The Expression of Sel1L and Tan-1 in Normal and Neoplastic Cells

Role of SEL1L in the progression of solid tumors, with a special focus on its recent therapeutic potential

Since suppressor/enhancer of Lin-12-like (SEL1L) was cloned in 1997, various pieces of evidence from lower species suggest it plays a significant role in protein degradation via the ubiquitin-proteasome system. The relevance of SEL1L in many aspects of malignant transformation and tumorigenic events has been the subject of research, which has shown compelling in vitro and in vivo findings relating its altered expression to changes in tumor aggressiveness. The Endoplasmic Reticulum (ER) in tumor cells is crucial for preserving cellular proteostasis by inducing the unfolded protein response (UPR), a stress response. A crucial component of the UPR is ER-associated degradation (ERAD), which guards against ER stress-induced apoptosis and the removal of unfolded or misfolded proteins by the ubiquitin-proteasome system. As a protein stabilizer of HMG-CoA reductase degradation protein 1 (HRD1), one of the main components of ERAD, SEL1L plays an important role in ER homeostasis. Notably, the expression levels of these two proteins fluctuate independently in various cancer types, yet changes in their expression affect the levels of other associated proteins during cancer pathogenesis. Recent studies have also outlined the function of SEL1L in cancer medication resistance. This review explores the value of targeting SEL1L as a novel treatment approach for cancer, focusing on the molecular processes of SEL1L and its involvement in cancer etiology.

The changing face of gastric cancer: epidemiologic trends and advances in novel therapies

Gastric cancer is an aggressive solid-tumor malignancy with poor prognosis. The epidemiologic face of gastric cancer is changing and further insight into its heterogenous immunohistopathologic nature is needed to develop personalized therapies for specific patient populations. In this review, we highlight changes in gastric cancer epidemiology with a special emphasis on racial and ethnic variations and discuss the implications of current clinical and preclinical treatment advances.

Production of a Monoclonal Antibody Directed against the Recombinant SEL1L Protein

SEL1L, highly similar to the C. elegans sel-1 gene, is a recently cloned human gene whose function is under investigation. SEL1L is differentially expressed in tumors and normal tissues and seems to play a role in tumor growth and aggressiveness. We used the recombinant N-terminus of the SEL1L protein to immunize a Balb/c mouse and produce a monoclonal antibody. A hybridoma secreting an antibody specifically reacting on the SEL1L recombinant fragment was selected. This monoclonal antibody, named MSel1, recognizes the SEL1L protein by Western blotting, immunofluorescence and immunohistochemistry on normal and tumor cells. MSel1 is able to recognize SEL1L even on archival tumor specimens and is therefore particularly appropriate to study SEL1L involvement in tumor progression.

Functional characterization of two secreted SEL1L isoforms capable of exporting unassembled substrate

SEL1L-A, a transmembrane glycoprotein residing in the endoplasmic reticulum (ER), is a component of the ER-associated degradation (ERAD) pathway. Alternative splicing generates two smaller SEL1L isoforms, -B and -C, that lack the SEL1L-A membrane-spanning region but retain some sel-1-like repeats, known to be involved in multi-protein interactions and signal transduction. In this study the functional characteristics of SEL1L-B and -C were investigated in human cell models. We show that these two isoforms are induced upon ER stress and activation of the unfolded protein response, together with SEL1L-A. Using transient transfection experiments (based on wild-type and mutant SEL1L constructs) combined with several biochemical tests we show that SEL1L-B and, more prominently, SEL1L-C are secreted glycoproteins. Although SEL1L-C is in monomeric form, SEL1L-B is engaged in intramolecular/intermolecular disulfide bonds. Both isoforms localize in secretory and degradative cellular compartments and in areas of cell-cell contact. However, whereas SEL1L-B is mainly associated with membranes, SEL1L-C shows the typical intralumenal localization of soluble proteins and is present in intercellular spaces. Furthermore, because of its peroxisomal domain, SEL1L-C localizes to peroxisomes. Both SEL1L-B and -C are involved in sorting and exporting unassembled Ig-mu(s) but do not affect two other ERAD substrates, the null Hong Kong variant of alpha(1)-antitrypsin, and mutant alpha(1)-AT Z. Overall these findings suggest that SEL1L-B and -C participate to novel molecular pathways that, in parallel with ERAD, contribute to the disposure of misfolded/unfolded or orphan proteins through degradation or secretion.

