Characterisation of mesothelioma-initiating cells and their susceptibility to anti-cancer agents

An Overview of Cellular and Molecular Determinants Regulating Chemoresistance in Pleural Mesothelioma

Malignant pleural mesothelioma (PM) is a highly aggressive disease of the lung pleura associated with poor prognosis. Despite advances in improving the clinical management of this malignancy, there is no effective chemotherapy for refractory or relapsing PM. The acquisition of resistance to standard and targeted therapy in this disease is a foremost concern; therefore, a deeper understanding of the complex factors surrounding the emergence of drug resistance is deemed necessary. In this review, we will present broad insights into various cellular and molecular concepts, accounting for the recalcitrance of PM to chemotherapy, including signaling networks regulating drug tolerance, drug resistance-associated proteins, genes, and miRNAs, as well as the critical role of cancer stem cells. Identification of the biological determinants and their associated mechanisms may provide a framework for the development of appropriate treatment.

Diving into cancer OXPHOS - The application of metabolic control analysis to cell and tissue research

Knowing how the oxidative phosphorylation (OXPHOS) system in cancer cells operates differently from that of normal cells would help find compounds that specifically paralyze the energy metabolism of cancer cells. The first experiments in the study of mitochondrial respiration using the metabolic control analysis (MCA) method were done with isolated liver mitochondria in the early 80s of the last century. Subsequent studies have shown that the regulation of mitochondrial respiration by ADP in isolated mitochondria differs significantly from a model of mitochondria in situ, where the contacts with components in the cytoplasm are largely preserved. The method of selective permeabilization of the outer membrane of the cells allows the application of MCA to evaluate the contribution of different components of the OXPHOS system to its functioning while mitochondria are in a natural state. In this review, we summarize the use of MCA to study OXPHOS in cancer using permeabilized cells and tissues. In addition, we give examples of how this data fits into cancer research with a completely different approach and methodology.

The Genes-Stemness-Secretome Interplay in Malignant Pleural Mesothelioma: Molecular Dynamics and Clinical Hints

MPM has a uniquely poor somatic mutational landscape, mainly driven by environmental selective pressure. This feature has dramatically limited the development of effective treatment. However, genomic events are known to be associated with MPM progression, and specific genetic signatures emerge from the exceptional crosstalk between neoplastic cells and matrix components, among which one main area of focus is hypoxia. Here we discuss the novel therapeutic strategies focused on the exploitation of MPM genetic asset and its interconnection with the surrounding hypoxic microenvironment as well as transcript products and microvesicles representing both an insight into the pathogenesis and promising actionable targets.

The application of cancer stem cell model in malignant mesothelioma

The high mortality rate of malignant pleural mesothelioma led to study the mechanisms for chemoresistance. The cancer stem cell (CSC) model has been proposed to explain chemoresistance. CSCs are characterized by self-renewal capacity, that is detected through tumor-initiating cell assays. As in other malignancies, many studies sought to identify surface markers to isolate CSCs from malignant mesothelioma. Other studies characterized malignant mesothelioma CSCs for the expression of specific genes involved in stemness and the expression of proteins involved in chemoresistance. However, the main methods to characterize isolated CSCs include sphere formation, invasiveness, tumor-initiating capacity and expression of specific surface markers. The better knowledge of malignant mesothelioma CSCs allowed exploring new potential targets to develop specific treatments.

Mesothelioma Malignancy and the Microenvironment: Molecular Mechanisms

Several studies have reported that cellular and soluble components of the tumor microenvironment (TME) play a key role in cancer-initiation and progression. Considering the relevance and the complexity of TME in cancer biology, recent research has focused on the investigation of the TME content, in terms of players and informational exchange. Understanding the crosstalk between tumor and non-tumor cells is crucial to design more beneficial anti-cancer therapeutic strategies. Malignant pleural mesothelioma (MPM) is a complex and heterogenous tumor mainly caused by asbestos exposure with few treatment options and low life expectancy after standard therapy. MPM leukocyte infiltration is rich in macrophages. Given the failure of macrophages to eliminate asbestos fibers, these immune cells accumulate in pleural cavity leading to the establishment of a unique inflammatory environment and to the malignant transformation of mesothelial cells. In this inflammatory landscape, stromal and immune cells play a driven role to support tumor development and progression via a bidirectional communication with tumor cells. Characterization of the MPM microenvironment (MPM-ME) may be useful to understand the complexity of mesothelioma biology, such as to identify new molecular druggable targets, with the aim to improve the outcome of the disease. In this review, we summarize the known evidence about the MPM-ME network, including its prognostic and therapeutic relevance.

