Sabotaging the protein factory to overcome glioma stem cell resistance

Glioblastomas (GBM) are among the most aggressive and frequent forms of CNS primary tumors.¹ The infiltrative nature of these tumors makes it almost impossible to remove the totality of the tumor cells, which leads to the eventual development of recurrence even following total tumor resection.^2,3

The high heterogeneity in GBM plays a critical role in tumor maintenance and progression.⁴ Particularly, GBMs are sustained by a cell population with stem-cell characteristics, such as self-renewal ability and the capacity to give rise to heterogeneous tumor cells with different degrees of differentiation, denominated glioma stem cells (GSCs). GSCs were also proposed to be particularly resistant to treatment, and due to their pluripotency, to be able to repopulate the tumor niche. Thus, targeting the GSC population has been proposed as a powerful strategy to target GBM.⁵

Ribosome biogenesis is one of the most complex and energy-consuming processes in the cell.⁶ It involves the coordinated work of the three RNA polymerases (RNA pol) and the assistance of more than 200 assembly factors.⁶ Many studies have shown that the biogenesis of the translation machinery, the ribosome, is decisive for sustaining GSC properties.⁶ Furthermore, recent works indicate that ribosome metabolism activation can promote stem-like characteristics in glioblastoma cells.⁷

Therefore, these observations emphasize the importance of translational control in stem cell homeostasis and fate decisions.⁸ However, the specific mechanisms that sustain ribosome production in GSCs remain unknown.

Tao et al. discovered and characterized a mechanism leading to increased production of ribosomal RNAs in GSCs.⁹ Their work provided an in-depth characterization of the novel histone acetyltransferase (INHAT) repressor (NIR) protein. NIR can bind to the histones blocking the access of the histone acetyltransferases P300/CBP and PCAF. This prevents histone acetylation and gene transcriptional activation. NIR has also been shown to interact with P53, repressing p53 transcriptional activator functions and promoting cell proliferation and survival.

Tao et al. found that NIR is expressed in GBM cells and not in normal brain cells, particularly by GSCs, and that it accumulated in the nucleolus of the cells. As nucleoli are the sites where ribosome biogenesis occurs, the authors speculated that NIR was associated with ribosomal metabolism.

Further experiments with NIR knocked-down cells demonstrated that NIR silencing inhibited GSC proliferation and self-renewal properties but did not affect non-stem tumor cells or neural precursor cells (NPCs). The effect of NIR silencing also extended to in vivo experiments, where NIR knockdown reduced tumor growth and cell proliferation.

Upon assessing the effect of NIR silencing, the authors investigated the function of this protein in ribosome biogenesis. NIR knockdown resulted in decreased levels of rRNA transcripts in GSCs but not in NPCs, and a reporter assay demonstrated that NIR directly regulates the rDNA promoter activity. NIR-silenced cells also showed decreased ribosome activity, reflected in decreased protein synthesis.

Via interaction experiments, the authors dissected in further detail the mechanism of action of NIR. NIR was shown to interact with NCL and NPM1, proteins involved in rDNA transcription, and these interactions were stronger in GSCs compared with NPCs. Consistently, NIR was shown to interact with the rDNA promoter in GSCs, and NIR modulates the binding of NCL and NPM1 to this promoter. Likewise, NCL or NPM1 silencing exhibited a phenotype that was comparable to that of NIR-silenced GBM.

Together, these results elegantly demonstrate that rDNA transcription is a critical mechanism on which GSCs rely on, to survive and sustain stem-cell properties. NIR was characterized as a regulator of rDNA transcription activity, and NIR disruption strongly affected GSC survival both in vitro and in vivo. This work positions NIR as a solid candidate for GSC targeting. The authors should be acknowledged for their elegant experimental design that led to a very detailed characterization of the function of NIR, which will undoubtedly help to design tailored therapies to modulate the described mechanisms The comparison of the role of NIR in GSCs, non-stem tumor cells and NPCs was pivotal to demonstrate the importance of rDNA transcription in GSCs.

This work opens the perspective to new questions such as which mechanisms mediate NIR increased expression in GSCs and whether these mechanisms can be epigenetically targeted, or whether NIR expression is preponderant in certain GBM subtypes. Additionally, the development of NIR pharmacological inhibitors is encouraged by the findings in this work.

As the production of ribosomal proteins and RNAs has been demonstrated to promote stemness in GBM,⁷ it would be interesting to investigate whether the tumor microenvironment (TME) can promote NIR activation and GSC proliferation. Single-cell profiling would be useful to characterize interactions between the TME, GSCs, and non-stem tumor cells.

In the future, it would be of most interest to evaluate the effect of NIR disruption/intervention in the resistance to treatments in GBM models. As GSCs are involved in repopulating the tumor mass after treatment,¹⁰ inhibiting the function of GSCs can be predicted to have a strong effect on tumor recurrence after treatment, extending even further the possibilities of rDNA transcription targeting for GBM.

In conclusion, this groundbreaking study not only sheds light on the intricate role of NIR in ribosome biogenesis within GSCs but also paves the way for innovative therapeutic strategies targeting the protein production machinery, offering hope for improved treatment outcomes for glioblastoma patients.

Acknowledgments

The text is the sole product of the authors and no third party had input or gave support to its writing.

Conflict of interest statement

The authors declare that no conflict of interest exists.

Funding

The laboratories of MGC and PRL are supported by the National Institutes of Health (NIH)/National Institute of Neurological Disorders and Stroke (NIH/NINDS) grants R37-NS094804, R01-NS105556, R01-NS122536, R01-NS124167, and R21-NS123879-01, and Rogel Cancer Center Scholar Award to M.G. Castro; NIH/NINDS grants R01-NS076991, R01-NS082311, R01-NS096756, R01NS122234, and National Institutes of Health/National Cancer Institute (NIH/NCI) R01-CA243916 to P.R. Lowenstein; the Department of Neurosurgery; and The Pediatric Brain Tumor Foundation, Leah's Happy Hearts Foundation, Ian's Friends Foundation (IFF), Chad Tough Foundation, and Smiles for Sophie Forever Foundation to M.G. Castro and P.R. Lowenstein.

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