The ubiquitin-proteasome pathway in cancer

Transcription factor E2F-1 acts as a growth-promoting factor and is associated with adverse prognosis in non-small cell lung carcinomas

Numerous upstream stimulatory and inhibitory signals converge to the pRb/E2F pathway, which governs cell-cycle progression, but the information concerning alterations of E2F-1 in primary malignancies is very limited. Several in vitro studies report that E2F-1 can act either as an oncoprotein or as a tumour suppressor protein. In view of this dichotomy in its functions and its critical role in cell cycle control, this study examined the following four aspects of E2F-1 in a panel of 87 non-small cell lung carcinomas (NSCLCs), previously analysed for defects in the pRb-p53-MDM2 network: firstly, the status of E2F-1 at the protein, mRNA and DNA levels; secondly, its relationship with the kinetic parameters and genomic instability of the tumours; thirdly, its association with the status of its transcriptional co-activator CBP, downstream target PCNA and main cell cycle regulatory and E2F-1-interacting molecules pRb, p53 and MDM2; and fourthly, its impact on clinical outcome. The protein levels of E2F-1 and its co-activator CBP were significantly higher in the tumour area than in the corresponding normal epithelium (p<0.001). E2F-1 overexpression was associated with increased E2F-1 mRNA levels in 82% of the cases examined. The latter finding, along with the low frequency of E2F-1 gene amplification observed (9%), suggests that the main mechanism of E2F-1 protein overexpression in NSCLCs is deregulation at the transcriptional level. Mutational analysis revealed only one sample with asomatic mutation at codon 371 (Glu-->Asp) and one carrying a polymorphism at codon 393 (Gly-->Ser). Carcinomas with increased E2F-1 positivity demonstrated a significant increase in their growth indexes (r=0.402, p=0.001) and were associated with adverse prognosis (p=0.033 by Cox regression analysis). The main determinant of the positive association with growth was the parallel increase between E2F-1 staining and proliferation (r=0.746, p<0.001), whereas apoptosis was not influenced by the status of E2F-1. Moreover, correlation with the status of the pRb-p53-MDM2 network showed that the cases with aberrant pRb expression displayed significantly higher E2F-1 indexes (p=0.033), while a similar association was noticed in the group of carcinomas with deregulation of the p53-MDM2 feedback loop. In conclusion, the results suggest that E2F-1 overexpression may contribute to the development of NSCLCs by promoting proliferation and provide evidence that this role is further enhanced in a genetic background with deregulated pRb-p53-MDM2 circuitry.

Genome-wide search of Schizosaccharomyces pombe genes causing overexpression-mediated cell cycle defects

Genetic studies in yeasts enable an in vivo analysis of gene functions required for the cell division cycle (cdc genes) in eukaryotes. In order to characterize new functions involved in cell cycle regulation, we searched for genes causing cell division defects by overexpression in the fission yeast Schizosaccharomyces pombe. By using this dominant genetic strategy, 26 independent clones were isolated from a Sz. pombe cDNA library. The cloned cDNAs were partially sequenced and identified by computer analysis. The 26 clones isolated corresponded to 21 different genes. Among them, six were genes previously characterized in Sz. pombe, 11 were homologues to genes identified and characterized in other organisms, and four represented genes with unknown functions. In addition to known cell cycle regulators encoding inhibitory protein kinases (wee1, pka1) and DNA checkpoint proteins (Pcna, rad24), we have identified genes that are involved in a number of cellular processes. This includes protein synthesis (ribosomal proteins L7, L10, L29, L41, S6, S11, S17 and the PolyA-Binding Protein PABP), protein degradation (UBI3), nucleolar rRNA expression (fib, imp1, dbp2), cell cytoskeleton (act1) and glycolysis (pfk1). The interference caused in the cell cycle by overexpression of these genes may elucidate novel mechanisms coupling different cellular processes with the control of the cell division. The effect caused by some of them is described in more detail.

Expression of RALT, a feedback inhibitor of ErbB receptors, is subjected to an integrated transcriptional and post-translational control

Over-expression studies have demonstrated that RALT (receptor associated late transducer) is a feedback inhibitor of ErbB-2 mitogenic and transforming signals. In growth-arrested cells, expression of endogenous RALT is induced by mitogenic stimuli, is high throughout mid to late G1 and returns to baseline as cells move into S phase. Here, we show that physiological levels of RALT effectively suppress ErbB-2 mitogenic signals. We also investigate the regulatory mechanisms that preside to the control of RALT expression. We demonstrate that pharmacological ablation of extracellular signal-regulated kinase (ERK) activation leads to blockade of RALT expression, unlike genetic and/or pharmacological interference with the activities of PKC, Src family kinases, p38 SAPK and PI-3K. Tamoxifen-dependent activation of an inducible Raf : ER chimera was sufficient to induce RALT expression. Thus, activation of the Ras-Raf-ERK pathway is necessary and sufficient to drive RALT expression. The RALT protein is labile and was found to accumulate robustly upon pharmacological inhibition of the proteasome. We were able to detect ubiquitin-conjugated RALT species in living cells, suggesting that ubiquitinylation targets RALT for proteasome-dependent degradation. Such an integrated transcriptional and post-translational control is likely to provide RALT with the ability to fluctuate timely in order to tune ErbB signals.

Cellular drug action profile paradigm applied to XK469

The cellular paradigm presented here defines the cellular action profile of new anticancer agents that complements our discovery and development paradigm. The main elements of this profile include a concentration clonogenicity response relationship on proliferating and plateau phase cells, flow cytometry studies assessing progression delay and apoptosis, macromolecular synthesis inhibition, and DNA damage assessment by the comet assay; other specific assessments then derive from these findings such as topoisomerase assays. XK469 is a new anticancer agent derived from the herbicide Assure that is the inactive parent compound of a family of quinoxaline analogs found to have anticancer activity in vivo. We have applied the described cellular action profile paradigm to XK469 to define a novel action at the cellular level. XK469 is a G2M phase-specific, antiproliferative agent whose activity is related to the 7-position of the chlorine ion in the benzene ring and expressed through a unique cellular action profile resulting in the irreversible increase in cyclin B1 (possibly by specific inhibition of its ubiquitination) and leading, in the absence of apoptosis, to the final mitotic arrest of HCT-116 cells in prophase with subsequent loss of clonogenicity.

Sequencing, transcript identification, and quantitative gene expression profiling in the breast cancer loss of heterozygosity region 16q24.3 reveal three potential tumor-suppressor genes

Loss of heterozygosity (LOH) of chromosome 16q24.3 is a common genetic alteration observed in invasive ductal and lobular breast carcinomas. We constructed a physical map and generated genomic DNA sequence data spanning 2.4 Mb in this region. Detailed in silico and in vitro analyses of the genomic sequence data enabled the identification of 104 genes. It was hypothesized that tumor-suppressor genes would exhibit marked mRNA expression variability in a panel of breast cancer cell lines as a result of downregulation due to mutation or hypermethylation. We examined the mRNA expression profiles of the genes identified at 16q24.3 in normal breast, a normal breast epithelial cell line, and several breast cancer cell lines exhibiting 16q24.3 LOH. Three of the genes, CYBA, Hs.7970, and CBFA2T3, exhibited variability ten times higher than the baseline. The possible role of these genes as tumor suppressors is discussed.

