Proteasome inhibitors sensitize colon carcinoma cells to TRAIL-induced apoptosis via enhanced release of smac/DIABLO from the mitochondria

Combination treatment of bortezomib and epirubicin increases the expression of TNFRSF10 A/B, and induces TRAIL-mediated cell death in colorectal cancer cells

Colorectal cancer is one of the most common cancers worldwide, affecting the colon and rectum. A major problem in the treatment of colorectal cancer is acquired chemoresistance, including resistance against death receptor-induced apoptosis. Therefore, investigating new biomarkers for the treatment of the disease and sensitization strategies against TRAIL might be of high clinical importance. TNFRSF10A/B are known as death receptors for TRAIL-induced apoptotic cell death. In this study, we used multiple bioinformatic tools and experimental analyses to investigate the role of TRAIL receptors TNFRSF10A and TNFRSF10B in colorectal cancer. We also identified the potential effect of bortezomib and epirubicin in the induction of TRAIL-mediated apoptotic cell death. Here, we showed that TNFRSF10 A/B expressions are upregulated in various tumor types, including COAD, and its high expression is decreased with the different clinicopathological parameters in COAD. We also found an association between TNFRSF10 A/B expression and tumor molecular subtypes. We further detected the association between the expression of TNFRSF10 A/B and immune cell tumor infiltration, including B cells, CD8+ T cells, neutrophils and dendritic cells. In addition, we showed that combining bortezomib and epirubicin treatment leads to the upregulation of TNFRSF10 A/B in colorectal cancer cells in vitro. The increase in the expression of death receptors was correlated with higher active caspase-3 levels following the incubation of cells with recombinant TRAIL protein, which is a ligand for TNFRSF10 A/B receptors. Our results suggest that TNFRSF10 A/B may be a marker to differentiate tumor molecular subtypes in colorectal cancer. The expression of TNFRSF10 A/B may be associated with the recruitment of immune cells into tumors and the development of tumor suppression. The combination of bortezomib and epirubicin treatment might sensitize colorectal cancer cells to TRAIL-induced apoptosis via the upregulation of death receptor.

Serine protease HtrA2/Omi regulates adaptive mitochondrial reprogramming in the brain cortex after ischemia/reperfusion injury via UCP2-SIRT3-PGC1 axis

This study is to investigate the underlying mechanisms of mitochondrial quality control (MQC) regulated by HtrA2/Omi during ischemia/reperfusion (I/R). We utilized the mnd2 mouse model, which has a missense mutation in HtrA2/Omi, to investigate the HtrA2/Omi regulation in mitochondria after I/R injury in the cerebral cortex. Compared to homozygous (HtrA2^mnd2) mice, heterozygous (HtrA2^Hetero) mice showed aging signs at a later age, increased HtrA2/Omi expression in the brain cortex, and lesser neurodegenerative signs. The brain cortex of HtrA2^Hetero mice had increased superoxide dismutase (SOD) activity; lower levels of malondialdehyde (MDA); higher expressions of mitochondrial unfolded protein response (mtUPR)-related proteins, NADH dehydrogenase [ubiquinone] iron-sulfur protein 7 (Ndufs7), and uncoupling protein 2 (UCP2) proteins; more mitochondrial fission; higher levels of ATP and mtDNA copies; elevated sirtuin 3 (SIRT3) activity; and increased NAD⁺/NADH ratio. After 1.5 h of I/R, the brain cortex of HtrA2^Hetero mice had a larger infarction size, reduced HtrA2/Omi expression, decreased S-X-linked inhibitor of apoptosis protein (XIAP), and increased C-Caspase3 than that of wild-type animals (WT). Mitochondria from the HtrA2^Hetero brain cortex showed decreased ATP production and MQC deficiency after 1.5 h I/R. Genipin pre-treatment reduced the aforementioned I/R injury in the HtrA2^Hetero brain cortex. In conclusion, mitochondrial function is compensated in the HtrA2^Hetero brain cortex via the upregulation of the UCP2-SIRT3-PGC1 axis. Decreased HtrA2/Omi function damages mitochondrial quality in the HtrA2^Hetero mouse brain cortex, leading to more brain I/R injury. Genipin pre-treatment ameliorates brain damages via the mitochondrial UCP2-SIRT3-PGC1 axis.

