Characterization and functional analysis of a slow-cycling subpopulation in colorectal cancer enriched by cell cycle inducer combined chemotherapy

Tumour response to hypoxia: understanding the hypoxic tumour microenvironment to improve treatment outcome in solid tumours

Hypoxia is a common feature of solid tumours affecting their biology and response to therapy. One of the main transcription factors activated by hypoxia is hypoxia-inducible factor (HIF), which regulates the expression of genes involved in various aspects of tumourigenesis including proliferative capacity, angiogenesis, immune evasion, metabolic reprogramming, extracellular matrix (ECM) remodelling, and cell migration. This can negatively impact patient outcomes by inducing therapeutic resistance. The importance of hypoxia is clearly demonstrated by continued research into finding clinically relevant hypoxia biomarkers, and hypoxia-targeting therapies. One of the problems is the lack of clinically applicable methods of hypoxia detection, and lack of standardisation. Additionally, a lot of the methods of detecting hypoxia do not take into consideration the complexity of the hypoxic tumour microenvironment (TME). Therefore, this needs further elucidation as approximately 50% of solid tumours are hypoxic. The ECM is important component of the hypoxic TME, and is developed by both cancer associated fibroblasts (CAFs) and tumour cells. However, it is important to distinguish the different roles to develop both biomarkers and novel compounds. Fibronectin (FN), collagen (COL) and hyaluronic acid (HA) are important components of the ECM that create ECM fibres. These fibres are crosslinked by specific enzymes including lysyl oxidase (LOX) which regulates the stiffness of tumours and induces fibrosis. This is partially regulated by HIFs. The review highlights the importance of understanding the role of matrix stiffness in different solid tumours as current data shows contradictory results on the impact on therapeutic resistance. The review also indicates that further research is needed into identifying different CAF subtypes and their exact roles; with some showing pro-tumorigenic capacity and others having anti-tumorigenic roles. This has made it difficult to fully elucidate the role of CAFs within the TME. However, it is clear that this is an important area of research that requires unravelling as current strategies to target CAFs have resulted in worsened prognosis. The role of immune cells within the tumour microenvironment is also discussed as hypoxia has been associated with modulating immune cells to create an anti-tumorigenic environment. Which has led to the development of immunotherapies including PD-L1. These hypoxia-induced changes can confer resistance to conventional therapies, such as chemotherapy, radiotherapy, and immunotherapy. This review summarizes the current knowledge on the impact of hypoxia on the TME and its implications for therapy resistance. It also discusses the potential of hypoxia biomarkers as prognostic and predictive indictors of treatment response, as well as the challenges and opportunities of targeting hypoxia in clinical trials.

Quiescent Cancer Cells-A Potential Therapeutic Target to Overcome Tumor Resistance and Relapse

Quiescent cancer cells (QCCs) are nonproliferating cells arrested in the G0 phase, characterized by ki67^low and p27^high. QCCs avoid most chemotherapies, and some treatments could further lead to a higher proportion of QCCs in tumors. QCCs are also associated with cancer recurrence since they can re-enter a proliferative state when conditions are favorable. As QCCs lead to drug resistance and tumor recurrence, there is a great need to understand the characteristics of QCCs, decipher the mechanisms that regulate the proliferative-quiescent transition in cancer cells, and develop new strategies to eliminate QCCs residing in solid tumors. In this review, we discussed the mechanisms of QCC-induced drug resistance and tumor recurrence. We also discussed therapeutic strategies to overcome resistance and relapse by targeting QCCs, including (i) identifying reactive quiescent cancer cells and removing them via cell-cycle-dependent anticancer reagents; (ii) modulating the quiescence-to-proliferation switch; and (iii) eliminating QCCs by targeting their unique features. It is believed that the simultaneous co-targeting of proliferating and quiescent cancer cells may ultimately lead to the development of more effective therapeutic strategies for the treatment of solid tumors.