Overexpression of p28/gankyrin in human hepatocellular carcinoma and its clinical significance

AIM: To investigate the expression of p28/gankyrin gene and its role in the carcinogenetic process of human hepatocellular carcinoma (HCC).

METHODS: 64 specimens of HCC and para-carcinoma tissues, 22 specimens of non-tumor liver tissues (7 normal, 15 cirrhosis), 10 specimens of normal human tissues and 5 hepatoma cell lines were studied for the expression of p28/gankyrin by Northern blot. The expression of p28/gankyrin protein was detected immunohistochemically by using the specific polyclonal antibody.

RESULTS: Northern blot analysis indicated that the expression of p28/gankyrin mRNA was intensively distributed in brain and heart, weakly in lung, spleen and muscle, undetectable in digestive system including liver, pancreas, stomach, small and large intestines. p28/gankyrin mRNA was absent in normal liver, weakly detected in liver cirrhosis and in 18 of 64 para-carcinoma liver tissues. In contrast, the expression of p28/gankyrin mRNA was intensively detected in all 5 hepatoma cell lines tested, markedly increased in 57 of 64 and moderately increased in 5 of 64 HCC samples. In comparison with liver cirrhosis and para-carcinoma liver tissues, the average expression of p28/gankyrin mRNA in HCC was increased 3.6- (2.901 ± 0.507 vs 0.805 ± 0.252, P < 0.05) and 5.2-fold (2.901 ± 0.507 vs 0.557 ± 0.203, P < 0.01), respectively. In addition, p28/gankyrin mRNA expression level was higher in HCC with portal vein tumor thrombus and microscopic hepatic vein involvement (P = 0.021 and P = 0.047, respectively). The overexpression of p28/gankyrin protein in HCC was targeted in hepatic tumor cells, not in bile duct cells and other interstitial cells.

CONCLUSION: Overexpression of p28/gankyrin in HCC plays an important role and contributes to the metastasis potential in the process of carcinogenesis. p28/gankyrin may become a specific biological tissue marker for the pathological diagnosis of HCC.

Role and function of the 26S proteasome in proliferation and apoptosis

The 26S proteasome constitutes the central proteolytic machinery of the highly conserved ubiquitin/proteasome system, the cell's major tool for extralysosomal protein degradation. Recently, a plethora of cell proteins implicated in the regulation of basic cellular processes, such as proliferation, differentiation, cell cycling, and apoptosis have been discovered to undergo processing and functional limitation by entering the ubiquitin/proteasome pathway with the final destination to be proteolytically degraded by the 26S proteasome. Because both negative and positive regulators of proliferation and apoptosis undergo proteasomal degradation in a tightly regulated and temporally controlled fashion, the 26S proteasome can play opposite roles in the regulation of proliferation and apoptosis. These roles are apparently defined by the cell's environment and proliferative state. Finally, proteasomal protein degradation is deregulated in a number of human diseases, including cancer and neurodegenerative and myodegenerative diseases, which all exhibit an imbalance of proliferation and apoptosis. An improved understanding of the modes of proteasomal action should lead to the development of beneficial therapeutic and diagnostic strategies in the future.

Regulating the 26S proteasome

Despite the fact that the composition of proteasomes purified from different species is almost identical, and the basic components of the proteasome are remarkably conserved among all eukaryotes, there are quite a few additional proteins that show up in certain purifications or in certain screens. There is increasing evidence that the proteasome is in fact a dynamic structure forming multiple interactions with transiently associated subunits and cellular factors that are necessary for functions such as cellular localization, presentation of substrates, substrate-specific interactions, or generation of varied products. Harnessing the eukaryotic proteasome to its defined regulatory roles has been achieved by a number of means: (a) increasing the complexity of the proteasome by gene duplication, and differentiation of members within each gene family (namely the CP and RPT subunits); (b) addition of regulatory particles, complexes, and factors that influence both what enters and what exits the proteasome; and (c) signal-dependent alterations in subunit composition (for example, the CP beta to beta i exchange). It is not be surprising that the proteasome plays diverse roles, and that its specific functions can be fine-tuned depending on biological context or need.

Identification of genes over-expressed in small cell lung carcinoma using suppression subtractive hybridization and cDNA microarray expression analysis

To identify genes that are differentially over-expressed in Small Cell Lung Carcinoma (SCLC) we have used a combination of suppression subtractive hybridization and cDNA microarray to analyse the expression profiles of 2400 cDNAs clones. Genes that are over-expressed in SCLC were identified using 32 pairs of fluorescence-labeled cDNA samples representing various lung tumors and normal tissues. This comprehensive approach has resulted in the identification of 209 genes that are differentially over-expressed in SCLC. Quantitative real-time PCR was used to further validate the expression of 43 genes in SCLC tumors and various normal tissues. Discussed in this report are nine genes, which showed the most promising SCLC tumor to normal tissue differential expression profiles, including seven known and two novel genes. The large number of differentially expressed genes identified from this analysis and the characterization of these genes will provide valuable information in better understanding the biology of SCLC and help us in developing these gene products as potential targets for diagnostic as well as therapeutic usage.

Papillomaviruses and cancer: from basic studies to clinical application

Links between human papillomaviruses (HPVs) and cervical cancer were first suspected almost 30 years ago. DNA of specific HPV types has since been found in almost all cervical cancer biopsies. HPV oncogenes that are expressed in these cells are involved in their transformation and immortalization, and are required for the progression towards malignancy. Epidemiological studies have underlined that HPVs are the main aetiological factor for cervical cancer. But how has this knowledge been translated into the clinic to allow the prevention, screening and treatment of cervical cancer?

Ubiquitin cytochemical changes during azaserine-initiated pancreatic carcinogenesis

The ubiquitin (Ub)-proteasome proteolytic system is highly selective, and the specific proteins involved in cell division, growth, activation, signaling and transcription are degraded at different rate depending on the physio-pathological state of the cell. Ubiquitination serves first of all as a signal for protein degradation of short-lived and abnormal proteins under several stressful conditions. The immunocytochemical localization of Ub in some malignant tumours has recently been presented and differences in Ub expression has been observed during malignant transformation. Change in the level of Ub and Ub-conjugated proteins might reflect a higher metabolic-catabolic ratio in neoplastic cells. Most studies have been focused on the malignant stage of tumour progression, and only a few papers have dealt with the change in Ub and Ub-protein conjugates level during the whole progression. To address this problem, we applied an azaserine-induced pancreatic carcinogenesis model, in which premalignant and malignant stages were investigated throughout the progression. The level of Ub immunoreactivity was measured in nucleus and cytoplasm by electron microscopic immunocytochemical and morphometrical methods. We found a significant increase of Ub level in the nucleus and the cytoplasmic area in premalignant atypical acinar cell nodule (AACN) cells and in malignant adenocarcinoma in situ (CIS) cells at month 20 after initiation.

