Cooperative pro-tumorigenic adaptation to oncogenic RAS through epithelial-to-mesenchymal plasticity

The epipliancy journey: Tumor initiation at the mercy of identity crisis and epigenetic drift

Cellular pliancy refers to the unique disposition of different stages of cellular differentiation to transform when exposed to specific oncogenic insults. This concept highlights a strong interconnection between cellular identity and tumorigenesis, and implies overcoming of epigenetic barriers defining cellular states. Emerging evidence suggests that the cell-type-specific response to intrinsic and extrinsic stresses is modulated by accessibility to certain areas of the genome. Understanding the interplay between epigenetic mechanisms, cellular differentiation, and oncogenic insults is crucial for deciphering the complex nature of tumorigenesis and developing targeted therapies. Hence, cellular pliancy relies on a dynamic cooperation between the cellular identity and the cellular context through epigenetic control, including the reactivation of cellular mechanisms, such as epithelial-to-mesenchymal transition (EMT). Such mechanisms and pathways confer plasticity to the cell allowing it to adapt to a hostile environment in a context of tumor initiation, thus changing its cellular identity. Indeed, growing evidence suggests that cancer is a disease of cell identity crisis, whereby differentiated cells lose their defined identity and gain progenitor characteristics. The loss of cell fate commitment is a central feature of tumorigenesis and appears to be a prerequisite for neoplastic transformation. In this context, EMT-inducing transcription factors (EMT-TFs) cooperate with mitogenic oncoproteins to foster malignant transformation. The aberrant activation of EMT-TFs plays an active role in tumor initiation by alleviating key oncosuppressive mechanisms and by endowing cancer cells with stem cell-like properties, including the ability to self-renew, thus changing the course of tumorigenesis. This highly dynamic phenotypic change occurs concomitantly to major epigenome reorganization, a key component of cell differentiation and cancer cell plasticity regulation. The concept of pliancy was initially proposed to address a fundamental question in cancer biology: why are some cells more likely to become cancerous in response to specific oncogenic events at particular developmental stages? We propose the concept of epipliancy, whereby a difference in epigenetic configuration leads to malignant transformation following an oncogenic insult. Here, we present recent studies furthering our understanding of how the epigenetic landscape may impact the modulation of cellular pliancy during early stages of cancer initiation.

Epigenomic disorder and partial EMT impair luminal progenitor integrity in Brca1-associated breast tumorigenesis

In breast cancer related to the BRCA1 mutation, luminal progenitor cells are believed to be the cells of origin, yet how these cells transform into invasive cancer cells remain poorly understood. Here, we combine single-cell epigenomic and transcriptomic data to reconstitute sequences of events in luminal cells that lead to tumorigenesis. Upon deletion of Trp53 and Brca1, we find that luminal progenitors display an extensive epigenomic disorder associated with a loss of cell identity. These cells then progress to tumor formation through a partial epithelial-to-mesenchymal transition, orchestrated by Snail and the timely activation of immunosuppressive and FGF signaling with their microenvironment. In human samples, pre-tumoral changes can be detected in early stage, basal-like tumors, which rarely recur, as well as in normal-like mammary glands of BRCA1 mutation carriers who have had cancer. Our study fills critical gaps in our understanding of BRCA1-driven tumorigenesis, opening perspectives for the early monitoring of individuals with high cancer risk.

Dynamics of epithelial-mesenchymal plasticity driving cancer drug resistance

Epithelial-mesenchymal transition (EMT) promotes several cancers by increasing tumor cell motility, disrupting epithelial cell phenotypes, apical-basal polarity, and intracellular connections, and enhancing tumor resistance to immunotherapy and chemotherapy. Mesenchymal-epithelial transition (MET), the opposite of EMT, causes tumor metastasis. EMT drives primary tumor cells, whereas MET inhibits them. Importantly, the complex network of EMT includes cell-cell interactions in the tumor microenvironment. Transcription factors, post-translational regulation, cytokine-mediated signaling, and microRNAs control EMT. In this review, we discussed how molecular mechanisms, signaling networks, and epithelial/mesenchymal states affect cancer treatment resistance and the tumor microenvironment. Research on immunotherapy and chemotherapy problems associated with EMT suggests that targeting EMT might be a potential cancer treatment resistance strategy.

