Growth, survival and progression in carcinoma depends in part on a complex crosstalk between the epithelial cells within the tumor and the stroma that surrounds them.1
By using gene expression profiles of soft tissue tumors as surrogates for stromal reaction patterns, we have previously described gene signatures that can be used to identify different stromal response patterns to breast carcinoma.5,6,13
While these studies have focused on the stromal signatures in primary tumors, the tumor microenvironment is clearly important in metastasis as well. There are competing theories regarding the choice of the metastatic site that address “tissue tropism” versus a more passive role of the microenvironment but it is clear that for a successful tumor growth at the site of metastasis there must be a productive interaction with the stroma. In the current study we asked whether stromal signatures established at the site of metastasis are similar to those established in the primary organ or whether novel interactions with the stroma are created. This would help determine whether the tumor brings with it the stromal expression signature that it established as it developed at the primary site and imposes this signature on the metastatic site or whether the tumor must develop an entirely novel gene expression pattern to interact with the metastatic stroma at the new site.
Analysis of the CSF1 macrophage stromal response signature demonstrated conservation of the signature between primary tumors and regional lymph node metastases in both breast and colon carcinoma types by two different assays, immunohistochemistry for protein expression and gene expression profiling for RNA expression. Although the correlations are not as tight for the immunohistochemistry study, they are still statistically significant and the difference is likely due to the limited number of immunohistochemistry markers. Taken together, these findings suggest that in both breast and colorectal cancer the CSF1 macrophage stromal response is determined by the intrinsic biology of the tumor and that it is independent of the TME that the tumor resides in. That we do not find a significant correlation in the distant metastases could be due to the small number of cases available for analysis. Despite this, 2 of the 4 cases with a positive CSF1 macrophage response signature in the primary also had a positive signature in the metastasis. It is possible that we greater numbers this would be significant. However, it is also possible that the differences reflect an important difference between metastases which would become apparent in a larger dataset.
Analysis of the DTF fibroblast stromal response signature demonstrated an absence of significant correlation between primary tumors and lymph node metastases by immunohistochemical analysis of the stromal compartment. This lack of correlation was observed in both breast cancer (rho=0.22, p=0.13) and colorectal cancer (rho=−0.23, p=0.47). These findings could suggest that the DTF fibroblast stromal response is independent of the intrinsic biology of the tumor and may vary with the tissue in which the response is observed. However, the gene microarray data demonstrated conservation of the DTF fibroblast gene signature between primary tumors and regional lymph node metastases (rho=0.72, p=0.003) in breast cancer. It is important to note that the gene microarray studies analyzed mRNA expression levels in homogenized tumors where epithelial tumor cells are intermixed with stroma, while the tissue microarray studies only evaluated the tumor stroma. This might indicate that the observed difference in the consistency of the DTF fibroblast stromal response for primary breast cancers and matched regional lymph node metastases may be a consequence of the measurement of mRNA levels within the epithelial elements of the tumor in the gene microarray analysis as compared to the evaluation of stromal elements in the tissue microarray analysis. It is possible that the DTF fibroblast stromal response in the primary tumor is initially generated from the stroma surrounding the tumor and not the tumor. In this scenario, the expression of the DTF fibroblast stromal response is mostly contained within the stroma but also involves cross-talk with the cancers cells. When the tumor metastasizes, it does not bring with it the stroma and so that gene signature is lost.
Others have found that the 70-gene and molecular subtype signatures are retained from the primary to the metastasis.12
However, these signatures are intrinsic to the cancer cells themselves and do not raise a conflict with our findings that are focused on the tumor stroma. In fact, our findings correlate with prior studies comparing primary and metastasis expression where it was noted that it only some ECM genes that vary between primary and metastasis.12
It is important to note that while we use the term “fibroblast” and “macrophage” to describe these signatures, these designations are not meant to suggest that the genes in these signatures are exclusively expressed in these cell types. However, the bulk of the genes from the 2 signatures appear to be involved in either fibroblast or macrophage function and this has been borne out by prior gene pathway analysis 5,6
and morphologic analysis in this study and prior studies.5–7
Our finding do not clarify the inducing event for the signatures and from what cell or cell interactions is this induction originates. Others have found that there is both CSF1 and CSF1R expression in breast cancer cells.14
It may be that the stromal CSF1 signature is associated primarily by the secretion of CSF1 by tumor cells.
The finding of a conserved CSF1 macrophage stromal response in 2 different types of metastatic cancer suggests that targeting therapeutics to this pathway may represent an effective therapeutic strategy for patients with metastatic disease. The CSF1 macrophage stromal response has opposite prognostic significance in multiple cancer types15–24
. For example it is associated with poor outcome in breast cancer but with improved outcome in colon cancer. The DTF fibroblast stromal response appears to be more complicated and is not entirely conserved between primary tumors and their metastases. Future studies are needed to determine which factors within these different tissues influence the fibroblast stromal response. The DTF fibroblast stromal response has been shown to be associated with a better prognosis in breast cancer, while no conclusions have been drawn about its prognostic significance in colorectal cancer.5