Faculty Bibliography

DEPTOR is a recently identified inhibitor of the mTOR kinase that is highly regulated at the posttranslational level. In response to mitogens, we found that DEPTOR was rapidly phosphorylated on three serines in a conserved degron, facilitating binding and ubiquitylation by the F box protein betaTrCP, with consequent proteasomal degradation of DEPTOR. Phosphorylation of the betaTrCP degron in DEPTOR is executed by CK1alpha after a priming phosphorylation event mediated by either the mTORC1 or mTORC2 complexes. Blocking the betaTrCP-dependent degradation of DEPTOR via betaTrCP knockdown or expression of a stable DEPTOR mutant that is unable to bind betaTrCP results in mTOR inhibition. Our findings reveal that mTOR cooperates with CK1alpha and betaTrCP to generate an auto-amplification loop to promote its own full activation. Moreover, our results suggest that pharmacologic inhibition of CK1 may be a viable therapeutic option for the treatment of cancers characterized by activation of mTOR-signaling pathways

Regulatory mechanisms to prevent centriole overduplication during the cell cycle are not completely understood. In this issue, FBXW5 is shown to control the degradation of the centriole assembly factor HsSAS-6. Moreover, the study proposes that FBXW5 is a substrate of both PLK4 and APC/C, two established regulators of centriole duplication

Targeted protein destruction of critical cellular regulators during the G1 phase of the cell cycle is achieved by anaphase-promoting complex/cyclosome(Cdh1) (APC/C(Cdh1)), a multisubunit E3 ubiquitin ligase. Cells lacking Cdh1 have been shown to accumulate deoxyribonucleic acid (DNA) damage, suggesting that it may play a previously unrecognized role in maintaining genomic stability. The ubiquitin-specific protease 1 (USP1) is a known critical regulator of DNA repair and genomic stability. In this paper, we report that USP1 was degraded in G1 via APC/C(Cdh1). USP1 levels were kept low in G1 to provide a permissive condition for inducing proliferating cell nuclear antigen (PCNA) monoubiquitination in response to ultraviolet (UV) damage before DNA replication. Importantly, expression of a USP1 mutant that cannot be degraded via APC/C(Cdh1) inhibited PCNA monoubiquitination during G1, likely compromising the recruitment of trans-lesion synthesis polymerase to UV repair sites. Thus, we propose a role for APC/C(Cdh1) in modulating the status of PCNA monoubiquitination and UV DNA repair before S phase entry

BACKGROUND: In several human cancers, overexpression of Skp2 (S-phase kinase associated protein 2), which targets p27 for degradation, portends a poorer prognosis. We examined whether Skp2 overexpression is associated with recurrence following radical prostatectomy (RP) for prostate cancer. METHODS: Immunohistochemical staining for Skp2, p27, and MIB-1 was performed on 109 men with node-negative prostate cancer surgically managed from 1985-1996. Associations between the stains were tested and Cox regression was used to determine the association between Skp2 expression and time to biochemical recurrence following RP. RESULTS: The 12 tumors (11%) with Skp2 overexpression all had correspondingly low p27 expression (P=0.006), and a similar inverse Skp2/p27 relationship was seen in vitro in LNCap cells. Skp2 overexpression in tissue was associated with higher Gleason score (P=0.002), more advanced pathological stage (P=0.01), and higher MIB-1 index (P=0.03), but a more favorable PSA profile (P=0.04). Five men received a TURP. Among 104 who received RP, median follow-up was 67 months (range: 0.2-218). After adjusting for PSA, pathologic stage, and Gleason score, Skp2 overexpression remained significantly associated with a shorter time to biochemical recurrence (adjusted hazard ratio 4.8 (95% C.I. 1.6-14, P=0.004)). The median time to recurrence with high vs. low Skp2 was 4 vs. 54 months. CONCLUSIONS: Skp2 overexpression was seen in a significant minority of surgically-managed men and was independently associated with a higher risk of recurrence, raising the possibility that Skp2 could be useful as a prognostic biomarker and as a potential molecular target for novel systemic agents in prostate cancer.

