Cancer cell heterogeneity and plasticity: From molecular understanding to therapeutic targeting

The phenomenon of cancer cell heterogeneity has been observed for decades in both experimental models and clinical tumor samples. However, it is only recently that technical advances in studying organogenesis, tissue development, stem cell hierarchy, epigenetics and cancer biology have allowed us to move from observational heterogeneity in cancer cell phenotypes to prospectively dissecting, functionally interrogating and molecularly understanding the impact of cancer cell heterogeneity. The explosion of studies in this fast-evolving area of cancer biology has led to the identification and characterization of distinct subpopulations of cancer cells including cancer stem cells (CSCs), tumor progenitors, slow-cycling (dormant) cancer cells, and differentiated and senescent cancer cells. Of clinical significance, recent studies have directly linked the intrinsic cancer cell heterogeneity to differential therapeutic responses and resistance, and to disease relapse and metastasis. Furthermore, it is also appreciated that cancer cells possess tremendous phenotypic and functional plasticity that can be induced by genetic mutations as well as by epigenetic mechanisms such as pro-inflammatory and immune-suppressive tumor microenvironment (TME), overexpression of stemness factors, and therapeutic treatments. Significant cancer cell plasticity, compared to normal cells, is likely associated with the increased epigenomic fluidity in cancer cells. Regardless, it has become clear that both intrinsic cancer cell heterogeneity and induced cancer cell plasticity have to be targeted in order to achieve enduring therapeutic efficacy in the clinic.

In this thematic issue entitled “Cancer Cell Heterogeneity and Plasticity: From Molecular Understanding to Therapeutic Targeting’, we have assembled 13 updated essays from leading experts in the field to expound on our current understanding of cancer cell heterogeneity and plasticity in representative tumor systems. The emphasis has been placed on the molecular circuitry that regulates the dynamic evolution and transition of various cancer cell subpopulations and their reciprocal interactions with the TME. Starting with breast cancer (BC), in which CSCs (i.e., breast cancer stem cells or BCSCs) were the first to be reported among all solid tumors [1], Zhao and Rosen dissect BC heterogeneity from the perspective of employing state-of-the-art technologies including single-cell (sc) analysis and spatial pathologies [2]. They succinctly summarized several major enabling technologies that have become powerful tools in helping dissect and understand BC cell heterogeneity and functional implications, which included scRNA-seq, scATAC-seq, CyTOF (suspension mass cytometry), imaging mass cytometry (IMC), and high-dimensional spatial transcriptomics and multi-omics platforms [2]. There is little doubt that these new technologies will facilitate identification of novel treatment biomarkers and prognostic factors and development of novel therapies. Zhang et al. [3] discuss novel molecular regulators of heterogeneous BCSC subpopulations, including transcription factors (TFs), signaling pathways (e.g., Wnt/β-catenin, Notch, Hippo etc), and epigenetic regulators such as non-coding RNAs. Importantly, they expounded on the importance of TME in diversifying BCSC heterogeneity and inducing BCSC plasticity. Hua and colleagues [4] focus on BCSCs in triple-negative BC (TNBC), the most aggressive subtype of BC due to their lack of hormone receptors such as estrogen receptor alpha (ERα), which is the major therapeutic target in patients. The authors made clear an important point that TNBC, in fact, is a very heterogenous class of BC that harbor many subpopulations of BCSCs, especially the dormant (slow-cycling) population that would not respond well to antimitotic chemotherapeutics drugs. Finally, Sukocheva et al. discuss the role of epigenetic mechanisms in generating BC cell heterogeneity and promoting BC resistance to anti-ERα therapies [5].