Therapeutic opportunities based on caspase modulation

Caspases are a family of proteolytic enzymes that play a critical role in the regulation of programmed cell death via apoptosis. Activation of caspases is frequently impaired in human cancers, contributing to cancer formation, progression and therapy resistance. A better understanding of the molecular mechanisms regulating caspase activation in cancer cells is therefore highly important. Thus, targeted modulation of caspase activation and apoptosis represents a promising approach for the development of new therapeutic options to elucidate cancer cell death.

Targeting extrinsic apoptosis in cancer: Challenges and opportunities

Apoptosis is a form of programmed cell death that plays a critical role in the regulation of various physiological and pathophysiological processes. Since apoptosis is typically disturbed in human cancers, therapeutic targeting of apoptosis represents a promising avenue for the development of novel therapeutic approaches. This strategy is particularly relevant, since many currently used anticancer therapies utilize apoptosis signaling pathways to exert their antitumor activities. A better understanding of these signaling networks and their deregulation in human cancers is anticipated to open new perspectives for the development of apoptosis-targeted therapies for the treatment of cancer.

Tumor-necrosis-factor-related apoptosis-inducing ligand (TRAIL)

The concept to exploit death receptors for cancer therapy is very attractive, since these cell surface receptors have a direct connection to the intracellular cell death machinery. Among the death receptor superfamily, the tumor-necrosis-factor-related apoptosis-inducing ligand (TRAIL) receptor/ligand system is of special interest. TRAIL receptor agonists have recently entered the stage of clinical evaluation for the treatment of human cancers. Further insights into the regulatory mechanisms of TRAIL signaling will help to better understand the determinants of TRAIL sensitivity versus resistance of human cancers.

Caffeic acid phenylethyl ester and MG132, two novel nonconventional chemotherapeutic agents, induce apoptosis of human leukemic cells by disrupting mitochondrial function

The ability to modulate balance between cell survival and death is recognized for its great therapeutic potential. Therefore, research continues to focus on elucidation of cell machinery and signaling pathways that control cell proliferation and apoptosis. Conventional chemotherapeutic agents often have a cytostatic effect over tumor cells. New natural or synthetic chemotherapeutic agents have a wider spectrum of interesting antitumor activities that merit in-depth studies. In the present work, we aimed at characterizing the molecular mechanism leading to induction of cell death upon treatment of the lymphoblastoid cell line PL104 with caffeic acid phenylethyl ester (CAPE), MG132 and two conventional chemotherapeutic agents, doxorubicine (DOX) and vincristine (VCR). Our results showed several apoptotic hallmarks such as phosphatidylserine (PS) exposure on the outer leaflet of the cell membrane, nuclear fragmentation, and increase sub-G1 DNA content after all treatments. In addition, all four drugs downregulated survivin expression. CAPE and both chemotherapeutic agents reduced Bcl-2, while only CAPE and MG132 significantly increased Bax level. CAPE and VCR treatment induced the collapse of mitochondrial membrane potential (∆ψm). All compounds induced cytochrome c release from mitochondrial compartment to cytosol. However, only MG132 caused the translocation of Smac/DIABLO. Except for VCR treatment, all other drugs increased reactive oxygen species (ROS) production level. All treatments induced activation of caspases 3/7, but only CAPE and MG132 led to the activation of caspase 9. In conclusion, our results indicate that CAPE and MG132 treatment of PL104 cells induced apoptosis through the mitochondrial intrinsic pathway, whereas the apoptotic mechanism induced by DOX and VCR may proceed through the extrinsic pathway.

