Targeting Acute Myelogenous Leukemia Using Potent Human Dihydroorotate Dehydrogenase Inhibitors Based on the 2-Hydroxypyrazolo[1,5-a]pyridine Scaffold: SAR of the Aryloxyaryl Moiety.

In recent years, human dihydroorotate dehydrogenase inhibitors have been associated with acute myelogenous leukemia as well as studied as potent host targeting antivirals. Starting from MEDS433 (IC50 1.2 nM), we kept improving the structure-activity relationship of this class of compounds characterized by 2-hydroxypyrazolo[1,5-a]pyridine scaffold. Using an in silico/crystallography supported design, we identified compound 4 (IC50 7.2 nM), characterized by the presence of a decorated aryloxyaryl moiety that replaced the biphenyl scaffold, with potent inhibition and pro-differentiating abilities on AML THP1 cells (EC50 74 nM), superior to those of brequinar (EC50 249 nM) and boosted when in combination with dipyridamole. Finally, compound 4 has an extremely low cytotoxicity on non-AML cells as well as MEDS433; it has shown a significant antileukemic activity in vivo in a xenograft mouse model of AML.

AMPK activation attenuates HER3 upregulation and Neuregulin-Mediated rescue of cell proliferation in HER2-Overexpressing breast cancer cell lines exposed to lapatinib.

Lapatinib is a highly selective reversible inhibitor of the tyrosine kinase domains of HER2 and EGFR, approved for the treatment of advanced stage HER2-overexpressing breast cancers. Although targeted therapy with lapatinib provides initial clinical advantage, cancer cells' adaptive responses can overcome the inhibitory effects of lapatinib. HER3 upregulation and autocrine induction of HER3 ligand neuregulin-1 (NRG), have been implicated in the restoration of AKT and ERK1/2 activity and rescue of cell proliferation. In this study we evaluated the effects of lapatinib alone and in combination with AMPK activator GSK-621 in HER2-overexpressing breast cancer cell lines SKBR3 and BT474. Our results show that in cells exposed to lapatinib and GSK-621 in combination, lapatinib-mediated HER3 upregulation was reduced and reactivation of AKT and ERK1/2 kinases was prevented. The two drugs in combination decreased cell viability in a synergistic manner and greatly reduced the ability of NRG to rescue cell proliferation. Finally, we provide evidence that in cells exposed to lapatinib and GSK-621 in combination the establishment of a transcriptionally permissive chromatin structure at the HER3 promoter is hampered. The results of this study highlight a potential role for AMPK activation in counteracting lapatinib-induced adaptive responses of HER2-overexpressing breast cancer cells.

Targeting Acute Myelogenous Leukemia Using Potent Human Dihydroorotate Dehydrogenase Inhibitors Based on the 2-Hydroxypyrazolo[1,5-a]pyridine Scaffold: SAR of the Biphenyl Moiety.

The connection with acute myelogenous leukemia (AML) of dihydroorotate dehydrogenase (hDHODH), a key enzyme in pyrimidine biosynthesis, has attracted significant interest from pharma as a possible AML therapeutic target. We recently discovered compound 1, a potent hDHODH inhibitor (IC50 = 1.2 nM), able to induce myeloid differentiation in AML cell lines (THP1) in the low nM range (EC50 = 32.8 nM) superior to brequinar's phase I/II clinical trial (EC50 = 265 nM). Herein, we investigate the 1 drug-like properties observing good metabolic stability and no toxic profile when administered at doses of 10 and 25 mg/kg every 3 days for 5 weeks (Balb/c mice). Moreover, in order to identify a backup compound, we investigate the SAR of this class of compounds. Inside the series, 17 is characterized by higher potency in inducing myeloid differentiation (EC50 = 17.3 nM), strong proapoptotic properties (EC50 = 20.2 nM), and low cytotoxicity toward non-AML cells (EC30(Jurkat) > 100 µM).

Targeting Myeloid Differentiation Using Potent 2-Hydroxypyrazolo[1,5- a]pyridine Scaffold-Based Human Dihydroorotate Dehydrogenase Inhibitors.

