RSK1 and RSK2 modulate the translatome of glioblastoma cells in an isoform-specific and mTORC1 independent manner.

Background: The p90 ribosomal S6 kinase (RSK) family, downstream target of Ras/ERK signaling, encompasses four human isoforms (RSK1-4). Glioblastomas (GBMs) predominantly express RSK1 and RSK2, whereby RSK1 is markedly upregulated in a subset of GBMs associated with dismal prognosis and immune infiltration, while RSK2 expression is constant. RSKs were proposed as regulators of mRNA translation through the activation of mTORC1 and other factors, such as eIF4B, but nothing is known about their effect on the translatome of GBM cells. Methods: Through the generation of RSK1 and RSK2 knockout as well as double knockout (DKO) GBM cells, we investigated RSK isoform-specific functions in cell signaling, followed by the identification of their distinct transcriptome and translatome. Results: We find that mTORC1 is not activated by RSK isoforms and that eIF4B phosphorylation at S422 is more potently targeted by RSK1 than mTORC1/S6K in GBM cells. Intriguingly, RSK isoforms display differential effects on translation, with RSK1 specifically sustaining translation of a subset of mRNAs upon mTORC1 inhibition. We demonstrate that RSK1 modulates expression in the translatome of mRNAs encoding proteins affecting cell cycle, DNA replication, and repair, while RSK2 impacts mitochondria-related functions. Notably, DKO cells exhibit compounded phenotypes, underscoring the existence of isoform-specific gene regulation. Conclusions: Our findings offer mechanistic insights into the role of RSK in GBMs and provide evidence for a mTORC1-independent and RSK1-dependent translation regulatory program.