Activating mutations in the RAS oncogene are common in cancer but are difficult to therapeutically target. RAS activation promotes autophagy, a highly regulated catabolic process that metabolically buffers cells in response to diverse stresses. Here we report that casein kinase 1α (CK1α), a ubiquitously expressed serine/threonine kinase, is a key negative regulator of oncogenic RAS–induced autophagy. Depletion or pharmacologic inhibition of CK1α enhanced autophagic flux in oncogenic RAS–driven human fibroblasts and multiple cancer cell lines. FOXO3A, a master longevity mediator that transcriptionally regulates diverse autophagy genes, was a critical target of CK1α, as depletion of CK1α reduced levels of phosphorylated FOXO3A and increased expression of FOXO3A-responsive genes. Oncogenic RAS increased CK1α protein abundance via activation of the PI3K/AKT/mTOR pathway. In turn, elevated levels of CK1α increased phosphorylation of nuclear FOXO3A, thereby inhibiting transactivation of genes critical for RAS-induced autophagy. In both RAS-driven cancer cells and murine xenograft models, pharmacologic CK1α inactivation synergized with lysosomotropic agents to inhibit growth and promote tumor cell death. Together, our results identify a kinase feedback loop that influences RAS-dependent autophagy and suggest that targeting CK1α-regulated autophagy offers a potential therapeutic opportunity to treat oncogenic RAS–driven cancers.
Authors
Jit Kong Cheong, Fuquan Zhang, Pei Jou Chua, Boon Huat Bay, Andrew Thorburn, David M. Virshup
Left: Oncogenic RAS, via its downstream PI3K/AKT/mTOR effector pathway, increases CK1α protein abundance to prevent hyperactivation of oncogenic RAS–induced autophagy. CK1α phosphorylates FOXO3A at serine residues 318 and 321 to destabilize FOXO3A, decreasing expression of autophagy genes and limiting autophagic recycling of nutrients. Right: RNAi depletion or pharmacological inhibition of CK1α activity by D4476 stabilizes FOXO3A to elevate basal autophagy and autophagic flux. D4476 sensitizes oncogenic RAS–driven cancer cells to lysosomotropic agents (such as CQ and NH4Cl) via hyperaccumulation of ineffective autophagic vesicles.