Mette Stoedter

Down-regulation of the hepatic selenoprotein biosynthesis machinery impairs selenium metabolism during the acute phase response in mice

The acute‐phase response (APR) is characterized by an impaired metabolism of the essential trace element selenium (Se). Moreover, low‐Se concentrations correlate to mortality risk in sepsis. Therefore, we analyzed the expression of the central Se transport and storage protein selenoprotein P (Seppl) during an APR in mice. Serum Se and Sepp1 concentrations declined in parallel after injection of lipopolysaccha‐ride to 50 and 39% of control‐injected littermates, respectively. This negative APR proceeded largely independent from hepatic Sepp1 transcript concentrations. Instead, we identified a set of hepatic transcripts involved in Se metabolism, which declined coordinately during the APR, including the selenocysteine‐specific elongation factor (EFsec), selenophosphate‐synthetase 2 (Sephs2),selenocysteine‐tRNA[Ser]Sec synthase (SecS), andphosphoseryl‐tRNA[Ser]Sec kinase (Pstk). Pstk reacted most strongly and qualified as a new limiting factor for Sepp1 biosynthesis in siRNA‐mediated knockdown experiments in hepatocytes in culture. Analogous experiments were performed with mice transgenic forhepato‐cyte‐specific human Sepp1 cDNA to verify this hypothesis. Similar kinetics and effect sizes of Sepp1 expression were observed as before in wild‐type mice. We conclude that hepatic translation of Sepp1 mRNA is specifically impaired during the APR. This deficit disrupts regular Se metabolism, transport, and supply to peripheral tissues and likely aggravates the pathological status.—Renko, K., Hofmann, P.J., Stoedter, M., Hollenbach, B., Behrends,T., Kohrle, J., Schweizer, U., Schomburg, L. Down‐regulation of the hepatic selenoprotein biosynthesis machinery impairs selenium metabolism during the acute phase response in mice. FASEB J. 23, 1758–1765 (2009)

Selenium controls the sex-specific immune response and selenoprotein expression during the acute-phase response in mice

Selenium modifies inflammatory reactions in rodents and humans. The liver controls metabolism and transport of selenium via hepatically-derived SEPP (selenoprotein P). Intracellular SEPS (selenoprotein S) modifies endoplasmic-reticulum function and immune-cell activity. Polymorphisms in SEPS have been associated with cytokine levels and inflammatory diseases in a subset of clinical studies. In the present study, we hypothesized that sex and selenium represent decisive parameters controlling the immune response and regulation of SEPS expression in vivo. Male and female mice fed a selenium-poor diet were supplemented or not with selenite for 3 days and injected with saline or LPS (lipopolysaccharide) 24 h before analysis. Selenium supplementation mitigated the LPS-induced rise in circulating cytokines in male mice. Serum SepP and selenium concentrations decreased in response to LPS, whereas hepatic SepS was specifically up-regulated despite declining selenium concentrations in the liver. Hepatic SepS induction was mainly controlled by post-transcriptional mechanisms and attributed to hepatocytes by analysing transgenic mice. Notably, selenium supplementation was essential for an optimal SepS induction. We conclude that selenoprotein biosynthesis becomes redirected in hepatocytes during the acute-phase response at the expense of dispensable selenoproteins (e.g. SepP) and in favour of SepS expression, thereby causing declining serum selenium and improving liver function. The selenium status and sex control SepS expression and modify cytokine response patterns in serum, which might explain contradictory results on associations of SEPS genotype and inflammatory diseases in clinical studies.

Selenoprotein P status correlates to cancer-specific mortality in renal cancer patients.

Selenium (Se) is an essential trace element for selenoprotein biosynthesis. Selenoproteins have been implicated in cancer risk and tumor development. Selenoprotein P (SePP) serves as the major Se transport protein in blood and as reliable biomarker of Se status in marginally supplied individuals. Among the different malignancies, renal cancer is characterized by a high mortality rate. In this study, we aimed to analyze the Se status in renal cell cancer (RCC) patients and whether it correlates to cancer-specific mortality. To this end, serum samples of RCC patients (n = 41) and controls (n = 21) were retrospectively analyzed. Serum Se and SePP concentrations were measured by X-ray fluorescence and an immunoassay, respectively. Clinical and survival data were compared to serum Se and SePP concentrations as markers of Se status by receiver operating characteristic (ROC) curve and Kaplan-Meier and Cox regression analyses. In our patients, higher tumor grade and tumor stage at diagnosis correlated to lower SePP and Se concentrations. Kaplan-Meier analyses indicated that low Se status at diagnosis (SePP<2.4 mg/l, bottom tertile of patient group) was associated with a poor 5-year survival rate of 20% only. We conclude that SePP and Se concentrations are of prognostic value in RCC and may serve as additional diagnostic biomarkers identifying a Se deficit in kidney cancer patients potentially affecting therapy regimen. As poor Se status was indicative of high mortality odds, we speculate that an adjuvant Se supplementation of Se-deficient RCC patients might be beneficial in order to stabilize their selenoprotein expression hopefully prolonging their survival. However, this assumption needs to be rigorously tested in prospective clinical trials.

