Alan W. Lau

SCF(FBW7) regulates cellular apoptosis by targeting MCL1 for ubiquitylation and destruction.

The effective use of targeted therapy is highly dependent on the identification of responder patient populations. Loss of FBW7, which encodes a tumour-suppressor protein, is frequently found in various types of human cancer, including breast cancer, colon cancer and T-cell acute lymphoblastic leukaemia (T-ALL). In line with these genomic data, engineered deletion of Fbw7 in mouse T cells results in T-ALL, validating FBW7 as a T-ALL tumour suppressor. Determining the precise molecular mechanisms by which FBW7 exerts antitumour activity is an area of intensive investigation. These mechanisms are thought to relate in part to FBW7-mediated destruction of key proteins relevant to cancer, including Jun, Myc, cyclin E and notch 1 (ref. 9), all of which have oncoprotein activity and are overexpressed in various human cancers, including leukaemia. In addition to accelerating cell growth, overexpression of Jun, Myc or notch 1 can also induce programmed cell death. Thus, considerable uncertainty surrounds how FBW7-deficient cells evade cell death in the setting of upregulated Jun, Myc and/or notch 1. Here we show that the E3 ubiquitin ligase SCFFBW7 (a SKP1–cullin-1–F-box complex that contains FBW7 as the F-box protein) governs cellular apoptosis by targeting MCL1, a pro-survival BCL2 family member, for ubiquitylation and destruction in a manner that depends on phosphorylation by glycogen synthase kinase 3. Human T-ALL cell lines showed a close relationship between FBW7 loss and MCL1 overexpression. Correspondingly, T-ALL cell lines with defective FBW7 are particularly sensitive to the multi-kinase inhibitor sorafenib but resistant to the BCL2 antagonist ABT-737. On the genetic level, FBW7 reconstitution or MCL1 depletion restores sensitivity to ABT-737, establishing MCL1 as a therapeutically relevant bypass survival mechanism that enables FBW7-deficient cells to evade apoptosis. Therefore, our work provides insight into the molecular mechanism of direct tumour suppression by FBW7 and has implications for the targeted treatment of patients with FBW7-deficient T-ALL.

Acetylation-Dependent Regulation of Skp2 Function

Aberrant Skp2 signaling has been implicated as a driving event in tumorigenesis. Although the underlying molecular mechanisms remain elusive, cytoplasmic Skp2 correlates with more aggressive forms of breast and prostate cancers. Here, we report that Skp2 is acetylated by p300 at K68 and K71, which is a process that can be antagonized by the SIRT3 deacetylase. Inactivation of SIRT3 leads to elevated Skp2 acetylation, which leads to increased Skp2 stability through impairment of the Cdh1-mediated proteolysis pathway. As a result, Skp2 oncogenic function is increased, whereby cells expressing an acetylation-mimetic mutant display enhanced cellular proliferation and tumorigenesis in vivo. Moreover, acetylation of Skp2 in the nuclear localization signal (NLS) promotes its cytoplasmic retention, and cytoplasmic Skp2 enhances cellular migration through ubiquitination and destruction of E-cadherin. Thus, our study identifies an acetylation-dependent regulatory mechanism governing Skp2 oncogenic function and provides insight into how cytoplasmic Skp2 controls cellular migration.

Negative Regulation of the Stability and Tumor Suppressor Function of Fbw7 by the Pin1 Prolyl Isomerase

Fbw7 is the substrate recognition component of the Skp1-Cullin-F-box (SCF)-type E3 ligase complex and a well-characterized tumor suppressor that targets numerous oncoproteins for destruction. Genomic deletion or mutation of FBW7 has been frequently found in various types of human cancers; however, little is known about the upstream signaling pathway(s) governing Fbw7 stability and cellular functions. Here we report that Fbw7 protein destruction and tumor suppressor function are negatively regulated by the prolyl isomerase Pin1. Pin1 interacts with Fbw7 in a phoshorylation-dependent manner and promotes Fbw7 self-ubiquitination and protein degradation by disrupting Fbw7 dimerization. Consequently, overexpressing Pin1 reduces Fbw7 abundance and suppresses Fbw7s ability to inhibit proliferation and transformation. By contrast, depletion of Pin1 in cancer cells leads to elevated Fbw7 expression, which subsequently reduces Mcl-1 abundance, sensitizing cancer cells to Taxol. Thus, Pin1-mediated inhibition of Fbw7 contributes to oncogenesis, and Pin1 may be a promising drug target for anticancer therapy.

The Fbw7 and betaTRCP E3 ubiquitin ligases and their roles in tumorigenesis.

