FANCD2 and HES1 Synergistically Regulate the Immunometabolism of Hematopoietic Stem Cells through Transcriptional Suppression of Pparg- mediated Fatty Acid Oxidation and Oxidative Phosphorylation

Emerging evidence has revealed that resting quiescent hematopoietic stem cells (HSCs) possess a distinct metabolic profile with a preference for anaerobic glycolysis rather than mitochondrial oxidative phosphorylation (OXPHOS). Recent few scattered studies on the relationship between metabolism and HSCs raise a completely different perspective on the immunological/metabolic interface between HSC defect and human diseases. How immunometabolism influences HSC function, and what signaling cascades help drive cell-intrinsic immunometabolic modes in HSCs, has begun to emerge as an area of intense interest. Here we have investigated the immnometabolic regulation of HSCs using a model of Fanconi anemia (FA), a cancer-prone disease known to impact the immune system and promote inflammation. We and others have demonstrated that inflammation in FA hematopoietic stem and progenitor cells (HSPCs) plays a crucial role in FA pathophysiology. Recently, we have shown that FA HSCs are more dependent on OXPHOS relative to glycolysis for energy metabolism. However, the mechanism underpinning the link between inflammation and the altered metabolic program in FA HSCs has not been defined. More recently, we exploited the repopulating defect of FA HSCs to conduct an unbiased in vivo shRNA screen in transplant recipients, and found enrichment of shRNAs targeting genes involved in the Pparg pathway. Pparg is a central transcriptional factor regulating adipocyte differentiation and energy metabolism but has not been linked to HSC homeostasis. Pparg inhibition by shRNA or chemical compounds significantly improved the repopulating ability of Fancd2-KO HSCs. Conversely, activation of Pparg in wild-type HSCs impaired hematopoietic repopulation. Suppression of inflammation-induced expression of Pparg required both Fancd2 and the Notch target Hes1. Deletion of Hes1 de-repressed Pparg expression in wild-type LSK cells, which are enriched for HSPCs, but had no further effect on Fancd2-KO LSK cells. Mechanistic studies showed that Fancd2 and Hes1 acted synergistically to repress the Pparg promoter. Furthermore, transcriptomic study revealed that inflammation orchestrated an overlapping transcriptional program in HSPCs deficient for Fancd2 and Hes1, featuring upregulated genes in fatty acid oxidation (FAO) and OXPHOS. Consistently, we observed a marked increase in FAO and OXPHOS in mouse HSPCs deficient for Fancd2 or Hes1 and in the BM samples of FA patients. Using a newly developed Fancd2-KI mouse model, we identified a novel and inflammation-responsive interaction between Fancd2 and Hes1. This inflammation-responsive FANCD2-HES1 interaction was abolished in FA-D2 patient-derived lymphoblast cells but rescued in those constituted with a functional FANCD2 (with a 3×FLAG tag) gene. Finally, a Fancd2 mutation lacking the Hes1-interacting domain abolished the activity of the repression of Pparg expression, FAO and OXPHOS. Our results indicate that the FA pathway may constitute a key component of a novel immunometabolism axis, connecting inflammation, cellular metabolism and HSC function.