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Supplementary Figure 1 from Degradation of Tob1 Mediated by SCFSkp2-Dependent Ubiquitination

The cyclin-dependent kinase inhibitor p27Kip1 is degraded in late G1 phase by the ubiquitin-proteasome pathway, allowing cells to enter S phase. Due to accelerated degradation of p27Kip1, various human cancers express low levels of p27Kip1 associated with poor prognosis. S-phase kinase–associated protein 2, the F-box protein component of an SCF ubiquitin ligase complex, is implicated in degradation of p27Kip1 during S-G2 phases. Recently, Kip1 ubiquitination–promoting complex has been reported as another ubiquitin ligase that targets cytoplasmic p27Kip1 exported from the nucleus in G0-G1 phases. Here, we identified a RING-H2–type ubiquitin ligase, Pirh2, as a p27Kip1-interacting protein. Endogenous Pirh2 physically interacted with endogenous p27Kip1 in mammalian cells. Pirh2 directly ubiquitinated p27Kip1 in an intact RING finger domain-dependent manner in vivo, as well as in vitro. Ablation of endogenous Pirh2 by small interfering RNA increased the steady-state level of p27Kip1 and decelerated p27Kip1 turnover. Depletion of Pirh2 induced accumulation of p27Kip1 in both the nucleus and cytoplasm. Pirh2 expression was induced from late G1-S phase, whereas p27Kip1 was decreased in synchronization with accumulation of Pirh2. Furthermore, reduction of Pirh2 resulted in an impairment of p27Kip1 degradation and an inhibition of cell cycle progression at G1-S transition in a p53-independent manner. Overall, the results indicate that Pirh2 acts as a negative regulator of p27Kip1 function by promoting ubiquitin-dependent proteasomal degradation. [Cancer Res 2007;67(22):10789–95]

Supplementary Figures 1-6 from Up-regulation of GPR48 Induced by Down-regulation of p27Kip1 Enhances Carcinoma Cell Invasiveness and Metastasis

Supplementary Figure 1 from Pirh2 Promotes Ubiquitin-Dependent Degradation of the Cyclin-Dependent Kinase Inhibitor p27Kip1

Tob1, a member of the Tob/BTG family, is involved in the control of G1-S progression by suppressing cyclin D1 expression and acts as a tumor suppressor gene. Tob1 was reported to have a quick turnover through the ubiquitin-proteasome pathway, but proteins involved in this process are still unknown. We showed that Skp2, a substrate-targeting subunit of the SCF (Skp1/Cul1/F-box protein) ubiquitin ligase complex, was involved in ubiquitin-dependent degradation of Tob1. Skp2 interacted with Tob1 and facilitated ubiquitination of Tob1 in intact cells as well as in vitro. Skp2 mutants without the F-box or leucine rich repeat were not able to bind to Tob1 and did not enhance ubiquitination of Tob1. Tob1 was stabilized in both Skp2−/− mouse fibroblasts and Skp2 knockdown HeLa cells. Moreover, cyclin D1 expression was suppressed in Skp2 knockdown HeLa cells. These data suggest that Tob1 is a novel target for degradation by the SCF-Skp2 ubiquitin ligase. (Cancer Res 2006; 66(17): 8477-83)

A reduced expression level of the cyclin-dependent kinase inhibitor p27Kip1 is associated with increased tumor malignancy and poor prognosis in individuals with various types of cancer. To investigate the basis for this relation, we applied microarray analysis to screen for genes differentially expressed between p27+/− and parental (p27+/+) HCT116 human colon carcinoma cells. Expression of the gene for G protein–coupled receptor 48 (GPR48) was increased in the p27+/− cells. Forced expression of GPR48 increased both in vitro invasive activity and lung metastasis potency of HCT116 cells. In contrast, depletion of endogenous GPR48 by RNA interference reduced the invasive potential of HeLa and Lewis lung carcinoma cells not only in vitro but also in vivo. Moreover, GPR48 expression was significantly associated with lymph node metastasis and inversely correlated with p27 expression in human colon carcinomas. GPR48 may thus play an important role in invasiveness and metastasis of carcinoma and might therefore represent a potential prognostic marker or therapeutic target. (Cancer Res 2006; 66(24): 11623-31)

Supplementary Figure 2 from Degradation of Tob1 Mediated by SCFSkp2-Dependent Ubiquitination

Terrestrial plants respond to and resist gravitational force. The response is termed "gravity resistance", and centrifugal hypergravity conditions are efficient for investigating its nature and mechanism. A functional screening of Arabidopsis T-DNA insertion lines for the suppression rate of elongation growth of hypocotyls under hypergravity conditions was performed in this study to identify the genes required for gravity resistance. As a result, we identified