Hui Kuan Lin

Skp2 targeting suppresses tumorigenesis by Arf-p53 -independent cellular senescence

Cellular senescence has been recently shown to have an important role in opposing tumour initiation and promotion. Senescence induced by oncogenes or by loss of tumour suppressor genes is thought to critically depend on induction of the p19Arf–p53 pathway. The Skp2 E3-ubiquitin ligase can act as a proto-oncogene and its aberrant overexpression is frequently observed in human cancers. Here we show that although Skp2 inactivation on its own does not induce cellular senescence, aberrant proto-oncogenic signals as well as inactivation of tumour suppressor genes do trigger a potent, tumour-suppressive senescence response in mice and cells devoid of Skp2. Notably, Skp2 inactivation and oncogenic-stress-driven senescence neither elicit activation of the p19Arf–p53 pathway nor DNA damage, but instead depend on Atf4, p27 and p21. We further demonstrate that genetic Skp2 inactivation evokes cellular senescence even in oncogenic conditions in which the p19Arf–p53 response is impaired, whereas a Skp2–SCF complex inhibitor can trigger cellular senescence in p53/Pten-deficient cells and tumour regression in preclinical studies. Our findings therefore provide proof-of-principle evidence that pharmacological inhibition of Skp2 may represent a general approach for cancer prevention and therapy.

PML regulates apoptosis at endoplasmic reticulum by modulating calcium release.

Promoting Apoptosis During acute disease, the promyelocytic leukemia (PML) protein becomes fused to another protein as a result of a chromosomal translocation. This protein appears to have multiple and varied functions, including the ability to form distinctive complexes in the nucleus that suppress tumorigenesis and promote apoptotic cell death. Giorgi et al. (p. 1247, published online 28 October; see the Perspective by Culjkovic-Kraljacic and Borden) have proposed a mechanism by which PML influences the cellular signals that promote apoptosis. The protein was localized at sites of contact between the endoplasmic reticulum and mitochondria, where it associated with a calcium channel, a protein kinase, and a protein phosphatase, to regulate calcium mobilization into the mitochondrion, which then triggers the cell death program. The promyelocytic leukemia protein likely influences apoptosis by influencing a calcium channel in the endoplasmic reticulum. The promyelocytic leukemia (PML) tumor suppressor is a pleiotropic modulator of apoptosis. However, the molecular basis for such a diverse proapoptotic role is currently unknown. We show that extranuclear Pml was specifically enriched at the endoplasmic reticulum (ER) and at the mitochondria-associated membranes, signaling domains involved in ER-to-mitochondria calcium ion (Ca2+) transport and in induction of apoptosis. We found Pml in complexes of large molecular size with the inositol 1,4,5-trisphosphate receptor (IP3R), protein kinase Akt, and protein phosphatase 2a (PP2a). Pml was essential for Akt- and PP2a-dependent modulation of IP3R phosphorylation and in turn for IP3R-mediated Ca2+ release from ER. Our findings provide a mechanistic explanation for the pleiotropic role of Pml in apoptosis and identify a pharmacological target for the modulation of Ca2+ signals.

LIFR is a breast cancer metastasis suppressor upstream of the Hippo-YAP pathway and a prognostic marker

There is a pressing need to identify prognostic markers of metastatic disease and targets for treatment. Combining high-throughput RNA sequencing, functional characterization, mechanistic studies and clinical validation, we identify leukemia inhibitory factor receptor (LIFR) as a breast cancer metastasis suppressor downstream of the microRNA miR-9 and upstream of Hippo signaling. Restoring LIFR expression in highly malignant tumor cells suppresses metastasis by triggering a Hippo kinase cascade that leads to phosphorylation, cytoplasmic retention and functional inactivation of the transcriptional coactivator YES-associated protein (YAP). Conversely, loss of LIFR in nonmetastatic breast cancer cells induces migration, invasion and metastatic colonization through activation of YAP. LIFR is downregulated in human breast carcinomas and inversely correlates with metastasis. Notably, in approximately 1,000 nonmetastatic breast tumors, LIFR expression status correlated with metastasis-free, recurrence-free and overall survival outcomes in the patients. These findings identify LIFR as a metastasis suppressor that functions through the Hippo-YAP pathway and has significant prognostic power.

The Skp2-SCF E3 ligase regulates Akt ubiquitination, glycolysis, herceptin sensitivity, and tumorigenesis.

