Downregulation of rRNA Transcription Triggers Cell Differentiation

Abstract

Responding to various stimuli is indispensable for the maintenance of homeostasis. The downregulation of ribosomal RNA (rRNA) transcription is one of the mechanisms involved in the response to stimuli by various cellular processes, such as cell cycle arrest and apoptosis. Cell differentiation is caused by intra- and extracellular stimuli and is associated with the downregulation of rRNA transcription as well as reduced cell growth. The downregulation of rRNA transcription during differentiation is considered to contribute to reduced cell growth. However, the downregulation of rRNA transcription can induce various cellular processes; therefore, it may positively regulate cell differentiation. To test this possibility, we specifically downregulated rRNA transcription using actinomycin D or a siRNA for Pol I-specific transcription factor IA (TIF-IA) in HL-60 and THP-1 cells, both of which have differentiation potential. The inhibition of rRNA transcription induced cell differentiation in both cell lines, which was demonstrated by the expression of the common differentiation marker CD11b. Furthermore, TIF-IA knockdown in an ex vivo culture of mouse hematopoietic stem cells increased the percentage of myeloid cells and reduced the percentage of immature cells. We also evaluated whether differentiation was induced via the inhibition of cell cycle progression because rRNA transcription is tightly coupled to cell growth. We found that cell cycle arrest without affecting rRNA transcription did not induce differentiation. To the best of our knowledge, our results demonstrate the first time that the downregulation of rRNA levels could be a trigger for the induction of differentiation in mammalian cells. Furthermore, this phenomenon was not simply a reflection of cell cycle arrest. Our results provide a novel insight into the relationship between rRNA transcription and cell differentiation.

Mammary epithelial morphogenesis and early breast cancer. Evidence of involvement of basal components of the RNA Polymerase I transcription machinery

“…To start tackling this hypothesis, we specifically focused on RRN3 because a) RRN3 is one of the Pol I components most prominently amplified/upregulated in both early and invasive breast cancer lesions, and b) it was previously reported that RRN3 expression and activity are critically linked to rRNA synthesis, cell differentiation, and cell proliferation in other cell models. 18,[20][21][22][23] RRN3 knock down in MCF7 breast cancer cells leads to decreased rRNA transcription and inhibition of cell proliferation…”

“…transcriptionally by MYC, 17 post-translationally by ERK-dependent phosphorylation, 18 and by AKT-induced stabilization and translocation to the nucleolus 19 ). There is evidence that RRN3 loss-of-function in different mammalian cell contexts leads to decreased rRNA synthesis, inhibition of cell proliferation, and induction of cell differentiation, 18,20,21 while RRN3 gain-offunction promotes rRNA synthesis and fosters cell proliferation.…”

Inhibition of Pol I transcription treats murine and human AML by targeting the leukemia-initiating cell population

“…Consistent with this concept, recent publications suggested that altered rRNA synthesis is sufficient to trigger differentiation in human AML-derived cancer cell lines, and determine cell fate within the stem cell lineage in Drosophila. [29][30][31] Differentiation therapy is a favorable nongenotoxic treatment option and well established for PML-RARa-driven acute promyelocytic leukemia, although all-trans retinoic acid-induced myeloid differentiation in MLL/AF9 but not MLL/AF4 AML. 32 The complex cellular response to CX-5461 involving cell death, cell-cycle delay, and blast differentiation is likely to explain the superior therapeutic efficacy of CX-5461 over cytotoxic therapy.…”

Emerging roles of nucleolar and ribosomal proteins in cancer, development, and aging

“…These data suggest that proper ribosome biogenesis is a critical regulator for the pluripotency of ES cells and that alteration of ribosome production induces the differentiation of ES cells in a p53-dependent manner. In mouse hematopoietic stem cells, inhibition of rRNA production by actinomycin D or siRNA against the gene encoding TIF-IA, an essential factor for Pol I transcription, decreases pre-rRNA levels and induces cell differentiation [10]. Importantly, inhibition of cell cycle without the inhibition of ribosome biogenesis does not induce cell differentiation, suggesting that changes in ribosome biogenesis positively regulate cell differentiation independently of cell cycle progression.…”

“…Recent studies have shown that nucleoli are actively involved in stem cell maintenance [8][9][10][11]75] and lifespan regulation in model organisms [12][13][14] and play a role in various cellular functions in addition to cancer progression. In this review, we discuss the functions of nucleoli in cancer cell progression, stem cell maintenance, and lifespan regulation, with an emphasis on how nucleoli regulate specific cellular processes rather than the overall cellular homeostasis.…”