Nihar Ranjan Jana

Increased expression of p62 in expanded polyglutamine-expressing cells and its association with polyglutamine inclusions

Huntingtons disease is a progressive neurodegenerative disorder that is associated with a CAG repeat expansion in the gene encoding huntingtin. We found that a 60‐kDa protein was increased in Neuro2a cells expressing the N‐terminal portion of huntingtin with expanded polyglutamine. We purified this protein, and, using mass spectrometry, identified it as p62, an ubiquitin‐associated domain‐containing protein. A specific p62 antibody stained the ubiquitylated polyQ inclusions in expanded polyglutamine‐expressing cells, as well as in the brain of the huntingtin exon 1 transgenic mice. Furthermore, the level of p62 protein and mRNA was increased in expanded polyglutamine‐expressing cells. We also found that p62 formed aggresome‐like inclusions when p62 was increased in normal Neuro2a cells by a proteasome inhibitor. Knock‐down of p62 does not affect the formation of aggresomes or polyglutamine inclusions, suggesting that p62 is recruited to the aggresome or inclusions secondary to their formation. These results suggest that p62 may play important roles as a responsive protein to a polyglutamine‐induced stress rather than as a cross‐linker between ubiquitylated proteins.

NSAIDs and apoptosis

Abstract.Regular use of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) has been associated with decreased incidence of cancer of the colon and other gastrointestinal organs. The chemopreventive properties of NSAIDs are due to their ability to induce apoptosis. Both COX-2-dependent and COX-2-independent mechanisms are involved in NSAIDs-induced neoplastic growth. This article reviews the recent literature that has revealed various important mechanisms of NSAIDs-induced apoptosis.

Aspirin induces apoptosis through the inhibition of proteasome function

Aspirin and other nonsteroidal anti-inflammatory drugs inhibit cell proliferation and induce apoptosis in various cancer cell lines, which is considered to be an important mechanism for their anti-tumor activity and prevention of carcinogenesis. However, the molecular mechanisms through which these compounds induce apoptosis are not well understood. Here we have found that aspirin treatment of the mouse Neuro 2a cells impaired the proteasome function and caused severe mitochondrial abnormalities. Treatment with aspirin lead to a dose- and time-dependent decrease in proteasome activity and an increase in the accumulation of ubiquitylated proteins in the cells, which correlated with its effect on cell death. Aspirin exposure also resulted in an increase in the half-life of pd1EGFP, a model substrate of proteasome, as well as various intracellular substrates like Bax, IκB-α, p53, and p27kip1. Aspirin-induced proteasomal malfunction might be responsible, at least in part, for the down-regulation of NF-κB activity and neurite outgrowth. Finally, we have shown that aspirin treatment caused changes in the mitochondrial membrane potential, release of cytochrome c from mitochondria, and activation of caspase-9 and -3, which could be because of the proteasomal dysfunction.

The Ubiquitin Ligase E6-AP Is Induced and Recruited to Aggresomes in Response to Proteasome Inhibition and May Be Involved in the Ubiquitination of Hsp70-bound Misfolded Proteins

Cells are equipped with an efficient quality control system to selectively eliminate abnormally folded and damaged proteins. Initially the cell tries to refold the unfolded proteins with the help of molecular chaperones, and failure to refold leads to their degradation by the ubiquitin proteasome system. But how this proteolytic machinery recognizes the abnormally folded proteins is poorly understood. Here, we report that E6-AP, a HECT domain family ubiquitin ligase implicated in Angelman syndrome, interacts with the substrate binding domain of Hsp70/Hsc70 chaperones and promotes the degradation of chaperone bound substrates. The expression of E6-AP was dramatically induced under a variety of stresses, and overexpression of E6-AP was found to protect against endoplasmic reticulum stress-induced cell death. The inhibition of proteasome function not only increases the expression of E6-AP but also causes its redistribution around microtubule-organizing center, a subcellular structure for the degradation of the cytoplasmic misfolded proteins. E6-AP is also recruited to aggresomes containing the cystic fibrosis transmembrane conductance regulator or expanded polyglutamine proteins. Finally, we demonstrate that E6-AP ubiquitinates misfolded luciferase that is bound by Hsp70. Our results suggest that E6-AP functions as a cellular quality control ubiquitin ligase and, therefore, can be implicated not only in the pathogenesis of Angelman syndrome but also in the biology of neurodegenerative disorders involving protein aggregation.