Arachidonic acid drives adaptive responses to chemotherapy-induced stress in malignant mesothelioma

Background High resistance to therapy and poor prognosis characterizes malignant pleural mesothelioma (MPM). In fact, the current lines of treatment, based on platinum and pemetrexed, have limited impact on the survival of MPM patients. Adaptive response to therapy-induced stress involves complex rearrangements of the MPM secretome, mediated by the acquisition of a senescence-associated-secretory-phenotype (SASP). This fuels the emergence of chemoresistant cell subpopulations, with specific gene expression traits and protumorigenic features. The SASP-driven rearrangement of MPM secretome takes days to weeks to occur. Thus, we have searched for early mediators of such adaptive process and focused on metabolites differentially released in mesothelioma vs mesothelial cell culture media, after treatment with pemetrexed.

METHODS

Mass spectrometry-based (LC/MS and GC/MS) identification of extracellular metabolites and unbiased statistical analysis were performed on the spent media of mesothelial and mesothelioma cell lines, at steady state and after a pulse with pharmacologically relevant doses of the drug. ELISA based evaluation of arachidonic acid (AA) levels and enzyme inhibition assays were used to explore the role of cPLA2 in AA release and that of LOX/COX-mediated processing of AA. QRT-PCR, flow cytometry analysis of ALDH expressing cells and 3D spheroid growth assays were employed to assess the role of AA at mediating chemoresistance features of MPM. ELISA based detection of p65 and IkBalpha were used to interrogate the NFkB pathway activation in AA-treated cells.

RESULTS

We first validated what is known or expected from the mechanism of action of the antifolate. Further, we found increased levels of PUFAs and, more specifically, arachidonic acid (AA), in the transformed cell lines treated with pemetrexed. We showed that pharmacologically relevant doses of AA tightly recapitulated the rearrangement of cell subpopulations and the gene expression changes happening in pemetrexed -treated cultures and related to chemoresistance. Further, we showed that release of AA following pemetrexed treatment was due to cPLA2 and that AA signaling impinged on NFkB activation and largely affected anchorage-independent, 3D growth and the resistance of the MPM 3D cultures to the drug.

CONCLUSIONS

AA is an early mediator of the adaptive response to pem in chemoresistant MPM and, possibly, other malignancies.

Therapeutic Landscape of Malignant Pleural Mesothelioma: Collateral Vulnerabilities and Evolutionary Dependencies in the Spotlight

Malignant pleural mesothelioma (MPM) is the epitome of a recalcitrant cancer driven by pharmacologically intractable tumor suppressor proteins. A significant but largely unmet challenge in the field is the translation of genetic information on alterations in tumor suppressor genes (TSGs) into effective cancer-specific therapies. The notion that abnormal tumor genome subverts physiological cellular processes, which creates collateral vulnerabilities contextually related to specific genetic alterations, offers a promising strategy to target TSG-driven MPM. Moreover, emerging evidence has increasingly appreciated the therapeutic potential of genetic and pharmacological dependencies acquired en route to cancer development and drug resistance. Here, we review the most recent progress on vulnerabilities co-selected by functional loss of major TSGs and dependencies evolving out of cancer development and resistance to cisplatin based chemotherapy, the only first-line regimen approved by the US Food and Drug Administration (FDA). Finally, we highlight CRISPR-based functional genomics that has emerged as a powerful platform for cancer drug discovery in MPM. The repertoire of MPM-specific “Achilles heel” rises on the horizon, which holds the promise to elucidate therapeutic landscape and may promote precision oncology for MPM.

