Parathyroid Hormone-Induced Modulation of the Bone Marrow Microenvironment Reduces Leukemic Stem Cells in Murine Chronic Myelogenous-Leukemia-Like Disease Via a TGFbeta-Dependent Pathway

Abstract 1670

It is known that osteoblastic cells regulate the normal hematopoietic stem cell (HSC) niche and control its size. Parathyroid hormone (PTH) is an important regulator of osteoblasts and osteoclasts maintaining calcium homeostasis and, additionally, increasing HSC number in transplant recipients and protecting HSCs from repeated exposure to cytotoxic chemotherapy. We, therefore, hypothesized, that PTH-treatment may allow normal HSCs to outcompete leukemic stem cells (LSCs) in a murine model of chronic myelogenous leukemia.

Mice with osteoblastic cell-specific constitutive activation of the receptor for PTH and PTH-related protein (Col1-caPPR mice) are characterized by activation of osteoblastic cells and increases in osteoclast and osteoblast number, trabecular bone, bone turnover and cortical porosity. Col1-caPPR mice have significantly prolonged survival and reduced leukemic mortality compared to wildtype (wt) littermates in a murine retroviral transduction/transplantation model of BCR-ABL1-induced CML-like disease (p=0.002) and B-cell acute lymphoblastic leukemia (B-ALL) (p=0.0004). However, a leukemogenic fusion transcription factor, MLL-AF9, known to cause acute myeloid leukemia in this model, led to more rapid death in the Col1-caPPR recipients compared with their wt counterparts (p<0.0001), indicating that the increased survival of Col1-caPPR recipients is specific for BCR-ABL1-induced leukemia.

Continuous infusion of human PTH(1–34) into wt mice with BCR-ABL1-induced CML led to a statistically significant decrease in spleen weights and decreased bone marrow infiltration by BCR-ABL⁺ cells. Limiting dilution secondary transplantation of BM cells from saline- or PTH-treated primary animals with fully established CML into wt recipients revealed a 15-fold reduction of LSCs in a PTH-treated environment. Secondary mice who received BM from saline-treated donors had an overall survival that was 1/4 that of recipients of marrow from a PTH-treated BM microenvironment.

Transforming growth factor beta-1 (TGFβ-1), whose largest and most concentrated tissue source is bone, was increased in the bones of Col1-caPPR mice. TGFβ-1 significantly decreased the in-vitro growth of the BCR-ABL⁺ cell line K562, but not the MLL-AF9⁺ cell line THP-1 suggesting that TGFβ-1, increased in the bone marrow microenvironment of Col1-caPPR mice, may be actively suppressing the growth of the BCR-ABL⁺ diseases, but not of MLL-AF9⁺ AML. Conversely, blockade of TGFβ-1, -2, and -3 by anti- TGFβ antibody treatment increased the incidence of CML in Col1-caPPR mice from 50% to 75%. Knockdown of TGF Receptor I in transplanted BCR-ABL⁺ BM in the CML model increased the percentage of BCR-ABL⁺ myeloid cells in peripheral blood in wt and, more strikingly, in Col1-caPPR recipient mice and increased the overall incidence of CML in Col1-caPPR mice. These results argue that reduction in TGFβ-1 signaling may rescue the CML phenotype in Col1-caPPR mice.

In conclusion, these studies suggest that modulation of the bone marrow microenvironment by PTH reduces the frequency of LSCs in CML, possibly by suppression of LSCs via TGFβ-1. Consequently, a clinical trial on the combined use of imatinib and PTH in patients with CML has been initiated at our institution.