Negative regulation of DAB2IP by Akt and SCFFbw7 pathways

Deletion of ovarian carcinoma 2/disabled homolog 2 (DOC-2/DAB2) interacting protein (DAB2IP), is a tumor suppressor that serves as a scaffold protein involved in coordinately regulating cell proliferation, survival and apoptotic pathways. DAB2IP is epigenetically down-regulated in a variety of tumors through the action of the histone methyltransferase EZH2. Although DAB2IP is transcriptionally down-regulated in a variety of tumors, it remains unclear if other mechanisms contribute to functional inactivation of DAB2IP. Here we demonstrate that DAB2IP can be functionally down-regulated by two independent mechanisms. First, we identified that Akt1 can phosphorylate DAB2IP on S847, which regulates the interaction between DAB2IP and its effector molecules H-Ras and TRAF2. Second, we demonstrated that DAB2IP can be degraded in part through ubiquitin-proteasome pathway by SCFFbw7. DAB2IP harbors two Fbw7 phosho-degron motifs, which can be regulated by the kinase, CK1δ. Our data hence indicate that in addition to epigenetic down-regulation, two additional pathways can functional inactivate DAB2IP. Given that DAB2IP has previously been identified to possess direct causal role in tumorigenesis and metastasis, our data indicate that a variety of pathways may pass through DAB2IP to govern cancer development, and therefore highlight DAB2IP agonists as potential therapeutic approaches for future anti-cancer drug development

Akt finds its new path to regulate cell cycle through modulating Skp2 activity and its destruction by APC/Cdh1

Skp2 over-expression has been observed in many human cancers. However, the mechanisms underlying elevated Skp2 expression have remained elusive. We recently reported that Akt1, but not Akt2, directly controls Skp2 stability by interfering with its association with APC/Cdh1. As a result, Skp2 degradation is protected in cancer cells with elevated Akt activity. This finding expands our knowledge of how specific kinase cascades influence proteolysis governed by APC/Cdh1 complexes. However, it awaits further investigation to elucidate whether the PI3K/Akt circuit affects other APC/Cdh1 substrates. Our results further strengthen the argument that different Akt isoforms might have distinct, even opposing functions in the regulation of cell growth or migration. In addition, we noticed that Ser72 is localized in a putative Nuclear Localization Sequence (NLS), and that phosphorylation of Ser72 disrupts the NLS and thus promotes Skp2 cytoplasmic translocation. This finding links elevated Akt activity with the observed cytoplasmic Skp2 staining in aggressive breast and prostate cancer patients. Furthermore, it provides the rationale for the development of specific Akt1 inhibitors as efficient anti-cancer therapeutic agents

Acetylation-dependent regulation of essential iPS-inducing factors: a regulatory crossroad for pluripotency and tumorigenesis

Induced pluripotent stem (iPS) cells can be generated from somatic cells by coexpression of four transcription factors: Sox2, Oct4, Klf4, and c-Myc. However, the low efficiency in generating iPS cells and the tendency of tumorigenesis hinder the therapeutic applications for iPS cells in treatment of human diseases. To this end, it remains largely unknown how the iPS process is subjected to regulation by upstream signaling pathway(s). Here, we report that Akt regulates the iPS process by modulating posttranslational modifications of these iPS factors in both direct and indirect manners. Specifically, Akt directly phosphorylates Oct4 to modulate the Oct4/Sox2 heterodimer formation. Furthermore, Akt either facilitates the p300-mediated acetylation of Oct4, Sox2, and Klf4, or stabilizes Klf4 by inactivating GSK3, thus indirectly modulating stemness. As tumorigenesis shares possible common features and mechanisms with iPS, our study suggests that Akt inhibition might serve as a cancer therapeutic approach to target cancer stem cells

Aging and cancer hallmarks as therapeutic targets

The prevalence of age-related diseases has been progressively increasing worldwide. The pathogenesis of these disorders, including cancer, is closely associated with an age-dependent decline in cellular functions. There are multiple layers of crosstalk between aging processes and oncogenesis. Therefore, a better understanding of the interplay between aging and cancer will facilitate the development of novel therapeutic intervention strategies for malignancies at advanced ages. Recent review articles have concisely summarized and updated aging and cancer hallmark features and adequately discussed the interplays at both the cellular and molecular levels. Because aging and cancer share several key mechanisms that define the hallmarks, targeting shared features, such as cellular senescence, may be beneficial for cancer prevention and treatments. Notably, senolysis, an innovative therapeutic intervention for selectively removing senescent cells, holds great promise for developing new therapeutic approaches for cancers and other age-related diseases, such as viral infections and cardiovascular diseases. Herein we briefly summarize the recently updated knowledge on aging and cancer hallmarks, as well as the advances in senolysis for age-related conditions

