FK506 Attenuates the MRP1-Mediated Chemoresistant Phenotype in Glioblastoma Stem-Like Cells

Enhanced fungal specificity and in vivo therapeutic efficacy of a C-22-modified FK520 analog against C. neoformans

IMPORTANCE

Fungal infections cause significant morbidity and mortality globally. The therapeutic armamentarium against these infections is limited, and the development of antifungal drugs has been hindered by the evolutionary conservation between fungi and the human host. With rising resistance to the current antifungal arsenal and an increasing at-risk population, there is an urgent need for the development of new antifungal compounds. The FK520 analogs described in this study display potent antifungal activity as a novel class of antifungals centered on modifying an existing orally active FDA-approved therapy. This research advances the development of much-needed newer antifungal treatment options with novel mechanisms of action.

Enhanced fungal specificity and in vivo therapeutic efficacy of a C-22 modified FK520 analog against C. neoformans

Fungal infections are of mounting global concern, and the current limited treatment arsenal poses challenges when treating such infections. In particular, infections by Cryptococcus neoformans are associated with high mortality, emphasizing the need for novel therapeutic options. Calcineurin is a protein phosphatase that mediates fungal stress responses, and calcineurin inhibition by the natural product FK506 blocks C. neoformans growth at 37°C. Calcineurin is also required for pathogenesis. However, because calcineurin is conserved in humans, and inhibition with FK506 results in immunosuppression, the use of FK506 as an anti-infective agent is precluded. We previously elucidated the structures of multiple fungal calcineurin-FK506-FKBP12 complexes and implicated the C-22 position on FK506 as a key point for differential modification of ligand inhibition of the mammalian versus fungal target proteins. Through in vitro antifungal and immunosuppressive testing of FK520 (a natural analog of FK506) derivatives, we identified JH-FK-08 as a lead candidate for further antifungal development. JH-FK-08 exhibited significantly reduced immunosuppressive activity and both reduced fungal burden and prolonged survival of infected animals. JH-FK-08 exhibited additive activity in combination with fluconazole in vivo . These findings further advance calcineurin inhibition as an antifungal therapeutic approach.

Importance

Fungal infections cause significant morbidity and mortality globally. The therapeutic armamentarium against these infections is limited and development of antifungal drugs has been hindered by the evolutionary conservation between fungi and the human host. With rising resistance to the current antifungal arsenal and an increasing at-risk population, there is an urgent need for the development of new antifungal compounds. The FK520 analogs described in this study display potent antifungal activity as a novel class of antifungals centered on modifying an existing orally-active FDA approved therapy. This research advances the development of much needed newer antifungal treatment options with novel mechanisms of action.

The Role of Hypoxia and Cancer Stem Cells in Development of Glioblastoma

Glioblastoma multiform (GBM) is recognized as the most malignant brain tumor with a high level of hypoxia, containing a small population of glioblastoma stem like cells (GSCs). These GSCs have the capacity of self-renewal, proliferation, invasion and recapitulating the parent tumor, and are major causes of radio-and chemoresistance of GBM. Upregulated expression of hypoxia inducible factors (HIFs) in hypoxia fundamentally contributes to maintenance and progression of GSCs. Therefore, we thoroughly reviewed the currently acknowledged roles of hypoxia-associated GSCs in development of GBM. In detail, we recapitulated general features of GBM, especially GSC-related features, and delineated essential responses resulted from interactions between GSC and hypoxia, including hypoxia-induced signatures, genes and pathways, and hypoxia-regulated metabolic alterations. Five hypothesized GSC niches are discussed and integrated into one comprehensive concept: hypoxic peri-arteriolar niche of GSCs. Autophagy, another protective mechanism against chemotherapy, is also closely related to hypoxia and is a potential therapeutic target for GBM. In addition, potential causes of therapeutic resistance (chemo-, radio-, surgical-, immuno-), and chemotherapeutic agents which can improve the therapeutic effects of chemo-, radio-, or immunotherapy are introduced and discussed. At last, as a potential approach to reverse the hypoxic microenvironment in GBM, hyperbaric oxygen therapy (HBOT) might be an adjuvant therapy to chemo-and radiotherapy after surgery. In conclusion, we focus on demonstrating the important role of hypoxia on development of GBM, especially by affecting the function of GSCs. Important advantages have been made to understand the complicated responses induced by hypoxia in GBM. Further exploration of targeting hypoxia and GSCs can help to develop novel therapeutic strategies to improve the survival of GBM patients.