Polyploid/Multinucleated Giant and Slow-Cycling Cancer Cell Enrichment in Response to X-ray Irradiation of Human Glioblastoma Multiforme Cells Differing in Radioresistance and TP53/PTEN Status

Radioresistance compromises the efficacy of radiotherapy for glioblastoma multiforme (GBM), the most devastating and common brain tumor. The present study investigated the relationship between radiation tolerance and formation of polyploid/multinucleated giant (PGCC/MGCC) and quiescent/senescent slow-cycling cancer cells in human U-87, LN-229, and U-251 cell lines differing in TP53/PTEN status and radioresistance. We found significant enrichment in MGCC populations of U-87 and LN-229 cell lines, and generation of numerous small mononuclear (called Raju cells, or RJ cells) U-87-derived cells that eventually form cell colonies, in a process termed neosis, in response to X-ray irradiation (IR) at single acute therapeutic doses of 2-6 Gy. For the first time, single-cell high-content imaging and analysis of Ki-67- and EdU-coupled fluorescence demonstrated that the IR exposure dose-dependently augments two distinct GBM cell populations. Bifurcation of Ki-67 staining suggests fast-cycling and slow-cycling populations with a normal-sized nuclear area, and with an enlarged nuclear area, including one resembling the size of PGCC/MGCCs, that likely underlie the highest radioresistance and propensity for repopulation of U-87 cells. Proliferative activity and anchorage-independent survival of GBM cell lines seem to be related to neosis, low level of apoptosis, fraction of prematurely stress-induced senescent MGCCs, and the expression of p63 and p73, members of p53 family transcription factors, but not to the mutant p53. Collectively, our data support the importance of the TP53wt/PTENmut genotype for the maintenance of cycling radioresistant U-87 cells to produce a significant amount of senescent MGCCs as an IR stress-induced adaptation response to therapeutic irradiation doses.

Protocol for isolation and functional validation of label-retaining quiescent colorectal cancer stem cells from patient-derived organoids for RNA-seq

Quiescent cancer stem cells (qCSCs) are a major source of posttreatment relapse, but methods to identify molecular targets for qCSC elimination are limited. Here, we present a protocol using the fluorescent dye PKH26 to isolate label-retaining qCSCs from colorectal cancer (CRC) patient-derived organoids (PDOs). We describe processing of organoids to single cells, followed by PKH26 labeling and FACS-based cell isolation. We then detail steps for functional assays and RNA sequencing. This protocol can also be applied to normal tissue-derived organoids. For complete details on the use and execution of this protocol, please refer to Regan et al. (2021).

Inhibition of CCL7 derived from Mo-MDSCs prevents metastatic progression from latency in colorectal cancer

In colorectal cancer (CRC), overt metastases often appear after years of latency. But the signals that cause micro-metastatic cells to remain indolent, thereby enabling them to survive for extended periods of time, are unclear. Immunofluorescence and co-immunoprecipitation assays were used to explore the co-localization of CCL7 and CCR2. Immunohistochemical (IHC) assays were employed to detect the characters of metastatic HT29 cells in mice liver. Flow cytometry assays were performed to detect the immune cells. Bruberin vivo MS FX Pro Imager was used to observe the liver metastasis of CRC in mice. Quantitative real-time PCR (qRT-PCR) and western blot were employed to detect the expressions of related proteins. Trace RNA sequencing was employed to identify differentially expressed genes in MDSCs from liver micro-M and macro-M of CRC in mice. Here, we firstly constructed the vitro dormant cell models and metastatic dormant animal models of colorectal cancer. Then we found that myeloid-derived suppressor cells (MDSCs) were increased significantly from liver micro-metastases to macro-metastases of CRC in mice. Moreover, monocytic MDSCs (Mo-MDSC) significantly promoted the dormant activation of micro-metastatic cells compared to polymorphonuclear MDSCs (PMN-MDSC). Mechanistically, CCL7 secreted by Mo-MDSCs bound with membrane protein CCR2 of micro-metastatic cells and then stimulated the JAK/STAT3 pathway to activate the dormant cells. Low-dose administration of CCL7 and MDSCs inhibitors in vivo could significantly maintain the CRC metastatic cells dormant status for a long time to reduce metastasis or recurrence after radical operation. Clinically, the level of CCL7 in blood was positively related to the number of Mo-MDSCs in CCR patients, and highly linked with the short-time recurrence and distant metastasis. CCL7 secreted by Mo-MDSCs plays an important role in initiating the outgrowth of metastatic latent CRC cells. Inhibition of CCL7 might provide a potential therapeutic strategy for the prevention of metastasis recurrence.