Human dihydroorotate dehydrogenase ( hDHODH) catalyzes the rate-limiting step in de novo pyrimidine biosynthesis, the conversion of dihydroorotate to orotate. hDHODH has recently been found to be associated with acute myelogenous leukemia, a disease for which the standard of intensive care has not changed over decades. This work presents a novel class of hDHODH inhibitors, which are based on an unusual carboxylic group bioisostere 2-hydroxypyrazolo[1,5- a]pyridine, that has been designed starting from brequinar, one of the most potent hDHODH inhibitors. A combination of structure-based and ligand-based strategies produced compound 4, which shows brequinar-like hDHODH potency in vitro and is superior in terms of cytotoxicity and immunosuppression. Compound 4 also restores myeloid differentiation in leukemia cell lines at concentrations that are one log digit lower than those achieved in experiments with brequinar. This Article reports the design, synthesis, SAR, X-ray crystallography, biological assays, and physicochemical characterization of the new class of hDHODH inhibitors.

Design, synthesis, biological evaluation and X-ray structural studies of potent human dihydroorotate dehydrogenase inhibitors based on hydroxylated azole scaffolds.

A new generation of potent hDHODH inhibitors designed by a scaffold-hopping replacement of the quinolinecarboxylate moiety of brequinar, one of the most potent known hDHODH inhibitors, is presented here. Their general structure is characterized by a biphenyl moiety joined through an amide bridge with an acidic hydroxyazole scaffold (hydroxylated thiadiazole, pyrazole and triazole). Molecular modelling suggested that these structures should adopt a brequinar-like binding mode involving interactions with subsites 1, 2 and 4 of the hDHODH binding site. Initially, the inhibitory activity of the compounds was studied on recombinant hDHODH. The most potent compound of the series in the enzymatic assays was the thiadiazole analogue 4 (IC50 16 nM). The activity was found to be dependent on the fluoro substitution pattern at the biphenyl moiety as well as on the choice/substitution of the heterocyclic ring. Structure determination of hDHODH co-crystallized with one representative compound from each series (4, 5 and 6) confirmed the brequinar-like binding mode as suggested by modelling. The specificity of the observed effects of the compound series was tested in cell-based assays for antiproliferation activity using Jurkat cells and PHA-stimulated PBMC. These tests were also verified by addition of exogenous uridine to the culture medium. In particular, the triazole analogue 6 (IC50 against hDHODH: 45 nM) exerted potent in vitro antiproliferative and immunosuppressive activity without affecting cell survival.

Exposure of neuroblastoma cell lines to imatinib results in the upregulation of the CDK inhibitor p27(KIP1) as a consequence of c-Abl inhibition.

Imatinib mesylate is a tyrosine kinase inhibitor with selectivity for abelson tyrosine-protein kinase 1 (c-Abl), breakpoint cluster region (Bcr)-Abl fusion protein (Bcr-Abl), mast/stem cell growth factor receptor Kit (c-Kit), and platelet-derived growth factor receptor (PDGFR). Previous studies demonstrated that imatinib in the low micromolar range exerted antiproliferative effects on neuroblastoma cell lines. However, although neuroblastoma cells express c-Kit and PDGFR, the imatinib concentrations required to achieve significant growth inhibitory effects (≥ 10 µM) are substantially higher than those required for inhibition of ligand-induced phosphorylation of wild type c-Kit and PDGFR (≤ 1 µM), suggesting that additional mechanisms are responsible for the antitumor activity of imatinib on these cells. In this study, we show that treatment of neuroblastoma cell lines with 1-15 µM imatinib resulted in a dose dependent inhibition of 5-bromo-2'-deoxyuridine (BrdU) incorporation into newly synthesized DNA. The antiproliferative effect of imatinib was dependent on the upregulation of the cyclin-dependent kinase (CDK) inhibitor p27(KIP1) in the nuclear compartment as a result of increased p27(KIP1) protein stability. We demonstrate that the mechanism of p27(KIP1) stabilization relied on inhibition of p27(KIP1) phosphorylation on tyrosine residues by c-Abl. We provide evidence that in neuroblastoma cell lines a significant fraction of cellular c-Abl is phosphorylated on Tyr-245, consistent with an open and active conformation. Notably, exposure to imatinib did not affect Tyr-245 phosphorylation. Given the low affinity of active c-Abl for imatinib, these data provide a molecular explanation for the relatively high imatinib concentrations required to inhibit neuroblastoma cell proliferation.

Reduced cellular Ca(2+) availability enhances TDP-43 cleavage by apoptotic caspases.