Akt kinase plays a central role in cell growth, metabolism, and tumorigenesis. The TRAF6 E3 ligase orchestrates IGF-1-mediated Akt ubiquitination and activation. Here, we show that Akt ubiquitination is also induced by activation of ErbB receptors; unexpectedly, and in contrast to IGF-1 induced activation, the Skp2 SCF complex, not TRAF6, is a critical E3 ligase for ErbB-receptor-mediated Akt ubiquitination and membrane recruitment in response to EGF. Skp2 deficiency impairs Akt activation, Glut1 expression, glucose uptake and glycolysis, and breast cancer progression in various tumor models. Moreover, Skp2 overexpression correlates with Akt activation and breast cancer metastasis and serves as a marker for poor prognosis in Her2-positive patients. Finally, Skp2 silencing sensitizes Her2-overexpressing tumors to Herceptin treatment. Our study suggests that distinct E3 ligases are utilized by diverse growth factors for Akt activation and that targeting glycolysis sensitizes Her2-positive tumors to Herceptin treatment.

Phosphorylation-dependent regulation of cytosolic localization and oncogenic function of Skp2 by Akt/PKB

Skp2 is an F-box protein that forms the SCF complex with Skp1 and Cullin-1 to constitute an E3 ligase for ubiquitylation. Ubiquitylation and degradation of the p27 are critical for Skp2-mediated entry to the cell cycle, and overexpression and cytosolic accumulation of Skp2 have been clearly associated with tumorigenesis, although the functional significance of the latter is still unknown. Here we show that Akt/protein kinase B (PKB) interacts with and directly phosphorylates Skp2. We find that Skp2 phosphorylation by Akt triggers SCF complex formation and E3 ligase activity. A phosphorylation-defective Skp2 mutant is drastically impaired in its ability to promote cell proliferation and tumorigenesis. Furthermore, we show that Akt-mediated phosphorylation triggers 14-3-3β-dependent Skp2 relocalization to the cytosol, and we attribute a specific role to cytosolic Skp2 in the positive regulation of cell migration. Finally, we demonstrate that high levels of activation of Akt correlate with the cytosolic accumulation of Skp2 in human cancer specimens. Our results therefore define a novel proto-oncogenic Akt/PKB-dependent signalling pathway.

Deacetylation of FOXO3 by SIRT1 or SIRT2 leads to Skp2-mediated FOXO3 ubiquitination and degradation.

Sirtuin deacetylases and FOXO (Forkhead box, class O) transcription factors have important roles in many biological pathways, including cancer development. SIRT1 and SIRT2 deacetylate FOXO factors to regulate FOXO function. Because acetylation and ubiquitination both modify the ɛ-amino group of lysine residues, we investigated whether FOXO3 deacetylation by SIRT1 or SIRT2 facilitates FOXO3 ubiquitination and subsequent proteasomal degradation. We found that SIRT1 and SIRT2 promote FOXO3 poly-ubiquitination and degradation. Proteasome-inhibitor treatment prevented sirtuin-induced FOXO3 degradation, indicating that this process is proteasome dependent. In addition, we demonstrated that E3 ubiquitin ligase subunit Skp2 binds preferentially to deacetylated FOXO3. Overexpression of Skp2 caused poly-ubiquitination of FOXO3 and degradation, whereas knockdown of Skp2 increased the amount of FOXO3 protein. We also present evidence that SCF-Skp2 ubiquitinates FOXO3 directly in vitro. Furthermore, mutating four known acetylated lysine residues (K242, K259, K290 and K569) of FOXO3 into arginines to mimic deacetylated FOXO3 resulted in enhanced Skp2 binding but with inhibition of FOXO3 ubiquitination; this suggests that some or all of these four lysine residues are likely the sites for ubiquitination. In the livers of mice deficient in SIRT1, we detected increased expression of FOXO3, indicating SIRT1 regulates FOXO3 protein levels in vivo. Furthermore, we found that the elevation of SIRT1 and Skp2 expression in malignant PC3 and DU145 prostate cells is responsible for the downregulation of FOXO3 protein levels in these cells. Taken together, our data support the notion that deacetylation of FOXO3 by SIRT1 or SIRT2 facilitates Skp2-mediated FOXO3 poly-ubiquitination and proteasomal degradation.

Regulation of Akt signaling activation by ubiquitination

Akt (also known as PKB) signaling orchestrates many aspects of biological functions and, importantly, its deregulation is linked to cancer development. Akt activity is well-known regulated through its phosphorylation at T308 and S473 by PDK1 and mTORC2, respectively. Although in the last decade the research has been primarily focused on Akt phosphorylation and its role in Akt activation and functions, other posttranslational modifications on Akt have never been reported. Until very recently, a novel posttranslational modification on Akt termed ubiquitination was identified and shown to play an important role in Akt activation. The cancer-associated Akt mutant recently identified in a subset of human cancers displays enhanced Akt ubiquitination, in turn contributing to Akt hyperactivation, suggesting a potential role of Akt ubiquitination in cancers. Thus, this novel posttranslational modification on Akt reveals an exciting avenue that has advanced our current understandings of how Akt signaling activation is regulated.

