Acute degradation of nucleolin reveals its novel functions in cell cycle progression and cell division in triple negative breast cancer.

Introduction: Nucleoli are large nuclear sub-compartments where vital processes, such as ribosome assembly, take place. Most nucleolar proteins are essential; thus, their abrogation cannot be achieved through conventional approaches. This technical obstacle has limited our understanding of the biological functions of nucleolar proteins in cell homeostasis and cancer pathogenesis.

Methods: We applied the Auxin Inducible Degron (AID) proteolytic system, paired with CRISPR/Cas9 knock-in gene-editing, to obtain an unprecedented characterization of the biological activities of Nucleolin (NCL), one of the most abundant nucleolar proteins, in Triple Negative Breast Cancer (TNBC) cells. Then, we combined live-cell imaging, RNA-sequencing, and quantitative proteomics, to characterize the impact of NCL acute abrogation on the behavior of TNBC cells. Finally, we used in silico analyses to validate NCL molecular role in TNBC patients.

Results: Acute abrogation of endogenous NCL impacted both the transcriptome and the proteome of TNBC cells, particularly affecting critical players involved in ribosome biogenesis and in cell cycle progression. Unexpectedly, NCL depletion limited cancer cell ability to effectively complete cytokinesis, ultimately leading to the accumulation of bi-nucleated cells. In silico analyses confirmed that the levels of regulators of cell cycle progression and chromosome segregation correlated with NCL abundance in TNBC patients. Finally, NCL degradation enhanced the activity of pharmaceutical inhibitors of cellular mitosis, such as the Anaphase Promoting Complex inhibitor APCin.

Conclusions: Our findings indicate a novel role for NCL in supporting the completion of the cell division in TNBC models, and that its abrogation could enhance the therapeutic activity of mitotic-progression inhibitors.