KRAS phosphorylation at Ser181 promotes colorectal cancer invasion via regulation of cathepsin B secretion

Oncogenic KRAS mutations are highly prevalent in colorectal cancer. Phosphorylation of KRAS at Ser181 regulates several of its oncogenic properties. Using CRISPR-edited SW480 cells expressing either the phosphorylatable (S181) or non-phosphorylatable (S181A) forms of oncogenic KRAS, we demonstrate that this post-translational modification modulates the invasive and metastatic capacity of the cells. To elucidate the underlying mechanism, we analyzed the secretome composition and identified 290 differentially secreted proteins. Notably, secretion of lysosomal proteins was specifically reduced in S181A cells. Among these, cathepsin B, a protein associated with invasiveness, was the most abundantly secreted in S181 cells. Protein kinase C (PKC) activity modulated cathepsin B secretion in S181 cells but had a diminished effect in S181A cells. Intracellular levels of cathepsin B and lysosomes were not significantly affected, indicating that secretion is the step regulated by KRAS phosphorylation. Cathepsin B secretion was also dependent on PI3K activity, which was lower in S181A cells, suggesting that KRAS phosphorylation facilitates cathepsin B secretion via PI3K activation. These findings indicate that KRAS phosphorylation by PKC regulates the invasive and metastatic potential of colorectal cancer cells by controlling secretion of lysosomal proteins, particularly cathepsin B.