Intracellular Autofluorescence as a New Biomarker for Cancer Stem Cells in Glioblastoma.

Joana Vieira De Castro,Christopher Heeschen,Nuno Sousa,Afonso A Pinto,Eduarda P Martins,Irene Miranda-Lorenzo,Rui L Reis,Céline S Gonçalves,Mariana T Cerqueira,Adhemar Longatto-Filho,Bruno M Costa

doi:10.3390/cancers13040828

Abstract

Simple SummarySince glioblastoma stem cells (GSCs) have paramount roles in tumor initiation, progression, recurrence, and therapy resistance, innovative approaches to specifically identify and isolate GSCs in a straightforward manner would be invaluable both for clinical and scientific applications. We demonstrate here that glioblastoma tumors have a subpopulation of cells with intracellular autofluorescence that display all the hallmark features of GSCs, and establish this biomarker as a new, simple, rapid and inexpensive way to identify and isolate this highly aggressive subpopulation of cells. Our findings are a great contribution to the neuro-oncology field, as they allow further studies on the molecular basis of GSCs, which may ultimately contribute to the identification of novel therapeutic targets and the development of smarter treatments to eliminate these cells. The identification of cancer stem cells (CSCs), which are implicated in tumor initiation, progression, therapy resistance, and relapse, is of great biological and clinical relevance. In glioblastoma (GBM), this is still a challenge, as no single marker is able to universally identify populations of GBM cancer stem cells (GSCs). Indeed, there is still controversy on whether biomarker-expressing cells fulfill the functional criteria of bona fide GSCs, despite being widely used. Here, we describe a novel subpopulation of autofluorescent (Fluo+) cells in GBM that bear all the functional characteristics of GSCs, including higher capacity to grow as neurospheres, long-term self-renewal ability, increased expression of stem cell markers, and enhanced in vivo tumorigenicity. Mechanistically, the autofluorescent phenotype is largely due to the intracellular accumulation of riboflavin, mediated by the ABC transporter ABCG2. In summary, our work identifies an intrinsic cellular autofluorescent phenotype enriched in GBM cells with functional stem cells features that can be used as a novel, simple and reliable biomarker to target these highly malignant tumors, with implications for GBM biological and clinical research.

Highlights

Gliomas are the most common primary tumors of the central nervous system, accounting for almost 80% of primary brain malignancies, of which glioblastoma (GBM) is the most aggressive subtype [1,2,3]
Human brain tumor stem cells (BTSCs) were originally isolated from primary tumors by cell sorting based on their CD133 membrane expression [7,8]
While GBMs are phenotypically rather distinct from those carcinomas, we demonstrate that intracellular autofluorescence exists in a subset of GBM cells and is a new biomarker for GBM cancer stem cells (GSCs), improving their specific identification and isolation, allowing subsequent characterization studies and providing a straightforward method for their tracking during clinical recurrence and treatment follow-up

Summary

Introduction

Gliomas are the most common primary tumors of the central nervous system, accounting for almost 80% of primary brain malignancies, of which glioblastoma (GBM) is the most aggressive subtype [1,2,3]. Recent evidence suggests that intratumor heterogeneity and poor response to current therapies are at least in part related to the existence of cancer stem cells (CSCs) in GBM [6,7,8,9,10,11,12]. These cells bear important characteristics similar to normal stem cells, most notably their unlimited self-renewal capacity, and have been associated with cancer initiation, progression, resistance to therapy, and relapse [6]. Several subsequent studies corroborated the existence of BTSCs, as well as their increased resistance to radiotherapy and chemotherapy with temozolomide (TMZ) [9,10,11,12]

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