Convergent inducers and effectors of T cell paralysis in the tumour microenvironment

Tumorigenesis embodies the formation of a heterotypic tumour microenvironment (TME) that, among its many functions, enables the evasion of T cell-mediated immune responses. Remarkably, most TME cell types, including cancer cells, fibroblasts, myeloid cells, vascular endothelial cells and pericytes, can be stimulated to deploy immunoregulatory programmes. These programmes involve regulatory inducers (signals-in) and functional effectors (signals-out) that impair CD8+ and CD4+ T cell activity through cytokines, growth factors, immune checkpoints and metabolites. Some signals target specific cell types, whereas others, such as transforming growth factor-β (TGFβ) and prostaglandin E2 (PGE2), exert broad, pleiotropic effects; as signals-in, they trigger immunosuppressive programmes in most TME cell types, and as signals-out, they directly inhibit T cells and also modulate other cells to reinforce immunosuppression. This functional diversity and redundancy pose a challenge for therapeutic targeting of the immune-evasive TME. Fundamentally, the commonality of regulatory programmes aimed at abrogating T cell activity, along with paracrine signalling between cells of the TME, suggests that many normal cell types are hard-wired with latent functions that can be triggered to prevent inappropriate immune attack. This intrinsic capability is evidently co-opted throughout the TME, enabling tumours to evade immune destruction.

A hybrid epithelial-mesenchymal transition program enables basal epithelial cells to bypass stress-induced stasis and contributes to a metaplastic breast cancer progenitor state

BACKGROUND

Human mammary epithelial cell (HMEC) cultures encounter a stress-associated barrier termed stasis, during which most cells adopt a senescence-like phenotype. From these cultures, rare variants emerge from the basal epithelial population, re-initiating growth. Variants exhibit pre-malignant properties, including an aberrant epigenetic program that enables continued proliferation and acquisition of genetic changes. Following oncogenic transformation, variants produce tumors that recapitulate the histopathological characteristics of metaplastic breast cancer (MBC), a rare and aggressive subtype marked by the differentiation of neoplastic epithelium into squamous and mesenchymal elements.

METHODS

Using a serum-free HMEC culture system, we probed the capacity for phenotypic plasticity inherent to basal epithelial cell populations from human breast tissue as they navigated stasis and emerged as variant populations.

RESULTS

We observed robust activation of a TGF-β-dependent epithelial-mesenchymal transition (EMT) program in basal epithelial cells during stasis, followed by subsequent attenuation of this program in emerging variants. Inhibition of the TGF-β pathway or depleting the EMT regulators Snail or Slug allowed basal epithelial cells to collectively bypass stasis, demonstrating that cellular dysfunction and arrest resulting from TGF-β and EMT activation are central to this in vitro barrier. The spontaneous emergence of variants from stasis cultures was associated with a restricted EMT trajectory, characterized by the stabilization of hybrid EMT states associated with greater proliferative capacity, rather than progressing to a complete mesenchymal state characterized by irreversible growth arrest. Epigenetic mechanisms, which contributed to the dysregulated growth control characteristic of the variant phenotype, also contributed to the stability of the hybrid EMT program in variants. By overcoming the cellular dysfunction and growth arrest resulting from TGF-β and complete EMT, variants exhibited a higher oncogenic transformation efficiency compared to pre-stasis basal epithelial cells. Inhibiting the TGF-β pathway prior to stasis significantly reduced EMT in the basal epithelial population, alleviated selective pressure driving variant emergence, and also enhanced oncogenic transformation efficiency, resulting in tumors with markedly diminished metaplastic differentiation.

CONCLUSIONS

This study reveals how an epigenetic program governs basal epithelial cell fate decisions and contributes to the development of MBC progenitors by restricting access to terminal mesenchymal states that induce growth arrest and, instead, favoring hybrid EMT states with enhanced tumorigenic potential.