Intracellular autofluorescence as a new biomarker for cancer stem cells in glioblastoma

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. This research was funded by FEDER funds through the Operational Programme Competitiveness Factors–COMPETE and National Funds through FCT under the projects UIDB/50026/2020, UIDP/50026/2020, and POCI-01-0145-FEDER-007038; by the project NORTE-01-0145-FEDER-000013, NORTE-01-0246-FEDER-000012, and NORTE-01-0145-FEDER-000023, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). J.V.d.C., C.S.G., E.P.M., and B.M.C. was funded by FCT-Foundation for Science and Technology (SFRH/BD/88121/2012 to J.V.d.C.; SFRH/BD/92786/2013 to C.S.G.; PD/BDE/143154/2019 to E.P.M.; and PTDC/SAUGMG/113795/2009, IF/00601/2012 and CEECIND/00072/2018 to B.M.C.). B.M.C. was also funded by Fundação Calouste Gulbenkian and Liga Portuguesa Contra o Cancro.