The Ribosome Biogenesis-Cancer Connection

Abstract

Multifaceted relations link ribosome biogenesis to cancer. Ribosome biogenesis takes place in the nucleolus. Clarifying the mechanisms involved in this nucleolar function and its relationship with cell proliferation: 1) allowed the understanding of the reasons for the nucleolar changes in cancer cells and their exploitation in tumor pathology, 2) defined the importance of the inhibition of ribosome biogenesis in cancer chemotherapy and 3) focused the attention on alterations of ribosome biogenesis in the pathogenesis of cancer. This review summarizes the research milestones regarding these relevant relationships between ribosome biogenesis and cancer. The structure and function of the nucleolus will also be briefly described.

Keywords: AgNORs; cancer chemotherapy; chronic inflammatory diseases; nucleolus; p53; ribosomal proteins; ribosome biogenesis; ribosomopathies; tumour pathology; tumourigenesis.

Figure 1

Nucleolar ultrastructure and location of the silver-stained Nucleolar Organizer Region (NOR) proteins in the fibrillar components; (A) electron microscope visualization of a nucleolus of a TG cell (established cell line from a human fallopian tube cancer) after uranium and lead staining. Arrows indicate fibrillar centres with the peripheral rim of dense fibrillar component (f). The granular component (g) surrounds the fibrillar components. Bar = 0.5 µm; (B) Electron microscope visualization of a TG cell nucleolus after specific silver staining for NOR proteins. Only the fibrillar centres and the associated dense fibrillar component are stained by silver. Bar = 0.5 µm.

Figure 2

The AgNORs are the sites where rRNA synthesis takes place in the nucleolus; (A) light microscopy visualization of a U2OS cell (established cell line from a human osteosarcoma) after specific silver staining for NOR proteins. The silver-stained structures appear as well-defined, darkly stained dots clustered in the three nucleoli. (B) Visualization of rRNA synthesis in a U2OS cell, labelled with 5-fluorouridine (FUrd) for 15 min. FUrd was revealed by specific FITCH-conjugated monoclonal antibodies. DAPI counter-staining. Note the spot-like distribution of the sites of the nucleolar rRNA transcription, which overlaps that of the silver-stained dots in Figure 4. Bar = 10 µm.

Figure 3

Histological sections from two cases of infiltrating ductal carcinomas after selective silver staining for NOR proteins; note the different quantitative distribution of the silver-stained structures in (A) (characterized by a high proliferation index, mutated p53 and RB loss) and (B) (characterized by a low proliferation index, wild type p53 and normal RB). Bar, 10 µm.

Figure 4

Chronic inflammatory diseases and nucleolar hypertrophy; (A) histological sections from normal human colon and (B) from human colon with ulcerative colitis (UC), after selective silver staining for NOR proteins. The size of nucleoli of the epithelial cells lining the surface of the normal mucosa is smaller than that of the epithelial cell nucleoli in UC. Bar, 10 µm. (C) Histological sections from normal human pancreas and (D) with chronic pancreatitis, after selective silver staining for NOR proteins (courtesy of Dr. M. Macchini, San Raffaele Scientific Institute, Department of Medical Oncology, Milan, Italy). Note the larger size of nucleoli of acinar cells in chronic pancreatitis than in normal pancreatic tissue. Bar, 10 µm; (E) histologic section from a human hepatitis B virus-related cirrhosis after selective silver staining for NOR proteins. Red arrows indicate hepatocytes with hypertrophic nucleoli, which are close to chronic inflammatory infiltrate. Green arrows indicate hepatocytes with normal-sized nucleoli. Bar, 10 µm.

Figure 5

Schematic representation of the RPs-MDM2-p53 pathway showing the relationship between ribosome biogenesis rate and the level of p53 stabilization; (A) in normal conditions, the p53 level within the cell is maintained low because p53 is a short-lived protein that is rapidly degraded by murine double minute 2 (MDM2) and HDM2 in humans. In fact, MDM2 acts as an E3 ubiquitin ligase facilitating p53 proteasomal degradation [75,76,77]. Only a limited amount of MDM2 is inhibited by RPs not used for ribosome biogenesis. Among the RPs binding to MDM2, RPL11-uL5 and RPL5-uL18 play a major role in MDM2 inactivation [41,42,43,49] by forming a complex with 5S rRNA, all the components of the complex being necessary for its inhibitory function [50,51]; (B) perturbations of ribosome biogenesis (for example those caused by the treatment of rRNA transcription or processing inhibitors) are responsible for the fact that rRNAs are no longer available for ribosome construction. Then a greater amount of ribosomal proteins (RPs) is left free bind to MDM2 thus relieving its inhibitory activity toward p53 and allowing p53 to accumulate within the cell nucleus (reviewed in [44,47]); (C) On the contrary, when an up-regulation of rRNA synthesis occurs, a larger amount of RPs are used for ribosome building, being no longer available for MDM2 inactivation. Consequently, the level of p53 is reduced as result of a greater portion of MDM2 which is left free to induce p53 proteasomal degradation [74].

Figure 6

How qualitative alterations of ribosome biogenesis in ribosomopathies may lead to cancer; conditions qualitatively altering ribosome biogenesis include alterations in the rRNA modifying/processing factors and mutations in genes encoding for ribosomal proteins. These conditions (green arrows) can give rise both to a reduced ribosome biogenesis and to the formation of altered ribosomal subunits. Deregulated ribosome biogenesis activates the RP-MDM2-p53 pathway with consequent sustained p53 stabilization. Cells bypass this activation by p53 loss of function. The formation of altered ribosomes leads to unbalanced translation of tumour contrasting and promoting factors. The combined effect of the two different mechanisms would favor cancer onset (red arrows).