E6-AP Promotes Misfolded Polyglutamine Proteins for Proteasomal Degradation and Suppresses Polyglutamine Protein Aggregation and Toxicity

The accumulation of intracellular protein deposits as inclusion bodies is the common pathological hallmark of most age-related neurodegenerative disorders including polyglutamine diseases. Appearance of aggregates of the misfolded mutant disease proteins suggest that cells are unable to efficiently degrade them, and failure of clearance leads to the severe disturbances of the cellular quality control system. Recently, the quality control ubiquitin ligase CHIP has been shown to suppress the polyglutamine protein aggregation and toxicity. Here we have identified another ubiquitin ligase, called E6-AP, which is able to promote the proteasomal degradation of misfolded polyglutamine proteins and suppress the polyglutamine protein aggregation and polyglutamine protein-induced cell death. E6-AP interacts with the soluble misfolded polyglutamine protein and associates with their aggregates in both cellular and transgenic mouse models. Partial knockdown of E6-AP enhances the rate of aggregate formation and cell death mediated by the polyglutamine protein. Finally, we have demonstrated the up-regulation of E6-AP in the expanded polyglutamine protein-expressing cells as well as cells exposed to proteasomal stress. These findings suggest that E6-AP is a critical mediator of the neuronal response to misfolded polyglutamine proteins and represents a potential therapeutic target in the polyglutamine diseases.

Regulation of miR-146a by RelA/NFkB and p53 in STHdhQ111/HdhQ111 Cells, a Cell Model of Huntington's Disease

Huntingtons disease (HD) is caused by the expansion of N-terminal polymorphic poly Q stretch of the protein huntingtin (HTT). Deregulated microRNAs and loss of function of transcription factors recruited to mutant HTT aggregates could cause characteristic transcriptional deregulation associated with HD. We observed earlier that expressions of miR-125b, miR-146a and miR-150 are decreased in STHdhQ111/HdhQ111 cells, a model for HD in comparison to those of wild type STHdhQ7/HdhQ7 cells. In the present manuscript, we show by luciferase reporter assays and real time PCR that decreased miR-146a expression in STHdhQ111/HdhQ111 cells is due to decreased expression and activity of p65 subunit of NFkB (RelA/NFkB). By reporter luciferase assay, RT-PCR and western blot analysis, we also show that both miR-150 and miR-125b target p53. This partially explains the up regulation of p53 observed in HD. Elevated p53 interacts with RelA/NFkB, reduces its expression and activity and decreases the expression of miR-146a, while knocking down p53 increases RelA/NFkB and miR-146a expressions. We also demonstrate that expression of p53 is increased and levels of RelA/NFkB, miR-146a, miR-150 and miR-125b are decreased in striatum of R6/2 mice, a mouse model of HD and in cell models of HD. In a cell model, this effect could be reversed by exogenous expression of chaperone like proteins HYPK and Hsp70. We conclude that (i) miR-125b and miR-150 target p53, which in turn regulates RelA/NFkB and miR-146a expressions; (ii) reduced miR-125b and miR-150 expressions, increased p53 level and decreased RelA/NFkB and miR-146a expressions originate from mutant HTT (iii) p53 directly or indirectly regulates the expression of miR-146a. Our observation of interplay between transcription factors and miRNAs using HD cell model provides an important platform upon which further work is to be done to establish if such regulation plays any role in HD pathogenesis.

