Breast cancer is a multifactorial disease and driven by aberrant regulation of cell signaling pathways due to the acquisition of genetic and epigenetic changes. An array of growth factors and their receptors is involved in cancer development and metastasis. Receptor Tyrosine Kinases (RTKs) constitute a class of receptors that play important role in cancer progression. RTKs are cell surface receptors with specialized structural and biological features which respond to environmental cues by initiating appropriate signaling cascades in tumor cells. RTKs are known to regulate various downstream signaling pathways such as MAPK, PI3K/Akt and JAK/STAT. These pathways have a pivotal role in the regulation of cancer stemness, angiogenesis and metastasis. These pathways are also imperative for a reciprocal interaction of tumor and stromal cells. Multi-faceted role of RTKs renders them amenable to therapy in breast cancer. However, structural mutations, gene amplification and alternate pathway activation pose challenges to anti-RTK therapy.

Breast cancer, the most frequent female malignancy, is often curable when detected at an early stage. The treatment of metastatic breast cancer is more challenging and may be unresponsive to conventional therapy. Immunotherapy is crucial for treating metastatic breast cancer, but its resistance is a major limitation. The tumor microenvironment (TME) is vital in modulating the immunotherapy response. Various tumor microenvironmental components, such as cancer-associated fibroblasts (CAFs), tumor-associated macrophages (TAMs), and myeloid-derived suppressor cells (MDSCs), are involved in TME modulation to cause immunotherapy resistance. This review highlights the role of stromal cells in modulating the breast tumor microenvironment, including the involvement of CAF-TAM interaction, alteration of tumor metabolism leading to immunotherapy failure, and other latest strategies, including high throughput genomic screening, single-cell and spatial omics techniques for identifying tumor immune genes regulating immunotherapy response. This review emphasizes the therapeutic approach to overcome breast cancer immune resistance through CAF reprogramming, modulation of TAM polarization, tumor metabolism, and genomic alterations.

Background: Cancer stem cells (CSCs) play crucial role in tumor progression, drug resistance and relapse in various cancers. CSC niche is comprised of various stromal cell types including Tumor-associated macrophages (TAMs). Extrinsic ques derived from these cells help in maintenance of CSC phenotype. TAMs have versatile roles in tumor progression however their function in enrichment of CSC is poorly explored. Methods: Mouse macrophages (RAW264.7) cells were activated by interaction with conditioned media (CM) of murine breast cancer cells (4T1) into TAMs and the effect of activated macrophage (TAM) derived factors was examined on enrichment of cancer stem cells (CSCs) and tumor growth using in vitro and in vivo models. Results: In this study, we report that macrophages upon interaction with breast cancer cells activate tumor promoting function and exhibit differential expression of various proteins as shown by secretome analysis using proteomics studies. Based on secretome data, we found that Interleukin-6 (IL-6) is one of the up-regulated genes expressed in activated macrophages. Further, we confirm that TAMs produce high levels of IL-6 and breast cancer cell derived factors induce IL-6 production in activated macrophages via p38-MAPK pathway. Furthermore, we demonstrate that tumor activated macrophages induce enrichment of CSCs and expression of CSC specific transcription factors such as Sox-2, Oct-3/4 and Nanog in breast cancer cells. We further prove that TAM derived IL-6 plays a key role in TAM mediated CSC enrichment through activation of Signal transducer and activator of transcription 3 (STAT-3) signaling. TAM derived IL-6 influences breast cancer cell migration and angiogenesis. Moreover, our in vivo findings indicated that TAM derived IL-6 induces CSC population and resulting tumor growth in breast cancer. Conclusion: These finding provide evidence that TAM derived IL-6 plays a major role in CSC enrichment and tumor progression in breast cancer and IL-6 and its regulated signalling network may act as potential therapeutic target for management of breast cancer.

Epithelial-to-mesenchymal transition (EMT) is a crucial cellular process for embryogenesis, wound healing, and cancer progression. It involves a shift in cell interactions, leading to the detachment of epithelial cells and activation of gene programs promoting a mesenchymal state. EMT plays a significant role in cancer metastasis triggering tumor initiation and stemness, and activates metastatic cascades resulting in resistance to therapy. Moreover, reversal of EMT contributes to the formation of metastatic lesions. Metastasis still needs to be better understood functionally in its major but complex steps of migration, invasion, intravasation, dissemination, which contributes to the establishment of minimal residual disease (MRD), extravasation, and successful seeding and growth of metastatic lesions at microenvironmentally heterogeneous sites. Therefore, the current review article intends to present, and discuss comprehensively, the status quo of experimental models able to investigate EMT and metastasis in vitro and in vivo, for researchers planning to enter the field. We emphasize various methods to understand EMT function and the major steps of metastasis, including diverse migration, invasion and matrix degradation assays, microfluidics, 3D co-culture models, spheroids, organoids, or latest spatial and imaging methods to analyze complex compartments. In vivo models such as the chorionallantoic membrane (CAM) assay, cell line-derived and patient-derived xenografts, syngeneic, genetically modified, and humanized mice, are presented as a promising arsenal of tools to analyze intravasation, site specific metastasis, and treatment response. Furthermore, we give a brief overview on methods detecting dissemination and MRD in carcinomas, highlighting its significance in tracking the course of disease and response to treatment. Enhanced lineage tracking tools, dynamic in vivo imaging, and therapeutically useful in vivo models as powerful preclinical tools may still better reveal functional interdependencies between metastasis and EMT. Future directions are discussed in light of emerging views on the biology, diagnosis, and treatment of EMT and metastasis.

Cancer is a complex pathological disorder, established as a result of accumulation of genetic and epigenetic changes, which lead to adverse alterations in the cellular phenotype. Tumor progression involves intricate signaling mediated through crosstalk between various growth factors, cytokines and chemokines. Osteopontin (OPN), a chemokine-like protein, is involved in promotion of neoplastic cancer into higher grade malignancies by regulating various facets of tumor progression such as cell proliferation, angiogenesis and metastasis. Tumors as well as stroma-derived OPN play key roles in various signaling pathways involved in tumor growth, angiogenesis and metastasis. OPN derived from tumor-activated macrophages modulates the tumor microenvironment and thereby regulate melanoma growth and angiogenesis. OPN also regulates hypoxia-inducible factor-1α-dependent VEGF expression leading to breast tumor growth and angiogenesis in response to hypoxia. Thus, a clear understanding of the molecular mechanism underlying OPN-mediated regulation will shed light on exciting avenues for further investigation of targeted therapies. Silencing of OPN using RNAi technology, blocking OPN activity using specific antibodies and small-molecule inhibitors might provide novel strategies, which would aid in developing effective therapeutics for the treatment of various types of cancer. This review focuses on new possibilities to exploit OPN as a tumor and stroma-derived therapeutic target to combat cancer.