The proteasome: a novel target for cancer chemotherapy

The ubiquitin-proteasome system is an important regulator of cell growth and apoptosis. The potential of specific proteasome inhibitors to act as novel anti-cancer agents is currently under intensive investigation. Several proteasome inhibitors exert anti-tumour activity in vivo and potently induce apoptosis in tumour cells in vitro, including those resistant to conventional chemotherapeutic agents. By inhibiting NF-kappaB transcriptional activity, proteasome inhibitors may also prevent angiogenesis and metastasis in vivo and further increase the sensitivity of cancer cells to apoptosis. Proteasome inhibitors also exhibit some level of selective cytotoxicity to cancer cells by preferentially inducing apoptosis in proliferating or transformed cells or by overcoming deficiencies in growth-inhibitory or pro-apoptotic molecules. High expression of oncogene products like c-Myc also makes cancer cells more susceptible to proteasome inhibitor-induced apoptosis. The induction of apoptosis by proteasome inhibitors varies between cell types but often occurs following an initial accumulation of short-lived proteins such as p53, p27, pro-apoptotic Bcl-2 family members or activation of the stress kinase JNK. These initial events often result in a perturbation of mitochondria with concomitant release of cytochrome c and activation of the Apaf-1 containing apoptosome complex. This results in activation of the apical caspase-9 followed by activation of effector caspases-3 and -7, which are responsible for the biochemical and morphological changes associated with apoptosis.

Identification of genes regulated by dexamethasone in multiple myeloma cells using oligonucleotide arrays

Our previous studies have characterized Dexamethasone (Dex)-induced apoptotic signaling pathways in multiple myeloma (MM) cells; however, related transcriptional events are not fully defined. In the present study, gene expression profiles of Dex-treated MM cells were determined using oligonucleotide arrays. Dex triggers early transient induction of many genes involved in cell defense/repair-machinery. This is followed by induction of genes known to mediate cell death and repression of growth/survival-related genes. The molecular and genetic alterations associated with Dex resistance in MM cells are also unknown. We compared the gene expression profiles of Dex-sensitive and Dex-resistant MM cells and identified a number of genes which may confer Dex-resistance. Finally, gene profiling of freshly isolated MM patient cells validates our in vitro MM cell line data, confirming an in vivo relevance of these studies. Collectively, these findings provide insights into the basic mechanisms of Dex activity against MM, as well as mechanisms of Dex-resistance in MM cells. These studies may therefore allow improved therapeutic uses of Dex, based upon targeting genes that regulate MM cell growth and survival.

Purification and identification of monoubiquitin-phosphoglycerate mutase B complex from human colorectal cancer tissues

Ubiquitin-conjugated proteins in human colorectal cancer tissues were analyzed by the immunoprecipitation with the antibody FK2 against conjugated ubiquitin followed with SDS-PAGE. In these immunoprecipitable proteins, a 38-kDa protein was abundant in the tumor regions but almost absent in the adjacent normal regions in 17/26 patients, thus we attempted to purify it. Using immunoaffinity chromatography with the antibody FK2 followed by gel filtration and SDS-PAGE, approximately 10 pmol of this protein was separated from 34 g of the pooled cancerous tissue and transferred onto a PVDF membrane. The 38-kDa protein was further digested with Achromobacter protease I, resulting in several peptide fragments. Amino acid sequences of these peptides showed complete sequence identity to those derived from either ubiquitin or phosphoglycerate mutase-B, suggesting that the 38-kDa protein is monoubiquitinated phosphoglycerate mutase-B, whose calculated mass is 37,369 Da. Western blot using an antibody against phosphoglycerate mutase-B revealed the presence of the 38-kDa protein in the anti-ubiquitin immunoprecipitates derived from the tumor regions, but not from normal counterparts. In addition, part of non-ubiquitinated phosphoglycerate mutase-B (29 kDa) was also found in the anti-ubiquitin immunoprecipitates, whose levels were higher in the tumor regions than in the adjacent normal regions. These results suggest that monoubiquitination of phosphoglycerate mutase-B as well as formation of a noncovalent complex containing ubiquitin and phosphoglycerate mutase-B increases in colorectal cancer and novel modification of phosphoglycerate mutase-B might have a pathophysiological role.

Butyrate suppression of colonocyte NF-kappa B activation and cellular proteasome activity

Butyrate is derived from the microbial metabolism of dietary fiber in the colon where it plays an important role in linking colonocyte turnover and differentiation to luminal content. In addition, butyrate appears to have both anti-inflammatory and cancer chemopreventive activities. Using confocal microscopy and cell fractionation studies, butyrate pretreatment of a human colon cell line (HT-29 cells) inhibited the tumor necrosis factor-alpha (TNF-alpha)-induced nuclear translocation of the proinflammatory transcription factor NF-kappaB. Butyrate inhibited NF-kappaB DNA binding within 30 min of TNF-alpha stimulation, consistent with an inhibition of nuclear translocation. IkappaB.NF-kappaB complexes extracted from butyrate-treated cells were relatively resistant to in vitro dissociation by deoxycholate, suggesting a change in cellular IkappaB composition. Butyrate treatment increased p100 expression, an IkappaB that was not degraded upon TNF-alpha treatment. Butyrate also reduced the extent of TNF-alpha-induced IkappaB-alpha degradation and enhanced the presence of ubiquitin-conjugated IkappaB-alpha. The suppression of IkappaB-alpha degradation corresponded with a reduction in cellular proteasome activity as determined by in vitro proteasome assays and the increased presence of ubiquitin-conjugated proteins. The butyrate suppression of IkappaB-alpha degradation and proteasome activity may derive from its ability to inhibit histone deacetylases since the specific deacetylase inhibitor trichostatin A had similar effects. These results suggest a potential mechanism for the anti-inflammatory activity of butyrate and demonstrate the interplay between short chain fatty acids and cellular proteasome activity.

Ubiquitin and malignant transformation of oral mucosa

BACKGROUND

Ubiquitinylation is a cyclical process operating in all cells to target specific proteins (eg, p53) for degradation. Abnormal accumulations of ubiquitinylated proteins have been identified in colorectal carcinoma. This study investigated abnormal ubiquitinylated proteins and changes in ubiquitinylation patterns associated with malignant transformation of oral tissue.

METHODS

Twenty-three fresh specimens of normal, premalignant, oral squamous cell carcinoma (SCC), and its adjacent normal mucosa were collected for immunohistochemistry with anti-ubiquitin and anti-UbcH7 (a human ubiquitin-conjugating enzyme) antibodies using light and electron microscopy and protein analysis by Western blot.

RESULTS

Ubiquitin and UbcH7 were observed in both nucleus and cytoplasm of normal, "premalignant" and malignant tissues of oral mucosa using immunoelectron microscopy. However there was no change in their distribution pattern.

