The proteasome system: a neglected tool for improvement of novel therapeutic strategies?

Ovarian cancer molecular pathology

Ovarian cancer (OVC) is the fourth leading cause of cancer mortality among women in Europe and the United States. Its early detection is difficult due to the lack of specificity of clinical symptoms. Unfortunately, late diagnosis is a major contributor to the poor survival rates for OVC, which can be attributed to the lack of specific sets of markers. Aside from patients sharing a strong family history of ovarian and breast cancer, including the BRCA1 and BRCA2 tumor suppressor genes mutations, the most used biomarker is the Cancer-antigen 125 (CA-125). CA-125 has a sensitivity of 80 % and a specificity of 97 % in epithelial cancer (stage III or IV). However, its sensitivity is 30 % in stage I cancer, as its increase is linked to several physiological phenomena and benign situations. CA-125 is particularly useful for at-risk population diagnosis and to assess response to treatment. It is clear that alone, CA-125 is inadequate as a biomarker for OVC diagnosis. There is an unmet need to identify additional biomarkers. Novel and more sensitive proteomic strategies such as MALDI mass spectrometry imaging studies are well suited to identify better markers for both diagnosis and prognosis. In the present review, we will focus on such proteomic strategies in regards to OVC signaling pathways, OVC development and escape from the immune response.

Synthesis and pharmacology of proteasome inhibitors

Shortly after the discovery of the proteasome it was proposed that inhibitors could stabilize proteins which ultimately would trigger apoptosis in tumor cells. The essential questions were whether small molecules would be able to inhibit the proteasome without generating prohibitive side effects and how one would derive these compounds. Fortunately, "Mother Nature" has generated a wide variety of natural products that provide distinct selectivities and specificities. The chemical synthesis of these natural products finally provided access to analogues and optimized drugs of which two different classes have been approved for the treatment of malignancies. Despite these achievements, additional lead structures derived from nature are under investigation and will be discussed with regard to their biological potential and chemical challenges.

Specific MALDI imaging and profiling for biomarker hunting and validation: fragment of the 11S proteasome activator complex, Reg alpha fragment, is a new potential ovary cancer biomarker

MALDI imaging mass spectrometry represents a new analytical tool to directly provide the spatial distribution and relative abundance of proteins in tissue. Twenty-five ovary carcinomas (stages III and IV) and 23 benign ovaries were directly analyzed using MALDI-TOF MS. The biomarker with the major prevalence (80%) has been fully identified using MALDI MS and nanoESI MS and MS/MS after separation by RP-HPLC and trypsin enzymatic digestion. This marker with an m/z of 9744 corresponds to 84 amino acid residues from the 11S proteasome activator complex, named PA28 or Reg-alpha. Validation of this marker has been performed using MALDI imaging, classical immunocytochemistry with an antibody raised against the C-terminal part of the protein, specific MALDI imaging, and Western blot analysis. The validation, using immunocytochemistry, confirmed the epithelial localization of this fragment with nucleus localization in benign epithelial cells and a cytoplasmic localization in carcinoma cells. This indicates that this antibody could be used to discriminate the borderline tumor cases. At this point, a multicentric study needs to be conducted in order to clearly establish the potential of this biomarker. Taken together these studies reflect that direct tissue analysis and specific MALDI imaging strategies facilitate biomarker hunting and validation which can be named pathological proteomics.

Proteasome inhibition in multiple myeloma

The ubiquitin-proteasome pathway is the major cellular degradative system for various proteins critical for proliferation, survival and homing of myeloma cells. Bortezomib is the first specific and reversible proteasome inhibitor for clinical application in humans. Phase I studies have defined the maximum tolerated dose and suggested activity against multiple myeloma. From single agent phase II studies, a rate of at least partial responses ranging from 27% for relapsed and refractory to 38% for second-line patients was derived. In comparison with pulsed dexamethasone, bortezomib enabled a higher response rate, a longer time to myeloma progression and a longer survival for patients after one to three prior lines of therapy. Preclinical and clinical phase I studies as well as initial phase II studies combining bortezomib with conventional chemotherapy or thalidomide support the assumption that bortezomib sensitizes myeloma cells to these drugs resulting in additive or synergistic activity.