Four papers [6–9] in this issue are dedicated to understanding and targeting the cellular heterogeneity and plasticity in prostate cancer (PCa), a cancer with significant cellular and molecular semblance to BC. Both cancers are driven by steroid hormones and both cancers are notoriously heterogeneous in molecular subtypes and cellular composition. Li and Shen discuss PCa cell heterogeneity and plasticity from studies that employ genetically engineered mouse models including many from the Shen lab [6]. Their elegant studies have linked the cell type of tumorigenic transformation (i.e., the cell-of-origin) to PCa subtypes that develop with distinct morphological and molecular phenotypes. Studies from the Shen lab have also demonstrated that oncogenic transformation of luminal cells, or a subset of cells in the luminal cell compartment, leads to development of the prostatic adenocarcinomas, the most prevalent human PCa. Tang provides a comprehensive review of human PCa cell heterogeneity and plasticity and discuss PCa development and progression from the perspective of organ (de-)differentiation [7]. One unique emphasis in his discourse is the mounting evidence that points to the population of luminal progenitor (LP) cells as the potential cells of origin for human PCa initiation, especially when instigated by chronic inflammatory insults [7]. His discussion ends with proposing several novel combinatorial treatment strategies to tackle both intrinsic PCa cell heterogeneity and induced plasticity. Peitzsch et al. systematically and comprehensively dissect how metabolism and metabolic reprogramming may influence PCa cell heterogeneity and plasticity [8]. It is interesting that the normal prostate epithelial cells are highly glycolytic due to the large amounts of Zn²⁺ in the organ. Early-stage (hormone-sensitive) prostate tumors manifest both increased glycolysis and oxidative phosphorylation in the mitochondria whereas advanced, therapy-resistant, and metastatic tumors tend to switch to a uniquely heightened glycolytic phenotype due to hypoxia and other changes in the TME [8]. Authors very nicely presented examples of the prognostic metabolic biomarkers, preclinical strategies of metabolic targeting agents and clinical application of metabolic targeting agents for PCa [8]. Lastly, in this section, Ji et al. [9] presented how LRIG1, a normal stem cell regulator and a tumor suppressor, is affected by, and also reciprocally influences, PCa cell heterogeneity, especially in the context of androgen receptor (AR) signaling and development of castration resistance.

Cancer cell heterogeneity and plasticity have also been observed in other cancers including hepatocellular carcinoma (HCC) [10], colorectal cancers (CRC) [11], and brain tumors such as glioblastoma multiforme (GBM) [12,13]. Chan and colleagues [10] not only discussed various subpopulations of HCC and liver CSCs but also nicely elaborated on the metabolic plasticity in HCC cells as well as the heterogeneity of immune cells in the HCC TME. Du et al. [11] focus their discussion on colorectal CSCs (CCSCs) by arguing that, despite the prominent heterogeneity in CCSC phenotypes, all CCSC subpopulations share the common property of ‘stemness’ (state), which should be selectively targeted. GBM represents the most aggressive brain tumor, and Lauko et al. [12] provided an updated overview of the diversity and plasticity of GBM cells and GBM stem cells. Importantly, authors also discussed the wide variety of cell types in the GBM TME and crosstalks and cross-regulation between the parenchymal tumor cells and the TME. Kondo dissected the cellular heterogeneity in GBM from the cell-of-origin perspective [13]. Kondo presented evidence that oligodendrocyte precursor cells as well as neural stem cells transform into GBM stem cells with acquiring neural stem cell characteristics, contributing GBM cell heterogeneity. He finally discussed about DHODH that is indispensable for heterogenous GBM stem cells, as a potential target for GBM therapy.

Last but not the least, Boyd and colleagues reviewed the heterogeneity and plasticity of carcinoma-associated fibroblasts (CAFs) in the pancreatic adenocarcinoma (PDAC) TME [14]. Authors presented the phenotypes of multiple reported PDAC-associated CAF populations, discussed their potential inter-relationship, and interpreted the potential tumor-promoting as well as tumor-repressing functions of various CAF populations [14].

We hope that the collection of essays in this issue will provide the readers an updated understanding of cancer cell (and CAF) heterogeneity and plasticity in several common tumor types, and that this updated molecular understanding will inform and facilitate the efforts in developing novel combinatorial strategies to target cancer cell heterogeneity and plasticity to achieve long-lasting clinical benefits.