Expression of the Notch signaling pathway and effect on exocrine cell proliferation in adult rat pancreas

When pancreatic tissue is injured after duct obstruction, acinoductal metaplasia is observed. Similar metaplastic changes occur when exocrine pancreatic cells are isolated and cultured. We demonstrate that under these experimental conditions the exocrine acinar cells lose their differentiated characteristics: expression of the acinar transcription factors p48/Ptf1alpha and Mist1 is decreased or lost, whereas expression of the embryonic transcription factor Pdx1 is increased. The receptors Notch1 and Notch2, members of the DSL family of Notch ligands, and the target genes in the Notch-signaling pathway Hes1, Hey1, and Hey2 become strongly up-regulated. We noted also reduced expression of Sel1L, a Notch repressor that is normally highly expressed in exocrine pancreas. Stimulation of Notch by its ligand Jagged1 diminished the proliferation of cultured metaplastic exocrine cells. Chemical inhibition of Notch signaling resulted in increased proliferation and induction of the cell-cycle regulator p21Cip1. This effect seems to be Hes1-independent and mainly coincides with decreased Hey1 and Hey2 mRNA expression. In conclusion, we demonstrate that during acinoductal metaplasia the Notch-signaling pathway is activated concomitantly with changes in transcription factor expression of pancreatic acinar cells. In addition, we show that Notch signaling is implicated in the suppression of proliferation of these metaplastic exocrine cells. The latter may be important in protection from neoplastic transformation.

SEL1L a multifaceted protein playing a role in tumor progression

Since the cloning in 1997 of SEL1L, the human ortholog of the sel-1 gene of C. elegans, most studies have focused on its role in cancer progression and have provided significant evidences to link its increased expression to a decrease in tumor aggressiveness. SEL1L resides on a "Genome Desert area" on chromosome 14q24.3-31 and is highly conserved in evolution. The function of the SEL1L encoded protein is still very elusive although, several evidences from lower organisms indicate that it plays a major role in protein degradation using the ubiquitin-proteosome system. SEL1L has a very complex structure made up of modules: genomically it consists of 21 exons featuring several alternative transcripts encoding for putative protein isoforms. This structural complexity ensures protein flexibility and specificity, indeed the protein was found in different sub-cellular compartments and may turn on a particular transcript in response to specific stimuli. The overall architecture of SEL1L guarantees an exquisite regulation in the expression of the gene.

Sel1-like repeat proteins in signal transduction

Solenoid proteins, which are distinguished from general globular proteins by their modular architectures, are frequently involved in signal transduction pathways. Proteins from the tetratricopeptide repeat (TPR) and Sel1-like repeat (SLR) families share similar alpha-helical conformations but different consensus sequence lengths and superhelical topologies. Both families are characterized by low sequence similarity levels, rendering the identification of functional homologous difficult. Therefore current knowledge of the molecular and cellular functions of the SLR proteins Sel1, Hrd3, Chs4, Nif1, PodJ, ExoR, AlgK, HcpA, Hsp12, EnhC, LpnE, MotX, and MerG has been reviewed. Although SLR proteins possess different cellular functions they all seem to serve as adaptor proteins for the assembly of macromolecular complexes. Sel1, Hrd3, Hsp12 and LpnE are activated under cellular stress. The eukaryotic Sel1 and Hrd3 proteins are involved in the ER-associated protein degradation, whereas the bacterial LpnE, EnhC, HcpA, ExoR, and AlgK proteins mediate the interactions between bacterial and eukaryotic host cells. LpnE and EnhC are responsible for the entry of L. pneumophila into epithelial cells and macrophages. ExoR from the symbiotic microorganism S. melioti and AlgK from the pathogen P. aeruginosa regulate exopolysaccaride synthesis. Nif1 and Chs4 from yeast are responsible for the regulation of mitosis and septum formation during cell division, respectively, and PodJ guides the cellular differentiation during the cell cycle of the bacterium C. crescentus. Taken together the SLR motif establishes a link between signal transduction pathways from eukaryotes and bacteria. The SLR motif is so far absent from archaea. Therefore the SLR could have developed in the last common ancestor between eukaryotes and bacteria.