Structure of the Mycobacterium tuberculosis proteasome and mechanism of inhibition by a peptidyl boronate

Mycobacterium tuberculosis (Mtb) has the remarkable ability to resist killing by human macrophages. The 750 kDa proteasome, not available in most eubacteria except Actinomycetes, appears to contribute to Mtb's resistance. The crystal structure of the Mtb proteasome at 3.0 A resolution reveals a substrate-binding pocket with composite features of the distinct beta1, beta2 and beta5 substrate binding sites of eukaryotic proteasomes, accounting for the broad specificity of the Mtb proteasome towards oligopeptides described in the companion article [Lin et al. (2006), Mol Microbiol doi:10.1111/j.1365-2958.2005.05035.x]. The substrate entrance at the end of the cylindrical proteasome appears open in the crystal structure due to partial disorder of the alpha-subunit N-terminal residues. However, cryo-electron microscopy of the core particle reveals a closed end, compatible with the density observed in negative-staining electron microscopy that depended on the presence of the N-terminal octapetides of the alpha-subunits in the companion article, suggesting that the Mtb proteasome has a gated structure. We determine for the first time the proteasomal inhibition mechanism of the dipeptidyl boronate N-(4-morpholine)carbonyl-beta-(1-naphthyl)-L-alanine-L-leucine boronic acid (MLN-273), an analogue of the antimyeloma drug bortezomib. The structure improves prospects for designing Mtb-specific proteasomal inhibitors as a novel approach to chemotherapy of tuberculosis.

Inhibitors of the eukaryotic 20S proteasome core particle: a structural approach

The ubiquitin-proteasome pathway is particularly important for the regulated degradation of various proteins which control a vast array of biological processes. Therefore, proteasome inhibitors are promising candidates for anti-tumoral or anti-inflammatory drugs. N-Acetyl-Leu-Leu-Norleucinal (Ac-LLN-al, also termed calpain inhibitor I) was one of the first proteasome inhibitors discovered and has been widely used to study the 20S proteasome core particle (CP) function in vivo, despite its lack of specificity. Vinyl sulfones, like Ac-PRLN-vs, show covalent binding of the beta-carbon atom of the vinyl sulfone group to the Thr1Ogamma only of subunit beta2. However, vinyl sulfones have similar limitations as peptide aldehydes as they have been reported also to bind and block intracellular cysteine proteases. A more specific proteasome inhibitor is the natural product lactacystin, which can be isolated from Streptomyces. It was found that this compound forms an ester bond only to the Thr1Ogamma of the chymotrypsin-like active subunit beta5 due to specific P1 interactions. In contrast to most other proteasome inhibitors, the natural alpha',beta'-epoxyketone peptide epoxomicin binds specifically to the small class of N-terminal nucleophilic (Ntn) hydrolases (CPs belong to this protease family) with the formation of a morpholino adduct. All previously described proteasome inhibitors bind covalently to the proteolytic active sites. However, as the proteasome is involved in a variety of biological important functions, it is of particular interest to block the CP only for limited time in order to reduce cytotoxic effects. Recently, the binding mode of the natural specific proteasome inhibitor TMC-95 obtained from Apiospora montagnei was investigated. The crystal structure revealed that the TMC-95 blocks the active sites of the CP noncovalently in the low nanomolar range. This review summarizes the current structural knowledge of inhibitory compounds bound to the CP, showing the proteasome as a potential target for drug development in medical research.

