Compromised proteasome degradation elevates neuronal nitric oxide synthase levels and induces apoptotic cell death

Increasing intratumor C/EBP-β LIP and nitric oxide levels overcome resistance to doxorubicin in triple negative breast cancer

BACKGROUND

Triple negative breast cancer (TNBC) easily develops resistance to the first-line drug doxorubicin, because of the high levels of the drug efflux transporter P-glycoprotein (Pgp) and the activation of pro-survival pathways dependent on endoplasmic reticulum (ER). Interfering with these mechanisms may overcome the resistance to doxorubicin, a still unmet need in TNBC.

METHODS

We analyzed a panel of human and murine breast cancer cells for their resistance to doxorubicin, Pgp expression, lysosome and proteasome activity, nitrite production, ER-dependent cell death and immunogenic cell death parameters. We evaluated the efficacy of genetic (C/EBP-β LIP induction) and pharmacological strategies (lysosome and proteasome inhibitors), in restoring the ER-dependent and immunogenic-dependent cell death induced by doxorubicin, in vitro and in syngeneic mice bearing chemoresistant TNBC. The results were analyzed by one-way analysis of variance test.

RESULTS

We found that TNBC cells characterized by high levels of Pgp and resistance to doxorubicin, had low induction of the ER-dependent pro-apoptotic factor C/EBP-β LIP upon doxorubicin treatment and high activities of lysosome and proteasome that constitutively destroyed LIP. The combination of chloroquine and bortezomib restored doxorubicin sensitivity by activating multiple and interconnected mechanisms. First, chloroquine and bortezomib prevented C/EBP-β LIP degradation and activated LIP-dependent CHOP/TRB3/caspase 3 axis in response to doxorubicin. Second, C/EBP-β LIP down-regulated Pgp and up-regulated calreticulin that triggered the dendritic cell (DC)-mediated phagocytosis of tumor cell, followed by the activation of anti-tumor CD8⁺T-lymphocytes upon doxorubicin treatment. Third, chloroquine and bortezomib increased the endogenous production of nitric oxide that further induced C/EBP-β LIP and inhibited Pgp activity, enhancing doxorubicin's cytotoxicity. In orthotopic models of resistant TNBC, intratumor C/EBP-β LIP induction - achieved by a specific expression vector or by chloroquine and bortezomib - effectively reduced tumor growth and Pgp expression, increased intra-tumor apoptosis and anti-tumor immune-infiltrate, rescuing the efficacy of doxorubicin.

CONCLUSIONS

We suggest that preventing C/EBP-β LIP degradation by lysosome and proteasome inhibitors triggers multiple virtuous circuitries that restore ER-dependent apoptosis, down-regulate Pgp and re-activate the DC/CD8⁺T-lymphocytes response against TNBC. Lysosome and proteasome inhibitors associated with doxorubicin may overcome the resistance to the drug in TNBC.

Neuronal nitric oxide synthase, as a downstream signaling molecule of c‑jun, regulates the survival of differentiated PC12 cells

The high expression of c-jun and neuronal nitric oxide synthase (nNOS) generally occurs in neurons following the generation of various animal models of central neuronal diseases. However, the mechanism between them in neuronal disease remains to be elucidated. Our previous studies demonstrated that the expression of c‑jun always occurs prior to expression of nNOS in motoneuron injuries and suppression of c‑jun expression by c‑jun siRNA decreased nNOS expression in differentiated PC12 cells. The present study aimed to examine whether there was an association of up and downstream regulation or crosstalk between c‑jun and nNOS in neurons. Using a culture of differentiated PC12 cells in vitro, the expression of nNOS and c-jun in cells was investigated by immunofluorescence. The nNOS inhibitor 7‑nitroindazole (7‑NI) was used in differentiated PC12 cells to downregulate the expression of nNOS. The optimal concentration of 7‑NI on the viability and survival of cultured differentiated PC12 cells was selected using a 3‑(4,5-dimethylthiazol-2-yl)‑2,5-diphenyltetrazolium assay and the effects of 7‑NI on the activity of constitutive nitric oxide synthase (cNOS) in differentiated PC12 cells were determined using a NOS Activity Detection kit. The effects of 7‑NI on the gene expression of nNOS and c‑jun were detected by western blot analysis. The results from the immunofluorescence demonstrated that the c‑jun and nNOS protein were constantly expressed in PC12 cells. The cell viability of differentiated PC12 cells were significantly inhibited by treatment with 200 and 400 µmol/l 7‑NI, and the expression levels of the nNOS protein were significantly inhibited by treatment with 200 µmol/l 7‑NI. However, 7‑NI had no significant effect on the protein expression level of c‑jun and the total activities of cNOS. Based on our previous studies, which revealed that the nNOS gene was a downstream signaling molecule of the JNK/c‑jun signaling pathway in cultured neurons, the expression of nNOS downstream was able to be regulated by c‑jun which was the upstream molecule. Therefore, these results indicated that the association between them involved up and downregulation instead of crosstalk.

Activation of neuronal nitric oxide synthase (nNOS) signaling pathway in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced neurotoxicity

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been reported to cause alterations in cognitive and motor behavior during both development and adulthood. In this study, the neuronal nitric oxide synthase (nNOS) signaling pathway was investigated in differentiated pheochromocytoma (PC12) cells to better understand the mechanisms of TCDD-induced neurotoxicity. TCDD exposure induced a time- and dose-dependent increase in nNOS expression. High levels of nitric oxide (NO) production by nNOS activation induced mitochondrial cytochrome c (Cyt-c) release and down-regulation of Bcl-2. Additionally, TCDD increased the expression of active caspase-3 and significantly led to apoptosis in PC12 cells. However, these effects above could be effectively inhibited by the addition of 7-nitroindazole (7-NI), a highly selective nNOS inhibitor. Moreover, in the brain cortex of Sprague-Dawley (SD) rats, nNOS was also found to have certain relationship with TCDD-induced neuronal apoptosis. Together, our findings establish a role for nNOS as an enhancer of TCDD-induced apoptosis in PC12 cells.

