Positive and negative regulation of gene expression by insulin and glucagon: the model of L-type pyruvate kinase gene

Spot 14 protein interacts and co-operates with chicken ovalbumin upstream promoter-transcription factor 1 in the transcription of the L-type pyruvate kinase gene through a specificity protein 1 (Sp1) binding site

In hepatocytes, the amount of the Spot 14 (S14) protein is closely related to the full expression of enzymes involved in the glycolytic and lipogenic pathways. In the present study we address the role played by this protein in the control of transcription of the L-type pyruvate kinase (L-PK) gene in primary hepatocytes. We show that human S14, which by itself does not bind to the L-PK promoter, physically interacts with the human chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1) and induces the switch of this factor from a repressor to an activator. However, the enhancing activity of S14 and COUP-TF1 depends on the presence of a proximal GC-rich box (the L0 element) that specifically binds nuclear proteins from the livers of rats fed a glucose-rich diet. Moreover, the L0 element, which strongly binds dephosphorylated specificity protein 1 (Sp1), loses all affinity when this factor is phosphorylated by cAMP-dependent protein kinase. Mutations that affect binding of Sp1 and nuclear proteins to the L0 box also decrease basal transcription and impair glucose responsiveness of the promoter. These results therefore shed light on the mechanism by which the S14 protein, whose concentration rapidly rises after glucose intake, contributes to the full activity of the L-PK promoter.

Genetic and epigenetic regulation schemes: need for an alternative paradigm

Self-organization of living cells results from the tangle of positive and negative feedback developed to ensure their homeostasis and/or their differentiation. There are three major means cellular regulation operates: the genetic, the epigenetic, and the metabolic ones. The regulation type in each of them has been overviewed. Further examination of relations between complexity and developmental stability points out sui generis properties of feedback loops, which are redundancy and pleiotropy. Prototypical schemes for positive and negative regulation with redundant and pleiotropic (including multifunctional) proteins are presented. They stress a theoretical shift from the analytical to the systemic framework. The systemic paradigm appears to be of increasing interest and importance in the study of concepts for the representation of genetic and epigenetic regulations.