The effects of human mesenchymal stem cells on estrogen receptor positive breast carcinoma: Progression to hormone independence and metastasis
Description
Adult human mesenchymal stem cells (hMSC) have been shown to home to sites of carcinoma, including breast, and integrate into the tumor stroma. Given the incomplete understanding of the biological consequences of hMSCs in cancer, we set out to investigate the impact of hMSCs on the estrogen receptor positive, hormone-dependent breast carcinoma cell line MCF-7. We demonstrated increased MCF-7 cell proliferation in culture and enhanced tumor growth in xenograft models with the addition of hMSCs. Not only was increased tumor growth observed in the presence of 17beta-estradiol, but we have established the ability of hMSCs to induce hormone-independent tumor growth of a carcinoma cell line. Furthermore, when cultured in the presence of hMSCs, MCF-7 cells were observed to have increased migration potential as well as improved ability to colonize at a distant metastatic site The observation of the hormone independent effect of hMSCs on an estrogen dependent cells line led us to examine the possibility of altered estrogen/estrogen receptor signaling. Quantitative PCR analysis on endpoint tumors revealed an increase in the expression of the known ER regulated gene progesterone receptor (PgR). These findings were validated by immunohistochemical staining for PgR in hMSC/MCF-7 tumor sections as well as increased PgR expression of MCF-7 cells cultured in the presence of hMSCs. Furthermore, inhibition of ER signaling by the pure anti-estrogen ICI 182,780 decreased the effect of hMSCs on MCF-7 cell proliferation and migration supporting a role for ER signaling in the hMSC/MCF-7 cell interaction Although direct co-culture conditions yielded the greatest increase in proliferation, the effects of hMSCs on MCF-7 proliferation were not strictly dependent on direct cell-to-cell contact. Transwell and conditioned media experiments indicate that hMSCs effect on MCF-7 cells is at least in part mediated by a secreted factor(s). hMSCs have been shown to secrete a wide variety of growth factors and chemokines including stromal derived factor 1 (SDF-1). This fact coupled with the knowledge that SDF-1 is an ER-mediated gene linked with hormone independence and metastasis led to the investigation of the SDF-1/CXCR4 signaling axis in the hMSC mediated effects. Experiments revealed an increase in SDF-1 gene expression both in vivo and in vitro when MCF-7 cells were grown in the presence of hMSCs. SDF-1 treatment of MCF-7 cells cultured alone increased proliferation to just below that seen with hMSC co-culture. Additionally, blocking SDF-1 signaling using a CXCR4 specific inhibitor (AMD3100) decreased hMSC induced proliferation and migration of MCF-7 cells to, but not below, basal levels. The combined treatment of ICI and AMD3100 reduced MCF-7 cell proliferation and migration to below basal levels, indicating targeting both pathways is more effective than either treatment alone in decreasing the hMSCs induction of MCF-7 cell proliferation and migration Our results indicate that the ER and SDF-1/CXCR4 signaling pathways are involved in the hMSC-mediated effects on MCF-7 cell proliferation and migration. This is further supported by the recent evidence of others that ER and CXCR4 signaling pathways can crosstalk to establish an autocrine feed forward loop resulting in hormone independence and enhanced cell proliferation. The sum of these data reveal the relationship between tumor microenvironment and tumor growth and the progression to hormone-independence. Better understanding of the mechanisms involved in this tumor stroma-cell interaction may provide novel targets for the development of treatment strategies for estrogen receptor-positive, hormone-independent, and endocrine-resistant breast carcinoma