Accumulation of transactive response DNA binding protein (TDP-43) fragments in motor neurons is a post mortem hallmark of different neurodegenerative diseases. TDP-43 fragments are the products of the apoptotic caspases-3 and -7. Either excessive or insufficient cellular Ca(2+) availability is associated with activation of apoptotic caspases. However, as far as we know, it is not described whether activation of caspases, due to restricted intracellular Ca(2+), affects TDP-43 cleavage. Here we show that in various cell lineages with restricted Ca(2+) availability, TDP-43 is initially cleaved by caspases-3 and -7 and then, also by caspases-6 and -8 once activated by caspase-3. Furthermore, we disclose the existence of a TDP-43 caspase-mediated fragment of 15kDa, in addition to the well-known fragments of 35 and 25kDa. Interestingly, with respect to the other two fragments this novel fragment is the major product of caspase activity on murine TDP-43 whereas in human cell lines the opposite occurs. This outcome should be considered when murine models are used to investigate TDP-43 proteinopathies.

IκB kinase ß is required for activation of NF-κB and AP-1 in CD3/CD28-stimulated primary CD4(+) T cells.

Engagement of the TCR and CD28 coreceptor by their respective ligands activates signal transduction cascades that ultimately lead to the activation of the transcription factors NFAT, AP-1, and NF-κB, which are required for the expression of cytokines and T cell clonal expansion. Previous studies have demonstrated that in mature T cells, activation of AP-1 and NF-κB is dependent on protein kinase C θ, suggesting the existence of a common signaling pathway. In this study, we show that in human primary CD4(+) T cells, exposure to the cell-permeable IKKß inhibitor PS-1145 or genetic ablation of IKKß abrogates cell proliferation and impairs the activation of NF-κB and AP-1 transcription factors in response to engagement of CD3 and CD28 coreceptor. In addition, we show that stimulation of T cells in the absence of IKKß activity promotes the time-dependent and cyclosporine-sensitive expression of negative regulators of T cell signaling leading to a hyporesponsive state of T cells.

Nitrooxyacyl derivatives of salicylic acid: aspirin-like molecules that covalently inactivate cyclooxygenase-1.

A recently described series of nitrooxyacyl derivatives of salicylic acid, displaying aspirin-like anti-inflammatory and platelet anti-aggregatory properties, were evaluated for their abilities to inhibit cyclooxygenase (COX). A number of these compounds irreversibly inhibited both COX-1 and COX-2 isoforms when tested in isolated human platelets and monocytes. Further studies using COX-1 expressed in human HEK293T cells showed that this inhibition mechanism is similar to that of aspirin; namely, the products are able to covalently bind to the Ser 530 residue present in the active cleft of the enzyme. Molecular modeling enabled us to rationalize this behavior. Because these products were previously found to display NO-dependent properties in rat animal models, particularly as they decreased in vivo gastrotoxicity and induced in vitro vasodilation, they represent a new and interesting class of potential aspirin-like antithrombotic agents worthy of further study.

Cytoplasmic accumulation of TDP-43 in circulating lymphomonocytes of ALS patients with and without TARDBP mutations.

TDP-43, encoded by TARDBP, is a ubiquitously expressed, primarily nuclear protein. In recent years, TDP-43 has been identified as the major pathological protein in ALS due to its mislocalisation in the cytoplasm of motor neurons of patients with and without TARDBP mutations and expression in forms that do not match its predicted molecular weight. In this study, the TDP-43 profile was investigated using western immunoblot analysis in whole lysates, nuclei and cytoplasm of circulating lymphomonocytes from 16 ALS patients, 4 with (ALS/TDP+) and 12 without (ALS/TDP-) TARDBP mutations in the protein C-terminal domain, and thirteen age-matched, healthy donors (controls). Three disease-unaffected first-degree relatives of an ALS/TDP+ patient were also included: one carried the parent mutation (Rel/TDP+) whereas the other two did not (Rel/TDP-). In all ALS patients, relatives and controls, TDP-43 retained the predicted molecular weight in whole cell lysates and nuclei, but in the cytoplasm its molecular weight was slightly smaller than expected. In quantitative terms, TDP-43 was expressed at approximately the same levels in whole cell lysates of ALS patients, relatives and controls. In contrast, TDP-43 accumulated in the cytoplasm with concomitant nuclear depletion in all ALS/TDP+ patients, in about 50% of ALS/TDP- patients and in the Rel/TDP+ subject compared to the controls. In the remaining ALS/TDP- patients and in the two Rel/TDP- subjects, TDP-43 matched the control levels in both subcellular compartments. Were these findings further confirmed, circulating lymphomonocytes could be informative of TDP-43 mislocalisation in nervous tissue and used as a biomarker for future disease risk.

