Oncogenic RAS-induced CK1\u03b1 drives nuclear FOXO proteolysis

F Zhang,D M Virshup,J K Cheong

doi:10.1038/onc.2017.334

Abstract

Evasion of forkhead box O (FOXO) family of longevity-related transcription factors-mediated growth suppression is necessary to promote cancer development. Since somatic alterations or mutations and transcriptional dysregulation of the FOXO genes are infrequent in human cancers, it remains unclear how these tumour suppressors are eliminated from cancer cells. The protein stability of FOXO3A is regulated by Casein Kinase 1 alpha (CK1α) in an oncogenic RAS-specific manner, but whether this mode of regulation extends to related FOXO family members is unknown. Here we report that CK1α similarly destabilizes FOXO4 in RAS-mutant cells by phosphorylation at serines 265/268. The CK1α-dependent phosphoregulation of FOXO4 is primed, in part, by the PI3K/AKT effector axis of oncogenic RAS signalling. In addition, mutant RAS coordinately elevates proteasome subunit expression and proteolytic activity to eradicate nuclear FOXO4 proteins from RAS-mutant cancer cells. Importantly, dual inhibition of CK1α and the proteasome synergistically inhibited the growth of multiple RAS-mutant human cancer cell lines of diverse tissue origin by blockade of nuclear FOXO4 degradation and induction of caspase-dependent apoptosis. Our findings challenge the current paradigm that nuclear export regulates the proteolysis of FOXO3A/4 tumour suppressors in the context of cancer and illustrates how oncogenic RAS-mediated degradation of FOXOs, via post-translational mechanisms, blocks these important tumour suppressors.

Highlights

We recently reported that oncogenic RAS (K-RASG13D and H-RASG12V), via its phosphoinositide-3 kinase (PI3K)/AKT/mTOR/CK1α effector pathway, downregulates FOXO3A protein abundance in human cancer cells
The forkhead box O (FOXO) family of longevity-promoting transcription factors has been shown to be bona fide tumour suppressors by an increasing number of reports, LOF somatic alterations or point mutations at FOXO gene loci and 371 transcriptional silencing of FOXO gene expression rarely occur in human cancers, regardless of whether aberrant activation of RAS or its downstream effectors exists (Supplementary Figure 1a–d)
We have previously shown that mutant RAS, via its PI3K/AKT/ mTOR effector signalling axis, upregulates the protein abundance of CK1α, leading to phosphorylation-driven destabilization of nuclear FOXO3A in RAS-mutant cancer cells.[29]

Summary

Introduction

The forkhead box O (FOXO) family of longevity-related transcription factors, in particular, FOXO1, FOXO3 and FOXO4, regulates a myriad of cellular processes that include nutrient metabolism,[1,2,3] DNA damage response,[4] oxidative stress response,[5] autophagy,[1,6,7] cell differentiation,[8,9] cell cycle progression[4,10] and cell death.[11,12,13,14,15] cell culture-based molecular and biochemical studies suggest functional redundancy among the FOXO proteins, somatic deletion of the respective FoxOs in mice revealed unique physiological roles of the FoxOs in vivo. The respective FoxO knockout mice exhibit little or no incidence of spontaneous tumours.[17] conditional compound deletion of FoxO1, FoxO3 and FoxO4 in mice resulted in the development of spontaneous lymphomas and hemangiomas, indicating that FOXOs are functionally redundant growth suppressors.[9] FOXO1 and FOXO3 have been recently identified to be targets of recurrent point mutations or homozygous deletions in a subset of human lymphoid neoplasms[20,21] and breast cancers,[22] suggesting that evasion of FOXO-mediated growth suppression is necessary to promote cancer initiation/progression in a subset of tissue types. While mouse knockout studies suggest its importance as a tumour suppressor, whether FOXO4 is altered in a broad range of human cancers is currently unknown

Methods

Results

Conclusion