Elsevier

Experimental Cell Research

Volume 316, Issue 20, 10 December 2010, Pages 3417-3424
Experimental Cell Research

Research Article
Interleukin 6 mediated recruitment of mesenchymal stem cells to the hypoxic tumor milieu

https://doi.org/10.1016/j.yexcr.2010.07.002Get rights and content

Abstract

Mesenchymal stem cells (MSCs) are a heterogeneous population of non-hematopoietic precursor cells predominantly found in the bone marrow. They have been recently reported to home towards the hypoxic tumor microenvironment in vivo. Interleukin-6 is a multifunctional cytokine normally involved in the regulation of the immune and inflammatory response. In addition to its normal function, IL-6 signaling has been implicated in tumorigenesis. Solid tumors develop hypoxia as a result of inadequate O2 supply. Interestingly, tumor types with increased levels of hypoxia are known to have increased resistance to chemotherapy as well as increased metastatic potential. Here, we present evidence that under hypoxic conditions (1.5% O2) breast cancer cells secrete high levels of IL-6, which serve to activate and attract MSCs. We now report that secreted IL-6 acts in a paracrine fashion on MSCs stimulating the activation of both Stat3 and MAPK signaling pathways to enhance migratory potential and cell survival. Inhibition of IL-6 signaling utilizing neutralizing antibodies leads to attenuation of MSC migration. Specifically, increased migration is dependent on IL-6 signaling through the IL-6 receptor. Collectively, our data demonstrate that hypoxic tumor cells specifically recruit MSCs, which through activation of signaling and survival pathways facilitate tumor progression.

Introduction

Human mesenchymal stem cells (hMSCs) are a heterogeneous population of progenitor cells, which predominantly reside in the bone marrow [1], [2] and under appropriate conditions have the capacity to differentiate into bone, fat and cartilage cells [3], [4], [5]. While tumor inhibition by hMSCs has been reported, more frequently hMSCs have been shown to have tumor-promoting effects [5], [6], [7], [8]. hMSCs have been shown to stimulate tumor growth by several mechanisms including differentiation into Carcinoma Associated Fibroblasts (CAFs) and contributing to the tumor stroma [9]. The stromally recruited hMSCs promote angiogenesis leading to tumor progression [6], [10], [11].

The TMEN is a dynamic system made up of a heterogeneous cellular population in a background of fluctuating gradients of O2 content, pH, and interstitial fluid pressure (IFP) [12], [13]. Hypoxia occurs within regions of the TMEN as a consequence of inadequate vascular supply [14], [15]. Hypoxia enhances the ability of tumor cells to develop resistance to radiotherapy, endocrine therapy and chemotherapy [12] and is associated with increased metastasis and poor outcome for various cancer types [16]. The hypoxic microenvironment may promote metastasis through the induction of chemokines and cytokines such as interleukin-8 (IL-8), interleukin-6 (IL-6) and through increased levels of genomic instability leading to mutations [17], [18], [19], [20]. In addition to hypoxia, the cellular makeup of the tumor microenvironment also plays a critical role in metastasis. In a current breast carcinoma model, tumor cells have been shown to release paracrine signals that mediate the recruitment of MSCs towards the microenvironment to enhance tumor metastasis [6].

In the present study we set out to better understand the interplay between tumor cells and hMSCs in the context of a hypoxic microenvironment. We show that IL-6, which is produced and secreted at high levels by breast cancer cells in response to hypoxia, regulates hMSC migration towards cancer cells. IL-6 binds to its cognate receptor on hMSCs, leading to Stat3 activation. Signaling events in hMSCs post-Stat3 activation, promote hMSC migration and survival. Inhibition of IL-6 production by breast cancer cells attenuated hMSC migration, demonstrating the significance of IL-6 signaling in mediating tumor-stroma interactions.

Section snippets

Cell lines and transfections

MDA-MB-231 (ER-/PR-, representing basal subtype B), MCF-7 (ER+/PR+; representing luminal subtype) and MDA MB 468 (ER−/PR−; representing basal subtype A) human breast cancer cell lines obtained from ATCC were cultured with DMEM media containing 10% FBS under normoxic (21% O2) or hypoxic (1.5% O2) conditions for 6 and 24 h, unless otherwise noted. Hypoxia was achieved by placing cells in a modular chamber obtained from Billups-Rothenburg according to manufacturer's instructions. Human bone-marrow

Hypoxia enhances hMSC migration towards breast cancer cells

Mesenchymal stem cells have been shown to localize to breast carcinomas in vivo [6]. Previous studies in our laboratory have shown that both human and rat derived MSCs migrate towards MDA-MB-231 cells or conditioned medium collected from these cells under normoxic conditions [21]. To investigate hMSC migration under hypoxia, we established an in vitro transwell migration assay in which hMSCs were allowed to migrate towards MDA-MB-231, MCF-7 and MDA-MB-468 breast cancer cells under normoxic and

Discussion

Hypoxia is a constant and universal feature of solid tumors [28], [29], [30]. It has recently become clear that hypoxia plays a role in tumor progression, metastasis and poor clinical outcome [31], [32]. Regions of hypoxia within the tumor microenvironment are known to activate several genes involved in enhancing tumor cell survival and angiogenesis [33], [34]. The tumor stroma also plays a key role in tumor progression via the increased presence of fibroblasts, macrophages, lymphocytes and

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    Present Address: Division of Hematology/Oncology, Department of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA.

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