Wnt/IL-1β/IL-8 autocrine circuitries control chemoresistance in mesothelioma initiating cells by inducing ABCB5

Malignant pleural mesothelioma (MPM) is a tumor with high chemoresistance and poor prognosis. MPM-initiating cells (ICs) are known to be drug resistant, but it is unknown if and how stemness-related pathways determine chemoresistance. Moreover, there are no predictive markers of IC-associated chemoresistance. Aim of this work is to clarify if and by which mechanisms the chemoresistant phenotype of MPM IC was due to specific stemness-related pathways. We generated MPM IC from primary MPM samples and compared the gene expression and chemo-sensitivity profile of IC and differentiated/adherent cells (AC) of the same patient. Compared to AC, IC had upregulated the drug efflux transporter ABCB5 that determined resistance to cisplatin and pemetrexed. ABCB5-knocked-out (KO) IC clones were resensitized to the drugs in vitro and in patient-derived xenografts. ABCB5 was transcriptionally activated by the Wnt/GSK3β/β-catenin/c-myc axis that also increased IL-8 and IL-1β production. IL-8 and IL-1β-KO IC clones reduced the c-myc-driven transcription of ABCB5 and reacquired chemosensitivity. ABCB5-KO clones had lower IL-8 and IL-1β secretion, and c-myc transcriptional activity, suggesting that either Wnt/GSK3β/β-catenin and IL-8/IL-1β signaling drive c-myc-mediated transcription of ABCB5. ABCB5 correlated with lower time-to-progression and overall survival in MPM patients treated with cisplatin and pemetrexed. Our work identified multiple autocrine loops linking stemness pathways and resistance to cisplatin and pemetrexed in MPM IC. ABCB5 may represent a new target to chemosensitize MPM IC and a potential biomarker to predict the response to the first-line chemotherapy in MPM patients.

The FAK inhibitor BI 853520 inhibits spheroid formation and orthotopic tumor growth in malignant pleural mesothelioma

No tyrosine kinase inhibitors are approved for malignant pleural mesothelioma (MPM). Preclinical studies identified focal adhesion kinase (FAK) as a target in MPM. Accordingly, we assessed the novel, highly selective FAK inhibitor (BI 853520) in 2D and 3D cultures and in vivo. IC50 values were measured by adherent cell viability assay. Cell migration and 3D growth were quantified by video microscopy and spheroid formation, respectively. Phosphorylation of FAK, Akt, S6, and Erk was measured by immunoblot. The mRNA expression of the putative tumor stem cell markers SOX2, Nanog, CD44, ALDH1, c-myc, and Oct4 was analyzed by qPCR. Cell proliferation, apoptosis, and tumor tissue microvessel density (MVD) were investigated in orthotopic MPM xenografts. In all 12 MPM cell lines, IC50 exceeded 5 μM and loss of NF2 did not correlate with sensitivity. No synergism was found with cisplatin in adherent cells. BI 853520 decreased migration in 3 out of 4 cell lines. FAK phosphorylation was reduced upon treatment but activation of Erk, Akt, or S6 remained unaffected. Nevertheless, BI 853520 inhibited spheroid growth and significantly reduced tumor weight, cell proliferation, and MVD in vivo. BI 853520 has limited effect in adherent cultures but demonstrates potent activity in spheroids and in orthotopic tumors in vivo. Based on our findings, further studies are warranted to explore the clinical utility of BI 853520 in human MPM. KEY MESSAGES: Response to FAK inhibition in MPM is independent of NF2 expression or histotype. FAK inhibition strongly interfered with MPM spheroid formation. BI 853520 has been shown to exert anti-tumor effect in MPM.

Transglutaminase is a mesothelioma cancer stem cell survival protein that is required for tumor formation

Mesothelioma is a rare cancer of the mesothelial cell layer of the pleura, peritoneum, pericardium and tunica vaginalis. It is typically caused by asbestos, notoriously resistant to chemotherapy and generally considered incurable with a poor life expectancy. Transglutaminase 2 (TG2), a GTP binding regulatory protein, is an important cancer stem cell survival and therapy resistance factor. We show that TG2 is highly expressed in human mesothelioma tumors and in mesothelioma cancer stem cells (MCS cells). TG2 knockdown or TG2 inhibitor treatment reduces MCS cell spheroid formation, matrigel invasion, migration and tumor formation. Time to tumor first appearance is doubled in TG2 knockout cells as compared to wild-type. In addition, TG2 loss is associated with reduced expression of stemness, and epithelial mesenchymal transition markers, and enhanced apoptosis. These studies indicate that TG2 is an important MCS cell survival protein and suggest that TG2 may serve as a mesothelioma cancer stem cell therapy target.

