TGFβ prevents proteasomal degradation of the cyclin-dependent kinase inhibitor p27kip1 for cell cycle arrest

Abstract

TGFβ mediates cell cycle arrest in late G 1 phase of the cell cycle with a simultaneous peak in the levels of the cyclin-dependent kinase inhibitor, p27 kip1 (p27). In this report, we show that whereas p27 resides in the cytoplasm in the endometrial carcinoma (ECA) cell line HEC-1A, TGFβ increases the total levels and translocation of p27 into the nucleus. Concomitantly, TGFβ activates the transcription factors Smad2 and Smad3, inhibits proliferation, and blocks Cdk2 activity; all these events are blocked by an inhibitor of TβRI serine kinase activity (SD208). In addition, we show that inhibiting p27 transcription with a specific siRNA completely blocks TGFβ-mediated growth inhibition in these cells. These data suggest that TGFβ inhibits cellular proliferation by increasing p27 levels through Smad2/3 signaling in HEC-1A cells. We further show that TGFβ decreases the levels of components of the SCF Skp2 targeting complex for ubiquitin-mediated degradation of p27 in proteasomes, at the protein but not the mRNA level. Therefore, TGFβ accumulates nuclear p27 by preventing its degradation to enable G 1 arrest in HEC-1A cells. Importantly, these data suggest a novel mechanism for TGFβ/Smad mediated growth inhibition that might be inoperable in the numerous human cancers demonstrating early dysregulated TGFβ signaling and loss of growth inhibition. The TGFβ/p27 axis might provide novel therapeutic targets for cancer.

AZGP1 is a tumor suppressor in pancreatic cancer inducing mesenchymal-to-epithelial transdifferentiation by inhibiting TGF-β-mediated ERK signaling

“…In line with our findings above, BMP4 and CDKN1B were increased, which reflected the activation of endogenous TGF-b signaling (Lecanda et al, 2009 Figures 5A and B), which is consistent with published results (Ellenrieder et al, 2001;Hamada et al, 2007). Furthermore, BCL2L1, CCND1 and MYC, which are survival pathway-related genes, were upregulated underscoring the importance of prosurvival signals during EMT (Biliran et al, 2005;Freemantle et al, 2007;Barbie et al, 2009).…”

SUMOylation regulates p27Kip1stability and localization in response to TGFβ

“…In the first context, p27 expression is essential to induce G1 arrest in response to TGFb treatment. Silencing of p27 in TGFb-responsive breast or endometrial cancer cells is sufficient to overcome the G1 arrest imposed by TGFb treatment (Donovan et al, 2002;Lecanda et al, 2009). These evidences are in complete accord with the notion that reduction of p27 levels may significantly contribute to loss of normal responsiveness to growth inhibitory stimuli during cancer progression, as demonstrated in several in vitro and in vivo models .…”

“…These evidences are in complete accord with the notion that reduction of p27 levels may significantly contribute to loss of normal responsiveness to growth inhibitory stimuli during cancer progression, as demonstrated in several in vitro and in vivo models . TGFb induces cell cycle arrest by both increasing p27 protein levels and favoring its nuclear accumulation (Lecanda et al, 2007(Lecanda et al, , 2009. Despite extensive evidences supporting that p27 is a key downstream molecule determining the fate of TGFb-responsive cells, the precise molecular mechanisms whereby TGFb regulates both p27 stability and nuclear accumulation are only poorly understood.…”

Transforming Growth Factor-β Regulates the Growth of Valve Interstitial Cells in Vitro

“…As expected, there are concomitant decreases in the expression of both cyclin D1 and CDK4. P27 is an inhibitor of the kinase complex, [47][48][49][50] and its expression is markedly up-regulated. Collectively, our data suggest that the control VICs are able to enter the S phase because of the presence of phosphorylated and inactive pRb, which releases the transcription factor E2F to stimulate the production of DNA replication proteins.…”