CONCLUSIONS

The lack of demonstrable difference in distribution pattern of ubiquitin enzymes indicates that at present, ubiquitin cannot be used as a diagnostic marker for malignant transformation of oral SCC.

Recent advances in breast cancer biology

Within the past year, the draft sequence of the human genome was completed and made available to researchers worldwide. Recent advances in technology along with the vast amount of sequence data on the human genome now provide a previously unimagined means of defining the genetic architecture of cancer cells. Implicit in this approach is the ability to describe the evolution of that architecture as normal breast cells progress toward the malignant phenotype. Ongoing experiments involving the simultaneous analysis of the entire genome in a high-throughput manner are expected to reveal those genes and regulatory mechanisms that are critical at each step of progression toward malignancy, including (1) providing a growth advantage over normal cells, (2) maintaining the malignant state, (3) modulating response to therapy, and (4) developing metastatic potential. Once these data are available, the ability to design preventive, diagnostic, prognostic, and therapeutic tools directed at those targets will be within reach.

Chemosensitization of pancreatic cancer by inhibition of the 26S proteasome

BACKGROUND

Pancreatic cancer is extremely resistant to the induction of apoptosis by chemotherapies; agents that regulate sensitivity to apoptosis may lead to chemosensitization of pancreatic cancer.

MATERIALS AND METHODS

MIA-PaCa-2 human pancreatic cancer cells were treated in vitro with the 26S-proteasome inhibitor PS-341. Levels of the apoptosis-regulating proteins (BCL-2, BAK, and BAX) were determined by Western blotting. The effect of PS-341 on BCL-2 gene transcription was examined using a BCL-2 promoter/luciferase reporter construct. The chemosensitizing effect of PS-341 was determined by measurement of the cytotoxic effect of gemcitabine in the presence of PS-341 (10-1000 nM) using the MTT assay. A corresponding in vivo experiment using tumor xenografts in athymic mice was also performed.

RESULTS

PS-341 decreased BCL-2, without effect on BAX or BAK. The downregulation of BCL-2 by PS-341 appears to be transcriptionally mediated. PS-341 induced apoptosis at high does (1000 nM) and increased the cytotoxicity of gemcitabine at low doses (10-100 nM). Xenograft growth was inhibited 59% by gemcitabine; the addition of PS-341 increased growth inhibition to 75%.

CONCLUSIONS

Inhibition of the 26S proteasome disrupts the cellular content of key regulators of cell cycle progression and apoptotic control leading to increased sensitivity to standard chemotherapeutic agents, such as gemcitabine, in pancreatic cancer. Combination therapy may lead to better response rates.

Nuclear import/export of hRPF1/Nedd4 regulates the ubiquitin-dependent degradation of its nuclear substrates

The ubiquitin-protein ligase (E3), hRPF1/Nedd4, is a component of the ubiquitin-proteasome pathway responsible for substrate recognition and specificity. Although previously characterized as a regulator of the stability of cytoplasmic proteins, hRPF1/Nedd4 has also been suggested to have a role in the nucleus. However, in light of the cytoplasmic localization of hRPF1/Nedd4, it is unclear whether bona fide nuclear substrates of hRPF1/Nedd4 exist, and if so, what mechanism may allow a cytoplasmic ubiquitin ligase to manifest nuclear activity. Our search for nuclear substrates led to the identification of the human proline-rich transcript, brain-expressed (hPRTB) protein, the ubiquitination and degradation of which is regulated by hRPF1/Nedd4. Interestingly, hPRTB colocalizes with the splicing factor SC35 in nuclear speckles. Finally, we demonstrate that hRPF1/Nedd4 is indeed capable of entering the nucleus; however, the presence of a functional Rev-like nuclear export sequence in hRPF1/Nedd4 ensures a predominant cytoplasmic localization. Cumulatively, these findings highlight a nuclear role for the ubiquitin ligase hRPF1/Nedd4 and underscore cytoplasmic/nuclear localization as an important regulatory component of hRPF1/Nedd4-substrate recognition.

The corepressor mSin3a interacts with the proline-rich domain of p53 and protects p53 from proteasome-mediated degradation

While the transactivation function of the tumor suppressor p53 is well understood, less is known about the transrepression functions of this protein. We have previously shown that p53 interacts with the corepressor protein mSin3a (hereafter designated Sin3) in vivo and that this interaction is critical for the ability of p53 to repress gene expression. In the present study, we demonstrate that expression of Sin3 results in posttranslational stabilization of both exogenous and endogenous p53, due to an inhibition of proteasome-mediated degradation of this protein. Stabilization of p53 by Sin3 requires the Sin3-binding domain, determined here to map to the proline-rich region of p53, from amino acids 61 to 75. The correlation between Sin3 binding and stabilization supports the hypothesis that this domain of p53 may normally be subject to a destabilizing influence. The finding that a synthetic mutant of p53 lacking the Sin3-binding domain has an increased half-life in cells, compared to wild-type p53, supports this premise. Interestingly, unlike retinoblastoma tumor suppressor protein, MDMX, and p14(ARF), Sin3 stabilizes p53 in an MDM2-independent manner. The ability of Sin3 to stabilize p53 is consistent with the model whereby these two proteins must exist on a promoter for extended periods, in order for repression to be an effective mechanism of gene regulation. This model is consistent with our data indicating that, unlike the p300-p53 complex, the p53-Sin3 complex is immunologically detectable for prolonged periods following exposure of cells to agents of DNA damage.

Proteasomes are a target of the anti-tumour drug vinblastine

Proteasomes, the proteolytic machinery of the ubiquitin/ATP-dependent pathway, have a relevant role in many processes crucial for cell physiology and cell cycle progression. Proteasome inhibitors are used to block cell cycle progression and to induce apoptosis in certain cell lines. Here we examine whether proteasomal function is affected by the anti-tumour drug vinblastine, whose cytostatic action relies mainly on the disruption of mitotic spindle dynamics. The effects of vinblastine on the peptidase activities of human 20 S and 26 S proteasomes and on the proteolytic activity of 26 S proteasome were assessed in the presence of specific fluorogenic peptides and (125)I-lysozyme-ubiquitin conjugates respectively. The assays of ubiquitin-protein conjugates and of inhibitory kappa B alpha (I kappa B alpha), which are characteristic intracellular proteasome substrates, by Western blotting on lysates from HL60 cells incubated with or without vinblastine, illustrated the effects of vinblastine on proteasomes in vivo. We also evaluated the effects of vinblastine on the signal-induced degradation of I kappa B alpha. Vinblastine at 3--110 microM reversibly inhibited the chymotrypsin-like activity of the 20 S proteasome and the trypsin-like and peptidyl-glutamyl-peptide hydrolysing activities of both proteasomes, but only at 110 microM vinblastine was the chymotrypsin-like activity of the 26 S proteasome inhibited; furthermore, at 25--200 microM the drug inhibited the degradation of ubiquitinated lysozyme. In HL60 cells exposed for 6 h to 0.5--10 microM vinblastine, the drug-dose-related accumulation of polyubiquitinated proteins, as well as that of a high-molecular-mass form of I kappa B alpha, occurred. Moreover, vinblastine impaired the signal-induced degradation of I kappa B alpha. Cell viability throughout the test was approx. 95%. Proteasomes can be considered to be a new and additional vinblastine target.