Cancer Stem Cell and Aggressiveness Traits Are Promoted by Stable Endothelin-Converting Enzyme-1c in Glioblastoma Cells

Glioblastoma (GBM) is the most common and aggressive type of brain tumor due to its elevated recurrence following treatments. This is mainly mediated by a subpopulation of cells with stemness traits termed glioblastoma stem-like cells (GSCs), which are extremely resistant to anti-neoplastic drugs. Thus, an advancement in the understanding of the molecular processes underlying GSC occurrence should contribute significantly towards progress in reducing aggressiveness. High levels of endothelin-converting enzyme-1 (ECE1), key for endothelin-1 (ET-1) peptide activation, have been linked to the malignant progression of GBM. There are four known isoforms of ECE1 that activate ET-1, which only differ in their cytoplasmic N-terminal sequences. Isoform ECE1c is phosphorylated at Ser-18 and Ser-20 by protein kinase CK2, which increases its stability and hence promotes aggressiveness traits in colon cancer cells. In order to study whether ECE1c exerts a malignant effect in GBM, we designed an ECE1c mutant by switching a putative ubiquitination lysine proximal to the phospho-serines Lys-6-to-Arg (i.e., K6R). This ECE1c^K6R mutant was stably expressed in U87MG, T98G, and U251 GBM cells, and their behavior was compared to either mock or wild-type ECE1c-expressing clone cells. ECE1c^K6R behaved as a highly stable protein in all cell lines, and its expression promoted self-renewal and the enrichment of a stem-like population characterized by enhanced neurospheroid formation, as well as increased expression of stem-like surface markers. These ECE1c^K6R-derived GSC-like cells also displayed enhanced resistance to the GBM-related chemotherapy drugs temozolomide and gemcitabine and increased expression of the ABCG2 efflux pump. In addition, ECE1c^K6R cells displayed enhanced metastasis-associated traits, such as the modulation of adhesion and the enhancement of cell migration and invasion. In conclusion, the acquisition of a GSC-like phenotype, together with heightened chemoresistance and invasiveness traits, allows us to suggest phospho-ECE1c as a novel marker for poor prognosis as well as a potential therapeutic target for GBM.

Knockout of ABCC1 in NCI-H441 cells reveals CF to be a suboptimal substrate to study MRP1 activity in organotypic in vitro models

