Regulation of FOXO protein stability via ubiquitination and proteasome degradation

Abstract

Forkhead box O-class (FOXO) proteins are evolutionally conserved transcription factors. They belong to a family of proteins consisting of FOXO1, FOXO3a, FOXO4 and FOXO6 in humans. Increasing evidence suggests that FOXO proteins function as tumor suppressors by transcriptionally regulating expression of genes involved in cell cycle arrest, apoptosis, DNA repair and oxidative stress resistance. Activation of various protein kinases, including Akt, IκB kinase (IKK) and ERK, leads to phosphorylation of FOXO proteins and their ubiquitination mediated by E3 ligases such as SKP2 and MDM2 in human primary tumors and cancer cell lines. As a result, the tumor suppressor functions of FOXO proteins are either diminished or abrogated due to their ubiquitination-proteasome degradation, thereby favoring cell transformation, proliferation and survival. Thus, ubiquitination and proteasome degradation of FOXO proteins play an important role in tumorigenesis and represent a viable target for cancer treatment. This article is part of a Special Issue entitled: PI3K-AKT-FoxO axis in cancer and aging.

Figure 1

Regulation of FOXO proteins by phosphorylation-dependent ubiquitin proteasome degradation pathways. Stimulation of cells by growth factors and insulin results in the activation of Akt and ERK through the Ras- and PI3K-dependent pathways. This in turn leads to phosphorylation of FOXO transcription factors. Akt-mediated phosphorylation promotes the localization of FOXO1 and other FOXO proteins in the cytoplasm, where the substrate-binding F-box protein SKP2 in the SCF^SKP2 E3 ligase binds to and induces ubiquitination and subsequent proteasome degradation of FOXO1. Activation of ERK by the Ras-Raf-MEK cascade also leads to phosphorylation of FOXO proteins, such as FOXO3a. This phosphorylation further facilitates the recognition of FOXO3a by the E3 ligase MDM2, which promotes either monoubiquitination or polyubiquitination in the presence of high levels of MDM2 or other E3 ligases such as SKP2. Monoubiquitination of FOXO proteins such as FOXO4 is subjected to deubiquitination mediated by the deubiquitination enzyme HAUSP/USP7.

Figure 2

Ubiquitination of FOXO1 by the SCF^SKP2 E3 complex. A, a diagram showing the components of the SKP1-CUL1-F-box protein SKP2 (SCF^SKP2) ubiquitin E3 ligase complex. F, F-box. LRR, leucine rich repeats. B, a putative positioning model for the destruction domain in the known substrates of SCF^β-TRCP and SCF^Skp2 complex E3 ligases. One prediction of this positioning model is that ubiquitin (Ub) accepting lysine residues are usually located 9–14 amino acids upstream of the phosphodegron site(s) in the substrates of the SCF E3 ligases.