Metabolic gatekeeping in murine stress erythropoiesis Open Access

In this issue of Blood RCI, Ruan et al¹ provide new insights into how cellular metabolism regulates the proliferation and differentiation of murine stress erythroid progenitors (SEPs). Ruan et al demonstrate that itaconate, an anti-inflammatory mitochondrial metabolite, activates the antioxidant transcription factor Nrf2 (nuclear factor erythroid 2–related factor 2), resolving inflammation and facilitating the differentiation of SEP (Figure 1).

Stress erythropoiesis is a rapid response to acute anemia, generating large numbers of mature erythrocytes when steady-state erythropoiesis is compromised.¹ Previous work by Paulson et al has focused on characterizing the mechanisms of extramedullary stress erythropoiesis in mice. During the first stages, SEP expand before committing to erythroid differentiation around monocyte-derived macrophages.^2,3,4 Although we are beginning to have a clear understanding of the cytokines and signaling pathways involved in SEP expansion and differentiation into erythroid precursors, only a few studies have identified metabolic changes critical for proliferating SEP and their transition.^3,5-8 Several groups have highlighted the importance of glucose, glutamine, lipid, and purinergic metabolism in erythroid commitment and differentiation during steady-state erythropoiesis,^9-13 but glutamine metabolism seems to be essential in stress erythropoiesis as glutaminolysis inhibition in phenylhydrazine-injected mice prevents erythroid commitment.¹⁰ Furthermore, GDF15 and Yap1 signaling has been found to regulate enzymes associated with glutamine and glucose metabolism in expanding SEP.^8,14 Other metabolic pathways appear important during SEP proliferation for rapid cell growth and division;¹⁵ however, multiple aspects remain unexplored, such as identifying the metabolic switches that drive SEP transition from proliferation to differentiation or investigating the roles of these metabolites in gene regulation.

In this study, Ruan et al used in vitro mouse stress erythropoiesis cultures to profile metabolite changes during SEP expansion and transition to differentiation phases. liquid chromatography–mass spectrometry identified itaconate as the most significantly decreased metabolite during SEP proliferation. Conversely, itaconate and immune-responsive gene 1 (IRG1) protein were elevated during the transition to differentiation.

Itaconate is a mitochondrial metabolite derived from the tricarboxylic acid cycle in macrophages. It is synthesized through decarboxylation of cis-aconitate by cis-aconitate decarboxylase (IRG1) in response to external proinflammatory stimuli. Itaconate induces anti-inflammatory and antioxidant effects.¹⁶ 

In this study, the authors demonstrate that during the expansion phase, low itaconate levels enable inflammatory signals, particularly nitric oxide (NO) production by Nos2, to promote SEP proliferation. Consequently, treatment with itaconate impaired the number of proliferating SEP, notably by decreasing NO. However, the transition to differentiation is associated with an increase in itaconate, which in turn inhibits NO production. Using both in vitro and in vivo Irg1^–/– models, the authors demonstrate that deficiency in itaconate results in a defective stress response. Specifically, it leads to an accumulation of immature SEP and a failure to adequately generate mature erythrocytes, ultimately delaying recovery from anemia in murine models of inflammatory and hemolytic anemia. Although not explored in this study, other groups have revealed that high itaconate levels must be reduced for erythroblast terminal differentiation, as exogenous itaconate inhibits heme biosynthesis in mouse erythroleukemia cells.¹⁷ 

How does itaconate promote stress erythropoiesis? In macrophages, itaconate activates Nrf2, a transcription factor involved in oxidative stress responses.¹⁸ Nrf2 is a known regulator of stress erythropoiesis and is critical for the establishment of macrophages in the niche. Nrf2^–/– mice exhibit macrophage deficiency and an impaired erythroid response to blood loss.^19,20 In this study, Nrf2-deficient SEP retained a proinflammatory signature and failed to differentiate. Furthermore, the findings from Ruan et al suggest that itaconate activates Nrf2, which in turn suppresses NO synthesis, thereby alleviating inflammation-driven inhibition on erythroid maturation, and promotes the expression of genes essential for SEP maturation. Notably, treatment with of 4-octyl-itaconate and other Nrf2 activators on Irg1^–/– SEP rescued the decreased expression of EPOR and GATA1 and the reduced frequency of burst-forming unit–erythroid in colony-forming unit assays. Thus, although this study clearly reveals that Nrf2 activation by itaconate is essential for SEP maturation, further research is needed to determine whether other itaconate targets, such as succinate dehydrogenase, glycolysis, or amino acid metabolism, are also implicated in itaconate-driven stress erythropoiesis.^16,17 

This exciting work expands our understanding of metabolic control in stress erythropoiesis. It demonstrates that resolution of inflammation through the itaconate-Nrf2 axis is an important mechanism governing the transition from proliferating SEP to differentiating SEP and suggests that targeting this pathway could be a promising strategy to treat anemia of inflammation or hemolytic anemia, such as sickle cell disease. In addition to replenishing erythrocytes through increased stress erythropoiesis, activating the itaconate-Nrf2 axis could provide multiple additional beneficial effects in sickle cell disease, as Nrf2 activation is known to induce fetal hemoglobin, decrease free heme, and reduce inflammation.^20-22 As such, it will be essential to translate these findings to human models of human erythropoiesis, under conditions of stress.

Conflict-of-interest disclosure: The authors declare no competing financial interests.