Parathyroid Hormone-Induced Modulation of the Bone Marrow Microenvironment Inhibits the Development of Murine Chronic Myelogenous-Leukemia-Like Disease

Abstract 937

The role of the bone marrow microenvironment, and in particular of osteoblastic cells, for normal hematopoiesis has recently been described. However, the role of elements of the bone marrow microenvironment on the initiation, maintenance or progression of leukemia is less clear. To test the influence of activated osteoblasts on the progression of chronic myelogenous leukemia (CML) we used the well-described murine retroviral transduction/transplantation model of BCR-ABL1-induced CML-like disease and mice with osteoblastic cell-specific constitutive activation of the parathyroid-hormone (PTH) receptor (PPR mice) as recipients. Compared to wildtype (wt) littermate control mice PPR mice had significantly prolonged survival (p=0.002) and reduced leukemic mortality with splenic leukemopoiesis contributing to leukemic fatality. Analysis of distinct proviral integration sites in splenic tissue by Southern blotting showed no difference in engraftment of viral clones in wt versus PPR mice. Survival of PPR recipients in the BCR-ABL1-induced model of B-cell acute lymphoblastic leukemia using non-5-fluorouracil-treated donor bone marrow was also significantly prolonged compared to wt mice (p=0.0004). However, a leukemogenic allele known to result in acute myeloid leukemia (AML), MLL-AF9, led to more rapid death in the PPR recipients compared with their wt counterparts arguing that the prolongation of survival in the BCR-ABL1-induced diseases was oncogene-specific. In-vitro assays including the Whitlock-Witte assay to test the role of PPR stroma or a cobblestone colony formation assay in osteogenic medium revealed no difference in the growth of plated BCR-ABL1⁺ or BCR-ABL1^- B-lymphoid progenitors or lin^- c-kit⁺ Sca-1⁺ cells on wt versus PPR stroma, respectively. In vivo, treatment of mice with the phosphodiesterase inhibitor forskolin, which increases intracellular cyclic adenosine monophosphate (cAMP) levels, similar to increased signaling from the PTH receptor, did not lead to prolonged survival in the murine model of CML-like disease. Prior splenectomy of wt, as well as PPR recipients of BCR-ABL1-induced CML-like disease did not significantly prolong survival, but drastically reduced the efficiency of induction of CML-like disease with the great majority of wt and PPR animals succumbing to non-CML causes. Secondary transplantation of CML-like disease from bone marrow or spleen from a wt or PPR microenvironment revealed, firstly, less efficient induction of secondary disease in wt recipients of PPR bone marrow compared to wt bone marrow and, secondly, superiority of CML-induction in secondary recipients of spleen compared to bone marrow from a PPR microenvironment. In order to test for an osteoblast-extrinsic cause of the prolonged survival of PPR recipients in the CML-model and to test for the role of bone remodeling, wt and PPR recipients of BCR-ABL1-transduced bone marrow were treated with saline or osteoprotegerin, an inhibitor of osteoclast differentiation and proliferation. Surprisingly, 100% of PPR and wt control mice succumbed to CML-like disease sooner than the control mice treated with saline. Additionally, continuous infusion of human PTH(1-34) into wt mice with BCR-ABL1-induced CML-like disease by minipump led to prolonged survival compared to saline-treated animals arguing that PTH may be an intervention beneficial in human CML. This, to our knowledge, represents first evidence that modulation of the bone marrow microenvironment may improve the outcome in leukemia.