Breast cancer stem cells and circulating tumor cells: Dual drivers of progression and relapse

Table 1 Key interactions between breast cancer stem cells and the tumor microenvironment

TME component	Mechanism/signal	Effect on BCSCs
Cancer-associated fibroblasts	IL-6, IL-8, IL-1β, FGF5 secretion[41,42]	ECM remodeling, enhanced stemness at tumor-stroma interface
	Wnt/β-catenin, HGF/Met, Hedgehog pathways[43]	Sustains stem-like phenotype
	Leukemia inhibitory factor[44]	Induces dedifferentiation; ↑ Nanog, Oct4, CD44+/CD24-
Macrophages	M1/M2 polarization[39]	Regulates BCSC dynamics
Endothelial cells	Jag1 release, ZEB1/VEGFA axis activation[45]	Enhances stemness via feedback loop
ECM	Collagen, hyaluronic acid[46]	Provides niche; supports metastasis
Hypoxia and cytokines	IL-6, C/EBPδ pathway[47]	Promotes dedifferentiation, stem-like traits
BCSC-secreted factors	CXCL1[37]	Promotes proliferation, alters transcription
Progranulin signaling	Induces IL-6/IL-8 via sortilin[48]	Enhances mammosphere formation
UCP1	Regulates FBP1; inhibits snail[49]	Suppresses BCSCs via metabolic modulation
RON + HGFL expression	Autocrine/paracrine signaling[50]	Promotes BCSC self-renewal; activates macrophages

TME: Tumor microenvironment; BCSCs: Breast cancer stem cells; IL: Interleukin; FGF5: Fibroblast growth factor 5; ECM: Extracellular matrix; HGF: Hepatocyte growth factor; ZEB1: Zinc-finger E-box-binding homeobox 1; VEGFA: Vascular endothelial growth factor A; C/EBPδ: CCAAT/enhancer-binding protein delta; CXCL1: C-X-C motif ligand 1; UCP1: Uncoupling protein 1; FBP1: Fructose-1,6-bisphosphatase; HGFL: Hepatocyte growth factor-like protein.

Table 2 Circulating tumor cell detection methods: Types, principles, and key features

Method type	Detection principle	Technologies	Advantages	Limitations
Label-dependent	Antigen-based (surface markers)	CellSearch® (EpCAM+). HER2, CD45-based	FDA-approved. High specificity for epithelial CTCs	Misses EMT/mesenchymal CTCs. Marker heterogeneity limits sensitivity
Label-independent	Physical properties: Size, density, deformability	Parsortix®. Filtration. Microfluidics	Captures broader CTC spectrum. Detects EMT-like CTCs	Lower specificity. Overlap with normal blood cells
Combination methods	Antigen + physical property integration	Microfluidic chips with antibodies	Improved sensitivity and specificity. Better for rare subtypes	More complex and costly
CTC cluster detection	Enrichment of tumor cell aggregates	Size-based filters. Inertial focusing	Important for early metastasis. Reflects high aggressiveness	Rare and fragile. Difficult to preserve integrity during handling
Single-cell analysis	High-resolution detection of individual CTCs	DEPArray. RareCyte. MCA/gel-based	Enables genomic/proteomic profiling. Detects rare variants	Low throughput. Requires optimization and high cost

FDA: Food and Drug Administration; CTC: Circulating tumor cell; EpCAM: Epithelial cell adhesion molecule; HER2: Human epidermal growth factor receptor 2; EMT: Epithelial-mesenchymal transition; MCA: Microcavity array.