Steroid hormone receptors (SRs) are classically defined as ligand-activated transcription factors that function as master regulators of gene programs important for a wide range of physiological processes governing cell or tissue homeostasis. As such, dysregulated SRs are targeted drivers of cancer cell proliferation. A second function of SRs includes their ability to rapidly activate cytoplasmic signaling pathways. In addition to making direct contact with diverse signaling molecules, SRs are fully integrated with signaling pathways by virtue of their N-terminal phosphorylation sites that act as regulatory hot-spots capable of sensing the signaling milieu. In particular, ER, PR, and closely related glucocorticoid receptors (GR) share the property of accepting (i.e. sensing) ligand-independent phosphorylation events by proline-directed kinases in the MAPK and CDK families. These signaling inputs act as a ‘second ligand’ that dramatically impacts cell fate. In the face of drugs that target SR ligand-binding domains to block proliferation, ligand-independent post-translational modifications alter SR-binding partners to guide changes in cell fate that confer increased survival, migration, stemness properties, and therapy resistance of SR+ cancer cells. Targeting phospho-SRs in addition to the key mediators of phosphorylation-dependent changes in breast cancer cell fate will be an impactful addition to modernized combination therapies.
Breast Cancer Hijacks a Trophoblast-Like Program of Immune Suppression
TNBC cells rapidly change their metabolism during anchorage independent survival. Proteins that increase during anchorage independent survival include LAT1, a transporter of large amino acids such as tryptophan. In TNBC tryptophan catabolism is largely mediated by the enzyme tryptophan-2,3-dioxygenase (TDO2). During embryonic development epithelial cells undergo epithelial to mesenchymal transition (EMT) to accomplish normal developmental events. Fetal trophoblasts undergo an EMT that facilitates invasion into the uterus and suppression of the maternal immune system to ensure fetal tolerance during pregnancy. Carcinomas undergo EMT to facilitate anchorage independence, invasion, and metastasis, but less is known regarding how oncogenic EMT facilitates tumor cell immune evasion. Restoration of the microRNA miR-200c to TNBC powerfully reverses EMT and thereby reveals exact mechanisms by which TNBC suppress the anti-tumor immune response. We find that miR-200c directly targets and represses TDO2, HMOX-1, IKBKB, PLCG1, and PD-L1 and other targets involved in immune suppression. Our findings provide insights into targetable transcriptional and post-transcriptional regulation of immuno-suppressive factors produced by TNBC. The data support our hypothesis that metabolic alterations in TNBC result in production of immune-suppressive metabolites, and that targeting these pathways may help prevent or contain metastases by boosting immune cell function.
Mifepristone primes Antitumor immunity in selected luminal mammary carcinomas opening the door to immune therapies
The role of active antitumor immunity in hormone receptor-positive (HR+) breast cancer has been historically underlooked. We are interested in evaluating the contribution of the immune system to antiprogestin-induced tumor growth inhibition using a hormone-dependent breast cancer model. We have generated a mouse model by transplanting BALB/c-GFP+ bone marrow (BM) cells were into immunodeficient NSG mice recipients to generate an immunocompetent NSG/BM-GFP+ (NSG-R) reconstituted model. Treatment with the antiprogestin Mifepristone (MFP) inhibited the growth of 59-2-HI tumors in immunocompetent or immunocompromised mice. However, in immunocompetent mice MFP treatment reshaped the tumor microenvironment, enhancing the production of proinflammatory cytokines and chemokines. Tumors showed increased infiltration of F4/80+ macrophages, NK, and CD8 T cells, displaying a central memory phenotype. Mechanistically, MFP induced an immunogenic cell death gene program, activating as a consequence immature dendritic cells, and induced a memory T cell response, that attenuates tumor onset and growth after re-challenge. Finally, MFP treatment increased the sensitivity of HR+ 59-2-HI tumor to PD-L1 blockade, suggesting that antiprogestins may improve immunotherapy response rates. Our work contributes to better understand the mechanisms underlying the antitumor effect of hormonal treatment in selected luminal mammary carcinoma opening the door to immune therapies.
Tumor-Associated Macrophages Induce Endocrine Therapy Resistance in ER+ Breast Cancer Cells
Antiestrogenic adjuvant treatments are first-line therapies in patients with breast cancer positive for estrogen receptor (ER+). But most patients eventually acquire endocrine resistance and many others are initially refractory to anti-estrogen treatments. The tumor microenvironment plays essential roles in cancer development and progress; however, the molecular mechanisms underlying such effects remain poorly understood. Breast cancer cell lines co-cultured with TNF-α-conditioned macrophages were used as pro-inflammatory tumor microenvironment models. Proliferation, migration, and colony formation assays were performed to evaluate tamoxifen and ICI182780 resistance and confirmed in a mouse xenograft model. In our simulated pro-inflammatory tumor microenvironment, tumor-associated macrophages promoted proliferation, migration, invasiveness, and breast tumor growth of ER+ cells, rendering these estrogen-dependent breast cancer cells resistant to estrogen withdrawal and tamoxifen or ICI 182780 treatment. Crosstalk between breast cancer cells and conditioned macrophages induced sustained release of pro-inflammatory cytokines from both cell types, activation of NF-κB/STAT3/ERK, and hyperphosphorylation of ERα in the cancer cells, which resulted in constitutively active. Our simulated tumor microenvironment strongly altered endocrine and inflammatory signaling pathways in breast cancer cells, leading to endocrine resistance in these cells.