SEL1L expression in non-small cell lung cancer

SEL1L gene product plays a role in cell transformation and tumor progression in human breast, pancreas, esophageal, and prostate cancer. SEL1L expression was evaluated in a series of 76 surgically resected non-small cell lung carcinomas to investigate its clinical significance. SEL1L is scarcely detectable in normal lung, whereas in the initial stages of cell transformation, it becomes consistently expressed with evident staining in bronchial squamous metaplasia and in associated dysplastic changes. SEL1L immunoreactivity can be detected both in the cytoplasm and less commonly in the nuclei; the subcellular location correlates with tumor histotype, with cytoplasmic immunoreactivity being most prevalent in squamous cell carcinomas (P = .0005) and nuclear immunoreactivity being associated with adenocarcinomas (P = .02). Nuclear import and export signals are present in the SEL1L coding sequence, justifying the different subcellular location of the protein. SEL1L immunoreactivity was inversely correlated with tumor grade (P = .05); when considering only the adenocarcinomas, a stronger association was found (P = .006). SEL1L messenger RNA and protein evaluation in lung cancer cell lines confirmed the expression of the gene and the dual subcellular location of the protein in lung tumors. The data here reported suggest that, in non-small cell lung carcinoma, SEL1L may be an indicator of cell transformation, thus having important biologic and clinical implications.

SEL1L and squamous cell carcinoma of the esophagus

The gene SEL1L is involved both in human breast and pancreatic cancer progression. It is located on 14q24.3-31, a region known to be lost in invasive cancer of the esophagus. We aimed to assess whether SEL1L could become a useful biomarker for this cancer. We assessed SEL1L mRNA and protein expression in 35 patients and found it to be weak in low-grade and strong in high-grade dysplasia. Although the majority of cancer patients showed differential expression (mRNA and protein) of SEL1L, in five cases it was completely absent; these patients had the worst outcomes. SEL1L immunoreactivity was negative in normal tissue samples from five patients with mild esophagitis as well as in normal mucosa adjacent to the tumor. We hypothesize that SEL1L could influence those cellular changes that mediate the transition from a normal mucosa to a neoplastic lesion and may help in the identification of those patients at higher risk of developing this cancer. The specific impact of SEL1L in esophageal cancer needs further investigation.

The crystal structure of Helicobacter cysteine-rich protein C at 2.0 A resolution: similar peptide-binding sites in TPR and SEL1-like repeat proteins

Helicobacter pylori is a Gram-negative human pathogen that infects the gastric mucosa and causes an inflammatory process leading to gastritis, ulceration and cancer. Bacterial cell-surface and secreted proteins often play an important role in pathogen-host interactions and are thought to be selective mediators for the pathology of the infection. The Helicobacter cysteine-rich proteins (Hcp) represent a large family of secreted proteins that seem to be specific for microorganisms from the epsilon-subfamily of proteobacteria. Although significantly elevated levels of anti-Hcp antibodies were observed in many patients infected with H.pylori, details on the biological functions of Hcp proteins are sparse. Hcps belong to a large family of Sel1-like multi-repeat proteins. The crystal structure of HcpC was refined at 2.0 A resolution and revealed a super-helical topology composed of seven disulfide bridged alpha/alpha-repeats, an N-terminal capping helix and an extended C-terminal coil consisting of alternating hydrophobic and hydrophilic residues. In the crystal packing, the C-terminal coil interacts with the concave surface of a symmetry-related HcpC super-helix. A hydrophobic pocket and a cluster of negatively charged residues recognize the side-chains of Val290 and Lys287 from the C-terminal coil, respectively. The peptide nitrogen atom of His291 forms a short hydrogen bond with the side-chain of Asn66. The interactions seen in this crystal contact are strikingly similar to the peptide-binding modes of the Hsp70/Hsp90 organizing protein and the PEX5 receptor. The conservation of the peptide-binding mode suggests that HcpC might recognize its binding partner in a similar way.