Differential regulation of the sphere formation and maintenance of cancer-initiating cells of malignant mesothelioma via CD44 and ALK4 signaling pathways

Malignant mesothelioma (MM) has a poor prognosis and is largely resistant to standard treatments, so it is important to seek novel therapeutic strategies for this disease. Cancer-initiating cells (CICs) were previously identified in MM using stem cell-associated markers in combination with spheroid cultures. However, the mechanisms underlying the induction and maintenance of CICs in MM remain to be fully explored. Here we showed that the CICs, which had high aldehyde dehydrogenase levels (ALDH^bright) and stem cell-associated genes, were expanded in MM cells cultured under sphere-forming conditions. The MM spheroids also initiated tumors in immunodeficient mice more efficiently than did conventional adherent MM cells. In the MM spheroids, the expression of hyaluronan (HA) synthases was upregulated. Inhibiting the HA synthesis or CD44 functions by gene knockdown or neutralizing antibody abolished the formation of large-sized spheroids and the expansion of ALDH^bright CICs. The expression of activin-A was also increased in the spheroids, and type I activin-A receptor subunit (ALK4) was upregulated in the ALDH^bright CICs. The neutralization of activin-A or functional inactivation of ALK4 diminished the ALDH^bright CICs without affecting spheroid formation. The knockdown of CD44 or ALK4 strongly suppressed the tumor growth in immunodeficient mice. These results together suggest that the HA–CD44 and activin-A–ALK4 pathways differentially regulate the spheroid formation and maintenance of ALDH^bright CICs in MM cells, and that both pathways play critical roles in tumor growth in immunodeficient hosts. Our findings provide a novel therapeutic option for MM that targets signaling pathways that promote the CIC compartment through CD44 and ALK4.

Alternative assembly of respiratory complex II connects energy stress to metabolic checkpoints

Cell growth and survival depend on a delicate balance between energy production and synthesis of metabolites. Here, we provide evidence that an alternative mitochondrial complex II (CII) assembly, designated as CII_low, serves as a checkpoint for metabolite biosynthesis under bioenergetic stress, with cells suppressing their energy utilization by modulating DNA synthesis and cell cycle progression. Depletion of CII_low leads to an imbalance in energy utilization and metabolite synthesis, as evidenced by recovery of the de novo pyrimidine pathway and unlocking cell cycle arrest from the S-phase. In vitro experiments are further corroborated by analysis of paraganglioma tissues from patients with sporadic, SDHA and SDHB mutations. These findings suggest that CII_low is a core complex inside mitochondria that provides homeostatic control of cellular metabolism depending on the availability of energy.

Mitochondrial complex II is normally composed of four subunits. Here the authors show that bioenergetic stress conditions give rise to a partially assembled variant of complex II, which shifts the anabolic pathways to less energy demanding processes.

Putative cancer stem cells may be the key target to inhibit cancer cell repopulation between the intervals of chemoradiation in murine mesothelioma

Background

Cancer cell repopulation during chemotherapy or radiotherapy is a major factor limiting the efficacy of treatment. Cancer stem cells (CSC) may play critical roles during this process. We aim to demonstrate the role of mesothelioma stem cells (MSC) in treatment failure and eventually to design specific target therapies against MSC to improve the efficacy of treatment in malignant mesothelioma.

Methods

Murine mesothelioma AB12 and RN5 cells were used to compare tumorigenicity in mice. The expression of CSC-associated genes was evaluated by quantitative real-time PCR in both cell lines treated with chemo-radiation. Stemness properties of MSC-enriched RN5-EOS-Puro2 cells were characterized with flow cytometry and immunostaining. A MSC-specific gene profile was screened by microarray assay and confirmed thereafter. Gene Ontology analysis of the selected genes was performed by GOMiner.