Lenalidomide in multiple myeloma: Current status and future potential

The clinical development of lenalidomide (Revlimid™), then pomalidomide (Actimid™) as members of immunomodulatory drugs (IMiDs) for the treatment of multiple myeloma (MM), exemplifies how insight into disease biology can lead to design of effective therapeutic agents. Increased experience and understanding of IMiD's diverse biological effects has lead to rational design of lenalidomide-based treatment-regimens over recent years. However, much about lenalidomide is yet to be understood and fully exploited. Here, we review what is known of lenalidomide's biological effects, clinical certainties and uncertainties in the treatment of MM, and explore its future potential with other synergistic therapeutic agents.

Bortezomib and TRAIL: a perfect match for apoptotic elimination of tumour cells?

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine that selectively eradicates tumour cells via specific cell surface receptors and is intensively explored for use as a novel anticancer approach. To enhance the efficacy of TRAIL receptor agonists the proteasome inhibitor bortezomib is one of the most potent sensitizers. Here we review the main mechanisms underlying bortezomib-dependent TRAIL sensitization, including stimulation of apoptosis by increasing expression of TRAIL receptors, reduction of cFLIP and enhancement of caspase 8 activation, and modulation of Bcl-2 family proteins and inhibitor of apoptosis proteins (IAPs). Concomitantly, pro-survival signals are suppressed such as elicited by NF-κB and Akt. The different preclinical tumour models explored with this combination, including primary tumour (stem) cells, stroma co-culture and mice models, are discussed, as well as possible hurdles for clinical activity. Collectively, anticipating a solid rationale for bortezomib-TRAIL combination and very promising preclinical results, its clinical activity remains to be demonstrated.

Sarcomas as a mise en abyme of mesenchymal stem cells: exploiting interrelationships for cell mediated anticancer therapy

Mise en abyme meaning "placed into abyss or infinite recurrence" is an apt paradigm for the relentless growth of sarcoma cells. Its alternative meaning, "self-reflexive embedding" fits the central role attributed to cancer stem cells (CSCs). Diversely sourced and defined, mesenchymal stem cells (MSCs) may be the cells of sarcoma origin, evolve a CSC phenotype and/or contribute to tumor growth through inherent qualities for homing, neovascularization, paracrine cross-feeding, microvesicle secretion, cell fusion, entosis and immune modulation. Exploiting these qualities, MSC expressing modified forms of the TNF-related apoptosis-inducing ligand (Apo2L/TRAIL) are being developed to complement more conventional radiation and chemotherapy.

Bortezomib-induced sensitization of malignant human glioma cells to vorinostat-induced apoptosis depends on reactive oxygen species production, mitochondrial dysfunction, Noxa upregulation, Mcl-1 cleavage, and DNA damage

Glioblastomas are invasive tumors with poor prognosis despite current therapies. Histone deacetylase inhibitors (HDACIs) represent a class of agents that can modulate gene expression to reduce tumor growth, and we and others have noted some antiglioma activity from HDACIs, such as vorinostat, although insufficient to warrant use as monotherapy. We have recently demonstrated that proteasome inhibitors, such as bortezomib, dramatically sensitized highly resistant glioma cells to apoptosis induction, suggesting that proteasomal inhibition may be a promising combination strategy for glioma therapeutics. In this study, we examined whether bortezomib could enhance response to HDAC inhibition in glioma cells. Although primary cells from glioblastoma multiforme (GBM) patients and established glioma cell lines did not show significant induction of apoptosis with vorinostat treatment alone, the combination of vorinostat plus bortezomib significantly enhanced apoptosis. The enhanced efficacy was due to proapoptotic mitochondrial injury and increased generation of reactive oxygen species. Our results also revealed that combination of bortezomib with vorinostat enhanced apoptosis by increasing Mcl-1 cleavage, Noxa upregulation, Bak and Bax activation, and cytochrome c release. Further downregulation of Mcl-1 using shRNA enhanced cell killing by the bortezomib/vorinostat combination. Vorinostat induced a rapid and sustained phosphorylation of histone H2AX in primary GBM and T98G cells, and this effect was significantly enhanced by co-administration of bortezomib. Vorinostat/bortezomib combination also induced Rad51 downregulation, which plays an important role in the synergistic enhancement of DNA damage and apoptosis. The significantly enhanced antitumor activity that results from the combination of bortezomib and HDACIs offers promise as a novel treatment for glioma patients.