In CD28-costimulated human naïve CD4+ T cells, I-κB kinase controls the expression of cell cycle regulatory proteins via interleukin-2-independent mechanisms.

Stimulation of naïve CD4(+) T cells through engagement of the T-cell receptor (TCR) and the CD28 co-receptor initiates cell proliferation which critically depends on interleukin (IL)-2 secretion and subsequent autocrine signalling via the IL-2 receptor. However, several studies indicate that in CD28-costimulated T cells additional IL-2-independent signals are also required for cell proliferation. In this study, using a neutralizing anti-human IL-2 antibody and two selective, structurally unrelated, cell-permeable I-κB kinase (IKK) inhibitors, BMS-345541 and PS-1145, we show that in human naïve CD4(+) T cells stimulated through a short engagement of the TCR and the CD28 co-receptor, IKK controls the expression of the cell cycle regulatory proteins cyclin D3, cyclin E and cyclin-dependent kinase 2 (CDK2) and the stability of the F-box protein S-phase kinase-associated protein 2 (SKP2) and its co-factor CDC28 protein kinase regulatory subunit 1B (CKS1B), through IL-2-independent mechanisms.

N-CAM dysfunction and unexpected accumulation of PSA-NCAM in brain of adult-onset autosomal-dominant leukodystrophy.

Previously, myelin from cerebral white matter (CWM) of two subjects of a family with orthochromatic adult-onset autosomal-dominant leukodystrophy (ADLD) was disclosed to exhibit defective large isoform of myelin-associated glycoprotein (L-MAG) and patchy distribution only in the elder subject. L-MAG and neural cell adhesion molecule (N-CAM) (N-CAM 180, 140, and 120) are structurally related and concur to myelin/axon interaction. In early developmental stages, in neurons and glia N-CAM is converted into polysialylated (PSA)-NCAM by two sialyltransferases sialyltransferase-X (STX) and polysialyltransferase-1 (PST). Notably, PSA-NCAM disrupts N-CAM adhesive properties and is nearly absent in the adult brain. Here, CWM extracts and myelin of the two subjects were searched for the expression pattern of the N-CAM isoforms and PSA-NCAM, and their CWM was evaluated for N-CAM, STX and PST gene copy number and gene expression as mRNA. Biochemically, we disclosed that in CWM extracts and myelin from both subjects, PSA-NCAM accumulates, N-CAM 180 considerably increases, N-CAM 140 is modestly modified and N-CAM 120 remarkably decreases; duplication of genes encoding N-CAM, STX and PST was not revealed, whereas PST mRNA was clearly increased. Immunohistochemically, in CWM of both subjects, we found an unusually diffuse accumulation of PSA-NCAM without inflammation markers. PSA-NCAM persistence, up-regulated PST mRNA and previously uncovered defective L-MAG may be early pathogenetic events in this ADLD form.

Alterations of myelin-specific proteins and sphingolipids characterize the brains of acid sphingomyelinase-deficient mice, an animal model of Niemann-Pick disease type A.

