Abstract
BackgroundIncreased numbers of tumour-associated macrophages correlate with shortened survival in some cancers. The molecular bases of this correlation are not thoroughly understood. Events triggered by CXCL12 may play a part, as CXCL12 drives the migration of both CXCR4-positive cancer cells and macrophages and may promote a molecular crosstalk between them.ResultsSamples of HER1-positive colon cancer metastases in liver, a tissue with high expression of CXCL12, were analysed by immunohistochemistry. In all of the patient biopsies, CD68-positive tumour-associated macrophages presented a mixed CXCL10 (M1)/CD163 (M2) pattern, expressed CXCR4, GM-CSF and HB-EGF, and some stained positive for CXCL12. Cancer cells stained positive for CXCR4, CXCL12, HER1, HER4 and GM-CSF. Regulatory interactions among these proteins were validated via experiments in vitro involving crosstalk between human mononuclear phagocytes and the cell lines DLD-1 (human colon adenocarcinoma) and HeLa (human cervical carcinoma), which express the above-mentioned ligand/receptor repertoire. CXCL12 induced mononuclear phagocytes to release HB-EGF, which activated HER1 and triggered anti-apoptotic and proliferative signals in cancer cells. The cancer cells then proliferated and released GM-CSF, which in turn activated mononuclear phagocytes and induced them to release more HB-EGF. Blockade of GM-CSF with neutralising antibodies or siRNA suppressed this loop.ConclusionsCXCL12-driven stimulation of cancer cells and macrophages may elicit and reinforce a GM-CSF/HB-EGF paracrine loop, whereby macrophages contribute to cancer survival and expansion. The involvement of mixed M1/M2 GM-CSF-stimulated macrophages in a tumour-promoting loop may challenge the paradigm of tumour-favouring macrophages as polarized M2 mononuclear phagocytes.
Highlights
Increased numbers of tumour-associated macrophages correlate with shortened survival in some cancers
Because mononuclear phagocytes express both CXCL12 guanine nucleotide-binding proteincoupled receptors (GPCR) and HB-EGF, we argued that the recruitment of mononuclear phagocytes to a site of metastasis such as liver through CXCL12 should induce a release of HB-EGF, which is expected to activate HER1 and favour tumour progression
We found that tumour-associated macrophages and metastatic HER1positive colon cancer in liver biopsies expressed a ligand/receptor repertoire that was consistent with our hypothesis and that in vitro CXCL12 could trigger a GM-CSF/HB-EGF paracrine loop whereby mononuclear phagocytes support cancer survival
Summary
Increased numbers of tumour-associated macrophages correlate with shortened survival in some cancers. A tissue with high expression of CXCL12 (for example, liver or bone marrow) may represent a site that preferentially attracts both macrophages [9] and cancer cells [10,11], which co-migrate depending on their expression of the CXCL12 receptors CXCR4 and/or CXCR7 [12]. Ligand binding to these receptors, which are heterotrimeric guanine nucleotide-binding proteincoupled receptors (GPCR), activates matrix metallopeptidases that cleave EGF-family ligands, such as EGF or HB-EGF, from the cell membrane [13], leading to transactivation of HER1 on neighbouring cells [14]. The macrophageregulator GM-CSF, which is produced by some types of cancer cells [18,19], induces HB-EGF in macrophages and neutrophils [20]
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