Novel insights into the synergistic interaction of Bortezomib and TRAIL: tBid provides the link

The proteasome inhibitor Bortezomib has been identified as a potent enhancer of TRAIL-induced apoptosis in several human cancers. However, the identification of the underlying molecular mechanisms of this synergistic cell death induction has been ongoing over the last years. A recent study identifies a new mechanism of action for the synergism of TRAIL and Bortezomib.

Bortezomib sensitizes malignant human glioma cells to TRAIL, mediated by inhibition of the NF-{kappa}B signaling pathway

Previous studies have shown that the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has significant apoptosis-inducing activity in some glioma cell lines, although many lines are either moderately or completely resistant, which has limited the therapeutic applicability of this agent. Because our recent studies showed that inhibition of proteasomal function may be independently active as an apoptosis-inducing stimulus in these tumors, we investigated the sensitivity of a panel of glioma cell lines (U87, T98G, U373, A172, LN18, LN229, LNZ308, and LNZ428) to TRAIL alone and in combination with the proteasome inhibitor bortezomib. Analysis of these cell lines revealed marked differences in their sensitivity to these treatments, with two (LNZ308 and U373) of the eight cell lines revealing no significant induction of cell death in response to TRAIL alone. No correlation was found between sensitivity of cells to TRAIL and expression of TRAIL receptors DR4, DR5, and decoy receptor DcR1, caspase 8, apoptosis inhibitory proteins XIAP, survivin, Mcl-1, Bcl-2, Bcl-Xl, and cFLIP. However, TRAIL-resistant cell lines exhibited a high level of basal NF-κB activity. Bortezomib was capable of potentiating TRAIL-induced apoptosis in TRAIL-resistant cells in a caspase-dependent fashion. Bortezomib abolished p65/NF-κB DNA-binding activity, supporting the hypothesis that inhibition of the NF-κB pathway is critical for the enhancement of TRAIL sensitization in glioma cells. Moreover, knockdown of p65/NF-κB by shRNA also enhanced TRAIL-induced apoptosis, indicating that p65/NF-κB may be important in mediating TRAIL sensitivity and the effect of bortezomib in promoting TRAIL sensitization and apoptosis induction.

Proteasome Inhibitor MG132 Induces Apoptosis and Inhibits Invasion of Human Malignant Pleural Mesothelioma Cells

Malignant pleural mesothelioma (MPM) is an aggressive malignancy tightly associated with asbestos exposure. The increasing incidence of MPM and its resistance to all therapeutic modalities necessitate an urgent development of new treatments for MPM. Proteasome inhibitors (PIs) have emerged as promising agents for treating human cancers that are refractory to current chemotherapies. In this study, we characterized MG132, a commonly used PI, for its proapoptotic and anti-invasion activities in NCI-H2452 and NCI-H2052 human thoracic MPM cell lines to determine the therapeutic effect of PIs on MPM. We found that as low as 0.5 microM MG132 caused a significant apoptosis in both cell lines as evidenced by DNA damage, cleavage of poly ADP-ribose polymerase and caspases 3, 7, and 9, and mitochondrial release of Smac/DIABLO and Cytochrome c. Mitochondrial caspase activation was found to be the underlying mechanism of the MG132-induced apoptosis. Mcl-1, among the Bcl-2 and IAP (inhibitor of apoptosis protein) antiapoptotic family proteins tested, was proved to be a major inhibitor of the MG132-induced apoptosis in MPM cells. Meanwhile, subapoptotic doses of MG132 inhibited the invasion of both MPM cell lines through reducing Rac1 activity. These observations demonstrate that MG132 possesses proapoptotic and anti-invasion activities in human MPM cells, therefore encouraging further investigations on the value of PIs for treating MPM.

