
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