NF-Y, a CCAAT box-binding protein, is one of the trans-acting factors necessary for the response of the murine ERp72 gene to protein traffic

Role of the CCAAT-binding protein NFY in SCA17 pathogenesis

Spinocerebellar ataxia 17 (SCA17) is caused by expansion of the polyglutamine (polyQ) tract in human TATA-box binding protein (TBP) that is ubiquitously expressed in both central nervous system and peripheral tissues. The spectrum of SCA17 clinical presentation is broad. The precise pathogenic mechanism in SCA17 remains unclear. Previously proteomics study using a cellular model of SCA17 has revealed reduced expression of heat shock 70 kDa protein 5 (HSPA5) and heat shock 70 kDa protein 8 (HSPA8), suggesting that impaired protein folding may contribute to the cell dysfunction of SCA17 (Lee et al., 2009). In lymphoblastoid cells, HSPA5 and HSPA8 expression levels in cells with mutant TBP were also significantly lower than that of the control cells (Chen et al., 2010). As nuclear transcription factor Y (NFY) has been reported to regulate HSPA5 transcription, we focused on if NFY activity and HSPA5 expression in SCA17 cells are altered. Here, we show that TBP interacts with NFY subunit A (NFYA) in HEK-293 cells and NFYA incorporated into mutant TBP aggregates. In both HEK-293 and SH-SY5Y cells expressing TBP/Q(61~79), the level of soluble NFYA was significantly reduced. In vitro binding assay revealed that the interaction between TBP and NFYA is direct. HSPA5 luciferase reporter assay and endogenous HSPA5 expression analysis in NFYA cDNA and siRNA transfection cells further clarified the important role of NFYA in regulating HSPA5 transcription. In SCA17 cells, HSPA5 promoter activity was activated as a compensatory response before aggregate formation. NFYA dysfunction was indicated in SCA17 cells as HSPA5 promoter activity reduced along with TBP aggregate formation. Because essential roles of HSPA5 in protection from neuronal apoptosis have been shown in a mouse model, NFYA could be a target of mutant TBP in SCA17.

Neuronal expression of TATA box-binding protein containing expanded polyglutamine in knock-in mice reduces chaperone protein response by impairing the function of nuclear factor-Y transcription factor

The polyglutamine diseases consist of nine neurodegenerative disorders including spinocerebellar ataxia type 17 that is caused by a polyglutamine tract expansion in the TATA box-binding protein. In all polyglutamine diseases, polyglutamine-expanded proteins are ubiquitously expressed throughout the body but cause selective neurodegeneration. Understanding the specific effects of polyglutamine-expanded proteins, when expressed at the endogenous levels, in neurons is important for unravelling the pathogenesis of polyglutamine diseases. However, addressing this important issue using mouse models that either overly or ubiquitously express mutant polyglutamine proteins in the brain and body has proved difficult. To investigate the pathogenesis of spinocerebellar ataxia 17, we generated a conditional knock-in mouse model that expresses one copy of the mutant TATA box-binding protein gene, which encodes a 105-glutamine repeat, selectively in neuronal cells at the endogenous level. Neuronal expression of mutant TATA box-binding protein causes age-dependent neurological symptoms in mice and the degeneration of cerebellar Purkinje cells. Mutant TATA box-binding protein binds more tightly to the transcription factor nuclear factor-Y, inhibits its association with the chaperone protein promoter, as well as the promoter activity and reduces the expression of the chaperones Hsp70, Hsp25 and HspA5, and their response to stress. These findings demonstrate how mutant TATA box-binding protein at the endogenous level affects neuronal function, with important implications for the pathogenesis and treatment of polyglutamine diseases.

Transformation-associated gene regulation by ATF6alpha during hepatocarcinogenesis

We have previously reported that the endoplasmic reticulum (ER) stress-regulated transmembrane transcription factor 6 alpha (ATF6alpha) is implicated in the pathogenesis of hepatocellular carcinomas (HCCs). In order to further identify genes that are regulated by ATF6alpha, the global gene expression profiles of the ATF6alpha-transfected and untransfected HCC cell line, HLF, were analyzed. These results were then compared with the differential gene expression patterns of poorly differentiated HCC and control non-tumorous liver tissue. Our findings demonstrate that at least 18 genes are specifically upregulated by ATF6alpha, while another UPR mediator, XBP1 or ER-stress inducer, thapsigargin could partially stimulate or even repress some of them in HCC cells. Moreover, six of these identified genes contain potential ER stress-responsive elements and/or unfolded protein response elements in their 5' regulatory regions.

