JAK2V617F-dependent down regulation of SHP-1 expression participates in the selection of myeloproliferative neoplasm cells in the presence of TGF-β.

Myeloproliferative neoplasms (MPNs) are characterized by an increased production of blood cells due to the acquisition of mutations such as JAK2. TGF-β, whose secretion is increased in MPN patients, is known to negatively regulate haematopoietic stem cell (HSC) proliferation. Using an isogenic JAK2 or JAK2 wild-type UT-7 cell line we observed that JAK2 cells resist to TGF-β antiproliferative activity. Although TGF-β receptors and SMAD2/3 expressions are similar in both cell types, TGF-β-induced phosphorylation of SMAD2/3 is reduced in UT-7 JAK2 cells compared with JAK2 WT cells. We confirmed that JAK2 mutated cells are resistant to the antiproliferative effect of TGF-β in a competitive assay as we observed a positive selection of JAK2 cells when exposed to TGF-β. Using cell lines, CD34-positive cells from MPN patients and bone marrow cells from JAK2 knock-in mice we identified a down regulation of the SHP-1 phosphatase, which is required for the regulation of HSC quiescence by TGF-β. The transduction of SHP-1 cDNA (but not a phosphatase inactive cDNA) restores the antiproliferative effect of TGF-β in JAK2 mutated cells. Finally, SC-1, a known agonist of SHP-1, antagonized the selection of JAK2 mutated cells in the presence of TGF-β. In conclusion, we show a JAK2-dependent down regulation of SHP-1 in MPN patients' cells which is related to their resistance to the antiproliferative effect of TGF-β. This may participate in the clonal selection of cancer cells in MPNs.