Fbxw7β resides in the endoplasmic reticulum membrane and protects cells from oxidative stress

Abstract

Oxidative stress has been implicated in cancer initiation and progression. Fbxw7 (also known as Fbw7, SEL‐10, hCdc4, or hAgo) is the F‐box protein subunit of an Skp1–Cul1–F‐box (SCF)‐type ubiquitin ligase complex that plays a central role in the degradation of oncoproteins such as c‐Myc, c‐Jun, Notch, and cyclin E. Fbxw7 is therefore thought to function as a tumor suppressor, and indeed the Fbxw7 gene is frequently mutated in many human malignancies. The Fbxw7 gene locus encodes three protein isoforms: Fbxw7α, Fbxw7β, and Fbxw7γ. Whereas Fbxw7α and Fbxw7γ are resident in the nucleus, Fbxw7β shows a cytoplasmic distribution suggestive of localization to the endoplasmic reticulum (ER). The specific function of Fbxw7β has remained unknown, however. We now show that Fbxw7β contains a putative transmembrane domain near its NH2‐terminus, and topological analysis revealed that Fbxw7β is inserted in the ER membrane. Fbxw7β assembled with Skp1, Cul1, and Rbx1 to form an SCF complex, although the efficiency of this process appeared lower than that for Fbxw7α or Fbxw7γ. To explore the physiological role of Fbxw7β, we generated mice specifically lacking this isoform of Fbxw7. Although these animals did not exhibit any apparent abnormalities in development, primary cultures of neurons prepared from the mutant mice were more vulnerable to oxidative stress than were those prepared from wild‐type mice. Conversely, overexpression of Fbxw7β rendered cells resistant to oxidative stress, without affecting sensitivity to ER stress or other apoptosis‐inducing agents. Our results thus suggest that Fbxw7β contributes to the protection of cells from oxidative stress. (Cancer Sci 2011; 102: 749–755)

Parkin-Dependent Degradation of the F-Box Protein Fbw7β Promotes Neuronal Survival in Response to Oxidative Stress by Stabilizing Mcl-1

“…1A). Fbw7 exists in three splice-variant isoforms, only one of which, Fbw7␤, is cytosolic (53,55). Based on mobility, the Fbw7 isoform associated with parkin is Fbw7␤, as expected.…”

“…The colocalization of Fbw7␤ and Mcl-1, presumably on the mitochondrial outer membrane, is consistent with a role for Fbw7␤ in the regulation of Mcl-1 turnover. It has been previously reported that Fbw7␤ is associated with the endoplasmic reticulum (ER) (55). However, localization experiments were carried out in nonneuronal cells ectopically expressing Fbw7␤.…”

“…Each isoform is encoded by a unique 5= exon that specifies the amino terminus and 10 common 3= exons that determine the rest of the protein. The amino terminus of each isoform specifies its cellular location, with the ␣ isoform being nucleoplasmic (53) but excluded from the nucleolus (35,54), the ␤ isoform being cytosolic (53) and enriched in the endoplasmic reticulum (55), and the ␥ isoform being nucleolar (35,54).…”

Role of the ubiquitin ligase Fbw7 in cancer progression

“…Fbw7β is primarily cytoplasmic protein and most abundant in brain and testis [23]. A recent study showed that Fbw7β contains a putative transmembrane domain in NH2 terminus region, and Fbw7β-deficient cells have higher sensitivity to oxidative stress [25]. Fbw7γ shows a nucleolar distribution and is expressed majorly in heart and skeletal muscle [23].…”

Fbw7α and Fbw7γ Collaborate To Shuttle Cyclin E1 into the Nucleolus for Multiubiquitylation

“…Fbw7␣ and Fbw7␥ have been shown to be nuclear, but with Fbw7␥ being primarily nucleolar and Fbw7␣ being excluded from the nucleolus (5). Fbw7␤ is cytosolic and localizes to membrane compartments, most likely via a predicted transmembrane domain near its amino terminus (25). Since most known targets of Fbw7 are nucleoplasmic, it would be logical that they be targeted by SCF Fbw7␣ .…”