Induction of endoplasmic reticulum-induced stress genes in Panc-1 pancreatic cancer cells is dependent on Sp proteins

Endoplasmic reticulum (ER) stress plays a critical role in multiple diseases, and pharmacologically active drugs can induce cell death through ER stress pathways. Stress-induced genes are activated through assembly of transcription factors on ER stress response elements (ERSEs) in target gene promoters. Gel mobility shift and chromatin immunoprecipitation assays have confirmed interactions of NF-Y and YY1 with the distal motifs of the tripartite ERSE from the glucose-related protein 78 (GRP78) gene promoter. The GC-rich nonanucleotide (N(9)) sequence, which forms the ER stress response binding factor (ERSF) complex binds TFII-I and ATF6; however, we have now shown that in Panc-1 pancreatic cancer cells, this complex also binds Sp1, Sp3, and Sp4 proteins. Sp proteins are constitutively bound to the ERSE; however, activation of GRP78 protein (or reporter gene) by thapsigargin or tunicamycin is inhibited after cotransfection with small inhibitory RNAs for Sp1, Sp3, and Sp4. This study demonstrates that Sp transcription factors are important for stress-induced responses through their binding to ERSEs.

The glucose-regulated proteins: stress induction and clinical applications

A protective mechanism used by cells to adapt to stress of the endoplasmic reticulum (ER) is the induction of members of the glucose-regulated protein (Grp) family. The induction of mammalian Grp proteins in response to ER stress involves a complex network of regulators and novel mechanisms. The elucidation of Grp function and regulation opens up new therapeutic approaches to diseases associated with ER stress and cancer.

Identification of TFII-I as the endoplasmic reticulum stress response element binding factor ERSF: its autoregulation by stress and interaction with ATF6

When mammalian cells are subjected to stress targeted to the endoplasmic reticulum (ER), such as depletion of the ER Ca(2+) store, the transcription of a family of glucose-regulated protein (GRP) genes encoding ER chaperones is induced. The GRP promoters contain multiple copies of the ER stress response element (ERSE), consisting of a unique tripartite structure, CCAAT(N(9))CCACG. Within a subset of mammalian ERSEs, N(9) represents a GC-rich sequence of 9 bp that is conserved across species. A novel complex (termed ERSF) exhibits enhanced binding to the ERSE of the grp78 and ERp72 promoters using HeLa nuclear extracts prepared from ER-stressed cells. Optimal binding of ERSF to ERSE and maximal ERSE-mediated stress inducibility require the conserved GGC motif within the 9-bp region. Through chromatographic purification and subsequent microsequencing, we have identified ERSF as TFII-I. Whereas TFII-I remains predominantly nuclear in both nontreated NIH 3T3 cells and cells treated with thapsigargin (Tg), a potent inducer of the GRP stress response through depletion of the ER Ca(2+) store, the level of TFII-I transcript was elevated in Tg-stressed cells, correlating with an increase in TFII-I protein level in the nuclei of Tg-stressed cells. Purified recombinant TFII-I isoforms bind directly to the ERSEs of grp78 and ERp72 promoters. The stimulation of ERSE-mediated transcription by TFII-I requires the consensus tyrosine phosphorylation site of TFII-I and the GGC sequence motif of the ERSE. We further discovered that TFII-I is an interactive protein partner of ATF6 and that optimal stimulation of ERSE by ATF6 requires TFII-I.

Conservation and divergence of the yeast and mammalian unfolded protein response. Activation of specific mammalian endoplasmic reticulum stress element of the grp78/BiP promoter by yeast Hac1

Yeast Hac1 (yHac1), the transcription factor that binds and activates the unfolded protein response element of endoplasmic reticulum (ER)-chaperone gene promoters, only accumulates in stressed cells after an unconventional splicesosome-free mRNA processing step and escape from translation block. In determining whether the novel regulatory mechanisms for yHac1 are conserved in mammalian cells, we discovered a unique unfolded protein response element-like sequence within the endoplasmic reticulum stress element 163, one of the three ER stress elements recently identified in the rat grp78 promoter. The unspliced form of yHac1 is stably expressed in nonstressed mammalian cells and is as active as the spliced form in stimulating the promoter activities of grp genes. Further, the yHac1 mRNA is not processed in response to ER stress in mammalian cells. We identified a CCAGC motif as the yHac1 binding site, which is contained within a YY1 binding site previously shown to be important for mammalian UPR. Dissection of the yHac1 and the YY1 binding sites uncovered specific contact points for an activator protein predicted to be the mammalian homolog of yHac1, the activity of which can be stimulated by YY1. A model of the conserved and unique features of the yeast and mammalian unfolded protein response transcription machinery is proposed.

Signal transduction from the endoplasmic reticulum to the cell nucleus

The endoplasmic reticulum (ER) serves several important functions. Cholesterol, an essential component of cellular membranes, is synthesized on the ER surface. Inside the organelle, proteins destined for secretion or transport to the cell surface are folded and become glycosylated. Because these processes are essential for cell viability, a disturbance in ER function presents significant stress to the cell. In response to ER stress, three distinct signal transduction pathways can be activated. Two of these, the unfolded protein response and the ER-overload response, respond to disturbances in protein processing. The third, the sterol regulatory cascade, is activated by depletion of cholesterol. This review summarizes the recent advances in our understanding of these ER-nuclear signal transduction pathways. In addition, it points to novel regulatory mechanisms discovered in these pathways, which may be widely used in other systems.