Induction of CXCL1 in Osteoblasts by Myeloma Cells Promotes Migration of Osteoclast Precursors

Multiple myeloma (MM) causes a dysbalance in the bone microenvironment between bone building osteoblasts and bone resorbing osteoclasts (OCs), with an increase in OC recruitment, differentiation and activation, leading to myeloma bone disease (MBD). Presence of MBD has a major impact on the quality of life of MM patients and novel treatment approaches for MBD are urgently needed. Several factors have been identified that play a role in this process, e.g. receptor activator of NF-kB ligand (RANKL). However, the pathomechanism of increased osteoclast recruitment and activation is not completely understood. Here, we investigated the role of the chemokine CXCL1 and its receptor CXCR2 in the bone microenvironment in MM.

Serum samples from 52 patients with newly diagnosed MM and from 22 healthy volunteers were assayed using a CXCL1 ELISA. Primary human mesenchymal stem cells (hMSCs) were cultured from bone marrow aspirates and primary human differentiated osteoblasts (hOBs) were cultured from trabecular bone fragments, both from healthy volunteers. Osteoclast precursors (pre-OCs) were generated by immunomagnetic sorting of CD14-positive cells from the peripheral blood of healthy volunteers. Human myeloma cell lines (HMCLs) U-266, RPMI-8226 and LP-1 and primary bone marrow myeloma cells (pMMCs) selected using CD138 immunomagnetic sorting were used for the experiments. Co-cultures of HMCLs and pMMCs with hMSCs or hOBs were performed using 0.45 μm transwell inserts, allowing for the exchange of soluble mediators. Migration assays were performed using 8 μm transwell inserts and human recombinant CXCL1. Immunohistochemistry was performed on paraffin-embedded bone marrow biopsies from MM patients using an anti-CXCR2 monoclonal antibody. All experimental procedures involving patient material were approved by the local ethics committee and conducted after informed consent was obtained.

CXCL1 serum levels were found to be significantly higher in MM patients than in healthy individuals (193.4 pg/mL vs. 137 pg/mL, respectively, p<0.05), indicating a role for CXCL1 in MM pathophysiology. We went on to investigate the role of CXCL1 in MBD and performed co-cultures of HMCLs and pMMCs with hMSCs or hOBs. Baseline CXCL1 expression was absent in HMCLs and low or absent in hMSCs or hOBs at baseline. RNA expression as well as protein excretion by hMSCs and hOBs were induced after co-culture with myeloma cells. For example, pMMCs from different individuals led to a mean 154-fold upregulation of CXCL1 mRNA levels in hMSCs and to a mean upregulation of CXCL1 protein in cell culture supernatants from <31.5 pg/mL at baseline to 2140 pg/mL after co-cultures. In order to investigate the potential function of elevated CXCL1 levels in the bone marrow microenvironment, the expression of CXCR2, the receptor for CXCL1, was analyzed. Pre-OCs as well as a majority of pMMCs expressed CXCR2 mRNA. CXCR2 protein expression in pMMCs was verified using immunohistochemistry on MM bone marrow biopsies. Human recombinant CXCL1 significantly increased pre-OC cell migration in a dose-dependent manner. For example, 50 ng/mL or 100 ng/mL of CXCL1 increased mean pre-OC migration along a CXCL1 gradient 2.5-fold and 5.6-fold over baseline, respectively. In addition, mean pMMC migration was increased 3.8-fold compared to baseline along a 100 ng/mL gradient of recombinant CXCL1. The osteoclastogenic capacity of the migrated pre-OCs was confirmed by TRAP expression after stimulation with RANKL and M-CSF.

We describe here a novel role for the chemokine CXCL1 in myeloma bone disease. We demonstrate that CXCL1 is induced in hMSCs and hOBs by co-culture with MM cells. CXCL1 leads to chemoattraction of both pre-OCs and pMMCs. These effects could lead to co-localization of OCs and MM cells in the bone marrow microenvironment and contribute to the tumor-promoting interaction between these cell types. Our data indicate the CXCL1-CXCR2 axis as a therapeutic target in myeloma bone disease.

No relevant conflicts of interest to declare.