Long-Lasting Dysregulation of the Hematopoietic Stem Cell Compartment in Obesity

Obesity is a complex pathological state defined by the excessive accumulation of adipose tissue and an array of hormonal, immunological and metabolic dysregulations. As such, obesity is a systemic stress that directly affects numerous organs and tissues. Notably, obesity and its sequelae modulate the immune system and the hematopoietic activity in the bone marrow (BM). Not surprisingly, obesity is also a well-established risk factor for leukemia associated with increased incidence and poor prognosis. However, despite their clinical relevance, mechanisms by which obesity affects the hematopoietic system remain elusive. Particularly, the impact of obesity on the hematopoietic stem cell (HSC) compartment has not been described.

Using genetic and dietary mouse models of obesity, we conducted a "HSC-centered study" to determine how obesity affects HSCs and how these cells develop specific compensatory mechanisms to respond to this environment. Although HSCs in an obese environment displayed limited phenotypic and functional perturbations at steady state, they showed an aberrant response to hematopoietic stresses. In serial competitive transplantation assays, obesity-primed HSCs (defined as Lin^- Sca-1⁺ c-Kit⁺ CD48^- CD150⁺) showed a higher level of engraftment than controls in primary recipient mice (control, 20.8% +/-6.2 vs obese, 45.5% +/-14.6, p=0.022) but a dramatically reduced level of engraftment in secondary recipient mice (control: 25.8% +/-14.0 vs obese: 5.4% +/-3.9, p=0.033). Interestingly, BM analysis of secondary recipients showed reduced chimerism in all hematopoietic compartments but not in the HSC compartment. Altogether these results uncovered a biphasic behavior of the obesity-primed HSCs, characterized by an excessive differentiation response followed by a functional decline in which HSCs self-renew but fail to produce downstream progenitors. To unveil the molecular mechanisms involved in this aberrant activity, we performed a genome-wide gene expression analysis on HSCs isolated from normal and obese mice. Although the phenotype observed upon serial transplantation partially mimics HSC aging, obesity-primed HSCs did not share the molecular signature of old HSCs. Furthermore, down-regulation of interferon response-related genes (e.g Irak4, Irf7, Ifi27) and stress response-related genes (e.g. Stip1, Cgrrf1) showed that, unlike what has been described for committed progenitors, HSCs do not elicit a dramatic response to the inflammatory environment associated with obesity. In contrast obesity leads to the activation of specific molecular programs in HSCs. Firstly, obesity-primed HSCs showed up-regulation of multiples genes involved in the phosphatidylinositol signaling pathway (e.g. Pi4ka, Pi4k2b, Pi3kap1, Pi3kip1). Phosphoflow cytometry analysis indicated that this gene expression pattern was associated with the constitutive activation of the protein kinase AKT. While AKT activation is linked to functional HSC exhaustion, obesity-primed HSCs appeared refractory to this signal, suggesting the existence of compensatory mechanisms that protect the integrity of the HSCs in an obese environment. In parallel, we found that the aberrant activity of the obesity-primed HSCs was correlated with an elevated expression of Gfi1, a transcription factor critical for HSC quiescence and differentiation. Interestingly, the 2-fold increase in Gfi1 expression (p<10^-5) observed in obesity-primed HSCs was maintained after serial transplantations in normal recipient mice indicating that the obese environment was able to promote the selection of a stable molecular program in the HSC compartment. Consistent with this idea, single-cell genome-wide analyses suggested a significant clonal shift within the obesity-primed HSC compartment. Finally, consistent with epidemiological data, we found that disruption of HSC homeostasis by obesity promotes the development of spontaneous hematopoietic pathologies resembling to myeloproliferative diseases. Altogether, our results establish the long lasting impact of obesity on the HSC compartment and uncover potential molecular mechanisms linking obesity to hematological diseases. Notably our results support the intriguing possibility that obesity, by directly acting on the HSC compartment, contributes to the development of a clonal hematopoiesis and favors the emergence of aberrant HSC clones.