Epigenetic and gene expression changes related to transgenerational carcinogenesis

Transgenerational carcinogenesis refers to transmission of cancer risk to the untreated progeny of parents exposed to carcinogens before mating. Accumulated evidence suggests that the mechanism of this process is epigenetic, and might involve hormonal and gene expression changes in offspring. To begin to test this hypothesis, we utilized a mouse model (NIH Swiss) in which exposure of fathers to Cr(III) chloride 2 wk before mating can alter incidence of neoplastic and nonneoplastic changes in offspring tissues. Utilizing a MS-RDA approach, we found that the sperm of these fathers had a significantly higher percentage of undermethylated copies of the 45S ribosomal RNA gene (rRNA); this finding was confirmed by bisulfite sequencing. Because gene methylation is a known mechanism of expression control in germ cells, and ribosomal RNA levels have been linked to cancer, these findings are consistent with the hypothesis. Secondly, we observed that offspring of Cr(III)-treated fathers were significantly heavier than controls, and had higher levels of serum T3. Possible effects of T3 levels on gene expression in the offspring were examined by microarray analysis of cDNAs from liver. A total of 58 genes, including 25 named genes, had expression ratios that correlated significantly with serum T3 ratios at P </= 0.001. Some of these genes have potential roles in growth and/or tumor suppression. These results also support the hypothesis of an epigenetic and/or gene expression-based mechanism for transgenerational carcinogenesis. Published 2004 Wiley-Liss, Inc.

Identification of a region within SEL1L protein required for tumour growth inhibition

The ectopic expression of the entire SEL1L cDNA significantly reduces the proliferate activity and aggressive behavior of the human breast carcinoma cell line MCF-7. However, the mechanism responsible for this biological effect remains unclear. A study was initiated to define those regions within SEL1L conferring such antiproliferative properties. Here we report that the region between amino acid residues 659-794 contains a functionally relevant domain since a deletion mutant impairs SEL1L's ability to suppress tumor cell growth. This region contains the Hrd3 motif, tetratricopeptide (TPR)-like SEL1 repeat, a transmembrane region, and a proline-rich tail.

SEL1L expression in pancreatic adenocarcinoma parallels SMAD4 expression and delays tumor growth in vitro and in vivo

Recent data suggest that SEL1L may play an important role in pancreatic carcinoma, similar to breast cancer, where the expression of SEL1L has been associated with a reduction in both proliferative activity in vitro and clinical tumor aggressiveness. To investigate this possibility, we examined the expression of Sel1L in a series of primary pancreatic carcinomas by immunohistochemistry and characterized the effects of Sel1L overexpression both in vitro and in vivo. In 74 pancreatic cancers analysed, 36% lacked Sel1L expression, although there was no significant correlation between the expression of Sel1L and any clinicopathologic parameter, including survival. However, immunohistochemical reactivity for Sel1L and Dpc4/Smad4 was concordant in 69% of cases (chi(2) test P&<0.004). Overexpression of SEL1L in stably transfected pancreatic cancer cells caused both a decrease in clonogenicity and anchorage-independent growth as well as a significant increase in the levels of activin A and SMAD4. When implanted in nude mice, Suit-2-SEL1L-overexpressing clones displayed a considerably reduced rate of tumor growth. Thus, it can be hypothesized that Sel1L plays an important function in the growth and aggressiveness of pancreatic carcinoma. Moreover, our data provide evidence that SEL1L has an impact on the expression of genes involved in regulation of cellular growth, possibly through the TGF-beta signaling pathway.

