Selenoprotein P expression is controlled through interaction of the coactivator PGC-1alpha with FoxO1a and hepatocyte nuclear factor 4alpha transcription factors.

UNLABELLED: Selenoprotein P (SeP), the major selenoprotein in plasma, is produced mainly by the liver, although SeP expression is detected in many organs. Recently, we reported stimulation of SeP promoter activity by the forkhead box transcription factor FoxO1a in hepatoma cells and its attenuation by insulin. Here, we demonstrate that this translates into fine-tuning of SeP production and secretion by insulin. Overexpression of peroxisomal proliferator activated receptor-gamma coactivator 1alpha (PGC-1alpha) enhanced the stimulatory effect of FoxO1a on SeP promoter activity. We identified a novel functional binding site for hepatocyte nuclear factor (HNF)-4alpha, termed hepatocyte nuclear factor binding element 1, in the human SeP promoter directly upstream of the FoxO-responsive element daf16-binding element 2 (DBE2). Point mutations in hepatocyte nuclear factor binding element 1 alone or together with DBE2 decreased basal activity and responsiveness of the SeP promoter to PGC-1alpha. Moreover, the PGC-1alpha-inducing glucocorticoid dexamethasone strongly enhanced SeP messenger RNA levels and protein secretion in cultured rat hepatocytes, whereas insulin suppressed the stimulation of both PGC-1alpha and SeP caused by dexamethasone treatment. In a brain-derived neuroblastoma cell line with low basal SeP expression, SeP transcription was stimulated by PGC-1alpha together with FoxO1a, and overexpression of HNF-4alpha potentiated this effect. CONCLUSION: High-level expression of SeP in liver is ensured by concerted action of the coactivator PGC-1alpha and the transcription factors FoxO1a and HNF-4alpha. Hence, the production of SeP is regulated similarly to that of the gluconeogenic enzyme glucose-6-phosphatase. As hepatic SeP production is crucial for selenium distribution throughout the body, the present study establishes PGC-1alpha as a key regulator of selenium homeostasis.

Stimulation of selenoprotein P promoter activity in hepatoma cells by FoxO1a transcription factor.

Selenoprotein P (SeP) is the major selenoprotein in human plasma, acting as an antioxidant and serving the transport of selenium from the liver to extrahepatic tissues. We here demonstrate that the human SeP promoter responds to overexpression of FoxO1a as well as of a constitutively active form of FoxO1a. Two FoxO-responsive elements were identified and characterized by generation of point mutation and deletion constructs. Similarly, SeP mRNA was upregulated in response to activation of FoxO1a in rat hepatoma cells stably transfected with a hydroxytamoxifen-regulatable form of FoxO1a. Insulin, stimulating the phosphorylation and inactivation of FoxO1a via phosphoinositide 3-kinase (PI3K) and Akt, suppressed SeP promoter activity and mRNA synthesis. This suppressive effect of insulin on SeP expression was attenuated by inhibitors of PI3K. In conclusion, the selenoprotein P promoter is a target of the Akt/FoxO signal transduction cascade and SeP expression is regulated at the level of transcription by the forkhead box protein FoxO1a in human and rat hepatoma cells.

Stimulation of phosphoinositide 3-kinase/Akt signaling by copper and zinc ions: mechanisms and consequences.

The phosphoinositide 3'-kinase (PI3K)/Akt signaling cascade controls cellular processes such as apoptosis and proliferation. Moreover, it is a mediator of insulin effects on target cells and as such is a major regulator of fuel metabolism. The PI3K/Akt cascade was demonstrated to be activated by stressful stimuli, including heat shock and reactive oxygen species (ROS). This minireview focuses on activation of the pathway by exposure of cells to heavy metal ions, Cu2+ and Zn2+. It is hypothesized that stimulation of PI3K/Akt is the molecular mechanism underlying the known insulin-mimetic effects of copper and zinc ions. Following a brief summary of PI3K/Akt signaling and of activation of the cascade by Cu2+ and Zn2+, mechanisms of metal-induced PI3K/Akt activation are discussed with a focus on the role of ROS and of cellular thiols (glutathione, thioredoxin) and protein tyrosine phosphatases in Cu2+ and Zn2+ signaling. Finally, consequences of metal-induced PI3K/Akt activation are discussed, focusing on the modulation of FoxO-family transcription factors by Cu2+ and Zn2+.

Modulation of FoxO signaling in human hepatoma cells by exposure to copper or zinc ions.

Cells respond to heavy metal stress by activating signaling cascades regulating cellular proliferation and survival. We here demonstrate that the anti-apoptotic kinase Akt is activated in HepG2 human hepatoma cells exposed to copper or zinc ions. Cu2+- and Zn2+-induced phosphorylation of Akt was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin and LY294002. Moreover, several endogenous Akt substrates were phosphorylated, including glycogen synthase kinase-3 and transcription factors of the FoxO family, FoxO1a and FoxO4. Exposure to Cu2+ or Zn2+ elicited the subcellular redistribution of an overexpressed FoxO1a-EGFP fusion protein from nucleus to cytoplasm, which was not seen with a mutant FoxO1a form devoid of Akt phosphorylation sites. Both FoxO phosphorylation and nuclear exclusion were blocked by wortmannin. Likewise, the subcellular translocation from nucleus to cytoplasm of the Caenorhabditis elegans FoxO ortholog, DAF-16, was caused in starved worms exposed to copper ions. Activity of the promoter of the human glucose 6-phosphatase gene, known to be regulated by insulin and FoxO1a, was demonstrated in reporter gene assays to be attenuated in hepatoma cells exposed to Cu2+. However, this suppression of glucose 6-phosphatase promoter activity was independent of modulation of the PI3K/Akt pathway. In summary, the PI3K/Akt pathway is activated in human hepatoma cells exposed to Cu2+ or Zn2+, resulting in the phosphorylation and subcellular relocalisation of transcription factor FoxO1a. Furthermore, copper is demonstrated to exert an insulin-mimetic effect also independently of the PI3K/Akt/FoxO pathway.