ALAS1 gene expression is down-regulated by Akt-mediated phosphorylation and nuclear exclusion of FOXO1 by vanadate in diabetic mice

Abstract

Porphyrias are diseases caused by partial deficiencies of haem biosynthesis enzymes. Acute porphyrias are characterized by a neuropsychiatric syndrome with intermittent induction of hepatic ALAS1 (ä-aminolaevulinate synthase 1), the first and rate-limiting enzyme of the haem pathway. Acute porphyria attacks are usually treated by the administration of glucose; its effect is apparently related to its ability to inhibit ALAS1 by modulating insulin plasma levels. It has been shown that insulin blunts hepatocyte ALAS1 induction, by disrupting the interaction of FOXO1 (forkhead box O1) and PGC-1á (peroxisome-proliferator-activated receptor ã co-activator 1á). We evaluated the expression of ALAS1 in a murine model of diabetes and determined the effects of the insulinomimetic vanadate on the enzyme regulation to evaluate its potential for the treatment of acute porphyria attacks. Both ALAS1 mRNA and protein content were induced in diabetic animals, accompanied by decreased Akt phosphorylation and increased nuclear FOXO1, PGC-1á and FOXO1-PGC-1á complex levels. Vanadate reversed ALAS1 induction, with a concomitant PI3K (phosphoinositide 3-kinase)/Akt pathway activation and subsequent reduction of nuclear FOXO1, PGC-1á and FOXO1-PGC-1á complex levels. These findings support the notion that the FOXO1-PGC-1á complex is involved in the control of ALAS1 expression and suggest further that a vanadate-based therapy could be beneficial for the treatment of acute porphyria attacks.