Role of Sp1 and SREBP-1a in the insulin-mediated regulation of glucokinase transcription in the liver of gilthead sea bream (Sparus aurata).

Insulin induction of glucokinase (GCK) transcription in the liver is essential for maintaining glucose homeostasis. To study the molecular mechanism underlying the regulation of hepatic GCK expression in the carnivorous fish gilthead sea bream (Sparus aurata), we analysed the role of sterol regulatory element binding protein-1a (SREBP-1a) and specificity protein (Sp) 1 in insulin-dependent GCK transcription. Transient transfection experiments performed in HepG2 cells and electrophoretic mobility shift assays allowed us to identify a cis-element in the proximal region of GCK promoter implicated in transactivation by SREBP-1a. Consistently, mutations in the SRE binding site completely abolished the enhancing effect of SREBP-1a. These results and previous findings suggest that SREBP-1a plays a role in the transcriptional regulation of key enzymes in glycolysis-gluconeogenesis. Since SREBP-1a and Sp1 may mediate insulin action on S. aurata GCK transcription, we analysed the effect of insulin on HepG2 cells transfected with GCK promoter reporter constructs carrying intact or mutated SRE or Sp boxes. Insulin transactivated GCK irrespective of the presence of an intact or mutated SRE box. However, insulin failed to induce GCK transcription when using reporter constructs that had either a mutated Sp site or no Sp site. Our findings indicate that Sp1, rather than SREBP-1a, mediates the insulin-dependent induction of S. aurata GCK.