Multidrug resistance-associated protein 1 (MRP1/ABCC1) is an efflux transporter responsible for the extrusion of endogenous substances as well as xenobiotics and their respective metabolites. Its high expression levels in lung tissue imply a key role in pulmonary drug disposition. Moreover, its association with inflammatory lung diseases underline MRP1's relevance in drug development and precision-medicine. With the aim to develop a tool to better understand MRP1's role in drug disposition and lung disease, we generated an ABCC1^-/- clone based on the human distal lung epithelial cell line NCI-H441 via a targeted CRISPR/Cas9 approach. Successful knockout (KO) of MRP1 was confirmed by qPCR, immunoblot and Sanger sequencing. To assess potential compensatory upregulation of transporters with a similar substrate recognition pattern as MRP1, expression levels of MRP2-9 as well as OAT1-4, 6, 7 and 10 were measured. Functional transporter activity was determined via release studies with two prodrug/substrate pairs, i.e. 5(6)-carboxyfluorescein (CF; formed from its diacetate prodrug) and S-(6-(7-methylpurinyl))glutathione (MPG; formed from its prodrug 6-bromo-7-methylpurine, BMP), respectively. Lastly, transepithelial electrical resistance (TEER) of monolayers of the KO clone were compared with wildtype (WT) NCI-H441 cells. Of eight initially generated clones, the M2 titled clone showed complete absence of mRNA and protein in accordance with the designed genome edit. In transport studies using the substrate CF, however, no differences between the KO clone and WT NCI-H441 cells were observed, whilst no differences in expression of potential compensatory transporters was noted. On the other hand, when using BMP/MPG, the release of MPG was reduced to 11.5% in the KO clone. Based on these results, CF appears to be a suboptimal probe for the study of MRP1 function, particularly in organotypic in vitro and ex vivo models. Our ABCC1^-/- NCI-H441 clone further retained the ability to form electrically tight barriers, making it a useful model to study MRP1 function in vitro.

Membrane transport of cobalamin

A wide variety of organisms encode cobalamin-dependent enzymes catalyzing essential metabolic reactions, but the cofactor cobalamin (vitamin B12) is only synthesized by a subset of bacteria and archaea. The biosynthesis of cobalamin is complex and energetically costly, making cobalamin variants and precursors metabolically valuable. Auxotrophs for these molecules have evolved uptake mechanisms to compensate for the lack of a synthesis pathway. Bacterial transport of cobalamin involves the passage over one or two lipidic membranes in Gram-positive and -negative bacteria, respectively. In higher eukaryotes, a complex system of carriers, receptors and transporters facilitates the delivery of the essential molecule to the tissues. Biochemical and genetic approaches have identified different transporter families involved in cobalamin transport. The majority of the characterized cobalamin transporters are active transport systems that belong to the ATP-binding cassette (ABC) superfamily of transporters. In this chapter, we describe the different cobalamin transport systems characterized to date that are present in bacteria and humans, as well as yet-to-be-identified transporters.

Cancer stem cells and strategies for targeted drug delivery

Cancer stem cells (CSCs) are a small proportion of cancer cells with high tumorigenic activity, self-renewal ability, and multilineage differentiation potential. Standard anti-tumor therapies including conventional chemotherapy, radiation therapy, and molecularly targeted therapies are not effective against CSCs, and often lead to enrichment of CSCs that can result in tumor relapse. Therefore, it is hypothesized that targeting CSCs is key to increasing the efficacy of cancer therapies. In this review, CSC properties including CSC markers, their role in tumor growth, invasiveness, metastasis, and drug resistance, as well as CSC microenvironment are discussed. Further, CSC-targeted strategies including the use of targeted drug delivery systems are examined.

Therapeutic Perspective of Temozolomide Resistance in Glioblastoma Treatment

Glioblastoma (GB) is the most lethal form of primary brain neoplasm. TMZ is the first-line standard treatment, but the strong resistance constrains the efficacy in clinical use. GB contains glioma stem cells (GSCs), which contribute to TMZ resistance, promote cell survival evolvement, and repopulate the tumor mass. This review summarizes the TMZ-resistance mechanisms and discusses several potential therapies from the conservative opinion of GSC-targeted therapy orientation to the current view of TMZ resistance-aimed efficacy, which will provide an understanding of the role of heterogeneity in drug resistance and improve therapeutic efficacy in general.

