Abstract P5-04-29: Osteoblasts regulate mobilization of the myeloid-derived suppressor cells from the bone marrow of breast cancer patients
Authors: Park Serkin , Lee Kyoung Jin , Lee Eun Jung , Whang Young Mi , Cho Sun Wook 
Published: March 2020

Abstract
MDSCs are T cell suppressive immature myeloid-lineage cells in the tumor microenvironent (TME). MDSCs originate in the bone marrow, circulate and accumulate in the spleen and tumor tissue. MDSCs consist of two morphologically and functionally distinct subpopulations, i.e. monocytic (M-) and polymorphonuclear (PMN-) MDSC. While PMN-MDSCs are more abundant and suppress T cells in an antigen-specific manner, M-MDSCs are earlier-stage MDSCs and suppress T cells in both antigen-specific and nonspecific manners. In contrast to clear functions in TME, how MDSCs are regulated in the bone marrow of tumor hosts remains unclear. We have previously reported that tumor-derived parathyroid hormone-related peptide (PTHrP) increased MDSCs in TME. In this study, we further investigated cellular and molecular mechanisms of PTHrP in mobilizing MDSCs from the bone marrow. Continuous infusion of PTHrP(1-34) in female Balb/C mice using subcutaneous osmotic pumps (80μg/kg, 3 weeks) increased M-MDSC, but not PMN-MDSC, in the circulation and bone marrow. Interestingly, a single administration of PTHrP(1-34) or PTH(1-34) in mice also rapidly increased M-MDSC in the circulation, suggesting that M-MDSC are mobilized from the bone marrow by PTH1R activation.RT-PCR analysis showed that PTH1R is expressed in osteoblasts, but not in human or mouse MDSC. In vitro cell binding assays measuring fluorescently labeled-human or mouse MDSC co-cultured with MC3T3E1 or hFOB osteoblasts were performed to determine the molecular mechanism of MDSC mobilization. We found that PTHrP(1-34) and PTH(1-34), but not PTHrP(7-34), released MDSC-osteoblast binding, and also that anti-VCAM 1 and/or -β1 integrin neutralizing antibodies blocked MDSC-osteoblast binding. Inhibitors of PKA, a downstream mediator of PTH1R, inhibited MDSC-osteoblast binding, while forskolin, a PKA activator, showed the opposite effects. Immunohistochemical staining of the bones of tumor-bearing mice showed that M-MDSCs localize adjacent to trabecular osteoblasts via VCAM-1 (expressed by osteoblasts) and β1 integrin (expressed by M-MDSC). Subsequent experiments demonstrated that PTH/PTHrP-stimulated osteoblasts release VEGF-A and IL-6 to stimulate M-MDSCs via phosphorylation of Src family kinase. Administration of dasatinib, a Src selective inhibitor, significantly suppressed PTHrP-mediated M-MDSC mobilization in vivo. Collectively, our data demonstrate that tumor-derived PTHrP circulate and activates osteoblasts, leading to release of M-MDSCs via disruption of VCAM-1 and β1 integrin retention axis by upregulation of VEGF-A and IL-6 in bone and by Src family kinase phosphorylation in MDCS. In conclusion, this study explains how MDSC, an essential bone marrow-derived cells in TME, are regulated in the bones of cancer patients. Further studies will provide a therapeutic approach to uncouple the partnership between tumor and bone.
Citation Format: Serkin Park, Kyoung Jin Lee, Eun Jung Lee, Young Mi Whang, Sun Wook Cho. Osteoblasts regulate mobilization of the myeloid-derived suppressor cells from the bone marrow of breast cancer patients [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-04-29.