Genetic imbalances in preleukemic thymuses

To understand the molecular mechanisms involved in preleukemia, the suppression subtractive hybridization method was used in a murine radiation-induced thymic lymphoma model. Seventeen mRNAs overexpressed in preleukemic thymuses were identified: mouse laminin binding protein (p40/37LBP), E25 protein, Rattus norvegicus clone BB.1.4.1, profilin, poly(A) binding protein (PABP), mouse high mobility group protein 1, topoisomerase I, clusterin, proteasome RC1 subunit, rat prostatein C3 and C1 subunits; two ESTs and four unknown genes. The overexpression of PABP, clusterin, profilin, and the p40/37LBP mRNAs was confirmed in preleukemic thymuses and can be related to some cellular events observed during the preleukemic period, i.e., alterations of cell cycle and apoptosis properties. The p40/37LBP and 67-kDa laminin receptor proteins were upregulated during the preleukemic period. The data suggest that additional studies on p40/37LBP and 67-kDa laminin receptor regulation are required to evaluate their potential role in the lymphoma prevention by TNF-alpha and IFN-gamma.

Mechanism of action of camptothecin

Camptothecin (CPT) class of compounds has been demonstrated to be effective against a broad spectrum of tumors. Their molecular target has been firmly established to be human DNA topoisomerase I (topo I). CPT inhibits topo I by blocking the rejoining step of the cleavage/religation reaction of topo-I, resulting in accumulation of a covalent reaction intermediate, the cleavable complex. The primary mechanism of cell killing by CPT is S-phase-specific killing through potentially lethal collisions between advancing replication forks and topo-I cleavable complexes. Collisions with the transcription machinery have also been shown to trigger the formation of long-lived covalent topo-I DNA complexes, which contribute to CPT cytotoxicity. Two novel repair responses to topo-I-mediated DNA damage involving covalent modifications of topo-I have been discovered. The first involves activation of the ubiquitin/26S proteasome pathway, leading to degradation of topo-I (CPT-induced topo-I downregulation). The second involves SUMO conjugation to topo-I. The potentials roles of these new mechanisms for repair of topo-I-mediated DNA damage in determining CPT sensitivity/resistance in tumor cells are discussed.

Identification of differentially expressed genes following treatment of monkey kidney cells with the mycotoxin fumonisin B(1)

Fumonisin B(1) (FB(1)) is a mycotoxin produced by the phytopathogenic fungus Fusarium moniliforme, which structurally resembles sphingoid bases. FB(1) perturbs sphingolipid synthesis by inhibiting the activity of ceramide synthase. Depending on the host, ingestion of FB(1) causes equine leukoencephalomalacia or porcine pulmonary edema. It is also carcinogenic to rats and may play a role in certain human cancers. Previous studies showed that FB(1) repressed specific isoforms of protein kinase C and cyclin-dependent kinase 2 (CDK2) activity. Conversely, FB(1) induced expression of CDK inhibitors, p21(Waf1/Cip1), p27(Kip1), and p57(Kip2) in monkey kidney cells (CV-1). Consequently, FB(1) treatment of CV-1 cells leads to cell-cycle arrest and apoptosis. The baculovirus IAP gene (inhibitor of apoptosis), which blocks tumor necrosis factor (TNF)-induced apoptosis, protects several fibroblast cell types from apoptosis, suggesting the TNF pathway is important for FB(1)-induced apoptosis. To identify genes that are induced by FB(1), we used a PCR-based subtraction approach. Eight genes that showed high similarity (> 90%) to known mammalian genes were identified. These genes included: tumor necrosis factor type 1 receptor associated protein 2 (TRAP2), human leukemia virus receptor (GLVR1), human Scaffold attachment factor A (SAF-A) also called heterogeneous nuclear ribonucleoprotein U (hnRNP-U), human protein kinase C-binding protein (RACK7), human oligosaccharyl transferase STT3 subunit, mouse WW-domain binding protein 2 (WBP2), human fibronectin, and an unknown human clone. The ability of FB(1) to alter gene expression and signal transduction pathways may be necessary for its carcinogenic and toxic effects.

Proteasome inhibition: a new strategy in cancer treatment

The ubiquitin proteasome pathway is a highly conserved intracellular pathway for the degradation of proteins. Many of the short-lived regulatory proteins which govern cell division, growth, activation, signaling and transcription are substrates that are temporally degraded by the proteasome. In recent years, new and selective inhibitors of the proteasome have been employed in cell culture systems to examine the anti-tumor potential of these agents. This review covers the chemistry of selected proteasome inhibitors, possible mechanisms of action in cell culture and the in vivo examination of proteasome inhibitors in murine and human xenograft tumor models in mice. One inhibitor, PS-341, has recently entered Phase I clinical trials in cancer patients with advanced disease to further test the potential of this approach.

Ubiquitin-proteasome system and increased sensitivity of B-CLL lymphocytes to apoptotic death activation

The ubiquitin-proteasome-dependent proteolytic system has been reported to regulate apoptotic cell death in many experimental cell models. We recently found that B-CLL (chronic lymphocytic leukemia) lymphocytes are hypersensitive to apoptotic death activation through specific inhibition of proteasome function by lactacystin. Lactacystin efficiently activates apoptotic death process in B-CLL lymphocytes at doses at which no apoptotic effect can be observed in normal human lymphocytes in which 10-fold higher doses of lactacystin are required to weakly induce apoptosis. This hypersensitivity of B-cell CLL may be a result of an altered ubiquitin pathway and proteasomal proteolysis in these malignant cells, and this alteration could be specific for this malignancy. Together with other published works, these results suggest that lactacystin, though not per se a discriminatory inhibitor of the ubiquitinated protein processing/degradation, can nonetheless be discriminatory in the apoptotic cell response between B-CLL and normal lymphocytes: the property that promises efficacy in clinical trials of B-cell CLL. This hypothesis is documented by the fact that lymphocytes from patients in complete remission become resistant to lactacystin-induced apoptosis as normal lymphocytes do.

