ABSTRACT: The nucleolus is a dynamic structure where ribosome subunits are produced. Indeed, nucleoli respond to any change in cellular homeostasis by altering the rate of ribosome biogenesis, thus working as a stress sensor. Consequently, imbalances in ribosome biogenesis promotes changes in morphology and function and can evoke a nucleolar stress and DNA damage responses. These changes in nucleolar architecture and composition, culminating in impaired ribosome biogenesis, prompt the emergence of the nucleolar stress, which manifests as a contributing factor in aging and cancer. Here, we illuminate the pivotal role that the RNA binding protein Hnrnpk plays in orchestrating nucleolar dynamics and sustaining ribosome function. As a ribonucleoprotein, Hnrnpk governs the chaperoning of nascent transcripts to facilitate their processing and subsequent nuclear export for ribosomal integration. When overexpressed, Hnrnpk induces disruptive alterations in nucleolar structure, resulting in stress-like phenotypes; such as unbalances of molecular components, accumulation and delocalization of nucleolin, and decrease expression and occupation of fibrillarin. Consequentially, these aberrations in nucleolar equilibrium engenders a disruption in ribosome biogenesis and concomitantly halts the protein translation machinery. Nucleolin (Ncl) haploinsufficiency is correlated with enlarged nucleoli, increased ribosome components, heightened translational rates translation and a concomitant decrease in lifespan. Thus, an Hnrnpk overexpression-dependent surge of Ncl levels stemming may instigate a decline in nucleolar integrity and perturbations in ribosome biogenesis—an event intimately associated with ribosomopathies and the ensuing syndrome of bone marrow failure syndrome. Intersting, the aging process shares a number of common biological hallmarks with bone marrow failure. Again, alterations in Hnrnpk expression, specifically overexpression triggers nucleolar stress, driving cell cycle arrest and senescence of the cells. Our investigations demonstrate that p53, c-Myc and Ncl haploinsufficiency rescue these Hnrnpk-mediated phenotypes. Collectively, the data begin to decipher novel signaling pathway involved in nucleolar stress - ribosome stress – and p53 driven cell cycle arrest that governs this process. Together, these findings support the idea that nucleolar disturbances contribute to ribosome impairment, thus catalyzing the genesis of hematopoietic anomalies and aging process. Significantly, this study elucidates Hnrnpk as a previously unrecognized master regulator within these intricate ribosomal mechanisms; providing a novel window to view the orchestration of nucleolar integrity, ribosome function, and cellular senescence.