UBE3A/E6-AP regulates cell proliferation by promoting proteasomal degradation of p27

The UBE3A/E6-AP is known to function both as an E3 ubiquitin ligase of the ubiquitin proteasome system and as a transcriptional coactivator. E6-AP shows brain-specific imprinting and loss of function of maternally inherited E6-AP causes Angelman syndrome. However, how the loss of function of E6-AP causes disease pathogenesis is poorly understood. Here, we show that E6-AP interacts with and promotes proteasome-mediated degradation of cyclin-dependent kinase inhibitor p27. E6-AP also directly ubiquitinates p27 in an in vitro ubiquitination assay. Partial knockdown of E6-AP increases the level of p27 leading to cell cycle arrest. Interestingly, partial knockdown also increases the transcription of p27. Finally, we have demonstrated the increased levels of p27 in E6-AP-maternal-deficient and null mice brain. Our result suggests that E6-AP not only enhances the degradation but also regulates the expression of p27 and its loss of function in Angelman syndrome might cause cell cycle alteration leading to disease pathogenesis.

Curcumin induces stress response, neurite outgrowth and prevent nf-κb activation by inhibiting the proteasome function

Curcumin, a natural polyphenolic compound, has long been known as an anti-tumour and anti-inflammatory compound; although, the common mechanism through which it exhibits such properties are remains unclear. Recently, we reported that the curcumin-induced apoptosis is mediated through the impairment of ubiquitin proteasome system (UPS). Here, we show that curcumin disrupts UPS function by directly inhibiting the enzyme activity of the proteasome’s 20S core catalytic component. Like other proteasome inhibitors, curcumin exposure induces neurite outgrowth and the stress response, as evident from the induction of various cytosolic and endoplasmic reticulum chaperones as well as induction of transcription factor CHOP/GADD153. The direct inhibition of proteasome activity also causes an increase in half-life of IκB-α that ultimately leads to the down-regulation of NF-κB activation. These results suggest that curcumin-induced proteasomal malfunction might be linked with both anti-proliferative and anti-inflammatory activities.

Recent advances in understanding the pathogenesis of polyglutamine diseases: involvement of molecular chaperones and ubiquitin-proteasome pathway

Polyglutamine diseases consist of a group of familial neurodegenerative disorders caused by expression of proteins containing expanded polyglutamine stretch. Over the past several years, tremendous progress has been made in identifying the molecular mechanisms by which the expanded polyglutamine tract leads to neuronal dysfunction and neurodegeneration. A common feature of most polyglutamine disorders is the occurrence of ubiquitin-positive neuronal intranuclear inclusions. The appearance of ubiquitinated aggregates implies an underline incapability of the cellular chaperones and proteasome machinery that normally functions to prevent the accumulation of misfolded proteins. Here we review the recent studies that have revealed a critical role for molecular chaperones and ubiquitin-proteasome pathway in the pathogenesis of polyglutamine diseases.

Loss of dopaminergic neurons and resulting behavioural deficits in mouse model of Angelman syndrome

E6 associated protein is an E3 ubiquitin ligase encoded by the gene Ube3a. Deletion or loss of function of the maternally inherited allele of Ube3a leads to Angelman syndrome. In the present study, we show that maternal loss of Ube3a (Ube3a(m-/p+)) in the mouse model leads to motor deficits that could be attributed to the dysfunction of the nigrostriatal pathway. The number of tyrosine hydroxylase positive neurons in the substantia nigra was significantly reduced in Ube3a(m-/p+) mice as compared to the wild type counterparts. The Ube3a(m-/p+) mice performed poorly in behavioural paradigms sensitive to nigrostriatal dysfunction. Even though the tyrosine hydroxylase staining was apparently the same in the striatum of both genotypes, the presynaptic and postsynaptic proteins were significantly reduced in Ube3a(m-/p+) mice. These findings suggest that the abnormality in the nigrostriatal pathway along with the cerebellum produces the observed motor dysfunctions in Ube3a(m-/p+) mice.