Inhibition of NF-kappaB signaling by quinacrine is cytotoxic to human colon carcinoma cell lines and is synergistic in combination with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) or oxaliplatin

Colorectal cancer is the third most common malignancy in the United States. Modest advances with therapeutic approaches that include oxaliplatin (L-OHP) have brought the median survival rate to 22 months, with drug resistance remaining a significant barrier. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is undergoing clinical evaluation. Although human colon carcinomas express TRAIL receptors, they can also demonstrate TRAIL resistance. Constitutive NF-kappaB activation has been implicated in resistance to TRAIL and to cytotoxic agents. We have demonstrated constitutive NF-kappaB activation in five of six human colon carcinoma cell lines; this activation is inhibited by quinacrine. Quinacrine induced apoptosis in colon carcinomas and potentiated the cytotoxic activity of TRAIL in RKO and HT29 cells and that of L-OHP in HT29 cells. Similarly, overexpression of IkappaBalpha mutant (IkappaBalphaM) or treatment with the IKK inhibitor, BMS-345541, also sensitized these cells to TRAIL and L-OHP. Importantly, 2 h of quinacrine pretreatment resulted in decreased expression of c-FLIP and Mcl-1, which were determined to be transcriptional targets of NF-kappaB. Extended exposure for 24 h to quinacrine did not further sensitize these cells to TRAIL- or L-OHP-induced cell death; however, exposure caused the down-regulation of additional NF-kappaB-dependent survival factors. Short hairpin RNA-mediated knockdown of c-FLIP or Mcl-1 significantly sensitized these cells to TRAIL and L-OHP. Taken together, data demonstrate that NF-kappaB is constitutively active in colon cancer cell lines and NF-kappaB, and its downstream targets may constitute an important target for the development of therapeutic approaches against this disease.

Bortezomib restores stroma-mediated APO2L/TRAIL apoptosis resistance in multiple myeloma

OBJECTIVES

Hematopoietic stroma promotes resistance to immune control by APO2L/TRAIL in multiple myeloma (MM) cells in part by increasing synthesis of the anti-apoptotic protein c-FLIP. Here, we tested whether bortezomib can reverse the APO2L/TRAIL environmental mediated-immune resistance (EM-IR).

MATERIAL AND METHODS

MM cell lines (RPMI 8226 and U266) and CD138+ patient's MM cells were directly adhered to HS5 stroma exposed to HS5 or bone marrow stroma of patients with MM released soluble factors in a transwell system. Cells were treated with either APO2L/TRAIL (10 ng/mL), bortezomib (10 nm) or both.

RESULTS

Pretreatment with bortezomib effectively overcomes APO2L/TRAIL apoptosis resistance in myeloma cell lines and in CD138+ cells while directly adhered or in transwell assay. Bortezomib was not cytotoxic to HS5 stroma cells and only altered monocyte chemotactic protein-2-3 and IL-10 levels in the stroma-myeloma milieu. Factors released by HS5 stroma increased expression of c-FLIP, induced STAT-3 and ERK phosphorylation and reduced DR4 receptor expression in MM cells. HS5 stroma-released factor(s) induced NF-kappaB activation after 20 h exposure in association with an enhanced c-FLIP transcription. Bortezomib effectively reduced c-FLIP protein expression without affecting other proteins. Bortezomib also increased DR4 and DR5 expression in the presence of stroma.

CONCLUSIONS

These findings provide the rationale to combine bortezomib and APO2L/TRAIL to disrupt the influence of the stroma microenvironment on MM cells.

