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Regulation of normal and leukaemic stem cells by the microenvironment: role of nestin+ mesenchymal stem cells and humoral signals

Final Report Summary - LSCMICROENV (Regulation of normal and leukaemic stem cells by the microenvironment: role of nestin+ mesenchymal stem cells and humoral signals)

In this project we proposed to study the regulation of normal and malignant haematopoietic stem cells (HSC) by the haematopoietic microenvironment in the bone marrow (BM) and to assess the possibility of targeting this microenvironment therapeutically in leukaemias. We focused on two aspects: (1) to characterize the role of nestin+ mesenchymal stem cells (MSC) and factors produced by them in leukaemia development, and (2) to investigate the regulation of normal and leukaemic haematopoietic progenitors by oestrogen hormones, and the possible antileukaemic action of oestrogen modulators (such as tamoxifen). The ultimate goal of the project was to identify novel therapeutic approaches for haematopoietic malignancies based on the regulation of the haematopoietic microenvironment, with particular emphasis on diseases that respond poorly to standard treatments (such as MLL-induced acute myeloid leukaemias [AML] and JAK2V617F-induced myeloproliferative neoplasms [MPN]).

Nestin+ cells are rare stromal cells that play an important role in the maintenance of normal HSC, and they persist during leukaemia development (in contrast to the loss of most other stromal cells in the BM during the disease). In order to investigate the role of nestin+ MSC in the development of MLL-induced leukaemias, we used genetically modified mouse models in which nestin+ cells could be eliminated during the course of the leukaemia and studied how this modification affected the pathology of the disease. Elimination of nestin+ MSC significantly reduced leukaemic burden (both in BM and in extramedullary locations) and decreased the number of leukaemic progenitors; therefore, nestin+ MSC contribute to the development of MLL-AF9-induced leukaemias. Depletion of nestin+ cells also enhanced the antileukemic effect of chemotherapy. These effects are not a consequence of changes in proliferation or survival of leukaemic progenitors. However, nestin+ cells appear to regulate multiple other processes in leukaemic progenitors, such as self-renewal, protein metabolism and mitochondrial respiration, among others. The interaction between nestin+ cells and leukaemic cells was not found to be mediated by the chemokine Cxcl12, but other candidate factors were identified and are currently being investigated.

We have extensively characterized the effects of oestrogen receptor signaling on normal and malignant haematopoietic progenitors (Sanchez-Aguilera, Cell Stem Cell, 2014). The selective oestrogen receptor modulator tamoxifen, acting as an oestrogen agonist in haematopoietic cells, induced apoptosis of multipotent haematopoietic progenitors and megakaryocytic precursors but enhanced cell cycle progression of primitive long-term HSC. The effects of tamoxifen are mediated by oestrogen receptor alpha and are exerted directly on the haematopoietic cells. Tamoxifen inhibits the self-renewal program of HSC and activates the Myc transcriptional program. Addition of tamoxifen enhanced the effect of conventional chemotherapy in a mouse model of MLL-AF9 induced leukaemia. Unexpectedly, tamoxifen treatment alone had a very potent effect in mouse models of JAK2V617F-induced myeloproliferative neoplasms, where it blocked all symptoms of the disease as a consequence of increased cell death of malignant haematopoietic progenitors. Although the action of tamoxifen was not completely specific toward malignant cells, it showed selective action toward the leukaemic clone, allowing a partial recovery of normal haematopoiesis. Similar doses of tamoxifen have only a minor impact on normal steady-state haematopoiesis, even after long treatments, highlighting its relative safety and selectivity. Tamoxifen also induced apoptosis of human leukaemic cells both in vitro and in xenotransplantation models in vivo.

These results have immediate therapeutic potential for the treatment of a group of neoplasias for which no curative treatment currently exists. Future work will attempt to translate these preclinical findings into clinical practice (particularly the use of oestrogen receptor modulators for the treatment of refractory AML and MPN, alone or in combination with conventional therapies). Tamoxifen and other oestrogen modulators are already approved for clinical use in the treatment and prevention of breast cancer, which greatly facilitates the possible clinical translation of our findings.