Disruption of the Akt-FoxO Axis As a Safeguard Mechanism for the Hematopoietic Stem Cell Compartment during Chronic Metabolic Stress

Lifelong maintenance of the blood system requires the preservation of a healthy hematopoietic stem cell (HSC) pool. Integrity of the HSC compartment is disrupted by severe homeostatic perturbations following acute infection or irradiation, which result in HSC loss and lead to dramatic hematological dysfunctions. HSC fitness is also affected by organismal patho-physiological conditions, which are associated with chronic low-grade stress. These chronic conditions present, over time, a unique challenge to the maintenance of the long-lived HSCs. However, the mechanisms by which the HSC pool adapts to chronic, low-grade stress conditions remain largely unknown.

We focus on obesity as a model of chronic stress that potentially affects the health of the HSC compartment. Obesity is a chronic pathological state, which is established and persists over long period of time and therefore, is a prime candidate to alter long-lived HSCs. Obesity becomes progressively associated with an array of systemic and local alterations that include metabolic dysregulations, unresolved low-grade inflammation andalteration of BM microenvironment, all conditions that have been linked to HSC defects (Nagareddy et al, Cell Metabol., 2013; Naveiras et al, Nature, 2009; Luo et al, Cell Metabol., 2015; Ambrosi et al, Cell Stem Cell, 2017).Consistent with these studies, we recently showed that obesity alters the composition of the HSC-SLAM (Lin- ckit+ Sca1+ CD48+ CD150-) compartment and leads to a progressive loss of HSC fitness upon serial competitive transplantation assays (Lee et al, J. Exp. Med.,2018). Mechanistically, we established that the oxidative stress induced by obesity dysregulates the expression of the transcription factor Gfi1 and contributes to the long-term alteration of the HSC functions.

Following this work, we confirmed that obesity mainly affects the short and long-term stress response of the most primitive HSC compartment (defined as HSC-SLAM CD34- CD49b-) but not of the downstream short-term HSC subsets, therefore indicating that obesitydisrupts intrinsic mechanisms regulating self-renewal activity. Genome-wide gene expression analyses indicated that HSCs isolated from obese mice dysregulate multiples genes involved in the phosphatidylinositol signaling pathway (e.g. Pik3c2a, Pik3c2b, Pi3kap1, Pi3kip1), upstream of the Akt signaling molecule. Phospho-flow cytometry analyses showed that HSCs freshly isolated from obese mice display constitutive Akt activation. In vivopharmacological Akt inhibition did not significantly change the size or phenotype of the HSC-SLAM compartment in obesity but led to the normalization of their functions in condition of transplantation-induced regenerative stress. These results indicate that the aberrant Akt activation in obesity-primed HSCs directly contributes to their hyperactivity upon transplantation, which ultimately results in the erosion of their self-renewal potential. We next explored the mechanisms that allow HSCs to remain quiescent in obesity despite the constitutive activation of Akt and the continued presence of oxidative stress condition. We focused on FoxO3, which is a key transcriptional regulator of HSC quiescence and direct Akt target. We found that FoxO3 in obesity-primed HSCs become insensitive to its normal upstream regulators such as Akt and p38, leading to the maintenance of its nuclear location in oxidative stress condition. As consequence, we observed that obesity-primed HSCs were more resistant to oxidative stress than their normal counterparts, suggesting an increase protection against ROS-mediated senescence and apoptosis. Altogether, these results suggest that chronic metabolic stresses associated with obesity progressively affect the wiring of the HSCs and modify their oxidative stress response. The uncoupling of FoxO3 to its environmental regulators in obesity could be viewed as a key adaptive strategy to ensure the survival and function of the HSC compartment in condition of chronic metabolic stress. More broadly, these results highlight how patho-physiological conditions associated with chronic low-grade stress (such as obesity) could shape the characteristic of the HSC compartment at steady state. Progressive alteration of the normal HSC stress response in such conditions could be an unrecognized contributing factor for the development of hematological diseases.