Niemann-Pick disease (NPD) type A is a neurodegenerative disorder caused by sphingomyelin (SM) accumulation in lysosomes relying on reduced or absent acid sphingomyelinase (ASM) activity. NPD-A patients develop progressive neurodegeneration including cerebral and cerebellar atrophy, relevant Purkinje cell and myelin deficiency with death within 3 years. ASM'knock-out' (ASMKO) mice, an animal model of NPD-A, develop a phenotype largely mimicking that of NPD-A. The mechanisms underlying myelin formation are poorly documented in ASMKO mice. In this study we determined the content of four myelin-specific proteins, myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP), myelin associated glycoprotein (MAG) and proteolipid protein (PLP), and that of myelin-enriched sphingolipids in the brains of ASMKO and wild-type mice in early stages of post-natal (pn) life. Protein and mRNA analysis revealed that in ASMKO mice beginning from 4 post-natal weeks (wk-pn), the expression levels of MAG, CNP, and MBP were below those observed in wild-type mice and the same applied to PLP at 10 wk-pn. Moreover, at 4 wk-pn the expression of SOX10, one of the transcription factors involved in oligodendrocyte development and maintenance was lower in ASMKO mice. Lipid analysis showed that SM and the gangliosides GM3 and GM2 accumulated in the brains of ASMKO mice, as opposed to galactocerebroside and galactosulfocerebroside that, in parallel with the mRNAs of UDP-galactose ceramide galactosyltransferase and galactose-3-O-sulfotransferase 1, the two transferases involved in their synthesis, decreased. Myelin lipid analysis showed a progressive sphingomyelin accumulation in ASMKO mice; noteworthy, of the two sphingomyelin species known to be resolved by TLC, only that with the lower Rf accumulated. The immunohistochemical analysis showed that the reduced expression of myelin specific proteins in ASMKO mice at 10 wk-pn was not restricted to the Purkinje layer of the cerebellar cortex but involved the cerebral cortex as well. In conclusion, reduced oligodendrocyte metabolic activity is likely to be the chief cause of myelin deficiency in ASMKO mice, thus shedding light on the molecular dysfunctions underlying neurodegeneration in NPD-A.

Structural and functional properties of proteasomes purified from the human kidney.

BACKGROUND: Proteasomes are 'proteolytic machineries' implicated in many cellular functions, including protein turnover, inflammatory response and immunosurveillance. They exist in various forms sharing the same catalytic core - the 20S proteasome. This core consists of 28 subunits codified by 14 different genes, 3 of which - beta 1, beta 2 and beta 5 - are catalytically active and show peptidyl-glutamyl peptide hydrolyzing (PGPH), trypsin-like and chymo-trypsin-like activities, respectively. Under IFN- delta and TNF- alfa stimuli, the 3 active constitutive subunits are replaced by the corresponding ones - i.e., LMP2, MECL-1, LMP7 - known as inducible subunits, thus resulting in the constitution of the 'immunoproteasome' that is specifically implicated in MHC class I-presented peptide generation. This process is enhanced when the proteasome is associated with the polymeric protein 11S regulator/PA28 made up of 4 alfa and 3 beta subunits. METHODS: The 20S proteasome was purified from post mortem specimens of human kidney cortex by chromatographic and ultracentrifugation techniques. It was then characterized on the basis of (i) multicatalytic activity evaluated using specific fluorogenic peptides, (ii) electrophoretic mobility on non-denaturating polyacrylamide gels followed by in-gel visualization by fluorogenic peptide overlaying and Coomassie blue staining and (iii) subunit composition as ascertained by SDS-PAGE and 2-dimensional electrophoresis followed by silver staining or Western immunoblotting using specific antibodies against the proteasome subunits. The 20S proteasome was also studied for its association with the 11S regulator by Western immunoblotting using an antibody to the regulator alfa subuniT. RESULTS: T he purified proteasome was shown to have PGPH, trypsin-like and chymotrypsin-like activities. Furthermore, it incorporated the inducible subunits and was associated with the 11S regulator. CONCLUSIONS: The features we observed make renal cells susceptible to an over-expression of inflammatory response to immunological challenges.

Characterization of the 20S proteasome in human glioblastomas.

Proteasomes are multisubunit proteases involved in many cellular processes, including tumorigenesis and immune surveillance. In their catalytic core, the 20S proteasome, the beta1, beta2 and beta5 subunits show peptidylglutamyl peptide hydrolyzing (PGPH), trypsin-like and chymotrypsin-like activities, respectively. By IFN-gamma and TNFalpha stimulus, these subunits are replaced by their counterparts LMP2, MECL-1 and LMP7, defined inducible subunits, thus originating the immunoproteasome, and expression of the proteasome activator PA28 is enhanced. These modifications strengthen MHC-class I restricted peptide generation. The 20S proteasome has been detected immunohistochemically in formalin-fixed samples purified from fresh surgical specimens of 18 tumors (G20S) and from 8 samples of normal peritumoral tissue. The G20S, LMP2, MECL-1 and LMP7 increased in only 12 cases, along with unvaried trypsin-like and decreased PGPH and chymotrypsin-like activities; PA28 was unvaried in all 18 samples. The immunoproteasome alterations may represent an anomalous immunological attitude of glioblastomas.

Down-regulation of pyruvate dehydrogenase phosphatase in obese subjects is a defect that signals insulin resistance.

