TRANSCRIPTION OF THE MITOCHONDRIAL CITRATE CARRIER GENE: ROLE OF SREBP-1, UPREGULATION BY INSULIN AND DOWNREGULATION BY PUFA

INTRODUCTION: Mature SREBPs are helix-loop-helix leucine zipper transcription factors that upregulate the transcription of some lipogenic and sterol biosynthesis genes. The mitochondrial citrate carrier (CIC), also known as the tricarboxylate carrier, is an integral protein of the mitochondrial inner membrane that is essential for fatty acid and sterol biosynthesis. In previous studies it was found that streptozotocin-treated diabetic rats exhibited a decrease in the activity of hepatic CIC, and this decrease was adjusted after insulin administration. Furthermore, a reduction of CIC activity, in parallel with CIC mRNA abundance, was demonstrated in the liver of rats fed with a diet enriched with polyunsaturated fatty acids (PUFA). However, the molecular mechanisms of the transcriptional control of the CIC gene are still not known. In this study we have investigated the role of SRE/SREBP-1 in the transcriptional regulation of the CIC gene. MATERIALS AND METHODS: Materials and methods employed are described in ref. 1. RESULTS: In EMSA experiments performed using nuclear extracts of HepG2 cells a specific band shift was observed with a probe from -1700 to -1679 bp of the human CIC gene promoter which encompasses the SRE region. Wild-type (but not mutated) CIC SRE cloned in front of the luciferase promoter conferred transcriptional activation of the gene reporter. Insulin activated and PUFA inhibited the gene reporter activity driven by the CIC promoter containing wild-type (but not mutated) SRE. Both insulin treatment and overexpression of SREBP-1 increased the CIC transcript and protein levels, whereas PUFA have an opposite effect. CONCLUSIONS: Our results show that a) the SRE site present in the human CIC gene promoter is an activation domain of this gene transcription, and b) the CIC transcriptional activation by insulin and the suppression by PUFA are mediated by the SRE/SREBP-1 regulatory system. [1] Iacobazzi, V., Infantino, V., Costanzo, P., Izzo, P., and Palmieri, F. (2005) Biochem. J. 391, 613- 621