Navigating the Intricacies of Tumor Heterogeneity: An Insight into Potential Prognostic Breast Cancer Biomarkers

Abstract

Breast cancer is a heterogeneous disease with diverse histological and molecular subtypes. Luminal breast tumors are the most diagnosed subtype. In luminal breast cancer, hormone receptors (including ER, PR, HER2) play a diagnostic and prognostic role. Despite the effectiveness of endocrine therapy in luminal breast tumors, tumor recurrence and resistance occur, and this may highlight evolutionary strategies for survival driven by stemness. In this review we thus consider the association between estrogen signaling and stemness in mediating tumor processes. Many studies report stemness as one of the factors promoting tumor progression. Its association with estrogen signaling warrants further investigation and provides an opportunity for the identification of novel biomarkers which may be used for diagnostic, prognostic, and therapeutic purposes. Breast cancer stem cells have been characterized (CD44⁺ CD24^-) and their role in promoting treatment resistance and tumor recurrence widely studied; however, the complexity of tumor progression which also involve microenvironmental factors suggests the existence of more varied cell phenotypes which mediate stemness and its role in tumor progression.

Keywords: Breast cancer; estrogen; evolution; intratumoral heterogeneity; stemness.

Figure 1.

Schematic representation of the role of stemness in breast tumor evolution. Tumor evolution relies on interactions between transformed cells and cells of the TME. From the onset, the heterogeneous population of transformed cells are exposed to a number of selective pressures exerted by non-tumor cells within the TME. Initially, macrophages, dendritic cells, natural killer cells, and cytotoxic T lymphocytes elicit an immune response against transformed cells in an attempt to inhibit tumor potentiation. Tumor cells that present with stem-like characteristics have a higher capacity to withstand anti-tumorigenic signals exerted by these immune cells. Through their secretion of various cytokines (TGF-β, IL-6), chemokines (CCL1), and their overexpression of PDL1, tumor cells facilitate the recruitment of myeloid-derived suppressor cells and regulatory T cells that inhibit anti-tumorigenic responses by cytotoxic T cells and natural killer cells thereby aiding in immune evasion. Using cytokines, stem-like tumor cells promote the polarization of macrophages into a pro-tumorigenic phenotype (TGF-β, MCSF-1) that stimulates angiogenesis (VEGF) and epithelial-to-mesenchymal transition (TGF-β, IL-10). The re-education of fibroblasts by tumor cells (TGF-β, bFGF, IL-6) to secrete pro-tumorigenic chemokines (CCL2) and matrix metalloproteinases that disrupt the ECM further aid in increasing the migratory capacity of tumor cells and the release of ECM-sequestered growth factors. Stem-like tumor cells display enhanced migratory capabilities. Upon the initiation of the invasion-metastasis cascade, tumor cells prepare the pre-metastatic niche and enter the lumen of surrounding vessels where they interact with endothelial cells and platelets to evade circulating immune cells. These circulating tumor cells extravasate at the secondary tumor site where they are able to adapt to the new microenvironment and colonize the tissue.

Figure 2.

Diagram illustrating the transient expression of cluster of differentiation (CD) surface markers during stem-like tumor cell migration. The metastatic capacity of stem-like tumor cells is dependent on their molecular and phenotypic plasticity. The adaptive response of these cells to signals from the tumor microenvironment is a key mechanism in promoting breast tumor progression. As the primary tumor outgrows its blood supply, the activation of HIF-1 directly upregulates the expression of CD44 and CD133 surface markers in stem-like tumor cells. Activation of CD44 through its binding to hyaluronic acid (HA) is a key mediator of breast tumor metastasis. Initially, the CD44-HA complex facilitates EMT in tumor cells by promoting the upregulation of TGF-β signaling. This results in the acquisition of a mesenchymal-like morphology. Thereafter, CD133 works in concert with CD44-HA to promote the secretion of MMPs to enable migration through the ECM. Alongside the increased expression of CD44 and CD133, stem-like cells present with low levels of CD24. This exposes CXCR4 on tumor cells which enables its binding to CCL12. This activation of CXCR4 is further increased via its interaction with CD133 which aids in guiding migratory tumor cells through the vascular system. At the sites of intra- and extravasation, tumor cells attach to endothelial cells through the binding of CD24 to E-selectin. This adhesion is strengthened by the increased expression of CD166, which binds CD6 on endothelial cells. Entry to and exit from the blood vessel lumen is facilitated by the interactions of CD44-HA and CD133 with VEGFR on endothelial cells. Circulating tumor cells continuously adapt to evade immune attack. The upregulation of CD61 expression promotes rapid platelet activation. Activated platelets express P-selectin, which binds CD24 on tumor cells thereby shielding them from immune attack. Additionally, the interaction of CD24 with Siglec-10 on macrophages acts to inhibit tumor cell phagocytosis. Other molecular mechanisms of immune evasion include the upregulated expression of CD47 and PD-L1, which is facilitated by CD44-HA. These stem-like tumor cells are guided to secondary sites in response to the chemoattractant gradient generated by stromal cells at the pre-metastatic niche.