Suppression of c-jun influences nNOS expression in differentiated PC12 cells

In various animal models of central neuronal diseases, both c-jun and nNOS genes are expressed inside injured neurons; however, the mechanism of these two genes in neuronal diseases remains uncertain. Our previous studies have shown that c-jun expression always occurs prior to expression of nNOS in motoneuron injuries. We aimed to determine whether there is a correlation between c-jun and nNOS, and whether the crosstalk between these two genes regulated the pathological progression of injury-induced neuronal degeneration. In the present study, we used the neuron-like differentiated PC12 cells, which express c-jun and nNOS, to examine whether c-jun is the upstream molecule modulating nNOS expression. The c-jun small interfering RNAs (c-jun siRNA) were transfected into PC12 cells and cells were treated for 72 h in vitro. Western blotting and immunofluorescence were used to check the protein levels and the expression of c-jun and nNOS in differentiated PC12 cells. The results from the immunofluorescence experiments showed that the c-jun and nNOS proteins were co-expressed in the differentiated PC12 cells. The results from the western blotting experiments revealed that the protein levels of c-jun were significantly decreased by c-jun siRNA. Moreover, the nNOS protein levels were also downregulated in differentiated PC12 cells following c-jun siRNA treatment. The present study found that siRNA used against c-jun not only knocked down c-jun, but also downregulated the nNOS protein expression in differentiated PC12 cells. These results indicate that there is a functional relationship between c-jun and nNOS in differentiated PC12 cells.

Proteasome inhibition modulates kinase activation in neural cells: relevance to ubiquitination, ribosomes, and survival

In this study we examined whether established signal transduction cascades, p44/42 mitogen-activated protein kinase (ERK1/2) and Jun N-terminal kinases (JNK) pathways, are altered in N2a neural cells in response to proteasome inhibition. Additionally, we sought to elucidate the relative contribution of these signal transduction pathways to the multiple downstream effects of proteasome inhibition. Our data indicate that ERK1/2 and JNK are activated in response to proteasome inhibition. Washout of proteasome inhibitor (MG132) results in an enhancement of ERK1/2 activation and amelioration of JNK activation. Treatment with an established MAPK inhibitor resulted in an increase in proteasome inhibitor toxicity, and incubation with JNK inhibitor was observed to attenuate proteasome inhibitor toxicity significantly. Subsequent studies demonstrated that inhibition of ERK1/2 and JNK activity does not alter the gross increase in ubiquitinated protein following proteasome inhibitor administration. Similarly, ERK1/2 and JNK activity do not appear to play a role in the disruption of polysomes following proteasome inhibitor administration in neural cells. Together these data indicate that ERK1/2 and JNK activation may play differential roles in modulating neurochemical disturbances and neurotoxicity induced by proteasome inhibition.

Preferentially increased nitration of alpha-synuclein at tyrosine-39 in a cellular oxidative model of Parkinson's disease

Alpha-synuclein is a major component of Lewy bodies, proteinacious inclusions which are a major hallmark of Parkinson's disease (PD). Lewy bodies contain high levels of nitrated tyrosine residues as determined by antibodies specific for 3-nitrotyrosine (3NT) and via mass spectrometry (MS). We have developed a multiple reaction monitoring (MRM) mass spectrometry method to sensitively quantitate the 3NT levels of specific alpha-synuclein tyrosine residues. We found a 9-fold increase (relative to controls) in levels of 3NT at Tyr-39 of alpha-synuclein in an inducible transgenic cellular model of Parkinson's disease in which monoamine oxidase B (MAO-B) is overexpressed and which emulates several features of PD. Increased nitration of Tyr-39 on endogenous alpha-synuclein via elevations in MAO-B levels could be abrogated by the addition of deprenyl, a specific MAO-B inhibitor. The increased levels of 3NT was selective for Tyr-39 as no significant increases in 3NT levels were detected at other tyrosine residues present in the protein (Tyr-125, Tyr-133, and Tyr-136). This is the first report of increased 3NT levels of a specific tyrosine in a PD model and the first use of MRM mass spectrometry to quantify changes in 3NT modifications at specific sites within a target protein.

The formation of peroxynitrite in the applied physiology of mitochondrial nitric oxide

Mitochondria require nitric oxide ((.)NO) to exert a delicate control of metabolic rate as well as to regulate life functions, cell cycle activation and arrest, and apoptosis. All activities depend on the matrical (.)NO steady state concentration as provided by mitochondrial (mtNOS) and cytosolic sources (eNOS) and reduced by forming superoxide anion and H2O2 and a low peroxynirite (ONOO(-)) yield. We review herein the biochemical pathways involved in the control of (.)NO mitochondrial level and its biological and physiological significance in hormone effects and aging. At high ()NO, the cost of this physiological regulation is that ONOO(-) excess will lead to nitrosation/nitration and oxidization of mitochondrial and cell proteins and lipids. The disruption of (.)NO modulation of mitochondrial respiration supports then, a platform for prevalent neurodegenerative and metabolic diseases.