HN1 Functions in Protein Synthesis Regulation via mTOR-RPS6 Axis and Maintains Nucleolar Integrity

Haematological and Neurological Expressed 1 (HN1) is an oncogene for various cancers and previously has been linked with centrosome clustering and cell cycle pathways. Moreover, HN1 has recently been reported to activate mTOR signalling, which is the regulator of ribosome biogenesis and maintenance. We explored the role of HN1 in mTOR signalling through various gain- and loss-of-function experiments using biochemical approaches in different cell lines. We demonstrated for the first time that HN1 is required for nucleolar organiser region (NOR) integrity and function. Immunoprecipitation-based association and colocalization studies demonstrated that HN1 is an important component of the mTOR-RPS6 axis, and its depletion results with reduced mRNA translation in mammalian cancer cell lines. This study also demonstrated that the depletion of HN1 leads to the irregular distribution of nucleolar structures, potentially leading to cell cycle deregulation as reported previously. Accordingly, components of the translation machinery aggregate with a distinct speckled pattern, lose their essential interactions and ultimately impair mRNA translation efficiency when the HN1 is depleted. These results suggest that HN1 is an essential component of the nucleolus, required for ribosome biogenesis as well as global mRNA translation.