Proteasome inhibitors induce apoptosis in human lung cancer cells through a positive feedback mechanism and the subsequent Mcl-1 protein cleavage

Proteasome inhibitors (PIs) are promising new therapeutic agents for treating non-small cell lung carcinoma (NSCLC). To investigate the mechanisms of action of PIs, we analyzed the proapoptotic activities of PIs (MG132 or Bortezomib) in NSCLC cells. We found that both MG132 (>1 microM) and Bortezomib (>0.025 microM) induced a significant apoptosis in NCI-H1703, a PI-sensitive NSCLC cell line, through initially activating the intrinsic apoptosis pathway, leading to the activation of a positive feedback mechanism (PFM), which then conveyed apoptosis signaling from the intrinsic pathway to the extrinsic pathway with formation of a signaling loop for maximal caspase activation. Mcl-1 and Noxa were identified to be the major anti-apoptotic and proapoptotic proteins, respectively, in PI-induced apoptosis and mutually exclusive in protein stability. Although the Mcl-1 protein was upregulated by proteasome inhibition, it was also subjected to caspase 3-dependent cleavage governed by the PFM. Moreover, it was revealed that Mcl-1 protein cleavage contributed to PFM-governed apoptosis in following inter-related ways: reducing the anti-apoptotic Mcl-1; generating the truncated proapoptotic Mcl-1(S); and inducing a shift of balance between Mcl-1 and Noxa. It was further manifested that tumor necrosis factor-related apoptosis-inducing ligand boosted MG132's proapoptotic activity through strengthening the PFM in both NCI-H1703 and NCI-H358, a PI-resistant NSCLC cell line. Therefore, this study provides a basis for enhancing the efficacy of PIs in treating NSCLC.

NFkappaB signaling in carcinogenesis and as a potential molecular target for cancer therapy

It has become increasingly clear that deregulation of the NFkappaB signaling cascade is a common underlying feature of many human ailments including cancers. The past two decades of intensive research on NFkappaB has identified the basic mechanisms that govern the functioning of this pathway but uncovering the details of why this pathway works differently in different cellular contexts or how it interacts with other signaling pathways remains a challenge. A thorough understanding of these processes is needed to design better and more efficient therapeutic approaches to treat complex diseases like cancer. In this review, we summarize the literature documenting the involvement of NFkappaB in cancer, and then focus on the approaches that are being undertaken to develop NFkappaB inhibitors towards treatment of human cancers.

Bortezomib overcomes tumor necrosis factor-related apoptosis-inducing ligand resistance in hepatocellular carcinoma cells in part through the inhibition of the phosphatidylinositol 3-kinase/Akt pathway

Hepatocellular carcinoma (HCC) is one of the most common and aggressive human malignancies. Recombinant tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anti-tumor agent. However, many HCC cells show resistance to TRAIL-induced apoptosis. In this study, we showed that bortezomib, a proteasome inhibitor, overcame TRAIL resistance in HCC cells, including Huh-7, Hep3B, and Sk-Hep1. The combination of bortezomib and TRAIL restored the sensitivity of HCC cells to TRAIL-induced apoptosis. Comparing the molecular change in HCC cells treated with these agents, we found that down-regulation of phospho-Akt (P-Akt) played a key role in mediating TRAIL sensitization of bortezomib. The first evidence was that bortezomib down-regulated P-Akt in a dose- and time-dependent manner in TRAIL-treated HCC cells. Second, LY294002, a PI3K inhibitor, also sensitized resistant HCC cells to TRAIL-induced apoptosis. Third, knocking down Akt1 by small interference RNA also enhanced TRAIL-induced apoptosis in Huh-7 cells. Finally, ectopic expression of mutant Akt (constitutive active) in HCC cells abolished TRAIL sensitization effect of bortezomib. Moreover, okadaic acid, a protein phosphatase 2A (PP2A) inhibitor, reversed down-regulation of P-Akt in bortezomib-treated cells, and PP2A knockdown by small interference RNA also reduced apoptosis induced by the combination of TRAIL and bortezomib, indicating that PP2A may be important in mediating the effect of bortezomib on TRAIL sensitization. Together, bortezomib overcame TRAIL resistance at clinically achievable concentrations in hepatocellular carcinoma cells, and this effect is mediated at least partly via inhibition of the PI3K/Akt pathway.

