PKC-alpha promotes phosphorylation of KRAS suppressing its oncogenic properties

Abstract

Oncogenic KRAS-driven cancers have long been considered as "undruggable" due to limited therapeutic options. While the recent success of KRAS-G12C inhibitors argues against the "undruggability" of KRAS, this treatment only benefits a small proportion of patients with KRAS mutant cancers, leaving an urgent need for modalities to target other KRAS mutants. KRAS-calmodulin (CaM) signaling axis reportedly regulates the oncogenic properties of KRAS through its C-terminal hypervariable region. Phosphorylation of KRAS by activated protein kinase C (PKC) uncouples KRAS-CaM, resulting in growth inhibition effective against the entire spectrum of KRAS hotspot mutations. However, broadly activating PKC could mediate tumor promoting signaling nodes and cause systemic toxicity, undermining its applicability as an anti-KRAS therapy. Here, we found that prostratin induces KRAS phosphorylation, resulting in an elevated level of active CaM in the cytosol of KRAS mutant cells, and consequentially suppresses their malignancies. A whole-genome wide CRISPR/Cas9 knockout screening, further confirmed by biochemical analysis, revealed that prostratin acts through activating PKC. Functional studies confirmed PKC as the sole kinase to phosphorylate KRAS and, therefore, a KRAS suppressor. Activation of PKC induces senescence in KRAS mutant tumor cells through PTPN14, accompanied by a secretory phenotype contributing to the growth inhibition, and parallelly mediates a nuclear translocation of a CaM-dependent transcription activator, CAMTA-1, which can be a biomarker to indicate the activity of PKC-KRAS-CaM axis. Our findings reveal a previously understudied regulation of KRAS-CaM axis by PKC, which can be an actionable target for developing anti-KRAS therapeutics. One Sentence SummaryThis study deciphers a PKC-led tumor suppressive effect specific to the "undruggable" KRAS-mutant tumor cells through the phosphorylation of KRAS and a consequently altered KRAS-CaM signaling axis.