Primary tumours and metastases within the same patient are often heterogeneous, which presents as an obstacle to the treatment of metastasis. Furthermore, mechanisms controlling colonization of cancer cells at distal organs, the last step of the metastatic cascade, are relatively understudied. Concentrating on these aspects, Obradović et al. have identified that breast cancer progression in mice is associated with increased levels of stress hormones resulting in activation of the glucocorticoid receptor at secondary sites, enhanced colonization and decreased survival.
The authors began by implanting patient-derived xenografts (PDXs) or human MDA-MB-231 breast cancer cells into mammary fat pads of immunodeficient mice, before primary tumour resection to compare matched tumours and metastases. Global transcriptomic analyses of isolated cancer cells revealed that the gene expression profiles of the cells clustered on the basis of the sites of metastasis, namely the lungs and liver, and as circulating tumour cells. Interestingly, the authors also noted that the cellular processes that varied most often between tumours and matched metastases were hypoxia, certain metabolic pathways and mTOR signalling.
Using an online tool called integrated system for motif activity response analysis (ISMARA) to model regulatory sites for transcription factors from their data set and other previously published data sets, the authors noted GR activity was commonly higher in metastases. Nuclear localization of the GR was also observed in lung metastases of the mice pointing to GR activation at distant metastatic sites. In addition, the plasma levels of the stress hormones cortisol and corticosterone as well as adrenocorticotropic hormone — which stimulates the production and release of these stress hormones — were increased in those mice with metastases compared with mice without metastases but with primary tumours. Importantly, this observation had been previously reported in patients with metastatic breast cancer.
Expression of GR target genes in the MDA-MB-231 cell line could be induced in vitro with the synthetic glucocorticoid dexamethasone. To test how GR activation in cancer cells might affect the later stages of the metastatic process, MDA-MB-231 cells or mouse 4T1 cells were cultured in the presence of dexamethasone for 7 days before being injected into the tail vein of immunodeficient or immunocompetent mice, respectively. GR activation in this case was sufficient to increase the efficiency of colonization of both cell lines, an effect that could be mitigated by downregulating GR in MDA-MB-231 cells with short hairpin RNA against the GR gene. As tail vein injections bypass the initial steps of the metastatic cascade, dexamethasone was also given to mice with orthotopic tumour implants following primary tumour resection. Under these conditions, dexamethasone hastened metastasis and the death of mice in a PDX and MDA-MB-231 or 4T1 xenografts, consolidating the finding that GR activation augments metastasis.
“activation of the glucocorticoid receptor (GR) at secondary sites, enhanced colonization and decreased survival”
To determine the mechanism by which GR activation by dexamethasone might promote increased metastatic colonization, lysates from GR-activated MDA-MB-231 cells were subjected to global proteomic analysis. Interestingly, signalling networks related to the processes of epithelial to mesenchymal transition, glucose and nicotinamide metabolism, actin cytoskeleton organization and metastatic signalling pathways were upregulated upon GR activation.
Among the differentially regulated genes were also 63 upregulated kinases. Narrowing their focus on one of these, ROR1, which in earlier studies had been associated with breast cancer metastasis, the authors were able to confirm that ROR1 expression was elevated in metastases compared with matched primary tumours in the PDX and MDA-MB-231 xenograft models. Furthermore, depletion of ROR1 with shRNA in GR-activated MDA-MB-231 cells was able to decrease metastatic outgrowth and extend the survival of mice in both the experimental metastasis and orthotopic models, implicating ROR1 in the increased colonization capacity of cancer cells observed upon GR activation.
Dexamethasone is widely used clinically in the treatment of side effects caused by chemotherapy and to reduce inflammation and immunosuppression associated with advanced cancer. Reasoning that GR activation initiated by this co-medication could potentially disrupt the effects of chemotherapy, the authors co-treated mice intravenously injected with MDA-MB-231 cells with dexamethasone and the chemotherapeutic agent paclitaxel. This combination treatment counteracted the effects of paclitaxel alone, by increasing metastatic colonization and shortening survival.
If translatable to humans, these findings suggest clinicians might need to exercise restraint when prescribing glucocorticoids as well as emphasize the importance of stress management for patients with breast cancer.
Obradović, M. M. S. et al. Glucocorticoids promote breast cancer metastasis. Nature https://doi.org/10.1038/s41586-019-1019-4 (2019)