ER stress inhibits mTORC2 and Akt signaling through GSK-3β-mediated phosphorylation of rictor

Cellular stress attenuates growth factor signaling through a phosphorylation event that blocks substrate access to the kinase complex mTORC2. No Access During Stressful Times Under conditions of cellular stress, cells tend to halt anabolic processes, such as cell growth and proliferation, to conserve resources. mTORC2 (mammalian target of rapamycin complex 2), which mediates its effects through activation of the kinase Akt, is a key signaling complex that promotes anabolic processes. Chen et al. investigated the mechanisms by which mTORC2 activity is inhibited by endoplasmic reticulum (ER) stress. They found that glycogen synthase kinase–3β (GSK-3β), which is activated by ER stress, phosphorylated rictor, a component of mTORC2 that helps to determine substrate specificity for the complex. This phosphorylation event decreased binding of Akt to mTORC2, resulting in reduced activation of Akt and cell proliferation. Furthermore, transformed cells expressing a mutant form of rictor lacking the GSK-3β phosphorylation site formed larger tumors in mice than did those expressing wild-type rictor or a rictor mutant that mimicked a constitutively phosphorylated form. These results define a pathway by which mTORC2 and Akt signaling can be attenuated by cellular stress and provide a potential therapeutic target for limiting cell proliferation (such as in cancer). In response to environmental cues, cells coordinate a balance between anabolic and catabolic pathways. In eukaryotes, growth factors promote anabolic processes and stimulate cell growth, proliferation, and survival through activation of the phosphoinositide 3-kinase (PI3K)–Akt pathway. Akt-mediated phosphorylation of glycogen synthase kinase–3β (GSK-3β) inhibits its enzymatic activity, thereby stimulating glycogen synthesis. We show that GSK-3β itself inhibits Akt by controlling the mammalian target of rapamycin complex 2 (mTORC2), a key activating kinase for Akt. We found that during cellular stress, GSK-3β phosphorylated the mTORC2 component rictor at serine-1235, a modification that interfered with the binding of Akt to mTORC2. The inhibitory effect of GSK-3β on mTORC2-Akt signaling and cell proliferation was eliminated by blocking phosphorylation of rictor at serine-1235. Thus, in response to cellular stress, GSK-3β restrains mTORC2-Akt signaling by specifically phosphorylating rictor, thereby balancing the activities of GSK-3β and Akt, two opposing players in glucose metabolism.

Critical Role of Monoubiquitination of Histone H2AX Protein in Histone H2AX Phosphorylation and DNA Damage Response

DNA damage response is an important surveillance mechanism used to maintain the integrity of the human genome in response to genotoxic stress. Histone variant H2AX is a critical sensor that undergoes phosphorylation at serine 139 upon genotoxic stress, which provides a docking site to recruit the mediator of DNA damage checkpoint protein 1 (MDC1) and DNA repair protein complex to sites of DNA breaks for DNA repair. Here, we show that monoubiquitination of H2AX is induced upon DNA double strand breaks and plays a critical role in H2AX Ser-139 phosphorylation (γ-H2AX), in turn facilitating the recruitment of MDC1 to DNA damage foci. Mechanistically, we show that monoubiquitination of H2AX induced by RING finger protein 2 (RNF2) is required for the recruitment of active ataxia telangiectasia mutated to DNA damage foci, thus affecting the formation of γ-H2AX. Importantly, a defect in monoubiquitination of H2AX profoundly enhances ionizing radiation sensitivity. Our study therefore suggests that monoubiquitination of H2AX is an important step for DNA damage response and may have important clinical implications for the treatment of cancers.

Skp2 E3 Ligase Integrates ATM Activation and Homologous Recombination Repair by Ubiquitinating NBS1

The Mre11/Rad50/NBS1 (MRN) complex is thought to be a critical sensor that detects damaged DNA and recruits ATM to DNA foci for activation. However, it remains to be established how the MRN complex regulates ATM recruitment to the DNA foci during DNA double-strand breaks (DSBs). Here we show that Skp2 E3 ligase is a key component for the MRN complex-mediated ATM activation in response to DSBs. Skp2 interacts with NBS1 and triggers K63-linked ubiquitination of NBS1 upon DSBs, which is critical for the interaction of NBS1 with ATM, thereby facilitating ATM recruitment to the DNA foci for activation. Finally, we show that Skp2 deficiency exhibits a defect in homologous recombination (HR) repair, thereby increasing IR sensitivity. Our results provide molecular insights into how Skp2 and the MRN complex coordinate to activate ATM, and identify Skp2-mediatetd NBS1 ubiquitination as a vital event for ATM activation in response to DNA damage.