FBXO7 triggers caspase 8-mediated proteolysis of the transcription factor FOXO4 and exacerbates neuronal cytotoxicity

Abstract

Parkinson’s disease (PD) is characterized by the progressive loss of midbrain dopamine neurons in the substantia nigra. Mutations in the F-box only protein 7 gene ( Fbxo7 ) have been reported to cause an autosomal recessive form of early-onset familial PD. FBXO7 is a part of the SKP1-Cullin1-F-box (SCF) E3 ubiquitin ligase complex, which mediates ubiquitination of numerous substrates. FBXO7 also regulates mitophagy, cell growth, and proteasome activity. A member of the FOXO family, the transcription factor FOXO4, is also known to modulate several cellular responses, including cell cycle progression and apoptosis; however, the relationship between FBXO7 and FOXO4 has not been investigated. In this study, we determined that FBXO7 binds to FOXO4 and negatively regulates intracellular FOXO4 levels. Interestingly, we also found that FBXO7-mediated degradation of FOXO4 did not occur through either of two major proteolysis systems, the ubiquitin-proteasome system or the lysosome-autophagy pathway, although it was blocked by a caspase 8-specific inhibitor and caspase 8 -knockdown. Moreover, intracellular FOXO4 levels were greatly reduced in dopaminergic MN9D cells following treatment with neurotoxic 6-hydroxydopamine (6-OHDA), which was produced upon FBXO7-mediated and caspase 8-mediated proteolysis. Taken together, these results suggest that FOXO4 is negatively regulated in FBXO7-linked PD through caspase 8 activation, suppressing the cytoprotective effect of FOXO4 during 6-OHDA-induced neuronal cell death.

E3 ligase adaptor FBXO7 contributes to ubiquitination and proteasomal degradation of SIRT7 and promotes cell death in response to hydrogen peroxide

“…Based on our previous finding that FBXO7 exacerbates 6-hydroxydopamine (6-OHDA)-induced neuronal cell death by interacting with aging-related FOXO4 and promoting its degradation ( 15 ), we speculated that FBXO7 may also be physically and/or functionally related to sirtuins, another family of crucial aging-related genes. To investigate whether this hypothesis is valid, we first checked whether FBXO7 affects the levels of the seven sirtuins when they were coexpressed.…”

“…Since several studies have reported that both FBXO7 and SIRT7 act as regulators of the stress response ( 15 , 21 , 33 , 34 , 35 ), we investigated whether FBXO7-induced SIRT7 degradation affects cell viability in response to various stress inducers. First, SH-SY5Y cells were treated with various neurotoxic stimuli, including MPP + , 6-OHDA, rotenone, staurosporine, and H 2 O 2 , which cause neuronal cell death.…”

“…FBXO7, a component of the SCF ubiquitin E3 ligase complex, has dual functions, including SCF-dependent canonical and SCF-independent noncanonical activity ( 8 ). As an example of its noncanonical function, we previously reported that FBXO7 facilitates the proteolysis of aging-related FOXO4 via caspase-8 activation in mammalian cells ( 15 ). This finding was further supported by reports that Nutcracker, the Drosophila ortholog of FBXO7, activates caspase, stimulating the proteasome during sperm differentiation in Drosophila ( 43 , 44 ).…”

“…In SCF-independent roles, FBXO7 regulates proteasome activity, cell cycle progression, and mitophagy ( 12 , 13 , 14 ). In addition, we recently revealed that FBXO7 interacts with caspase-8 to cause its activation, leading to degradation of the aging-related transcription factor FOXO4 in mammalian cells ( 15 ). In a standard SCF-dependent manner, FBXO7 interacts with several substrates and mediates their ubiquitination, thus playing a role in various cellular processes, including cell cycle regulation and mitophagy ( 16 , 17 , 18 , 19 , 20 ).…”

Fbxo7 promotes Cdk6 activity to inhibit PFKP and glycolysis in T cells

“…One possibility is that proteasome inhibition activates a pathway that ultimately targets Fbxo7 for degradation, but only a subtle increase in LC3II is seen upon proteasome inhibition, suggesting autophagy was not greatly enhanced upon MG132 treatment. An alternate possibility is that Fbxo7 has recently been shown to bind and activate a degradative caspase pathway in neuroblastoma and HEK293T cells ( Lee et al, 2021 ). Whether in specialized cell types, variations in glucose concentrations engage different degradative pathways for Fbxo7 with differential effects on glucose metabolism is an area for future investigation.…”