RESUMEN
Ambiguity in genetic codes exists in cases where certain stop codons are alternatively used to encode non-canonical amino acids. In selenoprotein transcripts, the UGA codon may either represent a translation termination signal or a selenocysteine (Sec) codon. Translating UGA to Sec requires selenium and specialized Sec incorporation machinery such as the interaction between the SECIS element and SBP2 protein, but how these factors quantitatively affect alternative assignments of UGA has not been fully investigated. We developed a model simulating the UGA decoding process. Our model is based on the following assumptions: (1) charged Sec-specific tRNAs (Sec-tRNASec) and release factors compete for a UGA site, (2) Sec-tRNASec abundance is limited by the concentrations of selenium and Sec-specific tRNA (tRNASec) precursors, and (3) all synthesis reactions follow first-order kinetics. We demonstrated that this model captured two prominent characteristics observed from experimental data. First, UGA to Sec decoding increases with elevated selenium availability, but saturates under high selenium supply. Second, the efficiency of Sec incorporation is reduced with increasing selenoprotein synthesis. We measured the expressions of four selenoprotein constructs and estimated their model parameters. Their inferred Sec incorporation efficiencies did not correlate well with their SECIS-SBP2 binding affinities, suggesting the existence of additional factors determining the hierarchy of selenoprotein synthesis under selenium deficiency. This model provides a framework to systematically study the interplay of factors affecting the dual definitions of a genetic codon.
Asunto(s)
Codón Iniciador/genética , Codón de Terminación/genética , Modelos Genéticos , Proteínas/genética , Selenocisteína/genética , Selenoproteínas/genética , Simulación por Computador , Biosíntesis de Proteínas/genética , Selenoproteínas/biosíntesis , Análisis de Secuencia de ARN/métodosRESUMEN
Gastric cancer is the second most common cause of cancer deaths worldwide and due to its poor prognosis, it is important that specific biomarkers are identified to enable its early detection. Through 2-D gel electrophoresis and MALDI-TOF-TOF-based proteomics approaches, we found that 14-3-3ß, which was one of the proteins that were differentially expressed by 5-fluorouracil-treated gastric cancer SC-M1 cells, was upregulated in gastric cancer cells. 14-3-3ß levels in tissues and serum were further validated in gastric cancer patients and controls. The results showed that 14-3-3ß levels were elevated in tumor tissues (n=40) in comparison to normal tissues (n=40; p<0.01), and serum 14-3-3ß levels in cancer patients (n=145) were also significantly higher than those in controls (n=63; p<0.0001). Elevated serum 14-3-3ß levels highly correlated with the number of lymph node metastases, tumor size and a reduced survival rate. Moreover, overexpression of 14-3-3ß enhanced the growth, invasiveness and migratory activities of tumor cells. Twenty-eight proteins involved in anti-apoptosis and tumor progression were also found to be differentially expressed in 14-3-3ß-overexpressing gastric cancer cells. Overall, these results highlight the significance of 14-3-3ß in gastric cancer cell progression and suggest that it has the potential to be used as a diagnostic and prognostic biomarker in gastric cancer.