Dual regulation of mouse D 5-and D 6-desaturase gene expression by SREBP-1 and PPAR a

In the process of seeking sterol regulatory elementbinding protein 1a (SREBP-1a) target genes, we identified and cloned a cDNA clone encoding mouse D 5 -desaturase (D5D). The hepatic expression of D5D as well as D 6 -desaturase (D6D) was highly activated in transgenic mice overexpressing nuclear SREBP-1a, -1c, and -2. Disruption of the SREBP-1 gene significantly reduced the expression of both desaturases in the livers of SREBP-1-deficient mice refed after fasting. The hepatic expression of both desaturases was downregulated by dietary PUFA, which were reported to suppress SREBP-1c gene expression. Sustained expression of hepatic nuclear SREBP-1c protein in the transgenic mice abolished the PUFA suppression of both desaturases. Although these data suggested that SREBP-1c regulates D5D and D6D expression, there was no difference in either the D5D or D6D mRNA level between fasted and refed normal mouse livers, indicating a mechanism for fasting induction of both desaturases. Administration of fibrate, a pharmacological ligand for peroxisome proliferator activating receptor a (PPAR a ), caused a significant increase in expression of both desaturases. The data suggested that D5D and D6D expression is dually regulated by SREBP-1c and PPAR a , two reciprocal transcription factors for fatty acid metabolism, and could be involved in lipogenic gene regulation by producing PUFA. —Matsuzaka, T. , H. Shimano, N. Yahagi, M. AmemiyaKudo, T. Yoshikawa, A. H. Hasty, Y. Tamura, J-i. Osuga, H. Okazaki, Y. Iizuka, A. Takahashi, H. Sone, T. Gotoda, S. Ishibashi, and N. Yamada. Dual regulation of mouse D 5 and D 6 desaturase gene expression by SREBP-1 and PPAR a . J. Lipid Res. 2002. 43: 107–114. Supplementary key words lipogenesis • nutrition • polyunsaturated fatty acids • sterol regulatory element-binding protein PUFA such as arachidonic acid (AA, 20:4n-6), EPA (20:5n-3), and DHA (22:6n-3) play pivotal roles in a number of biological functions including brain development, cognition, reproduction, inflammatory responses, and hemostasis. In addition to being crucial components of membrane phospholipids and functioning in key steps of cell signaling, PUFA have a unique ability to suppress the transcription of hepatic genes encoding lipogenic enzymes (1–4). The availability of 20and 22-carbon (n-3 and n-6) PUFA is determined by the synthesis of g -linolenic acid (18:3n-6) and stearidonic acid (18:4n-3) and by the subsequent elongation and desaturation of these fatty acids to 20:4n-6 and 22:6n-3 (5). One of two rate-limiting steps in the production of these polyunsaturated fatty acids is the desaturation of eicosatrienoic acid (20:3n-6) and eicosatetraenoic acid (20:4n-3) to 20:4n-6 and 20:5n-3, determined by the activity of D 5 -desaturase (D5D). The other step is the desaturation of linoleic acid (18:2n-6) and a linolenic acid (18:3n-3) to 18:3n-6 and 18:4n-3 catalyzed by D 6 -desaturase (D6D). Studies based on enzymatic activity suggest that the liver is the primary site for 20-carbon polyenoic fatty acid synthesis because the liver is an organ with a great amount of D6D and D5D activity (6). Both enzymes are coordinately regulated by various dietary conditions and hormonal levels in the liver. For instance, D6D activity is induced by peroxisome proliferators and by the supplementation of insulin to diabetic rats (7, 8). It is also highly suppressed by dietary PUFA, indicating that these enzymes are involved in feedback regulation in the production of AA, EPA, and DHA. Remarkable progress in understanding the molecular basis of the nutritional regulation of these desaturases has been made by cloning mammalian D5D and D6D cDNAs (9, 10). The predicted amino acid sequences were suggested to contain a cytochrome b 5 -like domain that might dispense with the presence of a separate cytochrome b 5 . Suppression of D5D and Abbreviations: AA, arachidonic acid; D5D, D 5 -desaturase; D6D, D 6 desaturase; PPAR, peroxisome proliferator activating receptor; SRE, sterol regulatory element; SREBP, sterol regulatory element-binding protein. 1 To whom correspondence should be addressed. e-mail: shimano-tky@umin.ac. jp