HEPATIC NUCLEAR FACTOR-3 AND INSULIN- REGULATED EXPRESSION OF THE 5-AMINOLEVULINATE SYNTHASE (ALAS) GENE.

SCASSA, M; CERUTI, JULIETA M; CÁNEPA, E.T

Córdoba, Argentina

Congreso; XXXVIII Reunión Anual de la Sociedad Argentina de Investigación en Bioquímica y Biología Molecular (SAIB); 2002

SAIB

The ALAS is the rate limiting enzyme for the synthesis of heme. Insulin affects the expression of over 150 genes although the molecular basis is poorly understood. Recently, we provided evidence that insulin causes a rapid inhibition of hepatic ALAS gene expression and that one or more sequences present in its promoter could be responsible for confering hormonal response. In this work we demonstrate that the region within the -469 to -354 bp confers hormonal sensitivity to a heterologous promoter in an orientation-dependent manner. This fragment includes an IRE-like sequence that has been associated with insulin-mediated transcriptional repression of a number of genes, and conserved HNF-3 (forkhead) cis-elements. Transcription from different ALAS/TK promoter CAT reporter constructs demonstrates that mutation or exclusion of the IRE abolish insulin action in HepG2 cells. Transfection  assays and Northern blots show a direct correlation between the amount of HNF3b overexpression and the ability of insulin to regulate transcription. A lost of the transactivation potential that HNF-3b possesses to modulate transcription was exhibited in transfected insulin-treated HeLa cells. However, this capability was recovered when MEK or PI3K inhibitors were added with the hormone. Collectivelly, these results led to the speculation that forkhead proteins might be target of insulin activated signal transduction pathways and be responsible for mediating the hormonal tissue-restricted regulation of ALAS expression.