Results

Tumor growth delay of murine mesothelioma AB12 cells was achieved after each cycle of cisplatin treatment, however, tumors grew back rapidly due to cancer cell repopulation between courses of chemotherapy. Strikingly, a 10-times lower number of irradiated cells in both cell lines led to a similar tumor incidence and growth rate as with untreated cells. The expression of CSC-associated genes such as CD24, CD133, CD90 and uPAR was dramatically up-regulated, while others did not change significantly after chemoradiation. Highly enriched MSC after selection with puromycin displayed an increasing GFP-positive population and showed typical properties of stemness. Comparatively, the proportion of MSC significantly increased after RN5-EOS parental cells were treated with either chemotherapy, γ-ray radiation, or a combination of the two, while MSC showed more resistance to the above treatments. A group of identified genes are most likely MSC-specific, and major pathways related to regulation of cell growth or apoptosis are involved. Upregulation of the gene transcripts Tnfsf18, Serpinb9b, Ly6a, and Nppb were confirmed.

Conclusion

Putative MSC possess the property of stemness showing more resistance to chemoradiation, suggesting that MSC may play critical roles in cancer cell repopulation. Further identification of selected genes may be used to design novel target therapies against MSC, so as to eliminate cancer cell repopulation in mesothelioma.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4354-1) contains supplementary material, which is available to authorized users.

In vitro experimental models of mesothelioma revisited

Malignant pleural mesothelioma (MPM) is a biologically unusual, highly aggressive cancer that defies current multimodality treatments. Epidemiologic data suggest that this malignancy has not abated despite increasingly strict environmental regulations on asbestos, the putative causative agent for sporadic cases. An incomplete understanding of all the factors mechanistically driving mesothelioma is largely responsible for the current lack of curative treatments. Many approaches have been employed to ascertain the step-by-step molecular events involved in mesothelioma oncogenesis including in vitro, small animal in vivo, and human experimental models; though clearly defined, druggable mechanisms still are elusive. Importantly, the foundation of the latest accepted model of tumor initiation is derived from in vitro systems. A thorough review of in vitro mesothelioma oncogenesis models may suggest further opportunities for discovery.

Stem Cell Factor-Based Identification and Functional Properties of In Vitro-Selected Subpopulations of Malignant Mesothelioma Cells

Malignant mesothelioma (MM) is an aggressive neoplasm characterized by a poor patient survival rate, because of rapid tumor recurrence following first-line therapy. Cancer stem cells (CSCs) are assumed to be responsible for initiating tumorigenesis and driving relapse after therapeutic interventions. CSC-enriched MM cell subpopulations were identified by an OCT4/SOX2 reporter approach and were characterized by (1) increased resistance to cisplatin, (2) increased sensitivity toward the FAK inhibitor VS-6063 in vitro, and (3) a higher tumor-initiating capacity in vivo in orthotopic xenograft and allograft mouse models. Overexpression of NF2 (neurofibromatosis 2, merlin), a tumor suppressor often mutated or lost in MM, did not affect proliferation and viability of CSC-enriched MM populations but robustly decreased the viability of reporter-negative cells. In contrast, downregulation of calretinin strongly decreased proliferation and viability of both populations. In summary, we have enriched and characterized a small MM cell subpopulation that bears the expected CSC characteristics.

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High SOX2 and OCT4 levels reveal a subpopulation of MM cells with CSC properties

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EGFP + MM cells are more resistant to cis-Pt, but more sensitive toward VS-6063

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EGFP + MM cells show a higher tumor-initiating capacity in xeno- and allograft models

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Calretinin downregulation, but not NF2 expression affects EGFP + MM cell viability

Correction: Characterisation of Mesothelioma-Initiating Cells and Their Susceptibility to Anti-Cancer Agents

[This corrects the article DOI: 10.1371/journal.pone.0119549.].

