Activation of Beta 2 Adrenergic Receptor in Osteoblasts Contributes to Bone Metastasis Progression via IL-6-dependent Upregulation of Myeloid-Derived Suppressor Cells

초록

The early phase of bone metastatic tumor growth occurs near the endosteal niche, sug-gesting that osteoblasts play active roles in regulating tumor cell dormancy and counterbal-ancing anti-tumoral immune surveillance. Stimulation of osteoblasts such as through beta 2 adrenergic receptors (Adrb2) and the sympathetic nerve system (SNS) was shown to increase breast cancer cell proliferation, angiogenesis and osteolysis in immune-compromised mouse models. We further investigated whether SNS-dependent Adrb2 stimulation in osteoblasts affects bone marrow immunity using immune-competent models of breast cancer. Activation of SNS by chronic immobilization stress (CIS) increased the bone metastatic seeding and initial growth of 4T1 syngeneic breast cancer cells in intra-cardiac and intra-tibial tumor injection models as well as proliferation of T cell-suppressive myeloid-derived suppressor cells (MDSC) in the bone marrow. The effects were reversed by ICI-118551, an Adrb2 in-hibitor or MDSC depletion using anti-Gr1 antibodies, suggesting that MDSC play a role in SNS activation-induced bone metastatic progression. More importantly, two-week CIS pre-treatment before intra-tibial tumor implantation increased MDSC expansion, supporting that CIS primes the pre-metastatic niche in bone by tipping anti-tumoral immunity balance in favor of tumor growth. RNA-sequencing transcriptome analysis of PBS- or clenbuter-ol- (an Adrb2 selective agonist) stimulated MC3T3E1 osteoblasts showed that interleukin-6 (IL-6) was the most significantly increased cytokine that potentially upregulate MDSC. Subsequent quantitative PCR and ELISA confirmed that IL-6 was increased by clenbuterol which was suppressed by ICI-118551. Our data collectively demonstrate that activation of osteoblasts via SNS contributes to breast cancer bone metastasis partly by increasing the number of MDSC, which was potentially mediated by osteoblastic expression of IL-6 via Adrb2 activation. Further in vivo studies using osteoblast-specific Adrb2 knockout mice and also in vitro studies on IL-6 and STAT3 pathways in MDSC will provide more clear molecular mechanism underlying the SNS-dependent bone marrow immunity regulation in the metastatic tumor environment.