Marked for death

Impact of genetic targets on cancer therapy in acute myelogenous leukemia

Acute myelogenous leukemia (AML) is characterized by uncontrolled proliferation of the cells of myeloid origin. It can present at all ages, but is more common in adults. It is one of the most common leukemias in adults and continues to pose significant challenge in diagnosis and long-term management.AML is a disease at the forefront of genetic and genomic approaches to medicine. It is a disease that has witnessed rapid advances in terms of diagnosis, classification, prognosis and ultimately individualized therapy. Newly diagnosed AML patients are now routinely stratified according to cytogenetics and molecular markers which guides long-term prognosis and treatment. On the other hand, with few exceptions, the initial treatment (also known as induction treatment) of AML has been 'one-size-fits-all'. It remains a great challenge for patients and physicians to consolidate and translate these advances into eventual success in clinic [1, 2].

Triptolide inhibits the proliferation of prostate cancer cells and down-regulates SUMO-specific protease 1 expression

Recently, traditional Chinese medicine and medicinal herbs have attracted more attentions worldwide for its anti-tumor efficacy. Celastrol and Triptolide, two active components extracted from the Chinese herb Tripterygium wilfordii Hook F (known as Lei Gong Teng or Thunder of God Vine), have shown anti-tumor effects. Celastrol was identified as a natural 26 s proteasome inhibitor which promotes cell apoptosis and inhibits tumor growth. The effect and mechanism of Triptolide on prostate cancer (PCa) is not well studied. Here we demonstrated that Triptolide, more potent than Celastrol, inhibited cell growth and induced cell death in LNCaP and PC-3 cell lines. Triptolide also significantly inhibited the xenografted PC-3 tumor growth in nude mice. Moreover, Triptolide induced PCa cell apoptosis through caspases activation and PARP cleavage. Unbalance between SUMOylation and deSUMOylation was reported to play an important role in PCa progression. SUMO-specific protease 1 (SENP1) was thought to be a potential marker and therapeutical target of PCa. Importantly, we observed that Triptolide down-regulated SENP1 expression in both mRNA and protein levels in dose-dependent and time-dependent manners, resulting in an enhanced cellular SUMOylation in PCa cells. Meanwhile, Triptolide decreased AR and c-Jun expression at similar manners, and suppressed AR and c-Jun transcription activity. Furthermore, knockdown or ectopic SENP1, c-Jun and AR expression in PCa cells inhibited the Triptolide anti-PCa effects. Taken together, our data suggest that Triptolide is a natural compound with potential therapeutic value for PCa. Its anti-tumor activity may be attributed to mechanisms involving down-regulation of SENP1 that restores SUMOylation and deSUMOyaltion balance and negative regulation of AR and c-Jun expression that inhibits the AR and c-Jun mediated transcription in PCa.

β-catenin inhibits promyelocytic leukemia protein tumor suppressor function in colorectal cancer cells

BACKGROUND & AIMS

Loss of promyelocytic leukemia protein (PML) nuclear body (NB) formation has been reported in colorectal and other solid tumors. However, genetic alteration of PML is rarely observed in these tumors; the exact mechanisms that mediate loss of PML function are not known.

METHODS

We previously used a comprehensive shotgun mass spectrometry approach to identify PML as 1 of 70 proteins that coimmunoprecipitate with anti-T-cell factor 4 in DLD-1 and HCT116 colorectal cancer cell lines; we investigated the effects of altered β-catenin expression on PML function in these cells.

RESULTS

β-catenin specifically interacted with the product of PML transcript variant IV (PML-IV) through the armadillo repeat domain of β-catenin. Overexpression of β-catenin in colorectal cancer cells disrupted the subcellular compartmentalization of PML-IV, whereas knockdown of β-catenin restored formation of PML-NB. Modification of PML by the small ubiquitin-related modifier (SUMO) is required for proper assembly of PML-NB. β-catenin inhibited Ran-binding protein 2-mediated SUMOylation of PML-IV.

CONCLUSIONS

β-catenin interacts with PML isoform IV and disrupts PML-IV function and PML-NB formation by inhibiting Ran-binding protein 2-mediated SUMO modification of PML-IV. These findings indicate the involvement of a posttranslational mechanism in disruption of PML-NB organization in cancer cells and provide more information about the oncogenic functions of β-catenin.

Coordinate to guard: crosstalk of phosphorylation, sumoylation, and ubiquitylation in DNA damage response

Small ubiquitin-like modifier-1/2/3 (SUMO-1/2/3) and ubiquitin share similar structure and utilize analogous machinery for protein lysine conjugation. Although sumoylation and ubiquitylation have distinct functions, they are often tightly associated with each other to fine-tune protein fate in transducing signals to regulate a wide variety of cellular functions, including DNA damage response, cell proliferation, DNA replication, embryonic development, and cell differentiation. In this Perspective, we specifically highlight the role of sumoylation and ubiquitylation in ataxia-telangiectasia mutated (ATM) signaling in response to DNA double-strand breaks and hypothesize that ATM-induced phosphorylation is a unique node in regulating SUMO-targeted ubiquitylation in mammalian cells to combat DNA damage and to maintain genome integrity. A potential role for the coordination of three types of post-translational modification in dictating the tempo and extent of cellular response to genotoxic stress is speculated.