In this study, we investigated the mechanism(s) of altered expression of protooncogene SKP2 in metastatic melanoma and its clinical relevance in patients with metastatic melanoma. The genomic status of SKP2 was assessed in cell lines by sequencing, single nucleotide polymorphism array, and genomic PCR. Copy number status was then evaluated for concordance with SKP2 mRNA and protein expression. SKP2 protein was further evaluated by immunohistochemistry in 93 human metastatic tissues. No mutations were identified in SKP2. Increased copy number at the SKP2 locus was observed in 6/14 (43%) metastatic cell lines and in 9/22 (41%) human metastatic tissues which was associated with overexpression of SKP2 protein. Overexpression of SKP2 protein in human tissues was associated with worse survival in a multivariate model controlling for the site of metastasis. Copy number gain is a major contributing mechanism of SKP2 overexpression in metastatic melanoma. Results may have implications for the development of therapeutics that target SKP2

Generally, F-box proteins are the substrate recognition subunits of SCF (Skp1-Cul1-F-box protein) ubiquitin ligase complexes, which mediate the timely proteolysis of important eukaryotic regulatory proteins. Mammalian genomes encode roughly 70 F-box proteins, but only a handful have established functions. The F-box protein family obtained its name from Cyclin F (also called Fbxo1), in which the F-box motif (the approximately 40-amino-acid domain required for binding to Skp1) was first described. Cyclin F, which is encoded by an essential gene, also contains a cyclin box domain, but in contrast to most cyclins, it does not bind or activate any cyclin-dependent kinases (CDKs). However, like other cyclins, Cyclin F oscillates during the cell cycle, with protein levels peaking in G2. Despite its essential nature and status as the founding member of the F-box protein family, Cyclin F remains an orphan protein, whose functions are unknown. Starting from an unbiased screen, we identified CP110, a protein that is essential for centrosome duplication, as an interactor and substrate of Cyclin F. Using a mode of substrate binding distinct from other F-box protein-substrate pairs, CP110 and Cyclin F physically associate on the centrioles during the G2 phase of the cell cycle, and CP110 is ubiquitylated by the SCF(Cyclin F) ubiquitin ligase complex, leading to its degradation. siRNA-mediated depletion of Cyclin F in G2 induces centrosomal and mitotic abnormalities, such as multipolar spindles and asymmetric, bipolar spindles with lagging chromosomes. These phenotypes were reverted by co-silencing CP110 and were recapitulated by expressing a stable mutant of CP110 that cannot bind Cyclin F. Finally, expression of a stable CP110 mutant in cultured cells also promotes the formation of micronuclei, a hallmark of chromosome instability. We propose that SCF(Cyclin F)-mediated degradation of CP110 is required for the fidelity of mitosis and genome integrity

Androgen receptor (AR), a member of the steroid receptor family, is a transcription factor that has an important role in the regulation of both prostate cell proliferation and growth suppression. AR coactivators may influence the transition between cell growth and growth suppression. We have shown previously that the internally spliced ARA70 isoform, ARA70beta, promotes prostate cancer cell growth and invasion. Here we report that the full length ARA70alpha, in contrast, represses prostate cancer cell proliferation and anchorage-independent growth in vitro and inhibits tumor growth in nude mice xenograft experiments in vivo. Further, the growth inhibition by ARA70alpha is AR-dependent and mediated through induction of apoptosis rather than cell cycle arrest. Interestingly, AR with T877A mutation in LNCaP cells decreased its physical and functional interaction with ARA70alpha, facilitating the growth of LNCaP cells. This is consistent with our previous findings that ARA70alpha expression is decreased in prostate cancer cells compared with benign prostate. ARA70alpha also reduced the invasion ability of LNCaP cells. Although growth inhibition by ARA70alpha is AR-dependent, the inhibition of cell invasion is an androgen-independent process. These results strongly suggest that ARA70alpha functions as a tumor suppressor gene

Ubiquitin-specific proteases (USPs) are a subclass of cysteine proteases that catalyze the removal of ubiquitin (either monomeric or chains) from substrates, thus counteracting the activity of E3 ubiquitin ligases. Although the importance of USPs in a multitude of processes, from hereditary cancer to neurodegeneration, is well established, our knowledge on their mode of regulation, substrate specificity and biological function is quite limited. In this study we identify USP47 as a novel interactor of the E3 ubiquitin ligase, Skp1/Cul1/F-box protein beta-transducin repeat-containing protein (SCF(beta-Trcp)). We found that both beta-Trcp1 and beta-Trcp2 bind specifically to USP47, and point mutations in the beta-Trcp WD-repeat region completely abolished USP47 binding, indicating an E3-substrate-type interaction. However, unlike canonical beta-Trcp substrates, USP47 protein levels were neither affected by silencing of beta-Trcp nor modulated in a variety of processes, such as cell-cycle progression, DNA damage checkpoint responses or tumor necrosis factor (TNF) pathway activation. Notably, genetic or siRNA-mediated depletion of USP47 induced accumulation of Cdc25A, decreased cell survival and augmented the cytotoxic effects of anticancer drugs. In conclusion, we showed that USP47, a novel beta-Trcp interactor, regulates cell growth and survival, potentially providing a novel target for anticancer therapies.