S6 Kinase- and β-TrCP2-Dependent Degradation of p19^Arf Is Required for Cell Proliferation

Abstract

The kinase mTOR (mammalian target of rapamycin) promotes translation as well as cell survival and proliferation under nutrient-rich conditions. Whereas mTOR activates translation through ribosomal protein S6 kinase (S6K) and eukaryotic translation initiation factor 4E-binding protein (4E-BP), how it facilitates cell proliferation has remained unclear. We have now identified p19Arf , an inhibitor of cell cycle progression, as a novel substrate of S6K that is targeted to promote cell proliferation. Serum stimulation induced activation of the mTOR-S6K axis and consequent phosphorylation of p19Arf at Ser 75 . Phosphorylated p19 Arf was then recognized by the F-box protein ␤-TrCP2 and degraded by the proteasome. Ablation of ␤-TrCP2 thus led to the arrest of cell proliferation as a result of the stabilization and accumulation of p19 Arf . The ␤-TrCP2 paralog ␤-TrCP1 had no effect on p19Arf stability, suggesting that phosphorylated p19 Arf is a specific substrate of ␤-TrCP2. Mice deficient in ␤-TrCP2 manifested accumulation of p19Arf in the yolk sac and died in utero. Our results suggest that the mTOR pathway promotes cell proliferation via ␤-TrCP2-dependent p19Arf degradation under nutrient-rich conditions.

The cross talk of two family members of β-TrCP in the regulation of cell autophagy and growth

“…Consistent with our findings that β-TrCP2 silencing inhibits cell growth (Fig. 6c), the proliferation of β-TrCP2 Δ/-MEFs is markedly lower than that of WT MEFs; this decrease can be rescued by β-TrCP2 overexpression, but not by β-TrCP1 overexpression [33]. Collectively, β-TrCP1 and β-TrCP2 play nonredundant roles in substrate recognition as well as in the regulation of cell processes.…”

“…Furthermore, the studies using knockout mice certified the vital role of β-TrCP2 in development. β-TrCP1 −/− mice displayed only minor defects in spermatogenesis [30,31], whereas β-TrCP2 −/− mice died before embryonic day 10.5 [33]. These findings and our study support the notion that β-TrCP2 is the dominant paralog of β-TrCP in governing cell processes.…”

“…Our results showed that β-TrCP2 silencing induces autophagy and senescence and suppresses cell growth and migration, indicating the oncogenic properties of β-TrCP2. Consistently, β-TrCP2 depletion inhibits the proliferation of MEFs [33]. Conversely, β-TrCP2 overexpression accelerates lymphocytic leukemia cell proliferation in vitro and tumorigenesis in vivo [39].…”

“…SK-BR3 cells were transfected with the indicated siRNA oligos for 48 h, transferred to serum-free medium and incubated for 48 h. The expression of senescence-associated β-gal was determined by SA-β-gal staining [33]. For each sample, SA-β-gal-positive and total cell numbers were counted from 5 random microscopic fields.…”

Locations of radical species in black pepper seeds investigated by CW EPR and 9GHz EPR imaging

“…Generally, the 9 GHz EPR imager is one to two-hundred times more sensitive than that of 1 GHz [10]. The sensitivity of the 9 GHz EPR is able to detect paramagnetic species existing in materials such as ascorbic acid in blood [11] and carbon centered organic radicals in foods [1]. …”