Human pathogens and the host cell SUMOylation system

Since posttranslational modification (PTM) by the small ubiquitin-related modifiers (SUMOs) was discovered over a decade ago, a huge number of cellular proteins have been found to be reversibly modified, resulting in alteration of differential cellular pathways. Although the molecular consequences of SUMO attachment are difficult to predict, the underlying principle of SUMOylation is altering inter- and/or intramolecular interactions of the modified substrate, changing localization, stability, and/or activity. Unsurprisingly, many different pathogens have evolved to exploit the cellular SUMO modification system due to its functional flexibility and far-reaching functional downstream consequences. Although the extensive knowledge gained so far is impressive, a definitive conclusion about the role of SUMO modification during virus infection in general remains elusive and is still restricted to a few, yet promising concepts. Based on the available data, this review aims, first, to provide a detailed overview of the current state of knowledge and, second, to evaluate the currently known common principles/molecular mechanisms of how human pathogenic microbes, especially viruses and their regulatory proteins, exploit the host cell SUMO modification system.

The sumoylation pathway is dysregulated in multiple myeloma and is associated with adverse patient outcome

Multiple myeloma (MM) is a plasma cell neoplasm that proceeds through a premalignant state of monoclonal gammopathy of unknown significance; however, the molecular events responsible for myelomagenesis remain uncharacterized. To identify cellular pathways deregulated in MM, we addressed that sumoylation is homologous to ubiquitination and results in the attachment of the ubiquitin-like protein Sumo onto target proteins. Sumoylation was markedly enhanced in MM patient lysates compared with normal plasma cells and expression profiling indicated a relative induction of sumoylation pathway genes. The Sumo-conjugating enzyme Ube2I, the Sumo-ligase PIAS1, and the Sumo-inducer ARF were elevated in MM patient samples and cell lines. Survival correlated with expression because 80% of patients with low UBE2I and PIAS1 were living 6 years after transplantation, whereas only 45% of patients with high expression survived 6 years. UBE2I encodes the sole Sumo-conjugating enzyme in mammalian cells and cells transfected with a dominant-negative sumoylation-deficient UBE2I mutant exhibited decreased survival after radiation exposure, impaired adhesion to bone marrow stroma cell and decreased bone marrow stroma cell-induced proliferation. UBE2I confers cells with multiple advantages to promote tumorigenesis and predicts decreased survival when combined with PIAS1. The sumoylation pathway is a novel therapeutic target with implications for existing proteasomal-based treatment strategies.

Control of genome stability by SLX protein complexes

The six Saccharomyces cerevisiae SLX genes were identified in a screen for factors required for the viability of cells lacking Sgs1, a member of the RecQ helicase family involved in processing stalled replisomes and in the maintenance of genome stability. The six SLX gene products form three distinct heterodimeric complexes, and all three have catalytic activity. Slx3-Slx2 (also known as Mus81-Mms4) and Slx1-Slx4 are both heterodimeric endonucleases with a marked specificity for branched replication fork-like DNA species, whereas Slx5-Slx8 is a SUMO (small ubiquitin-related modifier)-targeted E3 ubiquitin ligase. All three complexes play important, but distinct, roles in different aspects of the cellular response to DNA damage and perturbed DNA replication. Slx4 interacts physically not only with Slx1, but also with Rad1-Rad10 [XPF (xeroderma pigmentosum complementation group F)-ERCC1 (excision repair cross-complementing 1) in humans], another structure-specific endonuclease that participates in the repair of UV-induced DNA damage and in a subpathway of recombinational DNA DSB (double-strand break) repair. Curiously, Slx4 is essential for repair of DSBs by Rad1-Rad10, but is not required for repair of UV damage. Slx4 also promotes cellular resistance to DNA-alkylating agents that block the progression of replisomes during DNA replication, by facilitating the error-free mode of lesion bypass. This does not require Slx1 or Rad1-Rad10, and so Slx4 has several distinct roles in protecting genome stability. In the present article, I provide an overview of our current understanding of the cellular roles of the Slx proteins, paying particular attention to the advances that have been made in understanding the cellular roles of Slx4. In particular, protein-protein interactions and underlying molecular mechanisms are discussed and I draw attention to the many questions that have yet to be answered.

Differentiation therapy of acute myeloid leukemia: past, present and future

PURPOSE OF REVIEW

Since the 1970s, the concept of differentiation therapy has been viewed as a promising and revolutionary approach for the treatment of acute myeloid leukemia (AML) and other cancers. However, the successful clinical application of differentiation therapy has only been realized since the late 1980s and only in one subtype of AML, acute promyelocytic leukemia (APL). The use of all-trans-retinoic acid (ATRA) and arsenic trioxide, both of which induce degradation of the progressive multifocal leukoencephalopathy/retinoic acid receptor alpha oncoprotein, in combination with chemotherapy is currently the accepted treatment of APL, presenting a potential paradigm for differentiation therapy in clinical oncology.

