Protein B23 (M.W./pI = 37 kD/5.1) is the only major protein extracted from HeLa nucleoli with 3M urea

Mutual dependency between lncRNA LETN and protein NPM1 in controlling the nucleolar structure and functions sustaining cell proliferation

Fundamental processes such as ribosomal RNA synthesis and chromatin remodeling take place in the nucleolus, which is hyperactive in fast-proliferating cells. The sophisticated regulatory mechanism underlying the dynamic nucleolar structure and functions is yet to be fully explored. The present study uncovers the mutual functional dependency between a previously uncharacterized human long non-coding RNA, which we renamed LETN, and a key nucleolar protein, NPM1. Specifically, being upregulated in multiple types of cancer, LETN resides in the nucleolus via direct binding with NPM1. LETN plays a critical role in facilitating the formation of NPM1 pentamers, which are essential building blocks of the nucleolar granular component and control the nucleolar functions. Repression of LETN or NPM1 led to similar and profound changes of the nucleolar morphology and arrest of the nucleolar functions, which led to proliferation inhibition of human cancer cells and neural progenitor cells. Interestingly, this inter-dependency between LETN and NPM1 is associated with the evolutionarily new variations of NPM1 and the coincidental emergence of LETN in higher primates. We propose that this human-specific protein-lncRNA axis renders an additional yet critical layer of regulation with high physiological relevance in both cancerous and normal developmental processes that require hyperactive nucleoli.

Nucleolar Stress: hallmarks, sensing mechanism and diseases

The nucleolus is a prominent subnuclear compartment, where ribosome biosynthesis takes place. Recently, the nucleolus has gained attention for its novel role in the regulation of cellular stress. Nucleolar stress is emerging as a new concept, which is characterized by diverse cellular insult-induced abnormalities in nucleolar structure and function, ultimately leading to activation of p53 or other stress signaling pathways and alterations in cell behavior. Despite a number of comprehensive reviews on this concept, straightforward and clear-cut way criteria for a nucleolar stress state, regarding the factors that elicit this state, the morphological and functional alterations as well as the rationale for p53 activation are still missing. Based on literature of the past two decades, we herein summarize the evolution of the concept and provide hallmarks of nucleolar stress. Along with updated information and thorough discussion of existing confusions in the field, we pay particular attention to the current understanding of the sensing mechanisms, i.e., how stress is integrated by p53. In addition, we propose our own emphasis regarding the role of nucleolar protein NPM1 in the hallmarks of nucleolar stress and sensing mechanisms. Finally, the links of nucleolar stress to human diseases are briefly and selectively introduced.

NPM1/B23: A Multifunctional Chaperone in Ribosome Biogenesis and Chromatin Remodeling

At a first glance, ribosome biogenesis and chromatin remodeling are quite different processes, but they share a common problem involving interactions between charged nucleic acids and small basic proteins that may result in unwanted intracellular aggregations. The multifunctional nuclear acidic chaperone NPM1 (B23/nucleophosmin) is active in several stages of ribosome biogenesis, chromatin remodeling, and mitosis as well as in DNA repair, replication and transcription. In addition, NPM1 plays an important role in the Myc-ARF-p53 pathway as well as in SUMO regulation. However, the relative importance of NPM1 in these processes remains unclear. Provided herein is an update on the expanding list of the diverse activities and interacting partners of NPM1. Mechanisms of NPM1 nuclear export functions of NPM1 in the nucleolus and at the mitotic spindle are discussed in relation to tumor development. It is argued that the suggested function of NPM1 as a histone chaperone could explain several, but not all, of the effects observed in cells following changes in NPM1 expression. A future challenge is to understand how NPM1 is activated, recruited, and controlled to carry out its functions.

Role of protein phosphorylation in post-translational regulation of protein B23 during programmed cell death in the prostate gland

Protein B23 is a nucleolar and nuclear matrix-associated phosphoprotein that is involved in ribosome synthesis. Its expression and phosphorylation in rat ventral prostate, an androgen target organ, are profoundly influenced by androgens. Induction of programmed cell death (apoptosis) in the prostatic epithelium by androgen deprivation in the animal induces an early decline in protein B23 in the absence of a corresponding loss of protein B23 mRNA. We have now demonstrated that prostatic nuclei retain the ability to transcribe the B23 mRNA and that a significant amount of this mRNA persists even after 7 days of androgen deprivation when > 80% of the prostatic epithelial cells have undergone apoptosis. The B23 mRNA from these nuclei is also translatable in vitro. However, the majority of the B23 mRNA is associated with free and short-stretch polysomes, which may account for the castration-induced decline in synthesis of protein B23 in vivo. In addition, the mechanism of down-regulation of protein B23 in apoptotic prostatic cells appears to relate to two coordinate signals, which include loss of phosphorylation of the protein as well as the expression of a protease active toward dephosphorylated protein B23, under these conditions.