Signal sequence deletion and fusion to tetanus toxoid epitope augment antitumor immune responses to a human carcinoembryonic antigen (CEA) plasmid DNA vaccine in a murine test system

Carcinoembryonic antigen (CEA, CEACAM5) is expressed on several human carcinomas including colon cancer. CEA contains signal peptides that target the protein through the endoplasmic reticulum and to the cell membrane. We constructed a plasmid DNA vaccine encoding a truncated CEA (deltaCEA), devoid of its signal peptides, and demonstrated that it was retained inside the cell, while full-length CEA (wtCEA) was expressed on the membrane. We hypothesized that intracellular retention of deltaCEA would enhance MHC class I presentation of CEA peptides, thus favoring cellular immune responses. In addition, a promiscuous T-helper epitope (Q830-L844 of tetanus toxoid) was fused to the N-terminal of the truncated CEA gene (tetdeltaCEA). C57BL/6 mice immunized with DNA encoding wtCEA or tetdeltaCEA developed both humoral and cellular immune responses to CEA. SCID mice transplanted with spleen cells from tetdeltaCEA but not wtCEA-immunized C57BL/6 mice showed strong suppression of tumor growth after inoculation of human CEA-expressing colon carcinoma cells. Immune spleen cell populations depleted for either B, T or both B and T cells were active, indicating that effector cells might also reside in other populations. The present approach to manipulating antigen presentation may open new possibilities for immunotherapy against colon and other CEA-secreting carcinomas.

Crystal structure of the 20 S proteasome:TMC-95A complex: a non-covalent proteasome inhibitor

The 20 S proteasome core particle (CP), a multicatalytic protease, is involved in a variety of biologically important processes, including immune response, cell-cycle control, metabolic adaptation, stress response and cell differentiation. Therefore, selective inhibition of the CP will be one possible way to influence these essential pathways. Recently, a new class of specific proteasome inhibitors, TMC-95s, was investigated and we now present a biochemical and crystallographic characterisation of the yeast proteasome core particle in complex with the natural product TMC-95A. This unusual heterocyclic compound specifically blocks the active sites of CPs non-covalently, without modifying the nucleophilic Thr1 residue. The inhibitor is bound to the CP by specific hydrogen bonds with the main-chain atoms of the protein. Analysis of the crystal structure of the complex has revealed which portions of TMC-95s are essential for binding to the proteasome. This will form the basis for the development of synthetic selective proteasome inhibitors as promising candidates for anti-tumoral or anti-inflammatory drugs.

Inhibitory and stimulatory effects of lactacystin on expression of nitric oxide synthase type 2 in brain glial cells. The role of Ikappa B-beta

Expression of inflammatory nitric oxide synthase (NOS2) is mediated by transcription factor NFkappaB. By using the specific proteasome inhibitor lactacystin to examine IkappaB degradation, we observed a paradoxical increase in lipopolysaccharide- and cytokine-dependent NOS2 expression at low concentrations or when lactacystin was added subsequent to cytokines. Lactacystin reduced the initial accumulation of NOS2 mRNA but reduced its subsequent decrease. Lactacystin increased NOS2 promoter activation after 24 h, but not after 4 h, and similarly prevented initial NFkappaB activation and at later times caused NFkappaB reactivation. Lactacystin reduced initial degradation of IkappaB-alpha and IkappaB-beta, however, at later times selectively increased IkappaB-beta, which was predominantly non-phosphorylated. Expression of full-length rat IkappaB-beta, but not a carboxyl-terminal truncated form, inhibited NOS2 induction and potentiation by lactacystin. Lactacystin increased IkappaB-beta expression in the absence of NOS2 inducers, as well as expression of heat shock protein 70, and the heat shock response due to hyperthermia increased IkappaB-beta expression. These results suggest that IkappaB-beta contributes to persistent NFkappaB activation and NOS2 expression in glial cells, that IkappaB-beta is a stress protein inducible by hyperthermia or proteasome inhibitors, and that delayed addition of proteasome inhibitors can have stimulatory rather than inhibitory actions.