Slow-cycling (dormant) cancer cells in therapy resistance, cancer relapse and metastasis

It is increasingly appreciated that cancer cell heterogeneity and plasticity constitute major barriers to effective clinical treatments and long-term therapeutic efficacy. Research in the past two decades suggest that virtually all treatment-naive human cancers harbor subsets of cancer cells that possess many of the cardinal features of normal stem cells. Such stem-like cancer cells, operationally defined as cancer stem cells (CSCs), are frequently quiescent and dynamically change and evolve during tumor progression and therapeutic interventions. Intrinsic tumor cell heterogeneity is reflected in a different aspect in that tumors also harbor a population of slow-cycling cells (SCCs) that are not in the proliferative cell cycle and thus are intrinsically refractory to anti-mitotic drugs. In this Perspective, we focus our discussions on SCCs in cancer and on various methodologies that can be employed to enrich and purify SCCs, compare the similarities and differences between SCCs, CSCs and cancer cells undergoing EMT, and present evidence for the involvement of SCCs in surviving anti-neoplastic treatments, mediating tumor relapse, maintaining tumor dormancy and mediating metastatic dissemination. Our discussions make it clear that an in-depth understanding of the biological properties of SCCs in cancer will be instrumental to developing new therapeutic strategies to prevent tumor relapse and distant metastasis.

Targeting Genome Stability in Melanoma-A New Approach to an Old Field

Despite recent groundbreaking advances in the treatment of cutaneous melanoma, it remains one of the most treatment-resistant malignancies. Due to resistance to conventional chemotherapy, the therapeutic focus has shifted away from aiming at melanoma genome stability in favor of molecularly targeted therapies. Inhibitors of the RAS/RAF/MEK/ERK (MAPK) pathway significantly slow disease progression. However, long-term clinical benefit is rare due to rapid development of drug resistance. In contrast, immune checkpoint inhibitors provide exceptionally durable responses, but only in a limited number of patients. It has been increasingly recognized that melanoma cells rely on efficient DNA repair for survival upon drug treatment, and that genome instability increases the efficacy of both MAPK inhibitors and immunotherapy. In this review, we discuss recent developments in the field of melanoma research which indicate that targeting genome stability of melanoma cells may serve as a powerful strategy to maximize the efficacy of currently available therapeutics.

Towards a Framework for Better Understanding of Quiescent Cancer Cells

Quiescent cancer cells (QCCs) are cancer cells that are reversibly suspended in G₀ phase with the ability to re-enter the cell cycle and initiate tumor growth, and, ultimately, cancer recurrence and metastasis. QCCs are also therapeutically challenging due to their resistance to most conventional cancer treatments that selectively act on proliferating cells. Considering the significant impact of QCCs on cancer progression and treatment, better understanding of appropriate experimental models, and the evaluation of QCCs are key questions in the field that have direct influence on potential pharmacological interventions. Here, this review focuses on existing and emerging preclinical models and detection methods for QCCs and discusses their respective features and scope for application. By providing a framework for selecting appropriate experimental models and investigative methods, the identification of the key players that regulate the survival and activation of QCCs and the development of more effective QCC-targeting therapeutic agents may mitigate the consequences of QCCs.

