ING1 regulates rRNA levels by altering nucleolar chromatin structure and mTOR localization

Area of Science:

• Molecular Biology
• Cell Biology
• Epigenetics

Background:

• The nucleolus is a key site for ribosome biogenesis, regulating rRNA transcription and cell growth.
• Epigenetic modifications and signaling pathways within the nucleolus are critical for controlling these processes.

Purpose of the Study:

• To investigate the role of the tumor suppressor ING1b in regulating nucleolar function, specifically rRNA transcription and its impact on cell growth.
• To elucidate the molecular mechanisms by which ING1b controls rDNA chromatin and associated signaling pathways.

Main Methods:

• Chromatin immunoprecipitation (ChIP) to assess histone modifications and protein binding at rDNA loci.
• Western blotting and co-immunoprecipitation to analyze protein interactions and signaling pathways (e.g., mTOR).
• RNA quantification to measure rRNA levels and transcriptional activity.
• Cellular differentiation assays (e.g., THP1 cell differentiation).

Main Results:

• ING1b binds to rDNA and recruits HDAC1, leading to deacetylation of H3K9 and H3K27 marks, thereby repressing rDNA transcription.
• Loss of ING1b enhances the nucleolar localization and association of phospho-mTOR with its regulatory partners, even under rapamycin treatment.
• ING1b inhibits rDNA transcription by reducing UBF activity and its interaction with mTOR.
• ING1b-mediated regulation of rDNA heterochromatin and rRNA synthesis is observed during normal growth, cell stress, and PMA-induced THP1 cell differentiation.

Conclusions:

• ING1b acts as a regulator of the nucleolar epigenome and rDNA transcription.
• ING1b modulates rRNA levels by affecting mTOR signaling and UBF activity.
• These findings suggest that ING1b's tumor suppressor function is linked to its control over protein synthesis via nucleolar regulation.