Interplay between Selenium Levels and Replicative Senescence in WI-38 Human Fibroblasts: A Proteomic Approach.

Selenoproteins are essential components of antioxidant defense, redox homeostasis, and cell signaling in mammals, where selenium is found in the form of a rare amino acid, selenocysteine. Selenium, which is often limited both in food intake and cell culture media, is a strong regulator of selenoprotein expression and selenoenzyme activity. Aging is a slow, complex, and multifactorial process, resulting in a gradual and irreversible decline of various functions of the body. Several cellular aspects of organismal aging are recapitulated in the replicative senescence of cultured human diploid fibroblasts, such as embryonic lung fibroblast WI-38 cells. We previously reported that the long-term growth of young WI-38 cells with high (supplemented), moderate (control), or low (depleted) concentrations of selenium in the culture medium impacts their replicative lifespan, due to rapid changes in replicative senescence-associated markers and signaling pathways. In order to gain insight into the molecular link between selenium levels and replicative senescence, in the present work, we have applied a quantitative proteomic approach based on 2-Dimensional Differential in-Gel Electrophoresis (2D-DIGE) to the study of young and presenescent cells grown in selenium-supplemented, control, or depleted media. Applying a restrictive cut-off (spot intensity ±50% and a p value < 0.05) to the 2D-DIGE analyses revealed 81 differentially expressed protein spots, from which 123 proteins of interest were identified by mass spectrometry. We compared the changes in protein abundance for three different conditions: (i) spots varying between young and presenescent cells, (ii) spots varying in response to selenium concentration in young cells, and (iii) spots varying in response to selenium concentration in presenescent cells. Interestingly, a 72% overlap between the impact of senescence and selenium was observed in our proteomic results, demonstrating a strong interplay between selenium, selenoproteins, and replicative senescence.

Selective up-regulation of human selenoproteins in response to oxidative stress.

Selenocysteine is inserted into selenoproteins via the translational recoding of a UGA codon, normally used as a stop signal. This process depends on the nature of the selenocysteine insertion sequence element located in the 3' UTR of selenoprotein mRNAs, selenium bioavailability, and, possibly, exogenous stimuli. To further understand the function and regulation of selenoproteins in antioxidant defense and redox homeostasis, we investigated how oxidative stress influences selenoprotein expression as a function of different selenium concentrations. We found that selenium supplementation of the culture media, which resulted in a hierarchical up-regulation of selenoproteins, protected HEK293 cells from reactive oxygen species formation. Furthermore, in response to oxidative stress, we identified a selective up-regulation of several selenoproteins involved in antioxidant defense (Gpx1, Gpx4, TR1, SelS, SelK, and Sps2). Interestingly, the response was more efficient when selenium was limiting. Although a modest change in mRNA levels was noted, we identified a novel translational control mechanism stimulated by oxidative stress that is characterized by up-regulation of UGA-selenocysteine recoding efficiency and relocalization of SBP2, selenocysteine-specific elongation factor, and L30 recoding factors from the cytoplasm to the nucleus.

Interplay between selenium levels, selenoprotein expression, and replicative senescence in WI-38 human fibroblasts.

Selenium is an essential trace element, which is incorporated as selenocysteine into at least 25 selenoproteins using a unique translational UGA-recoding mechanism. Selenoproteins are important enzymes involved in antioxidant defense, redox homeostasis, and redox signaling pathways. Selenium levels decline during aging, and its deficiency is associated with a marked increase in mortality for people over 60 years of age. Here, we investigate the relationship between selenium levels in the culture medium, selenoprotein expression, and replicative life span of human embryonic lung fibroblast WI-38 cells. Selenium levels regulate the entry into replicative senescence and modify the cellular markers characteristic for senescent cells. Whereas selenium supplementation extends the number of population doublings, its deficiency impairs the proliferative capacity of WI-38 cells. We observe that the expression of several selenoproteins involved in antioxidant defense is specifically affected in response to cellular senescence. Their expression is selectively controlled by the modulation of mRNA levels and translational recoding efficiencies. Our data provide novel mechanistic insights into how selenium impacts the replicative life span of mammalian cells by identifying several selenoproteins as new targets of senescence.

Phenotype-haplotype correlation of IRF5 in systemic sclerosis: role of 2 haplotypes in disease severity.

OBJECTIVE: Identification of an association between IRF5 rs2004640 and systemic sclerosis (SSc) has highlighted a key role for type 1 interferon (IFN). Additional functional IRF5 variants have been identified as autoimmune susceptibility factors. Our aim was to investigate whether IRF5 haplotypes confer susceptibility to SSc, and to perform genotype haplotype-phenotype correlation analyses. METHODS: We genotyped IRF5 rs377385, rs2004640, and rs10954213 in 1623 individuals of French European Caucasian origin. SSc patient subphenotypes were analyzed according to cutaneous subsets and for SSc-related pulmonary fibrosis. RESULTS: Case-control studies of single markers revealed an association between IRF5 rs3757385, rs2004640, and rs10954213 variants and SSc. We identified an IRF5 risk haplotype "R" (p(adj) = 0.024, OR 1.23, 95% CI 1.07-1.40) and a mirrored protective haplotype "P" (p(adj) = 8.8 x 10(-3), OR 0.78, 95% CI 0.68-0.90) for SSc susceptibility. Genotype-phenotype correlation analyses failed to detect any association with a single marker. By contrast, phenotype-haplotype correlation analysis was able to detect intra-cohort association and to discriminate SSc patients with from those without the following clinical traits: "R" and/or "P" haplotypes identified diffuse cutaneous SSc (p = 0.0081) and fibrosing alveolitis (p = 0.018). CONCLUSION: IRF5 haplotypes are more informative than single markers, suggesting that they could be helpful for risk stratification of SSc patients. Our study provides further evidence of a key role of IRF5 in SSc severity.