Application of the RBBP9 Serine Hydrolase Inhibitor, ML114, Decouples Human Pluripotent Stem Cell Proliferation and Differentiation

Retinoblastoma binding protein 9 (RBBP9) is required for maintaining the expression of both pluripotency and cell cycle genes in human pluripotent stem cells (hPSCs). An siRNA-based study from our group showed it does so by influencing cell cycle progression through the RB/E2F pathway. In non-pluripotent cells, RBBP9 is also known to have serine hydrolase (SH) activity, acting on currently undefined target proteins. The role of RBBP9 SH activity in hPSCs, and during normal development, is currently unknown. To begin assessing whether RBBP9 SH activity might contribute to hPSC maintenance, hPSCs were treated with ML114—a selective chemical inhibitor of RBBP9 SH activity. Stem cells treated with ML114 showed significantly reduced population growth rate, colony size and progression through the cell cycle, with no observable change in cell morphology or decrease in pluripotency antigen expression—suggesting no initiation of hPSC differentiation. Consistent with this, hPSCs treated with ML114 retained the capacity for tri-lineage differentiation, as seen through teratoma formation. Subsequent microarray and Western blot analyses of ML114-treated hPSCs suggest the nuclear transcription factor Y subunit A (NFYA) may be a candidate effector of RBBP9 SH activity in hPSCs. These data support a role for RBBP9 in regulating hPSC proliferation independent of differentiation, whereby inhibition of RBBP9 SH activity de-couples decreased hPSC proliferation from initiation of differentiation.

Multi-color clonal tracking reveals intra-stage proliferative heterogeneity during mammary tumor progression

Despite major progress in breast cancer research, the functional contribution of distinct cancer cell clones to malignant tumor progression and metastasis remains largely elusive. We have assessed clonal heterogeneity within individual primary tumors and metastases and also during the distinct stages of malignant tumor progression using clonal tracking of cancer cells in the MMTV-PyMT mouse model of metastatic breast cancer. Comparative gene expression analysis of clonal subpopulations reveals a substantial level of heterogeneity across and also within the various stages of breast carcinogenesis. The intra-stage heterogeneity is primarily manifested by differences in cell proliferation, also found within invasive carcinomas of luminal A-, luminal B-, and HER2-enriched human breast cancer. Surprisingly, in the mouse model of clonal tracing of cancer cells, chemotherapy mainly targets the slow-proliferative clonal populations and fails to efficiently repress the fast-proliferative populations. These insights may have considerable impact on therapy selection and response in breast cancer patients.

FBX8 promotes metastatic dormancy of colorectal cancer in liver

Patients with colorectal cancer (CRC) often develop malignant regrowth of metastatic dormant tumor cells in liver years after primary treatment. FBX8 is involved in suppressing tumor metastasis. Short-term chemotherapy experiments and liver metastasis mice model of orthotopic injection into the cecum were performed to construct the dormant models. GST-pull-down assay, Co-IP and immunofluorescence were used to confirm the bindings among FBX8 and its substrates. FBX8 upregulated the expression of epithelial and stemness markers, while downregulated the expression of mesenchymal and proliferative markers associated with tumor cell dormancy. FBX8 promoted the maintenance of metastatic dormancy of CRC cells. Mechanistically, FBX8 directly bound to HIF-1α, CDK4 and C-myc through its Sec7 domain and led to the ubiquitin degradation of these proteins, thereby inhibiting cell cycle progression, proliferation, angiogenesis, and metastasis. Clinically, FBX8 expression was negatively correlated with the HIF-1α, CDK4, and c-Myc in CRC tissues. Our study reveals a novel mechanism of FBX8 in regulating tumor metastatic dormancy in liver and provides new strategies for the treatment of CRC metastasis.