Deregulation of the ubiquitin system and p53 proteolysis modify the apoptotic response in B-CLL lymphocytes

We recently reported increased sensitivity of B-cell chronic lymphocytic leukemia (B-CLL) lymphocytes to apoptotic death activation by the proteasome-specific inhibitor lactacystin. Here, we show that only specific—not nonspecific—proteasomal inhibitors can discriminate between malignant and normal lymphocytes in inducing the apoptotic death response. Indeed, lactacystin and its active metaboliteclasto-lactacystin β-lactone induced apoptotic death in CLL but not in normal lymphocytes. This difference was completely abolished when tripeptide aldehydes such as MG132 or LLnL (which can also inhibit calpains) were used as less specific proteasomal inhibitors. Moreover, B-CLL cells exhibited a constitutive altered ubiquitin-proteasome system, including a threefold higher chymotrypsin-like proteasomal activity and high levels of nuclear ubiquitin-conjugated proteins compared with normal lymphocytes. Interestingly, B-CLL cells also displayed altered proteolytic regulation of wild-type p53, an apoptotic factor reported to be a substrate for the ubiquitin-proteasome system. Nuclear wild-type p53 accumulated after lactacystin treatment used at the discriminating concentration in malignant, but not in normal, lymphocytes. In contrast, p53 was stabilized by MG132 or LLnL in malignant and normal cells undergoing apoptosis, indicating that in normal lymphocytes p53 is regulated mainly by calpains and not by the ubiquitin-proteasome system. This work raises the possibility that two different proteolytic pathways controlling p53 stability may be pathologically imbalanced. This could result in modification of apoptosis control, since in CLL-lymphocytes a highly upregulated ubiquitin-proteasome system, which controls p53 stability among other apoptotic factors, was correlated with an increased propensity of these cells to apoptosis triggered by lactacystin.

Deregulation of the ubiquitin system and p53 proteolysis modify the apoptotic response in B-CLL lymphocytes

We recently reported increased sensitivity of B-cell chronic lymphocytic leukemia (B-CLL) lymphocytes to apoptotic death activation by the proteasome-specific inhibitor lactacystin. Here, we show that only specific—not nonspecific—proteasomal inhibitors can discriminate between malignant and normal lymphocytes in inducing the apoptotic death response. Indeed, lactacystin and its active metaboliteclasto-lactacystin β-lactone induced apoptotic death in CLL but not in normal lymphocytes. This difference was completely abolished when tripeptide aldehydes such as MG132 or LLnL (which can also inhibit calpains) were used as less specific proteasomal inhibitors. Moreover, B-CLL cells exhibited a constitutive altered ubiquitin-proteasome system, including a threefold higher chymotrypsin-like proteasomal activity and high levels of nuclear ubiquitin-conjugated proteins compared with normal lymphocytes. Interestingly, B-CLL cells also displayed altered proteolytic regulation of wild-type p53, an apoptotic factor reported to be a substrate for the ubiquitin-proteasome system. Nuclear wild-type p53 accumulated after lactacystin treatment used at the discriminating concentration in malignant, but not in normal, lymphocytes. In contrast, p53 was stabilized by MG132 or LLnL in malignant and normal cells undergoing apoptosis, indicating that in normal lymphocytes p53 is regulated mainly by calpains and not by the ubiquitin-proteasome system. This work raises the possibility that two different proteolytic pathways controlling p53 stability may be pathologically imbalanced. This could result in modification of apoptosis control, since in CLL-lymphocytes a highly upregulated ubiquitin-proteasome system, which controls p53 stability among other apoptotic factors, was correlated with an increased propensity of these cells to apoptosis triggered by lactacystin.

Proteasome inhibitors as anti-cancer agents

The ubiquitin (Ub)-proteasome pathway is the major nonlysosomal pathway of proteolysis in human cells and accounts for the degradation of most short-lived, misfolded or damaged proteins. This pathway is important in the regulation of a number of key biological regulatory mechanisms. Proteins are usually targeted for proteasome-mediated degradation by polyubiquitinylation, the covalent addition of multiple units of the 76 amino acid protein Ub, which are ligated to 1-amino groups of lysine residues in the substrate. Polyubiquitinylated proteins are degraded by the 26S proteasome, a large, ATP-dependent multicatalytic protease complex, which also regenerates monomeric Ub. The targets of this pathway include key regulators of cell proliferation and cell death. An alternative form of the proteasome, termed the immunoproteasome, also has important functions in the generation of peptides for presentation by MHC class I molecules. In recent years there has been a great deal of interest in the possibility that proteasome inhibitors, through elevation of the levels of proteasome targets, might prove useful as a novel class of anti-cancer drugs. Here we review the progress made to date in this area and highlight the potential advantages and weaknesses of this approach.

Proteasome inhibition: a new strategy in cancer treatment

The ubiquitin proteasome pathway is a highly conserved intracellular pathway for the degradation of proteins. Many of the short-lived regulatory proteins which govern cell division, growth, activation, signaling and transcription are substrates that are temporally degraded by the proteasome. In recent years, new and selective inhibitors of the proteasome have been employed in cell culture systems to examine the anti-tumor potential of these agents. This review covers the chemistry of selected proteasome inhibitors, possible mechanisms of action in cell culture and the in vivo examination of proteasome inhibitors in murine and human xenograft tumor models in mice. One inhibitor, PS-341, has recently entered Phase I clinical trials in cancer patients with advanced disease to further test the potential of this approach.

Phosphorylation of human progesterone receptors at serine-294 by mitogen-activated protein kinase signals their degradation by the 26S proteasome

Ligand-dependent down-regulation that leads to rapid and extensive loss of protein is characteristic of several nuclear steroid receptors, including human progesterone receptors (PRs). In breast cancer cells, >95% of PRs are degraded 6 h after the start of progestin treatment. The mechanism for down-regulation is unknown. We examined the role of PR phosphorylation by mitogen-activated protein kinases (MAPKs) in this process. Lactacystin and calpain inhibitor I, specific inhibitors of the 26S proteasome, blocked progestin-induced down-regulation, and ubiquitinated conjugates of PR accumulated in cells. Ligand-dependent PR degradation was also blocked by specific inhibition of p42 and p44 MAPKs. To define the targets of phosphorylation by this kinase, two serine/proline MAPK consensus sites on PR were mutated. We demonstrate that mutation of PR serine-294 to alanine (S294A) specifically and completely prevents ligand-dependent receptor down-regulation. We also find that rapid, ligand-independent degradation of immature PR intermediates occurs by a proteasome-mediated pathway. These results demonstrate that PR destruction, by either of two alternate routes, is mediated by the 26S proteasome. Specifically, down-regulation of mature PRs occurs by a mechanism in which ligand binding activates PR phosphorylation by MAPKs at a unique serine residue, which then targets the receptors for degradation.

Reduced stability of retinoblastoma protein by gankyrin, an oncogenic ankyrin-repeat protein overexpressed in hepatomas

Hepatocellular carcinoma (HCC) is one of the most common cancers in Asia and Africa, where hepatitis virus infection and exposure to specific liver carcinogens are prevalent. Although inactivation of some tumor suppressor genes such as p53 and p16INK4Ahas been identified, no known oncogene is commonly activated in hepatocellular carcinomas. Here we have isolated genes overexpressed in hepatocellular carcinomas by cDNA subtractive hybridization, and identified an oncoprotein consisting of six ankyrin repeats (gankyrin). The expression of gankyrin was increased in all 34 hepatocellular carcinomas studied. Gankyrin induced anchorage-independent growth and tumorigenicity in NIH/3T3 cells. Gankyrin bound to the product of the retinoblastoma gene (RB1), increasing its phosphorylation and releasing the activity of the transcription factor E2F-1. Gankyrin accelerated the degradation of RB1 in vitro and in vivo, and was identical to or interacted with a subunit of the 26S proteasome. These results demonstrate the importance of ubiquitin-proteasome pathway in the regulation of cell growth and oncogenic transformation, and indicate that gankyrin overexpression contributes to hepatocarcinogenesis by destabilizing RB1.

