Reacting to Inflammatory Signals

Specialized cellular niches in the bone marrow (BM) modulate critical functions of the hematopoietic stem and progenitor cells, such as self-renewal, cell-fate decisions and the balance between proliferation and differentiation. Constituents of the BM niche, such as osteoblasts, endothelial cells and macrophages, respond to inflammation with secretion of pro-inflammatory cytokines, and serve as effectors of the inflammatory circuitry; thus their excessive activation can potentially impact on the regulation of hematopoietic cells, and on the initiation and progression of myeloid malignancies.

Our current understanding of the mechanisms for Bcr/Ab-negative myeloproliferative neoplasia (MPN) involves recognition of driver mutations (targeting i.e. Tet2, Jak2, Mpl, or Asxl) and of an inflammatory microenvironment. However, despite accumulating evidences on the role of inflammation and of the microenvironment in the maintenance and progression of myeloid malignancies, it is still unclear how the inflamed BM niche contributes to disease establishment and progression. Similarly, the causes leading to an inflammatory microenvironment in MPN are not fully understood.

We use genetically-controlled animal model of BM inflammation to study the contribution of the inflammatory microenvironment to MPN. We found that loss of Notch signaling in the BM microenvironment results in an inflammatory state of the BM niche promoting myeloproliferation and marrow fibrosis. Using this model, we discovered that loss of Notch/RBPJ signaling leads to transcriptional upregulation of the pro-inflammatory microRNA miR-155, especially in BM endothelial and mesenchymal cells, resulting in miR-155-dependent inhibition of the NF-κB inhibitor κB-Ras1. Decreased levels of kB-Ras1 resulted in the heightened and persistent activation of NF-κB and in the increased production of NF-kB dependent pro-inflammatory cytokines, leading to uncontrolled myeloproliferation. Deletion of miR-155 in the stroma of RBPJ^-/- mice prevented the development of the myeloproliferative disease, and patients affected by MPN exhibit elevated expression of miR155 in their BM. Therefore, we hypothesize that the Notch/miR155/NF-kB axis regulates the level of the inflammatory tonus in the BM niche, and that persistent deregulation of this pathway may contribute to the establishment and progression of MPN. In this presentation, we will discuss our findings and ongoing studies in the context of other recent insights on the crosstalk between tumor cells and the BM niche, and their implications for disease initiation, progression and therapeutic approaches.