Opposing functions of Fbxw7 in keratinocyte growth, differentiation and skin tumorigenesis mediated through negative regulation of c-Myc and Notch.

The tumor suppressor Fbxw7 (also known as Sel-10, hCdc4, hAgo, or Fbw7) is an F-box protein that functions as the substrate-recognition subunit of an SCF ubiquitin ligase complex and targets a group of oncoproteins for degradation. We now show that Fbxw7 regulates the proliferation and differentiation of keratinocytes by mediating the degradation of c-Myc and Notch proteins. Fbxw7-deficient keratinocytes showed an increased proliferative capacity that was dependent on the accumulation of c-Myc but not on that of Notch. Fbxw7 deficiency also resulted in the premature differentiation of keratinocytes in a manner dependent on both c-Myc and Notch. Although Fbxw7-deficient keratinocytes proliferated excessively in vitro, loss of Fbxw7 did not predispose keratinocytes to the formation of squamous cell carcinoma in vivo induced by the expression of oncogenic Ras, possibly because the stem cell population of keratinocytes becomes exhausted as a result of enhanced Notch activity. Indeed, suppression of Notch signaling by additional ablation of RBP-J in Fbxw7-deficient keratinocytes conferred a more aggressive tumorigenic capacity. Collectively, these results indicate that Fbxw7 controls the proliferation and differentiation of keratinocytes, and that it exerts both inhibitory and stimulatory actions in skin carcinogenesis by counteracting the proliferation-promoting effect of c-Myc and the tumor-suppressive effect of Notch, respectively.

Complex regulation of cell-cycle inhibitors by Fbxw7 in mouse embryonic fibroblasts.

The F-box protein Fbxw7 (also known as Fbw7, SEL-10, hCdc4 or hAgo) mediates the ubiquitylation and thereby contributes to the degradation of proteins that positively regulate cell cycle. Conditional ablation of Fbxw7 in mouse embryonic fibroblasts (MEFs) induces cell-cycle arrest accompanied by abnormal accumulation of the intracellular domain of Notch1 (NICD1) and c-Myc. However, the molecular mechanisms by which the accumulation of NICD1 and c-Myc induces cell-cycle arrest have remained unclear. We have now examined the expression of cell-cycle inhibitors in Fbxw7-deficient MEFs and found that the abundance of p27(Kip1) and p57(Kip2) is paradoxically decreased. This phenomenon appears to be attributable to the accumulation of NICD1, given that it was recapitulated by overexpression of NICD1 and blocked by ablation of RBP-J. Conversely, the expression of p16(Ink4a) and p19(ARF) was increased in an NICD1-independent manner in Fbxw7-null MEFs. The increased expression of p19(ARF) was recapitulated by overexpression of c-Myc and abolished by ablation of c-Myc, suggesting that the accumulation of c-Myc is primarily responsible for that of p19(ARF). In contrast, the upregulation of p16(Ink4a) appeared to be independent of c-Myc. These results indicate that cell-cycle inhibitors undergo complex regulation by the Fbxw7-mediated proteolytic system.

Notch-dependent cell cycle arrest and apoptosis in mouse embryonic fibroblasts lacking Fbxw7.

The F-box protein Fbxw7 mediates the ubiquitylation and consequent degradation of proteins that regulate cell cycle progression, including cyclin E, c-Myc, c-Jun and Notch. Moreover, certain human cancer cell lines harbor loss-of-function mutations in FBXW7 that result in excessive accumulation of Fbxw7 substrates, implicating Fbxw7 in tumor suppression. To elucidate the physiological function of Fbxw7, we conditionally ablated Fbxw7 in mouse embryonic fibroblasts (MEFs). Unexpectedly, loss of Fbxw7 induced cell cycle arrest and apoptosis that were accompanied by abnormal accumulation of the intracellular domain of Notch1 (NICD1). Forced expression of NICD1 in wild-type MEFs recapitulated the phenotype of the Fbxw7-deficient (Fbxw7(Delta/Delta)) MEFs. Conversely, deletion of Rbpj normalized the phenotype of Fbxw7(Delta/Delta) MEFs, indicating that this phenotype is dependent on the Notch1-RBP-J signaling pathway. Deletion of the p53 gene prevented cell cycle arrest but not the induction of apoptosis in Fbxw7(Delta/Delta) cells. These observations suggest that Fbxw7 does not function as an oncosuppressor in MEFs. Instead, it promotes cell cycle progression and cell survival through degradation of Notch1, with loss of Fbxw7 resulting in NICD1 accumulation, cell cycle arrest and apoptosis.