A cell-based high-throughput screen to identify synergistic TRAIL sensitizers

We have developed a high-throughput screen (HTS) to search for novel molecules that can synergize with TRAIL, thus promoting apoptosis of ACHN renal tumor cells in a combinatorial fashion. The HTS detects synthetic compounds and pure natural products that can pre-sensitize the cancer cells to TRAIL-mediated apoptosis, yet have limited toxicity on their own. We have taken into account the individual effects of the single agents, versus the combination, and have identified hits that are synergistic, synergistic-toxic, or additive when combined with TRAIL in promoting tumor cell death. Preliminary mechanistic studies indicate that a subset of the synergistic TRAIL sensitizers act very rapidly to promote cleavage and activation of caspase-8 following TRAIL binding. Caspase-8 is an apical enzyme that initiates programmed cell death via the extrinsic apoptotic pathway. Thus, these TRAIL sensitizers may potentially reduce resistance of tumor cells to TRAIL-mediated apoptosis. Two representative sensitizers were found to increase levels of p53 but did not inhibit the proteasome, suggesting that early DNA damage-sensing pathways may be involved in their mechanisms of action.

PDTC enables type I TRAIL signaling in type II follicular lymphoma cells

Based on Bcl-X(L) overexpression studies we identified type I and type II follicular lymphoma cell lines in response to TRAIL. We demonstrate here that either amount of caspase-8 activation or Bid cleavage could not define the dependence on mitochondria. Furthermore, an inhibitor of NF-kappaB, PDTC, enabled TRAIL to activate type I apoptotic pathway in type II cells. However, an inhibitor of IKK did not switch apoptosis to type I pathway in type II cells, indicating that NF-kappaB might not be responsible for the switch.

Bortezomib-resistant nuclear factor-kappaB activity in multiple myeloma cells

Bortezomib (Velcade/PS341), a proteasome inhibitor used in the treatment of multiple myeloma (MM), can inhibit activation of nuclear factor-kappaB (NF-kappaB), a family of transcription factors often deregulated and constitutively activated in primary MM cells. NF-kappaB can be activated via several distinct mechanisms, including the proteasome inhibitor-resistant (PIR) pathway. It remains unknown what fraction of primary MM cells harbor constitutive NF-kappaB activity maintained by proteasome-dependent mechanisms. Here, we report an unexpected finding that constitutive NF-kappaB activity in 10 of 14 primary MM samples analyzed is refractory to inhibition by bortezomib. Moreover, when MM cells were cocultured with MM patient-derived bone marrow stromal cells (BMSC), microenvironment components critical for MM growth and survival, further increases in NF-kappaB activity were observed that were also refractory to bortezomib. Similarly, MM-BMSCs caused PIR NF-kappaB activation in the RPMI8226 MM cell line, leading to increased NF-kappaB-dependent transcription and resistance to bortezomib-induced apoptosis. Our findings show that primary MM cells frequently harbor PIR NF-kappaB activity that is further enhanced by the presence of patient-derived BMSCs. They also suggest that this activity is likely relevant to the drug resistance development in some patients. Further elucidation of the mechanism of PIR NF-kappaB regulation could lead to the identification of novel diagnostic biomarkers and/or therapeutic targets for MM treatment.

Inhibition of Yin Yang 1-dependent repressor activity of DR5 transcription and expression by the novel proteasome inhibitor NPI-0052 contributes to its TRAIL-enhanced apoptosis in cancer cells