Sonic hedgehog pathway mediates genistein inhibition of renal cancer stem cells

Cancer stem cells (CSCs) have been implicated in the genesis, progression and recurrence of renal cancer. The sonic hedgehog (Shh) pathway serves a critical role in maintaining the stemness of CSCs. Genistein, a major isoflavone component extracted from soybeans and soy products, has been demonstrated to possess anticancer activity. However, the effects of genistein on renal CSCs and its underlying mechanisms remain to be fully elucidated. The aim of the present study was to investigate the role of the Shh pathway in genistein inhibition of renal CSCs. The results of the present study demonstrated that expression levels of renal CSC markers were markedly upregulated in the sphere-forming cells, which were isolated and enriched from 786-O and ACHN cells in a tumor sphere formation assay, and more cells were arrested at the G₀/G₁ phase instead of the S₁ phase compared with the adherent cells. Furthermore, the present study demonstrated that genistein could effectively diminish the activity of renal CSCs by suppressing tumor sphere formation, decreasing renal CSCs markers, inhibiting proliferation and inducing apoptosis. Additionally, the downregulation of Shh pathway activity could inhibit renal CSCs. Genistein exhibited an inhibitory effect on renal CSCs by attenuating the activation of the Shh pathway. In conclusion, the results illustrated the role of the Shh pathway in regulating renal CSC traits and the intervention of renal CSCs by genistein, which could provide novel insights into the molecular mechanisms of renal CSC intervention.

Implication for Cancer Stem Cells in Solid Cancer Chemo-Resistance: Promising Therapeutic Strategies Based on the Use of HDAC Inhibitors

Resistance to therapy in patients with solid cancers represents a daunting challenge that must be addressed. Indeed, current strategies are still not effective in the majority of patients; which has resulted in the need for novel therapeutic approaches. Cancer stem cells (CSCs), a subset of tumor cells that possess self-renewal and multilineage differentiation potential, are known to be intrinsically resistant to anticancer treatments. In this review, we analyzed the implications for CSCs in drug resistance and described that multiple alterations in morphogenetic pathways (i.e., Hippo, Wnt, JAK/STAT, TGF-β, Notch, Hedgehog pathways) were suggested to be critical for CSC plasticity. By interrogating The Cancer Genome Atlas (TCGA) datasets, we first analyzed the prevalence of morphogenetic pathways alterations in solid tumors with associated outcomes. Then, by highlighting epigenetic relevance in CSC development and maintenance, we selected histone deacetylase inhibitors (HDACi) as potential agents of interest to target this subpopulation based on the pleiotropic effects exerted specifically on altered morphogenetic pathways. In detail, we highlighted the role of HDACi in solid cancers and, specifically, in the CSC subpopulation and we pointed out some mechanisms by which HDACi are able to overcome drug resistance and to modulate stemness. Although, further clinical and preclinical investigations should be conducted to disclose the unclear mechanisms by which HDACi modulate several signaling pathways in different tumors. To date, several lines of evidence support the testing of novel combinatorial therapeutic strategies based on the combination of drugs commonly used in clinical practice and HDACi to improve therapeutic efficacy in solid cancer patients.

Engineered In Vitro Models of Tumor Dormancy and Reactivation

Metastatic recurrence is a major hurdle to overcome for successful control of cancer-associated death. Residual tumor cells in the primary site, or disseminated tumor cells in secondary sites, can lie in a dormant state for long time periods, years to decades, before being reactivated into a proliferative growth state. The microenvironmental signals and biological mechanisms that mediate the fate of disseminated cancer cells with respect to cell death, single cell dormancy, tumor mass dormancy and metastatic growth, as well as the factors that induce reactivation, are discussed in this review. Emphasis is placed on engineered, in vitro, biomaterial-based approaches to model tumor dormancy and subsequent reactivation, with a focus on the roles of extracellular matrix, secondary cell types, biochemical signaling and drug treatment. A brief perspective of molecular targets and treatment approaches for dormant tumors is also presented. Advances in tissue-engineered platforms to induce, model, and monitor tumor dormancy and reactivation may provide much needed insight into the regulation of these processes and serve as drug discovery and testing platforms.