Autophagy initiation correlates with the autophagic flux in 3D models of mesothelioma and with patient outcome

Understanding the role of autophagy in cancer has been limited by the inability to measure this dynamic process in formalin-fixed tissue. We considered that 3-dimensional models including ex vivo tumor, such as we have developed for studying mesothelioma, would provide valuable insights. Using these models, in which we could use lysosomal inhibitors to measure the autophagic flux, we sought a marker of autophagy that would be valid in formalin-fixed tumor and be used to assess the role of autophagy in patient outcome. Autophagy was studied in mesothelioma cell lines, as 2-dimensional (2D) monolayers and 3-dimensional (3D) multicellular spheroids (MCS), and in tumor from 25 chemonaive patients, both as ex vivo 3D tumor fragment spheroids (TFS) and as formalin-fixed tissue. Autophagy was evaluated as autophagic flux by detection of the accumulation of LC3 after lysosomal inhibition and as autophagy initiation by detection of ATG13 puncta. We found that autophagic flux in 3D, but not in 2D, correlated with ATG13 positivity. In each TFS, ATG13 positivity was similar to that of the original tumor. When tested in tissue microarrays of 109 chemonaive patients, higher ATG13 positivity correlated with better prognosis and provided information independent of known prognostic factors. Our results show that ATG13 is a static marker of the autophagic flux in 3D models of mesothelioma and may also reflect autophagy levels in formalin-fixed tumor. If confirmed, this marker would represent a novel prognostic factor for mesothelioma, supporting the notion that autophagy plays an important role in this cancer.

Influence of AQP1 on cell adhesion, migration, and tumor sphere formation in malignant pleural mesothelioma is substratum- and histological-type dependent

Malignant pleural mesothelioma (MPM) is an aggressive cancer. MPM cells express aquaporin-1 (AQP1) that in other cancers has been shown to participate in the tumor metastasis processes. However, in MPM patients AQP1 overexpression is an independent prognostic factor favoring survival. In this study we aimed at evaluating the role of AQP1 in cell adhesion, migration, and tumor sphere formation in nonmalignant mesothelial cells (MeT-5A) and in epithelioid (M14K) and sarcomatoid (ZL34) MPM cell lines. We used fibronectin (FN) or homologous cell-derived extracellular martrix (ECM) substratum to investigate the role of AQP1 in these experimental phenotypes, inhibiting AQP1 by 10(-5) M mercury chloride (MC). Deposited ECM during cell culture exhibited significant concentration differences among cell types. ZL34 cell adhesion was significantly higher than MeT-5A or M14K cells on FN and ECM. MeT-5A and M14K cell adhesion on FN was sensitive to AQP1 inhibition, whereas AQP1 inhibition on ECM was limited to M14K cells. Wound healing in ZL34 cells was significantly higher than MeT-5A and M14K cells on FN and ECM. AQP1 inhibition significantly lowered cell migration in ZL34 cells on FN and ECM. Sphere formation was not dependent on FN or ECM in the media. AQP1 inhibition in FN media reduced sphere formation in M14K cells, whereas, in ECM, all three cell types were sensitive to AQP1 inhibition.

Five years update on relationships between malignant pleural mesothelioma and exposure to asbestos and other elongated mineral particles

Despite the reduction of global asbestos consumption and production due to the ban or restriction of asbestos uses in more than 50 countries since the 1970s, malignant mesothelioma remains a disease of concern. Asbestos is still used, imported, and exported in several countries, and the number of mesothelioma deaths may be expected to increase in the next decades in these countries. Asbestos exposure is the main risk factor for malignant pleural mesothelioma, but other types of exposures are linked to the occurrence of this type of cancer. Although recent treatments improve the quality of life of patients with mesothelioma, malignant pleural mesothelioma remains an aggressive disease. Recent treatments have not resulted in appreciable improvement in survival, and thus development of more efficient therapies is urgently needed. The development of novel therapeutic strategies is dependent on our level of knowledge of the physiopathological and molecular changes that mesothelial cells acquired during the neoplastic process. During the past 5 years, new findings have been published on the etiology, epidemiology, molecular changes, and innovative treatments of malignant pleural mesothelioma. This review aims to update the findings of recent investigations on etiology, epidemiology, and molecular changes with a focus on (1) attributable risk of asbestos exposure in men and women and (2) coexposure to other minerals and other elongated mineral particles or high aspect ratio nanoparticles. Recent data obtained on genomic and gene alterations, pathways deregulations, and predisposing factors are summarized.