Mcl-1 Ubiquitination and Destruction

Loss of the Fbw7 tumor suppressor is common in diverse human cancer types, including T-Cell Acute Lymphoblastic Leukemia (T-ALL), although the mechanistic basis of its anti-oncogenic activity remains largely unclear. We recently reported that SCF$^{Fbw7}$ regulates cellular apoptosis by controlling the ubiquitination and destruction of the pro-survival protein, Mcl-1, in a GSK3 phosphorylation-dependent manner. We found that human T-ALL cell lines displayed a close relationship between Fbw7 loss and Mcl-1 overexpression. More interestingly, T-ALL cell lines that are deficient in Fbw7 are particularly sensitive to sorafenib, a multi-kinase inhibitor that has been demonstrated to reduce Mcl-1 expression through an unknown mechanism. On the other hand, Fbw7-deficient T-ALL cell lines are much more resistant to the Bcl-2 antagonist, ABT-737. Furthermore, reconstitution of Fbw7 or depletion of Mcl-1 in Fbw7-deficient cells restores ABT-737 sensitivity, suggesting that elevated Mcl-1 expression is important for Fbw7-deficient cells to evade apoptosis. Therefore, our work provides a novel molecular mechanism for the tumor suppression function of Fbw7. Furthermore, it provides the rationale for targeted usage of Mcl-1 antagonists to treat Fbw7-deficient T-ALL patients

DNA damage-induced activation of ATM promotes β-TRCP-mediated Mdm2 ubiquitination and destruction

The Mdm2 oncoprotein promotes p53 ubiquitination and destruction. Yet, exact molecular mechanisms of Mdm2 destruction itself, under DNA damaging conditions, remain unclear. Recently, we identified SCFβ-TRCP as a novel E3 ligase that targets Mdm2 for ubiquitination and destruction in a Casein Kinase Iδ (CKIδ)-dependent manner. However, it remains elusive how the β-TRCP/CKIδ/Mdm2 signaling axis is regulated by DNA damage signals to govern p53 activity. Consistent with previous studies, we found that inactivation of the Ataxia Telangiectasia Mutated (ATM) kinase, in turn, impaired DNA damage-induced Mdm2 destruction. Although phosphorylation of Mdm2 at Ser395 (an ATM phosphorylation site) facilitated Mdm2 interaction with β-TRCP, Ser395A-Mdm2 was degraded non-distinguishably from WT-Mdm2 by SCFβ-TRCP upon DNA damaging treatments. This indicates that in addition to phosphorylating Mdm2 at Ser395, ATM may govern Mdm2 stability through other unknown mechanisms. We further demonstrated that DNA damage-induced activation of ATM directly phosphorylated CKIδ at two well-conserved S/TQ sites, which promotes CKIδ nuclear localization to increase CKIδ-mediated phosphorylation of Mdm2, thereby facilitating subsequent Mdm2 ubiquitination by SCFβ-TRCP. Our studies provide a molecular mechanism of how ATM could govern DNA damage-induced destruction of Mdm2 in part by phosphorylating both Mdm2 and CKIδ to modulate SCFβ-TRCP–mediated Mdm2 ubiquitination. Given the pivotal role of Mdm2 in the negative regulation of p53, this work will also provide a rationale for developing CKIδ or ATM agonists as anti-cancer agents

SCFβ-TRCP-mediated degradation of NEDD4 inhibits tumorigenesis through modulating the PTEN/Akt signaling pathway

The HECT domain-containing ubiquitin E3 ligase NEDD4 is widely expressed in mammalian tissues and plays a crucial role in governing a wide spectrum of cellular processes including cell growth, tissue development and homeostasis. Recent reports have indicated that NEDD4 might facilitate tumorigenesis through targeted degradation of multiple tumor suppressor proteins including PTEN. However, the molecular mechanism by which NEDD4 stability is regulated has not been fully elucidated. Here we report that SCFβ-TRCP governs NEDD4 protein stability by targeting it for ubiquitination and subsequent degradation in a Casein Kinase-I (CKI) phosphorylation-dependent manner. Specifically, depletion of β-TRCP, or inactivation of CKI, stabilized NEDD4, leading to down-regulation of its ubiquitin target PTEN and subsequent activation of the mTOR/Akt oncogenic pathway. Furthermore, we found that CKIδ-mediated phosphorylation of Ser347 and Ser348 on NEDD4 promoted its interaction with SCFβ-TRCP for subsequent ubiquitination and degradation. As a result, compared to ectopic expression of wild-type NEDD4, introducing a non-degradable NEDD4 (S347A/S348A-NEDD4) promoted cancer cell growth and migration. Hence, our findings revealed the CKI/SCFβ-TRCP signaling axis as the upstream negative regulator of NEDD4, and further suggested that enhancing NEDD4 degradation, presumably with CKI or SCFβ-TRCP agonists, could be a promising strategy for treating human cancers

Identification and characterization of novel components of a Ca2+/calmodulin-dependent protein kinase cascade in HeLa cells

AbstractIn this report, we cloned a novel calmodulin-kinase (CaM-KIδ) from HeLa cells and characterized its activation mechanism. CaM-KIδ exhibits Ca2+/CaM-dependent activity that is enhanced (∼30-fold) in vitro by phosphorylation of its Thr180 by CaM-K kinase (CaM-KK)α, consistent with detection of CaM-KIδ-activating activity in HeLa cells. We also identified a novel CaM-KKβ isoform (CaM-KKβ-3) in HeLa cells whose activity was highly Ca2+/CaM-independent. Transiently expressed CaM-KIδ exhibited enhanced protein kinase activity in HeLa cells without ionomycin stimulation. This sustained activation of CaM-KIδ was completely abolished by Thr180Ala mutation and inhibited by CaM-KK inhibitor, STO-609, indicating a functional CaM-KK/CaM-KIδ cascade in HeLa cells