Estrogen receptor of primary breast cancers: evidence for intracellular proteolysis

Iodinated oestradiol-labeled oestrogen receptor (ER) isoforms devoid of amino-terminal ABC domains represent about two-thirds of the whole receptor population detected in cytosol samples from human breast cancers. This high frequency could not be ascribed to the expression of truncated mRNAs, or to the proteolysis of the native ER peptide at the time of homogenization or assay, suggesting an intracellular proteolysis. Free amino-terminal and ligand-binding domains maintained together within oligomeric structure(s); increase of ionic strength separated them. The amino-terminal region was consistently detected in the cell nucleus by specific immunohistochemistry leading to the concept of a potential intranuclear association between ER cleavage products and/or other regulatory proteins.

Molecular switches that govern the balance between proliferation and apoptosis

Tissue modelling during embryogenesis and tissue homeostasis during adult life is governed by a dynamic equilibrium between growth and programmed cell death (apoptosis). Growth control and apoptosis are intimately associated, and a disturbance of the balance between these two processes often leads to pathological situations, such as for example cell accumulations in cancer. To date many of the molecular mechanisms controlling growth control on the one hand, and apoptosis on the other hand are known, whereas the switch that controls the decision between both pathways remains elusive. A cell is continuously exposed to multiple opposing "death" and "survival" triggers. A challenging question is how a cell senses these signals and decides to live or die. A decision in favour of survival should automatically result in a shut down of the death pathways. Alternatively, a decision for death should result in inhibition of futile attempts to survive. The molecular events controlling this balance of signals will be discussed with special emphasis on the role of cyclin-dependent kinases and the ubiquitin-dependent and proteasome-mediated protein degradation pathway.

Discovering novel chemotherapeutic drugs for the third millennium

There is enormous potential for the discovery of innovative cancer drugs with improved efficacy and selectivity for the third millennium. In this review we show how novel mechanism-based agents are being discovered by focusing on the molecular targets and pathways that are causally involved in cancer formation, maintenance and progression. We also show how new technologies, from genomics through high through-put bioscience, combinatorial chemistry, rational drug design and molecular pharmacodynamic and imaging techniques, are accelerating the pace of cancer drug discovery. The process of contemporary small molecule drug discovery is described and progress and current issues are reviewed. New and potential targets and pathways for therapeutic intervention are illustrated. The first examples of a new generation of molecular therapeutics are now entering hypothesis-testing clinical trials and showing activity. The early years of the new millennium will see a range of exciting new agents moving from bench to bedside and beginning to impact on the management and cure of cancer.

Ubiquitin-mediated proteolysis in learning and memory

Sensitization of defensive reflexes in Aplysia is a simple behavioral paradigm for studying both short- and long-term memory. In the marine mollusk, as in other animals, memory has at least two phases: a short-term phase lasting minutes and a long-term phase lasting several days or longer. Short-term memory is produced by covalent modification of pre-existing proteins. In contrast, long-term memory needs gene induction, synthesis of new protein, and the growth of new synapses. The switch from short-term (STF) to long-term facilitation (LTF) in Aplysia sensory neurons requires not only positive regulation through gene induction, but also the specific removal of several inhibitory proteins. One important inhibitory protein is the regulatory (R) subunit of the cAMP-dependent protein kinase (PKA). Degradation of R subunits, which is essential for initiating long-term stable memory, occurs through the ubiquitin-proteasome pathway.

PGP9.5 as a candidate tumor marker for non-small-cell lung cancer

PGP9.5 is a neurospecific peptide that functions to remove ubiquitin from ubiquitinated proteins and prevents them from targeted degradation by proteasomes. Using the serial analysis of gene expression method (SAGE), we observed that the PGP9.5 transcript was highly expressed in primary lung cancers and lung cancer cell lines but was not detectable in the normal lung. Here we examined the expression of PGP9.5 protein in normal lung epithelium, lung tumor cell lines, and 98 resected primary non-small-cell lung carcinomas (NSCLCs). We found PGP9.5 reactivity in normal lung in a pattern compatible with K-cells of the diffuse neuroendocrine system. However, the PGP9.5 was present in both small-cell lung cancer (SCLC) and NSCLC cell lines (22/24) independent of neuronal differentiation. In primary NSCLCs, 54% (53/98) of the cases had positive PGP9.5 staining, and the expression of protein was strongly associated with pathological stage of the cancer. It was present in 44% (29/66) of stage I NSCLCs and in 75% (24/32) of stage II and IIIA NSCLCs (p = 0.0032). These results suggest that the increased expression of PGP9.5 is specifically associated with lung cancer development and may serve as a potential marker for the detection of lung cancer.

Epoxomicin, a potent and selective proteasome inhibitor, exhibits in vivo antiinflammatory activity

The proteasome regulates cellular processes as diverse as cell cycle progression and NF-kappaB activation. In this study, we show that the potent antitumor natural product epoxomicin specifically targets the proteasome. Utilizing biotinylated-epoxomicin as a molecular probe, we demonstrate that epoxomicin covalently binds to the LMP7, X, MECL1, and Z catalytic subunits of the proteasome. Enzymatic analyses with purified bovine erythrocyte proteasome reveal that epoxomicin potently inhibits primarily the chymotrypsin-like activity. The trypsin-like and peptidyl-glutamyl peptide hydrolyzing catalytic activities also are inhibited at 100- and 1,000-fold slower rates, respectively. In contrast to peptide aldehyde proteasome inhibitors, epoxomicin does not inhibit nonproteasomal proteases such trypsin, chymotrypsin, papain, calpain, and cathepsin B at concentrations of up to 50 microM. In addition, epoxomicin is a more potent inhibitor of the chymotrypsin-like activity than lactacystin and the peptide vinyl sulfone NLVS. Epoxomicin also effectively inhibits NF-kappaB activation in vitro and potently blocks in vivo inflammation in the murine ear edema assay. These results thus define epoxomicin as a novel proteasome inhibitor that likely will prove useful in exploring the role of the proteasome in various in vivo and in vitro systems.

p53 and survival in early onset breast cancer: analysis of gene mutations, loss of heterozygosity and protein accumulation

The p53 protein has proven to be central in tumorigenesis by its cell cycle regulatory properties and both gene mutations and protein accumulation have been associated with poor prognosis in breast cancer. The present study was undertaken to investigate the prognostic significance of gene mutations, p53 protein accumulation and of loss of heterozygosity (LOH) at the TP53 locus in young (age < 37 years) breast cancer patients. In total, gene mutations were found in 21 of the 123 patients (17%), LOH in 20 of the 47 informative cases (43%) and protein accumulation in 47 of the 102 available cases (46%). Log rank analysis revealed no significant association between survival and TP53 mutations (in general), p53 protein accumulation or LOH. However, missense mutations localised to the zinc binding domain were significantly (P = 0.0007) associated with poorer prognosis. As indicated in this as well as other studies, p53 protein accumulation is frequently found in young breast cancer patients, but this protein overexpression appears to be of minor significance for survival. Nevertheless, the present report also suggests that specific mutations contribute substantially to tumour aggressiveness.

