Role of TPO signaling in DNA repair of hematopoietic stem cells
Hematopoietic stem cells (HSC) are at the beginning of hematopoeisis. They constitute a pool of rare cells in bone marrow in specifics zones of bones called niche. Niche’s cells produce cytokines, like thrombopoietin (TPO). These cytokines regulates HSC by controlling quiescence and self-renewal. Few are known about mechanism used by HSC and its environment to prevent DNA damage, and especially those induced by radio- or chemo-therapies. In this study, we discover a new role of TPO and its receptor Mpl in DNA repair of HSC in response to genotoxic stress. HSC without Mpl, or wild type HSC cultured without TPO, show an important defect of DNA repair and genomic instability. In response to irradiation, TPO increases activation of NF-KB pathway that increases induction of IEX-1 early gene. TPO is also the major activator of ERK pathway in HSC. IEX-1 and p-ERK can form a tripartite complex with DNA-PK, a key protein of Non homologous end joining pathway (NHEJ). DNA-PK is fully activated by TPO which increase fidelity and efficacy of NHEJ pathway leading to better genomic integrity of HSC. We also show in this study that a simple injection of TPO or Romiplostim before irradiation or Doxorubicin injection, decrease mutagenesis of HSC and their loss of function associated. This effect of TPO is specific of TPO because other cytokines like SCF or Flt3-L have no effect on the DNA repair. These results show that TPO can directly control signaling pathway leading to repair of HSC’s DNA and open new avenues for TPO agonist using. They can be used to protect HSC before radio- or chemo-therapies and to minimize development of secondary acute myeloid leukemia. Expression of Mpl being haplo-insufficient for DNA repair functions, this result suggests that Mpl could be a tumor suppressor in response to radio- or chemo-therapies treatments.