Cancer-associated fibroblasts (CAF) have been suggested to originate from mesenchymal stromal cells (MSC), but their relationship to MSC is not clear. Here we have isolated from primary human neuroblastoma (NB) tumors a population of αFAP- and FSP-1-expressing CAF that share phenotypic and functional characteristics with bone marrow-derived MSC (BM-MSC). Analysis of human NB tumors also confirmed the presence of αFAP- and FSP-1-positive cells in the tumor stroma, and their presence correlated with that of M2 tumor-associated macrophages. These cells (designated CAF-MSC) enhanced in vitro NB cell proliferation, survival, and resistance to chemotherapy and stimulated NB tumor engraftment and growth in immunodeficient mice, indicating an effect independent of the immune system. The pro-tumorigenic activity of MSC in vitro and in xenografted mice was dependent on the co-activation of JAK2/STAT3 and MEK/ERK1/2 in NB cells. In a mouse model of orthotopically implanted NB cells, inhibition of JAK2/STAT3 and MEK/ERK/1/2 by ruxolitinib and trametinib potentiated tumor response to etoposide and increased overall survival. These data point to a new type pro-tumorigenic CAF in the tumor microenvironment (TME) of NB and to STAT3 and ERK1/2 as mediators of their activity.

Drug resistance is a major cause of treatment failure in cancer. Here we have evaluated the role of STAT3 in environment-mediated drug resistance (EMDR) in human neuroblastoma. We determined that STAT3 was not constitutively active in most neuroblastoma cell lines but was rapidly activated upon treatment with interleukin-6 (IL-6) alone and in combination with the soluble IL-6 receptor (sIL-6R). Treatment of neuroblastoma cells with IL-6 protected them from drug-induced apoptosis in a STAT3-dependent manner because the protective effect of IL-6 was abrogated in the presence of a STAT3 inhibitor and upon STAT3 knockdown. STAT3 was necessary for the upregulation of several survival factors such as survivin (BIRC5) and Bcl-xL (BCL2L1) when cells were exposed to IL-6. Importantly, IL-6-mediated STAT3 activation was enhanced by sIL-6R produced by human monocytes, pointing to an important function of monocytes in promoting IL-6-mediated EMDR. Our data also point to the presence of reciprocal activation of STAT3 between tumor cells and bone marrow stromal cells including not only monocytes but also Treg cells and non-myeloid stromal cells. Thus, the data identify an IL-6/sIL-6R/STAT3 interactive pathway between neuroblastoma cells and their microenvironment that contributes to drug resistance.

Interleukin-6 (IL-6) is a pleiotropic cytokine with a broad range of physiological and pathological functions. Because in cancer IL-6 contributes to a microenvironment that promotes tumor cell survival, angiogenesis and inflammation, understanding the mechanism responsible for its production is important. In neuroblastoma, the second most common solid tumor in children, IL-6 is produced not by tumor cells but by stromal cells such as monocytes and bone marrow mesenchymal stem cells (BMMSC). Here we show that the production of IL-6 in BMMSC is in part stimulated by galectin-3 binding protein (Gal-3BP) secreted by neuroblastoma cells. We identified a distal region of the IL-6 promoter that contains 3 CCATT/enhancer binding protein (C/EBP) binding domains involved in the transcriptional upregulation of IL-6 by Gal-3BP.Gal-3BP interacted with Galectin-3 (Gal-3) present in BMMSC, and a Gal-3BP/Gal-3/Ras/MEK/ERK signaling pathway was responsible for the transcriptional upregulation of IL-6 in BMMSC where Gal-3 has a necessary function. In support of the role of this pathway in human neuroblastoma tumors, Gal-3BP was found to be present in tumor cells and in the adjacent extracellular matrix of 96% of 78 primary neuroblastoma tumor samples examined by immunohistochemistry. Considering the protumorigenic function of IL-6 in cancer, this tumor cell-stromal cell interactive pathway could be a target for anticancer therapy.