Differential regulation of Notch signal transduction in leukaemia and lymphoma cells in culture

The transduction of Notch signal plays an intricate role in cell differentiation and pathogenesis of haematological malignancies as well as in certain congenital conditions. We found no genomic changes in either gene in 34 leukaemic samples and 25 leukaemia and lymphoma cell lines. The functionality of Notch signalling was tested using HES1 gene activation. We show that Notch signalling is differentially regulated in T-acute lymphoblastic leukaemia (ALL) and B-lymphoma cells. The Notch pathway is intact in a majority of B-lymphoma cell lines, but EBNA2, which mimics notch function, can occasionally activate the pathway. In contrast, the Notch pathway is constitutively active in T-ALL. This is the first demonstration of a distinction between B-lymphomas and T-cell leukaemias in the functioning of the Notch-signalling pathway. This might be related to their pathogenesis.

Cross-species conservation of SEL1L, a human pancreas-specific expressing gene

SEL1L is a recently cloned and organ-specific expressing human gene whose function is still at an embryonic stage but displays several interesting characteristics, among which a remarkable cross-species conservation. During evolution, the gene structural complexity increased, suggesting a diversification of its function; however, several amino acid motifs remain perfectly conserved from the bacteria to the human protein. SEL1L is the human ortholog of the C. elegans gene sel-1; the latter is implicated in the negative regulation of LIN-12/GLP-1/Notch receptor proteins. These receptor proteins play fundamental roles in signal transduction pathways and are key players in cell fate determination during the development of various organs. Studies in model organisms, such as C. elegans, helped to illuminate fundamental mechanisms involved in normal cellular functions and human diseases. This paper describes the conserved nature of SEL1L across a wide range of species suggesting, that the encoded protein most likely exerts a very important biological function; it may belong to a subclass of genes considered to be "essential."

Allelic polymorphisms in the transcriptional regulatory region of human SEL1L

In this work, we explored the existence of genetic variants within the SEL1L transcriptional regulatory region by direct sequencing of the basal promoter. SEL1L is the human ortholog of the Caenorhabditis elegans gene sel-1, a negative regulator of LIN-12/NOTCH receptor proteins. To understand the relation in SEL1L transcription pattern observed in different epithelial cells, we analysed its promoter activity. We found it to be considerably higher only in pancreatic cells. We then looked for the presence of genetic variability within this region by sequencing the minimal promoter of 63 individuals (126 alleles); two new and associated polymorphic variants were found only in few lung carcinoma bearing patients. The functional effects of this polymorphism was analysed by transient transfection assay which resulted in a significant increase in the transcriptional activity of the gene.

Cloning and functional analysis of SEL1L promoter region, a pancreas-specific gene

We examined the promoter activity of SEL1L, the human ortholog of the C. elegans gene sel-1, a negative regulator of LIN-12/NOTCH receptor proteins. To understand the relation in SEL1L transcription pattern observed in different epithelial cells, we determined the transcription start site and sequenced the 5' flanking region. Sequence analysis revealed the presence of consensus promoter elements--GC boxes and a CAAT box--but the absence of a TATA motif. Potential binding sites for transcription factors that are involved in tissue-specific gene expression were identified, including: activator protein-2 (AP-2), hepatocyte nuclear factor-3 (HNF3 beta), homeobox Nkx2-5 and GATA-1. Transcription activity of the TATA-less SEL1L promoter was analyzed by transient transfection using luciferase reporter gene constructs. A core basal promoter of 302 bp was sufficient for constitutive promoter activity in all the cell types studied. This genomic fragment contains a CAAT and several GC boxes. The activity of the SEL1L promoter was considerably higher in mouse pancreatic beta cells (beta TC3) than in several human pancreatic neoplastic cell lines; an even greater reduction of its activity was observed in cells of nonpancreatic origin. These results suggest that SEL1L promoter may be a useful tool in gene therapy applications for pancreatic pathologies.