RECENT FINDINGS

We have begun to understand why ATRA fails to induce differentiation in AML. The underlying reasons identified thus far are associated with an inability to target the removal of leukemogenic fusion proteins, aberrant epigenetic regulation of genes involved in the ATRA signaling pathway and the presence of factors that interfere with proper retinoic acid receptor alpha function.

SUMMARY

Here, we examine the reasons why the exquisite sensitivity of APL to ATRA-based differentiation therapy has not been extended to other of AML subtypes. Current differentiation-based combinatorial approaches to target AML will also be analyzed. Finally, we will evaluate the potential of novel strategies, high-throughput screening, and functional genomics to uncover new differentiation-based therapies for AML.

Ubiquitin-family modifications of topoisomerase I in camptothecin-treated human breast cancer cells

Camptothecins kill mammalian cells by stabilizing topoisomerase I-DNA strand passing intermediates that are converted to lethal double strand DNA breaks in DNA replication fork collisions. Camptothecin-stabilized topoisomerase I-DNA cleavage intermediates in mammalian cells are uniquely modified by ubiquitin-family proteins. The structure, composition, and function of these ubiquitin-family modifications are poorly understood. We have used capillary liquid chromatography-nanospray tandem mass spectrometry to analyze the endogenous ubiquitin-family modifications of topoisomerase I purified from camptothecin-stabilized topoisomerase I-DNA cleavage complexes in human breast cancer cells. Peptides shared by SUMO-2 and SUMO-3 were abundant, and a peptide unique to SUMO-2 was identified. Ubiquitin was also identified in these complexes. No SUMO-1 peptide was detected in human topoisomerase I-DNA cleavage complexes. Identical experiments with purified SUMO paralogues showed that SUMO-1 was well digested by our protocol and that fragments were easily analyzed by LC-MS/MS. Spiking experiments with purified SUMO paralogues determined that we could detect as little as 0.5 SUMO-1 residue per topoisomerase I molecule. These results indicate that SUMO-1 is below this detection level and that SUMO-2 or a mixture of SUMO-2 and SUMO-3 predominates. SUMO-1 capping seems unlikely to be limiting the growth of SUMO-2/3 chains formed on camptothecin-stabilized topoisomerase I-DNA cleavage complexes.

Role of SUMO in RNF4-mediated promyelocytic leukemia protein (PML) degradation: sumoylation of PML and phospho-switch control of its SUMO binding domain dissected in living cells

Promyelocytic leukemia protein (PML) is a tumor suppressor acting as the organizer of subnuclear structures called PML nuclear bodies (NBs). Both covalent modification of PML by the small ubiquitin-like modifier (SUMO) and non-covalent binding of SUMO to the PML SUMO binding domain (SBD) are necessary for PML NB formation and maturation. PML sumoylation and proteasome-dependent degradation induced by the E3 ubiquitin ligase, RNF4, are enhanced by the acute promyelocytic leukemia therapeutic agent, arsenic trioxide (As2O3). Here, we established a novel bioluminescence resonance energy transfer (BRET) assay to dissect and monitor PML/SUMO interactions dynamically in living cells upon addition of therapeutic agents. Using this sensitive and quantitative SUMO BRET assay that distinguishes PML sumoylation from SBD-mediated PML/SUMO non-covalent interactions, we probed the respective roles of covalent and non-covalent PML/SUMO interactions in PML degradation and interaction with RNF4. We found that, although dispensable for As2O3-enhanced PML sumoylation and RNF4 interaction, PML SBD core sequence was required for As2O3- and RNF4-induced PML degradation. As confirmed with a phosphomimetic mutant, phosphorylation of a stretch of serine residues, contained within PML SBD was needed for PML interaction with SUMO-modified protein partners and thus for NB maturation. However, mutation of these serine residues did not impair As2O3- and RNF4-induced PML degradation, contrasting with the known role of these phosphoserine residues for casein kinase 2-promoted PML degradation. Altogether, these data suggest a model whereby sumoylation- and SBD-dependent PML oligomerization within NBs is sufficient for RNF4-mediated PML degradation and does not require the phosphorylation-dependent association of PML with other sumoylated partners.

Regulation of apoptosis by PML and the PML-NBs

The promyelocytic leukemia protein (PML) is a tumor suppressor identified in acute PML and implicated in the pathogenesis of a variety of tumors. PML is essential for the proper assembly of a nuclear macromolecular structure called the PML nuclear body (PML-NB). PML and PML-NBs are functionally promiscuous and have been associated with the regulation of several cellular functions. Above all these is the control of apoptosis, a function of PML whose physiological relevance is emphasized by in vivo studies that demonstrate that mice and cells lacking Pml are resistant to a vast variety of apoptotic stimuli. The function of PML in regulating apoptosis is not confined to a linear pathway; rather, PML works within a regulatory network that finely tunes various apoptotic pathways, depending on the cellular context and the apoptotic stimulus. Here, we will summarize earlier and recent advances on the molecular mechanisms by which PML regulates apoptosis and the implication of these findings for cancer pathogenesis.