Activation of the cell death program by nitric oxide involves inhibition of the proteasome

The ubiquitin/proteasome pathway mediates the degradation of many short-lived proteins that are critically involved in the regulation of cell proliferation and cell death, including the tumor suppressor protein p53. Accumulation of p53 and induction of apoptosis in RAW 264.7 macrophages in response to nitric oxide are well established. However, the molecular mechanisms involved in nitric oxide-induced p53 accumulation are unknown. Here we show that, similar to nitric oxide, treatment of macrophages with specific proteasome inhibitors, including clastolactacystin-beta-lactone, induces p53 accumulation and apoptosis, suggesting that nitric oxide may affect the activity of the proteasome. In support of this hypothesis, both exposure of cells to S-nitrosoglutathione and stimulation of endogenous nitric oxide production by lipopolysaccharide/interferon-gamma treatment result in inhibition of proteasome activity as measured in vitro by the degradation of the proteasome-specific substrate succinyl-Leu-Leu-Val-Tyr-4-methylcoumarin-7-amide. Moreover, chemically diverse nitric oxide donors interfere with proteasome-mediated degradation of polyubiquitinated p53 in vitro. These data imply that nitric oxide-induced apoptosis and accumulation of p53 are, at least in part, mediated by inhibition of the proteasome.

Activation of the ubiquitin proteolytic system in murine acquired immunodeficiency syndrome affects IkappaBalpha turnover

Murine acquired immunodeficiency syndrome (MAIDS) is a complex immunopathology caused by a defective murine leukemia virus (LP-BM5) that mainly targets B-lymphocytes. Lymphadenophathy, splenomegaly, hypergammaglobulinemia and progressive immunodeficiency are prominent features of MAIDS. Previously, we showed that the ubiquitin proteolytic system was upregulated in infected lymph nodes [Crinelli, R., Fraternale, A., Casabianca, A. & Magnani, M. (1997) Eur. J. Biochem. 247, 91-97]. In this report, we demonstrate that increased 26S proteasome activity is responsible for accelerated turnover of the IkappaBalpha inhibitor in lymph node extracts derived from animals with MAIDS. The molecular mechanisms mediating IkappaBalpha proteolysis involved constitutive phosphorylation of IkappaBalpha at Ser32 and Ser36 and subsequent ubiquitination, suggesting persistent activation of an NF-kappaB inducing pathway. Interestingly, enhanced IkappaBalpha degradation did not result in enhanced NF-kappaB DNA binding activity, but rather in a different subunit composition. The modulation of NF-kappaB/IkappaB system may affect multiple immunoregulatory pathways and may in part explain the mechanisms leading to the profound immune dysregulation involved in MAIDS pathogenesis.

Glucose starvation and hypoxia induce nuclear accumulation of proteasome in cancer cells

Solid tumors commonly contain regions with glucose-starved and hypoxic conditions. Tumor cells under the adverse conditions can survive through the stress response, such as cell cycle arrest. In this study, we found that the stress conditions stimulated nuclear accumulation of proteasomes, large multicatalytic protease complexes, in human colon cancer HT-29 cells. The nuclear proteasome levels both in amount and in activity were increased approximately 4 and 2 times by glucose starvation and hypoxia, respectively. No changes were detected in the total expression levels of proteasome. The nuclear proteasome accumulation was also observed in ovarian cancer A2780 cells under glucose starvation, suggesting that this response was regardless of the origin of cancer cells. Our results indicate that the nuclear proteasome distribution is enhanced by glucose starvation and hypoxia, and suggest that the proteolysis by proteasome in the nucleus may play roles in the stress response of solid tumor cells.

Cyclin E dependent kinase activity in human breast cancer in relation to cyclin E, p27 and p21 expression and retinoblastoma protein phosphorylation

The cell cycle machinery is regulated by cyclin dependent kinases and sets of activating and inhibitory proteins. The G1-S control mechanism is often deregulated in tumours supposedly leading to increased kinase activity, phosphorylation of substrates and subsequent S phase entrance. Increased kinase activity has been proposed to be essential in cell cycle aberrations, but few studies have actually shown enhanced kinase activity related to specific cell cycle defects in primary tumours. In the present study we have determined the cyclin E dependent kinase activity (cyclin E(kinase)) in 59 primary breast cancers, using an H1-kinase assay, and related the activity to the expression of cyclin E, p27 and p21. In a subgroup of 48 tumours, we further characterized the association between cyclin E(kinase), in vivo phosphorylation of the retinoblastoma protein (pRb) and proliferation. The cyclin E(kinase) correlated significantly with cyclin E content and inversely with p27 and p21 expression. P27, but not p21, was associated with low cyclin E(kinase) in specimens with normal/low levels of cyclin E. At elevated cyclin E levels, suppression of cyclin E(kinase) seemed to require high levels of both p21 and p27. The cyclin E(kinase) correlated with the phosphorylation status of pRb as well as with proliferation. Surprisingly, pRb phosphorylation did not correlate with proliferation. Our results support that pRb is a substrate for cyclin E(kinase) in primary breast cancer and that deregulation of cyclin E and p27 act through increased CDK-kinase activity, but cyclin E associated events beside pRb phosphorylation might be rate-limiting for entrance into S phase.

Molecular mechanisms of CD8+ T cell-mediated delayed hypersensitivity: implications for allergies, asthma, and autoimmunity

Delayed-type hypersensitivity (DTH) is defined as the recruitment of T cells into tissues to be activated by antigen-presenting cells to produce cytokines that mediate local inflammation. CD8+ T cells are now known to mediate DTH responses in allergic contact dermatitis, drug eruptions, asthma, and autoimmune diseases. This inflammatory effector capability of CD8+ cytotoxic T cells was previously poorly recognized, but there is now considerable evidence that these diseases may be mediated by CD8+ DTH. The difference between CD8+ T cells and CD4+ T cells mediating DTH relates to the molecular mechanisms by which antigens are processed and presented to the T cells. Antigens external to the cell are phagocytosed and processed for presentation on MHC class II molecules (eg, HLA-DR) to CD4+ T cells. In contrast, internal cytoplasmic antigens are processed by the endogenous pathway for presentation on MHC class I molecules (eg, HLA-A, -B, and -C) to CD8+ T cells. External allergens can also enter the endogenous pathway to be presented to CD8+ T cells. These include many contact sensitizers, chemical and protein respiratory allergens, viral antigens, metabolic products of drugs, and autoantigens. The resulting CD8+ T-cell response explains the role of CD8+ T-cell DTH mechanisms in allergic contact dermatitis, asthma, drug eruptions, and autoimmune diseases.