TRAIL promotes apoptotic tumor cell death; however, TRAIL-resistant tumors need to be sensitized to reverse resistance. Proteasome inhibitors potentiate TRAIL apoptosis in vitro and in vivo and correlate with up-regulation of death receptor 5 (DR5) via an unknown mechanism. We hypothesized that the proteasome inhibitor NPI-0052 inhibits the transcription repressor Yin Yang 1 (YY1) which regulates TRAIL resistance and negatively regulates DR5 transcription. Treatment of PC-3 and Ramos cells with NPI-0052 (</=2.5 nM) and TRAIL sensitizes the tumor cells to TRAIL-induced apoptosis. By comparison to bortezomib, a 400-fold less concentration of NPI-0052 was used. NPI-0052 up-regulated DR5 reporter activity and both surface and total DR5 protein expression. NPI-0052-induced inhibition of NF-kappaB activity was involved in TRAIL sensitization as corroborated by the use of the NF-kappaB inhibitor dehydroxymethylepoxyquinomicin. NPI-0052 inhibited YY1 promoter activity as well as both YY1 mRNA and protein expression. The direct role of NPI-0052-induced inhibition of YY1 and up-regulation of DR5 in the regulation of TRAIL sensitivity was demonstrated by the use of YY1 small interfering RNA. The NPI-0052-induced sensitization to TRAIL involved activation of the intrinsic apoptotic pathway and dysregulation of genes that regulate apoptosis. The NPI-0052 concentrations used for TRAIL sensitization were not toxic to human hematopoetic stem cells. The present findings demonstrate, for the first time, the potential mechanism by which a proteasome inhibitor, like NPI-0052, inhibits the transcription repressor YY1 involved in TRAIL resistance and DR5 regulation. The findings also suggest the therapeutic application of subtoxic NPI-0052 concentrations in combination with TRAIL/agonist DR4/DR5 mAbs in the treatment of TRAIL-resistant tumors.

Proteasome modulates mitochondrial function during cellular senescence

Proteasome plays fundamental roles in the removal of oxidized proteins and in the normal degradation of short-lived proteins. Previously we have provided evidence that the impairment in proteasome observed during the replicative senescence of human fibroblasts has significant effects on MAPK signaling, proliferation, life span, senescent phenotype, and protein oxidative status. These studies have demonstrated that proteasome inhibition and replicative senescence caused accumulation of intracellular protein carbonyl content. In this study, we have investigated the mechanisms by which proteasome dysfunction modulates protein oxidation during cellular senescence. The results indicate that proteasome inhibition during replicative senescence has significant effects on intra- and extracellular ROS production in vitro. The data also show that ROS impaired the proteasome function, which is partially reversible by antioxidants. Increases in ROS after proteasome inhibition correlated with a significant negative effect on the activity of most mitochondrial electron transporters. We propose that failures in proteasome during cellular senescence lead to mitochondrial dysfunction, ROS production, and oxidative stress. Furthermore, it is likely that changes in proteasome dynamics could generate a prooxidative condition at the immediate extracellular microenvironment that could cause tissue injury during aging, in vivo.

Effect of proteasome inhibitors on proliferation and apoptosis of human cutaneous melanoma-derived cell lines

BACKGROUND

Cutaneous malignant melanoma is an aggressive type of skin cancer which causes disproportionate mortality in young and middle-aged adults. Once disseminated, melanoma can be considered an incurable disease, highly resistant to standard antineoplastic treatment, such as chemotherapy or radiation therapy. The proteasome represents a novel target for cancer therapy that can potentially be used in melanoma.

OBJECTIVES

To assess the effect of four structurally different proteasome inhibitors on human cutaneous melanoma-derived cell lines.

METHODS

Sixteen human cutaneous melanoma-derived cell lines which are original were obtained from patients who were treated by two of the authors. Cells were cultured, exposed to proteasome inhibitors (bortezomib, ALLN, MG-132 and epoxomicin) and then assayed for cell cycle and cell death analyses.

RESULTS

Proteasome inhibitors inhibited the in vitro growth of melanoma cells, and this effect was due to a reduction in cell proliferation rate and an induction of both caspase-dependent and caspase-independent cell death. Moreover, release of apoptosis-inducing factor was observed in the presence of the broad-specificity caspase inhibitor BAF (Boc-D-fmk). In addition, the four different proteasome inhibitors induced caspase 2 processing.

CONCLUSIONS

This study provides information regarding the in vitro effects of proteasome inhibitors on melanoma cell lines, and the molecular mechanisms involved. It also gives support to the future use of such inhibitors in the treatment of patients with melanoma, either administered alone or in combination with other drugs.