The Ubiquitin Proteasome System (UPS) is a major regulator of protein abundance in the cell. The UPS influences the functions of multiple biological processes by targeting key regulators for destruction. E3 ubiquitin ligases are a vital component of the UPS machinery, working with E1 and E2 enzymes to bind substrates and facilitate the transfer of ubiquitin molecules onto the target protein. This poly-ubiquitination, in turn, directs the modified proteins for proteolysis by the 26S proteasome. As the UPS regulates the degradation of multiple oncogenes and tumor suppressors, the dysregulation of this pathway is known to promote various diseases including cancer. While E1 and E2 enzymes have only been minimally linked to cancer development, burgeoning amounts of evidence have implicated loss or gain of E3 function as a key factor in cancer initiation and progression. This review will examine the literature on two SCF-type E3 ligases, SCFFbw7 and SCFbeta-TRCP. In particular, we will highlight novel substrates recently identified for these two E3 ligases, and further discuss how UPS regulation of these targets may promote carcinogenesis.

SCF-Mediated Cdh1 Degradation Defines a Negative Feedback System that Coordinates Cell-Cycle Progression

Proper cell-cycle transitions are driven by waves of ubiquitin-dependent degradation of key regulators by the anaphase-promoting complex (APC) and Skp1-Cullin1-F-box (SCF) E3 ubiquitin ligase complexes. But precisely how APC and SCF activities are coordinated to regulate cell-cycle progression remains largely unclear. We previously showed that APC/Cdh1 earmarks the SCF component Skp2 for degradation. Here, we continue to report that SCF(β-TRCP) reciprocally controls APC/Cdh1 activity by governing Cdh1 ubiquitination and subsequent degradation. Furthermore, we define both cyclin A and Plk1, two well-known Cdh1 substrates, as upstream modifying enzymes that promote Cdh1 phosphorylation to trigger Cdh1 ubiquitination and subsequent degradation by SCF(β-TRCP). Thus, our work reveals a negative repression mechanism for SCF to control APC, thereby illustrating an elegant dual repression system between these two E3 ligase complexes to create the ordered cascade of APC and SCF activities governing timely cell-cycle transitions.

The two faces of FBW7 in cancer drug resistance

Chemotherapy is an important therapeutic approach for cancer treatment. However, drug resistance is an obstacle that often impairs the successful use of chemotherapies. Therefore, overcoming drug resistance would lead to better therapeutic outcomes for cancer patients. Recently, studies by our own and other groups have demonstrated that there is an intimate correlation between the loss of the F‐box and WD repeat domain‐containing 7 (FBW7) tumor suppressor and the incurring drug resistance. While loss of FBW7 sensitizes cancer cells to certain drugs, FBW7‐/‐ cells are more resistant to other types of chemotherapies. FBW7 exerts its tumor suppressor function by promoting the degradation of various oncoproteins that regulate many cellular processes, including cell cycle progression, cellular metabolism, differentiation, and apoptosis. Since loss of the FBW7 tumor suppressor is linked to drug resistance, FBW7 may represent a novel therapeutic target to increase drug sensitivity of cancer cells to conventional chemotherapeutics. This paper thus focuses on the new functional aspects of FBW7 in drug resistance.

Regulation of APC(Cdh1) E3 ligase activity by the Fbw7/cyclin E signaling axis contributes to the tumor suppressor function of Fbw7.

Fbw7 and Cdh1 are substrate-recognition subunits of the SCF- and APC-type E3 ubiquitin ligases, respectively. There is emerging evidence suggesting that both Fbw7 and Cdh1 function as tumor suppressors by targeting oncoproteins for destruction. Loss of Fbw7, but not Cdh1, is frequently observed in various human tumors. However, it remains largely unknown how Fbw7 mechanistically functions as a tumor suppressor and whether there is a signaling crosstalk between Fbw7 and Cdh1. Here, we report that Fbw7-deficient cells not only display elevated expression levels of SCFFbw7 substrates, including cyclin E, but also have increased expression of various APCCdh1 substrates. We further defined cyclin E as the critical signaling link by which Fbw7 governs APCCdh1 activity, as depletion of cyclin E in Fbw7-deficient cells results in decreased expression of APCCdh1 substrates to levels comparable to those in wild-type (WT) cells. Conversely, ectopic expression of cyclin E recapitulates the aberrant APCCdh1 substrate expression observed in Fbw7-deficient cells. More importantly, 4A-Cdh1 that is resistant to Cdk2/cyclin E-mediated phosphorylation, but not WT-Cdh1, reversed the elevated expression of various APCCdh1 substrates in Fbw7-deficient cells. Overexpression of 4A-Cdh1 also resulted in retarded cell growth and decreased anchorage-independent colony formation. Altogether, we have identified a novel regulatory mechanism by which Fbw7 governs Cdh1 activity in a cyclin E-dependent manner. As a result, loss of Fbw7 can lead to aberrant increase in the expression of both SCFFbw7 and APCCdh1 substrates. Our study provides a better understanding of the tumor suppressor function of Fbw7, and suggests that Cdk2/cyclin E inhibitors could serve as effective therapeutic agents for treating Fbw7-deficient tumors.

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.