Structure- and cell-specific effects of imidoselenocarbamates on selenoprotein expression and activity in liver cells in culture

The essential micronutrient selenium (Se) exerts its biological effects mainly through selenoproteins thereby affecting a number of physiological pathways including intracellular redox control, stress response and cancer cell proliferation. Besides affecting selenoprotein expression, some selenocompounds have been synthesized and analyzed in order to serve as chemotherapeutic substances preferentially targeting cancer cells. This promising chemotherapeutic potential has recently been verified for a particular imidoselenocarbamate in a mouse tumor model. In the present study we tested the effects of this and a number of related Se-methyl- and Se-benzyl-imidoselenocarbamates on selenoprotein expression in nontransformed and hepatic carcinoma cells in culture. Most of the Se-benzyl-imidoselenocarbamates strongly stimulated selenoprotein P (SePP) secretion while the Se-methyl-imidoselenocarbamates elicited less pronounced effects in hepatocarcinoma HepG2 cells. However, most of the Se-methyl-imidoselenocarbamates increased glutathione peroxidase (GPx) activity and decreased thioredoxin reductase (TXNRD) activity in parallel, while the majority of the Se-benzyl-imidoselenocarbamates were without a respective effect in HepG2 cells. Performing inhibitor assays in vitro, GPx activity was unaffected by the imidoselenocarbamates. In contrast, most of the Se-methyl-imidoselenocarbamates inhibited TXNRD activity in vitro in line with the results in HepG2 cells. Both classes of imidoselenocarbamates strongly induced selenoprotein S (SELS) expression without a respective increase in ER stress or unfolded protein response which are known inducers of SELS biosynthesis. Notably, many of these effects were cancer cell-specific, and not observed in nontransformed AML12 hepatocytes. Our results indicate that these novel selenocompounds affect expression and activity of crucial selenoenzymes in a compound- and cell-specific way in hepatocytes. Especially the Se-methyl-imidoselenocarbamates elicit a unique spectrum of activities by stimulating GPx activity, SELS expression and SePP secretion while inhibiting TXNRD activity in hepatocarcinoma cells. These effects represent a promising finding with respect to the identification of therapeutic selenocompounds, as cancer-cell specificity is combined with desired effects on selenoprotein expression and activity.

Aminoglycoside-driven biosynthesis of selenium-deficient Selenoprotein P

Selenoprotein biosynthesis relies on the co-translational insertion of selenocysteine in response to UGA codons. Aminoglycoside antibiotics interfere with ribosomal function and may cause codon misreading. We hypothesized that biosynthesis of the selenium (Se) transporter selenoprotein P (SELENOP) is particularly sensitive to antibiotics due to its ten in frame UGA codons. As liver regulates Se metabolism, we tested the aminoglycosides G418 and gentamicin in hepatoma cell lines (HepG2, Hep3B and Hepa1-6) and in experimental mice. In vitro, SELENOP levels increased strongly in response to G418, whereas expression of the glutathione peroxidases GPX1 and GPX2 was marginally affected. Se content of G418-induced SELENOP was dependent on Se availability, and was completely suppressed by G418 under Se-poor conditions. Selenocysteine residues were replaced mainly by cysteine, tryptophan and arginine in a codon-specific manner. Interestingly, in young healthy mice, antibiotic treatment failed to affect Selenop biosynthesis to a detectable degree. These findings suggest that the interfering activity of aminoglycosides on selenoprotein biosynthesis can be severe, but depend on the Se status, and other parameters likely including age and general health. Focused analyses with aminoglycoside-treated patients are needed next to evaluate a possible interference of selenoprotein biosynthesis by the antibiotics and elucidate potential side effects.