OBJECTIVE: The objective of this study was to determine whether down-regulation of pyruvate dehydrogenase phosphatase (PDP) is responsible for poorly active pyruvate dehydrogenase (PDH) in circulating lymphocytes (CLs) of obese subjects (ObS), and if so, whether it improves when their plasma insulin rises. RESEARCH METHODS AND PROCEDURES: PDH activity was compared in lysed CLs of 10 euglycemic ObS and 10 sex- and age-matched controls before and during plasma insulin enhancement in an oral glucose tolerance test. It was evaluated without (PDHa) or with Mg/Ca or Mg at various concentrations to assess PDP1 or PDP2 activities or with Mg/Ca and exogenous PDP to determine total PDH activity (PDHt), which is an indirect measure of the amount of PDH. The insulin sensitivity index was calculated, and PDP1 and PDP2 mRNA was sought in the CLs. RESULTS: At T0 in ObS, PDHt was normal, whereas PDHa and PDP1 activity was below normal at all Mg/Ca concentrations. PDP2 activity was undetectable in both groups. PDP1 and PDP2 mRNA was identified, and insulin sensitivity index and PDHa were directly correlated. During the oral glucose tolerance test, plasma insulin rose considerably more in ObS than in controls; PDHa and PDP1 activity also increased but remained significantly below normal, and PDHt was unvaried in both groups. DISCUSSION: PDP1 is down-regulated in CLs of ObS because it is poorly sensitive to Mg/Ca; this defect is attenuated when plasma insulin is greatly enhanced.

Myelin-associated glycoprotein is altered in a familial late-onset orthochromatic leukodystrophy.

Adult-onset dominant leukodystrophies are a heterogeneous group of rare disorders, whose etiology, pathogenesis and molecular background are still unknown. We report the neuropathological and biochemical investigations of the brains and their myelin proteins components in 2 members of an Italian family affected by an adult-onset autosomal dominant leukoencephalopathy. Clinical signs included spastic paraparesis, pseudobulbar syndrome, action tremor of head and hands, and moderate memory impairment. No mental deterioration or neuropathy was present. Onset was subacute (range 42-53 years) and progression spanned 4 to 7 years. The neuropathological phenotype overlapped that of orthochromatic leukodystrophies. The biochemical analysis revealed an abnormal myelin-associated glycoprotein (MAG); the defect was localized at the C-terminal domain of the L-MAG isoform, resulting in a protein approximately 5 kDa shorter than the normal counterpart. No mutation in the MAG gene-coding regions was uncovered, and linkage analysis formally excluded the entire MAG locus. We show that the identified MAG protein alteration is probably due to an abnormal post-translational event. Considering MAG function in the maintenance of myelin, the abnormal protein may have a role in the pathogenesis of this disease. This is the first report of a possible pathogenetic role of MAG in a hereditary disease affecting the central white matter.

The HIV protease inhibitors nelfinavir and saquinavir, but not a variety of HIV reverse transcriptase inhibitors, adversely affect human proteasome function.

BACKGROUND: In HIV-infected patients some clinical and immunological benefits of antiretroviral therapy, which frequently include a combination of HIV protease inhibitors (PIs) and reverse transcriptase inhibitors (RTIs), cannot be solely explained by the drugs' action on viral enzymes. Proteasomes constitute the central protease of the ubiquitin ATP-dependent pathway involved in many cellular processes, as well as in HIV maturation and aggressiveness. OBJECTIVE: To explore whether the PIs nelfinavir and saquinavir and the RTIs abacavir, nevirapine, delavirdine, stavudine and didanosine affect proteasome function in vitro and in vivo. METHODS: Peptidase activity of purified human 26S and 20S proteasomes was assayed with and without the drugs at different concentrations. Intracellular proteasome proteolytic activity was evaluated by searching for ubiquitin-tagged proteins in HL60 cells incubated with and without the drugs. RESULTS: At therapeutic dosages, nelfinavir and saquinavir inhibited proteasome peptidase activity and caused intracellular accumulation of polyubiquitinated proteins, a hallmark of proteasome proteolytic inhibition in vivo; the RTIs failed to evoke either effect. CONCLUSION: Proteasomes are targeted by the two PIs but not the RTIs. Therefore, in HIV-infected patients the beneficial effect of a therapy including one of the two PIs should partly rely on inhibition of host proteasome function.