Identification of a Novel Intracellular Function of the Secreted Ribonuclease RNASE1 in Inhibiting Gene Expression.

RNASE1 is a ribonuclease secreted by cells and degrades extracellular RNAs. Here, we unexpectedly found that RNASE1, in addition to being secreted, is predominantly localized to the nucleus and functions to inhibit gene expression in human colorectal cancer (CRC) cells. RNASE1 expression is highly cell type-specific and is restricted to well-differentiated CRC cells where its transcription is activated by the pioneer transcription factor FOXA1. Using CRISPR interference utilizing three independent sgRNAs targeting the RNASE1 locus followed by RNA-seq, we found that upon depletion of RNASE1, most of the differentially expressed RNAs are modestly but significantly upregulated suggesting that RNASE1 predominantly functions to inhibit gene expression. In CRC patients, RNASE1 is significantly downregulated and high RNASE1 expression is associated with better patient survival, indicating a potential tumor suppressive function. Consistent with this, RNASE1 depletion results in increased proliferation and clonogenicity indicating that RNASE1 inhibits the growth of CRC cells. Finally, a promising RNASE1 target among the most significantly upregulated mRNAs upon RNASE1 depletion is DKK1 (Dickkopf inhibitor 1) which is upregulated in CRC and negatively regulated by RNASE1. Collectively, this initial characterization of endogenous RNASE1 uncovers a function of RNASE1 in inhibition of gene expression and CRC cell proliferation.