Proteasome Inhibitor Bortezomib Can Inhibit Bone Marrow Fibrosis Development in a Murine Model of Myelofibrosis.

Several lines of evidence obtained from idiopathic myelofibrosis (IM) studies are in favour of a crucial role of the NF-κB pathway activation in myelofibrosis induction. It has been demonstrated that megakaryocytes, monocytes but also CD34⁺ cells from IM patients present a spontaneous NF-κB pathway activation associated with transforming growth factor-β1 (TGF-β1) secretion. This growth factor has been previously shown as the main fibrogenic cytokine involved in the myelofibrosis development. Mice exposed to high systemic levels of thrombopoietin (TPO) mediated by a retroviral vector (TPO^high mice) develop a myeloproliferative disorder featuring numerous aspects of the human disease including bone marrow fibrosis, extramedullary hematopoiesis and dysmegakaryopoiesis. Moreover, TPO^high mice display high plasma levels of IL-1α suggesting that the NF-κB pathway may play a role in this model of fibrosis. We then conducted a study to investigate whether NF-κB inhibition in this mice model could have an impact on myelofibrosis development using the proteasome inhibitor bortezomib. One month after engraftment with TPO-overexpressing hematopoietic cells, 2 groups of 20 immunocompetent C57BL/6J mice displaying similar myeloproliferation induced by TPO overexpression were constituted. Mice engrafted were treated twice a week, with either bortezomib (1 mg/kg) or a placebo for 4 weeks. At the end of this protocol, mice from both groups were examined for histological and haematological analysis including TGF-β1 and IL-1α levels determination. Here, we demonstrate that: i) the NF-κB pathway is activated in TPO^high spleen cells and bortezomib treatment is able to inhibit this activation; ii) bortezomib treatment is able to decrease plasma concentration of TGF-β1 and IL-1α in mice as well as TGF-β1 content in extracellular fluids of marrow and spleen; iii) myelofibrosis development is inhibited after bortezomib treatment. These results emphasize the interest of developing bortezomib treatment in IM patients.