Adenosinergic Pathway: A Hope in the Immunotherapy of Glioblastoma

Brain tumors comprise different types of malignancies, most of which are originated from glial cells. Glioblastoma multiforme (GBM) is the most aggressive type of brain tumor with a poor response to conventional therapies and dismal survival rates (15 months) despite multimodal therapies. The development of immunotherapeutic strategies seems to be necessary to enhance the overall survival of GBM patients. So far, the immunotherapies applied in GBM had promising results in the primary phases of clinical trials but failed to continue their beneficial effects in later phases. GBM-microenvironment (GME) is a heterogenic and rigorously immunosuppressive milieu wrapping by an impenetrable blood-brain barrier. Hence, in-depth knowledge about the dominant immunosuppressive mechanisms in the GME could foster GBM immunotherapy. Recently, the adenosinergic pathway (AP) is found to be a major player in the suppression of antitumor immune responses in the GME. Tumor cells evolve to metabolize pro-inflammatory ATP to anti-inflammatory adenosine. Adenosine can suppress immune responses through the signaling of adenosine receptors on immune cells. The preclinical results targeting AP in GBM showed promising results in reinvigorating antitumor responses, overriding chemoresistance, and increasing survival. We reviewed the current GBM immunotherapies and elaborated on the role of AP in the immunopathogenesis, treatment, and even prognosis of GBM. We suggest that future clinical studies should consider this pathway in their combination therapies along with other immunotherapeutic approaches.

Identification of a Metabolism-Related Risk Signature Associated With Clinical Prognosis in Glioblastoma Using Integrated Bioinformatic Analysis

Altered metabolism of glucose, lipid and glutamine is a prominent hallmark of cancer cells. Currently, cell heterogeneity is believed to be the main cause of poor prognosis of glioblastoma (GBM) and is closely related to relapse caused by therapy resistance. However, the comprehensive model of genes related to glucose-, lipid- and glutamine-metabolism associated with the prognosis of GBM remains unclear, and the metabolic heterogeneity of GBM still needs to be further explored. Based on the expression profiles of 1,395 metabolism-related genes in three datasets of TCGA/CGGA/GSE, consistent cluster analysis revealed that GBM had three different metabolic status and prognostic clusters. Combining univariate Cox regression analysis and LASSO-penalized Cox regression machine learning methods, we identified a 17-metabolism-related genes risk signature associated with GBM prognosis. Kaplan-Meier analysis found that obtained signature could differentiate the prognosis of high- and low-risk patients in three datasets. Moreover, the multivariate Cox regression analysis and receiver operating characteristic curves indicated that the signature was an independent prognostic factor for GBM and had a strong predictive power. The above results were further validated in the CGGA and GSE13041 datasets, and consistent results were obtained. Gene set enrichment analysis (GSEA) suggested glycolysis gluconeogenesis and oxidative phosphorylation were significantly enriched in high- and low-risk GBM. Lastly Connectivity Map screened 54 potential compounds specific to different subgroups of GBM patients. Our study identified a novel metabolism-related gene signature, in addition the existence of three different metabolic status and two opposite biological processes in GBM were recognized, which revealed the metabolic heterogeneity of GBM. Robust metabolic subtypes and powerful risk prognostic models contributed a new perspective to the metabolic exploration of GBM.

Adenosine Depletion as A New Strategy to Decrease Glioblastoma Stem-Like Cells Aggressiveness

Glioblastoma is the brain tumor with the worst prognosis. This is mainly due to a cell subpopulation with an extremely aggressive potential, called glioblastoma stem-like cells (GSCs). These cells produce high levels of extracellular adenosine, which are increased even more under hypoxic conditions. Under hypoxia, adenosine signaling is related to HIF-2α expression, enhancing cell aggressiveness. Adenosine can be degraded using recombinant adenosine deaminase (ADA) to revert its pathological effects. The aim of this study was to degrade adenosine using ADA in order to decrease malignancy of GSCs. Adenosine depletion was performed using recombinant ADA. Migration and invasion were measured by transwell and matrigel-coated transwell assay, respectively. HIF-2α-dependent cell migration/invasion decreased in GSCs treated with ADA under hypoxia. MRPs-mediated chemoresistance and colony formation decreased in treatment with ADA. In conclusion, adenosine depletion using adenosine deaminase decreases GSCs aggressiveness.