Abstract
The tumor suppressor phosphatase and tensin homolog (PTEN) plays a central role in regulating phosphatidylinositol 3-kinase (PI3K) signaling, and its gene is very frequently mutated in various human cancers. Numerous studies have revealed that PTEN levels are tightly regulated by both transcriptional and posttranslational modifications, with especially ubiquitylation significantly regulating PTEN protein levels. Although several ubiquitin ligases have been reported to mediate PTEN ubiquitylation in vitro, the ubiquitin ligase that promotes PTEN degradation in vivo has not been reported. Here we took advantage of specific knockout mouse models to demonstrate that WW domain-containing E3 ubiquitin protein ligase 2 (WWP2) promotes PTEN degradation under physiological conditions, whereas another ubiquitin ligase, carboxyl terminus of Hsp70-interacting protein (CHIP), had no such effect. WWP2 knockout mice exhibited reduced body size, elevated PTEN protein levels, and reduced phosphorylation levels of the serine/threonine kinase and PTEN target AKT. In contrast, we observed no elevation of PTEN protein levels in CHIP knockout tissues and mouse embryonic fibroblasts. Furthermore, PTEN protein levels in CHIP/WWP2 double knockout mice were very similar to those in WWP2 single knockout mice and significantly higher than in WT and CHIP knockout mice. Our results demonstrate that WWP2, rather than CHIP, is an ubiquitin ligase that promotes PTEN degradation in vivo Considering PTEN's significant role in tumor development, we propose that WWP2 may be a potential target for fine-tuning PTEN levels in anticancer therapies.
Highlights
The tumor suppressor phosphatase and tensin homolog (PTEN) plays a central role in regulating phosphatidylinositol 3-kinase (PI3K) signaling, and its gene is very frequently mutated in various human cancers
To further confirm that WWP2, but not carboxyl terminus of Hsp70-interacting protein (CHIP), intrinsically ubiquitylates PTEN and controls its activity in vivo, we examined the ubiquitylation level of PTEN in four types of mouse embryonic fibroblasts (MEFs) and found a remarkable decrease of PTEN ubiquitylation in WWP2Ϫ/Ϫ MEFs and CHIPϪ/ϪWWP2Ϫ/Ϫ MEFs compared with WT and CHIPϪ/Ϫ MEFs (Fig. 7A)
We revealed that WWP2 is an ubiquitin ligase and degrades the tumor suppressor PTEN under physiological conditions
Summary
According to previous studies of the regulation between PTEN and its ubiquitin ligases, deletion of the ubiquitin ligase(s) for PTEN would lead to elevation of PTEN, meaning that the phenotypes of PTEN E3(s) knockout (KO) mice might be similar to that of Super-PTEN mice. Consistent with PTEN Tg mice (Fig. 2, A and B), increased PTEN protein levels and decreased phosphorylated AKT (Ser-473) levels were observed in WWP2Ϫ/Ϫ MEFs (Fig. 2D), whereas the PTEN mRNA level showed no difference between WT and WWP2Ϫ/Ϫ MEFs and multiple tissues (Fig. 2E), indicating that WWP2 deletion does not regulate PTEN expression at the transcriptional level. To further confirm these results under physiological conditions, we analyzed PTEN protein levels in several major organs and tissues. There is the remarkable fact that the p-AKT (Ser-473) level shows a small increase in CHIPϪ/Ϫ MEFs, PTEN remains at normal levels
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